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+1,"A computer is a machine that can be programmed to automatically carry out sequences of arithmetic or logical operations . Modern digital electronic computers can perform generic sets of operations known as programs. These programs enable computers to perform a wide range of tasks. The term computer system may refer to a nominally complete computer that includes the hardware, operating system, software, and peripheral equipment needed and used for full operation; or to a group of computers that are linked and function together, such as a computer network or computer cluster."
+2,"A broad range of industrial and consumer products use computers as control systems, including simple special-purpose devices like microwave ovens and remote controls, and factory devices like industrial robots. Computers are at the core of general-purpose devices such as personal computers and mobile devices such as smartphones. Computers power the Internet, which links billions of computers and users."
+3,"Early computers were meant to be used only for calculations. Simple manual instruments like the abacus have aided people in doing calculations since ancient times. Early in the Industrial Revolution, some mechanical devices were built to automate long, tedious tasks, such as guiding patterns for looms. More sophisticated electrical machines did specialized analog calculations in the early 20th century. The first digital electronic calculating machines were developed during World War II, both electromechanical and using thermionic valves. The first semiconductor transistors in the late 1940s were followed by the silicon-based MOSFET  and monolithic integrated circuit chip technologies in the late 1950s, leading to the microprocessor and the microcomputer revolution in the 1970s. The speed, power and versatility of computers have been increasing dramatically ever since then, with transistor counts increasing at a rapid pace , leading to the Digital Revolution during the late 20th to early 21st centuries."
+4,"Conventionally, a modern computer consists of at least one processing element, typically a central processing unit  in the form of a microprocessor, together with some type of computer memory, typically semiconductor memory chips. The processing element carries out arithmetic and logical operations, and a sequencing and control unit can change the order of operations in response to stored information. Peripheral devices include input devices , output devices , and input/output devices that perform both functions . Peripheral devices allow information to be retrieved from an external source and they enable the result of operations to be saved and retrieved."
+5,"According to the Oxford English Dictionary, the first known use of computer was in a 1613 book called The Yong Mans Gleanings by the English writer Richard Brathwait: ""I haue   read the truest computer of Times, and the best Arithmetician that euer   breathed, and he reduceth thy dayes into a short number."" This usage of the term referred to a human computer, a person who carried out calculations or computations. The word continued with the same meaning until the middle of the 20th century. During the latter part of this period women were often hired as computers because they could be paid less than their male counterparts. By 1943, most human computers were women."
+6,"The Online Etymology Dictionary gives the first attested use of computer in the 1640s, meaning 'one who calculates'; this is an ""agent noun from compute "". The Online Etymology Dictionary states that the use of the term to mean ""'calculating machine'  is from 1897."" The Online Etymology Dictionary indicates that the ""modern use"" of the term, to mean 'programmable digital electronic computer' dates from ""1945 under this name;  theoretical  from 1937, as Turing machine""."
+7,"Devices have been used to aid computation for thousands of years, mostly using one-to-one correspondence with fingers. The earliest counting device was most likely a form of tally stick. Later record keeping aids throughout the Fertile Crescent included calculi  which represented counts of items, likely livestock or grains, sealed in hollow unbaked clay containers. The use of counting rods is one example."
+8,"The abacus was initially used for arithmetic tasks. The Roman abacus was developed from devices used in Babylonia as early as 2400 BCE. Since then, many other forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, and markers moved around on it according to certain rules, as an aid to calculating sums of money."
+9,"The Antikythera mechanism is believed to be the earliest known mechanical analog computer, according to Derek J. de Solla Price. It was designed to calculate astronomical positions. It was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to approximately c. 100 BCE. Devices of comparable complexity to the Antikythera mechanism would not reappear until the fourteenth century."
+10,"Many mechanical aids to calculation and measurement were constructed for astronomical and navigation use. The planisphere was a star chart invented by Abū Rayhān al-Bīrūnī in the early 11th century. The astrolabe was invented in the Hellenistic world in either the 1st or 2nd centuries BCE and is often attributed to Hipparchus. A combination of the planisphere and dioptra, the astrolabe was effectively an analog computer capable of working out several different kinds of problems in spherical astronomy. An astrolabe incorporating a mechanical calendar computer and gear-wheels was invented by Abi Bakr of Isfahan, Persia in 1235. Abū Rayhān al-Bīrūnī invented the first mechanical geared lunisolar calendar astrolabe, an early fixed-wired knowledge processing machine with a gear train and gear-wheels, c. 1000 AD."
+11,"The sector, a calculating instrument used for solving problems in proportion, trigonometry, multiplication and division, and for various functions, such as squares and cube roots, was developed in the late 16th century and found application in gunnery, surveying and navigation."
+12,The planimeter was a manual instrument to calculate the area of a closed figure by tracing over it with a mechanical linkage.
+13,"The slide rule was invented around 1620–1630 by the English clergyman William Oughtred, shortly after the publication of the concept of the logarithm. It is a hand-operated analog computer for doing multiplication and division. As slide rule development progressed, added scales provided reciprocals, squares and square roots, cubes and cube roots, as well as transcendental functions such as logarithms and exponentials, circular and hyperbolic trigonometry and other functions. Slide rules with special scales are still used for quick performance of routine calculations, such as the E6B circular slide rule used for time and distance calculations on light aircraft."
+14,"In the 1770s, Pierre Jaquet-Droz, a Swiss watchmaker, built a mechanical doll  that could write holding a quill pen. By switching the number and order of its internal wheels different letters, and hence different messages, could be produced. In effect, it could be mechanically ""programmed"" to read instructions. Along with two other complex machines, the doll is at the Musée d'Art et d'Histoire of Neuchâtel, Switzerland, and still operates."
+15,"In 1831–1835, mathematician and engineer Giovanni Plana devised a Perpetual Calendar machine, which, through a system of pulleys and cylinders and over, could predict the perpetual calendar for every year from 0 CE  to 4000 CE, keeping track of leap years and varying day length. The tide-predicting machine invented by the Scottish scientist Sir William Thomson in 1872 was of great utility to navigation in shallow waters. It used a system of pulleys and wires to automatically calculate predicted tide levels for a set period at a particular location."
+16,"The differential analyser, a mechanical analog computer designed to solve differential equations by integration, used wheel-and-disc mechanisms to perform the integration. In 1876, Sir William Thomson had already discussed the possible construction of such calculators, but he had been stymied by the limited output torque of the ball-and-disk integrators. In a differential analyzer, the output of one integrator drove the input of the next integrator, or a graphing output. The torque amplifier was the advance that allowed these machines to work. Starting in the 1920s, Vannevar Bush and others developed mechanical differential analyzers."
+17,"In the 1890s, the Spanish engineer Leonardo Torres Quevedo began to develop a series of advanced analog machines that could solve real and complex roots of polynomials, which were published in 1901 by the Paris Academy of Sciences."
+18,"Charles Babbage, an English mechanical engineer and polymath, originated the concept of a programmable computer. Considered the ""father of the computer"", he conceptualized and invented the first mechanical computer in the early 19th century."
+19,"After working on his difference engine he announced his invention in 1822, in a paper to the Royal Astronomical Society, titled ""Note on the application of machinery to the computation of astronomical and mathematical tables"", he also designed to aid in navigational calculations, in 1833 he realized that a much more general design, an analytical engine, was possible. The input of programs and data was to be provided to the machine via punched cards, a method being used at the time to direct mechanical looms such as the Jacquard loom. For output, the machine would have a printer, a curve plotter and a bell. The machine would also be able to punch numbers onto cards to be read in later. The Engine incorporated an arithmetic logic unit, control flow in the form of conditional branching and loops, and integrated memory, making it the first design for a general-purpose computer that could be described in modern terms as Turing-complete."
+20,"The machine was about a century ahead of its time. All the parts for his machine had to be made by hand – this was a major problem for a device with thousands of parts. Eventually, the project was dissolved with the decision of the British Government to cease funding. Babbage's failure to complete the analytical engine can be chiefly attributed to political and financial difficulties as well as his desire to develop an increasingly sophisticated computer and to move ahead faster than anyone else could follow. Nevertheless, his son, Henry Babbage, completed a simplified version of the analytical engine's computing unit  in 1888. He gave a successful demonstration of its use in computing tables in 1906."
+21,"During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers, which used a direct mechanical or electrical model of the problem as a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers. The first modern analog computer was a tide-predicting machine, invented by Sir William Thomson  in 1872. The differential analyser, a mechanical analog computer designed to solve differential equations by integration using wheel-and-disc mechanisms, was conceptualized in 1876 by James Thomson, the elder brother of the more famous Sir William Thomson."
+22,"The art of mechanical analog computing reached its zenith with the differential analyzer, built by H. L. Hazen and Vannevar Bush at MIT starting in 1927. This built on the mechanical integrators of James Thomson and the torque amplifiers invented by H. W. Nieman. A dozen of these devices were built before their obsolescence became obvious. By the 1950s, the success of digital electronic computers had spelled the end for most analog computing machines, but analog computers remained in use during the 1950s in some specialized applications such as education  and aircraft ."
+23,"By 1938, the United States Navy had developed an electromechanical analog computer small enough to use aboard a submarine. This was the Torpedo Data Computer, which used trigonometry to solve the problem of firing a torpedo at a moving target. During World War II similar devices were developed in other countries as well."
+24,"Early digital computers were electromechanical; electric switches drove mechanical relays to perform the calculation. These devices had a low operating speed and were eventually superseded by much faster all-electric computers, originally using vacuum tubes. The Z2, created by German engineer Konrad Zuse in 1939 in Berlin, was one of the earliest examples of an electromechanical relay computer."
+25,"In 1941, Zuse followed his earlier machine up with the Z3, the world's first working electromechanical programmable, fully automatic digital computer. The Z3 was built with 2000 relays, implementing a 22 bit word length that operated at a clock frequency of about 5–10 Hz. Program code was supplied on punched film while data could be stored in 64 words of memory or supplied from the keyboard. It was quite similar to modern machines in some respects, pioneering numerous advances such as floating-point numbers. Rather than the harder-to-implement decimal system , using a binary system meant that Zuse's machines were easier to build and potentially more reliable, given the technologies available at that time. The Z3 was not itself a universal computer but could be extended to be Turing complete."
+26,"Zuse's next computer, the Z4, became the world's first commercial computer; after initial delay due to the Second World War, it was completed in 1950 and delivered to the ETH Zurich. The computer was manufactured by Zuse's own company, Zuse KG, which was founded in 1941 as the first company with the sole purpose of developing computers in Berlin."
+27,"Purely electronic circuit elements soon replaced their mechanical and electromechanical equivalents, at the same time that digital calculation replaced analog. The engineer Tommy Flowers, working at the Post Office Research Station in London in the 1930s, began to explore the possible use of electronics for the telephone exchange. Experimental equipment that he built in 1934 went into operation five years later, converting a portion of the telephone exchange network into an electronic data processing system, using thousands of vacuum tubes. In the US, John Vincent Atanasoff and Clifford E. Berry of Iowa State University developed and tested the Atanasoff–Berry Computer  in 1942, the first ""automatic electronic digital computer"". This design was also all-electronic and used about 300 vacuum tubes, with capacitors fixed in a mechanically rotating drum for memory."
+28,"During World War II, the British code-breakers at Bletchley Park achieved a number of successes at breaking encrypted German military communications. The German encryption machine, Enigma, was first attacked with the help of the electro-mechanical bombes which were often run by women. To crack the more sophisticated German Lorenz SZ 40/42 machine, used for high-level Army communications, Max Newman and his colleagues commissioned Flowers to build the Colossus. He spent eleven months from early February 1943 designing and building the first Colossus. After a functional test in December 1943, Colossus was shipped to Bletchley Park, where it was delivered on 18 January 1944 and attacked its first message on 5 February."
+29,"Colossus was the world's first electronic digital programmable computer. It used a large number of valves . It had paper-tape input and was capable of being configured to perform a variety of boolean logical operations on its data, but it was not Turing-complete. Nine Mk II Colossi were built . Colossus Mark I contained 1,500 thermionic valves , but Mark II with 2,400 valves, was both five times faster and simpler to operate than Mark I, greatly speeding the decoding process."
+30,"The ENIAC  was the first electronic programmable computer built in the U.S. Although the ENIAC was similar to the Colossus, it was much faster, more flexible, and it was Turing-complete. Like the Colossus, a ""program"" on the ENIAC was defined by the states of its patch cables and switches, a far cry from the stored program electronic machines that came later. Once a program was written, it had to be mechanically set into the machine with manual resetting of plugs and switches. The programmers of the ENIAC were six women, often known collectively as the ""ENIAC girls""."
+31,"It combined the high speed of electronics with the ability to be programmed for many complex problems. It could add or subtract 5000 times a second, a thousand times faster than any other machine. It also had modules to multiply, divide, and square root. High speed memory was limited to 20 words . Built under the direction of John Mauchly and J. Presper Eckert at the University of Pennsylvania, ENIAC's development and construction lasted from 1943 to full operation at the end of 1945. The machine was huge, weighing 30 tons, using 200 kilowatts of electric power and contained over 18,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, and inductors."
+32,"The principle of the modern computer was proposed by Alan Turing in his seminal 1936 paper, On Computable Numbers. Turing proposed a simple device that he called ""Universal Computing machine"" and that is now known as a universal Turing machine. He proved that such a machine is capable of computing anything that is computable by executing instructions  stored on tape, allowing the machine to be programmable. The fundamental concept of Turing's design is the stored program, where all the instructions for computing are stored in memory. Von Neumann acknowledged that the central concept of the modern computer was due to this paper. Turing machines are to this day a central object of study in theory of computation. Except for the limitations imposed by their finite memory stores, modern computers are said to be Turing-complete, which is to say, they have algorithm execution capability equivalent to a universal Turing machine."
+33,"Early computing machines had fixed programs. Changing its function required the re-wiring and re-structuring of the machine. With the proposal of the stored-program computer this changed. A stored-program computer includes by design an instruction set and can store in memory a set of instructions  that details the computation. The theoretical basis for the stored-program computer was laid out by Alan Turing in his 1936 paper. In 1945, Turing joined the National Physical Laboratory and began work on developing an electronic stored-program digital computer. His 1945 report ""Proposed Electronic Calculator"" was the first specification for such a device. John von Neumann at the University of Pennsylvania also circulated his First Draft of a Report on the EDVAC in 1945."
+34,"The Manchester Baby was the world's first stored-program computer. It was built at the University of Manchester in England by Frederic C. Williams, Tom Kilburn and Geoff Tootill, and ran its first program on 21 June 1948. It was designed as a testbed for the Williams tube, the first random-access digital storage device. Although the computer was described as ""small and primitive"" by a 1998 retrospective, it was the first working machine to contain all of the elements essential to a modern electronic computer. As soon as the Baby had demonstrated the feasibility of its design, a project began at the university to develop it into a practically useful computer, the Manchester Mark 1."
+35,"The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer. Built by Ferranti, it was delivered to the University of Manchester in February 1951. At least seven of these later machines were delivered between 1953 and 1957, one of them to Shell labs in Amsterdam. In October 1947 the directors of British catering company J. Lyons & Company decided to take an active role in promoting the commercial development of computers. Lyons's LEO I computer, modelled closely on the Cambridge EDSAC of 1949, became operational in April 1951 and ran the world's first routine office computer job."
+36,Grace Hopper was the first to develop a compiler for a programming language.
+37,"The concept of a field-effect transistor was proposed by Julius Edgar Lilienfeld in 1925. John Bardeen and Walter Brattain, while working under William Shockley at Bell Labs, built the first working transistor, the point-contact transistor, in 1947, which was followed by Shockley's bipolar junction transistor in 1948. From 1955 onwards, transistors replaced vacuum tubes in computer designs, giving rise to the ""second generation"" of computers. Compared to vacuum tubes, transistors have many advantages: they are smaller, and require less power than vacuum tubes, so give off less heat. Junction transistors were much more reliable than vacuum tubes and had longer, indefinite, service life. Transistorized computers could contain tens of thousands of binary logic circuits in a relatively compact space. However, early junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis, which limited them to a number of specialized applications."
+38,"At the University of Manchester, a team under the leadership of Tom Kilburn designed and built a machine using the newly developed transistors instead of valves. Their first transistorized computer and the first in the world, was operational by 1953, and a second version was completed there in April 1955. However, the machine did make use of valves to generate its 125 kHz clock waveforms and in the circuitry to read and write on its magnetic drum memory, so it was not the first completely transistorized computer. That distinction goes to the Harwell CADET of 1955, built by the electronics division of the Atomic Energy Research Establishment at Harwell."
+39,"The metal–oxide–silicon field-effect transistor , also known as the MOS transistor, was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959. It was the first truly compact transistor that could be miniaturized and mass-produced for a wide range of uses. With its high scalability, and much lower power consumption and higher density than bipolar junction transistors, the MOSFET made it possible to build high-density integrated circuits. In addition to data processing, it also enabled the practical use of MOS transistors as memory cell storage elements, leading to the development of MOS semiconductor memory, which replaced earlier magnetic-core memory in computers. The MOSFET led to the microcomputer revolution, and became the driving force behind the computer revolution. The MOSFET is the most widely used transistor in computers, and is the fundamental building block of digital electronics."
+40,"The next great advance in computing power came with the advent of the integrated circuit .
+The idea of the integrated circuit was first conceived by a radar scientist working for the Royal Radar Establishment of the Ministry of Defence, Geoffrey W.A. Dummer. Dummer presented the first public description of an integrated circuit at the Symposium on Progress in Quality Electronic Components in Washington, D.C., on 7 May 1952."
+41,"The first working ICs were invented by Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Semiconductor. Kilby recorded his initial ideas concerning the integrated circuit in July 1958, successfully demonstrating the first working integrated example on 12 September 1958. In his patent application of 6 February 1959, Kilby described his new device as ""a body of semiconductor material ... wherein all the components of the electronic circuit are completely integrated"". However, Kilby's invention was a hybrid integrated circuit , rather than a monolithic integrated circuit  chip. Kilby's IC had external wire connections, which made it difficult to mass-produce."
+42,"Noyce also came up with his own idea of an integrated circuit half a year later than Kilby. Noyce's invention was the first true monolithic IC chip. His chip solved many practical problems that Kilby's had not. Produced at Fairchild Semiconductor, it was made of silicon, whereas Kilby's chip was made of germanium. Noyce's monolithic IC was fabricated using the planar process, developed by his colleague Jean Hoerni in early 1959. In turn, the planar process was based on Mohamed M. Atalla's work on semiconductor surface passivation by silicon dioxide in the late 1950s."
+43,"Modern monolithic ICs are predominantly MOS  integrated circuits, built from MOSFETs . The earliest experimental MOS IC to be fabricated was a 16-transistor chip built by Fred Heiman and Steven Hofstein at RCA in 1962. General Microelectronics later introduced the first commercial MOS IC in 1964, developed by Robert Norman. Following the development of the self-aligned gate  MOS transistor by Robert Kerwin, Donald Klein and John Sarace at Bell Labs in 1967, the first silicon-gate MOS IC with self-aligned gates was developed by Federico Faggin at Fairchild Semiconductor in 1968. The MOSFET has since become the most critical device component in modern ICs."
+44,"The development of the MOS integrated circuit led to the invention of the microprocessor, and heralded an explosion in the commercial and personal use of computers. While the subject of exactly which device was the first microprocessor is contentious, partly due to lack of agreement on the exact definition of the term ""microprocessor"", it is largely undisputed that the first single-chip microprocessor was the Intel 4004, designed and realized by Federico Faggin with his silicon-gate MOS IC technology, along with Ted Hoff, Masatoshi Shima and Stanley Mazor at Intel. In the early 1970s, MOS IC technology enabled the integration of more than 10,000 transistors on a single chip."
+45,"System on a Chip  are complete computers on a microchip  the size of a coin. They may or may not have integrated RAM and flash memory. If not integrated, the RAM is usually placed directly above  or below  the SoC, and the flash memory is usually placed right next to the SoC, this all done to improve data transfer speeds, as the data signals do not have to travel long distances. Since ENIAC in 1945, computers have advanced enormously, with modern SoCs  being the size of a coin while also being hundreds of thousands of times more powerful than ENIAC, integrating billions of transistors, and consuming only a few watts of power."
+46,"The first mobile computers were heavy and ran from mains power. The 50 lb  IBM 5100 was an early example. Later portables such as the Osborne 1 and Compaq Portable were considerably lighter but still needed to be plugged in. The first laptops, such as the Grid Compass, removed this requirement by incorporating batteries – and with the continued miniaturization of computing resources and advancements in portable battery life, portable computers grew in popularity in the 2000s. The same developments allowed manufacturers to integrate computing resources into cellular mobile phones by the early 2000s."
+47,"These smartphones and tablets run on a variety of operating systems and recently became the dominant computing device on the market. These are powered by System on a Chip , which are complete computers on a microchip the size of a coin."
+48,"Computers can be classified in a number of different ways, including:"
+49,"The term hardware covers all of those parts of a computer that are tangible physical objects. Circuits, computer chips, graphic cards, sound cards, memory , motherboard, displays, power supplies, cables, keyboards, printers and ""mice"" input devices are all hardware."
+50,"A general-purpose computer has four main components: the arithmetic logic unit , the control unit, the memory, and the input and output devices . These parts are interconnected by buses, often made of groups of wires. Inside each of these parts are thousands to trillions of small electrical circuits which can be turned off or on by means of an electronic switch. Each circuit represents a bit  of information so that when the circuit is on it represents a ""1"", and when off it represents a ""0"" . The circuits are arranged in logic gates so that one or more of the circuits may control the state of one or more of the other circuits."
+51,"When unprocessed data is sent to the computer with the help of input devices, the data is processed and sent to output devices. The input devices may be hand-operated or automated. The act of processing is mainly regulated by the CPU. Some examples of input devices are:"
+52,The means through which computer gives output are known as output devices. Some examples of output devices are:
+53,"The control unit  manages the computer's various components; it reads and interprets  the program instructions, transforming them into control signals that activate other parts of the computer. Control systems in advanced computers may change the order of execution of some instructions to improve performance."
+54,"A key component common to all CPUs is the program counter, a special memory cell  that keeps track of which location in memory the next instruction is to be read from."
+55,"The control system's function is as follows— this is a simplified description, and some of these steps may be performed concurrently or in a different order depending on the type of CPU:"
+56,"Since the program counter is  just another set of memory cells, it can be changed by calculations done in the ALU. Adding 100 to the program counter would cause the next instruction to be read from a place 100 locations further down the program. Instructions that modify the program counter are often known as ""jumps"" and allow for loops  and often conditional instruction execution ."
+57,"The sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program, and indeed, in some more complex CPU designs, there is another yet smaller computer called a microsequencer, which runs a microcode program that causes all of these events to happen."
+58,"The control unit, ALU, and registers are collectively known as a central processing unit . Early CPUs were composed of many separate components. Since the 1970s, CPUs have typically been constructed on a single MOS integrated circuit chip called a microprocessor."
+59,"The ALU is capable of performing two classes of operations: arithmetic and logic. The set of arithmetic operations that a particular ALU supports may be limited to addition and subtraction, or might include multiplication, division, trigonometry functions such as sine, cosine, etc., and square roots. Some can operate only on whole numbers  while others use floating point to represent real numbers, albeit with limited precision. However, any computer that is capable of performing just the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, any computer can be programmed to perform any arithmetic operation—although it will take more time to do so if its ALU does not directly support the operation. An ALU may also compare numbers and return Boolean truth values  depending on whether one is equal to, greater than or less than the other . Logic operations involve Boolean logic: AND, OR, XOR, and NOT. These can be useful for creating complicated conditional statements and processing Boolean logic."
+60,"Superscalar computers may contain multiple ALUs, allowing them to process several instructions simultaneously. Graphics processors and computers with SIMD and MIMD features often contain ALUs that can perform arithmetic on vectors and matrices."
+61,"A computer's memory can be viewed as a list of cells into which numbers can be placed or read. Each cell has a numbered ""address"" and can store a single number. The computer can be instructed to ""put the number 123 into the cell numbered 1357"" or to ""add the number that is in cell 1357 to the number that is in cell 2468 and put the answer into cell 1595."" The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease. Since the CPU does not differentiate between different types of information, it is the software's responsibility to give significance to what the memory sees as nothing but a series of numbers."
+62,"In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits . Each byte is able to represent 256 different numbers ; either from 0 to 255 or −128 to +127. To store larger numbers, several consecutive bytes may be used . When negative numbers are required, they are usually stored in two's complement notation. Other arrangements are possible, but are usually not seen outside of specialized applications or historical contexts. A computer can store any kind of information in memory if it can be represented numerically. Modern computers have billions or even trillions of bytes of memory."
+63,"The CPU contains a special set of memory cells called registers that can be read and written to much more rapidly than the main memory area. There are typically between two and one hundred registers depending on the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. As data is constantly being worked on, reducing the need to access main memory  greatly increases the computer's speed."
+64,Computer main memory comes in two principal varieties:
+65,"RAM can be read and written to anytime the CPU commands it, but ROM is preloaded with data and software that never changes, therefore the CPU can only read from it. ROM is typically used to store the computer's initial start-up instructions. In general, the contents of RAM are erased when the power to the computer is turned off, but ROM retains its data indefinitely. In a PC, the ROM contains a specialized program called the BIOS that orchestrates loading the computer's operating system from the hard disk drive into RAM whenever the computer is turned on or reset. In embedded computers, which frequently do not have disk drives, all of the required software may be stored in ROM. Software stored in ROM is often called firmware, because it is notionally more like hardware than software. Flash memory blurs the distinction between ROM and RAM, as it retains its data when turned off but is also rewritable. It is typically much slower than conventional ROM and RAM however, so its use is restricted to applications where high speed is unnecessary."
+66,"In more sophisticated computers there may be one or more RAM cache memories, which are slower than registers but faster than main memory. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part."
+67,"I/O is the means by which a computer exchanges information with the outside world. Devices that provide input or output to the computer are called peripherals. On a typical personal computer, peripherals include input devices like the keyboard and mouse, and output devices such as the display and printer. Hard disk drives, floppy disk drives and optical disc drives serve as both input and output devices. Computer networking is another form of I/O.
+I/O devices are often complex computers in their own right, with their own CPU and memory. A graphics processing unit might contain fifty or more tiny computers that perform the calculations necessary to display 3D graphics. Modern desktop computers contain many smaller computers that assist the main CPU in performing I/O. A 2016-era flat screen display contains its own computer circuitry."
+68,"While a computer may be viewed as running one gigantic program stored in its main memory, in some systems it is necessary to give the appearance of running several programs simultaneously. This is achieved by multitasking i.e. having the computer switch rapidly between running each program in turn. One means by which this is done is with a special signal called an interrupt, which can periodically cause the computer to stop executing instructions where it was and do something else instead. By remembering where it was executing prior to the interrupt, the computer can return to that task later. If several programs are running ""at the same time"". then the interrupt generator might be causing several hundred interrupts per second, causing a program switch each time. Since modern computers typically execute instructions several orders of magnitude faster than human perception, it may appear that many programs are running at the same time even though only one is ever executing in any given instant. This method of multitasking is sometimes termed ""time-sharing"" since each program is allocated a ""slice"" of time in turn."
+69,"Before the era of inexpensive computers, the principal use for multitasking was to allow many people to share the same computer. Seemingly, multitasking would cause a computer that is switching between several programs to run more slowly, in direct proportion to the number of programs it is running, but most programs spend much of their time waiting for slow input/output devices to complete their tasks. If a program is waiting for the user to click on the mouse or press a key on the keyboard, then it will not take a ""time slice"" until the event it is waiting for has occurred. This frees up time for other programs to execute so that many programs may be run simultaneously without unacceptable speed loss."
+70,"Some computers are designed to distribute their work across several CPUs in a multiprocessing configuration, a technique once employed in only large and powerful machines such as supercomputers, mainframe computers and servers. Multiprocessor and multi-core  personal and laptop computers are now widely available, and are being increasingly used in lower-end markets as a result."
+71,"Supercomputers in particular often have highly unique architectures that differ significantly from the basic stored-program architecture and from general-purpose computers. They often feature thousands of CPUs, customized high-speed interconnects, and specialized computing hardware. Such designs tend to be useful for only specialized tasks due to the large scale of program organization required to use most of the available resources at once. Supercomputers usually see usage in large-scale simulation, graphics rendering, and cryptography applications, as well as with other so-called ""embarrassingly parallel"" tasks."
+72,"Software refers to parts of the computer which do not have a material form, such as programs, data, protocols, etc. Software is that part of a computer system that consists of encoded information or computer instructions, in contrast to the physical hardware from which the system is built. Computer software includes computer programs, libraries and related non-executable data, such as online documentation or digital media. It is often divided into system software and application software. Computer hardware and software require each other and neither can be realistically used on its own. When software is stored in hardware that cannot easily be modified, such as with BIOS ROM in an IBM PC compatible computer, it is sometimes called ""firmware""."
+73,"There are thousands of different programming languages—some intended for general purpose, others useful for only highly specialized applications."
+74,"The defining feature of modern computers which distinguishes them from all other machines is that they can be programmed. That is to say that some type of instructions  can be given to the computer, and it will process them. Modern computers based on the von Neumann architecture often have machine code in the form of an imperative programming language. In practical terms, a computer program may be just a few instructions or extend to many millions of instructions, as do the programs for word processors and web browsers for example. A typical modern computer can execute billions of instructions per second  and rarely makes a mistake over many years of operation. Large computer programs consisting of several million instructions may take teams of programmers years to write, and due to the complexity of the task almost certainly contain errors."
+75,This section applies to most common RAM machine–based computers.
+76,"In most cases, computer instructions are simple: add one number to another, move some data from one location to another, send a message to some external device, etc. These instructions are read from the computer's memory and are generally carried out  in the order they were given. However, there are usually specialized instructions to tell the computer to jump ahead or backwards to some other place in the program and to carry on executing from there. These are called ""jump"" instructions . Furthermore, jump instructions may be made to happen conditionally so that different sequences of instructions may be used depending on the result of some previous calculation or some external event. Many computers directly support subroutines by providing a type of jump that ""remembers"" the location it jumped from and another instruction to return to the instruction following that jump instruction."
+77,"Program execution might be likened to reading a book. While a person will normally read each word and line in sequence, they may at times jump back to an earlier place in the text or skip sections that are not of interest. Similarly, a computer may sometimes go back and repeat the instructions in some section of the program over and over again until some internal condition is met. This is called the flow of control within the program and it is what allows the computer to perform tasks repeatedly without human intervention."
+78,"Comparatively, a person using a pocket calculator can perform a basic arithmetic operation such as adding two numbers with just a few button presses. But to add together all of the numbers from 1 to 1,000 would take thousands of button presses and a lot of time, with a near certainty of making a mistake. On the other hand, a computer may be programmed to do this with just a few simple instructions. The following example is written in the MIPS assembly language:"
+79,"Once told to run this program, the computer will perform the repetitive addition task without further human intervention. It will almost never make a mistake and a modern PC can complete the task in a fraction of a second."
+80,"In most computers, individual instructions are stored as machine code with each instruction being given a unique number . The command to add two numbers together would have one opcode; the command to multiply them would have a different opcode, and so on. The simplest computers are able to perform any of a handful of different instructions; the more complex computers have several hundred to choose from, each with a unique numerical code. Since the computer's memory is able to store numbers, it can also store the instruction codes. This leads to the important fact that entire programs  can be represented as lists of numbers and can themselves be manipulated inside the computer in the same way as numeric data. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture. In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. This is called the Harvard architecture after the Harvard Mark I computer. Modern von Neumann computers display some traits of the Harvard architecture in their designs, such as in CPU caches."
+81,"While it is possible to write computer programs as long lists of numbers  and while this technique was used with many early computers, it is extremely tedious and potentially error-prone to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is indicative of its function and easy to remember – a mnemonic such as ADD, SUB, MULT or JUMP. These mnemonics are collectively known as a computer's assembly language. Converting programs written in assembly language into something the computer can actually understand  is usually done by a computer program called an assembler."
+82,"Programming languages provide various ways of specifying programs for computers to run. Unlike natural languages, programming languages are designed to permit no ambiguity and to be concise. They are purely written languages and are often difficult to read aloud. They are generally either translated into machine code by a compiler or an assembler before being run, or translated directly at run time by an interpreter. Sometimes programs are executed by a hybrid method of the two techniques."
+83,"Machine languages and the assembly languages that represent them  are generally unique to the particular architecture of a computer's central processing unit . For instance, an ARM architecture CPU  cannot understand the machine language of an x86 CPU that might be in a PC. Historically a significant number of other cpu architectures were created and saw extensive use, notably including the MOS Technology 6502 and 6510 in addition to the Zilog Z80."
+84,"Although considerably easier than in machine language, writing long programs in assembly language is often difficult and is also error prone. Therefore, most practical programs are written in more abstract high-level programming languages that are able to express the needs of the programmer more conveniently . High level languages are usually ""compiled"" into machine language  using another computer program called a compiler. High level languages are less related to the workings of the target computer than assembly language, and more related to the language and structure of the problem to be solved by the final program. It is therefore often possible to use different compilers to translate the same high level language program into the machine language of many different types of computer. This is part of the means by which software like video games may be made available for different computer architectures such as personal computers and various video game consoles."
+85,"Program design of small programs is relatively simple and involves the analysis of the problem, collection of inputs, using the programming constructs within languages, devising or using established procedures and algorithms, providing data for output devices and solutions to the problem as applicable. As problems become larger and more complex, features such as subprograms, modules, formal documentation, and new paradigms such as object-oriented programming are encountered. Large programs involving thousands of line of code and more require formal software methodologies. The task of developing large software systems presents a significant intellectual challenge. Producing software with an acceptably high reliability within a predictable schedule and budget has historically been difficult; the academic and professional discipline of software engineering concentrates specifically on this challenge."
+86,"Errors in computer programs are called ""bugs"". They may be benign and not affect the usefulness of the program, or have only subtle effects. However, in some cases they may cause the program or the entire system to ""hang"", becoming unresponsive to input such as mouse clicks or keystrokes, to completely fail, or to crash. Otherwise benign bugs may sometimes be harnessed for malicious intent by an unscrupulous user writing an exploit, code designed to take advantage of a bug and disrupt a computer's proper execution. Bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design. Admiral Grace Hopper, an American computer scientist and developer of the first compiler, is credited for having first used the term ""bugs"" in computing after a dead moth was found shorting a relay in the Harvard Mark II computer in September 1947."
+87,"Computers have been used to coordinate information between multiple locations since the 1950s. The U.S. military's SAGE system was the first large-scale example of such a system, which led to a number of special-purpose commercial systems such as Sabre. In the 1970s, computer engineers at research institutions throughout the United States began to link their computers together using telecommunications technology. The effort was funded by ARPA , and the computer network that resulted was called the ARPANET. The technologies that made the Arpanet possible spread and evolved."
+88,"In time, the network spread beyond academic and military institutions and became known as the Internet. The emergence of networking involved a redefinition of the nature and boundaries of the computer. Computer operating systems and applications were modified to include the ability to define and access the resources of other computers on the network, such as peripheral devices, stored information, and the like, as extensions of the resources of an individual computer. Initially these facilities were available primarily to people working in high-tech environments, but in the 1990s the spread of applications like e-mail and the World Wide Web, combined with the development of cheap, fast networking technologies like Ethernet and ADSL saw computer networking become almost ubiquitous. In fact, the number of computers that are networked is growing phenomenally. A very large proportion of personal computers regularly connect to the Internet to communicate and receive information. ""Wireless"" networking, often utilizing mobile phone networks, has meant networking is becoming increasingly ubiquitous even in mobile computing environments."
+89,"A computer does not need to be electronic, nor even have a processor, nor RAM, nor even a hard disk. While popular usage of the word ""computer"" is synonymous with a personal electronic computer, a typical modern definition of a computer is: ""A device that computes, especially a programmable  electronic machine that performs high-speed mathematical or logical operations or that assembles, stores, correlates, or otherwise processes information."" According to this definition, any device that processes information qualifies as a computer."
+90,"There is active research to make non-classical computers out of many promising new types of technology, such as optical computers, DNA computers, neural computers, and quantum computers. Most computers are universal, and are able to calculate any computable function, and are limited only by their memory capacity and operating speed. However different designs of computers can give very different performance for particular problems; for example quantum computers can potentially break some modern encryption algorithms  very quickly."
+91,There are many types of computer architectures:
+92,"Of all these abstract machines, a quantum computer holds the most promise for revolutionizing computing. Logic gates are a common abstraction which can apply to most of the above digital or analog paradigms. The ability to store and execute lists of instructions called programs makes computers extremely versatile, distinguishing them from calculators. The Church–Turing thesis is a mathematical statement of this versatility: any computer with a minimum capability  is, in principle, capable of performing the same tasks that any other computer can perform. Therefore, any type of computer  is able to perform the same computational tasks, given enough time and storage capacity."
+93,"A computer will solve problems in exactly the way it is programmed to, without regard to efficiency, alternative solutions, possible shortcuts, or possible errors in the code. Computer programs that learn and adapt are part of the emerging field of artificial intelligence and machine learning. Artificial intelligence based products generally fall into two major categories: rule-based systems and pattern recognition systems. Rule-based systems attempt to represent the rules used by human experts and tend to be expensive to develop. Pattern-based systems use data about a problem to generate conclusions. Examples of pattern-based systems include voice recognition, font recognition, translation and the emerging field of on-line marketing."
+94,"As the use of computers has spread throughout society, there are an increasing number of careers involving computers."
+95,"The need for computers to work well together and to be able to exchange information has spawned the need for many standards organizations, clubs and societies of both a formal and informal nature."
+96,"Testbank contains randomized versions of this quiz for classroom use.  For more information on printing these quizzes, see How to use testbank."
+97,"For on-screen use, this version of the quiz can be used as a pre-reading activity for Wikipedia:Computer.  An excellent homework assignment might be to ask students to submit more quiz questions to Wikiversity."
+98,"Some speech recognition systems require ""training""  where an individual speaker reads text or isolated vocabulary into the system. The system analyzes the person's specific voice and uses it to fine-tune the recognition of that person's speech, resulting in increased accuracy. Systems that do not use training are called ""speaker-independent"" systems. Systems that use training are called ""speaker dependent""."
+99,"Speech recognition applications include voice user interfaces such as voice dialing , call routing , domotic appliance control, search key words , simple data entry , preparation of structured documents , determining speaker characteristics, speech-to-text processing , and aircraft . Automatic pronunciation assessment is used in education such as for spoken language learning."
+100,"The term voice recognition or speaker identification refers to identifying the speaker, rather than what they are saying. Recognizing the speaker can simplify the task of translating speech in systems that have been trained on a specific person's voice or it can be used to authenticate or verify the identity of a speaker as part of a security process."
+101,"From the technology perspective, speech recognition has a long history with several waves of major innovations. Most recently, the field has benefited from advances in deep learning and big data. The advances are evidenced not only by the surge of academic papers published in the field, but more importantly by the worldwide industry adoption of a variety of deep learning methods in designing and deploying speech recognition systems."
+102,"The key areas of growth were: vocabulary size, speaker independence, and processing speed."
+103,Raj Reddy was the first person to take on continuous speech recognition as a graduate student at Stanford University in the late 1960s. Previous systems required users to pause after each word. Reddy's system issued spoken commands for playing chess.
+104,"Around this time Soviet researchers invented the dynamic time warping  algorithm and used it to create a recognizer capable of operating on a 200-word vocabulary. DTW processed speech by dividing it into short frames, e.g. 10ms segments, and processing each frame as a single unit. Although DTW would be superseded by later algorithms, the technique carried on. Achieving speaker independence remained unsolved at this time period."
+105,"During the late 1960s Leonard Baum developed the mathematics of Markov chains at the Institute for Defense Analysis. A decade later, at CMU, Raj Reddy's students James Baker and Janet M. Baker began using the hidden Markov model  for speech recognition. James Baker had learned about HMMs from a summer job at the Institute of Defense Analysis during his undergraduate education. The use of HMMs allowed researchers to combine different sources of knowledge, such as acoustics, language, and syntax, in a unified probabilistic model."
+106,The 1980s also saw the introduction of the n-gram language model.
+107,"Much of the progress in the field is owed to the rapidly increasing capabilities of computers. At the end of the DARPA program in 1976, the best computer available to researchers was the PDP-10 with 4 MB ram. It could take up to 100 minutes to decode just 30 seconds of speech."
+108,Two practical products were:
+109,"By this point, the vocabulary of the typical commercial speech recognition system was larger than the average human vocabulary. Raj Reddy's former student, Xuedong Huang, developed the Sphinx-II system at CMU. The Sphinx-II system was the first to do speaker-independent, large vocabulary, continuous speech recognition and it had the best performance in DARPA's 1992 evaluation. Handling continuous speech with a large vocabulary was a major milestone in the history of speech recognition. Huang went on to found the speech recognition group at Microsoft in 1993. Raj Reddy's student Kai-Fu Lee joined Apple where, in 1992, he helped develop a speech interface prototype for the Apple computer known as Casper."
+110,"Lernout & Hauspie, a Belgium-based speech recognition company, acquired several other companies, including Kurzweil Applied Intelligence in 1997 and Dragon Systems in 2000. The L&H speech technology was used in the Windows XP operating system. L&H was an industry leader until an accounting scandal brought an end to the company in 2001. The speech technology from L&H was bought by ScanSoft which became Nuance in 2005. Apple originally licensed software from Nuance to provide speech recognition capability to its digital assistant Siri."
+111,"In the 2000s DARPA sponsored two speech recognition programs: Effective Affordable Reusable Speech-to-Text  in 2002 and Global Autonomous Language Exploitation . Four teams participated in the EARS program: IBM, a team led by BBN with LIMSI and Univ. of Pittsburgh, Cambridge University, and a team composed of ICSI, SRI and University of Washington. EARS funded the collection of the Switchboard telephone speech corpus containing 260 hours of recorded conversations from over 500 speakers. The GALE program focused on Arabic and Mandarin broadcast news speech. Google's first effort at speech recognition came in 2007 after hiring some researchers from Nuance. The first product was GOOG-411, a telephone based directory service. The recordings from GOOG-411 produced valuable data that helped Google improve their recognition systems. Google Voice Search is now supported in over 30 languages."
+112,"In the United States, the National Security Agency has made use of a type of speech recognition for keyword spotting since at least 2006. This technology allows analysts to search through large volumes of recorded conversations and isolate mentions of keywords. Recordings can be indexed and analysts can run queries over the database to find conversations of interest. Some government research programs focused on intelligence applications of speech recognition, e.g. DARPA's EARS's program and IARPA's Babel program."
+113,"By early 2010s speech recognition, also called voice recognition was clearly differentiated from speaker recognition, and speaker independence was considered a major breakthrough. Until then, systems required a ""training"" period.  A 1987 ad for a doll had carried the tagline ""Finally, the doll that understands you."" – despite the fact that it was described as ""which children could train to respond to their voice""."
+114,"In 2017, Microsoft researchers reached a historical human parity milestone of transcribing conversational telephony speech on the widely benchmarked Switchboard task. Multiple deep learning models were used to optimize speech recognition accuracy. The speech recognition word error rate was reported to be as low as 4 professional human transcribers working together on the same benchmark, which was funded by IBM Watson speech team on the same task."
+115,Both acoustic modeling and language modeling are important parts of modern statistically based speech recognition algorithms. Hidden Markov models  are widely used in many systems. Language modeling is also used in many other natural language processing applications such as document classification or statistical machine translation.
+116,"Modern general-purpose speech recognition systems are based on hidden Markov models. These are statistical models that output a sequence of symbols or quantities. HMMs are used in speech recognition because a speech signal can be viewed as a piecewise stationary signal or a short-time stationary signal. In a short time scale , speech can be approximated as a stationary process. Speech can be thought of as a Markov model for many stochastic purposes."
+117,"Another reason why HMMs are popular is that they can be trained automatically and are simple and computationally feasible to use. In speech recognition, the hidden Markov model would output a sequence of n-dimensional real-valued vectors , outputting one of these every 10 milliseconds. The vectors would consist of cepstral coefficients, which are obtained by taking a Fourier transform of a short time window of speech and decorrelating the spectrum using a cosine transform, then taking the first  coefficients. The hidden Markov model will tend to have in each state a statistical distribution that is a mixture of diagonal covariance Gaussians, which will give a likelihood for each observed vector. Each word, or , each phoneme, will have a different output distribution; a hidden Markov model for a sequence of words or phonemes is made by concatenating the individual trained hidden Markov models for the separate words and phonemes."
+118,"Described above are the core elements of the most common, HMM-based approach to speech recognition. Modern speech recognition systems use various combinations of a number of standard techniques in order to improve results over the basic approach described above. A typical large-vocabulary system would need context dependency for the phonemes ; it would use cepstral normalization to normalize for a different speaker and recording conditions; for further speaker normalization, it might use vocal tract length normalization  for male-female normalization and maximum likelihood linear regression  for more general speaker adaptation. The features would have so-called delta and delta-delta coefficients to capture speech dynamics and in addition, might use heteroscedastic linear discriminant analysis ; or might skip the delta and delta-delta coefficients and use splicing and an LDA-based projection followed perhaps by heteroscedastic linear discriminant analysis or a global semi-tied co variance transform . Many systems use so-called discriminative training techniques that dispense with a purely statistical approach to HMM parameter estimation and instead optimize some classification-related measure of the training data. Examples are maximum mutual information , minimum classification error , and minimum phone error ."
+119,"Decoding of the speech  would probably use the Viterbi algorithm to find the best path, and here there is a choice between dynamically creating a combination hidden Markov model, which includes both the acoustic and language model information and combining it statically beforehand ."
+120,"A possible improvement to decoding is to keep a set of good candidates instead of just keeping the best candidate, and to use a better scoring function  to rate these good candidates so that we may pick the best one according to this refined score. The set of candidates can be kept either as a list  or as a subset of the models . Re scoring is usually done by trying to minimize the Bayes risk : Instead of taking the source sentence with maximal probability, we try to take the sentence that minimizes the expectancy of a given loss function with regards to all possible transcriptions . The loss function is usually the Levenshtein distance, though it can be different distances for specific tasks; the set of possible transcriptions is, of course, pruned to maintain tractability. Efficient algorithms have been devised to re score lattices represented as weighted finite state transducers with edit distances represented themselves as a finite state transducer verifying certain assumptions."
+121,Dynamic time warping is an approach that was historically used for speech recognition but has now largely been displaced by the more successful HMM-based approach.
+122,"Dynamic time warping is an algorithm for measuring similarity between two sequences that may vary in time or speed. For instance, similarities in walking patterns would be detected, even if in one video the person was walking slowly and if in another he or she were walking more quickly, or even if there were accelerations and deceleration during the course of one observation. DTW has been applied to video, audio, and graphics – indeed, any data that can be turned into a linear representation can be analyzed with DTW."
+123,"A well-known application has been automatic speech recognition, to cope with different speaking speeds. In general, it is a method that allows a computer to find an optimal match between two given sequences  with certain restrictions. That is, the sequences are ""warped"" non-linearly to match each other. This sequence alignment method is often used in the context of hidden Markov models."
+124,"Neural networks emerged as an attractive acoustic modeling approach in ASR in the late 1980s. Since then, neural networks have been used in many aspects of speech recognition such as phoneme classification, phoneme classification through multi-objective evolutionary algorithms, isolated word recognition, audiovisual speech recognition, audiovisual speaker recognition and speaker adaptation."
+125,"Neural networks make fewer explicit assumptions about feature statistical properties than HMMs and have several qualities making them attractive recognition models for speech recognition. When used to estimate the probabilities of a speech feature segment, neural networks allow discriminative training in a natural and efficient manner. However, in spite of their effectiveness in classifying short-time units such as individual phonemes and isolated words, early neural networks were rarely successful for continuous recognition tasks because of their limited ability to model temporal dependencies."
+126,"One approach to this limitation was to use neural networks as a pre-processing, feature transformation or dimensionality reduction, step prior to HMM based recognition. However, more recently, LSTM and related recurrent neural networks , Time Delay Neural Networks, and transformers have demonstrated improved performance in this area."
+127,"Deep neural networks and denoising autoencoders are also under investigation. A deep feedforward neural network  is an artificial neural network with multiple hidden layers of units between the input and output layers. Similar to shallow neural networks, DNNs can model complex non-linear relationships. DNN architectures generate compositional models, where extra layers enable composition of features from lower layers, giving a huge learning capacity and thus the potential of modeling complex patterns of speech data."
+128,"Since 2014, there has been much research interest in ""end-to-end"" ASR. Traditional phonetic-based  approaches required separate components and training for the pronunciation, acoustic, and language model. End-to-end models jointly learn all the components of the speech recognizer. This is valuable since it simplifies the training process and deployment process. For example, a n-gram language model is required for all HMM-based systems, and a typical n-gram language model often takes several gigabytes in memory making them impractical to deploy on mobile devices. Consequently, modern commercial ASR systems from Google and Apple  are deployed on the cloud and require a network connection as opposed to the device locally."
+129,"The first attempt at end-to-end ASR was with Connectionist Temporal Classification -based systems introduced by Alex Graves of Google DeepMind and Navdeep Jaitly of the University of Toronto in 2014. The model consisted of recurrent neural networks and a CTC layer. Jointly, the RNN-CTC model learns the pronunciation and acoustic model together, however it is incapable of learning the language due to conditional independence assumptions similar to a HMM. Consequently, CTC models can directly learn to map speech acoustics to English characters, but the models make many common spelling mistakes and must rely on a separate language model to clean up the transcripts. Later, Baidu expanded on the work with extremely large datasets and demonstrated some commercial success in Chinese Mandarin and English. In 2016, University of Oxford presented LipNet, the first end-to-end sentence-level lipreading model, using spatiotemporal convolutions coupled with an RNN-CTC architecture, surpassing human-level performance in a restricted grammar dataset. A large-scale CNN-RNN-CTC architecture was presented in 2018 by Google DeepMind achieving 6 times better performance than human experts."
+130,"An alternative approach to CTC-based models are attention-based models. Attention-based ASR models were introduced simultaneously by Chan et al. of Carnegie Mellon University and Google Brain and Bahdanau et al. of the University of Montreal in 2016. The model named ""Listen, Attend and Spell"" , literally ""listens"" to the acoustic signal, pays ""attention"" to different parts of the signal and ""spells"" out the transcript one character at a time. Unlike CTC-based models, attention-based models do not have conditional-independence assumptions and can learn all the components of a speech recognizer including the pronunciation, acoustic and language model directly. This means, during deployment, there is no need to carry around a language model making it very practical for applications with limited memory. By the end of 2016, the attention-based models have seen considerable success including outperforming the CTC models . Various extensions have been proposed since the original LAS model. Latent Sequence Decompositions  was proposed by Carnegie Mellon University, MIT and Google Brain to directly emit sub-word units which are more natural than English characters; University of Oxford and Google DeepMind extended LAS to ""Watch, Listen, Attend and Spell""  to handle lip reading surpassing human-level performance."
+131,"Typically a manual control input, for example by means of a finger control on the steering-wheel, enables the speech recognition system and this is signaled to the driver by an audio prompt. Following the audio prompt, the system has a ""listening window"" during which it may accept a speech input for recognition. "
+132,"Simple voice commands may be used to initiate phone calls, select radio stations or play music from a compatible smartphone, MP3 player or music-loaded flash drive. Voice recognition capabilities vary between car make and model. Some of the most recent car models offer natural-language speech recognition in place of a fixed set of commands, allowing the driver to use full sentences and common phrases. With such systems there is, therefore, no need for the user to memorize a set of fixed command words."
+133,"Automatic pronunciation assessment is the use of speech recognition to verify the correctness of pronounced speech, as distinguished from manual assessment by an instructor or proctor. Also called speech verification, pronunciation evaluation, and pronunciation scoring, the main application of this technology is computer-aided pronunciation teaching  when combined with computer-aided instruction for computer-assisted language learning , speech remediation, or accent reduction. Pronunciation assessment does not determine unknown speech  but instead, knowing the expected word in advance, it attempts to verify the correctness of the learner's pronunciation and ideally their intelligibility to listeners, sometimes along with often inconsequential prosody such as intonation, pitch, tempo, rhythm, and stress. Pronunciation assessment is also used in reading tutoring, for example in products such as Microsoft Teams and from Amira Learning. Automatic pronunciation assessment can also be used to help diagnose and treat speech disorders such as apraxia."
+134,"Assessing authentic listener intelligibility is essential for avoiding inaccuracies from accent bias, especially in high-stakes assessments; from words with multiple correct pronunciations; and from phoneme coding errors in machine-readable pronunciation dictionaries. In 2022, researchers found that some newer speech to text systems, based on end-to-end reinforcement learning to map audio signals directly into words, produce word and phrase confidence scores very closely correlated with genuine listener intelligibility. In the Common European Framework of Reference for Languages  assessment criteria for ""overall phonological control"", intelligibility outweighs formally correct pronunciation at all levels."
+135,"In the health care sector, speech recognition can be implemented in front-end or back-end of the medical documentation process. Front-end speech recognition is where the provider dictates into a speech-recognition engine, the recognized words are displayed as they are spoken, and the dictator is responsible for editing and signing off on the document. Back-end or deferred speech recognition is where the provider dictates into a digital dictation system, the voice is routed through a speech-recognition machine and the recognized draft document is routed along with the original voice file to the editor, where the draft is edited and report finalized. Deferred speech recognition is widely used in the industry currently."
+136,"One of the major issues relating to the use of speech recognition in healthcare is that the American Recovery and Reinvestment Act of 2009  provides for substantial financial benefits to physicians who utilize an EMR according to ""Meaningful Use"" standards. These standards require that a substantial amount of data be maintained by the EMR . The use of speech recognition is more naturally suited to the generation of narrative text, as part of a radiology/pathology interpretation, progress note or discharge summary: the ergonomic gains of using speech recognition to enter structured discrete data  are relatively minimal for people who are sighted and who can operate a keyboard and mouse."
+137,"A more significant issue is that most EHRs have not been expressly tailored to take advantage of voice-recognition capabilities. A large part of the clinician's interaction with the EHR involves navigation through the user interface using menus, and tab/button clicks, and is heavily dependent on keyboard and mouse: voice-based navigation provides only modest ergonomic benefits. By contrast, many highly customized systems for radiology or pathology dictation implement voice ""macros"", where the use of certain phrases – e.g., ""normal report"", will automatically fill in a large number of default values and/or generate boilerplate, which will vary with the type of the exam – e.g., a chest X-ray vs. a gastrointestinal contrast series for a radiology system."
+138,Prolonged use of speech recognition software in conjunction with word processors has shown benefits to short-term-memory restrengthening in brain AVM patients who have been treated with resection. Further research needs to be conducted to determine cognitive benefits for individuals whose AVMs have been treated using radiologic techniques.
+139,"Substantial efforts have been devoted in the last decade to the test and evaluation of speech recognition in fighter aircraft. Of particular note have been the US program in speech recognition for the Advanced Fighter Technology Integration /F-16 aircraft , the program in France for Mirage aircraft, and other programs in the UK dealing with a variety of aircraft platforms. In these programs, speech recognizers have been operated successfully in fighter aircraft, with applications including setting radio frequencies, commanding an autopilot system, setting steer-point coordinates and weapons release parameters, and controlling flight display."
+140,"Working with Swedish pilots flying in the JAS-39 Gripen cockpit, Englund  found recognition deteriorated with increasing g-loads. The report also concluded that adaptation greatly improved the results in all cases and that the introduction of models for breathing was shown to improve recognition scores significantly. Contrary to what might have been expected, no effects of the broken English of the speakers were found. It was evident that spontaneous speech caused problems for the recognizer, as might have been expected. A restricted vocabulary, and above all, a proper syntax, could thus be expected to improve recognition accuracy substantially."
+141,"The Eurofighter Typhoon, currently in service with the UK RAF, employs a speaker-dependent system, requiring each pilot to create a template. The system is not used for any safety-critical or weapon-critical tasks, such as weapon release or lowering of the undercarriage, but is used for a wide range of other cockpit functions. Voice commands are confirmed by visual and/or aural feedback. The system is seen as a major design feature in the reduction of pilot workload, and even allows the pilot to assign targets to his aircraft with two simple voice commands or to any of his wingmen with only five commands."
+142,Speaker-independent systems are also being developed and are under test for the F35 Lightning II  and the Alenia Aermacchi M-346 Master lead-in fighter trainer. These systems have produced word accuracy scores in excess of 98%.
+143,"The problems of achieving high recognition accuracy under stress and noise are particularly relevant in the helicopter environment as well as in the jet fighter environment. The acoustic noise problem is actually more severe in the helicopter environment, not only because of the high noise levels but also because the helicopter pilot, in general, does not wear a facemask, which would reduce acoustic noise in the microphone. Substantial test and evaluation programs have been carried out in the past decade in speech recognition systems applications in helicopters, notably by the U.S. Army Avionics Research and Development Activity  and by the Royal Aerospace Establishment  in the UK. Work in France has included speech recognition in the Puma helicopter. There has also been much useful work in Canada. Results have been encouraging, and voice applications have included: control of communication radios, setting of navigation systems, and control of an automated target handover system."
+144,"As in fighter applications, the overriding issue for voice in helicopters is the impact on pilot effectiveness. Encouraging results are reported for the AVRADA tests, although these represent only a feasibility demonstration in a test environment. Much remains to be done both in speech recognition and in overall speech technology in order to consistently achieve performance improvements in operational settings."
+145,"Training for air traffic controllers  represents an excellent application for speech recognition systems. Many ATC training systems currently require a person to act as a ""pseudo-pilot"", engaging in a voice dialog with the trainee controller, which simulates the dialog that the controller would have to conduct with pilots in a real ATC situation. Speech recognition and synthesis techniques offer the potential to eliminate the need for a person to act as a pseudo-pilot, thus reducing training and support personnel. In theory, Air controller tasks are also characterized by highly structured speech as the primary output of the controller, hence reducing the difficulty of the speech recognition task should be possible. In practice, this is rarely the case. The FAA document 7110.65 details the phrases that should be used by air traffic controllers. While this document gives less than 150 examples of such phrases, the number of phrases supported by one of the simulation vendors speech recognition systems is in excess of 500,000."
+146,"The USAF, USMC, US Army, US Navy, and FAA as well as a number of international ATC training organizations such as the Royal Australian Air Force and Civil Aviation Authorities in Italy, Brazil, and Canada are currently using ATC simulators with speech recognition from a number of different vendors."
+147,"ASR is now commonplace in the field of telephony and is becoming more widespread in the field of computer gaming and simulation. In telephony systems, ASR is now being predominantly used in contact centers by integrating it with IVR systems. Despite the high level of integration with word processing in general personal computing, in the field of document production, ASR has not seen the expected increases in use."
+148,"The improvement of mobile processor speeds has made speech recognition practical in smartphones. Speech is used mostly as a part of a user interface, for creating predefined or custom speech commands."
+149,"People with disabilities can benefit from speech recognition programs. For individuals that are Deaf or Hard of Hearing, speech recognition software is used to automatically generate a closed-captioning of conversations such as discussions in conference rooms, classroom lectures, and/or religious services."
+150,"Students who are blind  or have very low vision can benefit from using the technology to convey words and then hear the computer recite them, as well as use a computer by commanding with their voice, instead of having to look at the screen and keyboard."
+151,"Students who are physically disabled have a Repetitive strain injury/other injuries to the upper extremities can be relieved from having to worry about handwriting, typing, or working with scribe on school assignments by using speech-to-text programs. They can also utilize speech recognition technology to enjoy searching the Internet or using a computer at home without having to physically operate a mouse and keyboard."
+152,"Speech recognition can allow students with learning disabilities to become better writers. By saying the words aloud, they can increase the fluidity of their writing, and be alleviated of concerns regarding spelling, punctuation, and other mechanics of writing. Also, see Learning disability."
+153,"The use of voice recognition software, in conjunction with a digital audio recorder and a personal computer running word-processing software has proven to be positive for restoring damaged short-term memory capacity, in stroke and craniotomy individuals."
+154,"Speech recognition is also very useful for people who have difficulty using their hands, ranging from mild repetitive stress injuries to involve disabilities that preclude using conventional computer input devices. In fact, people who used the keyboard a lot and developed RSI became an urgent early market for speech recognition. Speech recognition is used in deaf telephony, such as voicemail to text, relay services, and captioned telephone. Individuals with learning disabilities who have problems with thought-to-paper communication  can possibly benefit from the software but the technology is not bug proof. Also the whole idea of speak to text can be hard for intellectually disabled person's due to the fact that it is rare that anyone tries to learn the technology to teach the person with the disability."
+155,"This type of technology can help those with dyslexia but other disabilities are still in question. The effectiveness of the product is the problem that is hindering it from being effective. Although a kid may be able to say a word depending on how clear they say it the technology may think they are saying another word and input the wrong one. Giving them more work to fix, causing them to have to take more time with fixing the wrong word."
+156,"The performance of speech recognition systems is usually evaluated in terms of accuracy and speed. Accuracy is usually rated with word error rate , whereas speed is measured with the real time factor. Other measures of accuracy include Single Word Error Rate  and Command Success Rate ."
+157,"Speech recognition by machine is a very complex problem, however. Vocalizations vary in terms of accent, pronunciation, articulation, roughness, nasality, pitch, volume, and speed. Speech is distorted by a background noise and echoes, electrical characteristics. Accuracy of speech recognition may vary with the following:"
+158,"As mentioned earlier in this article, the accuracy of speech recognition may vary depending on the following factors:"
+159,"With discontinuous speech full sentences separated by silence are used, therefore it becomes easier to recognize the speech as well as with isolated speech. 
+
+With continuous speech naturally spoken sentences are used, therefore it becomes harder to recognize the speech, different from both isolated and discontinuous speech."
+160,Constraints are often represented by grammar.
+161,Speech recognition is a multi-leveled pattern recognition task.
+162,"e.g. Known word pronunciations or legal word sequences, which can compensate for errors or uncertainties at a lower level;"
+163,For telephone speech the sampling rate is 8000 samples per second;
+164,"computed every 10 ms, with one 10 ms section called a frame;"
+165,"Analysis of four-step neural network approaches can be explained by further information. Sound is produced by air  vibration, which we register by ears, but machines by receivers. Basic sound creates a wave which has two descriptions: amplitude , and frequency .
+Accuracy can be computed with the help of word error rate . Word error rate can be calculated by aligning the recognized word and referenced word using dynamic string alignment. The problem may occur while computing the word error rate due to the difference between the sequence lengths of the recognized word and referenced word."
+166,The formula to compute the word error rate  is:
+167,"W
+        E
+        R
+        =
+        
+          
+            
+              
+            
+            n
+          
+        
+      
+    
+    {\displaystyle WER={ \over n}}"
+168,"where s is the number of substitutions, d is the number of deletions, i is the number of insertions, and n is the number of word references."
+169,"While computing, the word recognition rate  is used. The formula is:"
+170,where h is the number of correctly recognized words:
+171,"Speech recognition can become a means of attack, theft, or accidental operation. For example, activation words like ""Alexa"" spoken in an audio or video broadcast can cause devices in homes and offices to start listening for input inappropriately, or possibly take an unwanted action. Voice-controlled devices are also accessible to visitors to the building, or even those outside the building if they can be heard inside. Attackers may be able to gain access to personal information, like calendar, address book contents, private messages, and documents. They may also be able to impersonate the user to send messages or make online purchases."
+172,"Two attacks have been demonstrated that use artificial sounds. One transmits ultrasound and attempt to send commands without nearby people noticing. The other adds small, inaudible distortions to other speech or music that are specially crafted to confuse the specific speech recognition system into recognizing music as speech, or to make what sounds like one command to a human sound like a different command to the system."
+173,"Popular speech recognition conferences held each year or two include SpeechTEK and SpeechTEK Europe, ICASSP, Interspeech/Eurospeech, and the IEEE ASRU. Conferences in the field of natural language processing, such as ACL, NAACL, EMNLP, and HLT, are beginning to include papers on speech processing. Important journals include the IEEE Transactions on Speech and Audio Processing , Computer Speech and Language, and Speech Communication."
+174,"Books like ""Fundamentals of Speech Recognition"" by Lawrence Rabiner can be useful to acquire basic knowledge but may not be fully up to date . Another good source can be ""Statistical Methods for Speech Recognition"" by Frederick Jelinek and ""Spoken Language Processing "" by Xuedong Huang etc., ""Computer Speech"", by Manfred R. Schroeder, second edition published in 2004, and ""Speech Processing: A Dynamic and Optimization-Oriented Approach"" published in 2003 by Li Deng and Doug O'Shaughnessey. The updated textbook Speech and Language Processing  by Jurafsky and Martin presents the basics and the state of the art for ASR. Speaker recognition also uses the same features, most of the same front-end processing, and classification techniques as is done in speech recognition. A comprehensive textbook, ""Fundamentals of Speaker Recognition"" is an in depth source for up to date details on the theory and practice. A good insight into the techniques used in the best modern systems can be gained by paying attention to government sponsored evaluations such as those organised by DARPA ."
+175,"A good and accessible introduction to speech recognition technology and its history is provided by the general audience book ""The Voice in the Machine. Building Computers That Understand Speech"" by Roberto Pieraccini ."
+176,"The most recent book on speech recognition is Automatic Speech Recognition: A Deep Learning Approach  written by Microsoft researchers D. Yu and L. Deng and published near the end of 2014, with highly mathematically oriented technical detail on how deep learning methods are derived and implemented in modern speech recognition systems based on DNNs and related deep learning methods. A related book, published earlier in 2014, ""Deep Learning: Methods and Applications"" by L. Deng and D. Yu provides a less technical but more methodology-focused overview of DNN-based speech recognition during 2009–2014, placed within the more general context of deep learning applications including not only speech recognition but also image recognition, natural language processing, information retrieval, multimodal processing, and multitask learning."
+177,"In terms of freely available resources, Carnegie Mellon University's Sphinx toolkit is one place to start to both learn about speech recognition and to start experimenting. Another resource  is the HTK book . For more recent and state-of-the-art techniques, Kaldi toolkit can be used. In 2017 Mozilla launched the open source project called Common Voice to gather big database of voices that would help build free speech recognition project DeepSpeech , using Google's open source platform TensorFlow. When Mozilla redirected funding away from the project in 2020, it was forked by its original developers as Coqui STT using the same open-source license."
+178,Google Gboard supports speech recognition on all Android applications. It can be activated through the microphone icon.
+179,The commercial cloud based speech recognition APIs are broadly available.
+180,"For more software resources, see List of speech recognition software."
+181,"Pattern recognition systems are commonly trained from labeled ""training"" data. When no labeled data are available, other algorithms can be used to discover previously unknown patterns. KDD and data mining have a larger focus on unsupervised methods and stronger connection to business use. Pattern recognition focuses more on the signal and also takes acquisition and signal processing into consideration. It originated in engineering, and the term is popular in the context of computer vision: a leading computer vision conference is named Conference on Computer Vision and Pattern Recognition."
+182,"In machine learning, pattern recognition is the assignment of a label to a given input value. In statistics, discriminant analysis was introduced for this same purpose in 1936. An example of pattern recognition is classification, which attempts to assign each input value to one of a given set of classes . Pattern recognition is a more general problem that encompasses other types of output as well. Other examples are regression, which assigns a real-valued output to each input; sequence labeling, which assigns a class to each member of a sequence of values ; and parsing, which assigns a parse tree to an input sentence, describing the syntactic structure of the sentence."
+183,"Pattern recognition algorithms generally aim to provide a reasonable answer for all possible inputs and to perform ""most likely"" matching of the inputs, taking into account their statistical variation. This is opposed to pattern matching algorithms, which look for exact matches in the input with pre-existing patterns. A common example of a pattern-matching algorithm is regular expression matching, which looks for patterns of a given sort in textual data and is included in the search capabilities of many text editors and word processors."
+184,A modern definition of pattern recognition is:
+185,"Pattern recognition is generally categorized according to the type of learning procedure used to generate the output value. Supervised learning assumes that a set of training data  has been provided, consisting of a set of instances that have been properly labeled by hand with the correct output. A learning procedure then generates a model that attempts to meet two sometimes conflicting objectives: Perform as well as possible on the training data, and generalize as well as possible to new data . Unsupervised learning, on the other hand, assumes training data that has not been hand-labeled, and attempts to find inherent patterns in the data that can then be used to determine the correct output value for new data instances. A combination of the two that has been explored is semi-supervised learning, which uses a combination of labeled and unlabeled data . In cases of unsupervised learning, there may be no training data at all."
+186,"Sometimes different terms are used to describe the corresponding supervised and unsupervised learning procedures for the same type of output. The unsupervised equivalent of classification is normally known as clustering, based on the common perception of the task as involving no training data to speak of, and of grouping the input data into clusters based on some inherent similarity measure , rather than assigning each input instance into one of a set of pre-defined classes. In some fields, the terminology is different. In community ecology, the term classification is used to refer to what is commonly known as ""clustering""."
+187,"The piece of input data for which an output value is generated is formally termed an instance. The instance is formally described by a vector of features, which together constitute a description of all known characteristics of the instance. These feature vectors can be seen as defining points in an appropriate multidimensional space, and methods for manipulating vectors in vector spaces can be correspondingly applied to them, such as computing the dot product or the angle between two vectors. Features typically are either categorical , ordinal , integer-valued  or real-valued . Often, categorical and ordinal data are grouped together, and this is also the case for integer-valued and real-valued data. Many algorithms work only in terms of categorical data and require that real-valued or integer-valued data be discretized into groups ."
+188,"Many common pattern recognition algorithms are probabilistic in nature, in that they use statistical inference to find the best label for a given instance. Unlike other algorithms, which simply output a ""best"" label, often probabilistic algorithms also output a probability of the instance being described by the given label. In addition, many probabilistic algorithms output a list of the N-best labels with associated probabilities, for some value of N, instead of simply a single best label. When the number of possible labels is fairly small , N may be set so that the probability of all possible labels is output. Probabilistic algorithms have many advantages over non-probabilistic algorithms:"
+189,"Feature selection algorithms attempt to directly prune out redundant or irrelevant features. A general introduction to feature selection which summarizes approaches and challenges, has been given. The complexity of feature-selection is, because of its non-monotonous character, an optimization problem where given a total of 
+  
+    
+      
+        n
+      
+    
+    {\displaystyle n}
+  
+ features the powerset consisting of all 
+  
+    
+      
+        
+          2
+          
+            n
+          
+        
+        −
+        1
+      
+    
+    {\displaystyle 2^{n}-1}
+  
+ subsets of features need to be explored. The Branch-and-Bound algorithm does reduce this complexity but is intractable for medium to large values of the number of available features 
+  
+    
+      
+        n
+      
+    
+    {\displaystyle n}"
+190,"Techniques to transform the raw feature vectors  are sometimes used prior to application of the pattern-matching algorithm. Feature extraction algorithms attempt to reduce a large-dimensionality feature vector into a smaller-dimensionality vector that is easier to work with and encodes less redundancy, using mathematical techniques such as principal components analysis . The distinction between feature selection and feature extraction is that the resulting features after feature extraction has taken place are of a different sort than the original features and may not easily be interpretable, while the features left after feature selection are simply a subset of the original features."
+191,"The problem of pattern recognition can be stated as follows: Given an unknown function 
+  
+    
+      
+        g
+        :
+        
+          
+            X
+          
+        
+        →
+        
+          
+            Y
+          
+        
+      
+    
+    {\displaystyle g:{\mathcal {X}}\rightarrow {\mathcal {Y}}}
+  
+  that maps input instances 
+  
+    
+      
+        
+          x
+        
+        ∈
+        
+          
+            X
+          
+        
+      
+    
+    {\displaystyle {\boldsymbol {x}}\in {\mathcal {X}}}
+  
+ to output labels 
+  
+    
+      
+        y
+        ∈
+        
+          
+            Y
+          
+        
+      
+    
+    {\displaystyle y\in {\mathcal {Y}}}
+  
+, along with training data 
+  
+    
+      
+        
+          D
+        
+        =
+        {
+        
+        ,
+        …
+        ,
+        
+        }
+      
+    
+    {\displaystyle \mathbf {D} =\{,\dots ,\}}
+  
+ assumed to represent accurate examples of the mapping, produce a function 
+  
+    
+      
+        h
+        :
+        
+          
+            X
+          
+        
+        →
+        
+          
+            Y
+          
+        
+      
+    
+    {\displaystyle h:{\mathcal {X}}\rightarrow {\mathcal {Y}}}
+  
+ that approximates as closely as possible the correct mapping 
+  
+    
+      
+        g
+      
+    
+    {\displaystyle g}
+  
+. . In order for this to be a well-defined problem, ""approximates as closely as possible"" needs to be defined rigorously. In decision theory, this is defined by specifying a loss function or cost function that assigns a specific value to ""loss"" resulting from producing an incorrect label. The goal then is to minimize the expected loss, with the expectation taken over the probability distribution of 
+  
+    
+      
+        
+          
+            X
+          
+        
+      
+    
+    {\displaystyle {\mathcal {X}}}
+  
+. In practice, neither the distribution of 
+  
+    
+      
+        
+          
+            X
+          
+        
+      
+    
+    {\displaystyle {\mathcal {X}}}
+  
+ nor the ground truth function 
+  
+    
+      
+        g
+        :
+        
+          
+            X
+          
+        
+        →
+        
+          
+            Y
+          
+        
+      
+    
+    {\displaystyle g:{\mathcal {X}}\rightarrow {\mathcal {Y}}}
+  
+ are known exactly, but can be computed only empirically by collecting a large number of samples of 
+  
+    
+      
+        
+          
+            X
+          
+        
+      
+    
+    {\displaystyle {\mathcal {X}}}
+  
+ and hand-labeling them using the correct value of 
+  
+    
+      
+        
+          
+            Y
+          
+        
+      
+    
+    {\displaystyle {\mathcal {Y}}}
+  
+ . The particular loss function depends on the type of label being predicted. For example, in the case of classification, the simple zero-one loss function is often sufficient. This corresponds simply to assigning a loss of 1 to any incorrect labeling and implies that the optimal classifier minimizes the error rate on independent test data . The goal of the learning procedure is then to minimize the error rate  on a ""typical"" test set."
+192,"For a probabilistic pattern recognizer, the problem is instead to estimate the probability of each possible output label given a particular input instance, i.e., to estimate a function of the form"
+193,"where the feature vector input is 
+  
+    
+      
+        
+          x
+        
+      
+    
+    {\displaystyle {\boldsymbol {x}}}
+  
+, and the function f is typically parameterized by some parameters 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+. In a discriminative approach to the problem, f is estimated directly. In a generative approach, however, the inverse probability 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+ is instead estimated and combined with the prior probability 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+ using Bayes' rule, as follows:"
+194,"When the labels are continuously distributed , the denominator involves integration rather than summation:"
+195,"The value of 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+ is typically learned using maximum a posteriori  estimation. This finds the best value that simultaneously meets two conflicting objects: To perform as well as possible on the training data  and to find the simplest possible model. Essentially, this combines maximum likelihood estimation with a regularization procedure that favors simpler models over more complex models. In a Bayesian context, the regularization procedure can be viewed as placing a prior probability 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+ on different values of 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+. Mathematically:"
+196,"where 
+  
+    
+      
+        
+          
+            θ
+          
+          
+            ∗
+          
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}^{*}}
+  
+ is the value used for 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+ in the subsequent evaluation procedure, and 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+, the posterior probability of 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+, is given by"
+197,"In the Bayesian approach to this problem, instead of choosing a single parameter vector 
+  
+    
+      
+        
+          
+            θ
+          
+          
+            ∗
+          
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}^{*}}
+  
+, the probability of a given label for a new instance 
+  
+    
+      
+        
+          x
+        
+      
+    
+    {\displaystyle {\boldsymbol {x}}}
+  
+ is computed by integrating over all possible values of 
+  
+    
+      
+        
+          θ
+        
+      
+    
+    {\displaystyle {\boldsymbol {\theta }}}
+  
+, weighted according to the posterior probability:"
+198,"The first pattern classifier – the linear discriminant presented by Fisher – was developed in the frequentist tradition. The frequentist approach entails that the model parameters are considered unknown, but objective. The parameters are then computed  from the collected data. For the linear discriminant, these parameters are precisely the mean vectors and the covariance matrix. Also the probability of each class 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+ is estimated from the collected dataset. Note that the usage of 'Bayes rule' in a pattern classifier does not make the classification approach Bayesian."
+199,"Bayesian statistics has its origin in Greek philosophy where a distinction was already made between the 'a priori' and the 'a posteriori' knowledge. Later Kant defined his distinction between what is a priori known – before observation – and the empirical knowledge gained from observations. In a Bayesian pattern classifier, the class probabilities 
+  
+    
+      
+        p
+        
+      
+    
+    {\displaystyle p}
+  
+ can be chosen by the user, which are then a priori. Moreover, experience quantified as a priori parameter values can be weighted with empirical observations – using e.g., the Beta-  and Dirichlet-distributions. The Bayesian approach facilitates a seamless intermixing between expert knowledge in the form of subjective probabilities, and objective observations."
+200,Probabilistic pattern classifiers can be used according to a frequentist or a Bayesian approach.
+201,"Within medical science, pattern recognition is the basis for computer-aided diagnosis  systems. CAD describes a procedure that supports the doctor's interpretations and findings. Other typical applications of pattern recognition techniques are automatic speech recognition, speaker identification, classification of text into several categories , the automatic recognition of handwriting on postal envelopes, automatic recognition of images of human faces, or handwriting image extraction from medical forms. The last two examples form the subtopic image analysis of pattern recognition that deals with digital images as input to pattern recognition systems."
+202,"Optical character recognition is an example of the application of a pattern classifier. The method of signing one's name was captured with stylus and overlay starting in 1990. The strokes, speed, relative min, relative max, acceleration and pressure is used to uniquely identify and confirm identity. Banks were first offered this technology, but were content to collect from the FDIC for any bank fraud and did not want to inconvenience customers."
+203,Pattern recognition has many real-world applications in image processing. Some examples include:
+204,"In psychology, pattern recognition is used to make sense of and identify objects, and is closely related to perception. This explains how the sensory inputs humans receive are made meaningful. Pattern recognition can be thought of in two different ways. The first concerns template matching and the second concerns feature detection. A template is a pattern used to produce items of the same proportions. The template-matching hypothesis suggests that incoming stimuli are compared with templates in the long-term memory. If there is a match, the stimulus is identified. Feature detection models, such as the Pandemonium system for classifying letters , suggest that the stimuli are broken down into their component parts for identification. One observation is a capital E having three horizontal lines and one vertical line."
+205,"Algorithms for pattern recognition depend on the type of label output, on whether learning is supervised or unsupervised, and on whether the algorithm is statistical or non-statistical in nature. Statistical algorithms can further be categorized as generative or discriminative."
+206,Parametric:
+207,Nonparametric:
+208,Unsupervised:
+209,Two different kinds of rule-based systems emerged within the field of artificial intelligence in the 1970s:
+210,The differences and relationships between these two kinds of rule-based system has been a major source of misunderstanding and confusion.
+211,"Both kinds of rule-based systems use either forward or backward  chaining, in contrast with imperative programs, which execute commands listed sequentially. However, logic programming systems have a logical interpretation, whereas production systems do not."
+212,"A classic example of a production rule-based system is the domain-specific expert system that uses rules to make deductions or choices.  For example, an expert system might help a doctor choose the correct diagnosis based on a cluster of symptoms, or select tactical moves to play a game."
+213,"Rule-based systems can be used to perform lexical analysis to compile or interpret computer programs, or in natural language processing."
+214,"Rule-based programming attempts to derive execution instructions from a starting set of data and rules. This is a more indirect method than that employed by an imperative programming language, which lists execution steps sequentially."
+215,A typical rule-based system has four basic components:
+216,"Whereas the matching phase of the inference engine has a logical interpretation, the conflict resolution and action phases do not. Instead, ""their semantics is usually described as a series of applications of various state-changing operators, which often gets quite involved , and they can hardly be regarded as declarative""."
+217,"The logic programming family of computer systems includes the programming language Prolog, the database language Datalog and the knowledge representation and problem-solving language Answer Set Programming . In all of these languages, rules are written in the form of clauses:"
+218,and are read as declarative sentences in logical form:
+219,"In the simplest case of Horn clauses , which are a subset of first-order logic, all of the A, B1, ..., Bn are atomic formulae."
+220,"Although Horn clause logic programs are  Turing complete, for many practical applications, it is useful to extend Horn clause programs by allowing negative conditions, implemented by negation as failure. Such extended logic programs have the knowledge representation capabilities of a non-monotonic logic."
+221,"The most obvious difference between the two kinds of systems is that production rules are typically written in the forward direction, if A then B,  and logic programming rules are typically written in the backward direction, B if A. In the case of logic programming rules, this difference is superficial and purely syntactic. It does not affect the semantics of the rules. Nor does it affect whether the rules are used to reason backwards, Prolog style, to reduce the goal B to the subgoals A, or whether they are used, Datalog style, to derive B from A."
+222,"In the case of production rules, the forward direction of the syntax reflects the stimulus-response character of most production rules, with the stimulus A coming before the response B. Moreover, even in cases when the response is simply to draw a conclusion B from an assumption A, as in modus ponens, the match-resolve-act cycle is restricted to reasoning forwards from A to B. Reasoning backwards in a production system would require the use of an entirely different kind of inference engine."
+223,"In his Introduction to Cognitive Science, Paul Thagard includes logic and rules as alternative approaches to modelling human thinking. He does not consider logic programs in general, but he considers Prolog to be, not a rule-based system, but ""a programming language that uses logic representations and deductive techniques"" ."
+224,"He argues that rules, which have the form IF condition THEN action, are ""very similar"" to logical conditionals, but they are simpler and have greater psychological plausability . Among other differences between logic and rules, he argues that logic uses deduction, but rules use search  and can be used to reason either forward or backward . Sentences in logic ""have to be interpreted as universally true"", but rules can be defaults, which admit exceptions . He does not observe that all of these features of rules apply to logic programming systems."
+225,"Machine learning  is a field of study in artificial intelligence concerned with the development and study of statistical algorithms that can learn from data and generalize to unseen data, and thus perform tasks without explicit instructions. Recently, artificial neural networks have been able to surpass many previous approaches in performance."
+226,"Machine learning approaches have been applied to many fields including natural language processing, computer vision, speech recognition, email filtering, agriculture, and medicine. ML is known in its application across business problems under the name predictive analytics. Although not all machine learning is statistically based, computational statistics is an important source of the field's methods."
+227,"The mathematical foundations of ML are provided by mathematical optimization  methods. Data mining is a related  field of study, focusing on exploratory data analysis  through unsupervised learning."
+228,"From a theoretical viewpoint, probably approximately correct  learning provides a framework for describing machine learning."
+229,"The term machine learning was coined in 1959 by Arthur Samuel, an IBM employee and pioneer in the field of computer gaming and artificial intelligence. The synonym self-teaching computers was also used in this time period."
+230,"Although the earliest machine learning model was introduced in the 1950s when Arthur Samuel invented a program that calculated the winning chance in checkers for each side, the history of machine learning roots back to decades of human desire and effort to study human cognitive processes. In 1949, Canadian psychologist Donald Hebb published the book The Organization of Behavior, in which he introduced a theoretical neural structure formed by certain interactions among nerve cells. Hebb's model of neurons interacting with one another set a groundwork for how AIs and machine learning algorithms work under nodes, or artificial neurons used by computers to communicate data. Other researchers who have studied human cognitive systems contributed to the modern machine learning technologies as well, including logician Walter Pitts and Warren McCulloch, who proposed the early mathematical models of neural networks to come up with algorithms that mirror human thought processes."
+231,"By the early 1960s an experimental ""learning machine"" with punched tape memory, called Cybertron, had been developed by Raytheon Company to analyze sonar signals, electrocardiograms, and speech patterns using rudimentary reinforcement learning. It was repetitively ""trained"" by a human operator/teacher to recognize patterns and equipped with a ""goof"" button to cause it to re-evaluate incorrect decisions. A representative book on research into machine learning during the 1960s was Nilsson's book on Learning Machines, dealing mostly with machine learning for pattern classification. Interest related to pattern recognition continued into the 1970s, as described by Duda and Hart in 1973. In 1981 a report was given on using teaching strategies so that an artificial neural network learns to recognize 40 characters  from a computer terminal."
+232,"Tom M. Mitchell provided a widely quoted, more formal definition of the algorithms studied in the machine learning field: ""A computer program is said to learn from experience E with respect to some class of tasks T and performance measure P if its performance at tasks in T, as measured by P,  improves with experience E."" This definition of the tasks in which machine learning is concerned offers a fundamentally operational definition rather than defining the field in cognitive terms. This follows Alan Turing's proposal in his paper ""Computing Machinery and Intelligence"", in which the question ""Can machines think?"" is replaced with the question ""Can machines do what we  can do?""."
+233,Modern-day machine learning has two objectives.  One is to classify data based on models which have been developed; the other purpose is to make predictions for future outcomes based on these models. A hypothetical algorithm specific to classifying data may use computer vision of moles coupled with supervised learning in order to train it to classify the cancerous moles. A machine learning algorithm for stock trading may inform the trader of future potential predictions.
+234,"As a scientific endeavor, machine learning grew out of the quest for artificial intelligence . In the early days of AI as an academic discipline, some researchers were interested in having machines learn from data. They attempted to approach the problem with various symbolic methods, as well as what were then termed ""neural networks""; these were mostly perceptrons and other models that were later found to be reinventions of the generalized linear models of statistics. Probabilistic reasoning was also employed, especially in automated medical diagnosis.: 488"
+235,"However, an increasing emphasis on the logical, knowledge-based approach caused a rift between AI and machine learning. Probabilistic systems were plagued by theoretical and practical problems of data acquisition and representation.: 488  By 1980, expert systems had come to dominate AI, and statistics was out of favor. Work on symbolic/knowledge-based learning did continue within AI, leading to inductive logic programming, but the more statistical line of research was now outside the field of AI proper, in pattern recognition and information retrieval.: 708–710, 755  Neural networks research had been abandoned by AI and computer science around the same time. This line, too, was continued outside the AI/CS field, as ""connectionism"", by researchers from other disciplines including Hopfield, Rumelhart, and Hinton. Their main success came in the mid-1980s with the reinvention of backpropagation.: 25"
+236,"Machine learning , reorganized and recognized as its own field, started to flourish in the 1990s. The field changed its goal from achieving artificial intelligence to tackling solvable problems of a practical nature. It shifted focus away from the symbolic approaches it had inherited from AI, and toward methods and models borrowed from statistics, fuzzy logic, and probability theory."
+237,"An alternative view can show compression algorithms implicitly map strings into implicit feature space vectors, and compression-based similarity measures compute similarity within these feature spaces. For each compressor C we define an associated vector space ℵ, such that C maps an input string x, corresponding to the vector norm ||~x||. An exhaustive examination of the feature spaces underlying all compression algorithms is precluded by space; instead, feature vectors chooses to examine three representative lossless compression methods, LZW, LZ77, and PPM."
+238,"According to AIXI theory, a connection more directly explained in Hutter Prize, the best possible compression of x is the smallest possible software that generates x. For example, in that model, a zip file's compressed size includes both the zip file and the unzipping software, since you can not unzip it without both, but there may be an even smaller combined form."
+239,"Examples of AI-powered audio/video compression software include VP9, NVIDIA Maxine, AIVC, AccMPEG. Examples of software that can perform AI-powered image compression include OpenCV, TensorFlow, MATLAB's Image Processing Toolbox  and High-Fidelity Generative Image Compression."
+240,"In unsupervised machine learning, k-means clustering can be utilized to compress data by grouping similar data points into clusters. This technique simplifies handling extensive datasets that lack predefined labels and finds widespread use in fields such as image compression."
+241,"Data compression aims to reduce the size of data files, enhancing storage efficiency and speeding up data transmission. K-means clustering, an unsupervised machine learning algorithm, is employed to partition a dataset into a specified number of clusters, k, each represented by the centroid of its points. This process condenses extensive datasets into a more compact set of representative points. Particularly beneficial in image and signal processing, k-means clustering aids in data reduction by replacing groups of data points with their centroids, thereby preserving the core information of the original data while significantly decreasing the required storage space."
+242,"Machine learning and data mining often employ the same methods and overlap significantly, but while machine learning focuses on prediction, based on known properties learned from the training data, data mining focuses on the discovery of  unknown properties in the data . Data mining uses many machine learning methods, but with different goals; on the other hand, machine learning also employs data mining methods as ""unsupervised learning"" or as a preprocessing step to improve learner accuracy. Much of the confusion between these two research communities  comes from the basic assumptions they work with: in machine learning, performance is usually evaluated with respect to the ability to reproduce known knowledge, while in knowledge discovery and data mining  the key task is the discovery of previously unknown knowledge. Evaluated with respect to known knowledge, an uninformed  method will easily be outperformed by other supervised methods, while in a typical KDD task, supervised methods cannot be used due to the unavailability of training data."
+243,Machine learning also has intimate ties to optimization: many learning problems are formulated as minimization of some loss function on a training set of examples. Loss functions express the discrepancy between the predictions of the model being trained and the actual problem instances .
+244,"The difference between optimization and machine learning arises from the goal of generalization: while optimization algorithms can minimize the loss on a training set, machine learning is concerned with minimizing the loss on unseen samples. Characterizing the generalization of various learning algorithms is an active topic of current research, especially for deep learning algorithms."
+245,"Machine learning and statistics are closely related fields in terms of methods, but distinct in their principal goal: statistics draws population inferences from a sample, while machine learning finds generalizable predictive patterns. According to Michael I. Jordan, the ideas of machine learning, from methodological principles to theoretical tools, have had a long pre-history in statistics. He also suggested the term data science as a placeholder to call the overall field."
+246,"Conventional statistical analyses require the a priori selection of a model most suitable for the study data set. In addition, only significant or theoretically relevant variables based on previous experience are included for analysis. In contrast, machine learning is not built on a pre-structured model; rather, the data shape the model by detecting underlying patterns. The more variables  used to train the model, the more accurate the ultimate model will be."
+247,"Leo Breiman distinguished two statistical modeling paradigms: data model and algorithmic model, wherein ""algorithmic model"" means more or less the machine learning algorithms like Random Forest."
+248,"Some statisticians have adopted methods from machine learning, leading to a combined field that they call statistical learning."
+249,"Analytical and computational techniques derived from deep-rooted physics of disordered systems can be extended to large-scale problems, including machine learning, e.g., to analyze the weight space of deep neural networks. Statistical physics is thus finding applications in the area of medical diagnostics."
+250,"A core objective of a learner is to generalize from its experience. Generalization in this context is the ability of a learning machine to perform accurately on new, unseen examples/tasks after having experienced a learning data set. The training examples come from some generally unknown probability distribution  and the learner has to build a general model about this space that enables it to produce sufficiently accurate predictions in new cases."
+251,"The computational analysis of machine learning algorithms and their performance is a branch of theoretical computer science known as computational learning theory via the Probably Approximately Correct Learning  model. Because training sets are finite and the future is uncertain, learning theory usually does not yield guarantees of the performance of algorithms. Instead, probabilistic bounds on the performance are quite common. The bias–variance decomposition is one way to quantify generalization error."
+252,"For the best performance in the context of generalization, the complexity of the hypothesis should match the complexity of the function underlying the data. If the hypothesis is less complex than the function, then the model has under fitted the data. If the complexity of the model is increased in response, then the training error decreases. But if the hypothesis is too complex, then the model is subject to overfitting and generalization will be poorer."
+253,"In addition to performance bounds, learning theorists study the time complexity and feasibility of learning. In computational learning theory, a computation is considered feasible if it can be done in polynomial time. There are two kinds of time complexity results: Positive results show that a certain class of functions can be learned in polynomial time. Negative results show that certain classes cannot be learned in polynomial time."
+254,"Machine learning approaches are traditionally divided into three broad categories, which correspond to learning paradigms, depending on the nature of the ""signal"" or ""feedback"" available to the learning system:"
+255,"Although each algorithm has advantages and limitations, no single algorithm works for all problems."
+256,"Supervised learning algorithms build a mathematical model of a set of data that contains both the inputs and the desired outputs. The data is known as training data, and consists of a set of training examples. Each training example has one or more inputs and the desired output, also known as a supervisory signal.  In the mathematical model, each training example is represented by an array or vector, sometimes called a feature vector, and the training data is represented by a matrix. Through iterative optimization of an objective function, supervised learning algorithms learn a function that can be used to predict the output associated with new inputs. An optimal function allows the algorithm to correctly determine the output for inputs that were not a part of the training data. An algorithm that improves the accuracy of its outputs or predictions over time is said to have learned to perform that task."
+257,"Types of supervised-learning algorithms include active learning, classification and regression. Classification algorithms are used when the outputs are restricted to a limited set of values, and regression algorithms are used when the outputs may have any numerical value within a range. As an example, for a classification algorithm that filters emails, the input would be an incoming email, and the output would be the name of the folder in which to file the email."
+258,"Similarity learning is an area of supervised machine learning closely related to regression and classification, but the goal is to learn from examples using a similarity function that measures how similar or related two objects are. It has applications in ranking, recommendation systems, visual identity tracking, face verification, and speaker verification."
+259,"Unsupervised learning algorithms find structures in data that has not been labeled, classified or categorized. Instead of responding to feedback, unsupervised learning algorithms identify commonalities in the data and react based on the presence or absence of such commonalities in each new piece of data. Central applications of unsupervised machine learning include clustering, dimensionality reduction, and density estimation. Unsupervised learning algorithms also streamlined the process of identifying large indel based haplotypes of a gene of interest from pan-genome."
+260,"Cluster analysis is the assignment of a set of observations into subsets  so that observations within the same cluster are similar according to one or more predesignated criteria, while observations drawn from different clusters are dissimilar. Different clustering techniques make different assumptions on the structure of the data, often defined by some similarity metric and evaluated, for example, by internal compactness, or the similarity between members of the same cluster, and separation, the difference between clusters. Other methods are based on estimated density and graph connectivity."
+261,"Semi-supervised learning falls between unsupervised learning  and supervised learning . Some of the training examples are missing training labels, yet many machine-learning researchers have found that unlabeled data, when used in conjunction with a small amount of labeled data, can produce a considerable improvement in learning accuracy."
+262,"In weakly supervised learning, the training labels are noisy, limited, or imprecise; however, these labels are often cheaper to obtain, resulting in larger effective training sets."
+263,"Reinforcement learning is an area of machine learning concerned with how software agents ought to take actions in an environment so as to maximize some notion of cumulative reward. Due to its generality, the field is studied in many other disciplines, such as game theory, control theory, operations research, information theory, simulation-based optimization, multi-agent systems, swarm intelligence, statistics and genetic algorithms. In reinforcement learning, the environment is typically represented as a Markov decision process . Many reinforcements learning algorithms use dynamic programming techniques. Reinforcement learning algorithms do not assume knowledge of an exact mathematical model of the MDP and are used when exact models are infeasible. Reinforcement learning algorithms are used in autonomous vehicles or in learning to play a game against a human opponent."
+264,"Dimensionality reduction is a process of reducing the number of random variables under consideration by obtaining a set of principal variables. In other words, it is a process of reducing the dimension of the feature set, also called the ""number of features"". Most of the dimensionality reduction techniques can be considered as either feature elimination or extraction. One of the popular methods of dimensionality reduction is principal component analysis . PCA involves changing higher-dimensional data  to a smaller space . This results in a smaller dimension of data , while keeping all original variables in the model without changing the data.
+The manifold hypothesis proposes that high-dimensional data sets lie along low-dimensional manifolds, and many dimensionality reduction techniques make this assumption, leading to the area of manifold learning and manifold regularization."
+265,"Other approaches have been developed which do not fit neatly into this three-fold categorization, and sometimes more than one is used by the same machine learning system. For example, topic modeling, meta-learning."
+266,"Self-learning, as a machine learning paradigm was introduced in 1982 along with a neural network capable of self-learning, named crossbar adaptive array . It is learning with no external rewards and no external teacher advice. The CAA self-learning algorithm computes, in a crossbar fashion, both decisions about actions and emotions  about consequence situations. The system is driven by the interaction between cognition and emotion.
+The self-learning algorithm updates a memory matrix W =||w|| such that in each iteration executes the following machine learning routine:"
+267,in situation s perform action a
+268,receive a consequence situation s'
+269,compute emotion of being in the consequence situation v
+270,update crossbar memory  w' = w + v
+271,"It is a system with only one input, situation, and only one output, action  a. There is neither a separate reinforcement input nor an advice input from the environment. The backpropagated value  is the emotion toward the consequence situation. The CAA exists in two environments, one is the behavioral environment where it behaves, and the other is the genetic environment, wherefrom it initially and only once receives initial emotions about situations to be encountered in the behavioral environment. After receiving the genome  vector from the genetic environment, the CAA learns a goal-seeking behavior, in an environment that contains both desirable and undesirable situations."
+272,"Several learning algorithms aim at discovering better representations of the inputs provided during training. Classic examples include principal component analysis and cluster analysis. Feature learning algorithms, also called representation learning algorithms, often attempt to preserve the information in their input but also transform it in a way that makes it useful, often as a pre-processing step before performing classification or predictions. This technique allows reconstruction of the inputs coming from the unknown data-generating distribution, while not being necessarily faithful to configurations that are implausible under that distribution. This replaces manual feature engineering, and allows a machine to both learn the features and use them to perform a specific task."
+273,"Feature learning can be either supervised or unsupervised. In supervised feature learning, features are learned using labeled input data. Examples include artificial neural networks, multilayer perceptrons, and supervised dictionary learning. In unsupervised feature learning, features are learned with unlabeled input data.  Examples include dictionary learning, independent component analysis, autoencoders, matrix factorization and various forms of clustering."
+274,"Manifold learning algorithms attempt to do so under the constraint that the learned representation is low-dimensional. Sparse coding algorithms attempt to do so under the constraint that the learned representation is sparse, meaning that the mathematical model has many zeros. Multilinear subspace learning algorithms aim to learn low-dimensional representations directly from tensor representations for multidimensional data, without reshaping them into higher-dimensional vectors. Deep learning algorithms discover multiple levels of representation, or a hierarchy of features, with higher-level, more abstract features defined in terms of  lower-level features. It has been argued that an intelligent machine is one that learns a representation that disentangles the underlying factors of variation that explain the observed data."
+275,"Feature learning is motivated by the fact that machine learning tasks such as classification often require input that is mathematically and computationally convenient to process. However, real-world data such as images, video, and sensory data has not yielded attempts to algorithmically define specific features. An alternative is to discover such features or representations through examination, without relying on explicit algorithms."
+276,"Sparse dictionary learning is a feature learning method where a training example is represented as a linear combination of basis functions, and is assumed to be a sparse matrix. The method is strongly NP-hard and difficult to solve approximately. A popular heuristic method for sparse dictionary learning is the K-SVD algorithm. Sparse dictionary learning has been applied in several contexts. In classification, the problem is to determine the class to which a previously unseen training example belongs. For a dictionary where each class has already been built, a new training example is associated with the class that is best sparsely represented by the corresponding dictionary. Sparse dictionary learning has also been applied in image de-noising. The key idea is that a clean image patch can be sparsely represented by an image dictionary, but the noise cannot."
+277,"In data mining, anomaly detection, also known as outlier detection, is the identification of rare items, events or observations which raise suspicions by differing significantly from the majority of the data. Typically, the anomalous items represent an issue such as bank fraud, a structural defect, medical problems or errors in a text. Anomalies are referred to as outliers, novelties, noise, deviations and exceptions."
+278,"In particular, in the context of abuse and network intrusion detection, the interesting objects are often not rare objects, but unexpected bursts of inactivity. This pattern does not adhere to the common statistical definition of an outlier as a rare object. Many outlier detection methods  will fail on such data unless aggregated appropriately. Instead, a cluster analysis algorithm may be able to detect the micro-clusters formed by these patterns."
+279,"Three broad categories of anomaly detection techniques exist. Unsupervised anomaly detection techniques detect anomalies in an unlabeled test data set under the assumption that the majority of the instances in the data set are normal, by looking for instances that seem to fit the least to the remainder of the data set. Supervised anomaly detection techniques require a data set that has been labeled as ""normal"" and ""abnormal"" and involves training a classifier . Semi-supervised anomaly detection techniques construct a model representing normal behavior from a given normal training data set and then test the likelihood of a test instance to be generated by the model."
+280,"Robot learning is inspired by a multitude of machine learning methods, starting from supervised learning, reinforcement learning, and finally meta-learning ."
+281,"Association rule learning is a rule-based machine learning method for discovering relationships between variables in large databases. It is intended to identify strong rules discovered in databases using some measure of ""interestingness""."
+282,"Rule-based machine learning is a general term for any machine learning method that identifies, learns, or evolves ""rules"" to store, manipulate or apply knowledge. The defining characteristic of a rule-based machine learning algorithm is the identification and utilization of a set of relational rules that collectively represent the knowledge captured by the system. This is in contrast to other machine learning algorithms that commonly identify a singular model that can be universally applied to any instance in order to make a prediction. Rule-based machine learning approaches include learning classifier systems, association rule learning, and artificial immune systems."
+283,"Based on the concept of strong rules, Rakesh Agrawal, Tomasz Imieliński and Arun Swami introduced association rules for discovering regularities between products in large-scale transaction data recorded by point-of-sale  systems in supermarkets. For example, the rule 
+  
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+    {\displaystyle \{\mathrm {onions,potatoes} \}\Rightarrow \{\mathrm {burger} \}}
+  
+ found in the sales data of a supermarket would indicate that if a customer buys onions and potatoes together, they are likely to also buy hamburger meat. Such information can be used as the basis for decisions about marketing activities such as promotional pricing or product placements. In addition to market basket analysis, association rules are employed today in application areas including Web usage mining, intrusion detection, continuous production, and bioinformatics. In contrast with sequence mining, association rule learning typically does not consider the order of items either within a transaction or across transactions."
+284,"Learning classifier systems  are a family of rule-based machine learning algorithms that combine a discovery component, typically a genetic algorithm, with a learning component, performing either supervised learning, reinforcement learning, or unsupervised learning. They seek to identify a set of context-dependent rules that collectively store and apply knowledge in a piecewise manner in order to make predictions."
+285,"Inductive logic programming  is an approach to rule learning using logic programming as a uniform representation for input examples, background knowledge, and hypotheses. Given an encoding of the known background knowledge and a set of examples represented as a logical database of facts, an ILP system will derive a hypothesized logic program that entails all positive and no negative examples. Inductive programming is a related field that considers any kind of programming language for representing hypotheses , such as functional programs."
+286,"Inductive logic programming is particularly useful in bioinformatics and natural language processing. Gordon Plotkin and Ehud Shapiro laid the initial theoretical foundation for inductive machine learning in a logical setting. Shapiro built their first implementation  in 1981: a Prolog program that inductively inferred logic programs from positive and negative examples. The term inductive here refers to philosophical induction, suggesting a theory to explain observed facts, rather than mathematical induction, proving a property for all members of a well-ordered set."
+287,"A machine learning model is a type of mathematical model which, after being ""trained"" on a given dataset, can be used to make predictions or classifications on new data. During training, a learning algorithm iteratively adjusts the model's internal parameters to minimize errors in its predictions. By extension the term model can refer to several level of specifity, from a general class of models and their associated learning algorithms, to a fully trained model with all its internal parameters tuned."
+288,"Various types of models have been used and researched for machine learning systems, picking the best model for a task is called model selection."
+289,"Artificial neural networks , or connectionist systems, are computing systems vaguely inspired by the biological neural networks that constitute animal brains. Such systems ""learn"" to perform tasks by considering examples, generally without being programmed with any task-specific rules."
+290,"An ANN is a model based on a collection of connected units or nodes called ""artificial neurons"", which loosely model the neurons in a biological brain. Each connection, like the synapses in a biological brain, can transmit information, a ""signal"", from one artificial neuron to another. An artificial neuron that receives a signal can process it and then signal additional artificial neurons connected to it. In common ANN implementations, the signal at a connection between artificial neurons is a real number, and the output of each artificial neuron is computed by some non-linear function of the sum of its inputs. The connections between artificial neurons are called ""edges"". Artificial neurons and edges typically have a weight that adjusts as learning proceeds. The weight increases or decreases the strength of the signal at a connection. Artificial neurons may have a threshold such that the signal is only sent if the aggregate signal crosses that threshold. Typically, artificial neurons are aggregated into layers. Different layers may perform different kinds of transformations on their inputs. Signals travel from the first layer  to the last layer , possibly after traversing the layers multiple times."
+291,"The original goal of the ANN approach was to solve problems in the same way that a human brain would. However, over time, attention moved to performing specific tasks, leading to deviations from biology. Artificial neural networks have been used on a variety of tasks, including computer vision, speech recognition, machine translation, social network filtering, playing board and video games and medical diagnosis."
+292,Deep learning consists of multiple hidden layers in an artificial neural network. This approach tries to model the way the human brain processes light and sound into vision and hearing. Some successful applications of deep learning are computer vision and speech recognition.
+293,"Decision tree learning uses a decision tree as a predictive model to go from observations about an item  to conclusions about the item's target value . It is one of the predictive modeling approaches used in statistics, data mining, and machine learning. Tree models where the target variable can take a discrete set of values are called classification trees; in these tree structures, leaves represent class labels, and branches represent conjunctions of features that lead to those class labels. Decision trees where the target variable can take continuous values  are called regression trees. In decision analysis, a decision tree can be used to visually and explicitly represent decisions and decision making. In data mining, a decision tree describes data, but the resulting classification tree can be an input for decision-making."
+294,"Support-vector machines , also known as support-vector networks, are a set of related supervised learning methods used for classification and regression. Given a set of training examples, each marked as belonging to one of two categories, an SVM training algorithm builds a model that predicts whether a new example falls into one category.  An SVM training algorithm is a non-probabilistic, binary, linear classifier, although methods such as Platt scaling exist to use SVM in a probabilistic classification setting. In addition to performing linear classification, SVMs can efficiently perform a non-linear classification using what is called the kernel trick, implicitly mapping their inputs into high-dimensional feature spaces."
+295,"Regression analysis encompasses a large variety of statistical methods to estimate the relationship between input variables and their associated features. Its most common form is linear regression, where a single line is drawn to best fit the given data according to a mathematical criterion such as ordinary least squares. The latter is often extended by regularization methods to mitigate overfitting and bias, as in ridge regression. When dealing with non-linear problems, go-to models include polynomial regression , logistic regression  or even kernel regression, which introduces non-linearity by taking advantage of the kernel trick to implicitly map input variables to higher-dimensional space."
+296,"A Bayesian network, belief network, or directed acyclic graphical model is a probabilistic graphical model that represents a set of random variables and their conditional independence with a directed acyclic graph . For example, a Bayesian network could represent the probabilistic relationships between diseases and symptoms. Given symptoms, the network can be used to compute the probabilities of the presence of various diseases. Efficient algorithms exist that perform inference and learning. Bayesian networks that model sequences of variables, like speech signals or protein sequences, are called dynamic Bayesian networks. Generalizations of Bayesian networks that can represent and solve decision problems under uncertainty are called influence diagrams."
+297,"A Gaussian process is a stochastic process in which every finite collection of the random variables in the process has a multivariate normal distribution, and it relies on a pre-defined covariance function, or kernel, that models how pairs of points relate to each other depending on their locations."
+298,"Given a set of observed points, or input–output examples, the distribution of the  output of a new point as function of its input data can be directly computed by looking like the observed points and the covariances between those points and the new, unobserved point."
+299,Gaussian processes are popular surrogate models in Bayesian optimization used to do hyperparameter optimization.
+300,"A genetic algorithm  is a search algorithm and heuristic technique that mimics the process of natural selection, using methods such as mutation and crossover to generate new genotypes in the hope of finding good solutions to a given problem. In machine learning, genetic algorithms were used in the 1980s and 1990s. Conversely, machine learning techniques have been used to improve the performance of genetic and evolutionary algorithms."
+301,"The theory of belief functions, also referred to as evidence theory or Dempster–Shafer theory, is a general framework for reasoning with uncertainty, with understood connections to other frameworks such as probability, possibility and  imprecise probability theories. These theoretical frameworks can be thought of as a kind of learner and have some analogous properties of how evidence is combined , just like how in a pmf-based Bayesian approach would combine probabilities. However, there are many caveats to these beliefs functions when compared to Bayesian approaches in order to incorporate ignorance and Uncertainty quantification. These belief function approaches that are implemented within the machine learning domain typically leverage a fusion approach of various ensemble methods to better handle the learner's decision boundary, low samples, and ambiguous class issues that standard machine learning approach tend to have difficulty resolving. However, the computational complexity of these algorithms are dependent on the number of propositions , and can lead a much higher computation time when compared to other machine learning approaches."
+302,"Typically, machine learning models require a high quantity of reliable data in order for the models to perform accurate predictions. When training a machine learning model, machine learning engineers need to target and collect a large and representative sample of data. Data from the training set can be as varied as a corpus of text, a collection of images, sensor data, and data collected from individual users of a service. Overfitting is something to watch out for when training a machine learning model. Trained models derived from biased or non-evaluated data can result in skewed or undesired predictions. Bias models may result in detrimental outcomes thereby furthering the negative impacts on society or objectives. Algorithmic bias is a potential result of data not being fully prepared for training. Machine learning ethics is becoming a field of study and notably be integrated within machine learning engineering teams."
+303,"Federated learning is an adapted form of distributed artificial intelligence to training machine learning models that decentralizes the training process, allowing for users' privacy to be maintained by not needing to send their data to a centralized server. This also increases efficiency by decentralizing the training process to many devices. For example, Gboard uses federated machine learning to train search query prediction models on users' mobile phones without having to send individual searches back to Google."
+304,"There are many applications for machine learning, including:"
+305,"In 2006, the media-services provider Netflix held the first ""Netflix Prize"" competition to find a program to better predict user preferences and improve the accuracy of its existing Cinematch movie recommendation algorithm by at least 10%. A joint team made up of researchers from AT&T Labs-Research in collaboration with the teams Big Chaos and Pragmatic Theory built an ensemble model to win the Grand Prize in 2009 for $1 million. Shortly after the prize was awarded, Netflix realized that viewers' ratings were not the best indicators of their viewing patterns  and they changed their recommendation engine accordingly. In 2010 The Wall Street Journal wrote about the firm Rebellion Research and their use of machine learning to predict the financial crisis. In 2012, co-founder of Sun Microsystems, Vinod Khosla, predicted that 80% of medical doctors jobs would be lost in the next two decades to automated machine learning medical diagnostic software. In 2014, it was reported that a machine learning algorithm had been applied in the field of art history to study fine art paintings and that it may have revealed previously unrecognized influences among artists. In 2019 Springer Nature published the first research book created using machine learning. In 2020, machine learning technology was used to help make diagnoses and aid researchers in developing a cure for COVID-19. Machine learning was recently applied to predict the pro-environmental behavior of travelers. Recently, machine learning technology was also applied to optimize smartphone's performance and thermal behavior based on the user's interaction with the phone. When applied correctly, machine learning algorithms  can utilize a wide range of company characteristics to predict stock returns without overfitting. By employing effective feature engineering and combining forecasts, MLAs can generate results that far surpass those obtained from basic linear techniques like OLS."
+306,"Although machine learning has been transformative in some fields, machine-learning programs often fail to deliver expected results. Reasons for this are numerous: lack of  data, lack of access to the data, data bias, privacy problems, badly chosen tasks and algorithms, wrong tools and people, lack of resources, and evaluation problems."
+307,"The ""black box theory"" poses another yet significant challenge. Black box refers to a situation where the algorithm or the process of producing an output is entirely opaque, meaning that even the coders of the algorithm cannot audit the pattern that the machine extracted out of the data. The House of Lords Select Committee, which claimed that such an “intelligence system” that could have a “substantial impact on an individual’s life” would not be considered acceptable unless it provided “a full and satisfactory explanation for the decisions” it makes."
+308,"In 2018, a self-driving car from Uber failed to detect a pedestrian, who was killed after a collision. Attempts to use machine learning in healthcare with the IBM Watson system failed to deliver even after years of time and billions of dollars invested. Microsoft's Bing Chat chatbot has been reported to produce hostile and offensive response against its users."
+309,"Machine learning has been used as a strategy to update the evidence related to a systematic review and increased reviewer burden related to the growth of biomedical literature. While it has improved with training sets, it has not yet developed sufficiently to reduce the workload burden without limiting the necessary sensitivity for the findings research themselves."
+310,"Machine learning approaches in particular can suffer from different data biases. A machine learning system trained specifically on current customers may not be able to predict the needs of new customer groups that are not represented in the training data. When trained on human-made data, machine learning is likely to pick up the constitutional and unconscious biases already present in society."
+311,"Language models learned from data have been shown to contain human-like biases. In an experiment carried out by ProPublica, an investigative journalism organization, a machine learning algorithm's insight towards the recidivism rates among prisoners falsely flagged “black defendants high risk twice as often as white defendants.” In 2015, Google photos would often tag black people as gorillas, and in 2018 this still was not well resolved, but Google reportedly was still using the workaround to remove all gorillas from the training data, and thus was not able to recognize real gorillas at all. Similar issues with recognizing non-white people have been found in many other systems. In 2016, Microsoft tested Tay, a chatbot that learned from Twitter, and it quickly picked up racist and sexist language."
+312,"Because of such challenges, the effective use of machine learning may take longer to be adopted in other domains. Concern for fairness in machine learning, that is, reducing bias in machine learning and propelling its use for human good is increasingly expressed by artificial intelligence scientists, including Fei-Fei Li, who reminds engineers that ""There's nothing artificial about AI...It's inspired by people, it's created by people, and—most importantly—it impacts people. It is a powerful tool we are only just beginning to understand, and that is a profound responsibility."""
+313,"Explainable AI , or Interpretable AI, or Explainable Machine Learning , is artificial intelligence  in which humans can understand the decisions or predictions made by the AI. It contrasts with the ""black box"" concept in machine learning where even its designers cannot explain why an AI arrived at a specific decision. By refining the mental models of users of AI-powered systems and dismantling their misconceptions, XAI promises to help users perform more effectively. XAI may be an implementation of the social right to explanation."
+314,"Settling on a bad, overly complex theory gerrymandered to fit all the past training data is known as overfitting. Many systems attempt to reduce overfitting by rewarding a theory in accordance with how well it fits the data but penalizing the theory in accordance with how complex the theory is."
+315,"Learners can also disappoint by ""learning the wrong lesson"". A toy example is that an image classifier trained only on pictures of brown horses and black cats might conclude that all brown patches are likely to be horses. A real-world example is that, unlike humans, current image classifiers often do not primarily make judgments from the spatial relationship between components of the picture, and they learn relationships between pixels that humans are oblivious to, but that still correlate with images of certain types of real objects. Modifying these patterns on a legitimate image can result in ""adversarial"" images that the system misclassifies."
+316,"Adversarial vulnerabilities can also result in nonlinear systems, or from non-pattern perturbations. For some systems, it is possible to change the output by only changing a single adversarially chosen pixel. Machine learning models are often vulnerable to manipulation and/or evasion via adversarial machine learning."
+317,"Researchers have demonstrated how backdoors can be placed undetectably into classifying  machine learning models which are often developed and/or trained by third parties. Parties can change the classification of any input, including in cases for which a type of data/software transparency is provided, possibly including white-box access."
+318,"Classification of machine learning models can be validated by accuracy estimation techniques like the holdout method, which splits the data in a training and test set  and evaluates the performance of the training model on the test set. In comparison, the K-fold-cross-validation method randomly partitions the data into K subsets and then K experiments are performed each respectively considering 1 subset for evaluation and the remaining K-1 subsets for training the model. In addition to the holdout and cross-validation methods, bootstrap, which samples n instances with replacement from the dataset, can be used to assess model accuracy."
+319,"In addition to overall accuracy, investigators frequently report sensitivity and specificity meaning True Positive Rate  and True Negative Rate  respectively. Similarly, investigators sometimes report the false positive rate  as well as the false negative rate . However, these rates are ratios that fail to reveal their numerators and denominators. The total operating characteristic  is an effective method to express a model's diagnostic ability. TOC shows the numerators and denominators of the previously mentioned rates, thus TOC provides more information than the commonly used receiver operating characteristic  and ROC's associated area under the curve ."
+320,"Machine learning poses a host of ethical questions. Systems that are trained on datasets collected with biases may exhibit these biases upon use , thus digitizing cultural prejudices. For example, in 1988, the UK's Commission for Racial Equality found that St. George's Medical School had been using a computer program trained from data of previous admissions staff and this program had denied nearly 60 candidates who were found to be either women or had non-European sounding names. Using job hiring data from a firm with racist hiring policies may lead to a machine learning system duplicating the bias by scoring job applicants by similarity to previous successful applicants. Another example includes predictive policing company Geolitica's predictive algorithm that resulted in “disproportionately high levels of over-policing in low-income and minority communities” after being trained with historical crime data."
+321,"While responsible collection of data and documentation of algorithmic rules used by a system is considered a critical part of machine learning, some researchers blame lack of participation and representation of minority population in the field of AI for machine learning's vulnerability to biases. In fact, according to research carried out by the Computing Research Association  in 2021, “female faculty merely make up 16.1%” of all faculty members who focus on AI among several universities around the world. Furthermore, among the group of “new U.S. resident AI PhD graduates,” 45% identified as white, 22.4% as Asian, 3.2% as Hispanic, and 2.4% as African American, which further demonstrates a lack of diversity in the field of AI."
+322,"AI can be well-equipped to make decisions in technical fields, which rely heavily on data and historical information. These decisions rely on objectivity and logical reasoning. Because human languages contain biases, machines trained on language corpora will necessarily also learn these biases."
+323,"Other forms of ethical challenges, not related to personal biases, are seen in health care. There are concerns among health care professionals that these systems might not be designed in the public's interest but as income-generating machines. This is especially true in the United States where there is a long-standing ethical dilemma of improving health care, but also increasing profits. For example, the algorithms could be designed to provide patients with unnecessary tests or medication in which the algorithm's proprietary owners hold stakes. There is potential for machine learning in health care to provide professionals an additional tool to diagnose, medicate, and plan recovery paths for patients, but this requires these biases to be mitigated."
+324,"Since the 2010s, advances in both machine learning algorithms and computer hardware have led to more efficient methods for training deep neural networks  that contain many layers of non-linear hidden units. By 2019, graphic processing units , often with AI-specific enhancements, had displaced CPUs as the dominant method of training large-scale commercial cloud AI. OpenAI estimated the hardware computing used in the largest deep learning projects from AlexNet  to AlphaZero , and found a 300,000-fold increase in the amount of compute required, with a doubling-time trendline of 3.4 months."
+325,"A physical neural network or Neuromorphic computer  is a type of artificial neural network in which an electrically adjustable material is used to emulate the function of a neural synapse. ""Physical"" neural network is used to emphasize the reliance on physical hardware used to emulate neurons as opposed to software-based approaches. More generally the term is applicable to other artificial neural networks in which a memristor or other electrically adjustable resistance material is used to emulate a neural synapse."
+326,"Embedded Machine Learning is a sub-field of machine learning, where the machine learning model is run on embedded systems with limited computing resources such as wearable computers, edge devices and microcontrollers. Running machine learning model in embedded devices removes the need for transferring and storing data on cloud servers for further processing, henceforth, reducing data breaches and privacy leaks happening because of transferring data, and also minimizes theft of intellectual properties, personal data and business secrets. Embedded Machine Learning could be applied through several techniques including hardware acceleration, using approximate computing, optimization of machine learning models and many more. Pruning, Quantization, Knowledge Distillation, Low-Rank Factorization, Network Architecture Search  & Parameter Sharing are few of the techniques used for optimization of machine learning models."
+327,Software suites containing a variety of machine learning algorithms include the following:
+328,"Artificial intelligence , in its broadest sense, is intelligence exhibited by machines, particularly computer systems. It is a field of research in computer science that develops and studies methods and software which enable machines to perceive their environment and uses learning and intelligence to take actions that maximize their chances of achieving defined goals. Such machines may be called AIs."
+329,"AI technology is widely used throughout industry, government, and science. Some high-profile applications include advanced web search engines ; recommendation systems ; interacting via human speech ; autonomous vehicles ; generative and creative tools ; and superhuman play and analysis in strategy games . However, many AI applications are not perceived as AI: ""A lot of cutting edge AI has filtered into general applications, often without being called AI because once something becomes useful enough and common enough it's not labeled AI anymore."""
+330,"Alan Turing was the first person to conduct substantial research in the field that he called machine intelligence. Artificial intelligence was founded as an academic discipline in 1956. The field went through multiple cycles of optimism, followed by periods of disappointment and loss of funding, known as AI winter. Funding and interest vastly increased after 2012 when deep learning surpassed all previous AI techniques, and after 2017 with the transformer architecture. This led to the AI boom of the early 2020s, with companies, universities, and laboratories overwhelmingly based in the United States pioneering significant advances in artificial intelligence."
+331,"The growing use of artificial intelligence in the 21st century is influencing a societal and economic shift towards increased automation, data-driven decision-making, and the integration of AI systems into various economic sectors and areas of life, impacting job markets, healthcare, government, industry, and education. This raises questions about the long-term effects, ethical implications, and risks of AI, prompting discussions about regulatory policies to ensure the safety and benefits of the technology."
+332,"The various sub-fields of AI research are centered around particular goals and the use of particular tools. The traditional goals of AI research include reasoning, knowledge representation, planning, learning, natural language processing, perception, and support for robotics. General intelligence—the ability to complete any task performable by a human on an at least equal level—is among the field's long-term goals."
+333,"To reach these goals, AI researchers have adapted and integrated a wide range of techniques, including search and mathematical optimization, formal logic, artificial neural networks, and methods based on statistics, operations research, and economics. AI also draws upon psychology, linguistics, philosophy, neuroscience, and other fields."
+334,The general problem of simulating  intelligence has been broken into sub-problems. These consist of particular traits or capabilities that researchers expect an intelligent system to display. The traits described below have received the most attention and cover the scope of AI research.
+335,"Early researchers developed algorithms that imitated step-by-step reasoning that humans use when they solve puzzles or make logical deductions. By the late 1980s and 1990s, methods were developed for dealing with uncertain or incomplete information, employing concepts from probability and economics."
+336,"Many of these algorithms are insufficient for solving large reasoning problems because they experience a ""combinatorial explosion"": they became exponentially slower as the problems grew larger. Even humans rarely use the step-by-step deduction that early AI research could model. They solve most of their problems using fast, intuitive judgments. Accurate and efficient reasoning is an unsolved problem."
+337,"Knowledge representation and knowledge engineering allow AI programs to answer questions intelligently and make deductions about real-world facts. Formal knowledge representations are used in content-based indexing and retrieval, scene interpretation, clinical decision support, knowledge discovery , and other areas."
+338,"A knowledge base is a body of knowledge represented in a form that can be used by a program. An ontology is the set of objects, relations, concepts, and properties used by a particular domain of knowledge. Knowledge bases need to represent things such as: objects, properties, categories and relations between objects; situations, events, states and time; causes and effects; knowledge about knowledge ; default reasoning ; and many other aspects and domains of knowledge."
+339,"Among the most difficult problems in knowledge representation are: the breadth of commonsense knowledge ; and the sub-symbolic form of most commonsense knowledge . There is also the difficulty of knowledge acquisition, the problem of obtaining knowledge for AI applications."
+340,"An ""agent"" is anything that perceives and takes actions in the world. A rational agent has goals or preferences and takes actions to make them happen. In automated planning, the agent has a specific goal. In automated decision making, the agent has preferences—there are some situations it would prefer to be in, and some situations it is trying to avoid. The decision making agent assigns a number to each situation  that measures how much the agent prefers it. For each possible action, it can calculate the ""expected utility"": the utility of all possible outcomes of the action, weighted by the probability that the outcome will occur. It can then choose the action with the maximum expected utility."
+341,"In classical planning, the agent knows exactly what the effect of any action will be. In most real-world problems, however, the agent may not be certain about the situation they are in  and it may not know for certain what will happen after each possible action . It must choose an action by making a probabilistic guess and then reassess the situation to see if the action worked."
+342,"In some problems, the agent's preferences may be uncertain, especially if there are other agents or humans involved. These can be learned  or the agent can seek information to improve its preferences. Information value theory can be used to weigh the value of exploratory or experimental actions. The space of possible future actions and situations is typically intractably large, so the agents must take actions and evaluate situations while being uncertain what the outcome will be."
+343,"A Markov decision process has a transition model that describes the probability that a particular action will change the state in a particular way, and a reward function that supplies the utility of each state and the cost of each action. A policy associates a decision with each possible state. The policy could be calculated , be heuristic, or it can be learned."
+344,"Game theory describes rational behavior of multiple interacting agents, and is used in AI programs that make decisions that involve other agents."
+345,Machine learning is the study of programs that can improve their performance on a given task automatically. It has been a part of AI from the beginning.
+346,"There are several kinds of machine learning. Unsupervised learning analyzes a stream of data and finds patterns and makes predictions without any other guidance. Supervised learning requires a human to label the input data first, and comes in two main varieties: classification  and regression ."
+347,"In reinforcement learning the agent is rewarded for good responses and punished for bad ones. The agent learns to choose responses that are classified as ""good"". Transfer learning is when the knowledge gained from one problem is applied to a new problem. Deep learning is a type of machine learning that runs inputs through biologically inspired artificial neural networks for all of these types of learning."
+348,"Computational learning theory can assess learners by computational complexity, by sample complexity , or by other notions of optimization."
+349,"Natural language processing  allows programs to read, write and communicate in human languages such as English. Specific problems include speech recognition, speech synthesis, machine translation, information extraction, information retrieval and question answering."
+350,"Early work, based on Noam Chomsky's generative grammar and semantic networks, had difficulty with word-sense disambiguation unless restricted to small domains called ""micro-worlds"" . Margaret Masterman believed that it was meaning, and not grammar that was the key to understanding languages, and that thesauri and not dictionaries should be the basis of computational language structure."
+351,"Modern deep learning techniques for NLP include word embedding , transformers , and others. In 2019, generative pre-trained transformer  language models began to generate coherent text, and by 2023 these models were able to get human-level scores on the bar exam, SAT test, GRE test, and many other real-world applications."
+352,Machine perception is the ability to use input from sensors  to deduce aspects of the world. Computer vision is the ability to analyze visual input.
+353,"The field includes speech recognition, image classification, facial recognition, object recognition, and robotic perception."
+354,"Affective computing is an interdisciplinary umbrella that comprises systems that recognize, interpret, process or simulate human feeling, emotion and mood. For example, some virtual assistants are programmed to speak conversationally or even to banter humorously; it makes them appear more sensitive to the emotional dynamics of human interaction, or to otherwise facilitate human–computer interaction."
+355,"However, this tends to give naïve users an unrealistic conception of the intelligence of existing computer agents. Moderate successes related to affective computing include textual sentiment analysis and, more recently, multimodal sentiment analysis, wherein AI classifies the affects displayed by a videotaped subject."
+356,A machine with artificial general intelligence should be able to solve a wide variety of problems with breadth and versatility similar to human intelligence.
+357,AI research uses a wide variety of techniques to accomplish the goals above.
+358,AI can solve many problems by intelligently searching through many possible solutions. There are two very different kinds of search used in AI: state space search and local search.
+359,"State space search searches through a tree of possible states to try to find a goal state. For example, planning algorithms search through trees of goals and subgoals, attempting to find a path to a target goal, a process called means-ends analysis."
+360,"Simple exhaustive searches are rarely sufficient for most real-world problems: the search space  quickly grows to astronomical numbers. The result is a search that is too slow or never completes. ""Heuristics"" or ""rules of thumb"" can help to prioritize choices that are more likely to reach a goal."
+361,"Adversarial search is used for game-playing programs, such as chess or Go. It searches through a tree of possible moves and counter-moves, looking for a winning position."
+362,Local search uses mathematical optimization to find a solution to a problem. It begins with some form of guess and refines it incrementally.
+363,Gradient descent is a type of local search that optimizes a set of numerical parameters by incrementally adjusting them to minimize a loss function. Variants of gradient descent are commonly used to train neural networks.
+364,"Another type of local search is evolutionary computation, which aims to iteratively improve a set of candidate solutions by ""mutating"" and ""recombining"" them, selecting only the fittest to survive each generation."
+365,Distributed search processes can coordinate via swarm intelligence algorithms. Two popular swarm algorithms used in search are particle swarm optimization  and ant colony optimization .
+366,"Formal logic is used for reasoning and knowledge representation.
+Formal logic comes in two main forms: propositional logic 
+and predicate logic ."
+367,"Deductive reasoning in logic is the process of proving a new statement  from other statements that are given and assumed to be true . Proofs can be structured as proof trees, in which nodes are labelled by sentences, and children nodes are connected to  parent nodes by inference rules."
+368,"Given a problem and a set of premises, problem-solving reduces to searching for a proof tree whose root node is labelled by a solution of the problem and whose leaf nodes are labelled by premises or axioms. In the case of Horn clauses, problem-solving search can be performed by reasoning forwards from the premises or backwards from the problem. In the more general case of the clausal form of first-order logic, resolution is a single, axiom-free rule of inference, in which a problem is solved by proving a contradiction from premises that include the negation of the problem to be solved."
+369,"Inference in both Horn clause logic and first-order logic is undecidable, and therefore intractable. However, backward reasoning with Horn clauses, which underpins computation in the logic programming language Prolog, is Turing complete. Moreover, its efficiency is competitive with computation in other symbolic programming languages."
+370,"Fuzzy logic assigns a ""degree of truth"" between 0 and 1. It can therefore handle propositions that are vague and partially true."
+371,"Non-monotonic logics, including logic programming with negation as failure, are designed to handle default reasoning.
+Other specialized versions of logic have been developed to describe many complex domains."
+372,"Many problems in AI  require the agent to operate with incomplete or uncertain information. AI researchers have devised a number of tools to solve these problems using methods from probability theory and economics. Precise mathematical tools have been developed that analyze how an agent can make choices and plan, using decision theory, decision analysis, and information value theory. These tools include models such as Markov decision processes, dynamic decision networks, game theory and mechanism design."
+373,"Bayesian networks are a tool that can be used for reasoning , learning , planning  and perception ."
+374,"Probabilistic algorithms can also be used for filtering, prediction, smoothing and finding explanations for streams of data, helping perception systems to analyze processes that occur over time ."
+375,"The simplest AI applications can be divided into two types: classifiers , on one hand, and controllers , on the other hand. Classifiers
+are functions that use pattern matching to determine the closest match. They can be fine-tuned based on chosen examples using supervised learning. Each pattern  is labeled with a certain predefined class. All the observations combined with their class labels are known as a data set. When a new observation is received, that observation is classified based on previous experience."
+376,"There are many kinds of classifiers in use. The decision tree is the simplest and most widely used symbolic machine learning algorithm. K-nearest neighbor algorithm was the most widely used analogical AI until the mid-1990s, and Kernel methods such as the support vector machine  displaced k-nearest neighbor in the 1990s.
+The naive Bayes classifier is reportedly the ""most widely used learner"" at Google, due in part to its scalability.
+Neural networks are also used as classifiers."
+377,"An artificial neural network is based on a collection of nodes also known as artificial neurons, which loosely model the neurons in a biological brain. It is trained to recognise patterns; once trained, it can recognise those patterns in fresh data. There is an input, at least one hidden layer of nodes and an output. Each node applies a function and once the weight crosses its specified threshold, the data is transmitted to the next layer. A network is typically called a deep neural network if it has at least 2 hidden layers."
+378,"Learning algorithms for neural networks use local search to choose the weights that will get the right output for each input during training. The most common training technique is the backpropagation algorithm.
+Neural networks learn to model complex relationships between inputs and outputs and find patterns in data. In theory, a neural network can learn any function."
+379,"In feedforward neural networks the signal passes in only one direction.
+Recurrent neural networks feed the output signal back into the input, which allows short-term memories of previous input events. Long short term memory is the most successful network architecture for recurrent networks.
+Perceptrons
+use only a single layer of neurons, deep learning uses multiple layers.
+Convolutional neural networks strengthen the connection between neurons that are ""close"" to each other—this is especially important in image processing, where a local set of neurons must identify an ""edge"" before the network can identify an object."
+380,"Deep learning
+uses several layers of neurons between the network's inputs and outputs. The multiple layers can progressively extract higher-level features from the raw input. For example, in image processing, lower layers may identify edges, while higher layers may identify the concepts relevant to a human such as digits or letters or faces."
+381,"Deep learning has profoundly improved the performance of programs in many important subfields of artificial intelligence, including computer vision, speech recognition, natural language processing, image classification
+and others. The reason that deep learning performs so well in so many applications is not known as of 2023.
+The sudden success of deep learning in 2012–2015 did not occur because of some new discovery or theoretical breakthrough 
+but because of two factors: the incredible increase in computer power  and the availability of vast amounts of training data, especially the giant curated datasets used for benchmark testing, such as ImageNet."
+382,"Generative pre-trained transformers  are large language models that are based on the semantic relationships between words in sentences . Text-based GPT models are pre-trained on a large corpus of text which can be from the internet. The pre-training consists in predicting the next token . Throughout this pre-training, GPT models accumulate knowledge about the world, and can then generate human-like text by repeatedly predicting the next token. Typically, a subsequent training phase makes the model more truthful, useful and harmless, usually with a technique called reinforcement learning from human feedback . Current GPT models are still prone to generating falsehoods called ""hallucinations"", although this can be reduced with RLHF and quality data. They are used in chatbots, which allow you to ask a question or request a task in simple text."
+383,"Current models and services include: Gemini , ChatGPT, Grok, Claude, Copilot and LLaMA. Multimodal GPT models can process different types of data  such as images, videos, sound and text."
+384,"In the late 2010s, graphics processing units  that were increasingly designed with AI-specific enhancements and used with specialized TensorFlow software, had replaced previously used central processing unit  as the dominant means for large-scale  machine learning models' training. Historically, specialized languages, such as Lisp, Prolog, Python and others, had been used."
+385,"AI and machine learning technology is used in most of the essential applications of the 2020s, including: search engines , targeting online advertisements, recommendation systems , driving internet traffic, targeted advertising , virtual assistants , autonomous vehicles , automatic language translation , facial recognition  and image labeling ."
+386,"The application of AI in medicine and medical research has the potential to increase patient care and quality of life. Through the lens of the Hippocratic Oath, medical professionals are ethically compelled to use AI, if applications can more accurately diagnose and treat patients."
+387,"For medical research, AI is an important tool for processing and integrating big data. This is particularly important for organoid and tissue engineering development which use microscopy imaging as a key technique in fabrication. It has been suggested that AI can overcome discrepancies in funding allocated to different fields of research. New AI tools can deepen our understanding of biomedically relevant pathways. For example, AlphaFold 2  demonstrated the ability to approximate, in hours rather than months, the 3D structure of a protein. In 2023, it was reported that AI guided drug discovery helped find a class of antibiotics capable of killing two different types of drug-resistant bacteria."
+388,"Game playing programs have been used since the 1950s to demonstrate and test AI's most advanced techniques. Deep Blue became the first computer chess-playing system to beat a reigning world chess champion, Garry Kasparov, on 11 May 1997. In 2011, in a Jeopardy! quiz show exhibition match, IBM's question answering system, Watson, defeated the two greatest Jeopardy! champions, Brad Rutter and Ken Jennings, by a significant margin. In March 2016, AlphaGo won 4 out of 5 games of Go in a match with Go champion Lee Sedol, becoming the first computer Go-playing system to beat a professional Go player without handicaps. Then in 2017 it defeated Ke Jie, who was the best Go player in the world. Other programs handle imperfect-information games, such as the poker-playing program Pluribus. DeepMind developed increasingly generalistic reinforcement learning models, such as with MuZero, which could be trained to play chess, Go, or Atari games. In 2019, DeepMind's AlphaStar achieved grandmaster level in StarCraft II, a particularly challenging real-time strategy game that involves incomplete knowledge of what happens on the map. In 2021 an AI agent competed in a PlayStation Gran Turismo competition, winning against four of the world's best Gran Turismo drivers using deep reinforcement learning."
+389,"Various countries are deploying AI military applications. The main applications enhance command and control, communications, sensors, integration and interoperability. Research is targeting intelligence collection and analysis, logistics, cyber operations, information operations, and semiautonomous and autonomous vehicles. AI technologies enable coordination of sensors and effectors, threat detection and identification, marking of enemy positions, target acquisition, coordination and deconfliction of distributed Joint Fires between networked combat vehicles involving manned and unmanned teams. AI was incorporated into military operations in Iraq and Syria."
+390,"In November 2023, US Vice President Kamala Harris disclosed a declaration signed by 31 nations to set guardrails for the military use of AI. The commitments include using legal reviews to ensure the compliance of military AI with international laws, and being cautious and transparent in the development of this technology."
+391,"In the early 2020s, generative AI gained widespread prominence. In March 2023, 58% of US adults had heard about ChatGPT and 14% had tried it. The increasing realism and ease-of-use of AI-based text-to-image generators such as Midjourney, DALL-E, and Stable Diffusion sparked a trend of viral AI-generated photos. Widespread attention was gained by a fake photo of Pope Francis wearing a white puffer coat, the fictional arrest of Donald Trump, and a hoax of an attack on the Pentagon, as well as the usage in professional creative arts."
+392,"There are also thousands of successful AI applications used to solve specific problems for specific industries or institutions. In a 2017 survey, one in five companies reported they had incorporated ""AI"" in some offerings or processes. A few examples are energy storage, medical diagnosis, military logistics, applications that predict the result of judicial decisions, foreign policy, or supply chain management."
+393,"In agriculture, AI has helped farmers identify areas that need irrigation, fertilization, pesticide treatments or increasing yield. Agronomists use AI to conduct research and development. AI has been used to predict the ripening time for crops such as tomatoes, monitor soil moisture, operate agricultural robots, conduct predictive analytics, classify livestock pig call emotions, automate greenhouses, detect diseases and pests, and save water."
+394,"Artificial intelligence is used in astronomy to analyze increasing amounts of available data and applications, mainly for ""classification, regression, clustering, forecasting, generation, discovery, and the development of new scientific insights"" for example for discovering exoplanets, forecasting solar activity, and distinguishing between signals and instrumental effects in gravitational wave astronomy. It could also be used for activities in space such as space exploration, including analysis of data from space missions, real-time science decisions of spacecraft, space debris avoidance, and more autonomous operation."
+395,"AI has potential benefits and potential risks. AI may be able to advance science and find solutions for serious problems: Demis Hassabis of Deep Mind hopes to ""solve intelligence, and then use that to solve everything else"". However, as the use of AI has become widespread, several unintended consequences and risks have been identified. In-production systems can sometimes not factor ethics and bias into their AI training processes, especially when the AI algorithms are inherently unexplainable in deep learning."
+396,"Machine-learning algorithms require large amounts of data. The techniques used to acquire this data have raised concerns about privacy, surveillance and copyright."
+397,"Technology companies collect a wide range of data from their users, including online activity, geolocation data, video and audio.
+For example, in order to build speech recognition algorithms, Amazon has recorded millions of private conversations and allowed temporary workers to listen to and transcribe some of them. Opinions about this widespread surveillance range from those who see it as a necessary evil to those for whom it is clearly unethical and a violation of the right to privacy."
+398,"AI developers argue that this is the only way to deliver valuable applications. and have developed several techniques that attempt to preserve privacy while still obtaining the data, such as data aggregation, de-identification and differential privacy. Since 2016, some privacy experts, such as Cynthia Dwork, have begun to view privacy in terms of fairness. Brian Christian wrote that experts have pivoted ""from the question of 'what they know' to the question of 'what they're doing with it'."""
+399,"Generative AI is often trained on unlicensed copyrighted works, including in domains such as images or computer code; the output is then used under the rationale of ""fair use"". Website owners who do not wish to have their copyrighted content AI-indexed or 'scraped' can add code to their site if they do not want their website to be indexed by a search engine, which is currently available through certain services such as OpenAI. Experts disagree about how well and under what circumstances this rationale will hold up in courts of law; relevant factors may include ""the purpose and character of the use of the copyrighted work"" and ""the effect upon the potential market for the copyrighted work"". In 2023, leading authors  sued AI companies for using their work to train generative AI."
+400,"YouTube, Facebook and others use recommender systems to guide users to more content. These AI programs were given the goal of maximizing user engagement . The AI learned that users tended to choose misinformation, conspiracy theories, and extreme partisan content, and, to keep them watching, the AI recommended more of it. Users also tended to watch more content on the same subject, so the AI led people into filter bubbles where they received multiple versions of the same misinformation. This convinced many users that the misinformation was true, and ultimately undermined trust in institutions, the media and the government. The AI program had correctly learned to maximize its goal, but the result was harmful to society. After the U.S. election in 2016, major technology companies took steps to mitigate the problem."
+401,"In 2022, generative AI began to create images, audio, video and text that are indistinguishable from real photographs, recordings, films or human writing. It is possible for bad actors to use this technology to create massive amounts of misinformation or propaganda. AI pioneer Geoffrey Hinton expressed concern about AI enabling ""authoritarian leaders to manipulate their electorates"" on a large scale, among other risks."
+402,"Machine learning applications will be biased if they learn from biased data.
+The developers may not be aware that the bias exists.
+Bias can be introduced by the way training data is selected and by the way a model is deployed. If a biased algorithm is used to make decisions that can seriously harm people  then the algorithm may cause discrimination.
+Fairness in machine learning is the study of how to prevent the harm caused by algorithmic bias. It has become serious area of academic study within AI. Researchers have discovered it is not always possible to define ""fairness"" in a way that satisfies all stakeholders."
+403,"On June 28, 2015, Google Photos's new image labeling feature mistakenly identified Jacky Alcine and a friend as ""gorillas"" because they were black. The system was trained on a dataset that contained very few images of black people, a problem called ""sample size disparity"". Google ""fixed"" this problem by preventing the system from labelling anything as a ""gorilla"". Eight years later, in 2023, Google Photos still could not identify a gorilla, and neither could similar products from Apple, Facebook, Microsoft and Amazon."
+404,"COMPAS is a commercial program widely used by U.S. courts to assess the likelihood of a defendant becoming a recidivist.
+In 2016, Julia Angwin at ProPublica discovered that COMPAS exhibited racial bias, despite the fact that the program was not told the races of the defendants. Although the error rate for both whites and blacks was calibrated equal at exactly 61%, the errors for each race were different—the system consistently overestimated the chance that a black person would re-offend and would underestimate the chance that a white person would not re-offend. In 2017, several researchers showed that it was mathematically impossible for COMPAS to accommodate all possible measures of fairness when the base rates of re-offense were different for whites and blacks in the data."
+405,"A program can make biased decisions even if the data does not explicitly mention a problematic feature . The feature will correlate with other features , and the program will make the same decisions based on these features as it would on ""race"" or ""gender"".
+Moritz Hardt said ""the most robust fact in this research area is that fairness through blindness doesn't work."""
+406,"Criticism of COMPAS highlighted that machine learning models are designed to make ""predictions"" that are only valid if we assume that the future will resemble the past. If they are trained on data that includes the results of racist decisions in the past, machine learning models must predict that racist decisions will be made in the future. If an application then uses these predictions as recommendations, some of these ""recommendations"" will likely be racist. Thus, machine learning is not well suited to help make decisions in areas where there is hope that the future will be better than the past. It is necessarily descriptive and not proscriptive."
+407,"Bias and unfairness may go undetected because the developers are overwhelmingly white and male: among AI engineers, about 4% are black and 20% are women."
+408,"At its 2022 Conference on Fairness, Accountability, and Transparency , the Association for Computing Machinery, in Seoul, South Korea, presented and published findings that recommend that until AI and robotics systems are demonstrated to be free of bias mistakes, they are unsafe, and the use of self-learning neural networks trained on vast, unregulated sources of flawed internet data should be curtailed."
+409,"Many AI systems are so complex that their designers cannot explain how they reach their decisions. Particularly with deep neural networks, in which there are a large amount of non-linear relationships between inputs and outputs. But some popular explainability techniques exist."
+410,"It is impossible to be certain that a program is operating correctly if no one knows how exactly it works. There have been many cases where a machine learning program passed rigorous tests, but nevertheless learned something different than what the programmers intended. For example, a system that could identify skin diseases better than medical professionals was found to actually have a strong tendency to classify images with a ruler as ""cancerous"", because pictures of malignancies typically include a ruler to show the scale. Another machine learning system designed to help effectively allocate medical resources was found to classify patients with asthma as being at ""low risk"" of dying from pneumonia. Having asthma is actually a severe risk factor, but since the patients having asthma would usually get much more medical care, they were relatively unlikely to die according to the training data. The correlation between asthma and low risk of dying from pneumonia was real, but misleading."
+411,"People who have been harmed by an algorithm's decision have a right to an explanation. Doctors, for example, are expected to clearly and completely explain to their colleagues the reasoning behind any decision they make. Early drafts of the European Union's General Data Protection Regulation in 2016 included an explicit statement that this right exists. Industry experts noted that this is an unsolved problem with no solution in sight. Regulators argued that nevertheless the harm is real: if the problem has no solution, the tools should not be used."
+412,DARPA established the XAI  program in 2014 to try and solve these problems.
+413,"There are several possible solutions to the transparency problem. SHAP tried to solve the transparency problems by visualising the contribution of each feature to the output. LIME can locally approximate a model with a simpler, interpretable model. Multitask learning provides a large number of outputs in addition to the target classification. These other outputs can help developers deduce what the network has learned. Deconvolution, DeepDream and other generative methods can allow developers to see what different layers of a deep network have learned and produce output that can suggest what the network is learning."
+414,"Artificial intelligence provides a number of tools that are useful to bad actors, such as authoritarian governments, terrorists, criminals or rogue states."
+415,"A lethal autonomous weapon is a machine that locates, selects and engages human targets without human supervision. Widely available AI tools can be used by bad actors to develop inexpensive autonomous weapons and, if produced at scale, they are potentially weapons of mass destruction. Even when used in conventional warfare, it is unlikely that they will be unable to reliably choose targets and could potentially kill an innocent person. In 2014, 30 nations  supported a ban on autonomous weapons under the United Nations' Convention on Certain Conventional Weapons, however the United States and others disagreed. By 2015, over fifty countries were reported to be researching battlefield robots."
+416,"AI tools make it easier for authoritarian governments to efficiently control their citizens in several ways. Face and voice recognition allow widespread surveillance. Machine learning, operating this data, can classify potential enemies of the state and prevent them from hiding. Recommendation systems can precisely target propaganda and misinformation for maximum effect. Deepfakes and generative AI aid in producing misinformation. Advanced AI can make authoritarian centralized decision making more competitive with liberal and decentralized systems such as markets. It lowers the cost and difficulty of digital warfare and advanced spyware. All these technologies have been available since 2020 or earlier -- AI facial recognition systems are already being used for mass surveillance in China."
+417,"There many other ways that AI is expected to help bad actors, some of which can not be foreseen. For example, machine-learning AI is able to design tens of thousands of toxic molecules in a matter of hours."
+418,Training AI systems requires an enormous amount of computing power. Usually only Big Tech companies have the financial resources to make such investments. Smaller startups such as Cohere and OpenAI end up buying access to data centers from Google and Microsoft respectively.
+419,"Economists have frequently highlighted the risks of redundancies from AI, and speculated about unemployment if there is no adequate social policy for full employment."
+420,"In the past, technology has tended to increase rather than reduce total employment, but economists acknowledge that ""we're in uncharted territory"" with AI. A survey of economists showed disagreement about whether the increasing use of robots and AI will cause a substantial increase in long-term unemployment, but they generally agree that it could be a net benefit if productivity gains are redistributed. Risk estimates vary; for example, in the 2010s, Michael Osborne and Carl Benedikt Frey estimated 47% of U.S. jobs are at ""high risk"" of potential automation, while an OECD report classified only 9% of U.S. jobs as ""high risk"". The methodology of speculating about future employment levels has been criticised as lacking evidential foundation, and for implying that technology, rather than social policy, creates unemployment, as opposed to redundancies. In April 2023, it was reported that 70% of the jobs for Chinese video game illustrators had been eliminated by generative artificial intelligence."
+421,"Unlike previous waves of automation, many middle-class jobs may be eliminated by artificial intelligence; The Economist stated in 2015 that ""the worry that AI could do to white-collar jobs what steam power did to blue-collar ones during the Industrial Revolution"" is ""worth taking seriously"". Jobs at extreme risk range from paralegals to fast food cooks, while job demand is likely to increase for care-related professions ranging from personal healthcare to the clergy."
+422,"From the early days of the development of artificial intelligence, there have been arguments, for example, those put forward by Joseph Weizenbaum, about whether tasks that can be done by computers actually should be done by them, given the difference between computers and humans, and between quantitative calculation and qualitative, value-based judgement."
+423,"It has been argued AI will become so powerful that humanity may irreversibly lose control of it. This could, as physicist Stephen Hawking stated, ""spell the end of the human race"". This scenario has been common in science fiction, when a computer or robot suddenly develops a human-like ""self-awareness""  and becomes a malevolent character. These sci-fi scenarios are misleading in several ways."
+424,"First, AI does not require human-like ""sentience"" to be an existential risk. Modern AI programs are given specific goals and use learning and intelligence to achieve them. Philosopher Nick Bostrom argued that if one gives almost any goal to a sufficiently powerful AI, it may choose to destroy humanity to achieve it . Stuart Russell gives the example of household robot that tries to find a way to kill its owner to prevent it from being unplugged, reasoning that ""you can't fetch the coffee if you're dead."" In order to be safe for humanity, a superintelligence would have to be genuinely aligned with humanity's morality and values so that it is ""fundamentally on our side""."
+425,"Second, Yuval Noah Harari argues that AI does not require a robot body or physical control to pose an existential risk. The essential parts of civilization are not physical. Things like ideologies, law, government, money and the economy are made of language; they exist because there are stories that billions of people believe. The current prevalence of misinformation suggests that an AI could use language to convince people to believe anything, even to take actions that are destructive."
+426,"The opinions amongst experts and industry insiders are mixed, with sizable fractions both concerned and unconcerned by risk from eventual superintelligent AI. Personalities such as Stephen Hawking, Bill Gates, and Elon Musk have expressed concern about existential risk from AI.
+AI pioneers including Fei-Fei Li, Geoffrey Hinton, Yoshua Bengio, Cynthia Breazeal, Rana el Kaliouby, Demis Hassabis, Joy Buolamwini, and Sam Altman have expressed concerns about the risks of AI. In 2023, many leading AI experts issued the joint statement that ""Mitigating the risk of extinction from AI should be a global priority alongside other societal-scale risks such as pandemics and nuclear war""."
+427,"Other researchers, however, spoke in favor of a less dystopian view. AI pioneer Juergen Schmidhuber did not sign the joint statement, emphasising that in 95% of all cases, AI research is about making ""human lives longer and healthier and easier."" While the tools that are now being used to improve lives can also be used by bad actors, ""they can also be used against the bad actors."" Andrew Ng also argued that ""it's a mistake to fall for the doomsday hype on AI—and that regulators who do will only benefit vested interests."" Yann LeCun ""scoffs at his peers' dystopian scenarios of supercharged misinformation and even, eventually, human extinction."" In the early 2010s, experts argued that the risks are too distant in the future to warrant research or that humans will be valuable from the perspective of a superintelligent machine. However, after 2016, the study of current and future risks and possible solutions became a serious area of research."
+428,"Friendly AI are machines that have been designed from the beginning to minimize risks and to make choices that benefit humans. Eliezer Yudkowsky, who coined the term, argues that developing friendly AI should be a higher research priority: it may require a large investment and it must be completed before AI becomes an existential risk."
+429,"Machines with intelligence have the potential to use their intelligence to make ethical decisions. The field of machine ethics provides machines with ethical principles and procedures for resolving ethical dilemmas.
+The field of machine ethics is also called computational morality,
+and was founded at an AAAI symposium in 2005."
+430,"Other approaches include Wendell Wallach's ""artificial moral agents""
+and Stuart J. Russell's three principles for developing provably beneficial machines."
+431,"Artificial Intelligence projects can have their ethical permissibility tested while designing, developing, and implementing an AI system. An AI framework such as the Care and Act Framework containing the SUM values—developed by the Alan Turing Institute tests projects in four main areas:"
+432,"Other developments in ethical frameworks include those decided upon during the Asilomar Conference, the Montreal Declaration for Responsible AI, and the IEEE's Ethics of Autonomous Systems initiative, among others; however, these principles do not go without their criticisms, especially regards to the people chosen contributes to these frameworks."
+433,"Promotion of the wellbeing of the people and communities that these technologies affect requires consideration of the social and ethical implications at all stages of AI system design, development and implementation, and collaboration between job roles such as data scientists, product managers, data engineers, domain experts, and delivery managers."
+434,"The regulation of artificial intelligence is the development of public sector policies and laws for promoting and regulating artificial intelligence ; it is therefore related to the broader regulation of algorithms.
+The regulatory and policy landscape for AI is an emerging issue in jurisdictions globally. According to AI Index at Stanford, the annual number of AI-related laws passed in the 127 survey countries jumped from one passed in 2016 to 37 passed in 2022 alone.
+Between 2016 and 2020, more than 30 countries adopted dedicated strategies for AI.
+Most EU member states had released national AI strategies, as had Canada, China, India, Japan, Mauritius, the Russian Federation, Saudi Arabia, United Arab Emirates, US and Vietnam. Others were in the process of elaborating their own AI strategy, including Bangladesh, Malaysia and Tunisia.
+The Global Partnership on Artificial Intelligence was launched in June 2020, stating a need for AI to be developed in accordance with human rights and democratic values, to ensure public confidence and trust in the technology. Henry Kissinger, Eric Schmidt, and Daniel Huttenlocher published a joint statement in November 2021 calling for a government commission to regulate AI.
+In 2023, OpenAI leaders published recommendations for the governance of superintelligence, which they believe may happen in less than 10 years. In 2023, the United Nations also launched an advisory body to provide recommendations on AI governance; the body comprises technology company executives, governments officials and academics."
+435,"In a 2022 Ipsos survey, attitudes towards AI varied greatly by country; 78% of Chinese citizens, but only 35% of Americans, agreed that ""products and services using AI have more benefits than drawbacks"". A 2023 Reuters/Ipsos poll found that 61% of Americans agree, and 22% disagree, that AI poses risks to humanity.
+In a 2023 Fox News poll, 35% of Americans thought it ""very important"", and an additional 41% thought it ""somewhat important"", for the federal government to regulate AI, versus 13% responding ""not very important"" and 8% responding ""not at all important""."
+436,"In November 2023, the first global AI Safety Summit was held in Bletchley Park in the UK to discuss the near and far term risks of AI and the possibility of mandatory and voluntary regulatory frameworks. 28 countries including the United States, China, and the European Union issued a declaration at the start of the summit, calling for international co-operation to manage the challenges and risks of artificial intelligence."
+437,"The study of mechanical or ""formal"" reasoning began with philosophers and mathematicians in antiquity. The study of logic led directly to Alan Turing's theory of computation, which suggested that a machine, by shuffling symbols as simple as ""0"" and ""1"", could simulate any conceivable form of mathematical reasoning. This, along with concurrent discoveries in cybernetics, information theory and neurobiology, led researchers to consider the possibility of building an ""electronic brain"". 
+They developed several areas of research that would become part of AI,
+such as McCullouch and Pitts design for ""artificial neurons"" in 1943, and Turing's influential 1950 paper 'Computing Machinery and Intelligence', which introduced the Turing test and showed that ""machine intelligence"" was plausible."
+438,"The field of AI research was founded at a workshop at Dartmouth College in 1956. The attendees became the leaders of AI research in the 1960s. They and their students produced programs that the press described as ""astonishing"": computers were learning checkers strategies, solving word problems in algebra, proving logical theorems and speaking English. Artificial intelligence laboratories were set up at a number of British and U.S. Universities in the latter 1950s and early 1960s."
+439,"Researchers in the 1960s and the 1970s were convinced that their methods would eventually succeed in creating a machine with general intelligence and considered this the goal of their field. Herbert Simon predicted, ""machines will be capable, within twenty years, of doing any work a man can do"". Marvin Minsky agreed, writing, ""within a generation ... the problem of creating 'artificial intelligence' will substantially be solved"". They had, however, underestimated the difficulty of the problem. In 1974, both the U.S. and British governments cut off exploratory research in response to the criticism of Sir James Lighthill and ongoing pressure from the U.S. Congress to fund more productive projects. Minsky's and Papert's book Perceptrons was understood as proving that artificial neural networks would never be useful for solving real-world tasks, thus discrediting the approach altogether. The ""AI winter"", a period when obtaining funding for AI projects was difficult, followed."
+440,"In the early 1980s, AI research was revived by the commercial success of expert systems, a form of AI program that simulated the knowledge and analytical skills of human experts. By 1985, the market for AI had reached over a billion dollars. At the same time, Japan's fifth generation computer project inspired the U.S. and British governments to restore funding for academic research. However, beginning with the collapse of the Lisp Machine market in 1987, AI once again fell into disrepute, and a second, longer-lasting winter began."
+441,"Up to this point, most of AI's funding had gone to projects which used high level symbols to represent mental objects like plans, goals, beliefs and known facts. In the 1980s, some researchers began to doubt that this approach would be able to imitate all the processes of human cognition, especially perception, robotics, learning and pattern recognition, and began to look into ""sub-symbolic"" approaches. Rodney Brooks rejected ""representation"" in general and focussed directly on engineering machines that move and survive. Judea Pearl, Lofti Zadeh and others developed methods that handled incomplete and uncertain information by making reasonable guesses rather than precise logic. But the most important development was the revival of ""connectionism"", including neural network research, by Geoffrey Hinton and others. In 1990, Yann LeCun successfully showed that convolutional neural networks can recognize handwritten digits, the first of many successful applications of neural networks."
+442,"AI gradually restored its reputation in the late 1990s and early 21st century by exploiting formal mathematical methods and by finding specific solutions to specific problems. This ""narrow"" and ""formal"" focus allowed researchers to produce verifiable results and collaborate with other fields .
+By 2000, solutions developed by AI researchers were being widely used, although in the 1990s they were rarely described as ""artificial intelligence"".
+However, several academic researchers became concerned that AI was no longer pursuing its original goal of creating versatile, fully intelligent machines. Beginning around 2002, they founded the subfield of artificial general intelligence , which had several well-funded institutions by the 2010s."
+443,"Deep learning began to dominate industry benchmarks in 2012 and was adopted throughout the field.
+For many specific tasks, other methods were abandoned.
+Deep learning's success was based on both hardware improvements  and access to large amounts of data . Deep learning's success led to an enormous increase in interest and funding in AI. The amount of machine learning research  increased by 50% in the years 2015–2019."
+444,"In 2016, issues of fairness and the misuse of technology were catapulted into center stage at machine learning conferences, publications vastly increased, funding became available, and many researchers re-focussed their careers on these issues. The alignment problem became a serious field of academic study."
+445,"In the late teens and early 2020s, AGI companies began to deliver programs that created enormous interest. In 2015, AlphaGo, developed by DeepMind, beat the world champion Go player. The program was taught only the rules of the game and developed strategy by itself. GPT-3 is a large language model that was released in 2020 by OpenAI and is capable of generating high-quality human-like text. 
+These programs, and others, inspired an aggressive AI boom, where large companies began investing billions in AI research. According to 'AI Impacts', about $50 billion annually was invested in ""AI"" around 2022 in the U.S. alone and about 20% of new US Computer Science PhD graduates have specialized in ""AI"".
+About 800,000 ""AI""-related US job openings existed in 2022."
+446,"Alan Turing wrote in 1950 ""I propose to consider the question 'can machines think'?"" He advised changing the question from whether a machine ""thinks"", to ""whether or not it is possible for machinery to show intelligent behaviour"". He devised the Turing test, which measures the ability of a machine to simulate human conversation. Since we can only observe the behavior of the machine, it does not matter if it is ""actually"" thinking or literally has a ""mind"". Turing notes that we can not determine these things about other people but ""it is usual to have a polite convention that everyone thinks"""
+447,"Russell and Norvig agree with Turing that intelligence must be defined in terms of external behavior, not internal structure. However, they are critical that the test requires the machine to imitate humans. ""Aeronautical engineering texts,"" they wrote, ""do not define the goal of their field as making 'machines that fly so exactly like pigeons that they can fool other pigeons.'"" AI founder John McCarthy agreed, writing that ""Artificial intelligence is not, by definition, simulation of human intelligence""."
+448,"McCarthy defines intelligence as ""the computational part of the ability to achieve goals in the world."" Another AI founder, Marvin Minsky similarly describes it as ""the ability to solve hard problems"". The leading AI textbook defines it as the study of agents that perceive their environment and take actions that maximize their chances of achieving defined goals. These definitions view intelligence in terms of well-defined problems with well-defined solutions, where both the difficulty of the problem and the performance of the program are direct measures of the ""intelligence"" of the machine—and no other philosophical discussion is required, or may not even be possible."
+449,"Another definition has been adopted by Google, a major practitioner in the field of AI. This definition stipulates the ability of systems to synthesize information as the manifestation of intelligence, similar to the way it is defined in biological intelligence."
+450,"No established unifying theory or paradigm has guided AI research for most of its history. The unprecedented success of statistical machine learning in the 2010s eclipsed all other approaches . This approach is mostly sub-symbolic, soft and narrow . Critics argue that these questions may have to be revisited by future generations of AI researchers."
+451,"Symbolic AI  simulated the high-level conscious reasoning that people use when they solve puzzles, express legal reasoning and do mathematics. They were highly successful at ""intelligent"" tasks such as algebra or IQ tests. In the 1960s, Newell and Simon proposed the physical symbol systems hypothesis: ""A physical symbol system has the necessary and sufficient means of general intelligent action."""
+452,"However, the symbolic approach failed on many tasks that humans solve easily, such as learning, recognizing an object or commonsense reasoning. Moravec's paradox is the discovery that high-level ""intelligent"" tasks were easy for AI, but low level ""instinctive"" tasks were extremely difficult. Philosopher Hubert Dreyfus had argued since the 1960s that human expertise depends on unconscious instinct rather than conscious symbol manipulation, and on having a ""feel"" for the situation, rather than explicit symbolic knowledge. Although his arguments had been ridiculed and ignored when they were first presented, eventually, AI research came to agree with him."
+453,"The issue is not resolved: sub-symbolic reasoning can make many of the same inscrutable mistakes that human intuition does, such as algorithmic bias. Critics such as Noam Chomsky argue continuing research into symbolic AI will still be necessary to attain general intelligence, in part because sub-symbolic AI is a move away from explainable AI: it can be difficult or impossible to understand why a modern statistical AI program made a particular decision. The emerging field of neuro-symbolic artificial intelligence attempts to bridge the two approaches."
+454,"""Neats"" hope that intelligent behavior is described using simple, elegant principles . ""Scruffies"" expect that it necessarily requires solving a large number of unrelated problems. Neats defend their programs with theoretical rigor, scruffies rely mainly on incremental testing to see if they work. This issue was actively discussed in the 1970s and 1980s, but eventually was seen as irrelevant. Modern AI has elements of both."
+455,"Finding a provably correct or optimal solution is intractable for many important problems. Soft computing is a set of techniques, including genetic algorithms, fuzzy logic and neural networks, that are tolerant of imprecision, uncertainty, partial truth and approximation. Soft computing was introduced in the late 1980s and most successful AI programs in the 21st century are examples of soft computing with neural networks."
+456,"AI researchers are divided as to whether to pursue the goals of artificial general intelligence and superintelligence directly or to solve as many specific problems as possible  in hopes these solutions will lead indirectly to the field's long-term goals. General intelligence is difficult to define and difficult to measure, and modern AI has had more verifiable successes by focusing on specific problems with specific solutions. The experimental sub-field of artificial general intelligence studies this area exclusively."
+457,"The philosophy of mind does not know whether a machine can have a mind, consciousness and mental states, in the same sense that human beings do. This issue considers the internal experiences of the machine, rather than its external behavior. Mainstream AI research considers this issue irrelevant because it does not affect the goals of the field: to build machines that can solve problems using intelligence. Russell and Norvig add that ""he additional project of making a machine conscious in exactly the way humans are is not one that we are equipped to take on."" However, the question has become central to the philosophy of mind. It is also typically the central question at issue in artificial intelligence in fiction."
+458,"David Chalmers identified two problems in understanding the mind, which he named the ""hard"" and ""easy"" problems of consciousness. The easy problem is understanding how the brain processes signals, makes plans and controls behavior. The hard problem is explaining how this feels or why it should feel like anything at all, assuming we are right in thinking that it truly does feel like something . Human information processing is easy to explain, however, human subjective experience is difficult to explain. For example, it is easy to imagine a color-blind person who has learned to identify which objects in their field of view are red, but it is not clear what would be required for the person to know what red looks like."
+459,Computationalism is the position in the philosophy of mind that the human mind is an information processing system and that thinking is a form of computing. Computationalism argues that the relationship between mind and body is similar or identical to the relationship between software and hardware and thus may be a solution to the mind–body problem. This philosophical position was inspired by the work of AI researchers and cognitive scientists in the 1960s and was originally proposed by philosophers Jerry Fodor and Hilary Putnam.
+460,"Philosopher John Searle characterized this position as ""strong AI"": ""The appropriately programmed computer with the right inputs and outputs would thereby have a mind in exactly the same sense human beings have minds."" Searle counters this assertion with his Chinese room argument, which attempts to show that, even if a machine perfectly simulates human behavior, there is still no reason to suppose it also has a mind."
+461,"It is difficult or impossible to reliably evaluate whether an advanced AI is sentient , and if so, to what degree. But if there is a significant chance that a given machine can feel and suffer, then it may be entitled to certain rights or welfare protection measures, similarly to animals. Sapience  may provide another moral basis for AI rights. Robot rights are also sometimes proposed as a practical way to integrate autonomous agents into society."
+462,"In 2017, the European Union considered granting ""electronic personhood"" to some of the most capable AI systems. Similarly to the legal status of companies, it would have conferred rights but also responsibilities. Critics argued in 2018 that granting rights to AI systems would downplay the importance of human rights, and that legislation should focus on user needs rather than speculative futuristic scenarios. They also noted that robots lacked the autonomy to take part to society on their own."
+463,"Progress in AI increased interest in the topic. Proponents of AI welfare and rights often argue that AI sentience, if it emerges, would be particularly easy to deny. They warn that this may be a moral blind spot analogous to slavery or factory farming, which could lead to large-scale suffering if sentient AI is created and carelessly exploited."
+464,A superintelligence is a hypothetical agent that would possess intelligence far surpassing that of the brightest and most gifted human mind.
+465,"If research into artificial general intelligence produced sufficiently intelligent software, it might be able to reprogram and improve itself. The improved software would be even better at improving itself, leading to what I. J. Good called an ""intelligence explosion"" and Vernor Vinge called a ""singularity""."
+466,"However, technologies cannot improve exponentially indefinitely, and typically follow an S-shaped curve, slowing when they reach the physical limits of what the technology can do."
+467,"Robot designer Hans Moravec, cyberneticist Kevin Warwick, and inventor Ray Kurzweil have predicted that humans and machines will merge in the future into cyborgs that are more capable and powerful than either. This idea, called transhumanism, has roots in Aldous Huxley and Robert Ettinger."
+468,"Edward Fredkin argues that ""artificial intelligence is the next stage in evolution"", an idea first proposed by Samuel Butler's ""Darwin among the Machines"" as far back as 1863, and expanded upon by George Dyson in his book of the same name in 1998."
+469,"Thought-capable artificial beings have appeared as storytelling devices since antiquity, and have been a persistent theme in science fiction."
+470,"A common trope in these works began with Mary Shelley's Frankenstein, where a human creation becomes a threat to its masters. This includes such works as Arthur C. Clarke's and Stanley Kubrick's 2001: A Space Odyssey , with HAL 9000, the murderous computer in charge of the Discovery One spaceship, as well as The Terminator  and The Matrix . In contrast, the rare loyal robots such as Gort from The Day the Earth Stood Still  and Bishop from Aliens  are less prominent in popular culture."
+471,"Isaac Asimov introduced the Three Laws of Robotics in many books and stories, most notably the ""Multivac"" series about a super-intelligent computer of the same name. Asimov's laws are often brought up during lay discussions of machine ethics; while almost all artificial intelligence researchers are familiar with Asimov's laws through popular culture, they generally consider the laws useless for many reasons, one of which is their ambiguity."
+472,"Several works use AI to force us to confront the fundamental question of what makes us human, showing us artificial beings that have the ability to feel, and thus to suffer. This appears in Karel Čapek's R.U.R., the films A.I. Artificial Intelligence and Ex Machina, as well as the novel Do Androids Dream of Electric Sheep?, by Philip K. Dick. Dick considers the idea that our understanding of human subjectivity is altered by technology created with artificial intelligence."
+473,The two most widely used textbooks in 2023. .
+474,These were the four of the most widely used AI textbooks in 2008:
+475,"A cellular automaton  is a discrete model of computation studied in automata theory. Cellular automata are also called cellular spaces, tessellation automata, homogeneous structures, cellular structures, tessellation structures, and iterative arrays. Cellular automata have found application in various areas, including physics, theoretical biology and microstructure modeling."
+476,"A cellular automaton consists of a regular grid of cells, each in one of a finite number of states, such as on and off . The grid can be in any finite number of dimensions. For each cell, a set of cells called its neighborhood is defined relative to the specified cell. An initial state  is selected by assigning a state for each cell. A new generation is created , according to some fixed rule  that determines the new state of each cell in terms of the current state of the cell and the states of the cells in its neighborhood. Typically, the rule for updating the state of cells is the same for each cell and does not change over time, and is applied to the whole grid simultaneously, though exceptions are known, such as the stochastic cellular automaton and asynchronous cellular automaton."
+477,"The concept was originally discovered in the 1940s by Stanislaw Ulam and John von Neumann while they were contemporaries at Los Alamos National Laboratory. While studied by some throughout the 1950s and 1960s, it was not until the 1970s and Conway's Game of Life, a two-dimensional cellular automaton, that interest in the subject expanded beyond academia. In the 1980s, Stephen Wolfram engaged in a systematic study of one-dimensional cellular automata, or what he calls elementary cellular automata; his research assistant Matthew Cook showed that one of these rules is Turing-complete."
+478,"The primary classifications of cellular automata, as outlined by Wolfram, are numbered one to four. They are, in order, automata in which patterns generally stabilize into homogeneity, automata in which patterns evolve into mostly stable or oscillating structures, automata in which patterns evolve in a seemingly chaotic fashion, and automata in which patterns become extremely complex and may last for a long time, with stable local structures.  This last class is thought to be computationally universal, or capable of simulating a Turing machine. Special types of cellular automata are reversible, where only a single configuration leads directly to a subsequent one, and totalistic, in which the future value of individual cells only depends on the total value of a group of neighboring cells. Cellular automata can simulate a variety of real-world systems, including biological and chemical ones."
+479,"One way to simulate a two-dimensional cellular automaton is with an infinite sheet of graph paper along with a set of rules for the cells to follow. Each square is called a ""cell"" and each cell has two possible states, black and white. The neighborhood of a cell is the nearby, usually adjacent, cells. The two most common types of neighborhoods are the von Neumann neighborhood and the Moore neighborhood. The former, named after the founding cellular automaton theorist, consists of the four orthogonally adjacent cells. The latter includes the von Neumann neighborhood as well as the four diagonally adjacent cells. For such a cell and its Moore neighborhood, there are 512  possible patterns. For each of the 512 possible patterns, the rule table would state whether the center cell will be black or white on the next time interval. Conway's Game of Life is a popular version of this model. Another common neighborhood type is the extended von Neumann neighborhood, which includes the two closest cells in each orthogonal direction, for a total of eight. The general equation for the total number of automata possible is kks, where k is the number of possible states for a cell, and s is the number of neighboring cells  used to determine the cell's next state. Thus, in the two-dimensional system with a Moore neighborhood, the total number of automata possible would be 229, or 1.34×10154."
+480,"It is usually assumed that every cell in the universe starts in the same state, except for a finite number of cells in other states; the assignment of state values is called a configuration. More generally, it is sometimes assumed that the universe starts out covered with a periodic pattern, and only a finite number of cells violate that pattern. The latter assumption is common in one-dimensional cellular automata."
+481,"Cellular automata are often simulated on a finite grid rather than an infinite one. In two dimensions, the universe would be a rectangle instead of an infinite plane. The obvious problem with finite grids is how to handle the cells on the edges. How they are handled will affect the values of all the cells in the grid. One possible method is to allow the values in those cells to remain constant. Another method is to define neighborhoods differently for these cells. One could say that they have fewer neighbors, but then one would also have to define new rules for the cells located on the edges. These cells are usually handled with periodic boundary conditions resulting in a toroidal arrangement: when one goes off the top, one comes in at the corresponding position on the bottom, and when one goes off the left, one comes in on the right.  This can be visualized as taping the left and right edges of the rectangle to form a tube, then taping the top and bottom edges of the tube to form a torus . Universes of other dimensions are handled similarly. This solves boundary problems with neighborhoods, but another advantage is that it is easily programmable using modular arithmetic functions. For example, in a 1-dimensional cellular automaton like the examples below, the neighborhood of a cell xit is {xi−1t−1, xit−1, xi+1t−1}, where t is the time step , and i is the index  in one generation."
+482,"Stanislaw Ulam, while working at the Los Alamos National Laboratory in the 1940s, studied the growth of crystals, using a simple lattice network as his model. At the same time, John von Neumann, Ulam's colleague at Los Alamos, was working on the problem of self-replicating systems. Von Neumann's initial design was founded upon the notion of one robot building another robot. This design is known as the kinematic model. As he developed this design, von Neumann came to realize the great difficulty of building a self-replicating robot, and of the great cost in providing the robot with a ""sea of parts"" from which to build its replicant. Neumann wrote a paper entitled ""The general and logical theory of automata"" for the Hixon Symposium in 1948. Ulam was the one who suggested using a discrete system for creating a reductionist model of self-replication. Nils Aall Barricelli performed many of the earliest explorations of these models of artificial life."
+483,"Ulam and von Neumann created a method for calculating liquid motion in the late 1950s. The driving concept of the method was to consider a liquid as a group of discrete units and calculate the motion of each based on its neighbors' behaviors. Thus was born the first system of cellular automata. Like Ulam's lattice network, von Neumann's cellular automata are two-dimensional, with his self-replicator implemented algorithmically. The result was a universal copier and constructor working within a cellular automaton with a small neighborhood , and with 29 states per cell. Von Neumann gave an existence proof that a particular pattern would make endless copies of itself within the given cellular universe by designing a 200,000 cell configuration that could do so. This design is known as the tessellation model, and is called a von Neumann universal constructor."
+484,"Also in the 1940s, Norbert Wiener and Arturo Rosenblueth developed a model of excitable media with some of the characteristics of a cellular automaton. Their specific motivation was the mathematical description of impulse conduction in cardiac systems. However their model is not a cellular automaton because the medium in which signals propagate is continuous, and wave fronts are curves. A true cellular automaton model of excitable media was developed and studied by J. M. Greenberg and S. P. Hastings in 1978; see Greenberg-Hastings cellular automaton. The original work of Wiener and Rosenblueth contains many insights and continues to be cited in modern research publications on cardiac arrhythmia and excitable systems."
+485,"In the 1960s, cellular automata were studied as a particular type of dynamical system and the connection with the mathematical field of symbolic dynamics was established for the first time. In 1969, Gustav A. Hedlund compiled many results following this point of view in what is still considered as a seminal paper for the mathematical study of cellular automata. The most fundamental result is the characterization in the Curtis–Hedlund–Lyndon theorem of the set of global rules of cellular automata as the set of continuous endomorphisms of shift spaces."
+486,"In 1969, German computer pioneer Konrad Zuse published his book Calculating Space, proposing that the physical laws of the universe are discrete by nature, and that the entire universe is the output of a deterministic computation on a single cellular automaton; ""Zuse's Theory"" became the foundation of the field of study called digital physics."
+487,"Also in 1969 computer scientist Alvy Ray Smith completed a Stanford PhD dissertation on Cellular Automata Theory, the first mathematical treatment of CA as a general class of computers. Many papers came from this dissertation: He showed the equivalence of neighborhoods of various shapes, how to reduce a Moore to a von Neumann neighborhood or how to reduce any neighborhood to a von Neumann neighborhood. He proved that two-dimensional CA are computation universal, introduced 1-dimensional CA, and showed that they too are computation universal, even with simple neighborhoods. He showed how to subsume the complex von Neumann proof of construction universality  into a consequence of computation universality in a 1-dimensional CA. Intended as the introduction to the German edition of von Neumann's book on CA, he wrote a survey of the field with dozens of references to papers, by many authors in many countries over a decade or so of work, often overlooked by modern CA researchers."
+488,"In the 1970s a two-state, two-dimensional cellular automaton named Game of Life became widely known, particularly among the early computing community. Invented by John Conway and popularized by Martin Gardner in a Scientific American article, its rules are as follows:"
+489,"Any live cell with fewer than two live neighbours dies, as if caused by underpopulation."
+490,Any live cell with two or three live neighbours lives on to the next generation.
+491,"Any live cell with more than three live neighbours dies, as if by overpopulation."
+492,"Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction."
+493,"Despite its simplicity, the system achieves an impressive diversity of behavior, fluctuating between apparent randomness and order. One of the most apparent features of the Game of Life is the frequent occurrence of gliders, arrangements of cells that essentially move themselves across the grid. It is possible to arrange the automaton so that the gliders interact to perform computations, and after much effort it has been shown that the Game of Life can emulate a universal Turing machine. It was viewed as a largely recreational topic, and little follow-up work was done outside of investigating the particularities of the Game of Life and a few related rules in the early 1970s."
+494,"Stephen Wolfram independently began working on cellular automata in mid-1981 after considering how complex patterns seemed formed in nature in violation of the Second Law of Thermodynamics. His investigations were initially spurred by a desire to model systems such as the neural networks found in brains. He published his first paper in Reviews of Modern Physics investigating elementary cellular automata  in June 1983. The unexpected complexity of the behavior of these simple rules led Wolfram to suspect that complexity in nature may be due to similar mechanisms. His investigations, however, led him to realize that cellular automata were poor at modelling neural networks. Additionally, during this period Wolfram formulated the concepts of intrinsic randomness and computational irreducibility, and suggested that rule 110 may be universal—a fact proved later by Wolfram's research assistant Matthew Cook in the 1990s."
+495,"Wolfram, in A New Kind of Science and several papers dating from the mid-1980s, defined four classes into which cellular automata and several other simple computational models can be divided depending on their behavior. While earlier studies in cellular automata tended to try to identify type of patterns for specific rules, Wolfram's classification was the first attempt to classify the rules themselves. In order of complexity the classes are:"
+496,"These definitions are qualitative in nature and there is some room for interpretation. According to Wolfram, ""...with almost any general classification scheme there are inevitably cases which get assigned to one class by one definition and another class by another definition. And so it is with cellular automata: there are occasionally rules...that show some features of one class and some of another."" Wolfram's classification has been empirically matched to a clustering of the compressed lengths of the outputs of cellular automata."
+497,"There have been several attempts to classify cellular automata in formally rigorous classes, inspired by Wolfram's classification. For instance, Culik and Yu proposed three well-defined classes , which are sometimes called Culik–Yu classes; membership in these proved undecidable.
+Wolfram's class 2 can be partitioned into two subgroups of stable  and oscillating  rules."
+498,"The idea that there are 4 classes of dynamical system came originally from Nobel-prize winning chemist Ilya Prigogine who identified these 4 classes of thermodynamical systems:  systems in thermodynamic equilibrium,  spatially/temporally uniform systems,  chaotic systems, and  complex far-from-equilibrium systems with dissipative structures ."
+499,"A cellular automaton is reversible if, for every current configuration of the cellular automaton, there is exactly one past configuration . If one thinks of a cellular automaton as a function mapping configurations to configurations, reversibility implies that this function is bijective. If a cellular automaton is reversible, its time-reversed behavior can also be described as a cellular automaton; this fact is a consequence of the Curtis–Hedlund–Lyndon theorem, a topological characterization of cellular automata. For cellular automata in which not every configuration has a preimage, the configurations without preimages are called Garden of Eden patterns."
+500,"For one-dimensional cellular automata there are known algorithms for deciding whether a rule is reversible or irreversible. However, for cellular automata of two or more dimensions reversibility is undecidable; that is, there is no algorithm that takes as input an automaton rule and is guaranteed to determine correctly whether the automaton is reversible. The proof by Jarkko Kari is related to the tiling problem by Wang tiles."
+501,"Reversible cellular automata are often used to simulate such physical phenomena as gas and fluid dynamics, since they obey the laws of thermodynamics. Such cellular automata have rules specially constructed to be reversible. Such systems have been studied by Tommaso Toffoli, Norman Margolus and others. Several techniques can be used to explicitly construct reversible cellular automata with known inverses. Two common ones are the second-order cellular automaton and the block cellular automaton, both of which involve modifying the definition of a cellular automaton in some way. Although such automata do not strictly satisfy the definition given above, it can be shown that they can be emulated by conventional cellular automata with sufficiently large neighborhoods and numbers of states, and can therefore be considered a subset of conventional cellular automata. Conversely, it has been shown that every reversible cellular automaton can be emulated by a block cellular automaton."
+502,"A special class of cellular automata are totalistic cellular automata. The state of each cell in a totalistic cellular automaton is represented by a number , and the value of a cell at time t depends only on the sum of the values of the cells in its neighborhood  at time t − 1. If the state of the cell at time t depends on both its own state and the total of its neighbors at time t − 1 then the cellular automaton is properly called outer totalistic. Conway's Game of Life is an example of an outer totalistic cellular automaton with cell values 0 and 1; outer totalistic cellular automata with the same Moore neighborhood structure as Life are sometimes called life-like cellular automata."
+503,There are many possible generalizations of the cellular automaton concept.
+504,"One way is by using something other than a rectangular  grid. For example, if a plane is tiled with regular hexagons, those hexagons could be used as cells. In many cases the resulting cellular automata are equivalent to those with rectangular grids with specially designed neighborhoods and rules. Another variation would be to make the grid itself irregular, such as with Penrose tiles."
+505,"Also, rules can be probabilistic rather than deterministic. Such cellular automata are called probabilistic cellular automata. A probabilistic rule gives, for each pattern at time t, the probabilities that the central cell will transition to each possible state at time t + 1. Sometimes a simpler rule is used; for example: ""The rule is the Game of Life, but on each time step there is a 0.001% probability that each cell will transition to the opposite color."""
+506,"The neighborhood or rules could change over time or space. For example, initially the new state of a cell could be determined by the horizontally adjacent cells, but for the next generation the vertical cells would be used."
+507,"In cellular automata, the new state of a cell is not affected by the new state of other cells. This could be changed so that, for instance, a 2 by 2 block of cells can be determined by itself and the cells adjacent to itself."
+508,"There are continuous automata. These are like totalistic cellular automata, but instead of the rule and states being discrete , continuous functions are used, and the states become continuous . The state of a location is a finite number of real numbers. Certain cellular automata can yield diffusion in liquid patterns in this way."
+509,"Continuous spatial automata have a continuum of locations. The state of a location is a finite number of real numbers. Time is also continuous, and the state evolves according to differential equations. One important example is reaction–diffusion textures, differential equations proposed by Alan Turing to explain how chemical reactions could create the stripes on zebras and spots on leopards. When these are approximated by cellular automata, they often yield similar patterns. MacLennan  considers continuous spatial automata as a model of computation."
+510,"There are known examples of continuous spatial automata, which exhibit propagating phenomena analogous to gliders in the Game of Life."
+511,Graph rewriting automata are extensions of cellular automata based on graph rewriting systems.
+512,"The simplest nontrivial cellular automaton would be one-dimensional, with two possible states per cell, and a cell's neighbors defined as the adjacent cells on either side of it. A cell and its two neighbors form a neighborhood of 3 cells, so there are 23 = 8 possible patterns for a neighborhood. A rule consists of deciding, for each pattern, whether the cell will be a 1 or a 0 in the next generation. There are then 28 = 256 possible rules."
+513,"These 256 cellular automata are generally referred to by their Wolfram code, a standard naming convention invented by Wolfram that gives each rule a number from 0 to 255. A number of papers have analyzed and compared these 256 cellular automata. The rule 30, rule 90, rule 110, and rule 184 cellular automata are particularly interesting. The images below show the history of rules 30 and 110 when the starting configuration consists of a 1  surrounded by 0s. Each row of pixels represents a generation in the history of the automaton, with t=0 being the top row. Each pixel is colored white for 0 and black for 1."
+514,"Rule 30 exhibits class 3 behavior, meaning even simple input patterns such as that shown lead to chaotic, seemingly random histories."
+515,"Rule 110, like the Game of Life, exhibits what Wolfram calls class 4 behavior, which is neither completely random nor completely repetitive. Localized structures appear and interact in various complicated-looking ways. In the course of the development of A New Kind of Science, as a research assistant to Wolfram in 1994, Matthew Cook proved that some of these structures were rich enough to support universality. This result is interesting because rule 110 is an extremely simple one-dimensional system, and difficult to engineer to perform specific behavior. This result therefore provides significant support for Wolfram's view that class 4 systems are inherently likely to be universal. Cook presented his proof at a Santa Fe Institute conference on Cellular Automata in 1998, but Wolfram blocked the proof from being included in the conference proceedings, as Wolfram did not want the proof announced before the publication of A New Kind of Science. In 2004, Cook's proof was finally published in Wolfram's journal Complex Systems , over ten years after Cook came up with it. Rule 110 has been the basis for some of the smallest universal Turing machines."
+516,"An elementary cellular automaton rule is specified by 8 bits, and all elementary cellular automaton rules can be considered to sit on the vertices of the 8-dimensional unit hypercube. This unit hypercube is the cellular automaton rule space. For next-nearest-neighbor cellular automata, a rule is specified by 25 = 32 bits, and the cellular automaton rule space is a 32-dimensional unit hypercube. A distance between two rules can be defined by the number of steps required to move from one vertex, which represents the first rule, and another vertex, representing another rule, along the edge of the hypercube. This rule-to-rule distance is also called the Hamming distance."
+517,"Cellular automaton rule space allows us to ask the question concerning whether rules with similar dynamical behavior are ""close"" to each other. Graphically drawing a high dimensional hypercube on the 2-dimensional plane remains a difficult task, and one crude locator of a rule in the hypercube is the number of bit-1 in the 8-bit string for elementary rules . Drawing the rules in different Wolfram classes in these slices of the rule space show that class 1 rules tend to have lower number of bit-1s, thus located in one region of the space, whereas class 3 rules tend to have higher proportion  of bit-1s."
+518,"For larger cellular automaton rule space, it is shown that class 4 rules are located between the class 1 and class 3 rules. This observation is the foundation for the phrase edge of chaos, and is reminiscent of the phase transition in thermodynamics."
+519,Several biological processes occur—or can be simulated—by cellular automata.
+520,Some examples of biological phenomena modeled by cellular automata with a simple state space are:
+521,"Additionally, biological phenomena which require explicit modeling of the agents' velocities  may be modeled by cellular automata with a more complex state space and rules, such as biological lattice-gas cellular automata. These include phenomena of great medical importance, such as:"
+522,"The Belousov–Zhabotinsky reaction is a spatio-temporal chemical oscillator that can be simulated by means of a cellular automaton. In the 1950s A. M. Zhabotinsky  discovered that when a thin, homogenous layer of a mixture of malonic acid, acidified bromate, and a ceric salt were mixed together and left undisturbed, fascinating geometric patterns such as concentric circles and spirals propagate across the medium. In the ""Computer Recreations"" section of the August 1988 issue of Scientific American, A. K. Dewdney discussed a cellular automaton developed by Martin Gerhardt and Heike Schuster of the University of Bielefeld . This automaton produces wave patterns that resemble those in the Belousov-Zhabotinsky reaction."
+523,"Probabilistic cellular automata are used in statistical and condensed matter physics to study phenomena like fluid dynamics and phase transitions. The Ising model is a prototypical example, in which each cell can be in either of two states called ""up"" and ""down"", making an idealized representation of a magnet. By adjusting the parameters of the model, the proportion of cells being in the same state can be varied, in ways that help explicate how ferromagnets become demagnetized when heated. Moreover, results from studying the demagnetization phase transition can be transferred to other phase transitions, like the evaporation of a liquid into a gas; this convenient cross-applicability is known as universality. The phase transition in the two-dimensional Ising model and other systems in its universality class has been of particular interest, as it requires conformal field theory to understand in depth. Other cellular automata that have been of significance in physics include lattice gas automata, which simulate fluid flows."
+524,"Cellular automaton processors are physical implementations of CA concepts, which can process information computationally. Processing elements are arranged in a regular grid of identical cells. The grid is usually a square tiling, or tessellation, of two or three dimensions; other tilings are possible, but not yet used. Cell states are determined only by interactions with adjacent neighbor cells. No means exists to communicate directly with cells farther away. One such cellular automaton processor array configuration is the systolic array. Cell interaction can be via electric charge, magnetism, vibration , or any other physically useful means. This can be done in several ways so that no wires are needed between any elements. This is very unlike processors used in most computers today  which are divided into sections with elements that can communicate with distant elements over wires."
+525,Rule 30 was originally suggested as a possible block cipher for use in cryptography. Two-dimensional cellular automata can be used for constructing a pseudorandom number generator.
+526,"Cellular automata have been proposed for public-key cryptography. The one-way function is the evolution of a finite CA whose inverse is believed to be hard to find. Given the rule, anyone can easily calculate future states, but it appears to be very difficult to calculate previous states.  Cellular automata have also been applied to design error correction codes."
+527,Other problems that can be solved with cellular automata include:
+528,Cellular automata have been used in generative music and evolutionary music composition and procedural terrain generation in video games.
+529,"For a random starting pattern, these maze-generating cellular automata will evolve into complex mazes with well-defined walls outlining corridors. Mazecetric, which has the rule B3/S1234 has a tendency to generate longer and straighter corridors compared with Maze, with the rule B3/S12345. Since these cellular automaton rules are deterministic, each maze generated is uniquely determined by its random starting pattern. This is a significant drawback since the mazes tend to be relatively predictable."
+530,Specific cellular automata rules include:
+531,"In computability theory, the Church–Turing thesis  is a thesis about the nature of computable functions. It states that a function on the natural numbers can be calculated by an effective method if and only if it is computable by a Turing machine. The thesis is named after American mathematician Alonzo Church and the British mathematician Alan Turing. Before the precise definition of computable function, mathematicians often used the informal term effectively calculable to describe functions that are computable by paper-and-pencil methods. In the 1930s, several independent attempts were made to formalize the notion of computability:"
+532,"Church, Kleene, and Turing proved that these three formally defined classes of computable functions coincide: a function is λ-computable if and only if it is Turing computable, and if and only if it is general recursive. This has led mathematicians and computer scientists to believe that the concept of computability is accurately characterized by these three equivalent processes. Other formal attempts to characterize computability have subsequently strengthened this belief ."
+533,"On the other hand, the Church–Turing thesis states that the above three formally-defined classes of computable functions coincide with the informal notion of an effectively calculable function. Although the thesis has near-universal acceptance, it cannot be formally proven, as the concept of effective calculability is only informally defined."
+534,"Since its inception, variations on the original thesis have arisen, including statements about what can physically be realized by a computer in our universe  and what can be efficiently computed ). These variations are not due to Church or Turing, but arise from later work in complexity theory and digital physics. The thesis also has implications for the philosophy of mind ."
+535,"J. B. Rosser  addresses the notion of ""effective computability"" as follows: ""Clearly the existence of CC and RC  presupposes a precise definition of 'effective'. 'Effective method' is here used in the rather special sense of a method each step of which is precisely predetermined and which is certain to produce the answer in a finite number of steps"". Thus the adverb-adjective ""effective"" is used in a sense of ""1a: producing a decided, decisive, or desired effect"", and ""capable of producing a result""."
+536,"In the following, the words ""effectively calculable"" will mean ""produced by any intuitively 'effective' means whatsoever"" and ""effectively computable"" will mean ""produced by a Turing-machine or equivalent mechanical device"". Turing's ""definitions"" given in a footnote in his 1938 Ph.D. thesis Systems of Logic Based on Ordinals, supervised by Church, are virtually the same:"
+537,"One of the important problems for logicians in the 1930s was the Entscheidungsproblem of David Hilbert and Wilhelm Ackermann, which asked whether there was a mechanical procedure for separating mathematical truths from mathematical falsehoods. This quest required that the notion of ""algorithm"" or ""effective calculability"" be pinned down, at least well enough for the quest to begin. But from the very outset Alonzo Church's attempts began with a debate that continues to this day. Was the notion of ""effective calculability"" to be  an ""axiom or axioms"" in an axiomatic system,  merely a definition that ""identified"" two or more propositions,  an empirical hypothesis to be verified by observation of natural events, or  just a proposal for the sake of argument ?"
+538,"In the course of studying the problem, Church and his student Stephen Kleene introduced the notion of λ-definable functions, and they were able to prove that several large classes of functions frequently encountered in number theory were λ-definable. The debate began when Church proposed to Gödel that one should define the ""effectively computable"" functions as the λ-definable functions. Gödel, however, was not convinced and called the proposal ""thoroughly unsatisfactory"". Rather, in correspondence with Church , Gödel proposed axiomatizing the notion of ""effective calculability""; indeed, in a 1935 letter to Kleene, Church reported that:"
+539,"But Gödel offered no further guidance. Eventually, he would suggest his recursion, modified by Herbrand's suggestion, that Gödel had detailed in his 1934 lectures in Princeton NJ . But he did not think that the two ideas could be satisfactorily identified ""except heuristically""."
+540,"Next, it was necessary to identify and prove the equivalence of two notions of effective calculability. Equipped with the λ-calculus and ""general"" recursion, Kleene with help of Church and J. Barkley Rosser produced proofs  to show that the two calculi are equivalent. Church subsequently modified his methods to include use of Herbrand–Gödel recursion and then proved  that the Entscheidungsproblem is unsolvable: there is no algorithm that can determine whether a well formed formula has a beta normal form."
+541,"Many years later in a letter to Davis , Gödel said that ""he was, at the time of these  lectures, not at all convinced that his concept of recursion comprised all possible recursions"". By 1963–1964 Gödel would disavow Herbrand–Gödel recursion and the λ-calculus in favor of the Turing machine as the definition of ""algorithm"" or ""mechanical procedure"" or ""formal system""."
+542,"A hypothesis leading to a natural law?: In late 1936 Alan Turing's paper  was delivered orally, but had not yet appeared in print. On the other hand, Emil Post's 1936 paper had appeared and was certified independent of Turing's work. Post strongly disagreed with Church's ""identification"" of effective computability with the λ-calculus and recursion, stating:"
+543,"Rather, he regarded the notion of ""effective calculability"" as merely a ""working hypothesis"" that might lead by inductive reasoning to a ""natural law"" rather than by ""a definition or an axiom"". This idea was ""sharply"" criticized by Church."
+544,Thus Post in his 1936 paper was also discounting Kurt Gödel's suggestion to Church in 1934–1935 that the thesis might be expressed as an axiom or set of axioms.
+545,"Turing adds another definition, Rosser equates all three: Within just a short time, Turing's 1936–1937 paper ""On Computable Numbers, with an Application to the Entscheidungsproblem"" appeared. In it he stated another notion of ""effective computability"" with the introduction of his a-machines . And in a proof-sketch added as an ""Appendix"" to his 1936–1937 paper, Turing showed that the classes of functions defined by λ-calculus and Turing machines coincided. Church was quick to recognise how compelling Turing's analysis was. In his review of Turing's paper he made clear that Turing's notion made ""the identification with effectiveness in the ordinary  sense evident immediately""."
+546,"In a few years  Turing would propose, like Church and Kleene before him, that his formal definition of mechanical computing agent was the correct one. Thus, by 1939, both Church  and Turing  had individually proposed that their ""formal systems"" should be definitions of ""effective calculability""; neither framed their statements as theses."
+547,Rosser  formally identified the three notions-as-definitions:
+548,"Kleene proposes Thesis I: This left the overt expression of a ""thesis"" to Kleene. In 1943 Kleene proposed his ""Thesis I"":"
+549,"Since a precise mathematical definition of the term effectively calculable  has been wanting, we can take this thesis ... as a definition of it ..."
+550,"The Church–Turing Thesis: Stephen Kleene, in Introduction To Metamathematics, finally goes on to formally name ""Church's Thesis"" and ""Turing's Thesis"", using his theory of recursive realizability. Kleene having switched from presenting his work in the terminology of Church-Kleene lambda definability, to that of Gödel-Kleene recursiveness . In this transition, Kleene modified Gödel's general recursive functions to allow for proofs of the unsolvability of problems in the Intuitionism of E. J. Brouwer. In his graduate textbook on logic, ""Church's thesis"" is introduced and basic mathematical results are demonstrated to be unrealizable. Next, Kleene proceeds to present ""Turing's thesis"", where results are shown to be uncomputable, using his simplified derivation of a Turing machine based on the work of Emil Post. Both theses are proven equivalent by use of ""Theorem XXX""."
+551,"Kleene, finally, uses for the first time the term the ""Church-Turing thesis"" in a section in which he helps to give clarifications to concepts in Alan Turing's paper ""The Word Problem in Semi-Groups with Cancellation"", as demanded in a critique from William Boone."
+552,"An attempt to understand the notion of ""effective computability"" better led Robin Gandy  in 1980 to analyze machine computation . Gandy's curiosity about, and analysis of, cellular automata , parallelism, and crystalline automata, led him to propose four ""principles  ... which it is argued, any machine must satisfy"". His most-important fourth, ""the principle of causality"" is based on the ""finite velocity of propagation of effects and signals; contemporary physics rejects the possibility of instantaneous action at a distance"". From these principles and some additional constraints— a lower bound on the linear dimensions of any of the parts,  an upper bound on speed of propagation ,  discrete progress of the machine, and  deterministic behavior—he produces a theorem that ""What can be calculated by a device satisfying principles I–IV is computable."""
+553,"In the late 1990s Wilfried Sieg analyzed Turing's and Gandy's notions of ""effective calculability"" with the intent of ""sharpening the informal notion, formulating its general features axiomatically, and investigating the axiomatic framework"". In his 1997 and 2002 work Sieg presents a series of constraints on the behavior of a computor—""a human computing agent who proceeds mechanically"". These constraints reduce to:"
+554,The matter remains in active discussion within the academic community.
+555,"The thesis can be viewed as nothing but an ordinary mathematical definition. Comments by Gödel on the subject suggest this view, e.g. ""the correct definition of mechanical computability was established beyond any doubt by Turing"". The case for viewing the thesis as nothing more than a definition is made explicitly by Robert I. Soare, where it is also argued that Turing's definition of computability is no less likely to be correct than the epsilon-delta definition of a continuous function."
+556,"Other formalisms  have been proposed for describing effective calculability/computability. Kleene  adds to the list the functions ""reckonable in the system S1"" of Kurt Gödel 1936, and Emil Post's  ""canonical  systems"". In the 1950s Hao Wang and Martin Davis greatly simplified the one-tape Turing-machine model . Marvin Minsky expanded the model to two or more tapes and greatly simplified the tapes into ""up-down counters"", which Melzak and Lambek further evolved into what is now known as the counter machine model. In the late 1960s and early 1970s researchers expanded the counter machine model into the register machine, a close cousin to the modern notion of the computer. Other models include combinatory logic and Markov algorithms. Gurevich adds the pointer machine model of Kolmogorov and Uspensky : ""... they just wanted to ... convince themselves that there is no way to extend the notion of computable function."""
+557,"All these contributions involve proofs that the models are computationally equivalent to the Turing machine; such models are said to be Turing complete. Because all these different attempts at formalizing the concept of ""effective calculability/computability"" have yielded equivalent results, it is now generally assumed that the Church–Turing thesis is correct. In fact, Gödel  proposed something stronger than this; he observed that there was something ""absolute"" about the concept of ""reckonable in S1"":"
+558,"Proofs in computability theory often invoke the Church–Turing thesis in an informal way to establish the computability of functions while avoiding the  details which would be involved in a rigorous, formal proof. To establish that a function is computable by Turing machine, it is usually considered sufficient to give an informal English description of how the function can be effectively computed, and then conclude ""by the Church–Turing thesis"" that the function is Turing computable ."
+559,Dirk van Dalen gives the following example for the sake of illustrating this informal use of the Church–Turing thesis:
+560,"Proof: Let A be infinite RE. We list the elements of A effectively, n0, n1, n2, n3, ..."
+561,"From this list we extract an increasing sublist: put m0 = n0, after finitely many steps we find an nk such that nk > m0, put m1 = nk. We repeat this procedure to find m2 > m1, etc. this yields an effective listing of the subset B={m0, m1, m2,...} of A, with the property mi < mi+1."
+562,"Claim. B is decidable. For, in order to test k in B we must check if k = mi for some i. Since the sequence of mi's is increasing we have to produce at most k+1 elements of the list and compare them with k. If none of them is equal to k, then k not in B. Since this test is effective, B is decidable and, by Church's thesis, recursive."
+563,"In order to make the above example completely rigorous, one would have to carefully construct a Turing machine, or λ-function, or carefully invoke recursion axioms, or at best, cleverly invoke various theorems of computability theory. But because the computability theorist believes that Turing computability correctly captures what can be computed effectively, and because an effective procedure is spelled out in English for deciding the set B, the computability theorist accepts this as proof that the set is indeed recursive."
+564,"The success of the Church–Turing thesis prompted variations of the thesis to be proposed. For example, the physical Church–Turing thesis states: ""All physically computable functions are Turing-computable."": 101"
+565,The Church–Turing thesis says nothing about the efficiency with which one model of computation can simulate another. It has been proved for instance that a  universal Turing machine only suffers a logarithmic slowdown factor in simulating any Turing machine.
+566,"A variation of the Church–Turing thesis addresses whether an arbitrary but ""reasonable"" model of computation can be efficiently simulated. This is called the feasibility thesis, also known as the  complexity-theoretic Church–Turing thesis or the extended Church���Turing thesis, which is not due to Church or Turing, but rather was realized gradually in the development of complexity theory. It states: ""A probabilistic Turing machine can efficiently simulate any realistic model of computation."" The word 'efficiently' here means up to polynomial-time reductions. This thesis was originally called computational complexity-theoretic Church–Turing thesis by Ethan Bernstein and Umesh Vazirani . The complexity-theoretic Church–Turing thesis, then, posits that all 'reasonable' models of computation yield the same class of problems that can be computed in polynomial time. Assuming the conjecture that probabilistic polynomial time  equals deterministic polynomial time , the word 'probabilistic' is optional in the complexity-theoretic Church–Turing thesis. A similar thesis, called the invariance thesis, was introduced by Cees F. Slot and Peter van Emde Boas. It states: ""'Reasonable' machines can simulate each other within a polynomially bounded overhead in time and a constant-factor overhead in space."" The thesis originally appeared in a paper at STOC'84, which was the first paper to show that polynomial-time overhead and constant-space overhead could be simultaneously achieved for a simulation of a Random Access Machine on a Turing machine."
+567,"If BQP is shown to be a strict superset of BPP, it would invalidate the complexity-theoretic Church–Turing thesis. In other words, there would be efficient quantum algorithms that perform tasks that do not have efficient probabilistic algorithms. This would not however invalidate the original Church–Turing thesis, since a quantum computer can always be simulated by a Turing machine, but it would invalidate the classical complexity-theoretic Church–Turing thesis for efficiency reasons. Consequently, the quantum complexity-theoretic Church–Turing thesis states: ""A quantum Turing machine can efficiently simulate any realistic model of computation."""
+568,"Eugene Eberbach and Peter Wegner claim that the Church–Turing thesis is sometimes interpreted too broadly,
+stating ""Though  Turing machines express the behavior of algorithms, the broader assertion that algorithms precisely capture what can be computed is invalid"". They claim that forms of computation not captured by the thesis are relevant today,
+terms which they call super-Turing computation."
+569,"Philosophers have interpreted the Church–Turing thesis as having implications for the philosophy of mind. B. Jack Copeland states that it is an open empirical question whether there are actual deterministic physical processes that, in the long run, elude simulation by a Turing machine; furthermore, he states that it is an open empirical question whether any such processes are involved in the working of the human brain. There are also some important open questions which cover the relationship between the Church–Turing thesis and physics, and the possibility of hypercomputation. When applied to physics, the thesis has several possible meanings:"
+570,"The universe is equivalent to a Turing machine; thus, computing non-recursive functions is physically impossible. This has been termed the strong Church–Turing thesis, or Church–Turing–Deutsch principle, and is a foundation of digital physics."
+571,"The universe is not equivalent to a Turing machine , but incomputable physical events are not ""harnessable"" for the construction of a hypercomputer. For example, a universe in which physics involves random real numbers, as opposed to computable reals, would fall into this category."
+572,"The universe is a hypercomputer, and it is possible to build physical devices to harness this property and calculate non-recursive functions. For example, it is an open question whether all quantum mechanical events are Turing-computable, although it is known that rigorous models such as quantum Turing machines are equivalent to deterministic Turing machines.  John Lucas and Roger Penrose have suggested that the human mind might be the result of some kind of quantum-mechanically enhanced, ""non-algorithmic"" computation."
+573,"There are many other technical possibilities which fall outside or between these three categories, but these serve to illustrate the range of the concept."
+574,"Philosophical aspects of the thesis, regarding both physical and biological computers, are also discussed in Odifreddi's 1989 textbook on recursion theory.: 101–123"
+575,"One can formally define functions that are not computable. A well-known example of such a function is the Busy Beaver function. This function takes an input n and returns the largest number of symbols that a Turing machine with n states can print before halting, when run with no input. Finding an upper bound on the busy beaver function is equivalent to solving the halting problem, a problem known to be unsolvable by Turing machines. Since the busy beaver function cannot be computed by Turing machines, the Church–Turing thesis states that this function cannot be effectively computed by any method."
+576,"Several computational models allow for the computation of  non-computable functions. These are known as
+hypercomputers.
+Mark Burgin argues that super-recursive algorithms such as inductive Turing machines disprove the Church–Turing thesis. His argument relies on a definition of algorithm broader than the ordinary one, so that non-computable functions obtained from some inductive Turing machines are called computable. This interpretation of the Church–Turing thesis differs from the interpretation commonly accepted in computability theory, discussed above. The argument that super-recursive algorithms are indeed algorithms in the sense of the Church–Turing thesis has not found broad acceptance within the computability research community."
+577,"An analog signal is any continuous-time signal representing some other quantity, i.e., analogous to another quantity. For example, in an analog audio signal, the instantaneous signal voltage varies continuously with the pressure of the sound waves."
+578,"In contrast, a digital signal represents the original time-varying quantity as a sampled sequence of quantized values. Digital sampling imposes some bandwidth and dynamic range constraints on the representation and adds quantization error."
+579,"The term analog signal usually refers to electrical signals; however, mechanical, pneumatic, hydraulic, and other systems may also convey or be considered analog signals."
+580,"An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. In an electrical signal, the voltage, current, or frequency of the signal may be varied to represent the information."
+581,"Any information may be conveyed by an analog signal; such a signal may be a measured response to changes in a physical variable, such as sound, light, temperature, position, or pressure. The physical variable is converted to an analog signal by a transducer. For example, sound striking the diaphragm of a microphone induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone or the voltage produced by a condenser microphone. The voltage or the current is said to be an analog of the sound."
+582,"An analog signal is subject to electronic noise and distortion introduced by communication channels, recording and signal processing operations, which can progressively degrade the signal-to-noise ratio . As the signal is transmitted, copied, or processed, the unavoidable noise introduced in the signal path will accumulate as a generation loss, progressively and irreversibly degrading the SNR, until in extreme cases, the signal can be overwhelmed. Noise can show up as hiss and intermodulation distortion in audio signals, or snow in video signals. Generation loss is irreversible as there is no reliable method to distinguish the noise from the signal."
+583,"Converting an analog signal to digital form introduces a low-level quantization noise into the signal due to finite resolution of digital systems. Once in digital form, the signal can be transmitted, stored, and processed without introducing additional noise or distortion using error detection and correction."
+584,"Noise accumulation in analog systems can be minimized by electromagnetic shielding, balanced lines, low-noise amplifiers and high-quality electrical components."
+585,"Abstract machines are typically categorized into two types based on the quantity of operations they can execute simultaneously at any given moment: deterministic abstract machines and non-deterministic abstract machines. A deterministic abstract machine is a system in which a particular beginning state or condition always yields the same outputs. There is no randomness or variation in how inputs are transformed into outputs. In contrast, a non-deterministic abstract machine can provide various outputs for the same input on different executions. Unlike a deterministic algorithm, which gives the same result for the same input regardless of the number of iterations, a non-deterministic algorithm takes various paths to arrive to different outputs. Non-deterministic algorithms are helpful for obtaining approximate answers when deriving a precise solution using a deterministic approach is difficult or costly."
+586,"Turing machines, for example, are some of the most fundamental abstract machines in computer science. These machines conduct operations on a tape  of any length. Their instructions provide for both modifying the symbols and changing the symbol that the machine’s pointer is currently at. For example, a rudimentary Turing machine could have a single command, ""convert symbol to 1 then move right"", and this machine would only produce a string of 1s. This basic Turing machine is deterministic; however, nondeterministic Turing machines that can execute several actions given the same input may also be built."
+587,"Any implementation of an abstract machine in the case of physical implementation  uses some kind of physical device  to execute the instructions of a programming language. An abstract machine, however, can also be implemented in software or firmware at levels between the abstract machine and underlying physical device."
+588,"An abstract machine is, intuitively, just an abstraction of the idea of a physical computer. For actual execution, algorithms must be properly formalised using the constructs offered by a programming language. This implies that the algorithms to be executed must be expressed using programming language instructions. The syntax of a programming language enables the construction of programs using a finite set of constructs known as instructions. Most abstract machines share a program store and a state, which often includes a stack and registers. In digital computers, the stack is simply a memory unit with an address register that can count only positive integers . The address register for the stack is known as a stack pointer because its value always refers to the top item on the stack. The program consists of a series of instructions, with a stack pointer indicating the next instruction to be performed. When the instruction is completed, a stack pointer is advanced. This fundamental control mechanism of an abstract machine is also known as its execution loop. Thus, an abstract machine for a programming language is any collection of data structures and algorithms capable of storing and running programs written in the programming language. It bridges the gap between the high level of a programming language and the low level of an actual machine by providing an intermediate language step for compilation. An abstract machine's instructions are adapted to the unique operations necessary to implement operations of a certain source language or set of source languages."
+589,"In the late 1950s, the Association for Computing Machinery  and other allied organisations developed many proposals for Universal Computer Oriented Language , such as Conway's machine. The UNCOL concept is good, but it has not been widely used due to the poor performance of the generated code. In many areas of computing, its performance will continue to be an issue despite the development of the Java Virtual Machine in the late 1990s. Algol Object Code , P4-machine , UCSD P-machine , and Forth  are some successful abstract machines of this kind."
+590,"Abstract machines for object-oriented programming languages are often stack-based and have special access instructions for object fields and methods. In these machines, memory management is often implicit performed by a garbage collector . Smalltalk-80 , Self , and Java  are examples of this implementation."
+591,"A string processing language is a computer language that focuses on processing strings rather than numbers. There have been string processing languages in the form of command shells, programming tools, macro processors, and scripting languages for decades. Using a suitable abstract machine has two benefits: increased execution speed and enhanced portability. Snobol4 and ML/I are two notable instances of early string processing languages that use an abstract machine to gain machine independence."
+592,"The early abstract machines for functional languages, including the SECD machine  and Cardelli's Functional Abstract Machine , defined strict evaluation, also known as eager or call-by-value evaluation, in which function arguments are evaluated before the call and precisely once. Recently, the majority of research has been on lazy  evaluation, such as the G-machine , Krivine machine , and Three Instruction Machine , in which function arguments are evaluated only if necessary and at most once. One reason is because effective implementation of strict evaluation is now well-understood, therefore the necessity for an abstract machine has diminished."
+593,"Predicate calculus  is the foundation of logic programming languages. The most well-known logic programming language is Prolog. The rules in Prolog are written in a uniform format known as universally quantified 'Horn clauses', which means to begin the calculation that attempts to discover a proof of the objective. The Warren Abstract Machine WAM , which has become the de facto standard in Prolog program compilation, has been the focus of most study. It provides special purpose instructions such as data unification instructions and control flow instructions to support backtracking ."
+594,"A generic abstract machine is made up of a memory and an interpreter. The memory is used to store data and programs, while the interpreter is the component that executes the instructions included in programs."
+595,"The interpreter must carry out the operations that are unique to the language it is interpreting. However, given the variety of languages, it is conceivable to identify categories of operations and an ""execution mechanism"" shared by all interpreters. The interpreter's operations and accompanying data structures are divided into the following categories:"
+596,Operations for processing primitive data:
+597,Operations and data structures for controlling the sequence of execution of operations;
+598,Operations and data structures for controlling data transfers;
+599,Operations and data structures for memory management.
+600,"An abstract machine must contain operations for manipulating primitive data types such as strings and integers. For example, integers are nearly universally considered a basic data type for both physical abstract machines and the abstract machines used by many programming languages. The machine carries out the arithmetic operations necessary, such as addition and multiplication, within a single time step."
+601,"Operations and structures for ""sequence control"" allow controlling the execution flow of program instructions. When certain conditions are met, it is necessary to change the typical sequential execution of a program. Therefore, the interpreter employs data structures  that are modified by operations distinct from those used for data manipulation ."
+602,Data transfer operations are used to control how operands and data are transported from memory to the interpreter and vice versa. These operations deal with the store and the retrieval order of operands from the store.
+603,"Memory management is concerned with the operations performed in memory to allocate data and applications. In the abstract machine, data and programmes can be held indefinitely, or in the case of programming languages, memory can be allocated or deallocated using a more complex mechanism."
+604,"Abstract machine hierarchies are often employed, in which each machine uses the functionality of the level immediately below and adds additional functionality of its own to meet the level immediately above. A hardware computer, constructed with physical electronic devices, can be added at the most basic level. Above this level, the abstract microprogrammed machine level may be introduced. The abstract machine supplied by the operating system, which is implemented by a program written in machine language, is located immediately above . On the one hand, the operating system extends the capability of the physical machine by providing higher-level primitives that are not available on the physical machine . The host machine is formed by the abstract machine given by the operating system, on which a high-level programming language is implemented using an intermediary machine, such as the Java Virtual machine and its byte code language. The level given by the abstract machine for the high-level language  is not usually the final level of hierarchy. At this point, one or more applications that deliver additional services together may be introduced. A ""web machine"" level, for example, can be added to implement the functionalities necessary to handle Web communications . The ""Web Service"" level is located above this, and it provides the functionalities necessary to make web services communicate, both in terms of interaction protocols and the behaviour of the processes involved. At this level, entirely new languages that specify the behaviour of so-called ""business processes"" based on Web services may be developed . Finally, a specialised application can be found at the highest level  which has very specific and limited functionality."
+605,"Cell, a cellular architecture containing 9 cores, is the processor used in the PlayStation 3. Another prominent cellular architecture is Cyclops64, a massively parallel architecture currently under development by IBM."
+606,"Cellular architectures follow the low-level programming paradigm, which exposes the programmer to much of the underlying hardware. This allows the programmer to greatly optimize their code for the platform, but at the same time makes it more difficult to develop software."
+607,"In mathematical logic and theoretical computer science, a register machine is a generic class of abstract machines used in a manner similar to a Turing machine. All models of register machines are Turing equivalent."
+608,"The register machine gets its name from its use of one or more ""registers"". In contrast to the tape and head used by a Turing machine, the model uses multiple, uniquely addressed registers, each of which holds a single positive integer."
+609,"There are at least four sub-classes found in literature, here listed from most primitive to the most like a computer:"
+610,Any properly defined register machine model is Turing equivalent. Computational speed is very dependent on the model specifics.
+611,"In practical computer science, a related concept known as a virtual machine is occasionally employed to reduce reliance on underlying machine architectures. These virtual machines are also utilized in educational settings. In textbooks, the term ""register machine"" is sometimes used interchangeably to describe a virtual machine."
+612,A register machine consists of:
+613,"An unbounded number of labelled, discrete, unbounded registers unbounded in extent : a finite  set of registers 
+  
+    
+      
+        
+          r
+          
+            0
+          
+        
+        …
+        
+          r
+          
+            n
+          
+        
+      
+    
+    {\displaystyle r_{0}\ldots r_{n}}
+  
+ each considered to be of infinite extent and each of which holds a single non-negative integer . The registers may do their own arithmetic, or there may be one or more special registers that do the arithmetic e.g. an ""accumulator"" and/or ""address register"". See also Random-access machine."
+614,"Tally counters or marks: discrete, indistinguishable objects or marks of only one sort suitable for the model. In the most-reduced counter machine model, per each arithmetic operation only one object/mark is either added to or removed from its location/tape. In some counter machine models  and most RAM and RASP models more than one object/mark can be added or removed in one operation with ""addition"" and usually ""subtraction""; sometimes with ""multiplication"" and/or ""division"". Some models have control operations such as ""copy""  that move ""clumps"" of objects/marks from register to register in one action."
+615,"A limited set of instructions: the instructions tend to divide into two classes: arithmetic and control. The instructions are drawn from the two classes to form ""instruction-sets"", such that an instruction set must allow the model to be Turing equivalent .
+Arithmetic: Arithmetic instructions may operate on all registers or on a specific register, such as an accumulator. Typically, they are selected from the following sets, though exceptions exist: Counter machine: { Increment , Decrement , Clear-to-zero  }  Reduced RAM, RASP: { Increment , Decrement , Clear-to-zero , Load-immediate-constant k, Add , Proper-Subtract , Increment accumulator, Decrement accumulator, Clear accumulator, Add the contents of register r to the accumulator, Proper-Subtract the contents of register r from the accumulator }  Augmented RAM, RASP: Includes all of the reduced instructions as well as: { Multiply, Divide, various Boolean bit-wise operations  }
+Control: Counter machine models: Optionally include { Copy  }.  RAM and RASP models: Most include { Copy  }, or { Load Accumulator from r, Store accumulator into r, Load Accumulator with an immediate constant }.  All models: Include at least one conditional ""jump""  following the test of a register, such as { Jump-if-zero, Jump-if-not-zero , Jump-if-equal, Jump-if-not-equal }.  All models optionally include: { unconditional program jump  }.
+
+Register-addressing method:
+Counter machine: no indirect addressing, immediate operands possible in highly atomized models
+RAM and RASP: indirect addressing available, immediate operands typical
+Input-output: optional in all models"
+616,"Arithmetic: Arithmetic instructions may operate on all registers or on a specific register, such as an accumulator. Typically, they are selected from the following sets, though exceptions exist: Counter machine: { Increment , Decrement , Clear-to-zero  }  Reduced RAM, RASP: { Increment , Decrement , Clear-to-zero , Load-immediate-constant k, Add , Proper-Subtract , Increment accumulator, Decrement accumulator, Clear accumulator, Add the contents of register r to the accumulator, Proper-Subtract the contents of register r from the accumulator }  Augmented RAM, RASP: Includes all of the reduced instructions as well as: { Multiply, Divide, various Boolean bit-wise operations  }"
+617,"Control: Counter machine models: Optionally include { Copy  }.  RAM and RASP models: Most include { Copy  }, or { Load Accumulator from r, Store accumulator into r, Load Accumulator with an immediate constant }.  All models: Include at least one conditional ""jump""  following the test of a register, such as { Jump-if-zero, Jump-if-not-zero , Jump-if-equal, Jump-if-not-equal }.  All models optionally include: { unconditional program jump  }."
+618,"Counter machine: no indirect addressing, immediate operands possible in highly atomized models"
+619,"RAM and RASP: indirect addressing available, immediate operands typical"
+620,Input-output: optional in all models
+621,"State register: A special Instruction Register , distinct from the registers mentioned earlier, stores the current instruction to be executed along with its address in the instruction table. This register, along with its associated table, is located within the finite state machine. The IR is inaccessible in all models. In the case of RAM and RASP, for determining the ""address"" of a register, the model can choose either  the address specified by the table and temporarily stored in the IR for direct addressing or  the contents of the register specified by the instruction in the IR for indirect addressing.  It's important to note that the IR is not the ""program counter""  of the RASP . The PC is merely another register akin to an accumulator, but specifically reserved for holding the number of the RASP's current register-based instruction. Thus, a RASP possesses two ""instruction/program"" registers:  the IR , and  a PC  for the program stored in the registers. Additionally, aside from the PC, a RASP may also dedicate another register to the ""Program-Instruction Register"" ."
+622,"Two trends appeared in the early 1950s—the first to characterize the computer as a Turing machine, the second to define computer-like models—models with sequential instruction sequences and conditional jumps—with the power of a Turing machine, i.e. a so-called Turing equivalence. Need for this work was carried out in context of two ""hard"" problems: the unsolvable word problem posed by Emil Post—his problem of ""tag""—and the very ""hard"" problem of Hilbert's problems—the 10th question around Diophantine equations. Researchers were questing for Turing-equivalent models that were less ""logical"" in nature and more ""arithmetic"": 281 : 218"
+623,"The first trend toward characterizing computers has originated with Hans Hermes , Rózsa Péter , and Heinz Kaphengst  , the second trend with Hao Wang  and, as noted above, furthered along by Zdzislaw Alexander Melzak  , Joachim Lambek  and Marvin Minsky ."
+624,The last five names are listed explicitly in that order by Yuri Matiyasevich. He follows up with:
+625,"Lambek, Melzak, Minsky and Shepherdson and Sturgis independently discovered the same idea at the same time. See note on precedence below."
+626,The history begins with Wang's model.
+627,Wang's work followed from Emil Post's  paper and led Wang to his definition of his Wang B-machine—a two-symbol Post–Turing machine computation model with only four atomic instructions:
+628,"To these four both Wang  and then C. Y. Lee  added another instruction from the Post set { ERASE }, and then a Post's unconditional jump { JUMP_to_ instruction_z } (or to make things easier, the conditional jump JUMP_IF_blank_to_instruction_z, or both. Lee named this a ""W-machine"" model:"
+629,"Wang expressed hope that his model would be ""a rapprochement"": 63  between the theory of Turing machines and the practical world of the computer."
+630,"Wang's work was highly influential. We find him referenced by Minsky  and , Melzak , Shepherdson and Sturgis . Indeed, Shepherdson and Sturgis  remark that:"
+631,Martin Davis eventually evolved this model into the  Post–Turing machine.
+632,Difficulties with the Wang/Post–Turing model:
+633,"Except there was a problem: the Wang model  was still a single-tape Turing-like device, however nice its sequential program instruction-flow might be. Both Melzak  and Shepherdson and Sturgis  observed this :"
+634,"Indeed, as examples at Turing machine examples, Post–Turing machine and partial function show, the work can be ""complicated""."
+635,"So why not 'cut the tape' so each is infinitely long  but left-ended, and call these three tapes ""Post–Turing  tapes""? The individual heads will move left  and right . In one sense the heads indicate ""the tops of the stack"" of concatenated marks. Or in Minsky  and Hopcroft and Ullman : 171ff  the tape is always blank except for a mark at the left end—at no time does a head ever print or erase."
+636,"Care must be taken to write the instructions so that a test-for-zero and jump occurs before decrementing, otherwise the machine will ""fall off the end"" or ""bump against the end""—creating an instance of a partial function."
+637,"Minsky  and Shepherdson–Sturgis  prove that only a few tapes—as few as one—still allow the machine to be Turing equivalent if the data on the tape is represented as a Gödel number ; this number will evolve as the computation proceeds. In the one tape version with Gödel number encoding the counter machine must be able to  multiply the Gödel number by a constant , and  divide by a constant  and jump if the remainder is zero. Minsky  shows that the need for this bizarre instruction set can be relaxed to { INC , JZDEC  } and the convenience instructions { CLR , J  } if two tapes are available. A simple Gödelization is still required, however. A similar result appears in Elgot–Robinson  with respect to their RASP model."
+638,"Melzak's  model is significantly different. He took his own model, flipped the tapes vertically, called them ""holes in the ground"" to be filled with ""pebble counters"". Unlike Minsky's ""increment"" and ""decrement"", Melzak allowed for proper subtraction of any count of pebbles and ""adds"" of any count of pebbles."
+639,"He defines indirect addressing for his model: 288  and provides two examples of its use;: 89  his ""proof"": 290–292  that his model is Turing equivalent is so sketchy that the reader cannot tell whether or not he intended the indirect addressing to be a requirement for the proof."
+640,Legacy of Melzak's model is Lambek's simplification and the reappearance of his mnemonic conventions in Cook and Reckhow 1973.
+641,"Lambek  took Melzak's ternary model and atomized it down to the two unary instructions—X+, X− if possible else jump—exactly the same two that Minsky  had come up with."
+642,"However, like the Minsky  model, the Lambek model does execute its instructions in a default-sequential manner—both X+ and X− carry the identifier of the next instruction, and X− also carries the jump-to instruction if the zero-test is successful."
+643,"A RASP or random-access stored-program machine begins as a counter machine with its ""program of instruction"" placed in its ""registers"". Analogous to, but independent of, the finite state machine's ""Instruction Register"", at least one of the registers ) and one or more ""temporary"" registers maintain a record of, and operate on, the current instruction's number. The finite state machine's TABLE of instructions is responsible for  fetching the current program instruction from the proper register,  parsing the program instruction,  fetching operands specified by the program  instruction, and  executing the program instruction."
+644,"Except there is a problem: If based on the counter machine chassis this computer-like, von Neumann machine will not be Turing equivalent. It cannot compute everything that is computable. Intrinsically the model is bounded by the size of its  finite state machine's instructions. The counter machine based RASP can compute any primitive recursive function  but not all mu recursive functions ."
+645,"Elgot–Robinson investigate the possibility of allowing their RASP model to ""self modify"" its program instructions. The idea was an old one, proposed by Burks–Goldstine–von Neumann , and sometimes called ""the computed goto."" Melzak  specifically mentions the ""computed goto"" by name but instead provides his model with indirect addressing."
+646,"Computed goto: A RASP program of instructions that modifies the ""goto address"" in a conditional- or unconditional-jump program instruction."
+647,"But this does not solve the problem . What is necessary is a method to fetch the address of a program instruction that lies  ""beyond/above"" the upper bound of the finite state machine instruction register and TABLE."
+648,"Minsky  hints at the issue in his investigation of a counter machine  equipped with the instructions { CLR , INC , and RPT  }. He doesn't tell us how to fix the problem, but he does observe that:"
+649,"But Elgot and Robinson solve the problem: They augment their P0 RASP with an indexed set of instructions—a somewhat more complicated  form of indirect addressing. Their P'0 model addresses the registers by adding the contents of the ""base"" register  to the ""index"" specified explicitly in the instruction . Thus the indexing P'0 instructions have one more parameter than the non-indexing P0 instructions:"
+650,By 1971 Hartmanis has simplified the indexing to indirection for use in his RASP model.
+651,"Indirect addressing: A pointer-register supplies the finite state machine with the address of the target register required for the instruction. Said another way: The contents of the pointer-register is the address of the ""target"" register to be used by the instruction. If the pointer-register is unbounded, the RAM, and a suitable RASP built on its chassis, will be Turing equivalent. The target register can serve either as a source or destination register, as specified by the instruction."
+652,"Note that the finite state machine does not have to explicitly specify this target register's address. It just says to the rest of the machine: Get me the contents of the register pointed to by my pointer-register and then do xyz with it. It must specify explicitly by name, via its instruction, this pointer-register  but it doesn't have to know what number the pointer-register actually contains ."
+653,Cook and Reckhow  cite Hartmanis  and simplify his model to what they call a random-access machine . In a sense we are back to Melzak  but with a much simpler model than Melzak's.
+654,"Minsky was working at the MIT Lincoln Laboratory and published his work there; his paper was received for publishing in the Annals of Mathematics on 15 August 1960, but not published until November 1961. While receipt occurred a full year before the work of Melzak and Lambek was received and published . That  both were Canadians and published in the Canadian Mathematical Bulletin,  neither would have had reference to Minsky's work because it was not yet published in a peer-reviewed journal, but  Melzak references Wang, and Lambek references Melzak, leads one to hypothesize that their work occurred simultaneously and independently."
+655,"Almost exactly the same thing happened to Shepherdson and Sturgis. Their paper was received in December 1961—just a few months after Melzak and Lambek's work was received. Again, they had little  or no benefit of reviewing the work of Minsky. They were careful to observe in footnotes that papers by Ershov, Kaphengst and Péter had ""recently appeared"": 219  These were published much earlier but appeared in the German language in German journals so issues of accessibility present themselves."
+656,"The final paper of Shepherdson and Sturgis did not appear in a peer-reviewed journal until 1963. And as they note in their Appendix A, the 'systems' of Kaphengst , Ershov , Péter  are all so similar to what results were obtained later as to be indistinguishable to a set of the following:"
+657,"Indeed, Shepherson and Sturgis conclude"
+658,"By order of publishing date the work of Kaphengst , Ershov , Péter  were first."
+659,"Background texts: The following bibliography of source papers includes a number of texts to be used as background. The mathematics that led to the flurry of papers about abstract machines in the 1950s and 1960s can be found in van Heijenoort —an assemblage of original papers spanning the 50 years from Frege  to Gödel . Davis  The Undecidable  carries the torch onward beginning with Gödel  through Gödel's  postscriptum;: 71  the original papers of Alan Turing  and Emil Post  are included in The Undecidable. The mathematics of Church, Rosser and Kleene that appear as reprints of original papers in The Undecidable is carried further in Kleene , a mandatory text for anyone pursuing a deeper understanding of the mathematics behind the machines. Both Kleene  and Davis  are referenced by a number of the papers."
+660,"For a good treatment of the counter machine see Minsky  Chapter 11 ""Models similar to Digital Computers""—he calls the counter machine a ""program computer"". A recent overview is found at van Emde Boas . A recent treatment of the Minsky /Lambek  model can be found Boolos–Burgess–Jeffrey ; they reincarnate Lambek's ""abacus model"" to demonstrate equivalence of Turing machines and partial recursive functions, and they provide a graduate-level introduction to both abstract machine models  and the mathematics of recursion theory. Beginning with the first edition Boolos–Burgess  this model appeared with virtually the same treatment."
+661,"The papers: The papers begin with Wang  and his dramatic simplification of the Turing machine. Turing , Kleene , Davis  and in particular Post  are cited in Wang ; in turn, Wang is referenced by Melzak , Minsky  and Shepherdson–Sturgis  as they independently reduce the Turing tapes to ""counters"". Melzak  provides his pebble-in-holes counter machine model with indirection but doesn't carry the treatment further. The work of Elgot–Robinson  define the RASP—the computer-like random-access stored-program machines—and appear to be the first to investigate the failure of the bounded counter machine to calculate the mu-recursive functions. This failure—except with the draconian use of Gödel numbers in the manner of Minsky —leads to their definition of ""indexed"" instructions  for their RASP model. Elgot–Robinson  and more so Hartmanis  investigate RASPs with self-modifying programs. Hartmanis  specifies an instruction set with indirection, citing lecture notes of Cook . For use in investigations of computational complexity Cook and his graduate student Reckhow  provide the definition of a RAM . The pointer machines are an offshoot of Knuth  and independently Schönhage ."
+662,"For the most part the papers contain mathematics beyond the undergraduate level—in particular the primitive recursive functions and mu recursive functions presented elegantly in Kleene  and less in depth, but useful nonetheless, in Boolos–Burgess–Jeffrey ."
+663,All texts and papers excepting the four starred have been witnessed. These four are written in German and appear as references in Shepherdson–Sturgis  and Elgot–Robinson ; Shepherdson–Sturgis  offer a brief discussion of their results in Shepherdson–Sturgis' Appendix A. The terminology of at least one paper  seems to hark back to the Burke–Goldstine–von Neumann  analysis of computer architecture.
+664,"NUMA architectures logically follow in scaling from symmetric multiprocessing  architectures. They were developed commercially during the 1990s by Unisys, Convex Computer , Honeywell Information Systems Italy  , Silicon Graphics , Sequent Computer Systems , Data General , Digital  and ICL. Techniques developed by these companies later featured in a variety of Unix-like operating systems, and to an extent in Windows NT."
+665,"The first commercial implementation of a NUMA-based Unix system was the Symmetrical Multi Processing XPS-100 family of servers, designed by Dan Gielan of VAST Corporation for Honeywell Information Systems Italy."
+666,"Modern CPUs operate considerably faster than the main memory they use. In the early days of computing and data processing, the CPU generally ran slower than its own memory. The performance lines of processors and memory crossed in the 1960s with the advent of the first supercomputers. Since then, CPUs increasingly have found themselves ""starved for data"" and having to stall while waiting for data to arrive from memory . Many supercomputer designs of the 1980s and 1990s focused on providing high-speed memory access as opposed to faster processors, allowing the computers to work on large data sets at speeds other systems could not approach."
+667,"Limiting the number of memory accesses provided the key to extracting high performance from a modern computer. For commodity processors, this meant installing an ever-increasing amount of high-speed cache memory and using increasingly sophisticated algorithms to avoid cache misses. But the dramatic increase in size of the operating systems and of the applications run on them has generally overwhelmed these cache-processing improvements. Multi-processor systems without NUMA make the problem considerably worse. Now a system can starve several processors at the same time, notably because only one processor can access the computer's memory at a time."
+668,"NUMA attempts to address this problem by providing separate memory for each processor, avoiding the performance hit when several processors attempt to address the same memory. For problems involving spread data , NUMA can improve the performance over a single shared memory by a factor of roughly the number of processors . Another approach to addressing this problem is the multi-channel memory architecture, in which a linear increase in the number of memory channels increases the memory access concurrency linearly."
+669,"Of course, not all data ends up confined to a single task, which means that more than one processor may require the same data. To handle these cases, NUMA systems include additional hardware or software to move data between memory banks. This operation slows the processors attached to those banks, so the overall speed increase due to NUMA heavily depends on the nature of the running tasks."
+670,"AMD implemented NUMA with its Opteron processor , using HyperTransport. Intel announced NUMA compatibility for its x86 and Itanium servers in late 2007 with its Nehalem and Tukwila CPUs. Both Intel CPU families share a common chipset; the interconnection is called Intel QuickPath Interconnect , which provides extremely high bandwidth to enable high on-board scalability and was replaced by a new version called Intel UltraPath Interconnect with the release of Skylake ."
+671,"Nearly all CPU architectures use a small amount of very fast non-shared memory known as cache to exploit locality of reference in memory accesses. With NUMA, maintaining cache coherence across shared memory has a significant overhead. Although simpler to design and build, non-cache-coherent NUMA systems become prohibitively complex to program in the standard von Neumann architecture programming model."
+672,"Typically, ccNUMA uses inter-processor communication between cache controllers to keep a consistent memory image when more than one cache stores the same memory location. For this reason, ccNUMA may perform poorly when multiple processors attempt to access the same memory area in rapid succession.  Support for NUMA in operating systems attempts to reduce the frequency of this kind of access by allocating processors and memory in NUMA-friendly ways and by avoiding scheduling and locking algorithms that make NUMA-unfriendly accesses necessary."
+673,"Alternatively, cache coherency protocols such as the MESIF protocol attempt to reduce the communication required to maintain cache coherency. Scalable Coherent Interface  is an IEEE standard defining a directory-based cache coherency protocol to avoid scalability limitations found in earlier multiprocessor systems. For example, SCI is used as the basis for the NumaConnect technology."
+674,"One can view NUMA as a tightly coupled form of cluster computing.  The addition of virtual memory paging to a cluster architecture can allow the implementation of NUMA entirely in software. However, the inter-node latency of software-based NUMA remains several orders of magnitude greater  than that of hardware-based NUMA."
+675,"Since NUMA largely influences memory access performance, certain software optimizations are needed to allow scheduling threads and processes close to their in-memory data."
+676,"As of 2011, ccNUMA systems are multiprocessor systems based on the AMD Opteron processor, which can be implemented without external logic, and the Intel Itanium processor, which requires the chipset to support NUMA. Examples of ccNUMA-enabled chipsets are the SGI Shub , the Intel E8870, the HP sx2000 , and those found in NEC Itanium-based systems. Earlier ccNUMA systems such as those from Silicon Graphics were based on MIPS processors and the DEC Alpha 21364  processor."
+677,"A scalar processor is classified as a single instruction, single data  processor in Flynn's taxonomy. The Intel 486 is an example of a scalar processor.  It is to be contrasted with a vector processor where a single instruction operates simultaneously on multiple data items  processor). The difference is analogous to the difference between scalar and vector arithmetic."
+678,The term scalar in computing dates to the 1970 and 1980s when vector processors were first introduced.  It was originally used to distinguish the older designs from the new vector processors.
+679,"A superscalar processor  may execute more than one instruction during a clock cycle by simultaneously dispatching multiple instructions to redundant functional units on the processor. Each functional unit is not a separate CPU core but an execution resource within a single CPU such as an arithmetic logic unit, a bit shifter, or a multiplier. The Cortex-M7, like many consumer CPUs today, is a superscalar processor."
+680,"A scalar data type, or just scalar, is any non-composite value."
+681,"Generally, all basic primitive data types are considered scalar:"
+682,"Some programming languages also treat strings as scalar types, while other languages treat strings as arrays or objects."
+683,A quantum computer is a computer that takes advantage of quantum mechanical phenomena.
+684,"On small scales, physical matter exhibits properties of both particles and waves, and quantum computing leverages this behavior, specifically quantum superposition and entanglement, using specialized hardware that supports the preparation and manipulation of quantum states."
+685,"Classical physics cannot explain the operation of these quantum devices, and a scalable quantum computer could perform some calculations exponentially faster  than any modern ""classical"" computer. In particular, a large-scale quantum computer could break widely used encryption schemes and aid physicists in performing physical simulations; however, the current state of the technology is largely experimental and impractical, with several obstacles to useful applications. Moreover, scalable quantum computers do not hold promise for many practical tasks, and for many important tasks quantum speedups are proven impossible."
+686,"The basic unit of information in quantum computing is the qubit, similar to the bit in traditional digital electronics. Unlike a classical bit, a qubit can exist in a superposition of its two ""basis"" states. When measuring a qubit, the result is a probabilistic output of a classical bit, therefore making quantum computers nondeterministic in general. If a quantum computer manipulates the qubit in a particular way, wave interference effects can amplify the desired measurement results. The design of quantum algorithms involves creating procedures that allow a quantum computer to perform calculations efficiently and quickly."
+687,"Physically engineering high-quality qubits has proven challenging. If a physical qubit is not sufficiently isolated from its environment, it suffers from quantum decoherence, introducing noise into calculations. Paradoxically, perfectly isolating qubits is also undesirable because quantum computations typically need to initialize qubits, perform controlled qubit interactions, and measure the resulting quantum states. Each of those operations introduces errors and suffers from noise, and such inaccuracies accumulate."
+688,"In principle, a non-quantum  computer can solve the same computational problems as a quantum computer, given enough time. Quantum advantage comes in the form of time complexity rather than computability, and quantum complexity theory shows that some quantum algorithms for carefully selected tasks require exponentially fewer computational steps than the best known non-quantum algorithms. Such tasks can in theory be solved on a large-scale quantum computer whereas classical computers would not finish computations in any reasonable amount of time. However, quantum speedup is not universal or even typical across computational tasks, since basic tasks such as sorting are proven to not allow any asymptotic quantum speedup. Claims of quantum supremacy have drawn significant attention to the discipline, but are demonstrated on contrived tasks, while near-term practical use cases remain limited."
+689,"For many years, the fields of quantum mechanics and computer science formed distinct academic communities. Modern quantum theory developed in the 1920s to explain the wave–particle duality observed at atomic scales, and digital computers emerged in the following decades to replace human computers for tedious calculations. Both disciplines had practical applications during World War II; computers played a major role in wartime cryptography, and quantum physics was essential for the nuclear physics used in the Manhattan Project."
+690,"A measurement-based quantum computer decomposes computation into a sequence of Bell state measurements and single-qubit quantum gates applied to a highly entangled initial state , using a technique called quantum gate teleportation."
+691,"An adiabatic quantum computer, based on quantum annealing, decomposes computation into a slow continuous transformation of an initial Hamiltonian into a final Hamiltonian, whose ground states contain the solution."
+692,"Neuromorphic quantum computing  is an unconventional computing type of computing that uses neuromorphic computing to perform quantum operations. It was suggested that quantum algorithms, which are algorithms that run on a realistic model of quantum computation, can be computed equally efficiently with neuromorphic quantum computing. Both, traditional quantum computing and neuromorphic quantum computing are physics-based unconventional computing approaches to computations and don’t follow the von Neumann architecture. They both construct a system  that represents the physical problem at hand, and then leverage their respective physics properties of the system to seek the “minimum”. Neuromorphic quantum computing and quantum computing share similar physical properties during computation."
+693,A topological quantum computer decomposes computation into the braiding of anyons in a 2D lattice.
+694,"A quantum Turing machine is the quantum analog of a Turing machine. All of these models of computation—quantum circuits, one-way quantum computation, adiabatic quantum computation, and topological quantum computation—have been shown to be equivalent to the quantum Turing machine; given a perfect implementation of one such quantum computer, it can simulate all the others with no more than polynomial overhead. This equivalence need not hold for practical quantum computers, since the overhead of simulation may be too large to be practical."
+695,"Quantum computing has significant potential applications in the fields of cryptography and cybersecurity. Quantum cryptography, which relies on the principles of quantum mechanics, offers the possibility of secure communication channels that are resistant to eavesdropping. Quantum key distribution  protocols, such as BB84, enable the secure exchange of cryptographic keys between parties, ensuring the confidentiality and integrity of communication. Moreover, quantum random number generators  can produce high-quality random numbers, which are essential for secure encryption."
+696,"However, quantum computing also poses challenges to traditional cryptographic systems. Shor's algorithm, a quantum algorithm for integer factorization, could potentially break widely used public-key cryptography schemes like RSA, which rely on the difficulty of factoring large numbers. Post-quantum cryptography, which involves the development of cryptographic algorithms that are resistant to attacks by both classical and quantum computers, is an active area of research aimed at addressing this concern."
+697,"Ongoing research in quantum cryptography and post-quantum cryptography is crucial for ensuring the security of communication and data in the face of evolving quantum computing capabilities. Advances in these fields, such as the development of new QKD protocols, the improvement of QRNGs, and the standardization of post-quantum cryptographic algorithms, will play a key role in maintaining the integrity and confidentiality of information in the quantum era."
+698,"Quantum cryptography enables new ways to transmit data securely; for example, quantum key distribution uses entangled quantum states to establish secure cryptographic keys. When a sender and receiver exchange quantum states, they can guarantee that an adversary does not intercept the message, as any unauthorized eavesdropper would disturb the delicate quantum system and introduce a detectable change. With appropriate cryptographic protocols, the sender and receiver can thus establish shared private information resistant to eavesdropping."
+699,"Modern fiber-optic cables can transmit quantum information over relatively short distances. Ongoing experimental research aims to develop more reliable hardware , hoping to scale this technology to long-distance quantum networks with end-to-end entanglement. Theoretically, this could enable novel technological applications, such as distributed quantum computing and enhanced quantum sensing."
+700,"Progress in finding quantum algorithms typically focuses on this quantum circuit model, though exceptions like the quantum adiabatic algorithm exist. Quantum algorithms can be roughly categorized by the type of speedup achieved over corresponding classical algorithms."
+701,"Quantum algorithms that offer more than a polynomial speedup over the best-known classical algorithm include Shor's algorithm for factoring and the related quantum algorithms for computing discrete logarithms, solving Pell's equation, and more generally solving the hidden subgroup problem for abelian finite groups. These algorithms depend on the primitive of the quantum Fourier transform. No mathematical proof has been found that shows that an equally fast classical algorithm cannot be discovered, but evidence suggests that this is unlikely. Certain oracle problems like Simon's problem and the Bernstein–Vazirani problem do give provable speedups, though this is in the quantum query model, which is a restricted model where lower bounds are much easier to prove and doesn't necessarily translate to speedups for practical problems."
+702,"Other problems, including the simulation of quantum physical processes from chemistry and solid-state physics, the approximation of certain Jones polynomials, and the quantum algorithm for linear systems of equations have quantum algorithms appearing to give super-polynomial speedups and are BQP-complete. Because these problems are BQP-complete, an equally fast classical algorithm for them would imply that no quantum algorithm gives a super-polynomial speedup, which is believed to be unlikely."
+703,"Some quantum algorithms, like Grover's algorithm and amplitude amplification, give polynomial speedups over corresponding classical algorithms. Though these algorithms give comparably modest quadratic speedup, they are widely applicable and thus give speedups for a wide range of problems."
+704,"Since chemistry and nanotechnology rely on understanding quantum systems, and such systems are impossible to simulate in an efficient manner classically, quantum simulation may be an important application of quantum computing. Quantum simulation could also be used to simulate the behavior of atoms and particles at unusual conditions such as the reactions inside a collider. In June 2023, IBM computer scientists reported that a quantum computer produced better results for a physics problem than a conventional supercomputer."
+705,About 2% of the annual global energy output is used for nitrogen fixation to produce ammonia for the Haber process in the agricultural fertilizer industry . Quantum simulations might be used to understand this process and increase the energy efficiency of production. It is expected that an early use of quantum computing will be modeling that improves the efficiency of the Haber–Bosch process by the mid 2020s although some have predicted it will take longer.
+706,"A notable application of quantum computation is for attacks on cryptographic systems that are currently in use. Integer factorization, which underpins the security of public key cryptographic systems, is believed to be computationally infeasible with an ordinary computer for large integers if they are the product of few prime numbers . By comparison, a quantum computer could solve this problem exponentially faster using Shor's algorithm to find its factors. This ability would allow a quantum computer to break many of the cryptographic systems in use today, in the sense that there would be a polynomial time  algorithm for solving the problem. In particular, most of the popular public key ciphers are based on the difficulty of factoring integers or the discrete logarithm problem, both of which can be solved by Shor's algorithm. In particular, the RSA, Diffie–Hellman, and elliptic curve Diffie–Hellman algorithms could be broken. These are used to protect secure Web pages, encrypted email, and many other types of data. Breaking these would have significant ramifications for electronic privacy and security."
+707,"Identifying cryptographic systems that may be secure against quantum algorithms is an actively researched topic under the field of post-quantum cryptography. Some public-key algorithms are based on problems other than the integer factorization and discrete logarithm problems to which Shor's algorithm applies, like the McEliece cryptosystem based on a problem in coding theory. Lattice-based cryptosystems are also not known to be broken by quantum computers, and finding a polynomial time algorithm for solving the dihedral hidden subgroup problem, which would break many lattice based cryptosystems, is a well-studied open problem. It has been proven that applying Grover's algorithm to break a symmetric  algorithm by brute force requires time equal to roughly 2n/2 invocations of the underlying cryptographic algorithm, compared with roughly 2n in the classical case, meaning that symmetric key lengths are effectively halved: AES-256 would have the same security against an attack using Grover's algorithm that AES-128 has against classical brute-force search ."
+708,Problems that can be efficiently addressed with Grover's algorithm have the following properties:
+709,"There is no searchable structure in the collection of possible answers,"
+710,"The number of possible answers to check is the same as the number of inputs to the algorithm, and"
+711,There exists a boolean function that evaluates each input and determines whether it is the correct answer.
+712,"For problems with all these properties, the running time of Grover's algorithm on a quantum computer scales as the square root of the number of inputs , as opposed to the linear scaling of classical algorithms. A general class of problems to which Grover's algorithm can be applied is a Boolean satisfiability problem, where the database through which the algorithm iterates is that of all possible answers. An example and possible application of this is a password cracker that attempts to guess a password. Breaking symmetric ciphers with this algorithm is of interest to government agencies."
+713,"Quantum annealing relies on the adiabatic theorem to undertake calculations. A system is placed in the ground state for a simple Hamiltonian, which slowly evolves to a more complicated Hamiltonian whose ground state represents the solution to the problem in question. The adiabatic theorem states that if the evolution is slow enough the system will stay in its ground state at all times through the process. Adiabatic optimization may be helpful for solving computational biology problems."
+714,"Since quantum computers can produce outputs that classical computers cannot produce efficiently, and since quantum computation is fundamentally linear algebraic, some express hope in developing quantum algorithms that can speed up machine learning tasks."
+715,"For example, the HHL Algorithm, named after its discoverers Harrow, Hassidim, and Lloyd, is believed to provide speedup over classical counterparts. Some research groups have recently explored the use of quantum annealing hardware for training Boltzmann machines and deep neural networks."
+716,"Deep generative chemistry models emerge as powerful tools to expedite drug discovery. However, the immense size and complexity of the structural space of all possible drug-like molecules pose significant obstacles, which could be overcome in the future by quantum computers. Quantum computers are naturally good for solving complex quantum many-body problems and thus may be instrumental in applications involving quantum chemistry. Therefore, one can expect that quantum-enhanced generative models including quantum GANs may eventually be developed into ultimate generative chemistry algorithms."
+717,"As of 2023, classical computers outperform quantum computers for all real-world applications. While current quantum computers may speed up solutions to particular mathematical problems, they give no computational advantage for practical tasks. For many tasks there is no promise of useful quantum speedup, and some tasks provably prohibit any quantum speedup in the sense that any speedup is ruled out by proven theorems. Scientists and engineers are exploring multiple technologies for quantum computing hardware and hope to develop scalable quantum architectures, but serious obstacles remain."
+718,There are a number of technical challenges in building a large-scale quantum computer. Physicist David DiVincenzo has listed these requirements for a practical quantum computer:
+719,"Sourcing parts for quantum computers is also very difficult. Superconducting quantum computers, like those constructed by Google and IBM, need helium-3, a nuclear research byproduct, and special superconducting cables made only by the Japanese company Coax Co."
+720,The control of multi-qubit systems requires the generation and coordination of a large number of electrical signals with tight and deterministic timing resolution. This has led to the development of quantum controllers that enable interfacing with the qubits. Scaling these systems to support a growing number of qubits is an additional challenge.
+721,"One of the greatest challenges involved with constructing quantum computers is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T2 , typically range between nanoseconds and seconds at low temperature. Currently, some quantum computers require their qubits to be cooled to 20 millikelvin  in order to prevent significant decoherence. A 2020 study argues that ionizing radiation such as cosmic rays can nevertheless cause certain systems to decohere within milliseconds."
+722,"As a result, time-consuming tasks may render some quantum algorithms inoperable, as attempting to maintain the state of qubits for a long enough duration will eventually corrupt the superpositions."
+723,"These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time."
+724,"As described by the threshold theorem, if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often-cited figure for the required error rate in each gate for fault-tolerant computation is 10−3, assuming the noise is depolarizing."
+725,"Meeting this scalability condition is possible for a wide range of systems. However, the use of error correction brings with it the cost of a greatly increased number of required qubits. The number required to factor integers using Shor's algorithm is still polynomial, and thought to be between L and L2, where L is the number of digits in the number to be factored; error correction algorithms would inflate this figure by an additional factor of L. For a 1000-bit number, this implies a need for about 104 bits without error correction. With error correction, the figure would rise to about 107 bits. Computation time is about L2 or about 107 steps and at 1 MHz, about 10 seconds. However, the encoding and error-correction overheads increase the size of a real fault-tolerant quantum computer by several orders of magnitude. Careful estimates show that at least 3 million physical qubits would factor 2,048-bit integer in 5 months on a fully error-corrected trapped-ion quantum computer. In terms of the number of physical qubits, to date, this remains the lowest estimate for practically useful integer factorization problem sizing 1,024-bit or larger."
+726,"Another approach to the stability-decoherence problem is to create a topological quantum computer with anyons, quasi-particles used as threads, and relying on braid theory to form stable logic gates."
+727,"Physicist John Preskill coined the term quantum supremacy to describe the engineering feat of demonstrating that a programmable quantum device can solve a problem beyond the capabilities of state-of-the-art classical computers. The problem need not be useful, so some view the quantum supremacy test only as a potential future benchmark."
+728,"In October 2019, Google AI Quantum, with the help of NASA, became the first to claim to have achieved quantum supremacy by performing calculations on the Sycamore quantum computer more than 3,000,000 times faster than they could be done on Summit, generally considered the world's fastest computer. This claim has been subsequently challenged: IBM has stated that Summit can perform samples much faster than claimed, and researchers have since developed better algorithms for the sampling problem used to claim quantum supremacy, giving substantial reductions to the gap between Sycamore and classical supercomputers and even beating it."
+729,"In December 2020, a group at USTC implemented a type of Boson sampling on 76 photons with a photonic quantum computer, Jiuzhang, to demonstrate quantum supremacy. The authors claim that a classical contemporary supercomputer would require a computational time of 600 million years to generate the number of samples their quantum processor can generate in 20 seconds."
+730,"Claims of quantum supremacy have generated hype around quantum computing, but they are based on contrived benchmark tasks that do not directly imply useful real-world applications."
+731,"In January 2024, a study published in Physical Review Letters provided direct verification of quantum supremacy experiments by computing exact amplitudes for experimentally generated bitstrings using a new-generation Sunway supercomputer, demonstrating a significant leap in simulation capability built on a multiple-amplitude tensor network contraction algorithm. This development underscores the evolving landscape of quantum computing, highlighting both the progress and the complexities involved in validating quantum supremacy claims."
+732,"Despite high hopes for quantum computing, significant progress in hardware, and optimism about future applications, a 2023 Nature spotlight article summarised current quantum computers as being ""For now,  absolutely nothing"". The article elaborated that quantum computers are yet to be more useful or efficient than conventional computers in any case, though it also argued that in the long term such computers are likely to be useful. A 2023 Communications of the ACM article found that current quantum computing algorithms are ""insufficient for practical quantum advantage without significant improvements across the software/hardware stack"". It argues that the most promising candidates for achieving speedup with quantum computers are ""small-data problems"", for example in chemistry and materials science. However, the article also concludes that a large range of the potential applications it considered, such as machine learning, ""will not achieve quantum advantage with current quantum algorithms in the foreseeable future"", and it identified I/O constraints that make speedup unlikely for ""big data problems, unstructured linear systems, and database search based on Grover's algorithm""."
+733,This state of affairs can be traced to several current and long-term considerations.
+734,"In particular, building computers with large numbers of qubits may be futile if those qubits are not connected well enough and cannot maintain sufficiently high degree of entanglement for long time. When trying to outperform conventional computers, quantum computing researchers often look for new tasks that can be solved on quantum computers, but this leaves the possibility that efficient non-quantum techniques will be developed in response, as seen for Quantum supremacy demonstrations. Therefore, it is desirable to prove lower bounds on the complexity of best possible non-quantum algorithms  and show that some quantum algorithms asymptomatically improve upon those bounds."
+735,"Some researchers have expressed skepticism that scalable quantum computers could ever be built, typically because of the issue of maintaining coherence at large scales, but also for other reasons."
+736,Bill Unruh doubted the practicality of quantum computers in a paper published in 1994. Paul Davies argued that a 400-qubit computer would even come into conflict with the cosmological information bound implied by the holographic principle. Skeptics like Gil Kalai doubt that quantum supremacy will ever be achieved. Physicist Mikhail Dyakonov has expressed skepticism of quantum computing as follows:
+737,"A practical quantum computer must use a physical system as a programmable quantum register. Researchers are exploring several technologies as candidates for reliable qubit implementations. Superconductors and trapped ions are some of the most developed proposals, but experimentalists are considering other hardware possibilities as well."
+738,"Any computational problem solvable by a classical computer is also solvable by a quantum computer. Intuitively, this is because it is believed that all physical phenomena, including the operation of classical computers, can be described using quantum mechanics, which underlies the operation of quantum computers."
+739,"Conversely, any problem solvable by a quantum computer is also solvable by a classical computer. It is possible to simulate both quantum and classical computers manually with just some paper and a pen, if given enough time. More formally, any quantum computer can be simulated by a Turing machine. In other words, quantum computers provide no additional power over classical computers in terms of computability. This means that quantum computers cannot solve undecidable problems like the halting problem, and the existence of quantum computers does not disprove the Church–Turing thesis."
+740,"While quantum computers cannot solve any problems that classical computers cannot already solve, it is suspected that they can solve certain problems faster than classical computers. For instance, it is known that quantum computers can efficiently factor integers, while this is not believed to be the case for classical computers."
+741,"The class of problems that can be efficiently solved by a quantum computer with bounded error is called BQP, for ""bounded error, quantum, polynomial time"". More formally, BQP is the class of problems that can be solved by a polynomial-time quantum Turing machine with an error probability of at most 1/3. As a class of probabilistic problems, BQP is the quantum counterpart to BPP , the class of problems that can be solved by polynomial-time probabilistic Turing machines with bounded error. It is known that 
+  
+    
+      
+        
+          
+            B
+            P
+            P
+            ⊆
+            B
+            Q
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {BPP\subseteq BQP}}}
+  
+ and is widely suspected that 
+  
+    
+      
+        
+          
+            B
+            Q
+            P
+            ⊊
+            B
+            P
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {BQP\subsetneq BPP}}}
+  
+, which intuitively would mean that quantum computers are more powerful than classical computers in terms of time complexity."
+742,"The exact relationship of BQP to P, NP, and PSPACE is not known. However, it is known that 
+  
+    
+      
+        
+          
+            P
+            ⊆
+            B
+            Q
+            P
+            ⊆
+            P
+            S
+            P
+            A
+            C
+            E
+          
+        
+      
+    
+    {\displaystyle {\mathsf {P\subseteq BQP\subseteq PSPACE}}}
+  
+; that is, all problems that can be efficiently solved by a deterministic classical computer can also be efficiently solved by a quantum computer, and all problems that can be efficiently solved by a quantum computer can also be solved by a deterministic classical computer with polynomial space resources. It is further suspected that BQP is a strict superset of P, meaning there are problems that are efficiently solvable by quantum computers that are not efficiently solvable by deterministic classical computers. For instance, integer factorization and the discrete logarithm problem are known to be in BQP and are suspected to be outside of P. On the relationship of BQP to NP, little is known beyond the fact that some NP problems that are believed not to be in P are also in BQP . It is suspected that 
+  
+    
+      
+        
+          
+            N
+            P
+            ⊈
+            B
+            Q
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {NP\nsubseteq BQP}}}
+  
+; that is, it is believed that there are efficiently checkable problems that are not efficiently solvable by a quantum computer. As a direct consequence of this belief, it is also suspected that BQP is disjoint from the class of NP-complete problems ."
+743,"The first documented computer architecture was in the correspondence between Charles Babbage and Ada Lovelace, describing the analytical engine. While building the computer Z1 in 1936, Konrad Zuse described in two patent applications for his future projects that machine instructions could be stored in the same storage used for data, i.e., the stored-program concept. Two other early and important examples are:"
+744,"The term ""architecture"" in computer literature can be traced to the work of Lyle R. Johnson and Frederick P. Brooks, Jr., members of the Machine Organization department in IBM's main research center in 1959. Johnson had the opportunity to write a proprietary research communication about the Stretch, an IBM-developed supercomputer for Los Alamos National Laboratory . To describe the level of detail for discussing the luxuriously embellished computer, he noted that his description of formats, instruction types, hardware parameters, and speed enhancements were at the level of ""system architecture"", a term that seemed more useful than ""machine organization""."
+745,"Subsequently, Brooks, a Stretch designer, opened Chapter 2 of a book called Planning a Computer System: Project Stretch by stating, ""Computer architecture, like other architecture, is the art of determining the needs of the user of a structure and then designing to meet those needs as effectively as possible within economic and technological constraints."""
+746,"Brooks went on to help develop the IBM System/360  line of computers, in which ""architecture"" became a noun defining ""what the user needs to know"". Later, computer users came to use the term in many less explicit ways."
+747,"The earliest computer architectures were designed on paper and then directly built into the final hardware form.
+Later, computer architecture prototypes were physically built in the form of a transistor–transistor logic  computer—such as the prototypes of the 6800 and the PA-RISC—tested, and tweaked, before committing to the final hardware form.
+As of the 1990s, new computer architectures are typically ""built"", tested, and tweaked—inside some other computer architecture in a computer architecture simulator; or inside a FPGA as a soft microprocessor; or both—before committing to the final hardware form."
+748,The discipline of computer architecture has three main subcategories:
+749,"There are other technologies in computer architecture. The following technologies are used in bigger companies like Intel, and were estimated in 2002 to count for 1% of all of computer architecture:"
+750,"Computer architecture is concerned with balancing the performance, efficiency, cost, and reliability of a computer system. The case of instruction set architecture can be used to illustrate the balance of these competing factors. More complex instruction sets enable programmers to write more space efficient programs, since a single instruction can encode some higher-level abstraction . However, longer and more complex instructions take longer for the processor to decode and can be more costly to implement effectively. The increased complexity from a large instruction set also creates more room for unreliability when instructions interact in unexpected ways."
+751,"The implementation involves integrated circuit design, packaging, power, and cooling. Optimization of the design requires familiarity with compilers, operating systems to logic design, and packaging."
+752,"An instruction set architecture  is the interface between the computer's software and hardware and also can be viewed as the programmer's view of the machine. Computers do not understand high-level programming languages such as Java, C++, or most programming languages used. A processor only understands instructions encoded in some numerical fashion, usually as binary numbers. Software tools, such as compilers, translate those high level languages into instructions that the processor can understand."
+753,"Besides instructions, the ISA defines items in the computer that are available to a program—e.g., data types, registers, addressing modes, and memory.  Instructions locate these available items with register indexes  and memory addressing modes."
+754,"The ISA of a computer is usually described in a small instruction manual, which describes how the instructions are encoded. Also, it may define short  mnemonic names for the instructions. The names can be recognized by a software development tool called an assembler.  An assembler is a computer program that translates a human-readable form of the ISA into a computer-readable form.  Disassemblers are also widely available, usually in debuggers and software programs to isolate and correct malfunctions in binary computer programs."
+755,"ISAs vary in quality and completeness.  A good ISA compromises between programmer convenience , size of the code , cost of the computer to interpret the instructions , and speed of the computer .  Memory organization defines how instructions interact with the memory, and how memory interacts with itself."
+756,"During design emulation, emulators can run programs written in a proposed instruction set. Modern emulators can measure size, cost, and speed to determine whether a particular ISA is meeting its goals."
+757,"Computer organization helps optimize performance-based products. For example, software engineers need to know the processing power of processors. They may need to optimize software in order to gain the most performance for the lowest price. This can require quite a detailed analysis of the computer's organization.  For example, in an SD card, the designers might need to arrange the card so that the most data can be processed in the fastest possible way."
+758,"Computer organization also helps plan the selection of a processor for a particular project. Multimedia projects may need very rapid data access, while virtual machines may need fast interrupts. Sometimes certain tasks need additional components as well.  For example, a computer capable of running a virtual machine needs virtual memory hardware so that the memory of different virtual computers can be kept separated. Computer organization and features also affect power consumption and processor cost."
+759,"Once an instruction set and micro-architecture have been designed, a practical machine must be developed. This design process is called the implementation. Implementation is usually not considered architectural design, but rather hardware design engineering. Implementation can be further broken down into several steps:"
+760,"For CPUs, the entire implementation process is organized differently and is often referred to as CPU design."
+761,"The exact form of a computer system depends on the constraints and goals. Computer architectures usually trade off standards, power versus performance, cost, memory capacity, latency  and throughput. Sometimes other considerations, such as features, size, weight, reliability, and expandability are also factors."
+762,The most common scheme does an in-depth power analysis and figures out how to keep power consumption low while maintaining adequate performance.
+763,"Modern computer performance is often described in instructions per cycle , which measures the efficiency of the architecture at any clock frequency; a faster IPC rate means the computer is faster. Older computers had IPC counts as low as 0.1 while modern processors easily reach nearly 1. Superscalar processors may reach three to five IPC by executing several instructions per clock cycle."
+764,"Counting machine-language instructions would be misleading because they can do varying amounts of work in different ISAs. The ""instruction"" in the standard measurements is not a count of the ISA's machine-language instructions, but a unit of measurement, usually based on the speed of the VAX computer architecture."
+765,"Many people used to measure a computer's speed by the clock rate . This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have greater performance. As a result, manufacturers have moved away from clock speed as a measure of performance."
+766,"Other factors influence speed, such as the mix of functional units, bus speeds, available memory, and the type and order of instructions in the programs."
+767,There are two main types of speed: latency and throughput. Latency is the time between the start of a process and its completion. Throughput is the amount of work done per unit time.  Interrupt latency is the guaranteed maximum response time of the system to an electronic event .
+768,"Performance is affected by a very wide range of design choices — for example, pipelining a processor usually makes latency worse, but makes throughput better. Computers that control machinery usually need low interrupt latencies. These computers operate in a real-time environment and fail if an operation is not completed in a specified amount of time. For example, computer-controlled anti-lock brakes must begin braking within a predictable and limited time period after the brake pedal is sensed or else failure of the brake will occur."
+769,"Benchmarking takes all these factors into account by measuring the time a computer takes to run through a series of test programs. Although benchmarking shows strengths, it should not be how you choose a computer. Often the measured machines split on different measures. For example, one system might handle scientific applications quickly, while another might render video games more smoothly. Furthermore, designers may target and add special features to their products, through hardware or software, that permit a specific benchmark to execute quickly but do not offer similar advantages to general tasks."
+770,Power efficiency is another important measurement in modern computers. Higher power efficiency can often be traded for lower speed or higher cost. The typical measurement when referring to power consumption in computer architecture is MIPS/W .
+771,"Modern circuits have less power required per transistor as the number of transistors per chip grows. This is because each transistor that is put in a new chip requires its own power supply and requires new pathways to be built to power it. However, the number of transistors per chip is starting to increase at a slower rate. Therefore, power efficiency is starting to become as important, if not more important than fitting more and more transistors into a single chip. Recent processor designs have shown this emphasis as they put more focus on power efficiency rather than cramming as many transistors into a single chip as possible. In the world of embedded computers, power efficiency has long been an important goal next to throughput and latency."
+772,"Increases in clock frequency have grown more slowly over the past few years, compared to power reduction improvements. This has been driven by the end of Moore's Law and demand for longer battery life and reductions in size for mobile technology. This change in focus from higher clock rates to power consumption and miniaturization can be shown by the significant reductions in power consumption, as much as 50%, that were reported by Intel in their release of the Haswell microarchitecture; where they dropped their power consumption benchmark from 30 to 40 watts down to 10-20 watts. Comparing this to the processing speed increase of 3 GHz to 4 GHz  it can be seen that the focus in research and development is shifting away from clock frequency and moving towards consuming less power and taking up less space."
+773,"Shor proposed multiple similar algorithms for solving the factoring problem, the discrete logarithm problem, and the period-finding problem. ""Shor's algorithm"" usually refers to the factoring algorithm, but may refer to any of the three algorithms. The discrete logarithm algorithm and the factoring algorithm are instances of the period-finding algorithm, and all three are instances of the hidden subgroup problem."
+774,"The basic characteristic of a computable function is that there must be a finite procedure  telling how to compute the function.  The models of computation listed above give different interpretations of what a procedure is and how it is used, but these interpretations share many properties.  The fact that these models give equivalent classes of computable functions stems from the fact that each model is capable of reading and mimicking a procedure for any of the other models, much as a compiler is able to read instructions in one computer language and emit instructions in another language."
+775,"Enderton  gives the following characteristics of a procedure for computing a computable function; similar characterizations have been given by Turing , Rogers , and others."
+776,Enderton goes on to list several clarifications of these 3 requirements of the procedure for a computable function:
+777,"The procedure must theoretically work for arbitrarily large arguments. It is not assumed that the arguments are smaller than the number of atoms in the Earth, for example."
+778,"The procedure is required to halt after finitely many steps in order to produce an output, but it may take arbitrarily many steps before halting. No time limitation is assumed."
+779,"Although the procedure may use only a finite amount of storage space during a successful computation, there is no bound on the amount of space that is used.  It is assumed that additional storage space can be given to the procedure whenever the procedure asks for it."
+780,"To summarise, based on this view a function is computable if:"
+781,The field of computational complexity studies functions with prescribed bounds on the time and/or space allowed in a successful computation.
+782,"A set A of natural numbers is called computable  if there is a computable, total function f such that for any natural number n, f = 1 if n is in A and f = 0 if n is not in A."
+783,"A set of natural numbers is called computably enumerable  if there is a computable function f such that for each number n, f is defined if and only if n is in the set.  Thus a set is computably enumerable if and only if it is the domain of some computable function.  The word enumerable is used because the following are equivalent for a nonempty subset B of the natural numbers:"
+784,"One such function, which is provable total but not primitive recursive, is the Ackermann function: since it is recursively defined, it is indeed easy to prove its computability ."
+785,"In a sound proof system, every provably total function is indeed total, but the converse is not true: in every first-order proof system that is strong enough and sound , one can prove  the existence of total functions that cannot be proven total in the proof system."
+786,"If the total computable functions are enumerated via the Turing machines that produces them, then the above statement can be shown, if the proof system is sound, by a similar diagonalization argument to that used above, using the enumeration of provably total functions given earlier. One uses a Turing machine that enumerates the relevant proofs, and for every input n calls  fn  by invoking the Turing machine that computes it according to the n-th proof. Such a Turing machine is guaranteed to halt if the proof system is sound."
+787,"Every computable function has a finite procedure giving explicit, unambiguous instructions on how to compute it. Furthermore, this procedure has to be encoded in the finite alphabet used by the computational model, so there are only countably many computable functions. For example, functions may be encoded using a string of bits ."
+788,"The real numbers are uncountable so most real numbers are not computable.  See computable number. The set of finitary functions on the natural numbers is uncountable so most are not computable.  Concrete examples of such functions are Busy beaver, Kolmogorov complexity, or any function that outputs the digits of a noncomputable number, such as Chaitin's constant."
+789,"Similarly, most subsets of the natural numbers are not computable. The halting problem was the first such set to be constructed. The Entscheidungsproblem, proposed by David Hilbert, asked whether there is an effective procedure to determine which mathematical statements  are true.  Turing and Church independently showed in the 1930s that this set of natural numbers is not computable.  According to the Church–Turing thesis, there is no effective procedure  which can perform these computations."
+790,The notion of computability of a function can be relativized to an arbitrary set of natural numbers A.  A function f is defined to be computable in A  when it satisfies the definition of a computable function with modifications allowing access to A as an oracle.  As with the concept of a computable function relative computability can be given equivalent definitions in many different models of computation.  This is commonly accomplished by supplementing the model of computation with an additional primitive operation which asks whether a given integer is a member of A.  We can also talk about f being computable in g by identifying g with its graph.
+791,"Hyperarithmetical theory studies those sets that can be computed from a computable ordinal number of iterates of the Turing jump of the empty set.  This is equivalent to sets defined by both a universal and existential formula in the language of second order arithmetic and to some models of Hypercomputation.  Even more general recursion theories have been studied, such as E-recursion theory in which any set can be used as an argument to an E-recursive function."
+792,"Although the Church–Turing thesis states that the computable functions include all functions with algorithms, it is possible to consider broader classes of functions that relax the requirements that algorithms must possess.  The field of Hypercomputation studies models of computation that go beyond normal Turing computation."
+793,A quantum computer is a computer that takes advantage of quantum mechanical phenomena.
+794,"On small scales, physical matter exhibits properties of both particles and waves, and quantum computing leverages this behavior, specifically quantum superposition and entanglement, using specialized hardware that supports the preparation and manipulation of quantum states."
+795,"Classical physics cannot explain the operation of these quantum devices, and a scalable quantum computer could perform some calculations exponentially faster  than any modern ""classical"" computer. In particular, a large-scale quantum computer could break widely used encryption schemes and aid physicists in performing physical simulations; however, the current state of the technology is largely experimental and impractical, with several obstacles to useful applications. Moreover, scalable quantum computers do not hold promise for many practical tasks, and for many important tasks quantum speedups are proven impossible."
+796,"The basic unit of information in quantum computing is the qubit, similar to the bit in traditional digital electronics. Unlike a classical bit, a qubit can exist in a superposition of its two ""basis"" states. When measuring a qubit, the result is a probabilistic output of a classical bit, therefore making quantum computers nondeterministic in general. If a quantum computer manipulates the qubit in a particular way, wave interference effects can amplify the desired measurement results. The design of quantum algorithms involves creating procedures that allow a quantum computer to perform calculations efficiently and quickly."
+797,"Physically engineering high-quality qubits has proven challenging. If a physical qubit is not sufficiently isolated from its environment, it suffers from quantum decoherence, introducing noise into calculations. Paradoxically, perfectly isolating qubits is also undesirable because quantum computations typically need to initialize qubits, perform controlled qubit interactions, and measure the resulting quantum states. Each of those operations introduces errors and suffers from noise, and such inaccuracies accumulate."
+798,"In principle, a non-quantum  computer can solve the same computational problems as a quantum computer, given enough time. Quantum advantage comes in the form of time complexity rather than computability, and quantum complexity theory shows that some quantum algorithms for carefully selected tasks require exponentially fewer computational steps than the best known non-quantum algorithms. Such tasks can in theory be solved on a large-scale quantum computer whereas classical computers would not finish computations in any reasonable amount of time. However, quantum speedup is not universal or even typical across computational tasks, since basic tasks such as sorting are proven to not allow any asymptotic quantum speedup. Claims of quantum supremacy have drawn significant attention to the discipline, but are demonstrated on contrived tasks, while near-term practical use cases remain limited."
+799,"For many years, the fields of quantum mechanics and computer science formed distinct academic communities. Modern quantum theory developed in the 1920s to explain the wave–particle duality observed at atomic scales, and digital computers emerged in the following decades to replace human computers for tedious calculations. Both disciplines had practical applications during World War II; computers played a major role in wartime cryptography, and quantum physics was essential for the nuclear physics used in the Manhattan Project."
+800,"An adiabatic quantum computer, based on quantum annealing, decomposes computation into a slow continuous transformation of an initial Hamiltonian into a final Hamiltonian, whose ground states contain the solution."
+801,"Neuromorphic quantum computing  is an unconventional computing type of computing that uses neuromorphic computing to perform quantum operations. It was suggested that quantum algorithms, which are algorithms that run on a realistic model of quantum computation, can be computed equally efficiently with neuromorphic quantum computing. Both, traditional quantum computing and neuromorphic quantum computing are physics-based unconventional computing approaches to computations and don’t follow the von Neumann architecture. They both construct a system  that represents the physical problem at hand, and then leverage their respective physics properties of the system to seek the “minimum”. Neuromorphic quantum computing and quantum computing share similar physical properties during computation."
+802,A topological quantum computer decomposes computation into the braiding of anyons in a 2D lattice.
+803,"A quantum Turing machine is the quantum analog of a Turing machine. All of these models of computation—quantum circuits, one-way quantum computation, adiabatic quantum computation, and topological quantum computation—have been shown to be equivalent to the quantum Turing machine; given a perfect implementation of one such quantum computer, it can simulate all the others with no more than polynomial overhead. This equivalence need not hold for practical quantum computers, since the overhead of simulation may be too large to be practical."
+804,"Quantum computing has significant potential applications in the fields of cryptography and cybersecurity. Quantum cryptography, which relies on the principles of quantum mechanics, offers the possibility of secure communication channels that are resistant to eavesdropping. Quantum key distribution  protocols, such as BB84, enable the secure exchange of cryptographic keys between parties, ensuring the confidentiality and integrity of communication. Moreover, quantum random number generators  can produce high-quality random numbers, which are essential for secure encryption."
+805,"However, quantum computing also poses challenges to traditional cryptographic systems. Shor's algorithm, a quantum algorithm for integer factorization, could potentially break widely used public-key cryptography schemes like RSA, which rely on the difficulty of factoring large numbers. Post-quantum cryptography, which involves the development of cryptographic algorithms that are resistant to attacks by both classical and quantum computers, is an active area of research aimed at addressing this concern."
+806,"Ongoing research in quantum cryptography and post-quantum cryptography is crucial for ensuring the security of communication and data in the face of evolving quantum computing capabilities. Advances in these fields, such as the development of new QKD protocols, the improvement of QRNGs, and the standardization of post-quantum cryptographic algorithms, will play a key role in maintaining the integrity and confidentiality of information in the quantum era."
+807,"Quantum cryptography enables new ways to transmit data securely; for example, quantum key distribution uses entangled quantum states to establish secure cryptographic keys. When a sender and receiver exchange quantum states, they can guarantee that an adversary does not intercept the message, as any unauthorized eavesdropper would disturb the delicate quantum system and introduce a detectable change. With appropriate cryptographic protocols, the sender and receiver can thus establish shared private information resistant to eavesdropping."
+808,"Modern fiber-optic cables can transmit quantum information over relatively short distances. Ongoing experimental research aims to develop more reliable hardware , hoping to scale this technology to long-distance quantum networks with end-to-end entanglement. Theoretically, this could enable novel technological applications, such as distributed quantum computing and enhanced quantum sensing."
+809,"Progress in finding quantum algorithms typically focuses on this quantum circuit model, though exceptions like the quantum adiabatic algorithm exist. Quantum algorithms can be roughly categorized by the type of speedup achieved over corresponding classical algorithms."
+810,"Quantum algorithms that offer more than a polynomial speedup over the best-known classical algorithm include Shor's algorithm for factoring and the related quantum algorithms for computing discrete logarithms, solving Pell's equation, and more generally solving the hidden subgroup problem for abelian finite groups. These algorithms depend on the primitive of the quantum Fourier transform. No mathematical proof has been found that shows that an equally fast classical algorithm cannot be discovered, but evidence suggests that this is unlikely. Certain oracle problems like Simon's problem and the Bernstein–Vazirani problem do give provable speedups, though this is in the quantum query model, which is a restricted model where lower bounds are much easier to prove and doesn't necessarily translate to speedups for practical problems."
+811,"Other problems, including the simulation of quantum physical processes from chemistry and solid-state physics, the approximation of certain Jones polynomials, and the quantum algorithm for linear systems of equations have quantum algorithms appearing to give super-polynomial speedups and are BQP-complete. Because these problems are BQP-complete, an equally fast classical algorithm for them would imply that no quantum algorithm gives a super-polynomial speedup, which is believed to be unlikely."
+812,"Some quantum algorithms, like Grover's algorithm and amplitude amplification, give polynomial speedups over corresponding classical algorithms. Though these algorithms give comparably modest quadratic speedup, they are widely applicable and thus give speedups for a wide range of problems."
+813,"Since chemistry and nanotechnology rely on understanding quantum systems, and such systems are impossible to simulate in an efficient manner classically, quantum simulation may be an important application of quantum computing. Quantum simulation could also be used to simulate the behavior of atoms and particles at unusual conditions such as the reactions inside a collider. In June 2023, IBM computer scientists reported that a quantum computer produced better results for a physics problem than a conventional supercomputer."
+814,About 2% of the annual global energy output is used for nitrogen fixation to produce ammonia for the Haber process in the agricultural fertilizer industry . Quantum simulations might be used to understand this process and increase the energy efficiency of production. It is expected that an early use of quantum computing will be modeling that improves the efficiency of the Haber–Bosch process by the mid 2020s although some have predicted it will take longer.
+815,"A notable application of quantum computation is for attacks on cryptographic systems that are currently in use. Integer factorization, which underpins the security of public key cryptographic systems, is believed to be computationally infeasible with an ordinary computer for large integers if they are the product of few prime numbers . By comparison, a quantum computer could solve this problem exponentially faster using Shor's algorithm to find its factors. This ability would allow a quantum computer to break many of the cryptographic systems in use today, in the sense that there would be a polynomial time  algorithm for solving the problem. In particular, most of the popular public key ciphers are based on the difficulty of factoring integers or the discrete logarithm problem, both of which can be solved by Shor's algorithm. In particular, the RSA, Diffie–Hellman, and elliptic curve Diffie–Hellman algorithms could be broken. These are used to protect secure Web pages, encrypted email, and many other types of data. Breaking these would have significant ramifications for electronic privacy and security."
+816,"Identifying cryptographic systems that may be secure against quantum algorithms is an actively researched topic under the field of post-quantum cryptography. Some public-key algorithms are based on problems other than the integer factorization and discrete logarithm problems to which Shor's algorithm applies, like the McEliece cryptosystem based on a problem in coding theory. Lattice-based cryptosystems are also not known to be broken by quantum computers, and finding a polynomial time algorithm for solving the dihedral hidden subgroup problem, which would break many lattice based cryptosystems, is a well-studied open problem. It has been proven that applying Grover's algorithm to break a symmetric  algorithm by brute force requires time equal to roughly 2n/2 invocations of the underlying cryptographic algorithm, compared with roughly 2n in the classical case, meaning that symmetric key lengths are effectively halved: AES-256 would have the same security against an attack using Grover's algorithm that AES-128 has against classical brute-force search ."
+817,There exists a boolean function that evaluates each input and determines whether it is the correct answer.
+818,"For problems with all these properties, the running time of Grover's algorithm on a quantum computer scales as the square root of the number of inputs , as opposed to the linear scaling of classical algorithms. A general class of problems to which Grover's algorithm can be applied is a Boolean satisfiability problem, where the database through which the algorithm iterates is that of all possible answers. An example and possible application of this is a password cracker that attempts to guess a password. Breaking symmetric ciphers with this algorithm is of interest to government agencies."
+819,"Quantum annealing relies on the adiabatic theorem to undertake calculations. A system is placed in the ground state for a simple Hamiltonian, which slowly evolves to a more complicated Hamiltonian whose ground state represents the solution to the problem in question. The adiabatic theorem states that if the evolution is slow enough the system will stay in its ground state at all times through the process. Adiabatic optimization may be helpful for solving computational biology problems."
+820,"Since quantum computers can produce outputs that classical computers cannot produce efficiently, and since quantum computation is fundamentally linear algebraic, some express hope in developing quantum algorithms that can speed up machine learning tasks."
+821,"For example, the HHL Algorithm, named after its discoverers Harrow, Hassidim, and Lloyd, is believed to provide speedup over classical counterparts. Some research groups have recently explored the use of quantum annealing hardware for training Boltzmann machines and deep neural networks."
+822,"Deep generative chemistry models emerge as powerful tools to expedite drug discovery. However, the immense size and complexity of the structural space of all possible drug-like molecules pose significant obstacles, which could be overcome in the future by quantum computers. Quantum computers are naturally good for solving complex quantum many-body problems and thus may be instrumental in applications involving quantum chemistry. Therefore, one can expect that quantum-enhanced generative models including quantum GANs may eventually be developed into ultimate generative chemistry algorithms."
+823,"As of 2023, classical computers outperform quantum computers for all real-world applications. While current quantum computers may speed up solutions to particular mathematical problems, they give no computational advantage for practical tasks. For many tasks there is no promise of useful quantum speedup, and some tasks provably prohibit any quantum speedup in the sense that any speedup is ruled out by proven theorems. Scientists and engineers are exploring multiple technologies for quantum computing hardware and hope to develop scalable quantum architectures, but serious obstacles remain."
+824,There are a number of technical challenges in building a large-scale quantum computer. Physicist David DiVincenzo has listed these requirements for a practical quantum computer:
+825,"Sourcing parts for quantum computers is also very difficult. Superconducting quantum computers, like those constructed by Google and IBM, need helium-3, a nuclear research byproduct, and special superconducting cables made only by the Japanese company Coax Co."
+826,The control of multi-qubit systems requires the generation and coordination of a large number of electrical signals with tight and deterministic timing resolution. This has led to the development of quantum controllers that enable interfacing with the qubits. Scaling these systems to support a growing number of qubits is an additional challenge.
+827,"One of the greatest challenges involved with constructing quantum computers is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T2 , typically range between nanoseconds and seconds at low temperature. Currently, some quantum computers require their qubits to be cooled to 20 millikelvin  in order to prevent significant decoherence. A 2020 study argues that ionizing radiation such as cosmic rays can nevertheless cause certain systems to decohere within milliseconds."
+828,"As a result, time-consuming tasks may render some quantum algorithms inoperable, as attempting to maintain the state of qubits for a long enough duration will eventually corrupt the superpositions."
+829,"These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time."
+830,"As described by the threshold theorem, if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often-cited figure for the required error rate in each gate for fault-tolerant computation is 10−3, assuming the noise is depolarizing."
+831,"Meeting this scalability condition is possible for a wide range of systems. However, the use of error correction brings with it the cost of a greatly increased number of required qubits. The number required to factor integers using Shor's algorithm is still polynomial, and thought to be between L and L2, where L is the number of digits in the number to be factored; error correction algorithms would inflate this figure by an additional factor of L. For a 1000-bit number, this implies a need for about 104 bits without error correction. With error correction, the figure would rise to about 107 bits. Computation time is about L2 or about 107 steps and at 1 MHz, about 10 seconds. However, the encoding and error-correction overheads increase the size of a real fault-tolerant quantum computer by several orders of magnitude. Careful estimates show that at least 3 million physical qubits would factor 2,048-bit integer in 5 months on a fully error-corrected trapped-ion quantum computer. In terms of the number of physical qubits, to date, this remains the lowest estimate for practically useful integer factorization problem sizing 1,024-bit or larger."
+832,"Another approach to the stability-decoherence problem is to create a topological quantum computer with anyons, quasi-particles used as threads, and relying on braid theory to form stable logic gates."
+833,"Physicist John Preskill coined the term quantum supremacy to describe the engineering feat of demonstrating that a programmable quantum device can solve a problem beyond the capabilities of state-of-the-art classical computers. The problem need not be useful, so some view the quantum supremacy test only as a potential future benchmark."
+834,"In October 2019, Google AI Quantum, with the help of NASA, became the first to claim to have achieved quantum supremacy by performing calculations on the Sycamore quantum computer more than 3,000,000 times faster than they could be done on Summit, generally considered the world's fastest computer. This claim has been subsequently challenged: IBM has stated that Summit can perform samples much faster than claimed, and researchers have since developed better algorithms for the sampling problem used to claim quantum supremacy, giving substantial reductions to the gap between Sycamore and classical supercomputers and even beating it."
+835,"In December 2020, a group at USTC implemented a type of Boson sampling on 76 photons with a photonic quantum computer, Jiuzhang, to demonstrate quantum supremacy. The authors claim that a classical contemporary supercomputer would require a computational time of 600 million years to generate the number of samples their quantum processor can generate in 20 seconds."
+836,"Claims of quantum supremacy have generated hype around quantum computing, but they are based on contrived benchmark tasks that do not directly imply useful real-world applications."
+837,"In January 2024, a study published in Physical Review Letters provided direct verification of quantum supremacy experiments by computing exact amplitudes for experimentally generated bitstrings using a new-generation Sunway supercomputer, demonstrating a significant leap in simulation capability built on a multiple-amplitude tensor network contraction algorithm. This development underscores the evolving landscape of quantum computing, highlighting both the progress and the complexities involved in validating quantum supremacy claims."
+838,"Despite high hopes for quantum computing, significant progress in hardware, and optimism about future applications, a 2023 Nature spotlight article summarised current quantum computers as being ""For now,  absolutely nothing"". The article elaborated that quantum computers are yet to be more useful or efficient than conventional computers in any case, though it also argued that in the long term such computers are likely to be useful. A 2023 Communications of the ACM article found that current quantum computing algorithms are ""insufficient for practical quantum advantage without significant improvements across the software/hardware stack"". It argues that the most promising candidates for achieving speedup with quantum computers are ""small-data problems"", for example in chemistry and materials science. However, the article also concludes that a large range of the potential applications it considered, such as machine learning, ""will not achieve quantum advantage with current quantum algorithms in the foreseeable future"", and it identified I/O constraints that make speedup unlikely for ""big data problems, unstructured linear systems, and database search based on Grover's algorithm""."
+839,This state of affairs can be traced to several current and long-term considerations.
+840,"In particular, building computers with large numbers of qubits may be futile if those qubits are not connected well enough and cannot maintain sufficiently high degree of entanglement for long time. When trying to outperform conventional computers, quantum computing researchers often look for new tasks that can be solved on quantum computers, but this leaves the possibility that efficient non-quantum techniques will be developed in response, as seen for Quantum supremacy demonstrations. Therefore, it is desirable to prove lower bounds on the complexity of best possible non-quantum algorithms  and show that some quantum algorithms asymptomatically improve upon those bounds."
+841,"Some researchers have expressed skepticism that scalable quantum computers could ever be built, typically because of the issue of maintaining coherence at large scales, but also for other reasons."
+842,Bill Unruh doubted the practicality of quantum computers in a paper published in 1994. Paul Davies argued that a 400-qubit computer would even come into conflict with the cosmological information bound implied by the holographic principle. Skeptics like Gil Kalai doubt that quantum supremacy will ever be achieved. Physicist Mikhail Dyakonov has expressed skepticism of quantum computing as follows:
+843,"A practical quantum computer must use a physical system as a programmable quantum register. Researchers are exploring several technologies as candidates for reliable qubit implementations. Superconductors and trapped ions are some of the most developed proposals, but experimentalists are considering other hardware possibilities as well."
+844,"Any computational problem solvable by a classical computer is also solvable by a quantum computer. Intuitively, this is because it is believed that all physical phenomena, including the operation of classical computers, can be described using quantum mechanics, which underlies the operation of quantum computers."
+845,"Conversely, any problem solvable by a quantum computer is also solvable by a classical computer. It is possible to simulate both quantum and classical computers manually with just some paper and a pen, if given enough time. More formally, any quantum computer can be simulated by a Turing machine. In other words, quantum computers provide no additional power over classical computers in terms of computability. This means that quantum computers cannot solve undecidable problems like the halting problem, and the existence of quantum computers does not disprove the Church–Turing thesis."
+846,"While quantum computers cannot solve any problems that classical computers cannot already solve, it is suspected that they can solve certain problems faster than classical computers. For instance, it is known that quantum computers can efficiently factor integers, while this is not believed to be the case for classical computers."
+847,"The class of problems that can be efficiently solved by a quantum computer with bounded error is called BQP, for ""bounded error, quantum, polynomial time"". More formally, BQP is the class of problems that can be solved by a polynomial-time quantum Turing machine with an error probability of at most 1/3. As a class of probabilistic problems, BQP is the quantum counterpart to BPP , the class of problems that can be solved by polynomial-time probabilistic Turing machines with bounded error. It is known that 
+  
+    
+      
+        
+          
+            B
+            P
+            P
+            ⊆
+            B
+            Q
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {BPP\subseteq BQP}}}
+  
+ and is widely suspected that 
+  
+    
+      
+        
+          
+            B
+            Q
+            P
+            ⊊
+            B
+            P
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {BQP\subsetneq BPP}}}
+  
+, which intuitively would mean that quantum computers are more powerful than classical computers in terms of time complexity."
+848,"The exact relationship of BQP to P, NP, and PSPACE is not known. However, it is known that 
+  
+    
+      
+        
+          
+            P
+            ⊆
+            B
+            Q
+            P
+            ⊆
+            P
+            S
+            P
+            A
+            C
+            E
+          
+        
+      
+    
+    {\displaystyle {\mathsf {P\subseteq BQP\subseteq PSPACE}}}
+  
+; that is, all problems that can be efficiently solved by a deterministic classical computer can also be efficiently solved by a quantum computer, and all problems that can be efficiently solved by a quantum computer can also be solved by a deterministic classical computer with polynomial space resources. It is further suspected that BQP is a strict superset of P, meaning there are problems that are efficiently solvable by quantum computers that are not efficiently solvable by deterministic classical computers. For instance, integer factorization and the discrete logarithm problem are known to be in BQP and are suspected to be outside of P. On the relationship of BQP to NP, little is known beyond the fact that some NP problems that are believed not to be in P are also in BQP . It is suspected that 
+  
+    
+      
+        
+          
+            N
+            P
+            ⊈
+            B
+            Q
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathsf {NP\nsubseteq BQP}}}
+  
+; that is, it is believed that there are efficiently checkable problems that are not efficiently solvable by a quantum computer. As a direct consequence of this belief, it is also suspected that BQP is disjoint from the class of NP-complete problems ."
+849,"The concept of wetware is an application of specific interest to the field of computer manufacturing. Moore's law, which states that the number of transistors which can be placed on a silicon chip is doubled roughly every two years, has acted as a goal for the industry for decades, but as the size of computers continues to decrease, the ability to meet this goal has become more difficult, threatening to reach a plateau. Due to the difficulty in reducing the size of computers because of size limitations of transistors and integrated circuits, wetware provides an unconventional alternative. A wetware computer composed of neurons is an ideal concept because, unlike conventional materials which operate in binary , a neuron can shift between thousands of states, constantly altering its chemical conformation, and redirecting electrical pulses through over 200,000 channels in any of its many synaptic connections. Because of this large difference in the possible settings for any one neuron, compared to the binary limitations of conventional computers, the space limitations are far fewer."
+850,"The concept of wetware is distinct and unconventional and draws slight resonance with both hardware and software from conventional computers. While hardware is understood as the physical architecture of traditional computational devices, built from electrical circuitry and silicone plates, software represents the encoded architecture of storage and instructions. Wetware is a separate concept that uses the formation of organic molecules, mostly complex cellular structures , to create a computational device such as a computer. In wetware, the ideas of hardware and software are intertwined and interdependent. The molecular and chemical composition of the organic or biological structure would represent not only the physical structure of the wetware but also the software, being continually reprogrammed by the discrete shifts in electrical pulses and chemical concentration gradients as the molecules change their structures to communicate signals. The responsiveness of a cell, proteins, and molecules to changing conformations, both within their structures and around them, ties the idea of internal programming and external structure together in a way that is alien to the current model of conventional computer architecture."
+851,"The structure of wetware represents a model where the external structure and internal programming are interdependent and unified; meaning that changes to the programming or internal communication between molecules of the device would represent a physical change in the structure. The dynamic nature of wetware borrows from the function of complex cellular structures in biological organisms. The combination of “hardware” and “software” into one dynamic, and interdependent system which uses organic molecules and complexes to create an unconventional model for computational devices is a specific example of applied biorobotics."
+852,"Cells in many ways can be seen as their form of naturally occurring wetware, similar to the concept that the human brain is the preexisting model system for complex wetware. In his book Wetware: A Computer in Every Living Cell  Dennis Bray explains his theory that cells, which are the most basic form of life, are just a highly complex computational structure, like a computer. To simplify one of his arguments a cell can be seen as a type of computer, using its structured architecture. In this architecture, much like a traditional computer, many smaller components operate in tandem to receive input, process the information, and compute an output. In an overly simplified, non-technical analysis, cellular function can be broken into the following components: Information and instructions for execution are stored as DNA in the cell, RNA acts as a source for distinctly encoded input, processed by ribosomes and other transcription factors to access and process the DNA and to output a protein. Bray's argument in favor of viewing cells and cellular structures as models of natural computational devices is important when considering the more applied theories of wetware to biorobotics."
+853,"Wetware and biorobotics are closely related concepts, which both borrow from similar overall principles. A biorobotic structure can be defined as a system modeled from a preexisting organic complex or model such as cells  or more complex structures like organs  or whole organisms.  Unlike wetware the concept of biorobotics is not always a system composed of organic molecules, but instead could be composed of conventional material which is designed and assembled in a structure similar or derived from a biological model. Biorobotics have many applications and are used to address the challenges of conventional computer architecture. Conceptually, designing a program, robot, or computational device after a preexisting biological model such as a cell, or even a whole organism, provides the engineer or programmer the benefits of incorporating into the structure the evolutionary advantages of the model."
+854,"In 1999 William Ditto and his team of researchers at Georgia Institute of Technology and Emory University created a basic form of a wetware computer capable of simple addition by harnessing leech neurons. Leeches were used as a model organism due to the large size of their neuron, and the ease associated with their collection and manipulation. However, these results have never been published in a peer-reviewed journal, prompting questions about the validity of the claims. The computer was able to complete basic addition through electrical probes inserted into the neuron. The manipulation of electrical currents through neurons was not a trivial accomplishment, however. Unlike conventional computer architecture, which is based on the binary on/off states, neurons are capable of existing in thousands of states and communicate with each other through synaptic connections which each contain over 200,000 channels. Each can be dynamically shifted in a process called self-organization to constantly form and reform new connections. A conventional computer program called the dynamic clamp was written by Eve Marder, a neurobiologist at Brandeis University that was capable of reading the electrical pulses from the neurons in real time and interpreting them. This program was used to manipulate the electrical signals being input into the neurons to represent numbers and to communicate with each other to return the sum. While this computer is a very basic example of a wetware structure it represents a small example with fewer neurons than found in a more complex organ. It is thought by Ditto that by increasing the number of neurons present the chaotic signals sent between them will self-organize into a more structured pattern, such as the regulation of heart neurons into a constant heartbeat found in humans and other living organisms."
+855,"After his work creating a basic computer from leech neurons, Ditto continued to work not only with organic molecules and wetware but also on the concept of applying the chaotic nature of biological systems and organic molecules to conventional material and logic gates. Chaotic systems have advantages for generating patterns and computing higher-order functions like memory, arithmetic logic, and input/output operations. In his article Construction of a Chaotic Computer Chip Ditto discusses the advantages in programming of using chaotic systems, with their greater sensitivity to respond and reconfigure logic gates in his conceptual chaotic chip. The main difference between a chaotic computer chip and a conventional computer chip is the reconfigurability of the chaotic system. Unlike a traditional computer chip, where a programmable gate array element must be reconfigured through the switching of many single-purpose logic gates, a chaotic chip can reconfigure all logic gates through the control of the pattern generated by the non-linear chaotic element."
+856,"Cognitive biology evaluates cognition as a basic biological function. W. Tecumseh Fitch, a professor of cognitive biology at the University of Vienna, is a leading theorist on ideas of cellular intentionality. The idea is that not only do whole organisms have a sense of ""aboutness"" of intentionality, but that single cells also carry a sense of intentionality through cells' ability to adapt and reorganize in response to certain stimuli. Fitch discusses the idea of nano-intentionality, specifically in regards to neurons, in their ability to adjust rearrangements to create neural networks. He discusses the ability of cells such as neurons to respond independently to stimuli such as damage to be what he considers ""intrinsic intentionality"" in cells, explaining that ""hile at a vastly simpler level than intentionality at the human cognitive level, I propose that this basic capacity of living things  provides the necessary building blocks for cognition and higher-order intentionality."" Fitch describes the value of his research to specific areas of computer science such as artificial intelligence and computer architecture. He states ""If a researcher aims to make a conscious machine, doing it with rigid switches  is barking up the wrong tree."" Fitch believes that an important aspect of the development of areas such as artificial intelligence is wetware with nano-intentionally, and autonomous ability to adapt and restructure itself."
+857,"In a review of the above-mentioned research conducted by Fitch, Daniel Dennett, a professor at Tufts University, discusses the importance of the distinction between the concept of hardware and software when evaluating the idea of wetware and organic material such as neurons. Dennett discusses the value of observing the human brain as a preexisting example of wetware. He sees the brain as having ""the competence of a silicon computer to take on an unlimited variety of temporary cognitive roles."" Dennett disagrees with Fitch on certain areas, such as the relationship of software/hardware versus wetware, and what a machine with wetware might be capable of. Dennett highlights the importance of additional research into human cognition to better understand the intrinsic mechanism by which the human brain can operate, to better create an organic computer."
+858,"Brain-on-a-chip devices have been developed that are ""aimed at testing and predicting the effects of biological and chemical agents, disease or pharmaceutical drugs on the brain over time"". Wetware computers may be useful for research about brain diseases and brain health/capacities , for drug discovery, for testing genome edits and research about brain aging."
+859,"Wetware computers may have substantial ethical implications, for instance related to possible potentials to sentience and suffering and dual-use technology."
+860,"Moreover, in some cases the human brain itself may be connected as a kind of ""wetware"" to other information technology systems which may also have large social and ethical implications, including issues related to intimate access to people's brains. For example, in 2021 Chile became the first country to approve neurolaw that establishes rights to personal identity, free will and mental privacy."
+861,"The concept of artificial insects may raise substantial ethical questions, including questions related to the decline in insect populations."
+862,"It is an open question whether human cerebral organoids could develop a degree or form of consciousness. Whether or how it could acquire its moral status with related rights and limits may also be potential future questions. There is research on how consciousness could be detected. As cerebral organoids may acquire human brain-like neural function subjective experience and consciousness may be feasible. Moreover, it may be possible that they acquire such upon transplantation into animals. A study notes that it may, in various cases, be morally permissible ""to create self-conscious animals by engrafting human cerebral organoids, but in the case, the moral status of such animals should be carefully considered""."
+863,"While there have been few major developments in the creation of an organic computer since the neuron-based calculator developed by Ditto in the 1990s, research continues to push the field forward, and in 2023 a functioning computer was constructed by researchers at the University of Illinois Urbana-Champaign using 80,000 mouse neurons as processor that can detect light and electrical signals. Projects such as the modeling of chaotic pathways in silicon chips by Ditto have made discoveries in ways of organizing traditional silicon chips and structuring computer architecture to be more efficient and better structured. Ideas emerging from the field of cognitive biology also help to continue to push discoveries in ways of structuring systems for artificial intelligence, to better imitate preexisting systems in humans."
+864,"In a proposed fungal computer using basidiomycetes, information is represented by spikes of electrical activity, a computation is implemented in a mycelium network, and an interface is realized via fruit bodies."
+865,"Connecting cerebral organoids  with other nerve tissues may become feasible in the future, as is the connection of physical artificial neurons  and the control of muscle tissue. External modules of biological tissue could trigger parallel trains of stimulation back into the brain. All-organic devices could be advantageous because it could be biocompatible which may allow it to be implanted into the human body. This may enable treatments of certain diseases and injuries to the nervous system."
+866,Three companies are focusing specifically on wetware computing using living neurons:
+867,"Leonard Adleman of the University of Southern California initially developed this field in 1994. Adleman demonstrated a proof-of-concept use of DNA as a form of computation which solved the seven-point Hamiltonian path problem. Since the initial Adleman experiments, advances have occurred and various Turing machines have been proven to be constructible."
+868,"Since then the field has expanded into several avenues. In 1995, the idea for DNA-based memory was proposed by Eric Baum who conjectured that a vast amount of data can be stored in a tiny amount of DNA due to its ultra-high density. This expanded the horizon of DNA computing into the realm of memory technology although the in vitro demonstrations were made almost after a decade."
+869,"The field of DNA computing can be categorized as a sub-field of the broader DNA nanoscience field started by Ned Seeman about a decade before Len Adleman's demonstration. Ned's original idea in the 1980s was to build arbitrary structures using bottom-up DNA self-assembly for applications in crystallography. However, it morphed into the field of structural DNA self-assembly which as of 2020 is extremely sophisticated. Self-assembled structure from a few nanometers tall all the way up to several tens of micrometers in size have been demonstrated in 2018."
+870,"In 1994, Prof. Seeman's group demonstrated early DNA lattice structures using a small set of DNA components. While the demonstration by Adleman showed the possibility of DNA-based computers, the DNA design was trivial because as the number of nodes in a graph grows, the number of DNA components required in Adleman's implementation would grow exponentially. Therefore, computer scientists and biochemists started exploring tile-assembly where the goal was to use a small set of DNA strands as tiles to perform arbitrary computations upon growth. Other avenues that were theoretically explored in the late 90's include DNA-based security and cryptography, computational capacity of DNA systems, DNA memories and disks, and DNA-based robotics."
+871,"Before 2002, Lila Kari showed that the DNA operations performed by genetic recombination in some organisms are Turing complete."
+872,"In 2003, John Reif's group first demonstrated the idea of a DNA-based walker that traversed along a track similar to a line follower robot. They used molecular biology as a source of energy for the walker. Since this first demonstration, a wide variety of DNA-based walkers have been demonstrated."
+873,"In 1994 Leonard Adleman presented the first prototype of a DNA computer. The TT-100 was a test tube filled with 100 microliters of a DNA solution. He managed to solve an instance of the directed Hamiltonian path problem. In Adleman's experiment, the Hamiltonian Path Problem was implemented notationally as the ""travelling salesman problem"". For this purpose, different DNA fragments were created, each one of them representing a city that had to be visited. Every one of these fragments is capable of a linkage with the other fragments created. These DNA fragments were produced and mixed in a test tube. Within seconds, the small fragments form bigger ones, representing the different travel routes. Through a chemical reaction, the DNA fragments representing the longer routes were eliminated. The remains are the solution to the problem, but overall, the experiment lasted a week. However, current technical limitations prevent the evaluation of the results. Therefore, the experiment isn't suitable for the application, but it is nevertheless a proof of concept."
+874,First results to these problems were obtained by Leonard Adleman.
+875,"In 2002, J. Macdonald, D. Stefanović and M. Stojanović created a DNA computer able to play tic-tac-toe against a human player. The calculator consists of nine bins corresponding to the nine squares of the game. Each bin contains a substrate and various combinations of DNA enzymes. The substrate itself is composed of a DNA strand onto which was grafted a fluorescent chemical group at one end, and the other end, a repressor group. Fluorescence is only active if the molecules of the substrate are cut in half. The DNA enzymes simulate logical functions. For example, such a DNA will unfold if two specific types of DNA strand are introduced to reproduce the logic function AND."
+876,"By default, the computer is considered to have played first in the central square. The human player starts with eight different types of DNA strands corresponding to the eight remaining boxes that may be played. To play box number i, the human player pours into all bins the strands corresponding to input #i. These strands bind to certain DNA enzymes present in the bins, resulting, in one of these bins, in the deformation of the DNA enzymes which binds to the substrate and cuts it. The corresponding bin becomes fluorescent, indicating which box is being played by the DNA computer. The DNA enzymes are divided among the bins in such a way as to ensure that the best the human player can achieve is a draw, as in real tic-tac-toe."
+877,Kevin Cherry and Lulu Qian at Caltech developed a DNA-based artificial neural network that can recognize 100-bit hand-written digits. They achieve this by programming on computer in advance with appropriate set of weights represented by varying concentrations weight molecules which will later be added to the test tube that holds the input DNA strands.
+878,"One of the challenges of DNA computing is its speed. While DNA as a substrate is biologically compatible i.e. it can be used at places where silicon technology cannot, its computation speed is still very slow. For example, the square-root circuit used as a benchmark in field took over 100 hours to complete. While newer ways with external enzyme sources are reporting faster and more compact circuits, Chatterjee et al. demonstrated an interesting idea in the field to speed up computation through localized DNA circuits,  a concept being further explored by other groups. This idea, while originally proposed in the field of computer architecture, has been adopted in this field as well. In computer architecture, it is very well-known that if the instructions are executed in sequence, having them loaded in the cache will inevitably lead to fast performance, also called the principle of localization. This is because with instructions in fast cache memory, there is no need swap them in and out of main memory, which can be slow. Similarly, in localized DNA computing, the DNA strands responsible for computation are fixed on a breadboard-like substrate ensuring physical proximity of the computing gates. Such localized DNA computing techniques have shown to potentially reduce the computation time by orders of magnitude."
+879,"Subsequent research on DNA computing has produced reversible DNA computing, bringing the technology one step closer to the silicon-based computing used in  PCs. In particular, John Reif and his group at Duke University have proposed two different techniques to reuse the computing DNA complexes. The first design uses dsDNA gates, while the second design uses DNA hairpin complexes.
+While both the designs face some issues , this appears to represent a significant breakthrough in the field of DNA computing. Some other groups have also attempted to address the gate reusability problem."
+880,"Using strand displacement reactions , reversible proposals are presented in the ""Synthesis Strategy of Reversible Circuits on DNA Computers"" paper  for implementing reversible gates and circuits on DNA computers by combining DNA computing and reversible computing techniques. This paper also proposes a universal reversible gate library  for synthesizing n-bit reversible circuits on DNA computers with an average length and cost of the constructed circuits better than the previous methods."
+881,"There are multiple methods for building a computing device based on DNA, each with its own advantages and disadvantages. Most of these build the basic logic gates  associated with digital logic from a DNA basis. Some of the different bases include DNAzymes, deoxyoligonucleotides, enzymes, and toehold exchange."
+882,"The most fundamental operation in DNA computing and molecular programming is the strand displacement mechanism. Currently, there are two ways to perform strand displacement:"
+883,"Besides simple strand displacement schemes, DNA computers have also been constructed using the concept of toehold exchange. In this system, an input DNA strand binds to a sticky end, or toehold, on another DNA molecule, which allows it to displace another strand segment from the molecule. This allows the creation of modular logic components such as AND, OR, and NOT gates and signal amplifiers, which can be linked into arbitrarily large computers. This class of DNA computers does not require enzymes or any chemical capability of the DNA."
+884,"The full stack for DNA computing looks very similar to a traditional computer architecture. At the highest level, a C-like general purpose programming language is expressed using a set of chemical reaction networks . This intermediate representation gets translated to domain-level DNA design and then implemented using a set of DNA strands. In 2010, Erik Winfree's group showed that DNA can be used as a substrate to implement arbitrary chemical reactions. This opened the way to design and synthesis of biochemical controllers since the expressive power of CRNs is equivalent to a Turing machine. Such controllers can potentially be used in vivo for applications such as preventing hormonal imbalance."
+885,"Catalytic DNA  catalyze a reaction when interacting with the appropriate input, such as a matching oligonucleotide. These DNAzymes are used to build logic gates analogous to digital logic in silicon; however, DNAzymes are limited to 1-, 2-, and 3-input gates with no current implementation for evaluating statements in series."
+886,"The DNAzyme logic gate changes its structure when it binds to a matching oligonucleotide and the fluorogenic substrate it is bonded to is cleaved free. While other materials can be used, most models use a fluorescence-based substrate because it is very easy to detect, even at the single molecule limit. The amount of fluorescence can then be measured to tell whether or not a reaction took place. The DNAzyme that changes is then ""used"", and cannot initiate any more reactions. Because of this, these reactions take place in a device such as a continuous stirred-tank reactor, where old product is removed and new molecules added."
+887,"Two commonly used DNAzymes are named E6 and 8-17. These are popular because they allow cleaving of a substrate in any arbitrary location. Stojanovic and MacDonald have used the E6 DNAzymes to build the MAYA I and MAYA II machines, respectively; Stojanovic has also demonstrated logic gates using the 8-17 DNAzyme. While these DNAzymes have been demonstrated to be useful for constructing logic gates, they are limited by the need for a metal cofactor to function, such as Zn2+ or Mn2+, and thus are not useful in vivo."
+888,"A design called a stem loop, consisting of a single strand of DNA which has a loop at an end, are a dynamic structure that opens and closes when a piece of DNA bonds to the loop part. This effect has been exploited to create several logic gates. These logic gates have been used to create the computers MAYA I and MAYA II which can play tic-tac-toe to some extent."
+889,"Enzyme-based DNA computers are usually of the form of a simple Turing machine; there is analogous hardware, in the form of an enzyme, and software, in the form of DNA."
+890,"Benenson, Shapiro and colleagues have demonstrated a DNA computer using the FokI enzyme and expanded on their work by going on to show automata that diagnose and react to prostate cancer: under expression of the genes PPAP2B and GSTP1 and an over expression of PIM1 and HPN. Their automata evaluated the expression of each gene, one gene at a time, and on positive diagnosis then released a single strand DNA molecule  that is an antisense for MDM2. MDM2 is a repressor of protein 53, which itself is a tumor suppressor. On negative diagnosis it was decided to release a suppressor of the positive diagnosis drug instead of doing nothing. A limitation of this implementation is that two separate automata are required, one to administer each drug. The entire process of evaluation until drug release took around an hour to complete. This method also requires transition molecules as well as the FokI enzyme to be present. The requirement for the FokI enzyme limits application in vivo, at least for use in ""cells of higher organisms"". It should also be pointed out that the 'software' molecules can be reused in this case."
+891,"DNA nanotechnology has been applied to the related field of DNA computing. DNA tiles can be designed to contain multiple sticky ends with sequences chosen so that they act as Wang tiles. A DX array has been demonstrated whose assembly encodes an XOR operation; this allows the DNA array to implement a cellular automaton which generates a fractal called the Sierpinski gasket. This shows that computation can be incorporated into the assembly of DNA arrays, increasing its scope beyond simple periodic arrays."
+892,"DNA computing is a form of parallel computing in that it takes advantage of the many different molecules of DNA to try many different possibilities at once. For certain specialized problems, DNA computers are faster and smaller than any other computer built so far. Furthermore, particular mathematical computations have been demonstrated to work on a DNA computer."
+893,"DNA computing does not provide any new capabilities from the standpoint of computability theory, the study of which problems are computationally solvable using different models of computation.
+For example,
+if the space required for the solution of a problem grows exponentially with the size of the problem  on von Neumann machines, it still grows exponentially with the size of the problem on DNA machines.
+For very large EXPSPACE problems, the amount of DNA required is too large to be practical."
+894,"A partnership between IBM and Caltech was established in 2009 aiming at ""DNA chips"" production. A Caltech group is working on the manufacturing of these nucleic-acid-based integrated circuits. One of these chips can compute whole square roots. A compiler has been written in Perl."
+895,"The slow processing speed of a DNA computer  is compensated by its potential to make a high amount of multiple parallel computations. This allows the system to take a similar amount of time for a complex calculation as for a simple one. This is achieved by the fact that millions or billions of molecules interact with each other simultaneously. However, it is much harder to analyze the answers given by a DNA computer than by a digital one."
+896,"Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. This approach appears to offer the best short-term prospects for commercial optical computing, since optical components could be integrated into traditional computers to produce an optical-electronic hybrid. However, optoelectronic devices consume 30% of their energy converting electronic energy into photons and back; this conversion also slows the transmission of messages. All-optical computers eliminate the need for optical-electrical-optical  conversions, thus reducing electrical power consumption."
+897,"Application-specific devices, such as synthetic-aperture radar  and optical correlators, have been designed to use the principles of optical computing. Correlators can be used, for example, to detect and track objects, and to classify serial time-domain optical data."
+898,"The fundamental building block of modern electronic computers is the transistor. To replace electronic components with optical ones, an equivalent optical transistor is required. This is achieved by crystal optics . In particular, materials exist where the intensity of incoming light affects the intensity of the light transmitted through the material in a similar manner to the current response of a bipolar transistor. Such an optical transistor can be used to create optical logic gates, which in turn are assembled into the higher level components of the computer's central processing unit . These will be nonlinear optical crystals used to manipulate light beams into controlling other light beams."
+899,"Like any computing system, an optical computing system needs three things to function well:"
+900,optical processor
+901,"optical data transfer, e.g. fiber-optic cable"
+902,"optical storage,"
+903,"Substituting electrical components will need data format conversion from photons to electrons, which will make the system slower."
+904,"There are some disagreements between researchers about the future capabilities of optical computers; whether or not they may be able to compete with semiconductor-based electronic computers in terms of speed, power consumption, cost, and size is an open question. Critics note that real-world logic systems require ""logic-level restoration, cascadability, fan-out and input–output isolation"", all of which are currently provided by electronic transistors at low cost, low power, and high speed. For optical logic to be competitive beyond a few niche applications, major breakthroughs in non-linear optical device technology would be required, or perhaps a change in the nature of computing itself."
+905,"A significant challenge to optical computing is that computation is a nonlinear process in which multiple signals must interact. Light, which is an electromagnetic wave, can only interact with another electromagnetic wave in the presence of electrons in a material, and the strength of this interaction is much weaker for electromagnetic waves, such as light, than for the electronic signals in a conventional computer. This may result in the processing elements for an optical computer requiring more power and larger dimensions than those for a conventional electronic computer using transistors."
+906,"A further misconception is that since light can travel much faster than the drift velocity of electrons, and at frequencies measured in THz, optical transistors should be capable of extremely high frequencies. However, any electromagnetic wave must obey the transform limit, and therefore the rate at which an optical transistor can respond to a signal is still limited by its spectral bandwidth. In fiber-optic communications, practical limits such as dispersion often constrain channels to bandwidths of tens of GHz, only slightly better than many silicon transistors. Obtaining dramatically faster operation than electronic transistors would therefore require practical methods of transmitting ultrashort pulses down highly dispersive waveguides."
+907,Photonic logic is the use of photons  in logic gates . Switching is obtained using nonlinear optical effects when two or more signals are combined.
+908,"Resonators are especially useful in photonic logic, since they allow a build-up of energy from constructive interference, thus enhancing optical nonlinear effects."
+909,"Other approaches that have been investigated include photonic logic at a molecular level, using photoluminescent chemicals. In a demonstration, Witlicki et al. performed logical operations using molecules and SERS."
+910,The basic idea is to delay light  in order to perform useful computations. Of interest would be to solve NP-complete problems as those are difficult problems for the conventional computers.
+911,There are two basic properties of light that are actually used in this approach:
+912,When solving a problem with time-delays the following steps must be followed:
+913,The first problem attacked in this way was the Hamiltonian path problem.
+914,"The simplest one is the subset sum problem. An optical device solving an instance with four numbers {a1, a2, a3, a4} is depicted below:"
+915,The travelling salesman problem has been solved by Shaked et al.  by using an optical approach. All possible TSP paths have been generated and stored in a binary matrix which was multiplied with another gray-scale vector containing the distances between cities. The multiplication is performed optically by using an optical correlator.
+916,"Many computations, particularly in scientific applications, require frequent use of the 2D discrete Fourier transform  – for example in solving differential equations describing propagation of waves or transfer of heat. Though modern GPU technologies typically enable high-speed computation of large 2D DFTs, techniques have been developed that can perform continuous Fourier transform optically by utilising the natural Fourier transforming property of lenses. The input is encoded using a liquid crystal spatial light modulator and the result is measured using a conventional CMOS or CCD image sensor. Such optical architectures can offer superior scaling of computational complexity due to the inherently highly interconnected nature of optical propagation, and have been used to solve 2D heat equations."
+917,Physical computers whose design was inspired by the theoretical Ising model are called Ising machines.
+918,"Yoshihisa Yamamoto's lab at Stanford pioneered building Ising machines using photons. Initially Yamamoto and his colleagues built an Ising machine using lasers, mirrors, and other optical components commonly found on an optical table."
+919,"Later a team at Hewlett Packard Labs developed photonic chip design tools and used them to build an Ising machine on a single chip, integrating 1,052 optical components on that single chip."
+920,"Some additional companies involved with optical computing development include IBM, Microsoft, Procyon Photonics, Lightelligence, Lightmatter, Optalysys, Xanadu Quantum Technologies, ORCA Computing, PsiQuantum, Quandela , and TundraSystems Global."
+921,Media related to Optical computing at Wikimedia Commons
+922,"A hard disk drive , hard disk, hard drive, or fixed disk, is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box."
+923,"Hard disk drives were introduced by IBM in 1956, and were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like mobile phones and tablets, rely on flash memory storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation, most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced  for servers. Though production is growing slowly , sales revenues and unit shipments are declining, because solid-state drives  have higher data-transfer rates, higher areal storage density, somewhat better reliability, and much lower latency and access times."
+924,"The revenues for SSDs, most of which use NAND flash memory, slightly exceeded those for HDDs in 2018. Flash storage products had more than twice the revenue of hard disk drives as of 2017. Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important. As of 2019, the cost per bit of SSDs is falling, and the price premium over HDDs has narrowed."
+925,"The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte  drive has a capacity of 1,000 gigabytes, where 1 gigabyte = 1 000 megabytes = 1 000 000 kilobytes  = 1 000 000 000 bytes . Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. There can be confusion regarding storage capacity, since capacities are stated in decimal gigabytes  by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified as the time required to move the heads to a track or cylinder , the time it takes for the desired sector to move under the head , and finally, the speed at which the data is transmitted ."
+926,"The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as SATA , USB, SAS , or PATA  cables."
+927,"The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two large refrigerators and stored five million six-bit characters  on a stack of 52 disks . The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set.  Variants of the IBM 350 were the IBM 355, IBM 7300 and IBM 1405."
+928,"Over time, as recording densities were greatly increased, further reductions in disk diameter to 3.5"" and 2.5"" were found to be optimum. Powerful rare earth magnet materials became affordable during this period, and were complementary to the swing arm actuator design to make possible the compact form factors of modern HDDs."
+929,"As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest computers."
+930,"Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter, internal HDDs proliferated on personal computers."
+931,"External HDDs remained popular for much longer on the Apple Macintosh. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays , so on those models, external SCSI disks were the only reasonable option for expanding upon any internal storage."
+932,"HDD improvements have been driven by increasing areal density, listed in the table above. Applications expanded through the 2000s, from the mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content."
+933,"In the 2000s and 2010s, NAND began supplanting HDDs in applications requiring portability or high performance. NAND performance is improving faster than HDDs, and applications for HDDs are eroding. In 2018, the largest hard drive had a capacity of 15 TB, while the largest capacity SSD had a capacity of 100 TB. As of 2018, HDDs were forecast to reach 100 TB capacities around 2025, but as of 2019, the expected pace of improvement was pared back to 50 TB by 2026. Smaller form factors, 1.8-inches and below, were discontinued around 2010. The cost of solid-state storage , represented by Moore's law, is improving faster than HDDs. NAND has a higher price elasticity of demand than HDDs, and this drives market growth. During the late 2000s and 2010s, the product life cycle of HDDs entered a mature phase, and slowing sales may indicate the onset of the declining phase."
+934,The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013.
+935,"In 2019, Western Digital closed its last Malaysian HDD factory due to decreasing demand, to focus on SSD production. All three remaining HDD manufacturers have had decreasing demand for their HDDs since 2014."
+936,"A modern HDD records data by magnetizing a thin film of ferromagnetic material on both sides of a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization.  User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions."
+937,"A typical HDD design consists of a spindle that holds flat circular disks, called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is 0.07–0.18 mm  thick."
+938,"The platters in contemporary HDDs are spun at speeds varying from 4,200 RPM in energy-efficient portable devices, to 15,000 rpm for high-performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. As of November 2019, the platters in most consumer-grade HDDs spin at 5,400 or 7,200 RPM."
+939,"Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying height often in the range of tens of nanometers. The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it."
+940,"In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm  moves the heads on an arc  across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or, in some older designs, a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track, but modern drives  use zone bit recording, increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones."
+941,"In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects⁠ ⁠— thermally induced magnetic instability which is commonly known as the ""superparamagnetic limit"". To counter this, the platters are coated with two parallel magnetic layers, separated by a three-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording , first shipped in 2005, and as of 2007, used in certain HDDs. Perpendicular recording may be accompanied by changes in the manufacturing of the read/write heads to increase the strength of the magnetic field created by the heads."
+942,"In 2004, a higher-density recording media was introduced, consisting of coupled soft and hard magnetic layers. So-called exchange spring media magnetic storage technology, also known as exchange coupled composite media, allows good writability due to the write-assist nature of the soft layer. However, the thermal stability is determined only by the hardest layer and not influenced by the soft layer."
+943,"The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead  then interact with the magnetic field of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head."
+944,The HDD's electronics controls the movement of the actuator and the rotation of the disk and transfers data to/from a disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles  or segments interspersed with real data . The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil motor to rotate the arm. A more modern servo system also employs milli and/or micro actuators to more accurately position the read/write heads. The spinning of the disks uses fluid-bearing spindle motors. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.
+945,"Modern drives make extensive use of error correction codes , particularly Reed–Solomon error correction. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity. For example, a typical 1 TB hard disk with 512-byte sectors provides additional capacity of about 93 GB for the ECC data."
+946,"In the newest drives, as of 2009, low-density parity-check codes  were supplanting Reed–Solomon; LDPC codes enable performance close to the Shannon limit and thus provide the highest storage density available."
+947,"Typical hard disk drives attempt to ""remap"" the data in a physical sector that is failing to a spare physical sector provided by the drive's ""spare sector pool"" , while relying on the ECC to recover stored data while the number of errors in a bad sector is still low enough. The S.M.A.R.T  feature counts the total number of errors in the entire HDD fixed by ECC , and the total number of performed sector remappings, as the occurrence of many such errors may predict an HDD failure."
+948,"The ""No-ID Format"", developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located."
+949,Only a tiny fraction of the detected errors end up as not correctable. Examples of specified uncorrected bit read error rates include:
+950,Within a given manufacturers model the uncorrected bit error rate is typically the same regardless of capacity of the drive.
+951,The worst type of errors are silent data corruptions which are errors undetected by the disk firmware or the host operating system; some of these errors may be caused by hard disk drive malfunctions while others originate elsewhere in the connection between the drive and the host.
+952,The World Wide Web  is an information system that enables content sharing over the Internet through user-friendly ways meant to appeal to users beyond IT specialists and hobbyists. It allows documents and other web resources to be accessed over the Internet according to specific rules of the Hypertext Transfer Protocol .
+953,"The Web was invented by English computer scientist Tim Berners-Lee while at CERN in 1989 and opened to the public in 1991. It was conceived as a ""universal linked information system"". Documents and other media content are made available to the network through web servers and can be accessed by programs such as web browsers. Servers and resources on the World Wide Web are identified and located through character strings called uniform resource locators ."
+954,"The original and still very common document type is a web page formatted in Hypertext Markup Language . This markup language supports plain text, images, embedded video and audio contents, and scripts  that implement complex user interaction. The HTML language also supports hyperlinks  which provide immediate access to other web resources. Web navigation, or web surfing, is the common practice of following such hyperlinks across multiple websites. Web applications are web pages that function as application software. The information in the Web is transferred across the Internet using HTTP. Multiple web resources with a common theme and usually a common domain name make up a website. A single web server may provide multiple websites, while some websites, especially the most popular ones, may be provided by multiple servers. Website content is provided by a myriad of companies, organizations, government agencies, and individual users; and comprises an enormous amount of educational, entertainment, commercial, and government information."
+955,The Web has become the world's dominant information systems platform. It is the primary tool that billions of people worldwide use to interact with the Internet.
+956,"The Web was invented by English computer scientist Tim Berners-Lee while working at CERN. He was motivated by the problem of storing, updating, and finding documents and data files in that large and constantly changing organization, as well as distributing them to collaborators outside CERN. In his design, Berners-Lee dismissed the common tree structure approach, used for instance in the existing CERNDOC documentation system and in the Unix filesystem, as well as approaches that relied in tagging files with keywords, as in the VAX/NOTES system. Instead he adopted concepts he had put into practice with his private ENQUIRE system  built at CERN. When he became aware of Ted Nelson's hypertext model , in which documents can be linked in unconstrained ways through hyperlinks associated with ""hot spots"" embedded in the text, it helped to confirm the validity of his concept."
+957,"The model was later popularized by Apple's HyperCard system. Unlike Hypercard, Berners-Lee's new system from the outset was meant to support links between multiple databases on independent computers, and to allow simultaneous access by many users from any computer on the Internet. He also specified that the system should eventually handle other media besides text, such as graphics, speech, and video. Links could refer to  mutable data files, or even fire up programs on their server computer. He also conceived ""gateways"" that would allow access through the new system to documents organized in other ways . Finally, he insisted that the system should be decentralized, without any central control or coordination over the creation of links."
+958,"Berners-Lee submitted a proposal to CERN in May 1989, without giving the system a name. He got a working system implemented by the end of 1990, including a browser called  WorldWideWeb  and an HTTP server running at CERN. As part of that development he defined the first version of the HTTP protocol, the basic URL syntax, and implicitly made HTML the primary document format. The technology was released outside CERN to other research institutions starting in January 1991, and then to the whole Internet on 23 August 1991. The Web was a success at CERN, and began to spread to other scientific and academic institutions. Within the next two years, there were 50 websites created."
+959,"CERN made the Web protocol and code available royalty free in 1993, enabling its widespread use. After the NCSA released the Mosaic web browser later that year, the Web's popularity grew rapidly as thousands of websites sprang up in less than a year. Mosaic was a graphical browser that could display inline images and submit forms that  were processed by the HTTPd server. Marc Andreessen and Jim Clark founded Netscape the following year and released the Navigator browser, which introduced Java and JavaScript to the Web. It quickly became the dominant browser. Netscape became a public company in 1995 which triggered a frenzy for the Web and started the dot-com bubble. Microsoft responded by developing its own browser, Internet Explorer, starting the browser wars. By bundling it with Windows, it became the dominant browser for 14 years."
+960,"Berners-Lee founded the World Wide Web Consortium  which created XML in 1996 and recommended replacing HTML with stricter XHTML. In the meantime, developers began exploiting an IE feature called XMLHttpRequest to make Ajax applications and launched the Web 2.0 revolution. Mozilla, Opera, and Apple rejected XHTML and created the WHATWG which developed HTML5. In 2009, the W3C conceded and abandoned XHTML. In 2019, it ceded control of the HTML specification to the WHATWG."
+961,The World Wide Web has been central to the development of the Information Age and is the primary tool billions of people use to interact on the Internet.
+962,"Tim Berners-Lee states that World Wide Web is officially spelled as three separate words, each capitalised, with no intervening hyphens. Nonetheless, it is often called simply the Web, and also often the web; see Capitalization of Internet for details. In Mandarin Chinese, World Wide Web is commonly translated via a phono-semantic matching to wàn wéi wǎng , which satisfies www and literally means ""10,000-dimensional net"", a translation that reflects the design concept and proliferation of the World Wide Web."
+963,"Use of the www prefix has been declining, especially when web applications sought to brand their domain names and make them easily pronounceable. As the mobile Web grew in popularity, services like Gmail.com, Outlook.com, Myspace.com, Facebook.com and Twitter.com are most often mentioned without adding ""www.""  to the domain."
+964,"In English, www is usually read as double-u double-u double-u. Some users pronounce it dub-dub-dub, particularly in New Zealand. Stephen Fry, in his ""Podgrams"" series of podcasts, pronounces it wuh wuh wuh. The English writer Douglas Adams once quipped in The Independent on Sunday : ""The World Wide Web is the only thing I know of whose shortened form takes three times longer to say than what it's short for""."
+965,"The terms Internet and World Wide Web are often used without much distinction. However, the two terms do not mean the same thing. The Internet is a global system of computer networks interconnected through telecommunications and optical networking. In contrast, the World Wide Web is a global collection of documents and other resources, linked by hyperlinks and URIs. Web resources are accessed using HTTP or HTTPS, which are application-level Internet protocols that use the Internet's transport protocols."
+966,"Viewing a web page on the World Wide Web normally begins either by typing the URL of the page into a web browser or by following a hyperlink to that page or resource. The web browser then initiates a series of background communication messages to fetch and display the requested page. In the 1990s, using a browser to view web pages—and to move from one web page to another through hyperlinks—came to be known as 'browsing,' 'web surfing' , or 'navigating the Web'. Early studies of this new behavior investigated user patterns in using web browsers. One study, for example, found five user patterns: exploratory surfing, window surfing, evolved surfing, bounded navigation and targeted navigation."
+967,The following example demonstrates the functioning of a web browser when accessing a page at the URL http://example.org/home.html. The browser resolves the server name of the URL  into an Internet Protocol address using the globally distributed Domain Name System . This lookup returns an IP address such as 203.0.113.4 or 2001:db8:2e::7334. The browser then requests the resource by sending an HTTP request across the Internet to the computer at that address. It requests service from a specific TCP port number that is well known for the HTTP service so that the receiving host can distinguish an HTTP request from other network protocols it may be servicing. HTTP normally uses port number 80 and for HTTPS it normally uses port number 443. The content of the HTTP request can be as simple as two lines of text:
+968,The computer receiving the HTTP request delivers it to web server software listening for requests on port 80. If the webserver can fulfil the request it sends an HTTP response back to the browser indicating success:
+969,followed by the content of the requested page. Hypertext Markup Language  for a basic web page might look like this:
+970,"The web browser parses the HTML and interprets the markup  that surrounds the words to format the text on the screen. Many web pages use HTML to reference the URLs of other resources such as images, other embedded media, scripts that affect page behaviour, and Cascading Style Sheets that affect page layout. The browser makes additional HTTP requests to the web server for these other Internet media types. As it receives their content from the web server, the browser progressively renders the page onto the screen as specified by its HTML and these additional resources."
+971,"Hypertext Markup Language  is the standard markup language for creating web pages and web applications. With Cascading Style Sheets  and JavaScript, it forms a triad of cornerstone technologies for the World Wide Web."
+972,Web browsers receive HTML documents from a web server or from local storage and render the documents into multimedia web pages. HTML describes the structure of a web page semantically and originally included cues for the appearance of the document.
+973,"HTML elements are the building blocks of HTML pages. With HTML constructs, images and other objects such as interactive forms may be embedded into the rendered page. HTML provides a means to create structured documents by denoting structural semantics for text such as headings, paragraphs, lists, links, quotes and other items. HTML elements are delineated by tags, written using angle brackets. Tags such as <img /> and <input /> directly introduce content into the page. Other tags such as <p> surround and provide information about document text and may include other tags as sub-elements. Browsers do not display the HTML tags, but use them to interpret the content of the page."
+974,"HTML can embed programs written in a scripting language such as JavaScript, which affects the behavior and content of web pages. Inclusion of CSS defines the look and layout of content. The World Wide Web Consortium , maintainer of both the HTML and the CSS standards, has encouraged the use of CSS over explicit presentational HTML since 1997."
+975,"Most web pages contain hyperlinks to other related pages and perhaps to downloadable files, source documents, definitions and other web resources. In the underlying HTML, a hyperlink looks like this:
+<a href=""http://example.org/home.html"">Example.org Homepage</a>."
+976,"Such a collection of useful, related resources, interconnected via hypertext links is dubbed a web of information. Publication on the Internet created what Tim Berners-Lee first called the WorldWideWeb  in November 1990."
+977,"The hyperlink structure of the web is described by the webgraph: the nodes of the web graph correspond to the web pages  the directed edges between them to the hyperlinks. Over time, many web resources pointed to by hyperlinks disappear, relocate, or are replaced with different content. This makes hyperlinks obsolete, a phenomenon referred to in some circles as link rot, and the hyperlinks affected by it are often called ""dead"" links. The ephemeral nature of the Web has prompted many efforts to archive websites. The Internet Archive, active since 1996, is the best known of such efforts."
+978,"Many hostnames used for the World Wide Web begin with www because of the long-standing practice of naming Internet hosts according to the services they provide. The hostname of a web server is often www, in the same way that it may be ftp for an FTP server, and news or nntp for a Usenet news server. These hostnames appear as Domain Name System  or subdomain names, as in www.example.com. The use of www is not required by any technical or policy standard and many web sites do not use it; the first web server was nxoc01.cern.ch. According to Paolo Palazzi, who worked at CERN along with Tim Berners-Lee, the popular use of www as subdomain was accidental; the World Wide Web project page was intended to be published at www.cern.ch while info.cern.ch was intended to be the CERN home page; however the DNS records were never switched, and the practice of prepending www to an institution's website domain name was subsequently copied. Many established websites still use the prefix, or they employ other subdomain names such as www2, secure or en for special purposes. Many such web servers are set up so that both the main domain name  and the www subdomain  refer to the same site; others require one form or the other, or they may map to different web sites. The use of a subdomain name is useful for load balancing incoming web traffic by creating a CNAME record that points to a cluster of web servers. Since, currently, only a subdomain can be used in a CNAME, the same result cannot be achieved by using the bare domain root."
+979,"When a user submits an incomplete domain name to a web browser in its address bar input field, some web browsers automatically try adding the prefix ""www"" to the beginning of it and possibly "".com"", "".org"" and "".net"" at the end, depending on what might be missing. For example, entering ""microsoft"" may be transformed to http://www.microsoft.com/ and ""openoffice"" to http://www.openoffice.org. This feature started appearing in early versions of Firefox, when it still had the working title 'Firebird' in early 2003, from an earlier practice in browsers such as Lynx.  It is reported that Microsoft was granted a US patent for the same idea in 2008, but only for mobile devices."
+980,"The scheme specifiers http:// and https:// at the start of a web URI refer to Hypertext Transfer Protocol or HTTP Secure, respectively. They specify the communication protocol to use for the request and response. The HTTP protocol is fundamental to the operation of the World Wide Web, and the added encryption layer in HTTPS is essential when browsers send or retrieve confidential data, such as passwords or banking information. Web browsers usually automatically prepend http:// to user-entered URIs, if omitted."
+981,A web page  is a document that is suitable for the World Wide Web and web browsers. A web browser displays a web page on a monitor or mobile device.
+982,"The term web page usually refers to what is visible, but may also refer to the contents of the computer file itself, which is usually a text file containing hypertext written in HTML or a comparable markup language. Typical web pages provide hypertext for browsing to other web pages via hyperlinks, often referred to as links. Web browsers will frequently have to access multiple web resource elements, such as reading style sheets, scripts, and images, while presenting each web page."
+983,"On a network, a web browser can retrieve a web page from a remote web server. The web server may restrict access to a private network such as a corporate intranet. The web browser uses the Hypertext Transfer Protocol  to make such requests to the web server."
+984,"A static web page is delivered exactly as stored, as web content in the web server's file system. In contrast, a dynamic web page is generated by a web application, usually driven by server-side software. Dynamic web pages are used when each user may require completely different information, for example, bank websites, web email etc."
+985,"A static web page  is a web page that is delivered to the user exactly as stored, in contrast to dynamic web pages which are generated by a web application."
+986,"Consequently, a static web page displays the same information for all users, from all contexts, subject to modern capabilities of a web server to negotiate content-type or language of the document where such versions are available and the server is configured to do so."
+987,"A server-side dynamic web page is a web page whose construction is controlled by an application server processing server-side scripts. In server-side scripting, parameters determine how the assembly of every new web page proceeds, including the setting up of more client-side processing."
+988,"A client-side dynamic web page processes the web page using JavaScript running in the browser. JavaScript programs can interact with the document via Document Object Model, or DOM, to query page state and alter it. The same client-side techniques can then dynamically update or change the DOM in the same way."
+989,"A dynamic web page is then reloaded by the user or by a computer program to change some variable content. The updating information could come from the server, or from changes made to that page's DOM. This may or may not truncate the browsing history or create a saved version to go back to, but a dynamic web page update using Ajax technologies will neither create a page to go back to nor truncate the web browsing history forward of the displayed page. Using Ajax technologies the end user gets one dynamic page managed as a single page in the web browser while the actual web content rendered on that page can vary. The Ajax engine sits only on the browser requesting parts of its DOM, the DOM, for its client, from an application server."
+990,"Dynamic HTML, or DHTML, is the umbrella term for technologies and methods used to create web pages that are not static web pages, though it has fallen out of common use since the popularization of AJAX, a term which is now itself rarely used. Client-side-scripting, server-side scripting, or a combination of these make for the dynamic web experience in a browser."
+991,"JavaScript is a scripting language that was initially developed in 1995 by Brendan Eich, then of Netscape, for use within web pages. The standardised version is ECMAScript. To make web pages more interactive, some web applications also use JavaScript techniques such as Ajax . Client-side script is delivered with the page that can make additional HTTP requests to the server, either in response to user actions such as mouse movements or clicks, or based on elapsed time. The server's responses are used to modify the current page rather than creating a new page with each response, so the server needs only to provide limited, incremental information. Multiple Ajax requests can be handled at the same time, and users can interact with the page while data is retrieved. Web pages may also regularly poll the server to check whether new information is available."
+992,"A website is a collection of related web resources including web pages, multimedia content, typically identified with a common domain name, and published on at least one web server. Notable examples are wikipedia.org, google.com, and amazon.com."
+993,"A website may be accessible via a public Internet Protocol  network, such as the Internet, or a private local area network , by referencing a uniform resource locator  that identifies the site."
+994,"Websites can have many functions and can be used in various fashions; a website can be a personal website, a corporate website for a company, a government website, an organization website, etc. Websites are typically dedicated to a particular topic or purpose, ranging from entertainment and social networking to providing news and education. All publicly accessible websites collectively constitute the World Wide Web, while private websites, such as a company's website for its employees, are typically a part of an intranet."
+995,"Web pages, which are the building blocks of websites, are documents, typically composed in plain text interspersed with formatting instructions of Hypertext Markup Language . They may incorporate elements from other websites with suitable markup anchors. Web pages are accessed and transported with the Hypertext Transfer Protocol , which may optionally employ encryption  to provide security and privacy for the user. The user's application, often a web browser, renders the page content according to its HTML markup instructions onto a display terminal."
+996,"Hyperlinking between web pages conveys to the reader the site structure and guides the navigation of the site, which often starts with a home page containing a directory of the site web content. Some websites require user registration or subscription to access content. Examples of subscription websites include many business sites, news websites, academic journal websites, gaming websites, file-sharing websites, message boards, web-based email, social networking websites, websites providing real-time price quotations for different types of markets, as well as sites providing various other services. End users can access websites on a range of devices, including desktop and laptop computers, tablet computers, smartphones and smart TVs."
+997,"A web browser  is a software user agent for accessing information on the World Wide Web. To connect to a website's server and display its pages, a user needs to have a web browser program. This is the program that the user runs to download, format, and display a web page on the user's computer."
+998,"In addition to allowing users to find, display, and move between web pages, a web browser will usually have features like keeping bookmarks, recording history, managing cookies , and home pages and may have facilities for recording passwords for logging into web sites."
+999,"The most popular browsers are Chrome, Firefox, Safari, Internet Explorer, and Edge."
+1000,"A Web server is server software, or hardware dedicated to running said software, that can satisfy World Wide Web client requests. A web server can, in general, contain one or more websites. A web server processes incoming network requests over HTTP and several other related protocols."
+1001,"The primary function of a web server is to store, process and deliver web pages to clients. The communication between client and server takes place using the Hypertext Transfer Protocol . Pages delivered are most frequently HTML documents, which may include images, style sheets and scripts in addition to the text content."
+1002,"A user agent, commonly a web browser or web crawler, initiates communication by making a request for a specific resource using HTTP and the server responds with the content of that resource or an error message if unable to do so. The resource is typically a real file on the server's secondary storage, but this is not necessarily the case and depends on how the webserver is implemented."
+1003,"While the primary function is to serve content, full implementation of HTTP also includes ways of receiving content from clients. This feature is used for submitting web forms, including uploading of files."
+1004,"Many generic web servers also support server-side scripting using Active Server Pages , PHP , or other scripting languages. This means that the behavior of the webserver can be scripted in separate files, while the actual server software remains unchanged. Usually, this function is used to generate HTML documents dynamically  as opposed to returning static documents. The former is primarily used for retrieving or modifying information from databases. The latter is typically much faster and more easily cached but cannot deliver dynamic content."
+1005,"Web servers can also frequently be found embedded in devices such as printers, routers, webcams and serving only a local network. The web server may then be used as a part of a system for monitoring or administering the device in question. This usually means that no additional software has to be installed on the client computer since only a web browser is required ."
+1006,"An HTTP cookie  is a small piece of data sent from a website and stored on the user's computer by the user's web browser while the user is browsing. Cookies were designed to be a reliable mechanism for websites to remember stateful information  or to record the user's browsing activity . They can also be used to remember arbitrary pieces of information that the user previously entered into form fields such as names, addresses, passwords, and credit card numbers."
+1007,"Cookies perform essential functions in the modern web. Perhaps most importantly, authentication cookies are the most common method used by web servers to know whether the user is logged in or not, and which account they are logged in with. Without such a mechanism, the site would not know whether to send a page containing sensitive information or require the user to authenticate themselves by logging in. The security of an authentication cookie generally depends on the security of the issuing website and the user's web browser, and on whether the cookie data is encrypted. Security vulnerabilities may allow a cookie's data to be read by a hacker, used to gain access to user data, or used to gain access  to the website to which the cookie belongs ."
+1008,"Tracking cookies, and especially third-party tracking cookies, are commonly used as ways to compile long-term records of individuals' browsing histories – a potential privacy concern that prompted European and U.S. lawmakers to take action in 2011. European law requires that all websites targeting European Union member states gain ""informed consent"" from users before storing non-essential cookies on their device."
+1009,"Google Project Zero researcher Jann Horn describes ways cookies can be read by intermediaries, like Wi-Fi hotspot providers. He recommends using the browser in incognito mode in such circumstances."
+1010,"A web search engine or Internet search engine is a software system that is designed to carry out web search , which means to search the World Wide Web in a systematic way for particular information specified in a web search query. The search results are generally presented in a line of results, often referred to as search engine results pages . The information may be a mix of web pages, images, videos, infographics, articles, research papers, and other types of files. Some search engines also mine data available in databases or open directories. Unlike web directories, which are maintained only by human editors, search engines also maintain real-time information by running an algorithm on a web crawler. Internet content that is not capable of being searched by a web search engine is generally described as the deep web."
+1011,"The deep web, invisible web, or hidden web are parts of the World Wide Web whose contents are not indexed by standard web search engines. The opposite term to the deep web is the surface web, which is accessible to anyone using the Internet. Computer scientist Michael K. Bergman is credited with coining the term deep web in 2001 as a search indexing term."
+1012,"The content of the deep web is hidden behind HTTP forms, and includes many very common uses such as web mail, online banking, and services that users must pay for, and which is protected by a paywall, such as video on demand, some online magazines and newspapers, among others."
+1013,The content of the deep web can be located and accessed by a direct URL or IP address and may require a password or other security access past the public website page.
+1014,"A web cache is a server computer located either on the public Internet or within an enterprise that stores recently accessed web pages to improve response time for users when the same content is requested within a certain time after the original request. Most web browsers also implement a browser cache by writing recently obtained data to a local data storage device. HTTP requests by a browser may ask only for data that has changed since the last access. Web pages and resources may contain expiration information to control caching to secure sensitive data, such as in online banking, or to facilitate frequently updated sites, such as news media. Even sites with highly dynamic content may permit basic resources to be refreshed only occasionally. Web site designers find it worthwhile to collate resources such as CSS data and JavaScript into a few site-wide files so that they can be cached efficiently. Enterprise firewalls often cache Web resources requested by one user for the benefit of many users. Some search engines store cached content of frequently accessed websites."
+1015,"For criminals, the Web has become a venue to spread malware and engage in a range of cybercrimes, including  identity theft, fraud, espionage and intelligence gathering. Web-based vulnerabilities now outnumber traditional computer security concerns, and as measured by Google, about one in ten web pages may contain malicious code. Most web-based attacks take place on legitimate websites, and most, as measured by Sophos, are hosted in the United States, China and Russia. The most common of all malware threats is SQL injection attacks against websites. Through HTML and URIs, the Web was vulnerable to attacks like cross-site scripting  that came with the introduction of JavaScript and were exacerbated to some degree by Web 2.0 and Ajax web design that favours the use of scripts. Today by one estimate, 70% of all websites are open to XSS attacks on their users. Phishing is another common threat to the Web. In February 2013, RSA  estimated the global losses from phishing at $1.5 billion in 2012. Two of the well-known phishing methods are Covert Redirect and Open Redirect."
+1016,"Proposed solutions vary. Large security companies like McAfee already design governance and compliance suites to meet post-9/11 regulations, and some, like Finjan have recommended active real-time inspection of programming code and all content regardless of its source. Some have argued that for enterprises to see Web security as a business opportunity rather than a cost centre, while others call for ""ubiquitous, always-on digital rights management"" enforced in the infrastructure to replace the hundreds of companies that secure data and networks. Jonathan Zittrain has said users sharing responsibility for computing safety is far preferable to locking down the Internet."
+1017,"Every time a client requests a web page, the server can identify the request's IP address. Web servers usually log IP addresses in a log file. Also, unless set not to do so, most web browsers record requested web pages in a viewable history feature, and usually cache much of the content locally. Unless the server-browser communication uses HTTPS encryption, web requests and responses travel in plain text across the Internet and can be viewed, recorded, and cached by intermediate systems. Another way to hide personally identifiable information is by using a virtual private network. A VPN encrypts online traffic and masks the original IP address lowering the chance of user identification."
+1018,"When a web page asks for, and the user supplies, personally identifiable information—such as their real name, address, e-mail address, etc. web-based entities can associate current web traffic with that individual. If the website uses HTTP cookies, username, and password authentication, or other tracking techniques, it can relate other web visits, before and after, to the identifiable information provided. In this way, a web-based organization can develop and build a profile of the individual people who use its site or sites. It may be able to build a record for an individual that includes information about their leisure activities, their shopping interests, their profession, and other aspects of their demographic profile. These profiles are of potential interest to marketers, advertisers, and others. Depending on the website's terms and conditions and the local laws that apply information from these profiles may be sold, shared, or passed to other organizations without the user being informed. For many ordinary people, this means little more than some unexpected e-mails in their in-box or some uncannily relevant advertising on a future web page. For others, it can mean that time spent indulging an unusual interest can result in a deluge of further targeted marketing that may be unwelcome. Law enforcement, counterterrorism, and espionage agencies can also identify, target, and track individuals based on their interests or proclivities on the Web."
+1019,"Social networking sites usually try to get users to use their real names, interests, and locations, rather than pseudonyms, as their executives believe that this makes the social networking experience more engaging for users. On the other hand, uploaded photographs or unguarded statements can be identified to an individual, who may regret this exposure. Employers, schools, parents, and other relatives may be influenced by aspects of social networking profiles, such as text posts or digital photos, that the posting individual did not intend for these audiences. Online bullies may make use of personal information to harass or stalk users. Modern social networking websites allow fine-grained control of the privacy settings for each posting, but these can be complex and not easy to find or use, especially for beginners. Photographs and videos posted onto websites have caused particular problems, as they can add a person's face to an online profile. With modern and potential facial recognition technology, it may then be possible to relate that face with other, previously anonymous, images, events, and scenarios that have been imaged elsewhere. Due to image caching, mirroring, and copying, it is difficult to remove an image from the World Wide Web."
+1020,"Web standards include many interdependent standards and specifications, some of which govern aspects of the Internet, not just the World Wide Web. Even when not web-focused, such standards directly or indirectly affect the development and administration of websites and web services. Considerations include the interoperability, accessibility and usability of web pages and web sites."
+1021,"Web standards, in the broader sense, consist of the following:"
+1022,Web standards are not fixed sets of rules but are constantly evolving sets of finalized technical specifications of web technologies. Web standards are developed by standards organizations—groups of interested and often competing parties chartered with the task of standardization—not technologies developed and declared to be a standard by a single individual or company. It is crucial to distinguish those specifications that are under development from the ones that already reached the final development status .
+1023,"There are methods for accessing the Web in alternative mediums and formats to facilitate use by individuals with disabilities. These disabilities may be visual, auditory, physical, speech-related, cognitive, neurological, or some combination. Accessibility features also help people with temporary disabilities, like a broken arm, or ageing users as their abilities change. The Web is receiving information as well as providing information and interacting with society. The World Wide Web Consortium claims that it is essential that the Web be accessible, so it can provide equal access and equal opportunity to people with disabilities. Tim Berners-Lee once noted, ""The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect."" Many countries regulate web accessibility as a requirement for websites. International co-operation in the W3C Web Accessibility Initiative led to simple guidelines that web content authors as well as software developers can use to make the Web accessible to persons who may or may not be using assistive technology."
+1024,"The W3C Internationalisation Activity assures that web technology works in all languages, scripts, and cultures. Beginning in 2004 or 2005, Unicode gained ground and eventually in December 2007 surpassed both ASCII and Western European as the Web's most frequently used character encoding. Originally RFC 3986 allowed resources to be identified by URI in a subset of US-ASCII. RFC 3987 allows more characters—any character in the Universal Character Set—and now a resource can be identified by IRI in any language."
+1025,The Advanced Research Projects Agency Network  was the first wide-area packet-switched network with distributed control and one of the first computer networks to implement the TCP/IP protocol suite. Both technologies became the technical foundation of the Internet. The ARPANET was established by the Advanced Research Projects Agency  of the United States Department of Defense.
+1026,"Building on the ideas of J. C. R. Licklider, Bob Taylor initiated the ARPANET project in 1966 to enable resource sharing between remote computers. Taylor appointed Larry Roberts as program manager. Roberts made the key decisions about the request for proposal to build the network. He incorporated Donald Davies' concepts and designs for packet switching, and sought input from Paul Baran. ARPA awarded the contract to build the network to Bolt Beranek & Newman. The design was led by Bob Kahn who developed the first protocol for the network. Roberts engaged Leonard Kleinrock at UCLA to develop mathematical methods for analyzing the packet network technology."
+1027,"The first computers were connected in 1969 and the Network Control Protocol was implemented in 1970, development of which was led by Steve Crocker at UCLA and other graduate students, including Jon Postel and Vint Cerf. The network was declared operational in 1971. Further software development enabled remote login and file transfer, which was used to provide an early form of email. The network expanded rapidly and operational control passed to the Defense Communications Agency in 1975."
+1028,"Bob Kahn moved to DARPA and, together with Vint Cerf at Stanford University, formulated the Transmission Control Program, which incorporated concepts from the French CYCLADES project directed by Louis Pouzin. As this work progressed, a protocol was developed by which multiple separate networks could be joined into a network of networks. Version 4 of TCP/IP was installed in the ARPANET for production use in January 1983 after the Department of Defense made it standard for all military computer networking."
+1029,"Access to the ARPANET was expanded in 1981 when the National Science Foundation  funded the Computer Science Network . In the early 1980s, the NSF funded the establishment of national supercomputing centers at several universities and provided network access and network interconnectivity with the NSFNET project in 1986. The ARPANET was formally decommissioned in 1990, after partnerships with the telecommunication and computer industry had assured private sector expansion and commercialization of an expanded worldwide network, known as the Internet."
+1030,"Historically, voice and data communications were based on methods of circuit switching, as exemplified in the traditional telephone network, wherein each telephone call is allocated a dedicated end-to-end electronic connection between the two communicating stations. The connection is established by switching systems that connected multiple intermediate call legs between these systems for the duration of the call."
+1031,"The traditional model of the circuit-switched telecommunication network was challenged in the early 1960s by Paul Baran at the RAND Corporation, who had been researching systems that could sustain operation during partial destruction, such as by nuclear war. He developed the theoretical model of distributed adaptive message block switching. However, the telecommunication establishment rejected the development in favor of existing models. Donald Davies at the United Kingdom's National Physical Laboratory  independently arrived at a similar concept in 1965."
+1032,"The earliest ideas for a computer network intended to allow general communications among computer users were formulated by computer scientist J. C. R. Licklider of Bolt Beranek and Newman , in April 1963, in memoranda discussing the concept of the ""Intergalactic Computer Network"". Those ideas encompassed many of the features of the contemporary Internet. In October 1963, Licklider was appointed head of the Behavioral Sciences and Command and Control programs at the Defense Department's Advanced Research Projects Agency . He convinced Ivan Sutherland and Bob Taylor that this network concept was very important and merited development, although Licklider left ARPA before any contracts were assigned for development."
+1033,"Sutherland and Taylor continued their interest in creating the network, in part, to allow ARPA-sponsored researchers at various corporate and academic locales to utilize computers provided by ARPA, and, in part, to quickly distribute new software and other computer science results. Taylor had three computer terminals in his office, each connected to separate computers, which ARPA was funding: one for the System Development Corporation  Q-32 in Santa Monica, one for Project Genie at the University of California, Berkeley, and another for Multics at the Massachusetts Institute of Technology. Taylor recalls the circumstance: ""For each of these three terminals, I had three different sets of user commands. So, if I was talking online with someone at S.D.C., and I wanted to talk to someone I knew at Berkeley, or M.I.T., about this, I had to get up from the S.D.C. terminal, go over and log into the other terminal and get in touch with them. I said, 'Oh Man!', it's obvious what to do: If you have these three terminals, there ought to be one terminal that goes anywhere you want to go. That idea is the ARPANET""."
+1034,"Donald Davies' work caught the attention of ARPANET developers at Symposium on Operating Systems Principles in October 1967. He gave the first public presentation, having coined the term packet switching, in August 1968 and incorporated it into the NPL network in England. The NPL network and ARPANET were the first two networks in the world to use packet switching. Roberts said the packet switching networks built in the 1970s were similar ""in nearly all respects"" to Davies' original 1965 design."
+1035,"In February 1966, Bob Taylor successfully lobbied ARPA's Director Charles M. Herzfeld to fund a network project. Herzfeld redirected funds in the amount of one million dollars from a ballistic missile defense program to Taylor's budget. Taylor hired Larry Roberts as a program manager in the ARPA Information Processing Techniques Office in January 1967 to work on the ARPANET. Roberts met Paul Baran in February 1967, but did not discuss networks."
+1036,"Roberts asked Frank Westervelt to explore the initial design questions for a network. In April 1967, ARPA held a design session on technical standards. The initial standards for identification and authentication of users, transmission of characters, and error checking and retransmission procedures were discussed. Roberts' proposal was that all mainframe computers would connect to one another directly. The other investigators were reluctant to dedicate these computing resources to network administration. Wesley Clark proposed minicomputers should be used as an interface to create a message switching network. Roberts modified the ARPANET plan to incorporate Clark's suggestion and named the minicomputers Interface Message Processors ."
+1037,"The plan was presented at the inaugural Symposium on Operating Systems Principles in October 1967. Donald Davies' work on packet switching and the NPL network, presented by a colleague , came to the attention of the ARPA investigators at this conference. Roberts applied Davies' concept of packet switching for the ARPANET, and sought input from Paul Baran. The NPL network was using line speeds of 768 kbit/s, and the proposed line speed for the ARPANET was upgraded from 2.4 kbit/s to 50 kbit/s."
+1038,"By mid-1968, Roberts and Barry Wessler wrote a final version of the IMP specification based on a Stanford Research Institute  report that ARPA commissioned to write detailed specifications describing the ARPANET communications network. Roberts gave a report to Taylor on 3 June, who approved it on 21 June. After approval by ARPA, a Request for Quotation  was issued for 140 potential bidders. Most computer science companies regarded the ARPA proposal as outlandish, and only twelve submitted bids to build a network; of the twelve, ARPA regarded only four as top-rank contractors. At year's end, ARPA considered only two contractors and awarded the contract to build the network to BBN in January 1969."
+1039,"The initial, seven-person BBN team were much aided by the technical specificity of their response to the ARPA RFQ, and thus quickly produced the first working system. The ""IMP guys"" were led by Frank Heart and the theoretical design of the network was led by Bob Kahn; the team included Dave Walden, Severo Omstein, William Crowther and several others. The BBN-proposed network closely followed Roberts' ARPA plan: a network composed of small computers, the IMPs , that functioned as gateways interconnecting local resources. Routing, flow control, software design and network control were developed by the BBN IMP team. At each site, the IMPs performed store-and-forward packet switching functions and were interconnected with leased lines via telecommunication data sets , with initial data rates of 56kbit/s. The host computers were connected to the IMPs via custom serial communication interfaces. The system, including the hardware and the packet switching software, was designed and installed in nine months. The BBN team continued to interact with the NPL team with meetings between them taking place in the U.S. and the U.K."
+1040,"The first-generation IMPs were built by BBN Technologies using a rugged computer version of the Honeywell DDP-516 computer, configured with 24KB of expandable magnetic-core memory, and a 16-channel Direct Multiplex Control  direct memory access unit. The DMC established custom interfaces with each of the host computers and modems. In addition to the front-panel lamps, the DDP-516 computer also features a special set of 24 indicator lamps showing the status of the IMP communication channels. Each IMP could support up to four local hosts and could communicate with up to six remote IMPs via early Digital Signal 0 leased telephone lines. The network connected one computer in Utah with three in California. Later, the Department of Defense allowed the universities to join the network for sharing hardware and software resources."
+1041,"According to Charles Herzfeld, ARPA Director :"
+1042,"According to Stephen J. Lukasik, who as deputy director  and Director of DARPA  was the person who signed most of the checks for Arpanet's development, the goal was ""command and control"":"
+1043,"The ARPANET did use distributed computation, and incorporated frequent re-computation of routing tables . These features increased the survivability of the network in the event of significant interruption. Furthermore, the ARPANET was designed to survive subordinate-network losses. However, the Internet Society agrees with Herzfeld in a footnote in their online article, A Brief History of the Internet:"
+1044,"Paul Baran, the first to put forward a theoretical model for communication using packet switching, conducted the RAND study referenced above. Though the ARPANET did not exactly share Baran's project's goal, he said his work did contribute to the development of the ARPANET. Minutes taken by Elmer Shapiro of Stanford Research Institute at the ARPANET design meeting of 9–10 October 1967 indicate that a version of Baran's routing method  may be used, consistent with the NPL team's proposal at the Symposium on Operating System Principles in Gatlinburg."
+1045,"The first four nodes were designated as a testbed for developing and debugging the 1822 protocol, which was a major undertaking. While they were connected electronically in 1969, network applications were not possible until the Network Control Protocol was implemented in 1970 enabling the first two host-host protocols, remote login  and file transfer  which were specified and implemented between 1969 and 1973. The network was declared operational in 1971. Network traffic began to grow once email was established at the majority of sites by around 1973."
+1046,The first four IMPs were:
+1047,"The first successful host-to-host connection on the ARPANET was made between Stanford Research Institute  and UCLA, by SRI programmer Bill Duvall and UCLA student programmer Charley Kline, at 10:30 pm PST on 29 October 1969 . Kline connected from UCLA's SDS Sigma 7 Host computer  to the Stanford Research Institute's SDS 940 Host computer. Kline typed the command ""login,"" but initially the SDS 940 crashed after he typed two characters. About an hour later, after Duvall adjusted parameters on the machine, Kline tried again and successfully logged in. Hence, the first two characters successfully transmitted over the ARPANET were ""lo"". The first permanent ARPANET link was established on 21 November 1969, between the IMP at UCLA and the IMP at the Stanford Research Institute. By 5 December 1969, the initial four-node network was established."
+1048,"Elizabeth Feinler created the first Resource Handbook for ARPANET in 1969 which led to the development of the ARPANET directory. The directory, built by Feinler and a team made it possible to navigate the ARPANET."
+1049,"Roberts engaged Howard Frank to consult on the topological design of the network. Frank made recommendations to increase throughput and reduce costs in a scaled-up network. By March 1970, the ARPANET reached the East Coast of the United States, when an IMP at BBN in Cambridge, Massachusetts was connected to the network. Thereafter, the ARPANET grew: 9 IMPs by June 1970 and 13 IMPs by December 1970, then 18 by September 1971 ; 29 IMPs by August 1972, and 40 by September 1973. By June 1974, there were 46 IMPs, and in July 1975, the network numbered 57 IMPs. By 1981, the number was 213 host computers, with another host connecting approximately every twenty days."
+1050,"Support for inter-IMP circuits of up to 230.4 kbit/s was added in 1970, although considerations of cost and IMP processing power meant this capability was not actively used."
+1051,"Larry Roberts saw the ARPANET and NPL projects as complementary and sought in 1970 to connect them via a satellite link. Peter Kirstein's research group at University College London  was subsequently chosen in 1971 in place of NPL for the UK connection. In June 1973, a transatlantic satellite link connected ARPANET to the Norwegian Seismic Array , via the Tanum Earth Station in Sweden, and onward via a terrestrial circuit to a TIP at UCL. UCL provided a gateway for interconnection of the ARPANET with British academic networks, the first international resource sharing network, and carried out some of the earliest experimental research work on internetworking."
+1052,"1971 saw the start of the use of the non-ruggedized  Honeywell 316 as an IMP.
+It could also be configured as a Terminal Interface Processor , which provided terminal server support for up to 63 ASCII serial terminals through a multi-line controller in place of one of the hosts. The 316 featured a greater degree of integration than the 516, which made it less expensive and easier to maintain. The 316 was configured with 40 kB of core memory for a TIP. The size of core memory was later increased, to 32 kB for the IMPs, and 56 kB for TIPs, in 1973."
+1053,The ARPANET was demonstrated at the International Conference on Computer Communications in October 1972.
+1054,"In 1975, BBN introduced IMP software running on the Pluribus multi-processor. These appeared in a few sites. In 1981, BBN introduced IMP software running on its own C/30 processor product."
+1055,"In 1968, Roberts contracted with Kleinrock to measure the performance of the network and find areas for improvement. Building on his earlier work on queueing theory, Kleinrock specified mathematical models of the performance of packet-switched networks, which underpinned the development of the ARPANET as it expanded rapidly in the early 1970s."
+1056,"ARPA was intended to fund advanced research. The ARPANET was a research project that was communications-oriented, rather than user-oriented in design. Nonetheless, in the summer of 1975, operational control of the ARPANET passed to the Defense Communications Agency. At about this time, the first ARPANET encryption devices were deployed to support classified traffic."
+1057,"The ARPANET Completion Report,  published in 1981 jointly by BBN and DARPA, concludes that:"
+1058,Access to the ARPANET was expanded in 1981 when the National Science Foundation  funded the Computer Science Network .
+1059,"The transatlantic connectivity with NORSAR and UCL later evolved into the SATNET. The ARPANET, SATNET and PRNET were interconnected in 1977."
+1060,The DoD made TCP/IP the standard communication protocol for all military computer networking in 1980. NORSAR and University College London left the ARPANET and began using TCP/IP over SATNET in early 1982.
+1061,"On January 1, 1983, known as flag day, TCP/IP protocols became the standard for the ARPANET, replacing the earlier Network Control Protocol."
+1062,In September 1984 work was completed on restructuring the ARPANET giving U.S. military sites their own Military Network  for unclassified defense department communications. Both networks carried unclassified information and were connected at a small number of controlled gateways which would allow total separation in the event of an emergency. MILNET was part of the Defense Data Network .
+1063,"Separating the civil and military networks reduced the 113-node ARPANET by 68 nodes. After MILNET was split away, the ARPANET would continue to be used as an Internet backbone for researchers, but be slowly phased out."
+1064,"In 1985, the NSF funded the establishment of national supercomputing centers at several universities and provided network access and network interconnectivity with the NSFNET project in 1986. NSFNET became the Internet backbone for government agencies and universities."
+1065,"The ARPANET project was formally decommissioned in 1990. The original IMPs and TIPs were phased out as the ARPANET was shut down after the introduction of the NSFNet, but some IMPs remained in service as late as July 1990."
+1066,"In the wake of the decommissioning of the ARPANET on 28 February 1990, Vinton Cerf wrote the following lamentation, entitled ""Requiem of the ARPANET"":"
+1067,#NAME?
+1068,"The technological advancements and practical applications achieved through the ARPANET were instrumental in shaping modern computer networking including the Internet. Development and implementation of the concepts of packet switching, decentralized communication, and the development of protocols like TCP/IP laid the foundation for a global network that revolutionized communication, information sharing and collaborative research across the world."
+1069,"The ARPANET was related to many other research projects, which either influenced the ARPANET design, or which were ancillary projects or spun out of the ARPANET."
+1070,"Senator Al Gore authored the High Performance Computing and Communication Act of 1991, commonly referred to as ""The Gore Bill"", after hearing the 1988 concept for a National Research Network submitted to Congress by a group chaired by Leonard Kleinrock. The bill was passed on 9 December 1991 and led to the National Information Infrastructure  which Gore called the information superhighway."
+1071,"The ARPANET project was honored with two IEEE Milestones, both dedicated in 2009."
+1072,"Because it was never a goal for the ARPANET to support IMPs from vendors other than BBN, the IMP-to-IMP protocol and message format were not standardized. However, the IMPs did nonetheless communicate amongst themselves to perform link-state routing, to do reliable forwarding of messages, and to provide remote monitoring and management functions to ARPANET's Network Control Center. Initially, each IMP had a 6-bit identifier and supported up to 4 hosts, which were identified with a 2-bit index. An ARPANET host address, therefore, consisted of both the port index on its IMP and the identifier of the IMP, which was written with either port/IMP notation or as a single byte; for example, the address of MIT-DMG  could be written as either 1/6 or 70. An upgrade in early 1976 extended the host and IMP numbering to 8-bit and 16-bit, respectively."
+1073,"In addition to primary routing and forwarding responsibilities, the IMP ran several background programs, titled TTY, DEBUG, PARAMETER-CHANGE, DISCARD, TRACE, and STATISTICS. These were given host numbers in order to be addressed directly and provided functions independently of any connected host. For example, ""TTY"" allowed an on-site operator to send ARPANET packets manually via the teletype connected directly to the IMP."
+1074,"The starting point for host-to-host communication on the ARPANET in 1969 was the 1822 protocol, which defined the transmission of messages to an IMP. The message format was designed to work unambiguously with a broad range of computer architectures. An 1822 message essentially consisted of a message type, a numeric host address, and a data field. To send a data message to another host, the transmitting host formatted a data message containing the destination host's address and the data message being sent, and then transmitted the message through the 1822 hardware interface. The IMP then delivered the message to its destination address, either by delivering it to a locally connected host, or by delivering it to another IMP. When the message was ultimately delivered to the destination host, the receiving IMP would transmit a Ready for Next Message  acknowledgment to the sending, host IMP."
+1075,"Unlike modern Internet datagrams, the ARPANET was designed to reliably transmit 1822 messages, and to inform the host computer when it loses a message; the contemporary IP is unreliable, whereas the TCP is reliable. Nonetheless, the 1822 protocol proved inadequate for handling multiple connections among different applications residing in a host computer. This problem was addressed with the Network Control Protocol , which provided a standard method to establish reliable, flow-controlled, bidirectional communications links among different processes in different host computers. The NCP interface allowed application software to connect across the ARPANET by implementing higher-level communication protocols, an early example of the protocol layering concept later incorporated in the OSI model."
+1076,"NCP was developed under the leadership of Steve Crocker, then a graduate student at UCLA. Crocker created and led the Network Working Group  which was made up of a collection of graduate students at universities and research laboratories, including Jon Postel and Vint Cerf at UCLA. They were sponsored by ARPA to carry out the development of the ARPANET and the software for the host computers that supported applications."
+1077,"NCP provided a standard set of network services that could be shared by several applications running on a single host computer. This led to the evolution of application protocols that operated, more or less, independently of the underlying network service, and permitted independent advances in the underlying protocols."
+1078,"The various application protocols such as TELNET for remote time-sharing access and File Transfer Protocol , the latter used to enable rudimentary electronic mail, were developed and eventually ported to run over the TCP/IP protocol suite. In the 1980s, FTP for email was replaced by the Simple Mail Transfer Protocol and, later, POP and IMAP."
+1079,"Telnet was developed in 1969 beginning with RFC 15, extended in RFC 855."
+1080,"The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971. By 1973, the File Transfer Protocol  specification had been defined  and implemented, enabling file transfers over the ARPANET."
+1081,"In 1971, Ray Tomlinson, of BBN sent the first network e-mail . An ARPA study in 1973, a year after network e-mail was introduced to the ARPANET community, found that three-quarters of the traffic over the ARPANET consisted of email messages. E-mail remained a very large part of the overall ARPANET traffic."
+1082,"The Network Voice Protocol  specifications were defined in 1977 , and implemented. But, because of technical shortcomings, conference calls over the ARPANET never worked well; the contemporary Voice over Internet Protocol  was decades away."
+1083,"Stephen J. Lukasik directed DARPA to focus on internetworking research in the early 1970s. Bob Kahn moved from BBN to DARPA in 1972, first as program manager for the ARPANET, under Larry Roberts, then as director of the IPTO when Roberts left to found Telenet. Kahn worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across both. Vint Cerf joined the International Networking Working Group in 1972 and became its Chair. This group considered how to interconnect packet switching networks with different specifications, that is, internetworking. Research led by Bob Kahn at DARPA and Vint Cerf at Stanford University resulted in the formulation of the Transmission Control Program, which incorporated concepts from the French CYCLADES project directed by Louis Pouzin. Its specification was written by Cerf with Yogen Dalal and Carl Sunshine at Stanford in December 1974 . The following year, testing began through concurrent implementations at Stanford, BBN and University College London. At first a monolithic design, the software was redesigned as a modular protocol stack in version 3 in 1978. Version 4 was installed in the ARPANET for production use in January 1983, replacing NCP.  The development of the complete Internet protocol suite by 1989, as outlined in RFC 1122 and RFC 1123, and partnerships with the telecommunication and computer industry laid the foundation for the adoption of TCP/IP as a comprehensive protocol suite as the core component of the emerging Internet."
+1084,"The Purdy Polynomial hash algorithm was developed for the ARPANET to protect passwords in 1971 at the request of Larry Roberts, head of ARPA at that time. It computed a polynomial of degree 224 + 17 modulo the 64-bit prime p = 264 − 59. The algorithm was later used by Digital Equipment Corporation  to hash passwords in the VMS operating system and is still being used for this purpose."
+1085,"Because of its government funding, certain forms of traffic were discouraged or prohibited."
+1086,"Leonard Kleinrock claims to have committed the first illegal act on the Internet, having sent a request for return of his electric razor after a meeting in England in 1973. At the time, use of the ARPANET for personal reasons was unlawful."
+1087,"In 1978, against the rules of the network, Gary Thuerk of Digital Equipment Corporation  sent out the first mass email to approximately 400 potential clients via the ARPANET. He claims that this resulted in $13 million worth of sales in DEC products, and highlighted the potential of email marketing."
+1088,A 1982 handbook on computing at MIT's AI Lab stated regarding network etiquette:
+1089,The Defense Advanced Research Projects Agency  is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military.
+1090,"Originally known as the Advanced Research Projects Agency , the agency was created on February 7, 1958, by President Dwight D. Eisenhower in response to the Soviet launching of Sputnik 1 in 1957. By collaborating with academia, industry, and government partners, DARPA formulates and executes research and development projects to expand the frontiers of technology and science, often beyond immediate U.S. military requirements."
+1091,"The Economist has called DARPA the agency that shaped the modern world, with technologies like ""weather satellites, GPS, drones, stealth technology, voice interfaces, the personal computer and the internet on the list of innovations for which DARPA can claim at least partial credit."" Its track record of success has inspired governments around the world to launch similar research and development agencies."
+1092,"DARPA is independent of other military research and development and reports directly to senior Department of Defense management. DARPA comprises approximately 220 government employees in six technical offices, including nearly 100 program managers, who together oversee about 250 research and development programs."
+1093,"The name of the organization first changed from its founding name, ARPA, to DARPA, in March 1972, changing back to ARPA in February 1993, then reverted to DARPA in March 1996."
+1094,"The agency's current director, appointed in March 2021, is Stefanie Tompkins."
+1095,"As of 2021, their mission statement is ""to make pivotal investments in breakthrough technologies for national security""."
+1096,"The Advanced Research Projects Agency  was suggested by the President's Scientific Advisory Committee to President Dwight D. Eisenhower in a meeting called after the launch of Sputnik.  ARPA was formally authorized by President Eisenhower in 1958 for the purpose of forming and executing research and development projects to expand the frontiers of technology and science, and able to reach far beyond immediate military requirements. The two relevant acts are the Supplemental Military Construction Authorization   and Department of Defense Directive 5105.15, in February 1958. It was placed within the Office of the Secretary of Defense  and counted approximately 150 people.  Its creation was directly attributed to the launching of Sputnik and to U.S. realization that the Soviet Union had developed the capacity to rapidly exploit military technology. Initial funding of ARPA was $520 million. ARPA's first director, Roy Johnson, left a $160,000 management job at General Electric for an $18,000 job at ARPA. Herbert York from Lawrence Livermore National Laboratory was hired as his scientific assistant."
+1097,"Johnson and York were both keen on space projects, but when NASA was established later in 1958 all space projects and most of ARPA's funding were transferred to it. Johnson resigned and ARPA was repurposed to do ""high-risk"", ""high-gain"", ""far out"" basic research, a posture that was enthusiastically embraced by the nation's scientists and research universities. ARPA's second director was Brigadier General Austin W. Betts, who resigned in early 1961 and was succeeded by Jack Ruina who served until 1963. Ruina, the first scientist to administer ARPA, managed to raise its budget to $250 million. It was Ruina who hired J. C. R. Licklider as the first administrator of the Information Processing Techniques Office, which played a vital role in creation of ARPANET, the basis for the future Internet."
+1098,"Additionally, the political and defense communities recognized the need for a high-level Department of Defense organization to formulate and execute R&D projects that would expand the frontiers of technology beyond the immediate and specific requirements of the Military Services and their laboratories. In pursuit of this mission, DARPA has developed and transferred technology programs encompassing a wide range of scientific disciplines that address the full spectrum of national security needs."
+1099,"From 1958 to 1965, ARPA's emphasis centered on major national issues, including space, ballistic missile defense, and nuclear test detection.  During 1960, all of its civilian space programs were transferred to the National Aeronautics and Space Administration  and the military space programs to the individual services."
+1100,"This allowed ARPA to concentrate its efforts on the Project Defender , Project Vela , and Project AGILE  programs, and to begin work on computer processing, behavioral sciences, and materials sciences. The DEFENDER and AGILE programs formed the foundation of DARPA sensor, surveillance, and directed energy R&D, particularly in the study of radar, infrared sensing, and x-ray/gamma ray detection."
+1101,"ARPA at this point  played an early role in Transit  a predecessor to the Global Positioning System .  ""Fast-forward to 1959 when a joint effort between DARPA and the Johns Hopkins Applied Physics Laboratory began to fine-tune the early explorers' discoveries. TRANSIT, sponsored by the Navy and developed under the leadership of Richard Kirschner at Johns Hopkins, was the first satellite positioning system."""
+1102,"During the late 1960s, with the transfer of these mature programs to the Services, ARPA redefined its role and concentrated on a diverse set of relatively small, essentially exploratory research programs. The agency was renamed the Defense Advanced Research Projects Agency  in 1972, and during the early 1970s, it emphasized direct energy programs, information processing, and tactical technologies."
+1103,"Concerning information processing, DARPA made great progress, initially through its support of the development of time-sharing.  All modern operating systems rely on concepts invented for the Multics system, developed by a cooperation among Bell Labs, General Electric and MIT, which DARPA supported by funding Project MAC at MIT with an initial two-million-dollar grant."
+1104,"DARPA supported the evolution of the ARPANET , Packet Radio Network, Packet Satellite Network and ultimately, the Internet and research in the artificial intelligence fields of speech recognition and signal processing, including parts of Shakey the robot. DARPA also supported the early development of both hypertext and hypermedia. DARPA funded one of the first two hypertext systems, Douglas Engelbart's NLS computer system, as well as The Mother of All Demos.  DARPA later funded the development of the Aspen Movie Map, which is generally seen as the first hypermedia system and an important precursor of virtual reality."
+1105,The Mansfield Amendment of 1973 expressly limited appropriations for defense research  only to projects with direct military application.
+1106,"The resulting ""brain drain"" is credited with boosting the development of the fledgling personal computer industry. Some young computer scientists left the universities to startups and private research laboratories such as Xerox PARC."
+1107,"Between 1976 and 1981, DARPA's major projects were dominated by air, land, sea, and space technology, tactical armor and anti-armor programs, infrared sensing for space-based surveillance, high-energy laser technology for space-based missile defense, antisubmarine warfare, advanced cruise missiles, advanced aircraft, and defense applications of advanced computing."
+1108,"Many of the successful programs were transitioned to the Services, such as the foundation technologies in automatic target recognition, space-based sensing, propulsion, and materials that were transferred to the Strategic Defense Initiative Organization , later known as the Ballistic Missile Defense Organization , now titled the Missile Defense Agency ."
+1109,"During the 1980s, the attention of the Agency was centered on information processing and aircraft-related programs, including the National Aerospace Plane  or Hypersonic Research Program. The Strategic Computing Program enabled DARPA to exploit advanced processing and networking technologies and to rebuild and strengthen relationships with universities after the Vietnam War. In addition, DARPA began to pursue new concepts for small, lightweight satellites  and directed new programs regarding defense manufacturing, submarine technology, and armor/anti-armor."
+1110,"In 1981, two engineers, Robert McGhee and Kenneth Waldron, started to develop the Adaptive Suspension Vehicle  nicknamed the ""Walker"" at the Ohio State University, under a research contract from DARPA. The vehicle was 17 feet long, 8 feet wide, and 10.5 feet high, and had six legs to support its three-ton aluminum body, in which it was designed to carry cargo over difficult terrains. However, DARPA lost interest in the ASV, after problems with cold-weather tests."
+1111,"On February 4, 2004, the agency shut down its so called ""LifeLog Project"". The project's aim would have been, ""to gather in a single place just about everything an individual says, sees or does""."
+1112,"On October 28, 2009, the agency broke ground on a new facility in Arlington County, Virginia a few miles from The Pentagon."
+1113,"In fall 2011, DARPA hosted the 100-Year Starship Symposium with the aim of getting the public to start thinking seriously about interstellar travel."
+1114,"On June 5, 2016, NASA and DARPA announced that it planned to build new X-planes with NASA's plan setting to create a whole series of X planes over the next 10 years."
+1115,"Between 2014 and 2016, DARPA shepherded the first machine-to-machine computer security competition, the Cyber Grand Challenge ,
+bringing a group of top-notch computer security experts to search for security vulnerabilities, exploit them, and create fixes that patch those vulnerabilities in a fully automated fashion. It is one of DARPA prize competitions to spur innovations."
+1116,"In June 2018, DARPA leaders demonstrated a number of new technologies that were developed within the framework of the GXV-T program. The goal of this program is to create a lightly armored combat vehicle of not very large dimensions, which, due to maneuverability and other tricks, can successfully resist modern anti-tank weapon systems."
+1117,"In September 2020, DARPA and the US Air Force announced that the Hypersonic Air-breathing Weapon Concept  are ready for free-flight tests within the next year."
+1118,Victoria Coleman became the director of DARPA in November 2020.
+1119,"In recent years, DARPA officials have contracted out core functions to corporations. For example, during fiscal year 2020, Chenega ran physical security on DARPA's premises, System High Corp. carried out program security, and Agile Defense ran unclassified IT services. General Dynamics runs classified IT services. Strategic Analysis Inc. provided support services regarding engineering, science, mathematics, and front office and administrative work."
+1120,"DARPA has six technical offices that manage the agency's research portfolio, and two additional offices that manage special projects.  All offices report to the DARPA director, including:"
+1121,A 1991 reorganization created several offices which existed throughout the early 1990s:
+1122,A 2010 reorganization merged two offices:
+1123,"A list of DARPA's active and archived projects is available on the agency's website. Because of the agency's fast pace, programs constantly start and stop based on the needs of the U.S. government. Structured information about some of the DARPA's contracts and projects is publicly available."
+1124,"DARPA publishes a list of current research programs, and a list of archived programs."
+1125,"DARPA is well known as a high-tech government agency, and as such has many appearances in popular fiction.  Some realistic references to DARPA in fiction are as ""ARPA"" in Tom Swift and the Visitor from Planet X , in episodes of television program The West Wing , the television program Numb3rs, and the Netflix film Spectral."
+1126,The system's parent company is organized into three business units:
+1127,"Sabre is headquartered in Southlake, Texas, and has many employees in various locations around the world."
+1128,"The name of the travel reservation system is an abbreviation for ""Semi-automated Business Research Environment"", and was originally styled in all-capital letters as SABRE. It was developed to automate the way American Airlines booked reservations."
+1129,"In the 1950s, American Airlines was facing a serious challenge in its ability to quickly handle airline reservations in an era that witnessed high growth in passenger volumes in the airline industry. Before the introduction of SABRE, the airline's system for booking flights was entirely manual, having developed from the techniques originally developed at its Little Rock, Arkansas, reservations center in the 1920s.  In this manual system, a team of eight operators would sort through a rotating file with cards for every flight. When a seat was booked, the operators would place a mark on the side of the card, and knew visually whether it was full. This part of the process was not all that slow, at least when there were not that many planes, but the entire end-to-end task of looking for a flight, reserving a seat, and then writing up the ticket could take up to three hours in some cases, and 90 minutes on average. The system also had limited room to scale. It was limited to about eight operators because that was the maximum that could fit around the file. To handle more queries the only solution was to add more layers of hierarchy to filter down requests into batches."
+1130,"American Airlines had already attacked the problem to some degree, and was in the process of introducing their new Magnetronic Reservisor, an electromechanical computer, in 1952 to replace the card files. This computer consisted of a single magnetic drum, each memory location holding the number of seats left on a particular flight. Using this system, a large number of operators could access information simultaneously, so the ticket agents could be told via phone if a seat was available. On the downside, a staff member was needed at each end of the phone line, and handling the ticket took considerable effort and filing. Something much more highly automated was needed if American Airlines was going to enter the jet age, booking many times more seats.: p.100"
+1131,"During the testing phase of the Reservisor a high-ranking IBM salesman, Blair Smith, was flying on an American Airlines flight from Los Angeles back to IBM in New York City in 1953.  He found himself sitting next to American Airlines president C. R. Smith.  Noting that they shared a family name, they began talking."
+1132,"Just prior to this chance meeting, IBM had been working with the United States Air Force on their Semi Automatic Ground Environment  project. SAGE used a series of large computers to coordinate the message flow from radar sites to interceptors, dramatically reducing the time needed to direct an attack on an incoming bomber. The system used teleprinter machines located around the world to feed information into the system, which then sent orders back to teleprinters located at the fighter bases. It was one of the first online systems."
+1133,"Smith and Watson observed that the SAGE system's basic architecture was suitable for use in American Airlines' booking services. Teleprinters would be placed at American Airlines' ticketing offices to send in requests and receive responses directly, without the need for anyone on the other end of the phone. The number of available seats on the aircraft could be tracked automatically, and if a seat was available the ticket agent could be notified. Booking simply took one more command, updating the availability and, if desired, could be followed by printing a ticket."
+1134,"Thirty days later IBM sent a research proposal to American Airlines, suggesting that they join forces to study the problem. A team was set up consisting of IBM engineers led by John Siegfried and a large number of American Airlines' staff led by Malcolm Perry, taken from booking, reservations, and ticket sales, calling the effort the Semi-Automated Business Research Environment, or SABRE."
+1135,"A formal development arrangement was signed in 1957. The first experimental system went online in 1960, based on two IBM 7090 mainframes in a new data center located in Briarcliff Manor, New York. The system was a success. Up to this point, it had cost $40 million to develop and install . The SABRE system by IBM in the 1960s was specified to process a very large number of transactions, such as handling 83,000 daily phone calls. The system took over all booking functions in 1964, when the name had changed to SABRE."
+1136,"In 1972, SABRE was migrated to IBM System/360 systems in a new underground location in Tulsa, Oklahoma. Max Hopper joined American Airlines in 1972 as director of SABRE, and pioneered its use. Originally used only by American Airlines, the system was expanded to access by travel agents in 1976."
+1137,"With SABRE up and running, IBM offered its expertise to other airlines, and soon developed Deltamatic for Delta Air Lines on the IBM 7074, and PANAMAC for Pan American World Airways using an IBM 7080. In 1968, they generalized their work into the PARS , which ran on any member of the IBM System/360 family and thus could support any sized airline. The operating system component of PARS evolved into ACP , and later to TPF . Application programs were originally written in assembly language, later in SabreTalk, a proprietary dialect of PL/I, and now in C and C++."
+1138,"By the 1980s, SABRE offered airline reservations through the CompuServe Information Service, and General Electric's GEnie under the Eaasy SABRE brand. This service was extended to America Online  in the 1990s."
+1139,"American and Sabre separated on March 15, 2000. Sabre had been a publicly traded corporation, Sabre Holdings, stock symbol TSG on the New York Stock Exchange until taken private in March 2007. The corporation introduced the new logo and changed from the all-caps acronym ""SABRE"" to the mixed-case ""Sabre Holdings"", when the new corporation was formed. The Travelocity website, introduced in 1996, was owned by Sabre Holdings. Travelocity was acquired by Expedia in January 2015. Sabre Holdings' three remaining business units, Sabre Travel Network, Sabre Airline Solutions and Sabre Hospitality, today serves as a global travel technology company."
+1140,"In 1982, Advertising Age reported that ""United Airlines operates a similar system, Apollo, while Eastern operates Mars and Delta operates Datas."" Braniff International's Cowboy system was considered by Electronic Data Systems for building an airline-neutral system."
+1141,"A 1982 study by American Airlines found that travel agents selected the flight appearing on the first line more than half the time.  Ninety-two percent of the time, the selected flight was on the first screen.  This provided a huge incentive for American to manipulate its ranking formula, or even corrupt the search algorithm outright, to favor American flights over its competitors in the results of flight search results, and the airline did not resist the temptation."
+1142,"At first this was limited to juggling the relative importance of factors such as the length of the flight, how close the actual departure time was to the desired time, and whether the flight had a connection, but with each success American became bolder.  In late 1981, New York Air added a flight from La Guardia to Detroit, challenging American in an important market.  Before long, the new flights suddenly started appearing at the bottom of the screen.  Its reservations dried up, and it was forced to cut back from eight Detroit flights a day to none."
+1143,"On one occasion, Sabre deliberately withheld Continental's discount fares on 49 routes where American competed.  A Sabre staffer had been directed to work on a program that would automatically suppress any discount fares loaded into the system."
+1144,"Congress investigated these practices, and in 1983 Bob Crandall, president of American, vocally defended the airline's preferential treatment of its own offerings in the system. ""The preferential display of our flights, and the corresponding increase in our market share, is the competitive raison d'être for having created the system in the first place,"" he told them. The U.S. government disagreed, and in 1984 it outlawed the biasing practices for the search results."
+1145,"The fairness rules were eliminated or allowed to expire in 2010. By then, none of the major distribution systems was majority owned by the airlines."
+1146,"In 1987 Sabre's success of selling to European travel agents was inhibited by the refusal of big European carriers led by British Airways to grant the system ticketing authority for their flights even though Sabre had obtained IATA Billing and Settlement Plan  clearance for the UK in 1986. American brought High Court action which alleged that after the arrival of Sabre on its doorstep British Airways immediately offered financial incentives to travel agents who continued to use Travicom and would tie any override commissions to it. Travicom was created by Videcom, British Airways and British Caledonian and launched in 1976 as the world's first multi-access reservations system based on Videcom technology which eventually became part of Galileo UK. It connected 49 subscribing international airlines  to thousands of travel agents in the UK. It allowed agents and airlines to communicate via a common distribution language and network, handling 97% of UK airline business trade bookings by 1987."
+1147,"British Airways eventually bought out the stakes in Travicom held by Videcom and British Caledonian, to become the sole owner. Although Sabre's vice-president in London, David Schwarte, made representations to the U.S. Department of Transportation and the British Monopolies Commission, British Airways defended the use of Travicom as a truly non-discriminatory system in flight selection because an agent had access to some 50 carriers worldwide, including Sabre, for flight information."
+1148,"The processing power behind SAGE was supplied by the largest discrete component-based computer ever built, the IBM-manufactured AN/FSQ-7. Each SAGE Direction Center  housed an FSQ-7 which occupied an entire floor, approximately 22,000 square feet  not including supporting equipment. The FSQ-7 was actually two computers, ""A"" side and ""B"" side. Computer processing was switched from ""A"" side to ""B"" side on a regular basis, allowing maintenance on the unused side. Information was fed to the DCs from a network of radar stations as well as readiness information from various defense sites. The computers, based on the raw radar data, developed ""tracks"" for the reported targets, and automatically calculated which defenses were within range. Operators used light guns to select targets on-screen for further information, select one of the available defenses, and issue commands to attack. These commands would then be automatically sent to the defense site via teleprinter."
+1149,"Connecting the various sites was an enormous network of telephones, modems and teleprinters. Later additions to the system allowed SAGE's tracking data to be sent directly to CIM-10 Bomarc missiles and some of the US Air Force's interceptor aircraft in-flight, directly updating their autopilots to maintain an intercept course without operator intervention. Each DC also forwarded data to a Combat Center  for ""supervision of the several sectors within the division"" .: 51"
+1150,"SAGE became operational in the late 1950s and early 1960s at a combined cost of billions of dollars. It was noted that the deployment cost more than the Manhattan Project—which it was, in a way, defending against. Throughout its development, there were continual concerns about its real ability to deal with large attacks, and the Operation Sky Shield tests showed that only about one-fourth of enemy bombers would have been intercepted. Nevertheless, SAGE was the backbone of NORAD's air defense system into the 1980s, by which time the tube-based FSQ-7s were increasingly costly to maintain and completely outdated. Today the same command and control task is carried out by microcomputers, based on the same basic underlying data."
+1151,"Just prior to World War II, Royal Air Force  tests with the new Chain Home  radars had demonstrated that relaying information to the fighter aircraft directly from the radar sites was not feasible. The radars determined the map coordinates of the enemy, but could generally not see the fighters at the same time. This meant the fighters had to be able to determine where to fly to perform an interception but were often unaware of their own exact location and unable to calculate an interception while also flying their aircraft."
+1152,"The solution was to send all of the radar information to a central control station where operators collated the reports into single tracks, and then reported these tracks to the airbases, or sectors. The sectors used additional systems to track their own aircraft, plotting both on a single large map. Operators viewing the map could then see what direction their fighters would have to fly to approach their targets and relay that simply by telling them to fly along a certain heading or vector. This Dowding system was the first ground-controlled interception  system of large scale, covering the entirety of the UK. It proved enormously successful during the Battle of Britain, and is credited as being a key part of the RAF's success."
+1153,"The system was slow, often providing information that was up to five minutes out of date. Against propeller driven bombers flying at perhaps 225 miles per hour  this was not a serious concern, but it was clear the system would be of little use against jet-powered bombers flying at perhaps 600 miles per hour . The system was extremely expensive in manpower terms, requiring hundreds of telephone operators, plotters and trackers in addition to the radar operators. This was a serious drain on manpower, making it difficult to expand the network."
+1154,"The idea of using a computer to handle the task of taking reports and developing tracks had been explored beginning late in the war. By 1944, analog computers had been installed at the CH stations to automatically convert radar readings into map locations, eliminating two people. Meanwhile, the Royal Navy began experimenting with the Comprehensive Display System , another analog computer that took X and Y locations from a map and automatically generated tracks from repeated inputs. Similar systems began development with the Royal Canadian Navy, DATAR, and the US Navy, the Naval Tactical Data System. A similar system was also specified for the Nike SAM project, specifically referring to a US version of CDS, coordinating the defense over a battle area so that multiple batteries did not fire on a single target. All of these systems were relatively small in geographic scale, generally tracking within a city-sized area."
+1155,"When the Soviet Union tested its first atomic bomb in August 1949, the topic of air defense of the US became important for the first time. A study group, the ""Air Defense Systems Engineering Committee"" was set up under the direction of Dr. George Valley to consider the problem, and is known to history as the ""Valley Committee""."
+1156,"Their December report noted a key problem in air defense using ground-based radars. A bomber approaching a radar station would detect the signals from the radar long before the reflection off the bomber was strong enough to be detected by the station. The committee suggested that when this occurred, the bomber would descend to low altitude, thereby greatly limiting the radar horizon, allowing the bomber to fly past the station undetected. Although flying at low altitude greatly increased fuel consumption, the team calculated that the bomber would only need to do this for about 10% of its flight, making the fuel penalty acceptable."
+1157,"The only solution to this problem was to build a huge number of stations with overlapping coverage. At that point the problem became one of managing the information. Manual plotting was ruled out as too slow, and a computerized solution was the only possibility. To handle this task, the computer would need to be fed information directly, eliminating any manual translation by phone operators, and it would have to be able to analyze that information and automatically develop tracks. A system tasked with defending cities against the predicted future Soviet bomber fleet would have to be dramatically more powerful than the models used in the NTDS or DATAR."
+1158,"The Committee then had to consider whether or not such a computer was possible. The Valley Committee was introduced to Jerome Wiesner, associate director of the Research Laboratory of Electronics at MIT. Wiesner noted that the Servomechanisms Laboratory had already begun development of a machine that might be fast enough. This was the Whirlwind I, originally developed for the Office of Naval Research as a general purpose flight simulator that could simulate any current or future aircraft by changing its software."
+1159,"Wiesner introduced the Valley Committee to Whirlwind's project lead, Jay Forrester, who convinced him that Whirlwind was sufficiently capable. In September 1950, an early microwave early-warning radar system at Hanscom Field was connected to Whirlwind using a custom interface developed by Forrester's team. An aircraft was flown past the site, and the system digitized the radar information and successfully sent it to Whirlwind. With this demonstration, the technical concept was proven. Forrester was invited to join the committee."
+1160,"With this successful demonstration, Louis Ridenour, chief scientist of the Air Force, wrote a memo stating ""It is now apparent that the experimental work necessary to develop, test, and evaluate the systems proposals made by ADSEC will require a substantial amount of laboratory and field effort."" Ridenour approached MIT President James Killian with the aim of beginning a development lab similar to the war-era Radiation Laboratory that made enormous progress in radar technology. Killian was initially uninterested, desiring to return the school to its peacetime civilian charter. Ridenour eventually convinced Killian the idea was sound by describing the way the lab would lead to the development of a local electronics industry based on the needs of the lab and the students who would leave the lab to start their own companies. Killian agreed to at least consider the issue, and began Project Charles to consider the size and scope of such a lab."
+1161,"Project Charles was placed under the direction of Francis Wheeler Loomis and included 28 scientists, about half of whom were already associated with MIT. Their study ran from February to August 1951, and in their final report they stated that ""We endorse the concept of a centralized system as proposed by the Air Defense Systems Engineering Committee, and we agree that the central coordinating apparatus of this system should be a high-speed electronic digital computer."" The report went on to describe a new lab that would be used for generic technology development for the Air Force, Army and Navy, and would be known as Project Lincoln."
+1162,"Loomis took over direction of Project Lincoln and began planning by following the lead of the earlier RadLab. By September 1951, only months after the Charles report, Project Lincoln had more than 300 employees. By the end of the summer of 1952 this had risen to 1300, and after another year, 1800. The only building suitable for classified work at that point was Building 22, suitable for a few hundred people at most, although some relief was found by moving the non-classified portions of the project, administration and similar, to Building 20. But this was clearly insufficient space. After considering a variety of suitable locations, a site at Laurence G. Hanscom Field was selected, with the groundbreaking taking place in 1951."
+1163,"The terms of the National Security Act were formulated during 1947, leading to the creation of the US Air Force out of the former US Army Air Force. During April of the same year,  US Air Force staff were identifying specifically the requirement for the creation of automatic equipment for radar-detection which would relay information to an air defence control system, a system which would function without the inclusion of persons for its operation. The December 1949 ""Air Defense Systems Engineering Committee"" led by Dr. George Valley had recommended computerized networking for ""radar stations guarding the northern air approaches to the United States"" .  After a January 1950 meeting, Valley and Jay Forrester proposed using the Whirlwind I  for air defense. On August 18, 1950, when the ""1954 Interceptor"" requirements were issued, the USAF ""noted that manual techniques of aircraft warning and control would impose ""intolerable"" delays"": 484   published Electronic Air Defense Environment for 1954 in December .)  During February–August 1951 at the new Lincoln Laboratory, the USAF conducted Project Claude which concluded an improved air defense system was needed."
+1164,"In a test for the US military at Bedford, Massachusetts on 20 April 1951, data produced by a radar was transmitted through telephone lines to a computer for the first time, showing the detection of a mock enemy aircraft. This first test was directed by C. Robert Wieser."
+1165,"The ""Summer Study Group"" of scientists in 1952 recommended ""computerized air direction centers…to be ready by 1954."""
+1166,"IBM's ""Project High"" assisted under their October 1952 Whirlwind subcontract with Lincoln Laboratory,: 210  and a 1952 USAF Project Lincoln ""fullscale study"" of ""a large scale integrated ground control system"" resulted in the SAGE approval ""first on a trial basis in 1953"".: 128   The USAF had decided by April 10, 1953, to cancel the competing ADIS , and the University of Michigan's Aeronautical Research Center withdrew in the spring.: 289  Air Research and Development Command  planned to ""finalize a production contract for the Lincoln Transition System"".: 201   Similarly, the July 22, 1953, report by the Bull Committee  identified completing the Mid-Canada Line radars as the top priority and ""on a second-priority-basis: the Lincoln automated system""  "
+1167,"The Priority Permanent System with the initial  radar stations was completed in 1952: 223  as a ""manual air defense system""   The Permanent System radar stations included 3 subsequent phases of deployments and by June 30, 1957, had 119 ""Fixed CONUS"" radars, 29 ""Gap-filler low altitude"" radars, and 23 control centers"".  At ""the end of 1957, ADC operated 182 radar stations  17 control centers … 32  had been added during the last half of the year as low-altitude, unmanned gap-filler radars.  The total consisted of 47 gap-filler stations, 75 Permanent System radars, 39 semimobile radars, 19 Pinetree stations,…1 Lashup -era radar and a single Texas Tower"".: 223   ""On 31 December 1958, USAF ADC had 187 operational land-based radar stations"" ."
+1168,"Systems scientist Jay Forrester was instrumental in directing the development of the key concept of an interception system during his work at Servomechanisms Laboratory of MIT. The concept of the system, according to the Lincoln Laboratory site was to 
+""develop a digital computer that could receive vast quantities of data from multiple radars and perform real-time processing to produce targeting information for intercepting aircraft and missiles."""
+1169,"The AN/FSQ-7 was developed by the Lincoln Laboratory's Digital Computer Laboratory and Division 6, working closely with IBM as the manufacturer. Each FSQ-7 actually consisted of two nearly identical computers operating in ""duplex"" for redundancy. The design used an improved version of the Whirlwind I magnetic core memory and was an extension of the Whirlwind II computer program, renamed AN/FSQ-7 in 1953 to comply with Air Force nomenclature. It has been suggested the FSQ-7 was based on the IBM 701 but, while the 701 was investigated by MIT engineers, its design was ultimately rejected due to high error rates and generally being ""inadequate to the task."" IBM's contributions were essential to the success of the FSQ-7, and IBM benefited immensely from its association with the SAGE project, most evidently during development of the IBM 704."
+1170,"On October 28, 1953, the Air Force Council recommended 1955 funding for ""ADC to convert to the Lincoln automated system"": 193   .: 201   The ""experimental SAGE subsector, located in Lexington, Mass., was completed in 1955…with a prototype AN/FSQ-7…known as XD-1"" .  In 1955, Air Force personnel began IBM training at the Kingston, New York, prototype facility, and the ""4620th Air Defense Wing  was established at Lincoln Laboratory"""
+1171,"On May 3, 1956, General Partridge presented CINCNORAD's Operational Concept for Control of Air Defense Weapons to the Armed Forces Policy Council, and a June 1956 symposium presentation identified advanced programming methods of SAGE code.    For SAGE consulting Western Electric and Bell Telephone Laboratories formed the Air Defense Engineering Service , which was contracted in January 1954.  IBM delivered the FSQ-7 computer's prototype in June 1956, and Kingston's XD-2 with dual computers guided a Cape Canaveral BOMARC to a successful aircraft intercept on August 7, 1958.: 197   Initially contracted to RCA, the AN/FSQ-7 production units were started by IBM in 1958   IBM's production contract developed 56 SAGE computers for $.5 billion —cf. the $2 billion WWII Manhattan Project."
+1172,"General Operational Requirements  79 and 97 were ""the basic USAF documents guiding development and improvement of  ground environment.: 97   Prior to fielding the AN/FSQ-7 centrals, the USAF initially deployed ""pre-SAGE semiautomatic intercept systems""  to Air Defense Direction Centers, ADDCs: 11  .  On April 22, 1958, NORAD approved Nike AADCPs to be collocated with the USAF manual ADDCs at Duncanville Air Force Station TX, Olathe Air Force Station KS, Belleville Air Force Station IL, and Osceola Air Force Station KS."
+1173,"In 1957, SAGE System groundbreaking at McChord AFB was for DC-12 where the ""electronic brain"" began arriving in November 1958, and the ""first SAGE regional battle post  began operating in Syracuse, New York in early 1959"".: 263   BOMARC ""crew training was activated January 1, 1958"", and AT&T ""hardened many of its switching centers, putting them in deep underground bunkers"",  The North American Defense Objectives Plan  submitted to Canada in December 1958 scheduled 5 Direction Centers and 1 Combat Center to be complete in Fiscal Year 1959, 12 DCs and 3 CCs complete at the end of FY 60, 19 DC/4 CC FY 61, 25/6 FY 62, and 30/10 FY 63.  On June 30 NORAD ordered that ""Air Defense Sectors  were to be designated as NORAD sectors"", : 7"
+1174,"SAGE Geographic Reorganization: The SAGE Geographic Reorganization Plan of July 25, 1958, by NORAD was ""to provide a means for the orderly transition and phasing from the manual to the SAGE system.""  The plan identified deactivation of the Eastern, Central, and Western Region/Defense Forces on July 1, 1960, and ""current manual boundaries"" were to be moved to the new ""eight SAGE divisions""  as soon as possible.  Manual divisions ""not to get SAGE computers were to be phased out"" along with their Manual Air Defense Control Centers at the headquarters base: ""9th  Geiger Field… 32d, Syracuse AFS… 35th, Dobbins AFB… 58th, Wright-Patterson AFB… 85th, Andrews AFB"".  The 26th SAGE Division --the 1st of the SAGE divisions—became operational at Hancock Field on 1 January 1959 after the redesignation started for AC&W Squadrons  October 1.): 156   Additional sectors included the Los Angeles Air Defense Sector  designated in February 1959.  A June 23 JCS memorandum approved the new ""March 1959 Reorganization Plan"" for HQ NORAD/CONAD/ADC.: 5"
+1175,"Project Wild Goose teams of Air Materiel Command personnel installed c. 1960 the Ground Air Transmit Receive stations for the SAGE TDDL . By the middle of 1960, AMC had determined that about 800,000 man-hours  would be required to bring the F-106 fleet to the point where it would be a valuable adjunct to the air defense system. Part of the work  was accomplished by Sacramento Air Materiel Area. The remainder  was done at ADC bases by roving AMC field assistance teams supported by ADC maintenance personnel.   After a September 1959 experimental ATABE test between an ""abbreviated"" AN/FSQ-7 staged at Fort Banks and the Lexington XD-1, the 1961 ""SAGE/Missile Master test program"" conducted large-scale field testing of the ATABE ""mathematical model"" using radar tracks of actual SAC and ADC aircraft flying mock penetrations into defense sectors.  Similarly conducted was the joint SAC-NORAD Sky Shield II exercise followed by Sky Shield III on 2 September 1962  On July 15, 1963, ESD's CMC Management Office assumed ""responsibilities in connection with BMEWS, Space Track, SAGE, and BUIC.""  The Chidlaw Building's computerized NORAD/ADC Combined Operations Center in 1963 became the highest echelon of the SAGE computer network when operations moved from Ent AFB's 1954 manual Command Center to the partially underground ""war room"".  Also in 1963, radar stations were renumbered  and the vacuum-tube SAGE System was completed .: 9"
+1176,"On ""June 26, 1958,…the New York sector became operational"": 207  and on December 1, 1958, the Syracuse sector's DC-03 was operational  Construction of CFB North Bay in Canada was started in 1959 for a bunker ~700 feet  underground , and by 1963 the system had 3 Combat Centers.  The 23 SAGE centers included 1 in Canada, and the ""SAGE control centers reached their full 22 site deployments in 1961 .""  The completed Minot AFB blockhouse received an AN/FSQ-7, but never received the FSQ-8 ."
+1177,The SAGE system included a direction center  assigned to air defense sectors as they were defined at the time.
+1178,"*Some of the originally planned 32 DCs were never completed and DCs were planned at installations for additional sectors: Calypso/Raleigh NC, England/Shreveport LA, Fort Knox KY, Kirtland/Albuquerque NM, Robins/Miami, Scott/St. Louis, Webb/San Antonio TX."
+1179,"The environment allowed radar station personnel to monitor the radar data and systems' status  and to use the range height equipment to process height requests from Direction Center  personnel.  DCs received the Long Range Radar Input from the sector's radar stations, and DC personnel monitored the radar tracks and IFF data provided by the stations, requested height-finder radar data on targets, and monitored the computer's evaluation of which fighter aircraft or Bomarc missile site could reach the threat first.  The DC's ""NORAD sector commander's operational staff"" could designate fighter intercept of a target or, using the Senior Director's keyed console in the Weapons Direction room, launch a Bomarc intercept with automatic Q-7 guidance of the surface-to-air missile to a final homing dive ."
+1180,"The ""NORAD sector direction center   air defense artillery director  consoles  ADA battle staff officer"", and the NSDC automatically communicated crosstelling of ""SAGE reference track data"" to/from adjacent sectors' DCs and to 10 Nike Missile Master AADCPs.  Forwardtelling automatically communicated data from multiple DCs to a 3-story Combat Center  usually at one of the sector's DCs  for coordinating the air battle in the NORAD region  and which forwarded data to the NORAD Command Center . NORAD's integration of air warning data  along with space surveillance, intelligence, and other data allowed attack assessment of an Air Defense Emergency for alerting the SAC command centers , The Pentagon/Raven Rock NMCC/ANMCC, and the public via CONELRAD radio stations."
+1181,"The Burroughs 416L SAGE component  was the Cold War network connecting IBM supplied computer system at the various DC and that created the display and control environment for operation of the separate radars and to provide outbound command guidance for ground-controlled interception by air defense aircraft in the ""SAGE Defense System"" . Burroughs Corporation was a prime contractor for SAGE network interface equipment which included 134 Burroughs AN/FST-2 Coordinate Data Transmitting Sets  at radar stations and other sites, the IBM supplied AN/FSQ-7 at 23 Direction Centers, and the AN/FSQ-8 Combat Control Computers at 8 Combat Centers. The 2 computers of each AN/FSQ-7 together weighing 275 short tons-force  used about ⅓ of the DC's 2nd floor space and at ~$50 per instruction had approximately 125,000 ""computer instructions support actual operational air-defense mission"" processing.  The AN/FSQ-7 at Luke AFB had additional memory  and was used as a ""computer center for all other"" DCs. Project 416L was the USAF predecessor of NORAD, SAC, and other military organizations' ""Big L"" computer systems ."
+1182,Network communications:
+1183,"The SAGE network of computers connected by a ""Digital Radar Relay""  used AT&T voice lines, microwave towers, switching centers , etc.; and AT&T's ""main underground station"" was in Kansas  with other bunkers in Connecticut , California , Iowa  and Maryland .  CDTS modems at automated radar stations transmitted range and azimuth, and the Air Movements Identification Service  provided air traffic data to the SAGE System.  Radar tracks by telephone calls  could be entered via consoles of the 4th floor ""Manual Inputs"" room adjacent to the ""Communication Recording-Monitoring and VHF"" room.  In 1966, SAGE communications were integrated into the AUTOVON Network."
+1184,"SAGE Sector Warning Networks  provided the radar netting communications for each DC and eventually also allowed transfer of command guidance to autopilots of TDDL-equipped interceptors for vectoring to targets via the Ground to Air Data Link Subsystem and the Ground Air Transmit Receive  network of radio sites for ""HF/VHF/UHF voice & TDDL"" each generally co-located at a CDTS site.  SAGE Direction Centers and Combat Centers were also nodes of NORAD's Alert Network Number 1, and SAC Emergency War Order Traffic included ""Positive Control/Noah's Ark instructions"" through northern NORAD radio sites to confirm or recall SAC bombers if ""SAC decided to launch the alert force before receiving an execution order from the JCS""."
+1185,"A SAGE System ergonomic test at Luke AFB in 1964 ""showed conclusively that the wrong timing of human and technical operations was leading to frequent truncation of the flight path tracking system"" .: 9   SAGE software development was ""grossly underestimated"": 370  :  ""the biggest mistake  the SAGE computer program was  jump from the 35,000  instructions … to the more than 100,000 instructions on the"" AN/FSQ-8.  NORAD conducted a Sage/Missile Master Integration/ECM-ECCM Test in 1963, and although SAGE used AMIS input of air traffic information, the 1959 plan developed by the July 1958 USAF Air Defense Systems Integration Division for SAGE Air Traffic Integration  was cancelled by the DoD."
+1186,"SAGE radar stations, including 78 DEW Line sites in December 1961, provided radar tracks to DCs and had frequency diversity  radars United States Navy picket ships also provided radar tracks, and seaward radar coverage was provided. By the late 1960s EC-121 Warning Star aircraft based at Otis AFB MA and McClellan AFB CA provided radar tracks via automatic data link to the SAGE System. Civil Aeronautics Administration radars were at some stations , and the ARSR-1 Air Route Surveillance Radar rotation rate had to be modified ""for SAGE  Modes III and IV"" : 21"
+1187,"ADC aircraft such as the F-94 Starfire, F-89 Scorpion, F-101B Voodoo, and F-4 Phantom were controlled by SAGE GCI.  The F-104 Starfighter was ""too small to be equipped with  data link equipment"" and used voice-commanded GCI,: 229  but the  F-106 Delta Dart was equipped for the automated data link .  The ADL was designed to allow Interceptors that reached targets to transmit real-time tactical friendly and enemy movements and to determine whether sector defence reinforcement was necessary."
+1188,Familiarization flights allowed SAGE weapons directors to fly on two-seat interceptors to observe GCI operations. Surface-to-air missile installations for CIM-10 Bomarc interceptors were displayed on SAGE consoles.
+1189,"Partially solid-state AN/FST-2B and later AN/FYQ-47 computers replaced the AN/FST-2, and sectors without AN/FSQ-7 centrals requiring a ""weapon direction control device"" for USAF air defense used the solid-state AN/GSG-5 CCCS instead of the AN/GPA-73 recommended by ADC in June 1958. Back-Up Interceptor Control  with CCCS dispersed to radar stations for survivability allowed a diminished but functional SAGE capability.  In 1962, Burroughs ""won the contract to provide a military version of its D825"" modular data processing system for BUIC II. BUIC II was first used at North Truro Z-10 in 1966, and the Hamilton AFB BUIC II was installed in the former MCC building when it was converted to a SAGE Combat Center in 1966 .  On June 3, 1963, the Direction Centers at Marysville CA, Marquette/K I Sawyer AFB  MI, Stewart AFB NY , and Moses Lake WA  were planned for closing and at the end of 1969, only 6 CONUS SAGE DCs remained  all with the vacuum tube AN/FSQ-7 centrals.: 47   In 1966, NORAD Combined Operations Center operations at Chidlaw transferred to the Cheyenne Mountain Operations Center  and in December 1963, the DoD approved solid state replacement of Martin AN/FSG-1 centrals: 317  with the AN/GSG-5 and subsequent Hughes AN/TSQ-51. The ""416L/M/N Program Office"" at Hanscom Field had deployed the BUIC III by 1971 , and the initial BUIC systems were phased out 1974–5.  ADC had been renamed Aerospace Defense Command on January 15, 1968, and its general surveillance radar stations transferred to ADTAC in 1979 when the ADC major command was broken up "
+1190,"For airborne command posts, ""as early as 1962 the Air Force began exploring possibilities for an Airborne Warning and Control System "",: 266  and the Strategic Defense Architecture  planned an integrated air defense and air traffic control network.  The USAF declared full operational capability of the first seven Joint Surveillance System ROCCs on December 23, 1980, with Hughes AN/FYQ-93 systems, and many of the SAGE radar stations became Joint Surveillance System  sites   The North Bay AN/FSQ-7 was dismantled and sent to Boston's Computer Museum. In 1996, AN/FSQ-7 components were moved to Moffett Federal Airfield for storage and later moved to the Computer History Museum in Mountain View, California.  The last AN/FSQ-7 centrals were demolished at McChord AFB  and Luke AFB .  Decommissioned AN/FSQ-7 equipment was also used as science fiction cinema and TV series props ."
+1191,"SAGE histories include a 1983 special issue of the Annals of the History of Computing, and various personal histories were published, e.g., Valley in 1985 and Jacobs in 1986.  In 1998, the SAGE System was identified as 1 of 4 ""Monumental Projects"", and a SAGE lecture presented the vintage film In Your Defense followed by anecdotal information from Les Earnest, Jim Wong, and Paul Edwards.  In 2013, a copy of a 1950s cover girl image programmed for SAGE display was identified as the ""earliest known figurative computer art"".  Company histories identifying employees' roles in SAGE include the 1981 System Builders: The Story of SDC and the 1998 Architects of Information Advantage: The MITRE Corporation Since 1958."
+1192,"The contract to build the Mark II was signed with Harvard in February 1945, after the successful demonstration of the Mark I in 1944. It was completed and debugged in 1947, and delivered to the US Navy Proving Ground at Dahlgren, Virginia in March 1948, becoming fully operational by the end of that year."
+1193,"The Mark II was constructed with high-speed electromagnetic relays instead of the electro-mechanical counters used in the Mark I, making it much faster than its predecessor. It weighed 25 short tons  and occupied over 4,000 square feet  of floor space. Its addition time was 0.125 seconds  and the multiplication time was 0.750 seconds. This was a factor of 2.6 faster for addition and a factor of 8 faster for multiplication compared to the Mark I. It was the second machine  to have floating-point hardware. A unique feature of the Mark II is that it had built-in hardware for several functions such as the reciprocal, square root, logarithm, exponential, and some trigonometric functions. These took between five and twelve seconds to execute. Additionally, the Mark II was actually composed of two sub-computers that could either work in tandem or operate on separate functions, to cross-check results and debug malfunctions."
+1194,"The Mark I and Mark II were not stored-program computers – they read instructions of the program one at a time from a tape and executed them. The Mark II had a peculiar programming method that was devised to ensure that the contents of a register were available when needed. The tape containing the program could encode only eight instructions, so what a particular instruction code meant depended on when it was executed. Each second was divided up into several periods, and a coded instruction could mean different things in different periods.  An addition could be started in any of eight periods in the second, a multiplication could be started in any of four periods of the second, and a transfer of data could be started in any of twelve periods of the second.  Although this system worked, it made the programming complicated, and it reduced the efficiency of the machine somewhat."
+1195,"The Mark II is also known for being the computer with the first recorded instance of an actual bug  disrupting its operation. The insect was extracted from the machine's electronics and taped to the log book, with the note ""first actual case of  bug being found"", on September 9, 1947."
+1196,There are several methods of classifying exploits. The most common is by how the exploit communicates to the vulnerable software.
+1197,A remote exploit works over a network and exploits the security vulnerability without any prior access to the vulnerable system.
+1198,"A local exploit requires prior access to the vulnerable system and usually increases the privileges of the person running the exploit past those granted by the system administrator. Exploits against client applications also exist, usually consisting of modified servers that send an exploit if accessed with a client application. A common form of exploits against client applications are browser exploits."
+1199,"Exploits against client applications may also require some interaction with the user and thus may be used in combination with the social engineering method. Another classification is by the action against the vulnerable system; unauthorized data access, arbitrary code execution, and denial of service are examples."
+1200,"Many exploits are designed to provide superuser-level access to a computer system. However, it is also possible to use several exploits, first to gain low-level access, then to escalate privileges repeatedly until one reaches the highest administrative level . In this case the attacker is chaining several exploits together to perform one attack, this is known as an exploit chain."
+1201,"After an exploit is made known to the authors of the affected software, the vulnerability is often fixed through a patch and the exploit becomes unusable. That is the reason why some black hat hackers as well as military or intelligence agencies' hackers do not publish their exploits but keep them private."
+1202,Exploits unknown to everyone except the people that found and developed them are referred to as zero day or “0day” exploits.
+1203,"Exploits are used by hackers to bypass security controls and manipulate system vulnerabilities. Researchers have estimated that this costs over $450 billion every year from the global economy. In response, organizations are using cyber threat intelligence to protect their vulnerabilities."
+1204,"Exploitations are commonly categorized and named by the type of vulnerability they exploit , whether they are local/remote and the result of running the exploit . One scheme that offers zero day exploits is exploit as a service."
+1205,"A zero-click attack is an exploit that requires no user interaction to operate – that is to say, no key-presses or mouse clicks. FORCEDENTRY, discovered in 2021, is an example of a zero-click attack."
+1206,These exploits are commonly the most sought after exploits  because the target typically has no way of knowing they have been compromised at the time of exploitation.
+1207,"In 2022, NSO Group was reportedly selling zero-click exploits to governments for breaking into individuals' phones."
+1208,"Pivoting is a method used by hackers and penetration testers to expand the attack surface of a target organization. A compromised system to attack other systems on the same network that are not directly reachable from the Internet due to restrictions such as firewall. There tend to be more machines reachable from inside a network as compared to Internet facing hosts. For example, if an attacker compromises a web server on a corporate network, the attacker can then use the compromised web server to attack any reachable system on the network. These types of attacks are often called multi-layered attacks. Pivoting is also known as island hopping."
+1209,Pivoting can further be distinguished into proxy pivoting and VPN pivoting:
+1210,"Typically, the proxy or VPN applications enabling pivoting are executed on the target computer as the payload of an exploit."
+1211,"Pivoting is usually done by infiltrating a part of a network infrastructure  and using a scanner to find other devices connected to attack them. By attacking a vulnerable piece of networking, an attacker could infect most or all of a network and gain complete control."
+1212,"In computing, a crash, or system crash, occurs when a computer program such as a software application or an operating system stops functioning properly and exits. On some operating systems or individual applications, a crash reporting service will report the crash and any details relating to it , usually to the developer of the application. If the program is a critical part of the operating system, the entire system may crash or hang, often resulting in a kernel panic or fatal system error."
+1213,"Most crashes are the result of a software bug. Typical causes include accessing invalid memory addresses, incorrect address values in the program counter, buffer overflow, overwriting a portion of the affected program code due to an earlier bug, executing invalid machine instructions , or triggering an unhandled exception. The original software bug that started this chain of events is typically considered to be the cause of the crash, which is discovered through the process of debugging. The original bug can be far removed from the code that actually triggered the crash."
+1214,"In early personal computers, attempting to write data to hardware addresses outside the system's main memory could cause hardware damage. Some crashes are exploitable and let a malicious program or hacker execute arbitrary code, allowing the replication of viruses or the acquisition of data which would normally be inaccessible."
+1215,An application typically crashes when it performs an operation that is not allowed by the operating system. The operating system then triggers an exception or signal in the application. Unix applications traditionally responded to the signal by dumping core. Most Windows and Unix GUI applications respond by displaying a dialogue box  with the option to attach a debugger if one is installed. Some applications attempt to recover from the error and continue running instead of exiting.
+1216,An application can also contain code to crash after detecting a severe error.
+1217,Typical errors that result in application crashes include:
+1218,"A ""crash to desktop"" is said to occur when a program  unexpectedly quits, abruptly taking the user back to the desktop. Usually, the term is applied only to crashes where no error is displayed, hence all the user sees as a result of the crash is the desktop. Many times there is no apparent action that causes a crash to desktop. During normal function, the program may freeze for a shorter period of time, and then close by itself. Also during normal function, the program may become a black screen and repeatedly play the last few seconds of sound  that was being played before it crashes to desktop. Other times it may appear to be triggered by a certain action, such as loading an area."
+1219,"Crash to desktop bugs are considered particularly problematic for users. Since they frequently display no error message, it can be very difficult to track down the source of the problem, especially if the times they occur and the actions taking place right before the crash do not appear to have any pattern or common ground. One way to track down the source of the problem for games is to run them in windowed-mode. Windows Vista has a feature that can help track down the cause of a CTD problem when it occurs on any program. Windows XP included a similar feature as well."
+1220,"Some computer programs, such as StepMania and BBC's Bamzooki, also crash to desktop if in full-screen, but display the error in a separate window when the user has returned to the desktop."
+1221,"The software running the web server behind a website may crash, rendering it inaccessible entirely or providing only an error message instead of normal content."
+1222,"For example: if a site is using an SQL database  for a script  and that SQL database server crashes, then PHP will display a connection error."
+1223,An operating system crash commonly occurs when a hardware exception occurs that cannot be handled. Operating system crashes can also occur when internal sanity-checking logic within the operating system detects that the operating system has lost its internal self-consistency.
+1224,"Modern multi-tasking operating systems, such as Linux, and macOS, usually remain unharmed when an application program crashes."
+1225,"Some operating systems, e.g., z/OS, have facilities for Reliability, availability and serviceability  and the OS can recover from the crash of a critical component, whether due to hardware failure, e.g., uncorrectable ECC error, or to software failure, e.g., a reference to an unassigned page."
+1226,"An Abnormal end or ABEND is an abnormal termination of software, or a program crash. Errors or crashes on the Novell NetWare network operating system are usually called ABENDs. Communities of NetWare administrators sprung up around the Internet, such as abend.org."
+1227,"This usage derives from the ABEND macro on IBM OS/360, ..., z/OS operating systems. Usually capitalized, but may appear as ""abend"". Some common ABEND codes are System ABEND 0C7  and System ABEND 0CB . Abends can be ""soft""  or ""hard"" .The term is jocularly claimed to be derived from the German word ""abend"" meaning ""evening""."
+1228,"Depending on the application, the crash may contain the user's sensitive and private information. Moreover, many software bugs which cause crashes are also exploitable for arbitrary code execution and other types of privilege escalation. For example, a stack buffer overflow can overwrite the return address of a subroutine with an invalid value, which will cause, e.g., a segmentation fault, when the subroutine returns. However, if an exploit overwrites the return address with a valid value, the code in that address will be executed."
+1229,"When crashes are collected in the field using a crash reporter, the next step for developers is to be able to reproduce them locally. For this, several techniques exist:
+STAR uses symbolic execution,
+EvoCrash performs evolutionary search."
+1230,"Hangs have varied causes and symptoms, including software or hardware defects, such as an infinite loop or long-running uninterruptible computation, resource exhaustion , under-performing hardware , external events such as a slow computer network, misconfiguration, and compatibility problems. The fundamental reason is typically resource exhaustion: resources necessary for some part of the system to run are not available, due to being in use by other processes or simply insufficient. Often the cause is an interaction of multiple factors, making ""hang"" a loose umbrella term rather than a technical one."
+1231,"A hang may be temporary if caused by a condition that resolves itself, such as slow hardware, or it may be permanent and require manual intervention, as in the case of a hardware or software logic error. Many modern operating systems provide the user with a means to forcibly terminate a hung program without rebooting or logging out; some operating systems, such as those designed for mobile devices, may even do this automatically. In more severe hangs affecting the whole system, the only solution might be to reboot the machine, usually by power cycling with an off/on or reset button."
+1232,"A hang differs from a crash, in which the failure is immediate and unrelated to the responsiveness of inputs."
+1233,"In a multitasking operating system, it is possible for an individual process or thread to get stuck, such as blocking on a resource or getting into an infinite loop, though the effect on the overall system varies significantly. In a cooperative multitasking system, any thread that gets stuck without yielding will hang the system, as it will wedge itself as the running thread and prevent other threads from running."
+1234,"By contrast, modern operating systems primarily use pre-emptive multitasking, such as Windows 2000 and its successors, as well as Linux and Apple Inc.'s macOS.  In these cases, a single thread getting stuck will not necessarily hang the system, as the operating system will preempt it when its time slice expires, allowing another thread to run. If a thread does hang, the scheduler may switch to another group of interdependent tasks so that all processes will not hang. However, a stuck thread will still consume resources: at least an entry in scheduling, and if it is running , it will consume processor cycles and power when it is scheduled, slowing the system though it does not hang it."
+1235,"However, even with preemptive multitasking, a system can hang, and a misbehaved or malicious task can hang the system, primarily by monopolizing some other resource, such as IO or memory, even though processor time cannot be monopolized. For example, a process that blocks the file system will often hang the system."
+1236,Moving around a window on top of a hanging program during a hang may cause a window trail from redrawing.
+1237,"Hardware can cause a computer to hang, either because it is intermittent or because it is mismatched with other hardware in the computer . Hardware can also become defective over time due to dirt or heat damage."
+1238,"A hang can also occur due to the fact that the programmer has incorrect termination conditions for a loop, or, in a co-operative multitasking operating system, forgetting to yield to other tasks. Said differently, many software-related hangs are caused by threads waiting for an event to occur which will never occur. This is also known as an infinite loop."
+1239,"Another cause of hangs is a race condition in communication between processes. One process may send a signal to a second process then stop execution until it receives a response. If the second process is busy the signal will be forced to wait until the process can get to it. However, if the second process was busy sending a signal to the first process then both processes would wait forever for the other to respond to signals and never see the other’s signal . If the processes are uninterruptible they will hang and have to be shut down. If at least one of the processes is a critical kernel process the whole system may hang and have to be restarted."
+1240,"A computer may seem to hang when in fact it is simply processing very slowly. This can be caused by too many programs running at once, not enough memory , or memory fragmentation, slow hardware access , slow system APIs, etc. It can also be caused by hidden programs which were installed surreptitiously, such as spyware."
+1241,"In many cases programs may appear to be hung, but are making slow progress, and waiting a few minutes will allow the task to complete."
+1242,"Modern operating systems provide a mechanism for terminating hung processes, for instance, with the Unix kill command, or through a graphical means such as the Task Manager's ""end task"" button in Windows .  Older systems, such as those running MS-DOS, early versions of Windows, or Classic Mac OS often needed to be completely restarted in the event of a hang."
+1243,"On embedded devices where human interaction is limited, a watchdog timer can reboot the computer in the event of a hang."
+1244,"A software bug is an error, flaw or fault in the design, development, or operation of computer software that causes it to produce an incorrect or unexpected result, or to behave in unintended ways. The process of finding and correcting bugs is termed ""debugging"" and often uses formal techniques or tools to pinpoint bugs. Since the 1950s, some computer systems have been designed to detect or auto-correct various software errors during operations."
+1245,"Bugs in software can arise from mistakes and errors made in interpreting and extracting users' requirements, planning a program's design, writing its source code, and from interaction with humans, hardware and programs, such as operating systems or libraries. A program with many, or serious, bugs is often described as buggy. Bugs can trigger errors that may have ripple effects. The effects of bugs may be subtle, such as unintended text formatting, through to more obvious effects such as causing a program to crash, freezing the computer, or causing damage to hardware. Other bugs qualify as security bugs and might, for example, enable a malicious user to bypass access controls in order to obtain unauthorized privileges."
+1246,"Some software bugs have been linked to disasters. Bugs in code that controlled the Therac-25 radiation therapy machine were directly responsible for patient deaths in the 1980s. In 1996, the European Space Agency's US$1 billion prototype Ariane 5 rocket was destroyed less than a minute after launch due to a bug in the on-board guidance computer program. In 1994, an RAF Chinook helicopter crashed, killing 29; this was initially blamed on pilot error, but was later thought to have been caused by a software bug in the engine-control computer. Buggy software caused the early 21st century British Post Office scandal, the most widespread miscarriage of justice in British legal history."
+1247,"In 2002, a study commissioned by the US Department of Commerce's National Institute of Standards and Technology concluded that ""software bugs, or errors, are so prevalent and so detrimental that they cost the US economy an estimated $59 billion annually, or about 0.6 percent of the gross domestic product""."
+1248,"The Middle English word bugge is the basis for the terms ""bugbear"" and ""bugaboo"" as terms used for a monster."
+1249,"The term ""bug"" to describe defects has been a part of engineering jargon since the 1870s and predates electronics and computers; it may have originally been used in hardware engineering to describe mechanical malfunctions. For instance, Thomas Edison wrote in a letter to an associate in 1878:"
+1250,"Baffle Ball, the first mechanical pinball game, was advertised as being ""free of bugs"" in 1931. Problems with military gear during World War II were referred to as bugs . In a book published in 1942, Louise Dickinson Rich, speaking of a powered ice cutting machine, said, ""Ice sawing was suspended until the creator could be brought in to take the bugs out of his darling."""
+1251,"Isaac Asimov used the term ""bug"" to relate to issues with a robot in his short story ""Catch That Rabbit"", published in 1944."
+1252,"The term ""bug"" was used in an account by computer pioneer Grace Hopper, who publicized the cause of a malfunction in an early electromechanical computer. A typical version of the story is:"
+1253,"Hopper was not present when the bug was found, but it became one of her favorite stories. The date in the log book was September 9, 1947. The operators who found it, including William ""Bill"" Burke, later of the Naval Weapons Laboratory, Dahlgren, Virginia, were familiar with the engineering term and amusedly kept the insect with the notation ""First actual case of bug being found."" This log book, complete with attached moth, is part of the collection of the Smithsonian National Museum of American History."
+1254,"The related term ""debug"" also appears to predate its usage in computing: the Oxford English Dictionary's etymology of the word contains an attestation from 1945, in the context of aircraft engines."
+1255,"The concept that software might contain errors dates back to Ada Lovelace's 1843 notes on the analytical engine, in which she speaks of the possibility of program ""cards"" for Charles Babbage's analytical engine being erroneous:"
+1256,"While the use of the term ""bug"" to describe software errors is common, many have suggested that it should be abandoned. One argument is that the word ""bug"" is divorced from a sense that a human being caused the problem, and instead implies that the defect arose on its own, leading to a push to abandon the term ""bug"" in favor of terms such as ""defect"", with limited success."
+1257,"The term ""bug"" may also be used to cover up an intentional design decision. In 2011, after receiving scrutiny from US Senator Al Franken for recording and storing users' locations in unencrypted files, Apple called the behavior a bug. However, Justin Brookman of the Center for Democracy and Technology directly challenged that portrayal, stating ""I'm glad that they are fixing what they call bugs, but I take exception with their strong denial that they track users."""
+1258,"In software engineering, mistake metamorphism  refers to the evolution of a defect in the final stage of software deployment. Transformation of a ""mistake"" committed by an analyst in the early stages of the software development lifecycle, which leads to a ""defect"" in the final stage of the cycle has been called 'mistake metamorphism'."
+1259,"Different stages of a ""mistake"" in the entire cycle may be described as ""mistakes"", ""anomalies"", ""faults"", ""failures"", ""errors"", ""exceptions"", ""crashes"", ""glitches"", ""bugs"", ""defects"", ""incidents"", or ""side effects""."
+1260,The software industry has put much effort into reducing bug counts. These include:
+1261,"Bugs usually appear when the programmer makes a logic error. Various innovations in programming style and defensive programming are designed to make these bugs less likely, or easier to spot. Some typos, especially of symbols or operators, allow the program to operate incorrectly, while others such as a missing symbol or misspelled name may prevent the program from operating. Compiled languages can reveal some typos when the source code is compiled."
+1262,"Several schemes assist managing programmer activity so that fewer bugs are produced. Software engineering  applies many techniques to prevent defects. For example, formal program specifications state the exact behavior of programs so that design bugs may be eliminated. Formal specifications are impractical for anything but the shortest programs, because of problems of combinatorial explosion and indeterminacy."
+1263,Unit testing involves writing a test for every function  that a program is to perform.
+1264,In test-driven development unit tests are written before the code and the code is not considered complete until all tests complete successfully.
+1265,Agile software development involves frequent software releases with relatively small changes. Defects are revealed by user feedback.
+1266,"Open source development allows anyone to examine source code. A school of thought popularized by Eric S. Raymond as Linus's law says that popular open-source software has more chance of having few or no bugs than other software, because ""given enough eyeballs, all bugs are shallow"". This assertion has been disputed, however: computer security specialist Elias Levy wrote that ""it is easy to hide vulnerabilities in complex, little understood and undocumented source code,"" because, ""even if people are reviewing the code, that doesn't mean they're qualified to do so."" An example of an open-source software bug was the 2008 OpenSSL vulnerability in Debian."
+1267,"Programming languages include features to help prevent bugs, such as static type systems, restricted namespaces and modular programming. For example, when a programmer writes  LET REAL_VALUE PI = ""THREE AND A BIT"", although this may be syntactically correct, the code fails a type check. Compiled languages catch this without having to run the program. Interpreted languages catch such errors at runtime. Some languages deliberately exclude features that easily lead to bugs, at the expense of slower performance: the general principle being that, it is almost always better to write simpler, slower code than inscrutable code that runs slightly faster, especially considering that maintenance cost is substantial. For example, the Java programming language does not support pointer arithmetic; implementations of some languages such as Pascal and scripting languages often have runtime bounds checking of arrays, at least in a debugging build."
+1268,"Tools for code analysis help developers by inspecting the program text beyond the compiler's capabilities to spot potential problems. Although in general the problem of finding all programming errors given a specification is not solvable , these tools exploit the fact that human programmers tend to make certain kinds of simple mistakes often when writing software."
+1269,"Tools to monitor the performance of the software as it is running, either specifically to find problems such as bottlenecks or to give assurance as to correct working, may be embedded in the code explicitly , or provided as tools. It is often a surprise to find where most of the time is taken by a piece of code, and this removal of assumptions might cause the code to be rewritten."
+1270,"Software testers are people whose primary task is to find bugs, or write code to support testing. On some efforts, more resources may be spent on testing than in developing the program."
+1271,Measurements during testing can provide an estimate of the number of likely bugs remaining; this becomes more reliable the longer a product is tested and developed.
+1272,"Finding and fixing bugs, or debugging, is a major part of computer programming. Maurice Wilkes, an early computing pioneer, described his realization in the late 1940s that much of the rest of his life would be spent finding mistakes in his programs."
+1273,"Usually, the most difficult part of debugging is finding the bug. Once it is found, correcting it is usually relatively easy. Programs known as debuggers help programmers locate bugs by executing code line by line, watching variable values, and other features to observe program behavior. Without a debugger, code may be added so that messages or values may be written to a console or to a window or log file to trace program execution or show values."
+1274,"However, even with the aid of a debugger, locating bugs is something of an art. It is not uncommon for a bug in one section of a program to cause failures in a completely different section, thus making it especially difficult to track , in an apparently unrelated part of the system."
+1275,"Sometimes, a bug is not an isolated flaw, but represents an error of thinking or planning on the part of the programmer. Such logic errors require a section of the program to be overhauled or rewritten. As a part of code review, stepping through the code and imagining or transcribing the execution process may often find errors without ever reproducing the bug as such."
+1276,"More typically, the first step in locating a bug is to reproduce it reliably. Once the bug is reproducible, the programmer may use a debugger or other tool while reproducing the error to find the point at which the program went astray."
+1277,"Some bugs are revealed by inputs that may be difficult for the programmer to re-create. One cause of the Therac-25 radiation machine deaths was a bug  that occurred only when the machine operator very rapidly entered a treatment plan; it took days of practice to become able to do this, so the bug did not manifest in testing or when the manufacturer attempted to duplicate it. Other bugs may stop occurring whenever the setup is augmented to help find the bug, such as running the program with a debugger; these are called heisenbugs ."
+1278,"Since the 1990s, particularly following the Ariane 5 Flight 501 disaster, interest in automated aids to debugging rose, such as static code analysis by abstract interpretation."
+1279,"Some classes of bugs have nothing to do with the code. Faulty documentation or hardware may lead to problems in system use, even though the code matches the documentation. In some cases, changes to the code eliminate the problem even though the code then no longer matches the documentation. Embedded systems frequently work around hardware bugs, since to make a new version of a ROM is much cheaper than remanufacturing the hardware, especially if they are commodity items."
+1280,"To facilitate reproducible research on testing and debugging, researchers use curated benchmarks of bugs:"
+1281,"Bug management includes the process of documenting, categorizing, assigning, reproducing, correcting and releasing the corrected code. Proposed changes to software – bugs as well as enhancement requests and even entire releases – are commonly tracked and managed using bug tracking systems or issue tracking systems. The items added may be called defects, tickets, issues, or, following the agile development paradigm, stories and epics. Categories may be objective, subjective or a combination, such as version number, area of the software, severity and priority, as well as what type of issue it is, such as a feature request or a bug."
+1282,"A bug triage reviews bugs and decides whether and when to fix them. The decision is based on the bug's priority, and factors such as development schedules. The triage is not meant to investigate the cause of bugs, but rather the cost of fixing them. The triage happens regularly, and goes through bugs opened or reopened since the previous meeting. The attendees of the triage process typically are the project manager, development manager, test manager, build manager, and technical experts."
+1283,"Severity is the intensity of the impact the bug has on system operation. This impact may be data loss, financial, loss of goodwill and wasted effort. Severity levels are not standardized. Impacts differ across industry. A crash in a video game has a totally different impact than a crash in a web browser, or real time monitoring system. For example, bug severity levels might be ""crash or hang"", ""no workaround"" , ""has workaround"" , ""visual defect"" , or ""documentation error"". Some software publishers use more qualified severities such as ""critical"", ""high"", ""low"", ""blocker"" or ""trivial"". The severity of a bug may be a separate category to its priority for fixing, and the two may be quantified and managed separately."
+1284,"Priority controls where a bug falls on the list of planned changes. The priority is decided by each software producer. Priorities may be numerical, such as 1 through 5, or named, such as ""critical"", ""high"", ""low"", or ""deferred"". These rating scales may be similar or even identical to severity ratings, but are evaluated as a combination of the bug's severity with its estimated effort to fix; a bug with low severity but easy to fix may get a higher priority than a bug with moderate severity that requires excessive effort to fix. Priority ratings may be aligned with product releases, such as ""critical"" priority indicating all the bugs that must be fixed before the next software release."
+1285,"A bug severe enough to delay or halt the release of the product is called a ""show stopper"" or ""showstopper bug"". It is named so because it ""stops the show"" – causes unacceptable product failure."
+1286,"It is common practice to release software with known, low-priority bugs. Bugs of sufficiently high priority may warrant a special release of part of the code containing only modules with those fixes. These are known as patches. Most releases include a mixture of behavior changes and multiple bug fixes. Releases that emphasize bug fixes are known as maintenance releases, to differentiate it from major releases that emphasize feature additions or changes."
+1287,Reasons that a software publisher opts not to patch or even fix a particular bug include:
+1288,"In software development, a mistake or error may be introduced at any stage. Bugs arise from oversight or misunderstanding by a software team during specification, design, coding, configuration, data entry or documentation. For example, a relatively simple program to alphabetize a list of words, the design might fail to consider what should happen when a word contains a hyphen. Or when converting an abstract design into code, the coder might inadvertently create an off-by-one error which can be a ""<"" where ""<="" was intended, and fail to sort the last word in a list."
+1289,"Another category of bug is called a race condition that may occur when programs have multiple components executing at the same time. If the components interact in a different order than the developer intended, they could interfere with each other and stop the program from completing its tasks. These bugs may be difficult to detect or anticipate, since they may not occur during every execution of a program."
+1290,"Conceptual errors are a developer's misunderstanding of what the software must do. The resulting software may perform according to the developer's understanding, but not what is really needed. Other types:"
+1291,"In operations on numerical values, problems can arise that result in unexpected output, slowing of a process, or crashing. These can be from a lack of awareness of the qualities of the data storage such as a loss of precision due to rounding, numerically unstable algorithms, arithmetic overflow and underflow, or from lack of awareness of how calculations are handled by different software coding languages such as division by zero which in some languages may throw an exception, and in others may return a special value such as NaN or infinity."
+1292,"Control flow bugs are those found in processes with valid logic, but that lead to unintended results, such as infinite loops and infinite recursion, incorrect comparisons for conditional statements such as using the incorrect comparison operator, and off-by-one errors ."
+1293,"The amount and type of damage a software bug may cause naturally affects decision-making, processes and policy regarding software quality. In applications such as human spaceflight, aviation, nuclear power, health care, public transport or automotive safety, since software flaws have the potential to cause human injury or even death, such software will have far more scrutiny and quality control than, for example, an online shopping website. In applications such as banking, where software flaws have the potential to cause serious financial damage to a bank or its customers, quality control is also more important than, say, a photo editing application."
+1294,"Other than the damage caused by bugs, some of their cost is due to the effort invested in fixing them. In 1978, Lientz et al. showed that the median of projects invest 17 percent of the development effort in bug fixing. In 2020, research on GitHub repositories showed the median is 20%."
+1295,"In 1994, NASA's Goddard Space Flight Center managed to reduce their average number of errors from 4.5 per 1000 lines of code  down to 1 per 1000 SLOC."
+1296,"Another study in 1990 reported that exceptionally good software development processes can achieve deployment failure rates as low as 0.1 per 1000 SLOC. This figure is iterated in literature such as Code Complete by Steve McConnell, and the NASA study on Flight Software Complexity. Some projects even attained zero defects: the firmware in the IBM Wheelwriter typewriter which consists of 63,000 SLOC, and the Space Shuttle software with 500,000 SLOC."
+1297,"A number of software bugs have become well-known, usually due to their severity: examples include various space and military aircraft crashes. Possibly the most famous bug is the Year 2000 problem or Y2K bug, which caused many programs written long before the transition from 19xx to 20xx dates to malfunction. For instance, the date ""25 December 04,"" referring to 2004, may have been incorrectly treated as 1904, or the year ""2000"" may have been incorrectly displayed as ""19100.""  As a result of a major effort taken by programmers near the end of the 20th century, the most severe consequences of this bug were averted. "
+1298,The 2012 stock trading disruption involved one such incompatibility between the old API and a new API.
+1299,"The Open Technology Institute, run by the group, New America, released a report ""Bugs in the System"" in August 2016 stating that U.S. policymakers should make reforms to help researchers identify and address software bugs. The report ""highlights the need for reform in the field of software vulnerability discovery and disclosure."" One of the report's authors said that Congress has not done enough to address cyber software vulnerability, even though Congress has passed a number of bills to combat the larger issue of cyber security."
+1300,"Government researchers, companies, and cyber security experts are the people who typically discover software flaws. The report calls for reforming computer crime and copyright laws."
+1301,"Software is a collection of programs and data that tell a computer how to perform specific tasks. Software often includes associated software documentation. This is in contrast to hardware, from which the system is built and which actually performs the work."
+1302,"At the lowest programming level, executable code consists of machine language instructions supported by an individual processor—typically a central processing unit  or a graphics processing unit . Machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. For example, an instruction may change the value stored in a particular storage location in the computer—an effect that is not directly observable to the user. An instruction may also invoke one of many input or output operations, for example, displaying some text on a computer screen, causing state changes that should be visible to the user. The processor executes the instructions in the order they are provided, unless it is instructed to ""jump"" to a different instruction or is interrupted by the operating system. As of 2024, most personal computers, smartphone devices, and servers have processors with multiple execution units, or multiple processors performing computation together, so computing has become a much more concurrent activity than in the past."
+1303,"The majority of software is written in high-level programming languages. They are easier and more efficient for programmers because they are closer to natural languages than machine languages. High-level languages are translated into machine language using a compiler, an interpreter, or a combination of the two. Software may also be written in a low-level assembly language that has a strong correspondence to the computer's machine language instructions and is translated into machine language using an assembler."
+1304,"An algorithm for what would have been the first piece of software was written by Ada Lovelace in the 19th century, for the planned Analytical Engine. She created proofs to show how the engine would calculate Bernoulli numbers. Because of the proofs and the algorithm, she is considered the first computer programmer."
+1305,"The first theory about software, prior to the creation of computers as we know them today, was proposed by Alan Turing in his 1936 essay, On Computable Numbers, with an Application to the Entscheidungsproblem . This eventually led to the creation of the academic fields of computer science and software engineering; both fields study software and its creation. Computer science is the theoretical study of computer and software , whereas software engineering is the application of engineering principles to development of software."
+1306,"In 2000, Fred Shapiro, a librarian at the Yale Law School, published a letter revealing that John Wilder Tukey's 1958 paper ""The Teaching of Concrete Mathematics"" contained the earliest known usage of the term ""software"" found in a search of JSTOR's electronic archives, predating the Oxford English Dictionary's citation by two years. This led many to credit Tukey with coining the term, particularly in obituaries published that same year, although Tukey never claimed credit for any such coinage. In 1995, Paul Niquette claimed he had originally coined the term in October 1953, although he could not find any documents supporting his claim. The earliest known publication of the term ""software"" in an engineering context was in August 1953 by Richard R. Carhart, in a Rand Corporation Research Memorandum."
+1307,"On virtually all computer platforms, software can be grouped into a few broad categories."
+1308,"Based on the goal, computer software can be divided into:"
+1309,"Programming tools are also software in the form of programs or applications that developers use to create, debug, maintain, or otherwise support software."
+1310,"Software is written in one or more programming languages; there are many programming languages in existence, and each has at least one implementation, each of which consists of its own set of programming tools. These tools may be relatively self-contained programs such as compilers, debuggers, interpreters, linkers, and text editors, that can be combined to accomplish a task; or they may form an integrated development environment , which combines much or all of the functionality of such self-contained tools. IDEs may do this by either invoking the relevant individual tools or by re-implementing their functionality in a new way. An IDE can make it easier to do specific tasks, such as searching in files in a particular project. Many programming language implementations provide the option of using both individual tools or an IDE."
+1311,"People who use modern general purpose computers  usually see three layers of software performing a variety of tasks: platform, application, and user software."
+1312,"Computer software has to be ""loaded"" into the computer's storage . Once the software has loaded, the computer is able to execute the software. This involves passing instructions from the application software, through the system software, to the hardware which ultimately receives the instruction as machine code. Each instruction causes the computer to carry out an operation—moving data, carrying out a computation, or altering the control flow of instructions."
+1313,"Data movement is typically from one place in memory to another. Sometimes it involves moving data between memory and registers which enable high-speed data access in the CPU. Moving data, especially large amounts of it, can be costly; this is sometimes avoided by using ""pointers"" to data instead. Computations include simple operations such as incrementing the value of a variable data element. More complex computations may involve many operations and data elements together."
+1314,"Software quality is very important, especially for commercial and system software. If software is faulty, it can delete a person's work, crash the computer and do other unexpected things. Faults and errors are called ""bugs"" which are often discovered during alpha and beta testing. Software is often also a victim to what is known as software aging, the progressive performance degradation resulting from a combination of unseen bugs."
+1315,"Many bugs are discovered and fixed through software testing. However, software testing rarely—if ever—eliminates every bug; some programmers say that ""every program has at least one more bug"" . In the waterfall method of software development, separate testing teams are typically employed, but in newer approaches, collectively termed agile software development, developers often do all their own testing, and demonstrate the software to users/clients regularly to obtain feedback. Software can be tested through unit testing, regression testing and other methods, which are done manually, or most commonly, automatically, since the amount of code to be tested can be large. Programs containing command software enable hardware engineering and system operations to function much easier together."
+1316,"The software's license gives the user the right to use the software in the licensed environment, and in the case of free software licenses, also grants other rights such as the right to make copies."
+1317,Proprietary software can be divided into two types:
+1318,"Open-source software comes with a free software license, granting the recipient the rights to modify and redistribute the software."
+1319,"Software patents, like other types of patents, are theoretically supposed to give an inventor an exclusive, time-limited license for a detailed idea  on how to implement a piece of software, or a component of a piece of software. Ideas for useful things that software could do, and user requirements, are not supposed to be patentable, and concrete implementations  are not supposed to be patentable either—the latter are already covered by copyright, generally automatically. So software patents are supposed to cover the middle area, between requirements and concrete implementation. In some countries, a requirement for the claimed invention to have an effect on the physical world may also be part of the requirements for a software patent to be held valid—although since all useful software has effects on the physical world, this requirement may be open to debate. Meanwhile, American copyright law was applied to various aspects of the writing of the software code."
+1320,"Software patents are controversial in the software industry with many people holding different views about them. One of the sources of controversy is that the aforementioned split between initial ideas and patent does not seem to be honored in practice by patent lawyers—for example the patent for aspect-oriented programming , which purported to claim rights over any programming tool implementing the idea of AOP, howsoever implemented. Another source of controversy is the effect on innovation, with many distinguished experts and companies arguing that software is such a fast-moving field that software patents merely create vast additional litigation costs and risks, and actually retard innovation. In the case of debates about software patents outside the United States, the argument has been made that large American corporations and patent lawyers are likely to be the primary beneficiaries of allowing or continue to allow software patents."
+1321,"Design and implementation of software vary depending on the complexity of the software. For instance, the design and creation of Microsoft Word took much more time than designing and developing Microsoft Notepad because the former has much more basic functionality."
+1322,"Software is usually developed in integrated development environments  like Eclipse, IntelliJ and Microsoft Visual Studio that can simplify the process and compile the software. As noted in a different section, software is usually created on top of existing software and the application programming interface  that the underlying software provides like GTK+, JavaBeans or Swing. Libraries  can be categorized by their purpose. For instance, the Spring Framework is used for implementing enterprise applications, the Windows Forms library is used for designing graphical user interface  applications like Microsoft Word, and Windows Communication Foundation is used for designing web services. When a program is designed, it relies upon the API. For instance, a Microsoft Windows desktop application might call API functions in the .NET Windows Forms library like Form1.Close and Form1.Show to close or open the application. Without these APIs, the programmer needs to write these functionalities entirely themselves. Companies like Oracle and Microsoft provide their own APIs so that many applications are written using their software libraries that usually have numerous APIs in them."
+1323,"Data structures such as hash tables, arrays, and binary trees, and algorithms such as quicksort, can be useful for creating software."
+1324,"Computer software has special economic characteristics that make its design, creation, and distribution different from most other economic goods."
+1325,"A person who creates software is called a programmer, software engineer or software developer, terms that all have a similar meaning. More informal terms for programmer also exist such as ""coder"" and ""hacker"" – although use of the latter word may cause confusion, because it is more often used to mean someone who illegally breaks into computer systems."
+1326,"A video game console is an electronic device that outputs a video signal or image to display a video game that can be played with a game controller. These may be home consoles, which are generally placed in a permanent location connected to a television or other display devices and controlled with a separate game controller, or handheld consoles, which include their own display unit and controller functions built into the unit and which can be played anywhere. Hybrid consoles combine elements of both home and handheld consoles."
+1327,"Video game consoles are a specialized form of a home computer geared towards video game playing, designed with affordability and accessibility to the general public in mind, but lacking in raw computing power and customization. Simplicity is achieved in part through the use of game cartridges or other simplified methods of distribution, easing the effort of launching a game. However, this leads to ubiquitous proprietary formats that create competition for market share. More recent consoles have shown further confluence with home computers, making it easy for developers to release games on multiple platforms. Further, modern consoles can serve as replacements for media players with capabilities to play films and music from optical media or streaming media services."
+1328,"Video game consoles are usually sold on a 5–7 year cycle called a generation, with consoles made with similar technical capabilities or made around the same time period grouped into one generation. The industry has developed a razor and blades model: manufacturers often sell consoles at low prices, sometimes at a loss, while primarily making a profit from the licensing fees for each game sold. Planned obsolescence then draws consumers into buying the next console generation. While numerous manufacturers have come and gone in the history of the console market, there have always been two or three dominant leaders in the market, with the current market led by Sony , Microsoft , and Nintendo . Previous console developers include Sega, Atari, Coleco, Mattel, NEC, SNK, Fujitsu, and 3D0."
+1329,"The first video game consoles were produced in the early 1970s. Ralph H. Baer devised the concept of playing simple, spot-based games on a television screen in 1966, which later became the basis of the Magnavox Odyssey in 1972. Inspired by the table tennis game on the Odyssey, Nolan Bushnell, Ted Dabney, and Allan Alcorn at Atari, Inc. developed the first successful arcade game, Pong, and looked to develop that into a home version, which was released in 1975. The first consoles were capable of playing only a very limited number of games built into the hardware. Programmable consoles using swappable ROM cartridges were introduced with the Fairchild Channel F in 1976, though popularized with the Atari 2600 released in 1977."
+1330,"Handheld consoles emerged from technology improvements in handheld electronic games as these shifted from mechanical to electronic/digital logic, and away from light-emitting diode  indicators to liquid-crystal displays  that resembled video screens more closely. Early examples include the Microvision in 1979 and Game & Watch in 1980, and the concept was fully realized by the Game Boy in 1989."
+1331,"Both home and handheld consoles have become more advanced following global changes in technology. These technological shifts include including improved electronic and computer chip manufacturing to increase computational power at lower costs and size, the introduction of 3D graphics and hardware-based graphic processors for real-time rendering, digital communications such as the Internet, wireless networking and Bluetooth, and larger and denser media formats as well as digital distribution."
+1332,"Following the same type of Moore's law progression, home consoles are grouped into generations; each lasting approximately five years. Consoles within each generation share similar specifications and features, such as processor word size. While no one grouping of consoles by generation is universally accepted, one breakdown of generations, showing representative consoles, of each is shown below."
+1333,"Most consoles are considered programmable consoles and have the means for the player to switch between different games. Traditionally, this has been done by switching a physical game cartridge or game card or through using optical media. It is now common to download games through digital distribution and store them on internal or external digital storage devices."
+1334,Dedicated consoles were very popular in the first generation until they were gradually replaced by second generation that use ROM cartridges. The fourth generation gradually merged to optical media. It is now common to download games through digital distribution and store them on internal or external digital storage devices.
+1335,"Some consoles are considered dedicated consoles, in which games available for the console are ""baked"" onto the hardware, either by being programmed via the circuitry or set in the read-only flash memory of the console. Thus, the console's game library cannot be added to or changed directly by the user. The user can typically switch between games on dedicated consoles using hardware switches on the console, or through in-game menus. Dedicated consoles were common in the first generation of home consoles, such as the Magnavox Odyssey and the home console version of Pong, and more recently have been used for retro-consoles such as the NES Classic Edition and Sega Genesis Mini."
+1336,"Home video game consoles are meant to be connected to a television or other type of monitor, with power supplied through an outlet. This requires the unit to be used in a fixed location, typically at home in one's living room. Separate game controllers, connected through wired or wireless connections, are used to provide input to the game. Early examples include the Atari 2600, the Nintendo Entertainment System, and the Sega Genesis; newer examples include the Wii U, the PlayStation 5, and the Xbox Series X. Specific types of home consoles include:"
+1337,"Handheld game consoles are devices that typically include a built-in screen and game controller in their case, and contain a rechargeable battery or battery compartment. This allows the unit to be carried around and played anywhere, in contrast to a home game console. Examples include the Game Boy, the PlayStation Portable, and the Nintendo 3DS."
+1338,"Hybrid video game consoles are devices that can be used either as a handheld or as a home console. They have either a wired connection or docking station that connects the console unit to a television screen and fixed power source, and the potential to use a separate controller. However, they can also be used as a handheld. While prior handhelds like the Sega Nomad and PlayStation Portable, or home consoles such as the Wii U, have had these features, some consider the Nintendo Switch to be the first true hybrid console."
+1339,"A microconsole is a home video game console that is typically powered by low-cost computing hardware, making the console lower-priced compared to other home consoles on the market. The majority of microconsoles, with a few exceptions such as the PlayStation TV and OnLive Game System, are Android-based digital media players that are bundled with gamepads and marketed as gaming devices. Such microconsoles can be connected to the television to play video games downloaded from an application store such as Google Play."
+1340,"During the later part of video game history, there have been specialized consoles using computing components to offer multiple games to players. Most of these plug directly into one's television, and thus are often called plug-and-play consoles.  They are also considered dedicated consoles since it is generally impossible to access the computing components by an average consumer, though tech-savvy consumers often have found ways to hack the console to install additional functionality, voiding the manufacturer's warranty. Plug-and-play consoles usually come with the console unit itself, one or more controllers, and the required components for power and video hookup. Many recent plug-and-play releases have been for distributing a number of retro games for a specific console platform. Examples of these include the Atari Flashback series, the NES Classic Edition, Sega Genesis Mini and also handheld retro console such as Nintendo Game & Watch color screen series."
+1341,"Early console hardware was designed as customized printed circuit boards s, selecting existing integrated circuit chips that performed known functions, or programmable chips like erasable programmable read-only memory  chips that could perform certain functions. Persistent computer memory was expensive, so dedicated consoles were generally limited to the use of processor registers for storage of the state of a game, thus limiting the complexities of such titles.  Pong in both its arcade and home format had a handful of logic and calculation chips that used the current input of the players' paddles and resisters storing the ball's position to update the game's state and sent to the display device. Even with more advanced integrated circuits s of the time, designers were limited to what could be done through the electrical process rather than through programming as normally associated with video game development."
+1342,"Improvements in console hardware followed with improvements in microprocessor technology and semiconductor device fabrication. Manufacturing processes have been able to reduce the feature size on chips , allowing more transistors and other components to fit on a chip, and at the same time increasing the circuit speeds and the potential frequency the chip can run at, as well as reducing thermal dissipation. Chips were able to be made on larger dies, further increasing the number of features and effective processing power. Random-access memory became more practical with the higher density of transistors per chip, but to address the correct blocks of memory, processors needed to be updated to use larger word sizes and allot for larger bandwidth in chip communications. All these improvements did increase the cost of manufacturing but at a rate far less than the gains in overall processing power, which helped to make home computers and consoles inexpensive for the consumer, all related to Moore's law of technological improvements."
+1343,"For the consoles of the 1980s to 1990s, these improvements were evident in the marketing in the late 1980s to 1990s during the ""bit wars"", where console manufacturers had focused on their console's processor's word size as a selling point. Consoles since the 2000s are more similar to personal computers, building in memory, storage features, and networking capabilities to avoid the limitations of the past. The confluence with personal computers eased software development for both computer and console games, allowing developers to target both platforms. However, consoles differ from computers as most of the hardware components are preselected and customized between the console manufacturer and hardware component provider to assure a consistent performance target for developers. Whereas personal computer motherboards are designed with the needs for allowing consumers to add their desired selection of hardware components, the fixed set of hardware for consoles enables console manufacturers to optimize the size and design of the motherboard and hardware, often integrating key hardware components into the motherboard circuitry itself. Often, multiple components such as the central processing unit and graphics processing unit can be combined into a single chip, otherwise known as a system on a chip , which is a further reduction in size and cost. In addition, consoles tend to focus on components that give the unit high game performance such as the CPU and GPU, and as a tradeoff to keep their prices in expected ranges, use less memory and storage space compared to typical personal computers."
+1344,"In comparison to the early years of the industry, where most consoles were made directly by the company selling the console, many consoles of today are generally constructed through a value chain that includes component suppliers, such as AMD and NVidia for CPU and GPU functions, and contract manufacturers including electronics manufacturing services, factories which assemble those components into the final consoles such as Foxconn and Flextronics. Completed consoles are then usually tested, distributed, and repaired by the company itself. Microsoft and Nintendo both use this approach to their consoles, while Sony maintains all production in-house with the exception of their component suppliers."
+1345,Some of the commons elements that can be found within console hardware include:
+1346,"All game consoles require player input through a game controller to provide a method to move the player character in a specific direction and a variation of buttons to perform other in-game actions such as jumping or interacting with the game world. Though controllers have become more featured over the years, they still provide less control over a game compared to personal computers or mobile gaming. The type of controller available to a game can fundamentally change the style of how a console game will or can be played. However, this has also inspired changes in game design to create games that accommodate for the comparatively limited controls available on consoles."
+1347,Controllers have come in a variety of styles over the history of consoles. Some common types include:
+1348,"Numerous other controller types exist, including those that support motion controls, touchscreen support on handhelds and some consoles, and specialized controllers for specific types of games, such as racing wheels for racing games, light guns for shooting games, and musical instrument controllers for rhythm games. Some newer consoles also include optional support for a mouse and keyboard devices. Some older consoles such as 1988 Sega Genesis aka Mega Drive and 1993 3DO Interactive Multiplayer, supported optional mice, both with special mice made for them, but the 3DO mouse like that console was a flop, and the mouse for the Sega had very limited game support. The Sega also supported the optional Menacer, a wireless infrared light gun, and such were at one point popular for games. It also support BatterUP, a baseball bat-shaped controller."
+1349,"A controller may be attached through a wired connection onto the console itself, or in some unique cases like the Famicom hardwired to the console, or with a wireless connection. Controllers require power, either provided by the console via the wired connection, or from batteries or a rechargeable battery pack for wireless connections. Controllers are nominally built into a handheld unit, though some newer ones allow for separate wireless controllers to also be used."
+1350,"While the first game consoles were dedicated game systems, with the games programmed into the console's hardware, the Fairchild Channel F introduced the ability to store games in a form separate from the console's internal circuitry, thus allowing the consumer to purchase new games to play on the system. Since the Channel F, nearly all game consoles have featured the ability to purchase and swap games through some form, through those forms have changes with improvements in technology."
+1351,"While magnetic storage, such as tape drives and floppy disks, had been popular for software distribution with early personal computers in the 1980s and 1990s, this format did not see much use in console system. There were some attempts, such as the Bally Astrocade and APF-M1000 using tape drives, as well as the Disk System for the Nintendo Famicom, and the Nintendo 64DD for the Nintendo 64, but these had limited applications, as magnetic media was more fragile and volatile than game cartridges."
+1352,"In addition to built-in internal storage, newer consoles often give the consumer the ability to use external storage media to save game date, downloaded games, or other media files from the console. Early iterations of external storage were achieved through the use of flash-based memory cards, first used by the Neo Geo but popularized with the PlayStation. Nintendo continues to support this approach with extending the storage capabilities of the 3DS and Switch, standardizing on the current SD card format. As consoles began incorporating the use of USB ports, support for USB external hard drives was also added, such as with the Xbox 360."
+1353,"With Internet-enabled consoles, console manufacturers offer both free and paid-subscription services that provide value-added services atop the basic functions of the console. Free services generally offer user identity services and access to a digital storefront, while paid services allow players to play online games, interact with other uses through social networking, use cloud saves for supported games, and gain access to free titles on a rotating basis. Examples of such services include the Xbox network, PlayStation Network, and Nintendo Switch Online."
+1354,"Certain consoles saw various add-ons or accessories that were designed to attach to the existing console to extend its functionality. The best example of this was through the various CD-ROM add-ons for consoles of the fourth generation such as the TurboGrafx CD, Atari Jaguar CD, and the Sega CD. Other examples of add-ons include the 32X for the Sega Genesis intended to allow owners of the aging console to play newer games but has several technical faults, and the Game Boy Player for the GameCube to allow it to play Game Boy games."
+1355,Consumers can often purchase a range of accessories for consoles outside of the above categories. These can include:
+1356,"Console or game development kits are specialized hardware units that typically include the same components as the console and additional chips and components to allow the unit to be connected to a computer or other monitoring device for debugging purposes. A console manufacturer will make the console's dev kit available to registered developers months ahead of the console's planned launch to give developers time to prepare their games for the new system. These initial kits will usually be offered under special confidentiality clauses to protect trade secrets of the console's design, and will be sold at a high cost to the developer as part of keeping this confidentiality. Newer consoles that share features in common with personal computers may no longer use specialized dev kits, though developers are still expected to register and purchase access to software development kits from the manufacturer. For example, any consumer Xbox One can be used for game development after paying a fee to Microsoft to register one intent to do so."
+1357,"Since the release of the Nintendo Famicom / Nintendo Entertainment System, most video game console manufacturers employ a strict licensing scheme that limit what games can be developed for it. Developers and their publishers must pay a fee, typically based on royalty per unit sold, back to the manufacturer. The cost varies by manufacturer but was estimated to be about US$3−10 per unit in 2012. With additional fees, such as branding rights, this has generally worked out to be an industry-wide 30% royalty rate paid to the console manufacturer for every game sold. This is in addition to the cost of acquiring the dev kit to develop for the system."
+1358,"The licensing fee may be collected in a few different ways. In the case of Nintendo, the company generally has controlled the production of game cartridges with its lockout chips and optical media for its systems, and thus charges the developer or publisher for each copy it makes as an upfront fee. This also allows Nintendo to review the game's content prior to release and veto games it does not believe appropriate to include on its system. This had led to over 700 unlicensed games for the NES, and numerous others on other Nintendo cartridge-based systems that had found ways to bypass the hardware lockout chips and sell without paying any royalties to Nintendo, such as by Atari in its subsidiary company Tengen. This licensing approach was similarly used by most other cartridge-based console manufacturers using lockout chip technology."
+1359,"With optical media, where the console manufacturer may not have direct control on the production of the media, the developer or publisher typically must establish a licensing agreement to gain access to the console's proprietary storage format for the media as well as to use the console and manufacturer's logos and branding for the game's packaging, paid back through royalties on sales. In the transition to digital distribution, where now the console manufacturer runs digital storefronts for games, license fees apply to registering a game for distribution on the storefront –  again gaining access to the console's branding and logo –  with the manufacturer taking its cut of each sale as its royalty. In both cases, this still gives console manufacturers the ability to review and reject games it believes unsuitable for the system and deny licensing rights."
+1360,"With the rise of indie game development, the major console manufacturers have all developed entry level routes for these smaller developers to be able to publish onto consoles at far lower costs and reduced royalty rates. Programs like Microsoft's ID@Xbox give developers most of the needed tools for free after validating the small development size and needs of the team."
+1361,Similar licensing concepts apply for third-party accessory manufacturers.
+1362,"Consoles, like most consumer electronic devices, have limited lifespans. There is great interest in preservation of older console hardware for archival and historical purposes, as games from older consoles, as well as arcade and personal computers, remain of interest. Computer programmers and hackers have developed emulators that can be run on personal computers or other consoles that simulate the hardware of older consoles that allow games from that console to be run. The development of software emulators of console hardware is established to be legal, but there are unanswered legal questions surrounding copyrights, including acquiring a console's firmware and copies of a game's ROM image, which laws such as the United States' Digital Millennium Copyright Act make illegal save for certain archival purposes. Even though emulation itself is legal, Nintendo is recognized to be highly protective of any attempts to emulate its systems and has taken early legal actions to shut down such projects."
+1363,"To help support older games and console transitions, manufacturers started to support backward compatibility on consoles in the same family. Sony was the first to do this on a home console with the PlayStation 2 which was able to play original PlayStation content, and subsequently became a sought-after feature across many consoles that followed. Backward compatibility functionality has included direct support for previous console games on the newer consoles such as within the Xbox console family, the distribution of emulated games such as Nintendo's Virtual Console, or using cloud gaming services for these older games as with the PlayStation Now service."
+1364,"Consoles may be shipped in a variety of configurations, but typically will include one base configuration that include the console, one controller, and sometimes a pack-in game. Manufacturers may offer alternate stock keeping unit  options that include additional controllers and accessories or different pack-in games. Special console editions may feature unique cases or faceplates with art dedicated to a specific video game or series and are bundled with that game as a special incentive for its fans. Pack-in games are typically first-party games, often featuring the console's primary mascot characters."
+1365,"The more recent console generations have also seen multiple versions of the same base console system either offered at launch or presented as a mid-generation refresh. In some cases, these simply replace some parts of the hardware with cheaper or more efficient parts, or otherwise streamline the console's design for production going forward; the PlayStation 3 underwent several such hardware refreshes during its lifetime due to technological improvements such as significant reduction of the process node size for the CPU and GPU. In these cases, the hardware revision model will be marked on packaging so that consumers can verify which version they are acquiring."
+1366,"In other cases, the hardware changes create multiple lines within the same console family. The base console unit in all revisions share fundamental hardware, but options like internal storage space and RAM size may be different. Those systems with more storage and RAM would be marked as a higher performance variant available at a higher cost, while the original unit would remain as a budget option. For example, within the Xbox One family, Microsoft released the mid-generation Xbox One X as a higher performance console, the Xbox One S as the lower-cost base console, and a special Xbox One S All-Digital Edition revision that removed the optical drive on the basis that users could download all games digitally, offered at even a lower cost than the Xbox One S. In these cases, developers can often optimize games to work better on the higher-performance console with patches to the retail version of the game. In the case of the Nintendo 3DS, the New Nintendo 3DS, featured upgraded memory and processors, with new games that could only be run on the upgraded units and cannot be run on an older base unit. There have also been a number of ""slimmed-down"" console options with significantly reduced hardware components that significantly reduced the price they could sell the console to the consumer, but either leaving certain features off the console, such as the Wii Mini that lacked any online components compared to the Wii, or that required the consumer to purchase additional accessories and wiring if they did not already own it, such as the New-Style NES that was not bundled with the required RF hardware to connect to a television."
+1367,"Consoles when originally launched in the 1970s and 1980s were about US$200−300, and with the introduction of the ROM cartridge, each game averaged about US$30−40. Over time the launch price of base consoles units has generally risen to about US$400−500, with the average game costing US$60. Exceptionally, the period of transition from ROM cartridges to optical media in the early 1990s saw several consoles with high price points exceeding US$400 and going as high as US$700. Resultingly, sales of these first optical media consoles were generally poor."
+1368,"When adjusted for inflation, the price of consoles has generally followed a downward trend, from US$800−1,000 from the early generations down to US$500−600 for current consoles. This is typical for any computer technology, with the improvements in computing performance and capabilities outpacing the additional costs to achieve those gains. Further, within the United States, the price of consoles has generally remained consistent, being within 0.8% to 1% of the median household income, based on the United States Census data for the console's launch year."
+1369,"Since the Nintendo Entertainment System, console pricing has stabilized on the razorblade model, where the consoles are sold at little to no profit for the manufacturer, but they gain revenue from each game sold due to console licensing fees and other value-added services around the console . Console manufacturers have even been known to take losses on the sale of consoles at the start of a console's launch with expectation to recover with revenue sharing and later price recovery on the console as they switch to less expensive components and manufacturing processes without changing the retail price. Consoles have been generally designed to have a five-year product lifetime, though manufacturers have considered their entries in the more recent generations to have longer lifetimes of seven to potentially ten years."
+1370,"The competition within the video game console market as subset of the video game industry is an area of interest to economics with its relatively modern history, its rapid growth to rival that of the film industry, and frequent changes compared to other sectors."
+1371,"Effects of unregulated competition on the market were twice seen early in the industry. The industry had its first crash in 1977 following the release of the Magnavox Odyssey, Atari's home versions of Pong and the Coleco Telstar, which led other third-party manufacturers, using inexpensive General Instruments processor chips, to make their own home consoles which flooded the market by 1977.: 81–89  The video game crash of 1983 was fueled by multiple factors including competition from lower-cost personal computers, but unregulated competition was also a factor, as numerous third-party game developers, attempting to follow on the success of Activision in developing third-party games for the Atari 2600 and Intellivision, flooded the market with poor quality games, and made it difficult for even quality games to sell. Nintendo implemented a lockout chip, the Checking Integrated Circuit, on releasing the Nintendo Entertainment System in Western territories, as a means to control which games were published for the console. As part of their licensing agreements, Nintendo further prevented developers from releasing the same game on a different console for a period of two years. This served as one of the first means of securing console exclusivity for games that existed beyond technical limitation of console development."
+1372,"The Nintendo Entertainment System also brought the concept of a video game mascot as the representation of a console system as a means to sell and promote the unit, and for the NES was Mario. The use of mascots in businesses had been a tradition in Japan, and this had already proven successful in arcade games like Pac-Man. Mario was used to serve as an identity for the NES as a humor-filled, playful console. Mario caught on quickly when the NES released in the West, and when the next generation of consoles arrived, other manufacturers pushed their own mascots to the forefront of their marketing, most notably Sega with the use of Sonic the Hedgehog. The Nintendo and Sega rivalry that involved their mascot's flagship games served as part of the fourth console generation's ""console wars"". Since then, manufacturers have typically positioned their mascot and other first-party games as key titles in console bundles used to drive sales of consoles at launch or at key sales periods such as near Christmas."
+1373,"Another type of competitive edge used by console manufacturers around the same time was the notion of ""bits"" or the size of the word used by the main CPU. The TurboGrafx-16 was the first console to push on its bit-size, advertising itself as a ""16-bit"" console, though this only referred to part of its architecture while its CPU was still an 8-bit unit. Despite this, manufacturers found consumers became fixated on the notion of bits as a console selling point, and over the fourth, fifth and sixth generation, these ""bit wars"" played heavily into console advertising. The use of bits waned as CPU architectures no longer needed to increase their word size and instead had other means to improve performance such as through multicore CPUs."
+1374,"Generally, increased console numbers gives rise to more consumer options and better competition, but the exclusivity of titles made the choice of console for consumers an ""all-or-nothing"" decision for most. Further, with the number of available consoles growing with the fifth and sixth generations, game developers became pressured to which systems to focus on, and ultimately narrowed their target choice of platforms to those that were the best-selling. This cased a contraction in the market, with major players like Sega leaving the hardware business after the Dreamcast but continuing in the software area. Effectively, each console generation was shown to have two or three dominant players."
+1375,"Competition in the console market in the 2010s and 2020s is considered an oligarchy between three main manufacturers: Nintendo, Sony, and Microsoft. The three use a combination of first-party games exclusive to their console and negotiate exclusive agreements with third-party developers to have their games be exclusive for at least an initial period of time to drive consumers to their console. They also worked with CPU and GPU manufacturers to tune and customize hardware for computers to make it more amenable and effective for video games, leading to lower-cost hardware needed for video game consoles. Finally, console manufacturers also work with retailers to help with promotion of consoles, games, and accessories. While there is little difference in pricing on the console hardware from the manufacturer's suggested retail price for the retailer to profit from, these details with the manufacturers can secure better profits on sales of game and accessory bundles for premier product placement. These all form network effects, with each manufacturer seeking to maximize the size of their network of partners to increase their overall position in the competition."
+1376,"Of the three, Microsoft and Sony, both with their own hardware manufacturing capabilities, remain at a leading edge approach, attempting to gain a first-mover advantage over the other with adaption of new console technology. Nintendo is more reliant on its suppliers and thus instead of trying to compete feature for feature with Microsoft and Sony, had instead taken a ""blue ocean"" strategy since the Nintendo DS and Wii."
+1377,"In computer science, a high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language elements, be easier to use, or may automate  significant areas of computing systems , making the process of developing a program simpler and more understandable than when using a lower-level language. The amount of abstraction provided defines how ""high-level"" a programming language is."
+1378,"Note that languages are not strictly interpreted languages or compiled languages. Rather, implementations of language behavior use interpreting or compiling. For example, ALGOL 60 and Fortran have both been interpreted . Similarly, Java shows the difficulty of trying to apply these labels to languages, rather than to implementations; Java is compiled to bytecode which is then executed by either interpreting ) or compiling . Moreover, compiling, transcompiling, and interpreting is not strictly limited to only a description of the compiler artifact ."
+1379,"Alternatively, it is possible for a high-level language to be directly implemented by a computer – the computer directly executes the HLL code. This is known as a high-level language computer architecture – the computer architecture itself is designed to be targeted by a specific high-level language. The Burroughs large systems were target machines for ALGOL 60, for example."
+1380,"x86  is a family of complex instruction set computer  instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term ""x86"" came into being because the names of several successors to Intel's 8086 processor end in ""86"", including the 80186, 80286, 80386 and 80486 processors. Colloquially, their names were ""186"", ""286"", ""386"" and ""486""."
+1381,"The term is not synonymous with IBM PC compatibility, as this implies a multitude of other computer hardware. Embedded systems and general-purpose computers used x86 chips before the PC-compatible market started, some of them before the IBM PC  debut."
+1382,"As of June 2022, most desktop and laptop computers sold are based on the x86 architecture family, while mobile categories such as smartphones or tablets are dominated by ARM. At the high end, x86 continues to dominate computation-intensive workstation and cloud computing segments. The fastest supercomputer in the TOP500 list for June 2022 was the first exascale system, Frontier, built using AMD Epyc CPUs based on the x86 ISA; it broke the 1 exaFLOPS barrier in May 2022."
+1383,"In the 1980s and early 1990s, when the 8088 and 80286 were still in common use, the term x86 usually represented any 8086-compatible CPU. Today, however, x86 usually implies a binary compatibility also with the 32-bit instruction set of the 80386. This is due to the fact that this instruction set has become something of a lowest common denominator for many modern operating systems and probably also because the term became common after the introduction of the 80386 in 1985."
+1384,"A few years after the introduction of the 8086 and 8088, Intel added some complexity to its naming scheme and terminology as the ""iAPX"" of the ambitious but ill-fated Intel iAPX 432 processor was tried on the more successful 8086 family of chips, applied as a kind of system-level prefix. An 8086 system, including coprocessors such as 8087 and 8089, and simpler Intel-specific system chips, was thereby described as an iAPX 86 system. There were also terms iRMX , iSBC , and iSBX , all together under the heading Microsystem 80.  However, this naming scheme was quite temporary, lasting for a few years during the early 1980s."
+1385,"Although the 8086 was primarily developed for embedded systems and small multi-user or single-user computers, largely as a response to the successful 8080-compatible Zilog Z80, the x86 line soon grew in features and processing power. Today, x86 is ubiquitous in both stationary and portable personal computers, and is also used in midrange computers, workstations, servers, and most new supercomputer clusters of the TOP500 list. A large amount of software, including a large list of x86 operating systems are using x86-based hardware."
+1386,"Modern x86 is relatively uncommon in embedded systems, however, and small low power applications , and low-cost microprocessor markets, such as home appliances and toys, lack significant x86 presence. Simple 8- and 16-bit based architectures are common here, as well as simpler RISC architectures like RISC-V, although the x86-compatible VIA C7, VIA Nano, AMD's Geode, Athlon Neo and Intel Atom are examples of 32- and 64-bit designs used in some relatively low-power and low-cost segments."
+1387,"There have been several attempts, including by Intel, to end the market dominance of the ""inelegant"" x86 architecture designed directly from the first simple 8-bit microprocessors. Examples of this are the iAPX 432 , the Intel 960, Intel 860 and the Intel/Hewlett-Packard Itanium architecture. However, the continuous refinement of x86 microarchitectures, circuitry and semiconductor manufacturing would make it hard to replace x86 in many segments. AMD's 64-bit extension of x86  and the scalability of x86 chips in the form of modern multi-core CPUs, is underlining x86 as an example of how continuous refinement of established industry standards can resist the competition from completely new architectures."
+1388,"The table below lists processor models and model series implementing various architectures in the x86 family, in chronological order. Each line item is characterized by significantly improved or commercially successful processor microarchitecture designs."
+1389,"At various times, companies such as IBM, VIA, NEC, AMD, TI, STM, Fujitsu, OKI, Siemens, Cyrix, Intersil, C&T, NexGen, UMC, and DM&P started to design or manufacture x86 processors  intended for personal computers and embedded systems. Other companies that designed or manufactured x86 or x87 processors include ITT Corporation, National Semiconductor, ULSI System Technology, and Weitek."
+1390,"Such x86 implementations were seldom simple copies but often employed different internal microarchitectures and different solutions at the electronic and physical levels. Quite naturally, early compatible microprocessors were 16-bit, while 32-bit designs were developed much later. For the personal computer market, real quantities started to appear around 1990 with i386 and i486 compatible processors, often named similarly to Intel's original chips."
+1391,"After the fully pipelined i486, in 1993 Intel introduced the Pentium brand name  for their new set of superscalar x86 designs. With the x86 naming scheme now legally cleared, other x86 vendors had to choose different names for their x86-compatible products, and initially some chose to continue with variations of the numbering scheme: IBM partnered with Cyrix to produce the 5x86 and then the very efficient 6x86  and 6x86MX  lines of Cyrix designs, which were the first x86 microprocessors implementing register renaming to enable speculative execution."
+1392,"AMD meanwhile designed and manufactured the advanced but delayed 5k86 , which, internally, was closely based on AMD's earlier 29K RISC design; similar to NexGen's Nx586, it used a strategy such that dedicated pipeline stages decode x86 instructions into uniform and easily handled micro-operations, a method that has remained the basis for most x86 designs to this day."
+1393,"Some early versions of these microprocessors had heat dissipation problems. The 6x86 was also affected by a few minor compatibility problems, the Nx586 lacked a floating-point unit  and  pin-compatibility, while the K5 had somewhat disappointing performance when it was  introduced."
+1394,"Customer ignorance of alternatives to the Pentium series further contributed to these designs being comparatively unsuccessful, despite the fact that the K5 had very good Pentium compatibility and the 6x86 was significantly faster than the Pentium on integer code. AMD later managed to grow into a serious contender with the K6 set of processors, which gave way to the very successful Athlon and Opteron."
+1395,"There were also other contenders, such as Centaur Technology , Rise Technology, and Transmeta. VIA Technologies' energy efficient C3 and C7 processors, which were designed by the Centaur company, were sold for many years following their release in 2005. Centaur's 2008 design, the VIA Nano, was their first processor with superscalar and speculative execution. It was introduced at about the same time  as Intel introduced the Intel Atom, its first ""in-order"" processor after the P5 Pentium."
+1396,"Many additions and extensions have been added to the original x86 instruction set over the years, almost consistently with full backward compatibility. The architecture family has been implemented in processors from Intel, Cyrix, AMD, VIA Technologies and many other companies; there are also open implementations, such as the Zet SoC platform . Nevertheless, of those, only Intel, AMD, VIA Technologies, and DM&P Electronics hold x86 architectural licenses, and from these, only the first two actively produce modern 64-bit designs, leading to what has been called a ""duopoly"" of Intel and AMD in x86 processors."
+1397,"However, in 2014 the Shanghai-based Chinese company Zhaoxin, a joint venture between a Chinese company and VIA Technologies, began designing VIA based x86 processors for desktops and laptops. The release of its newest ""7"" family of x86 processors , which are not quite as fast as AMD or Intel chips but are still state of the art, had been planned for 2021; as of March 2022 the release had not taken place, however."
+1398,"The instruction set architecture has twice been extended to a larger word size. In 1985, Intel released the 32-bit 80386  which gradually replaced the earlier 16-bit chips in computers  during the following years; this extended programming model was originally referred to as the i386 architecture  but Intel later dubbed it IA-32 when introducing its  IA-64 architecture."
+1399,"In 1999–2003, AMD extended this 32-bit architecture to 64 bits and referred to it as x86-64 in early documents and later as AMD64. Intel soon adopted AMD's architectural extensions under the name IA-32e, later using the name EM64T and finally using Intel 64. Microsoft and Sun Microsystems/Oracle also use term ""x64"", while many Linux distributions, and the BSDs also use the ""amd64"" term. Microsoft Windows, for example, designates its 32-bit versions as ""x86"" and 64-bit versions as ""x64"", while installation files of 64-bit Windows versions are required to be placed into a directory called ""AMD64""."
+1400,"In 2023, Intel proposed a major change to the architecture referred to as x86-S , which aims to remove support for legacy execution modes and instructions. A processor implementing this proposal would start execution directly in long mode and would only support 64-bit operating systems. 32-bit code would only be supported for user applications running in ring 3, and would use the same simplified segmentation as long mode."
+1401,"The x86 architecture is a variable instruction length, primarily ""CISC"" design with emphasis on backward compatibility. The instruction set is not typical CISC, however, but basically an extended version of the simple eight-bit 8008 and 8080 architectures. Byte-addressing is enabled and words are stored in memory with little-endian byte order. Memory access to unaligned addresses is allowed for almost all instructions. The largest native size for integer arithmetic and memory addresses  is 16, 32 or 64 bits depending on architecture generation . Multiple scalar values can be handled simultaneously via the SIMD unit present in later generations, as described below. Immediate addressing offsets and immediate data may be expressed as 8-bit quantities for the frequently occurring cases or contexts where a −128..127 range is enough. Typical instructions are therefore 2 or 3 bytes in length ."
+1402,"To further conserve encoding space, most registers are expressed in opcodes using three or four bits, the latter via an opcode prefix in 64-bit mode, while at most one operand to an instruction can be a memory location. However, this memory operand may also be the destination , while the other operand, the source, can be either register or immediate. Among other factors, this contributes to a code size that rivals eight-bit machines and enables efficient use of instruction cache memory. The relatively small number of general registers  has made register-relative addressing  an important method of accessing operands, especially on the stack. Much work has therefore been invested in making such accesses as fast as register accesses—i.e., a one cycle instruction throughput, in most circumstances where the accessed data is available in the top-level cache."
+1403,"A dedicated floating-point processor with 80-bit internal registers, the 8087, was developed for the original 8086. This microprocessor subsequently developed into the extended 80387, and later processors incorporated a backward compatible version of this functionality on the same microprocessor as the main processor.  In addition to this, modern x86 designs also contain a SIMD-unit  where instructions can work in parallel on  128-bit words, each containing two or four floating-point numbers , or alternatively, 2, 4, 8 or 16 integers ."
+1404,"The presence of wide SIMD registers means that existing x86 processors can load or store up to 128 bits of memory data in a single instruction and also perform bitwise operations  on full 128-bits quantities in parallel. Intel's Sandy Bridge processors added the Advanced Vector Extensions  instructions, widening the SIMD registers to 256 bits. The Intel Initial Many Core Instructions implemented by the Knights Corner Xeon Phi processors, and the AVX-512 instructions implemented by the Knights Landing Xeon Phi processors and by Skylake-X processors, use 512-bit wide SIMD registers."
+1405,"During execution, current x86 processors employ a few extra decoding steps to split most instructions into smaller pieces called micro-operations. These are then handed to a control unit that buffers and schedules them in compliance with x86-semantics so that they can be executed, partly in parallel, by one of several  execution units. These modern x86 designs are thus pipelined, superscalar, and also capable of out of order and speculative execution , which means they may execute multiple  x86 instructions simultaneously, and not necessarily in the same order as given in the instruction stream.
+Some Intel CPUs  and AMD CPUs  are also capable of simultaneous multithreading with two threads per core .  Some Intel CPUs support transactional memory ."
+1406,"When introduced, in the mid-1990s, this method was sometimes referred to as a ""RISC core"" or as ""RISC translation"", partly for marketing reasons, but also because these micro-operations share some properties with certain types of RISC instructions. However, traditional microcode  also inherently shares many of the same properties; the new method differs mainly in that the translation to micro-operations now occurs asynchronously. Not having to synchronize the execution units with the decode steps opens up possibilities for more analysis of the  code stream, and therefore permits detection of operations that can be performed in parallel, simultaneously feeding more than one execution unit."
+1407,The latest processors also do the opposite when appropriate; they combine certain x86 sequences  into a more complex micro-op which fits the execution model better and thus can be executed faster or with fewer machine resources involved.
+1408,"Another way to try to improve performance is to cache the decoded micro-operations, so the processor can directly access the decoded micro-operations from a special cache, instead of decoding them again. Intel followed this approach with the Execution Trace Cache feature in their NetBurst microarchitecture  and later in the Decoded Stream Buffer ."
+1409,Transmeta used a completely different method in their Crusoe x86 compatible CPUs. They used just-in-time translation to convert x86 instructions to the CPU's native VLIW instruction set. Transmeta argued that their approach allows for more power efficient designs since the CPU can forgo the complicated decode step of more traditional x86 implementations.
+1410,Addressing modes for 16-bit processor modes can be summarized by the formula:
+1411,Addressing modes for 32-bit x86 processor modes can be summarized by the formula:
+1412,Addressing modes for the 64-bit processor mode can be summarized by the formula:
+1413,Instruction relative addressing in 64-bit code  simplifies the implementation of position-independent code .
+1414,"The 8086 had 64 KB of eight-bit  I/O space, and a 64 KB  stack in memory supported by computer hardware. Only words  can be pushed to the stack. The stack grows toward numerically lower addresses, with SS:SP pointing to the most recently pushed item. There are 256 interrupts, which can be invoked by both hardware and software. The interrupts can cascade, using the stack to store the return address."
+1415,"The original Intel 8086 and 8088 have fourteen 16-bit registers. Four of them  are general-purpose registers , although each may have an additional purpose; for example, only CX can be used as a counter with the loop instruction. Each can be accessed as two separate bytes . Two pointer registers have special roles: SP  points to the ""top"" of the stack, and BP  is often used to point at some other place in the stack, typically above the local variables . The registers SI, DI, BX and BP are address registers, and may also be used for array indexing."
+1416,"One of four possible 'segment registers'  is used to form a memory address. In the original 8086 / 8088 / 80186 / 80188 every address was built from a segment register and one of the general purpose registers. For example ds:si is the notation for an address formed as  to allow 20-bit addressing rather than 16 bits, although this changed in later processors. At that time only certain combinations were supported."
+1417,"The FLAGS register contains flags such as carry flag, overflow flag and zero flag. Finally, the instruction pointer  points to the next instruction that will be fetched from memory and then executed;  this register cannot be directly accessed  by a program."
+1418,"The Intel 80186 and 80188 are essentially an upgraded 8086 or 8088 CPU, respectively, with on-chip peripherals added, and they have the same CPU registers as the 8086 and 8088 ."
+1419,"The 8086, 8088, 80186, and 80188 can use an optional floating-point coprocessor, the 8087.  The 8087 appears to the programmer as part of the CPU and adds eight 80-bit wide registers, st to st, each of which can hold numeric data in one of seven formats: 32-, 64-, or 80-bit floating point, 16-, 32-, or 64-bit  integer, and 80-bit packed decimal integer.: S-6, S-13..S-15  It also has its own 16-bit status register accessible through the fstsw instruction, and it is common to simply use some of its bits for branching by copying it into the normal FLAGS."
+1420,"In the Intel 80286, to support protected mode, three special registers hold descriptor table addresses , and a fourth task register  is used for task switching.  The 80287 is the floating-point coprocessor for the 80286 and has the same registers as the 8087 with the same data formats."
+1421,"With the advent of the 32-bit 80386 processor, the 16-bit general-purpose registers, base registers, index registers, instruction pointer, and FLAGS register, but not the segment registers, were expanded to 32 bits. The nomenclature represented this by prefixing an ""E""  to the register names in x86 assembly language. Thus, the AX register corresponds to the lower 16 bits of the new 32-bit EAX register, SI corresponds to the lower 16 bits of ESI, and so on. The general-purpose registers, base registers, and index registers can all be used as the base in addressing modes, and all of those registers except for the stack pointer can be used as the index in addressing modes."
+1422,"Two new segment registers  were added. With a greater number of registers, instructions and operands, the machine code format was expanded. To provide backward compatibility, segments with executable code can be marked as containing either 16-bit or 32-bit instructions. Special prefixes allow inclusion of 32-bit instructions in a 16-bit segment or vice versa."
+1423,"The 80386 had an optional floating-point coprocessor, the 80387; it had eight 80-bit wide registers: st to st, like the 8087 and 80287. The 80386 could also use an 80287 coprocessor. With the 80486 and all subsequent x86 models, the floating-point processing unit  is integrated on-chip."
+1424,"The Pentium MMX added eight 64-bit MMX integer vector registers . With the Pentium III, Intel added a 32-bit Streaming SIMD Extensions  control/status register  and eight 128-bit SSE floating-point registers ."
+1425,"Starting with the AMD Opteron processor, the x86 architecture extended the 32-bit registers into 64-bit registers in a way similar to how the 16 to 32-bit extension took place. An R-prefix  identifies the 64-bit registers , and eight additional 64-bit general registers  were also introduced in the creation of x86-64. Also, eight more SSE vector registers  were added. However, these extensions are only usable in 64-bit mode, which is one of the two modes only available in long mode. The addressing modes were not dramatically changed from 32-bit mode, except that addressing was extended to 64 bits, virtual addresses are now sign extended to 64 bits , and other selector details were dramatically reduced. In addition, an addressing mode was added to allow memory references relative to RIP , to ease the implementation of position-independent code, used in shared libraries in some operating systems."
+1426,SIMD registers XMM0–XMM15 .
+1427,SIMD registers YMM0–YMM15 . Lower half of each of the YMM registers maps onto the corresponding XMM register.
+1428,SIMD registers ZMM0–ZMM31. Lower half of each of the ZMM registers maps onto the corresponding YMM register.
+1429,"x86 processors that have a protected mode, i.e. the 80286 and later processors, also have three descriptor registers  and a task register ."
+1430,"32-bit x86 processors  also include various special/miscellaneous registers such as control registers , debug registers , test registers , and model-specific registers ."
+1431,"AVX-512 has eight extra 64-bit mask registers K0–K7 for selecting elements in a vector register. Depending on the vector register and element widths, only a subset of bits of the mask register may be used by a given instruction."
+1432,"Although the main registers  are ""general-purpose"" in the 32-bit and 64-bit versions of the instruction set and can be used for anything, it was originally envisioned that they be used for the following purposes:"
+1433,Segment registers:
+1434,No particular purposes were envisioned for the other 8 registers available only in 64-bit mode.
+1435,"Some instructions compile and execute more efficiently when using these registers for their designed purpose. For example, using AL as an accumulator and adding an immediate byte value to it produces the efficient add to AL opcode of 04h, whilst using the BL register produces the generic and longer add to register opcode of 80C3h. Another example is double precision division and multiplication that works specifically with the AX and DX registers."
+1436,"Modern compilers benefited from the introduction of the sib byte  that allows registers to be treated uniformly .  However, using the sib byte universally is non-optimal, as it produces longer encodings than only using it selectively when necessary.    Some special instructions lost priority in the hardware design and became slower than equivalent small code sequences. A notable example is the LODSW instruction."
+1437,Note: The ?PL registers are only available in 64-bit mode.
+1438,Note: The ?IL registers are only available in 64-bit mode.
+1439,"Real Address mode, commonly called Real mode, is an operating mode of 8086 and later x86-compatible CPUs. Real mode is characterized by a 20-bit segmented memory address space , direct software access to peripheral hardware, and no concept of memory protection or multitasking at the hardware level. All x86 CPUs in the 80286 series and later start up in real mode at power-on; 80186 CPUs and earlier had only one operational mode, which is equivalent to real mode in later chips. "
+1440,"In order to use more than 64 KB of memory, the segment registers must be used. This created great complications for compiler implementors who introduced odd pointer modes such as ""near"", ""far"" and ""huge"" to leverage the implicit nature of segmented architecture to different degrees, with some pointers containing 16-bit offsets within implied segments and other pointers containing segment addresses and offsets within segments.  It is technically possible to use up to 256 KB of memory for code and data, with up to 64 KB for code, by setting all four segment registers once and then only using 16-bit offsets  to address memory, but this puts substantial restrictions on the way data can be addressed and memory operands can be combined, and it violates the architectural intent of the Intel designers, which is for separate data items  to be contained in separate segments and addressed by their own segment addresses, in new programs that are not ported from earlier 8-bit processors with 16-bit address spaces."
+1441,Unreal mode is used by some 16-bit operating systems and some 32-bit boot loaders.
+1442,"The System Management Mode  is only used by the system firmware , not by operating systems and applications software. The SMM code is running in SMRAM."
+1443,"In addition to real mode, the Intel 80286 supports protected mode, expanding addressable physical memory to 16 MB and addressable virtual memory to 1 GB, and providing protected memory, which prevents programs from corrupting one another. This is done by using the segment registers only for storing an index into a descriptor table that is stored in memory. There are two such tables, the Global Descriptor Table  and the Local Descriptor Table , each holding up to 8192 segment descriptors, each segment giving access to 64 KB of memory. In the 80286, a segment descriptor provides a 24-bit base address, and this base address is added to a 16-bit offset to create an absolute address.  The base address from the table fulfills the same role that the literal value of the segment register fulfills in real mode; the segment registers have been converted from direct registers to indirect registers. Each segment can be assigned one of four ring levels used for hardware-based computer security. Each segment descriptor also contains a segment limit field which specifies the maximum offset that may be used with the segment. Because offsets are 16 bits, segments are still limited to 64 KB each in 80286 protected mode."
+1444,"Each time a segment register is loaded in protected mode, the 80286 must read a 6-byte segment descriptor from memory into a set of hidden internal registers. Thus, loading segment registers is much slower in protected mode than in real mode, and changing segments very frequently is to be avoided.  Actual memory operations using protected mode segments are not slowed much because the 80286 and later have hardware to check the offset against the segment limit in parallel with instruction execution."
+1445,"The Intel 80386 extended offsets and also the segment limit field in each segment descriptor to 32 bits, enabling a segment to span the entire memory space. It also introduced support in protected mode for paging, a mechanism making it possible to use paged virtual memory . Paging allows the CPU to map any page of the virtual memory space to any page of the physical memory space.  To do this, it uses additional mapping tables in memory called page tables.  Protected mode on the 80386 can operate with paging either enabled or disabled; the segmentation mechanism is always active and generates virtual addresses that are then mapped by the paging mechanism if it is enabled.  The segmentation mechanism can also be effectively disabled by setting all segments to have a base address of 0 and size limit equal to the whole address space; this also requires a minimally-sized segment descriptor table of only four descriptors ."
+1446,"Paging is used extensively by modern multitasking operating systems. Linux, 386BSD and Windows NT were developed for the 386 because it was the first Intel architecture CPU to support paging and 32-bit segment offsets. The 386 architecture became the basis of all further development in the x86 series."
+1447,"x86 processors that support protected mode boot into real mode for backward compatibility with the older 8086 class of processors. Upon power-on , the processor initializes in real mode, and then begins executing instructions. Operating system boot code, which might be stored in read-only memory, may place the processor into the protected mode to enable paging and other features. Conversely, segment arithmetic, a common practice in real mode code, is not allowed in protected mode."
+1448,"There is also a sub-mode of operation in 32-bit protected mode  called virtual 8086 mode, also known as V86 mode. This is basically a special hybrid operating mode that allows real mode programs and operating systems to run while under the control of a protected mode supervisor operating system. This allows for a great deal of flexibility in running both protected mode programs and real mode programs simultaneously. This mode is exclusively available for the 32-bit version of protected mode; it does not exist in the 16-bit version of protected mode, or in long mode."
+1449,"In the mid 1990s, it was obvious that the 32-bit address space of the x86 architecture was limiting its performance in applications requiring large data sets. A 32-bit address space would allow the processor to directly address only 4 GB of data, a size surpassed by applications such as video processing and database engines. Using 64-bit addresses, it is possible to directly address 16 EiB of data, although most 64-bit architectures do not support access to the full 64-bit address space; for example, AMD64 supports only 48 bits from a 64-bit address, split into four paging levels."
+1450,"In 1999, AMD published a  complete specification for a 64-bit extension of the x86 architecture which they called x86-64 with claimed intentions to produce. That design is currently used in almost all x86 processors, with some exceptions intended for embedded systems."
+1451,"Mass-produced x86-64 chips for the general market were available four years later, in 2003, after the time was spent for working prototypes to be tested and refined; about the same time, the initial name x86-64 was changed to AMD64. The success of the AMD64 line of processors coupled with lukewarm reception of the IA-64 architecture forced Intel to release its own implementation of the AMD64 instruction set. Intel had previously implemented support for AMD64 but opted not to enable it in hopes that AMD would not bring AMD64 to market before Itanium's new IA-64 instruction set was widely adopted. It branded its implementation of AMD64 as EM64T, and later rebranded it Intel 64."
+1452,"In its literature and product version names, Microsoft and Sun refer to AMD64/Intel 64 collectively as x64 in the Windows and Solaris operating systems. Linux distributions refer to it either as ""x86-64"", its variant ""x86_64"", or ""amd64"". BSD systems use ""amd64"" while macOS uses ""x86_64""."
+1453,"Long mode is mostly an extension of the 32-bit instruction set, but unlike the 16–to–32-bit transition, many instructions were dropped in the 64-bit mode. This does not affect actual binary backward compatibility , but it changes the way assembler and compilers for new code have to work."
+1454,This was the first time that a major extension of the x86 architecture was initiated and originated by a manufacturer other than Intel. It was also the first time that Intel accepted technology of this nature from an outside source.
+1455,"Early x86 processors could be extended with floating-point hardware in the form of a series of floating-point numerical co-processors with names like 8087, 80287 and 80387, abbreviated x87. This was also known as the NPX , an apt name since the coprocessors, while used mainly for floating-point calculations, also performed integer operations on both binary and decimal formats.  With very few exceptions, the 80486 and subsequent x86 processors then integrated this x87 functionality on chip which made the x87 instructions a de facto integral part of the x86 instruction set."
+1456,"Each x87 register, known as ST through ST, is 80 bits wide and stores numbers in the IEEE floating-point standard double extended precision format. These registers are organized as a stack with ST as the top. This was done in order to conserve opcode space, and the registers are therefore randomly accessible only for either operand in a register-to-register instruction; ST0 must always be one of the two operands, either the source or the destination, regardless of whether the other operand is ST or a memory operand.  However, random access to the stack registers can be obtained through an instruction which exchanges any specified ST with ST."
+1457,"The operations include arithmetic and transcendental functions, including trigonometric and exponential functions, and instructions that load common constants ; and log10) into one of the stack registers. While the integer ability is often overlooked, the x87 can operate on larger integers with a single instruction than the 8086, 80286, 80386, or any x86 CPU without to 64-bit extensions can, and repeated integer calculations even on small values  can be accelerated by executing integer instructions on the x86 CPU and the x87 in parallel.  "
+1458,MMX is a SIMD instruction set designed by Intel and introduced in 1997 for the Pentium MMX microprocessor. The MMX instruction set was developed from a similar concept first used on the Intel i860. It is supported on most subsequent IA-32 processors by Intel and other vendors. MMX is typically used for video processing .
+1459,"MMX added 8 new registers to the architecture, known as MM0 through MM7 . In reality, these new registers were just aliases for the existing x87 FPU stack registers. Hence, anything that was done to the floating-point stack would also affect the MMX registers. Unlike the FP stack, these MMn registers were fixed, not relative, and therefore they were randomly accessible. The instruction set did not adopt the stack-like semantics so that existing operating systems could still correctly save and restore the register state when multitasking without modifications."
+1460,"Each of the MMn registers are 64-bit integers. However, one of the main concepts of the MMX instruction set is the concept of packed data types, which means instead of using the whole register for a single 64-bit integer , one may use it to contain two 32-bit integers , four 16-bit integers  or eight 8-bit integers . Given that the MMX's 64-bit MMn registers are aliased to the FPU stack and each of the floating-point registers are 80 bits wide, the upper 16 bits of the floating-point registers are unused in MMX. These bits are set to all ones by any MMX instruction, which correspond to the floating-point representation of NaNs or infinities."
+1461,"In 1997, AMD introduced 3DNow!. The introduction of this technology coincided with the rise of 3D entertainment applications and was designed to improve the CPU's vector processing performance of graphic-intensive applications. 3D video game developers and 3D graphics hardware vendors use 3DNow! to enhance their performance on AMD's K6 and Athlon series of processors."
+1462,"3DNow! was designed to be the natural evolution of MMX from integers to floating point. As such, it uses exactly the same register naming convention as MMX, that is MM0 through MM7. The only difference is that instead of packing integers into these registers, two single-precision floating-point numbers are packed into each register. The advantage of aliasing the FPU registers is that the same instruction and data structures used to save the state of the FPU registers can also be used to save 3DNow! register states. Thus no special modifications are required to be made to operating systems which would otherwise not know about them."
+1463,"In 1999, Intel introduced the Streaming SIMD Extensions  instruction set, following in 2000 with SSE2. The first addition allowed offloading of basic floating-point operations from the x87 stack and the second made MMX almost obsolete and allowed the instructions to be realistically targeted by conventional compilers. Introduced in 2004 along with the Prescott revision of the Pentium 4 processor, SSE3 added specific memory and thread-handling instructions to boost the performance of Intel's HyperThreading technology. AMD licensed the SSE3 instruction set and implemented most of the SSE3 instructions for its revision E and later Athlon 64 processors. The Athlon 64 does not support HyperThreading and lacks those SSE3 instructions used only for HyperThreading."
+1464,"SSE discarded all legacy connections to the FPU stack. This also meant that this instruction set discarded all legacy connections to previous generations of SIMD instruction sets like MMX. But it freed the designers up, allowing them to use larger registers, not limited by the size of the FPU registers. The designers created eight 128-bit registers, named XMM0 through XMM7.  However, the downside was that operating systems had to have an awareness of this new set of instructions in order to be able to save their register states. So Intel created a slightly modified version of Protected mode, called Enhanced mode which enables the usage of SSE instructions, whereas they stay disabled in regular Protected mode. An OS that is aware of SSE will activate Enhanced mode, whereas an unaware OS will only enter into traditional Protected mode."
+1465,"SSE is a SIMD instruction set that works only on floating-point values, like 3DNow!. However, unlike 3DNow! it severs all legacy connection to the FPU stack. Because it has larger registers than 3DNow!, SSE can pack twice the number of single precision floats into its registers. The original SSE was limited to only single-precision numbers, like 3DNow!. The SSE2 introduced the capability to pack double precision numbers too, which 3DNow! had no possibility of doing since a double precision number is 64-bit in size which would be the full size of a single 3DNow! MMn register. At 128 bits, the SSE XMMn registers could pack two double precision floats into one register. Thus SSE2 is much more suitable for scientific calculations than either SSE1 or 3DNow!, which were limited to only single precision. SSE3 does not introduce any additional registers."
+1466,"The Advanced Vector Extensions  doubled the size of SSE registers to 256-bit YMM registers. It also introduced the VEX coding scheme to accommodate the larger registers, plus a few instructions to permute elements. AVX2 did not introduce extra registers, but was notable for the addition for masking, gather, and shuffle instructions."
+1467,"AVX-512 features yet another expansion to 32 512-bit ZMM registers and a new EVEX scheme. Unlike its predecessors featuring a monolithic extension, it is divided into many subsets that specific models of CPUs can choose to implement."
+1468,"Physical Address Extension or PAE was first added in the Intel Pentium Pro, and later by AMD in the Athlon processors, to allow up to 64 GB of RAM to be addressed. Without PAE, physical RAM in 32-bit protected mode is usually limited to 4 GB. PAE defines a different page table structure with wider page table entries and a third level of page table, allowing additional bits of physical address. Although the initial implementations on 32-bit processors theoretically supported up to 64 GB of RAM, chipset and other platform limitations often restricted what could actually be used. x86-64 processors define page table structures that theoretically allow up to 52 bits of physical address, although again, chipset and other platform concerns  prevent such a large physical address space to be realized. On x86-64 processors PAE mode must be active before the switch to long mode, and must remain active while long mode is active, so while in long mode there is no ""non-PAE"" mode. PAE mode does not affect the width of linear or virtual addresses."
+1469,"By the 2000s, 32-bit x86 processors' limits in memory addressing were an obstacle to their use in high-performance computing clusters and powerful desktop workstations. The aged 32-bit x86 was competing with much more advanced 64-bit RISC architectures which could address much more memory. Intel and the whole x86 ecosystem needed 64-bit memory addressing if x86 was to survive the 64-bit computing era, as workstation and desktop software applications were soon to start hitting the limits of 32-bit memory addressing. However, Intel felt that it was the right time to make a bold step and use the transition to 64-bit desktop computers for a transition away from the x86 architecture in general, an experiment which ultimately failed."
+1470,"In 2001, Intel attempted to introduce a non-x86 64-bit architecture named IA-64 in its Itanium processor, initially aiming for the high-performance computing market, hoping that it would eventually replace the 32-bit x86. While IA-64 was incompatible with x86, the Itanium processor did provide emulation abilities for translating x86 instructions into IA-64, but this affected the performance of x86 programs so badly that it was rarely, if ever, actually useful to the users: programmers should rewrite x86 programs for the IA-64 architecture or their performance on Itanium would be orders of magnitude worse than on a true x86 processor. The market rejected the Itanium processor since it broke backward compatibility and preferred to continue using x86 chips, and very few programs were rewritten for IA-64."
+1471,"AMD decided to take another path toward 64-bit memory addressing, making sure backward compatibility would not suffer. In April 2003, AMD released the first x86 processor with 64-bit general-purpose registers, the Opteron, capable of addressing much more than 4 GB of virtual memory using the new x86-64 extension . The 64-bit extensions to the x86 architecture were enabled only in the newly introduced long mode, therefore 32-bit and 16-bit applications and operating systems could simply continue using an AMD64 processor in protected or other modes, without even the slightest sacrifice of performance and with full compatibility back to the original instructions of the 16-bit Intel 8086.: 13–14  The market responded positively, adopting the 64-bit AMD processors for both high-performance applications and business or home computers."
+1472,"Seeing the market rejecting the incompatible Itanium processor and Microsoft supporting AMD64, Intel had to respond and introduced its own x86-64 processor, the Prescott Pentium 4, in July 2004. As a result, the Itanium processor with its IA-64 instruction set is rarely used and x86, through its x86-64 incarnation, is still the dominant CPU architecture in non-embedded computers."
+1473,"x86-64 also introduced the NX bit, which offers some protection against security bugs caused by buffer overruns."
+1474,"As a result of AMD's 64-bit contribution to the x86 lineage and its subsequent acceptance by Intel, the 64-bit RISC architectures ceased to be a threat to the x86 ecosystem and almost disappeared from the workstation market. x86-64 began to be utilized in powerful supercomputers , a market which was previously the natural habitat for 64-bit RISC designs . The great leap toward 64-bit computing and the maintenance of backward compatibility with 32-bit and 16-bit software enabled the x86 architecture to become an extremely flexible platform today, with x86 chips being utilized from small low-power systems  to fast gaming desktop computers , and even dominate large supercomputing clusters, effectively leaving only the ARM 32-bit and 64-bit RISC architecture as a competitor in the smartphone and tablet market."
+1475,"Prior to 2005, x86 architecture processors were unable to meet the Popek and Goldberg requirements – a specification for virtualization created in 1974 by Gerald J. Popek and Robert P. Goldberg. However, both proprietary and open-source x86 virtualization hypervisor products were developed using software-based virtualization. Proprietary systems include Hyper-V, Parallels Workstation, VMware ESX, VMware Workstation, VMware Workstation Player and Windows Virtual PC, while free and open-source systems include QEMU, Kernel-based Virtual Machine, VirtualBox, and Xen."
+1476,The introduction of the AMD-V and Intel VT-x instruction sets in 2005 allowed x86 processors to meet the Popek and Goldberg virtualization requirements.
+1477,"APX  are extensions to double the number of general-purpose registers from 16 to 32 and add new features to improve general-purpose performance. These extensions have been called ""generational"" and ""the biggest x86 addition since 64 bits"". Intel contributed APX support to GNU Compiler Collection  14."
+1478,"According to the architecture specification, the main features of APX are:"
+1479,"Extended GPRs for general purpose instructions are encoded using 2-byte REX2 prefix, while new instructions and extended operands for existing AVX/AVX2/AVX-512 instructions are encoded with extended EVEX prefix which has four variants used for different groups of instructions."
+1480,"A handheld game console, or simply handheld console, is a small, portable self-contained video game console with a built-in screen, game controls and speakers. Handheld game consoles are smaller than home video game consoles and contain the console, screen, speakers, and controls in one unit, allowing players to carry them and play them at any time or place."
+1481,"In 1976, Mattel introduced the first handheld electronic game with the release of Auto Race. Later, several companies—including Coleco and Milton Bradley—made their own single-game, lightweight table-top or handheld electronic game devices. The first commercial successful handheld console was Merlin from 1978 which sold more than 5 million units. The first handheld game console with interchangeable cartridges is the Milton Bradley Microvision in 1979."
+1482,"Nintendo is credited with popularizing the handheld console concept with the release of the Game Boy in 1989 and continues to dominate the handheld console market. The first internet-enabled handheld console and the first with a touchscreen was the Game.com released by Tiger Electronics in 1997. The Nintendo DS, released in 2004, introduced touchscreen controls and wireless online gaming to a wider audience, becoming the best-selling handheld console with over 150 million units sold worldwide."
+1483,"This table describes handheld games consoles by generation, with over 1 million sales. No handheld achieved this prior to the fourth generation of game consoles. This list does not include dedicated consoles, such as LCD games and the Tamagotchi."
+1484,"The origins of handheld game consoles are found in handheld and tabletop electronic game devices of the 1970s and early 1980s.  These electronic devices are capable of playing only a single game, they fit in the palm of the hand or on a tabletop, and they may make use of a variety of video displays such as LED, VFD, or LCD. In 1978, handheld electronic games were described by Popular Electronics magazine as ""nonvideo electronic games"" and ""non-TV games"" as distinct from devices that required use of a television screen. Handheld electronic games, in turn, find their origins in the synthesis of previous handheld and tabletop electro-mechanical devices such as Waco's Electronic Tic-Tac-Toe  Cragstan's Periscope-Firing Range , and the emerging optoelectronic-display-driven calculator market of the early 1970s.  This synthesis happened in 1976, when ""Mattel began work on a line of calculator-sized sports games that became the world's first handheld electronic games.  The project began when Michael Katz, Mattel's new product category marketing director, told the engineers in the electronics group to design a game the size of a calculator, using LED  technology."""
+1485,"The result was the 1976 release of Auto Race.  Followed by Football later in 1977, the two games were so successful that according to Katz, ""these simple electronic handheld games turned into a '$400 million category.'"" Mattel would later win the honor of being recognized by the industry for innovation in handheld game device displays. Soon, other manufacturers including Coleco, Parker Brothers, Milton Bradley, Entex, and Bandai began following up with their own tabletop and handheld electronic games."
+1486,"In 1979 the LCD-based Microvision, designed by Smith Engineering and distributed by Milton-Bradley, became the first handheld game console and the first to use interchangeable game cartridges. The Microvision game Cosmic Hunter  also introduced the concept of a directional pad on handheld gaming devices, and is operated by using the thumb to manipulate the on-screen character in any of four directions."
+1487,"In 1979, Gunpei Yokoi, traveling on a bullet train, saw a bored businessman playing with an LCD calculator by pressing the buttons. Yokoi then thought of an idea for a watch that doubled as a miniature game machine for killing time. Starting in 1980, Nintendo began to release a series of electronic games designed by Yokoi called the Game & Watch games. Taking advantage of the technology used in the credit-card-sized calculators that had appeared on the market, Yokoi designed the series of LCD-based games to include a digital time display in the corner of the screen. For later, more complicated Game & Watch games, Yokoi invented a cross shaped directional pad or ""D-pad"" for control of on-screen characters. Yokoi also included his directional pad on the NES controllers, and the cross-shaped thumb controller soon became standard on game console controllers and ubiquitous across the video game industry since. When Yokoi began designing Nintendo's first handheld game console, he came up with a device that married the elements of his Game & Watch devices and the Famicom console, including both items' D-pad controller. The result was the Nintendo Game Boy."
+1488,"In 1982, the Bandai LCD Solarpower was the first solar-powered gaming device. Some of its games, such as the horror-themed game Terror House, features two LCD panels, one stacked on the other, for an early 3D effect. In 1983, Takara Tomy's Tomytronic 3D simulates 3D by having two LCD panels that were lit by external light through a window on top of the device, making it the first dedicated home video 3D hardware."
+1489,"The late 1980s and early 1990s saw the beginnings of the modern-day handheld game console industry, after the demise of the Microvision. As backlit LCD game consoles with color graphics consume a lot of power, they were not battery-friendly like the non-backlit original Game Boy whose monochrome graphics allowed longer battery life. By this point, rechargeable battery technology had not yet matured and so the more advanced game consoles of the time such as the Sega Game Gear and Atari Lynx did not have nearly as much success as the Game Boy."
+1490,"Even though third-party rechargeable batteries were available for the battery-hungry alternatives to the Game Boy, these batteries employed a nickel-cadmium process and had to be completely discharged before being recharged to ensure maximum efficiency; lead-acid batteries could be used with automobile circuit limiters ; but the batteries had mediocre portability. The later NiMH batteries, which do not share this requirement for maximum efficiency, were not released until the late 1990s, years after the Game Gear, Atari Lynx, and original Game Boy had been discontinued. During the time when technologically superior handhelds had strict technical limitations, batteries had a very low mAh rating since batteries with heavy power density were not yet available."
+1491,"Modern game systems such as the Nintendo DS and PlayStation Portable have rechargeable Lithium-Ion batteries with proprietary shapes. Other seventh-generation consoles, such as the GP2X, use standard alkaline batteries. Because the mAh rating of alkaline batteries has increased since the 1990s, the power needed for handhelds like the GP2X may be supplied by relatively few batteries."
+1492,"Nintendo released the Game Boy on April 21, 1989 . The design team headed by Gunpei Yokoi had also been responsible for the Game & Watch system, as well as the Nintendo Entertainment System games Metroid and Kid Icarus. The Game Boy came under scrutiny by Nintendo president Hiroshi Yamauchi, saying that the monochrome screen was too small, and the processing power was inadequate. The design team had felt that low initial cost and battery economy were more important concerns, and when compared to the Microvision, the Game Boy was a huge leap forward."
+1493,"Yokoi recognized that the Game Boy needed a killer app—at least one game that would define the console, and persuade customers to buy it. In June 1988, Minoru Arakawa, then-CEO of Nintendo of America saw a demonstration of the game Tetris at a trade show. Nintendo purchased the rights for the game, and packaged it with the Game Boy system as a launch title. It was almost an immediate hit. By the end of the year more than a million units were sold in the US. As of March 31, 2005, the Game Boy and Game Boy Color combined to sell over 118 million units worldwide."
+1494,"In 1987, Epyx created the Handy Game; a device that would become the Atari Lynx in 1989. It is the first color handheld console ever made, as well as the first with a backlit screen. It also features networking support with up to 17 other players, and advanced hardware that allows the zooming and scaling of sprites.  The Lynx can also be turned upside down to accommodate left-handed players. However, all these features came at a very high price point, which drove consumers to seek cheaper alternatives. The Lynx is also very unwieldy, consumes batteries very quickly, and lacked the third-party support enjoyed by its competitors. Due to its high price, short battery life, production shortages, a dearth of compelling games, and Nintendo's aggressive marketing campaign, and despite a redesign in 1991, the Lynx became a commercial failure. Despite this, companies like Telegames helped to keep the system alive long past its commercial relevance, and when new owner Hasbro released the rights to develop for the public domain, independent developers like Songbird have managed to release new commercial games for the system every year until 2004's Winter Games."
+1495,"The TurboExpress is a portable version of the TurboGrafx, released in 1990 for $249.99. Its Japanese equivalent is the PC Engine GT."
+1496,"It is the most advanced handheld of its time and can play all the TurboGrafx-16's games . It has a 66 mm  screen, the same as the original Game Boy, but in a much higher resolution, and can display 64 sprites at once, 16 per scanline, in 512 colors. Although the hardware can only handle 481 simultaneous colors. It has 8 kilobytes of RAM. The Turbo runs the HuC6820 CPU at 1.79 or 7.16 MHz."
+1497,"The optional ""TurboVision"" TV tuner includes RCA audio/video input, allowing users to use TurboExpress as a video monitor. The ""TurboLink"" allowed two-player play. Falcon, a flight simulator, included a ""head-to-head"" dogfight mode that can only be accessed via TurboLink. However, very few TG-16 games offered co-op play modes especially designed with the TurboExpress in mind."
+1498,The Bitcorp Gamate is one of the first handheld game systems created in response to the Nintendo Game Boy. It was released in Asia in 1990 and distributed worldwide by 1991.
+1499,"Like the Sega Game Gear, it was horizontal in orientation and like the Game Boy, required 4 AA batteries. Unlike many later Game Boy clones, its internal components were professionally assembled . Unfortunately the system's fatal flaw is its screen. Even by the standards of the day, its screen is rather difficult to use, suffering from similar ghosting problems that were common complaints with the first generation Game Boys. Likely because of this fact sales were quite poor, and Bitcorp closed by 1992. However, new games continued to be published for the Asian market, possibly as late as 1994. The total number of games released for the system remains unknown."
+1500,"Gamate games were designed for stereo sound, but the console is only equipped with a mono speaker."
+1501,"The Game Gear is the third color handheld console, after the Lynx and the TurboExpress; produced by Sega. Released in Japan in 1990 and in North America and Europe in 1991, it is based on the Master System, which gave Sega the ability to quickly create Game Gear games from its large library of games for the Master System. While never reaching the level of success enjoyed by Nintendo, the Game Gear proved to be a fairly durable competitor, lasting longer than any other Game Boy rivals."
+1502,"While the Game Gear is most frequently seen in black or navy blue, it was also released in a variety of additional colors: red, light blue, yellow, clear, and violet. All of these variations were released in small quantities and frequently only in the Asian market."
+1503,"Following Sega's success with the Game Gear, they began development on a successor during the early 1990s, which was intended to feature a touchscreen interface, many years before the Nintendo DS. However, such a technology was very expensive at the time, and the handheld itself was estimated to have cost around $289 were it to be released. Sega eventually chose to shelve the idea and instead release the Genesis Nomad, a handheld version of the Genesis, as the successor."
+1504,"The Watara Supervision was released in 1992 in an attempt to compete with the Nintendo Game Boy. The first model was designed very much like a Game Boy, but it is grey in color and has a slightly larger screen. The second model was made with a hinge across the center and can be bent slightly to provide greater comfort for the user. While the system did enjoy a modest degree of success, it never impacted the sales of Nintendo or Sega. The Supervision was redesigned a final time as ""The Magnum"". Released in limited quantities it was roughly equivalent to the Game Boy Pocket. It was available in three colors: yellow, green and grey. Watara designed many of the games themselves, but did receive some third party support, most notably from Sachen."
+1505,"A TV adapter was available in both PAL and NTSC formats that could transfer the Supervision's black-and-white palette to 4 colors, similar in some regards to the Super Game Boy from Nintendo."
+1506,"The Hartung Game Master is an obscure handheld released at an unknown point in the early 1990s. Its graphics fidelity was much lower than most of its contemporaries, displaying just 64x64 pixels. It was available in black, white, and purple, and was frequently rebranded by its distributors, such as Delplay, Videojet and Systema."
+1507,"The exact number of games released is not known, but is likely around 20. The system most frequently turns up in Europe and Australia."
+1508,"By this time, the lack of significant development in Nintendo's product line began allowing more advanced systems such as the Neo Geo Pocket Color and the WonderSwan Color to be developed."
+1509,"The Nomad was released in October 1995 in North America only. The release was six years into the market span of the Genesis, with an existing library of more than 500 Genesis games. According to former Sega of America research and development head Joe Miller, the Nomad was not intended to be the Game Gear's replacement; he believed that there was little planning from Sega of Japan for the new handheld. Sega was supporting five different consoles: Saturn, Genesis, Game Gear, Pico, and the Master System, as well as the Sega CD and 32X add-ons. In Japan, the Mega Drive had never been successful and the Saturn was more successful than Sony's PlayStation, so Sega Enterprises CEO Hayao Nakayama decided to focus on the Saturn. By 1999, the Nomad was being sold at less than a third of its original price."
+1510,"The Game Boy Pocket is a redesigned version of the original Game Boy having the same features. It was released in 1996. Notably, this variation is smaller and lighter. It comes in seven different colors; red, yellow, green, black, clear, silver, blue, and pink. It has space for two AAA batteries, which provide approximately 10 hours of game play. The screen was changed to a true black-and-white display, rather than the ""pea soup"" monochromatic display of the original Game Boy. Although, like its predecessor, the Game Boy Pocket has no backlight to allow play in a darkened area, it did notably improve visibility and pixel response-time ."
+1511,"The first model of the Game Boy Pocket did not have an LED to show battery levels, but the feature was added due to public demand. The Game Boy Pocket was not a new software platform and played the same software as the original Game Boy model."
+1512,"The Game.com  is a handheld game console released by Tiger Electronics in September 1997. It featured many new ideas for handheld consoles and was aimed at an older target audience, sporting PDA-style features and functions such as a touch screen and stylus. However, Tiger hoped it would also challenge Nintendo's Game Boy and gain a following among younger gamers too. Unlike other handheld game consoles, the first game.com consoles included two slots for game cartridges, which would not happen again until the Tapwave Zodiac, the DS and DS Lite, and could be connected to a 14.4 kbit/s modem. Later models had only a single cartridge slot."
+1513,"The Game Boy Color  is Nintendo's successor to the Game Boy and was released on October 21, 1998, in Japan and in November of the same year in the United States. It features a color screen, and is slightly bigger than the Game Boy Pocket. The processor is twice as fast as a Game Boy's and has twice as much memory. It also had an infrared communications port for wireless linking which did not appear in later versions of the Game Boy, such as the Game Boy Advance."
+1514,"The Game Boy Color was a response to pressure from game developers for a new system, as they felt that the Game Boy, even in its latest incarnation, the Game Boy Pocket, was insufficient. The resulting product was backward compatible, a first for a handheld console system, and leveraged the large library of games and great installed base of the predecessor system. This became a major feature of the Game Boy line, since it allowed each new launch to begin with a significantly larger library than any of its competitors. As of March 31, 2005, the Game Boy and Game Boy Color combined to sell 118.69 million units worldwide."
+1515,"The console is capable of displaying up to 56 different colors simultaneously on screen from its palette of 32,768, and can add basic four-color shading to games that had been developed for the original Game Boy. It can also give the sprites and backgrounds separate colors, for a total of more than four colors."
+1516,"The Neo Geo Pocket Color  was released in 1999 in Japan, and later that year in the United States and Europe. It is a 16-bit color handheld game console designed by SNK, the maker of the Neo Geo home console and arcade machine. It came after SNK's original Neo Geo Pocket monochrome handheld, which debuted in 1998 in Japan."
+1517,"In 2000 following SNK's purchase by Japanese Pachinko manufacturer Aruze, the Neo Geo Pocket Color was dropped from both the US and European markets, purportedly due to commercial failure."
+1518,"The system seemed well on its way to being a success in the U.S. It was more successful than any Game Boy competitor since Sega's Game Gear, but was hurt by several factors, such as SNK's infamous lack of communication with third-party developers, and anticipation of the Game Boy Advance. The decision to ship U.S. games in cardboard boxes in a cost-cutting move rather than hard plastic cases that Japanese and European releases were shipped in may have also hurt US sales."
+1519,"The WonderSwan Color is a handheld game console designed by Bandai. It was released on December 9, 2000, in Japan, Although the WonderSwan Color was slightly larger and heavier  compared to the original WonderSwan, the color version featured 512 KB of RAM and a larger color LCD screen. In addition, the WonderSwan Color is compatible with the original WonderSwan library of games."
+1520,"Prior to WonderSwan's release, Nintendo had virtually a monopoly in the Japanese video game handheld market. After the release of the WonderSwan Color, Bandai took approximately 8% of the market share in Japan partly due to its low price of 6800 yen . Another reason for the WonderSwan's success in Japan was the fact that Bandai managed to get a deal with Square to port over the original Famicom Final Fantasy games with improved graphics and controls. However, with the popularity of the Game Boy Advance and the reconciliation between Square and Nintendo, the WonderSwan Color and its successor, the SwanCrystal quickly lost its competitive advantage."
+1521,"The 2000s saw a major leap in innovation, particularly in the second half with the release of the DS and PSP."
+1522,"In 2001, Nintendo released the Game Boy Advance , which added two shoulder buttons, a larger screen, and more computing power than the Game Boy Color."
+1523,"The design was revised two years later when the Game Boy Advance SP , a more compact version, was released. The SP features a ""clamshell"" design , as well as a frontlit color display and rechargeable battery. Despite the smaller form factor, the screen remained the same size as that of the original. In 2005, the Game Boy Micro was released. This revision sacrifices screen size and backwards compatibility with previous Game Boys for a dramatic reduction in total size and a brighter backlit screen. A new SP model with a backlit screen was released in some regions around the same time."
+1524,"Along with the GameCube, the GBA also introduced the concept of ""connectivity"": using a handheld system as a console controller. A handful of games use this feature, most notably Animal Crossing, Pac-Man Vs., Final Fantasy Crystal Chronicles, The Legend of Zelda: Four Swords Adventures, The Legend of Zelda: The Wind Waker, Metroid Prime, and Sonic Adventure 2: Battle."
+1525,"As of December 31, 2007, the GBA, GBA SP, and the Game Boy Micro combined have sold 80.72 million units worldwide."
+1526,"The original GP32 was released in 2001 by the South Korean company Game Park a few months after the launch of the Game Boy Advance. It featured a 32-bit CPU, 133 MHz processor, MP3 and Divx player, and e-book reader. SmartMedia cards were used for storage, and could hold up to 128mb of anything downloaded through a USB cable from a PC. The GP32 was redesigned in 2003. A front-lit screen was added and the new version was called GP32 FLU . In summer 2004, another redesign, the GP32 BLU, was made, and added a backlit screen. This version of the handheld was planned for release outside South Korea; in Europe, and it was released for example in Spain . While not a commercial success on a level with mainstream handhelds , it ended up being used mainly as a platform for user-made applications and emulators of other systems, being popular with developers and more technically adept users."
+1527,"Nokia released the N-Gage in 2003. It was designed as a combination MP3 player, cellphone, PDA, radio, and gaming device. The system received much criticism alleging defects in its physical design and layout, including its vertically oriented screen and requirement of removing the battery to change game cartridges. The most well known of these was ""sidetalking"", or the act of placing the phone speaker and receiver on an edge of the device instead of one of the flat sides, causing the user to appear as if they are speaking into a taco."
+1528,"The N-Gage QD was later released to address the design flaws of the original. However, certain features available in the original N-Gage, including MP3 playback, FM radio reception, and USB connectivity were removed."
+1529,"Second generation of N-Gage launched on April 3, 2008 in the form of a service for selected Nokia Smartphones."
+1530,"In 2003, Tapwave released the Zodiac. It was designed to be a PDA-handheld game console hybrid. It supported photos, movies, music, Internet, and documents. The Zodiac used a special version Palm OS 5, 5.2T, that supported the special gaming buttons and graphics chip. Two versions were available, Zodiac 1 and 2, differing in memory and looks. The Zodiac line ended in July 2005 when Tapwave declared bankruptcy."
+1531,"The Nintendo DS was released in November 2004. Among its new features were the incorporation of two screens, a touchscreen, wireless connectivity, and a microphone port. As with the Game Boy Advance SP, the DS features a clamshell design, with the two screens aligned vertically on either side of the hinge."
+1532,"The DS's lower screen is touch sensitive, designed to be pressed with a stylus, a user's finger or a special ""thumb pad"" . More traditional controls include four face buttons, two shoulder buttons, a D-pad, and ""Start"" and ""Select"" buttons. The console also features online capabilities via the Nintendo Wi-Fi Connection and ad-hoc wireless networking for multiplayer games with up to sixteen players. It is backwards-compatible with all Game Boy Advance games, but like the Game Boy Micro, it is not compatible with games designed for the Game Boy or Game Boy Color."
+1533,"In January 2006, Nintendo revealed an updated version of the DS: the Nintendo DS Lite  with an updated, smaller form factor , a cleaner design, longer battery life, and brighter, higher-quality displays, with adjustable brightness. It is also able to connect wirelessly with Nintendo's Wii console."
+1534,"On October 2, 2008, Nintendo announced the Nintendo DSi, with larger, 3.25-inch screens and two integrated cameras. It has an SD card storage slot in place of the Game Boy Advance slot, plus internal flash memory for storing downloaded games. It was released on November 1, 2008, in Japan, April 2, 2009, in Australia, April 3, 2009, in Europe, and April 5, 2009, in North America. On October 29, 2009, Nintendo announced a larger version of the DSi, called the DSi XL, which was released on November 21, 2009, in Japan, March 5, 2010, in Europe, March 28, 2010, in North America, and April 15, 2010, in Australia."
+1535,"As of December 31, 2009, the Nintendo DS, Nintendo DS Lite, and Nintendo DSi combined have sold 125.13 million units worldwide."
+1536,"The GameKing is a handheld game console released by the Chinese company TimeTop in 2004. The first model while original in design owes a large debt to Nintendo's Game Boy Advance. The second model, the GameKing 2, is believed to be inspired by Sony's PSP. This model also was upgraded with a backlit screen, with a distracting background transparency . A color model, the GameKing 3 apparently exists, but was only made for a brief time and was difficult to purchase outside of Asia.  Whether intentionally or not, the GameKing has the most primitive graphics of any handheld released since the Game Boy of 1989. "
+1537,"As many of the games have an ""old school"" simplicity, the device has developed a small cult following. The Gameking's speaker is quite loud and the cartridges' sophisticated looping soundtracks  are seemingly at odds with its primitive graphics."
+1538,"TimeTop made at least one additional device sometimes labeled as ""GameKing"", but while it seems to possess more advanced graphics, is essentially an emulator that plays a handful of multi-carts . Outside of Asia  however the Gameking remains relatively unheard of due to the enduring popularity of Japanese handhelds such as those manufactured by Nintendo and Sony."
+1539,"The PlayStation Portable  is a handheld game console manufactured and marketed by Sony Computer Entertainment. Development of the console was first announced during E3 2003, and it was unveiled on May 11, 2004, at a Sony press conference before E3 2004. The system was released in Japan on December 12, 2004, in North America on March 24, 2005, and in the PAL region on September 1, 2005."
+1540,"The PlayStation Portable is the first handheld video game console to use an optical disc format, Universal Media Disc , for distribution of its games. UMD Video discs with movies and television shows were also released. The PSP utilized the Sony/SanDisk Memory Stick Pro Duo format as its primary storage medium. Other distinguishing features of the console include its large viewing screen, multi-media capabilities, and connectivity with the PlayStation 3, other PSPs, and the Internet."
+1541,"Tiger's Gizmondo came out in the UK during March 2005 and it was released in the U.S. during October 2005. It is designed to play music, movies, and games, have a camera for taking and storing photos, and have GPS functions. It also has Internet capabilities. It has a phone for sending text and multimedia messages. Email was promised at launch, but was never released before Gizmondo, and ultimately Tiger Telematics', downfall in early 2006. Users obtained a second service pack, unreleased, hoping to find such functionality. However, Service Pack B did not activate the e-mail functionality."
+1542,"The GP2X is an open-source, Linux-based handheld video game console and media player created by GamePark Holdings of South Korea, designed for homebrew developers as well as commercial developers. It is commonly used to run emulators for game consoles such as Neo-Geo, Genesis, Master System, Game Gear, Amstrad CPC, Commodore 64, Nintendo Entertainment System, TurboGrafx-16, MAME and others."
+1543,"A new version called the ""F200"" was released October 30, 2007, and features a touchscreen, among other changes.  Followed by GP2X Wiz  and GP2X Caanoo ."
+1544,"The Dingoo A320 is a micro-sized gaming handheld that resembles the Game Boy Micro and is open to game development. It also supports music, radio, emulators  and video playing capabilities with its own interface much like the PSP. There is also an onboard radio and recording program. It is currently available in two colors — white and black. Other similar products from the same manufacturer are the Dingoo A330 , Dingoo A360, Dingoo A380, and Dingoo A320E."
+1545,"The PSP Go is a version of the PlayStation Portable handheld game console manufactured by Sony. It was released on October 1, 2009, in American and European territories, and on November 1 in Japan. It was revealed prior to E3 2009 through Sony's Qore VOD service. Although its design is significantly different from other PSPs, it is not intended to replace the PSP 3000, which Sony continued to manufacture, sell, and support. On April 20, 2011, the manufacturer announced that the PSP Go would be discontinued so that they may concentrate on the PlayStation Vita. Sony later said that only the European and Japanese versions were being cut, and that the console would still be available in the US.
+Unlike previous PSP models, the PSP Go does not feature a UMD drive, but instead has 16 GB of internal flash memory to store games, video, pictures, and other media. This can be extended by up to 32 GB with the use of a Memory Stick Micro  flash card. Also unlike previous PSP models, the PSP Go's rechargeable battery is not removable or replaceable by the user. The unit is 43% lighter and 56% smaller than the original PSP-1000, and 16% lighter and 35% smaller than the PSP-3000. It has a 3.8"" 480 × 272 LCD . The screen slides up to reveal the main controls. The overall shape and sliding mechanism are similar to that of Sony's mylo COM-2 internet device."
+1546,"The Pandora is a handheld game console/UMPC/PDA hybrid designed to take advantage of existing open source software and to be a target for home-brew development. It runs a full distribution of Linux, and in functionality is like a small PC with gaming controls. It is developed by OpenPandora, which is made up of former distributors and community members of the GP32 and GP2X handhelds."
+1547,"OpenPandora began taking pre-orders for one batch of 4000 devices in November 2008 and after manufacturing delays, began shipping to customers on May 21, 2010."
+1548,"The FC-16 Go is a portable Super NES hardware clone manufactured by Yobo Gameware in 2009. It features a 3.5-inch display, two wireless controllers, and CRT cables that allow cartridges to be played on a television screen. Unlike other Super NES clone consoles, it has region tabs that only allow NTSC North American cartridges to be played. Later revisions feature stereo sound output, larger shoulder buttons, and a slightly re-arranged button, power, and A/V output layout."
+1549,"The Nintendo 3DS is the successor to Nintendo's DS handheld. The autostereoscopic device is able to project stereoscopic three-dimensional effects without requirement of active shutter or passive polarized glasses, which are required by most current 3D televisions to display the 3D effect. The 3DS was released in Japan on February 26, 2011; in Europe on March 25, 2011; in North America on March 27, 2011, and in Australia on March 31, 2011. The system features backward compatibility with Nintendo DS series software, including Nintendo DSi software except those that require the Game Boy Advance slot. It also features an online service called the Nintendo eShop, launched on June 6, 2011, in North America and June 7, 2011, in Europe and Japan, which allows owners to download games, demos, applications and information on upcoming film and game releases. On November 24, 2011, a limited edition Legend of Zelda 25th Anniversary 3DS was released that contained a unique Cosmo Black unit decorated with gold Legend of Zelda related imagery, along with a copy of The Legend of Zelda: Ocarina of Time 3D."
+1550,"There are also other models including the Nintendo 2DS and the New Nintendo 3DS, the latter with a larger  variant, like the original Nintendo 3DS, as well as the New Nintendo 2DS XL."
+1551,"The Sony Ericsson Xperia PLAY is a handheld game console smartphone produced by Sony Ericsson under the Xperia smartphone brand. The device runs Android 2.3 Gingerbread, and is the first to be part of the PlayStation Certified program which means that it can play PlayStation Suite games. The device is a horizontally sliding phone with its original form resembling the Xperia X10 while the slider below resembles the slider of the PSP Go. The slider features a D-pad on the left side, a set of standard PlayStation buttons  on the right, a long rectangular touchpad in the middle, start and select buttons on the bottom right corner, a menu button on the bottom left corner, and two shoulder buttons  on the back of the device. It is powered by a 1 GHz Qualcomm Snapdragon processor, a Qualcomm Adreno 205 GPU, and features a display measuring 4.0 inches  , an 8-megapixel camera, 512 MB RAM, 8 GB internal storage, and a micro-USB connector. It supports microSD cards, versus the Memory Stick variants used in PSP consoles. The device was revealed officially for the first time in a Super Bowl ad on Sunday, February 6, 2011. On February 13, 2011, at Mobile World Congress  2011, it was announced that the device would be shipping globally in March 2011, with a launch lineup of around 50 software titles."
+1552,"The PlayStation Vita is the successor to Sony's PlayStation Portable  Handheld series. It was released in Japan on December 17, 2011, and in Europe, Australia, North, and South America on February 22, 2012."
+1553,"The handheld includes two analog sticks, a 5-inch  OLED/LCD multi-touch capacitive touchscreen, and supports Bluetooth, Wi-Fi and optional 3G. Internally, the PS Vita features a 4 core ARM Cortex-A9 MPCore processor and a 4 core SGX543MP4+ graphics processing unit, as well as LiveArea software as its main user interface, which succeeds the XrossMediaBar."
+1554,"The device is fully backwards-compatible with PlayStation Portable games digitally released on the PlayStation Network via the PlayStation Store. However, PSone Classics and PS2 titles were not compatible at the time of the primary public release in Japan. The Vita's dual analog sticks will be supported on selected PSP games. The graphics for PSP releases will be up-scaled, with a smoothing filter to reduce pixelation."
+1555,"On September 20, 2018, Sony announced at Tokyo Game Show 2018 that the Vita would be discontinued in 2019, ending its hardware production. Production of Vita hardware officially ended on March 1, 2019."
+1556,"The Razer Switchblade was a prototype pocket-sized like a Nintendo DSi XL designed to run Windows 7, featured a multi-touch LCD screen and an adaptive keyboard that changed keys depending on the game the user would play. It also was to feature a full mouse."
+1557,"It was first unveiled on January 5, 2011, on the Consumer Electronics Show . The Switchblade won The Best of CES 2011 People's Voice award. It has since been in development and the release date is still unknown. The device has likely been suspended indefinitely."
+1558,"Project Shield is a handheld system developed by Nvidia announced at CES 2013. It runs on Android 4.2 and uses Nvidia Tegra 4 SoC. The hardware includes a 5-inches multitouch screen with support for HD graphics . The console allows for the streaming of games running on a compatible desktop PC, or laptop."
+1559,"Nvidia Shield Portable has received mixed reception from critics. Generally, reviewers praised the performance of the device, but criticized the cost and lack of worthwhile games. Engadget's review noted the system's ""extremely impressive PC gaming"", but also that due to its high price, the device was ""a hard sell as a portable game console"", especially when compared to similar handhelds on the market. CNET's Eric Franklin states in his review of the device that ""The Nvidia Shield is an extremely well made device, with performance that pretty much obliterates any mobile product before it; but like most new console launches, there is currently a lack of available games worth your time."" Eurogamer's comprehensive review of the device provides a detailed account of the device and its features; concluded by saying: ""In the here and now, the first-gen Shield Portable is a gloriously niche, luxury product - the most powerful Android system on the market by a clear stretch and possessing a unique link to PC gaming that's seriously impressive in beta form, and can only get better."""
+1560,"The Nintendo Switch is a hybrid console that can either be used in a handheld form, or inserted into a docking station attached to a television to play on a bigger screen. The Switch features two detachable wireless controllers, called Joy-Con, which can be used individually or attached to a grip to provide a traditional gamepad form. A handheld-only revision named Nintendo Switch Lite was released on September 20, 2019."
+1561,"The Switch Lite had sold about 1.95 million units worldwide by September 30, 2019, only 10 days after its launch."
+1562,"Evercade is a handheld game console developed and manufactured by UK company Blaze Entertainment. It focuses on retrogaming with ROM cartridges that each contain a number of emulated games. Development began in 2018, and the console was released in May 2020, after a few delays. Upon its launch, the console offered 10 game cartridges with a combined total of 122 games."
+1563,"Arc System Works, Atari, Data East, Interplay Entertainment, Bandai Namco Entertainment and Piko Interactive have released emulated versions of their games for the Evercade. Pre-existing homebrew games have also been re-released for the console by Mega Cat Studios. The Evercade is capable of playing games originally released for the Atari 2600, the Atari 7800, the Atari Lynx, the NES, the SNES, and the Sega Genesis/Mega Drive."
+1564,"The Analogue Pocket is a FPGA-based handheld game console designed and manufactured by Analogue, It is designed to play games designed for handhelds of the fourth, fifth and sixth generation of video game consoles. The console features a design reminiscent of the Game Boy, with additional buttons for the supported platforms. It features a 3.5"" 1600x1440 LTPS LCD display, an SD card port, and a link cable port compatible with Game Boy link cables. The Analogue Pocket uses an Altera Cyclone V processor, and is compatible with the original Game Boy, Game Boy Color and Game Boy Advance cartridges out of the box. With cartridge adapters  the Analogue Pocket can play Game Gear, Neo Geo Pocket, Neo Geo Pocket Color and Atari Lynx game cartridges. The Analogue Pocket includes an additional FPGA, allowing 3rd party FPGA development. The Analogue Pocket was released in December 2021."
+1565,"The Steam Deck is a handheld computer device, developed by Valve, which runs SteamOS 3.0, a tailored distro of Arch Linux and includes support for Proton, a compatibility layer that allows most Microsoft Windows games to be played on the Linux-based operating system. In terms of hardware, the Deck includes a custom AMD APU based on their Zen 2 and RDNA 2 architectures, with the CPU running a four-core/eight-thread unit and the GPU running on eight compute units with a total estimated performance of 1.6 TFLOPS. Both the CPU and GPU use variable timing frequencies, with the CPU running between 2.4 and 3.5 GHz and the GPU between 1.0 and 1.6 GHz based on current processor needs. Valve stated that the CPU has comparable performance to Ryzen 3000 desktop computer processors and the GPU performance to the Radeon RX 6000 series. The Deck includes 16 GB of LPDDR5 RAM in a quad channel configuration."
+1566,"Valve revealed the Steam Deck on July 15, 2021, with pre-orders being made option the next day. The Deck was expected to ship in December 2021 to the US, Canada, the EU and the UK but was delayed to February 2022, with other regions to follow in 2022. Pre-orders were limited to those with Steam accounts opened before June 2021 to prevent resellers from controlling access to the device. Pre-orders reservations on July 16, 2021, through the Steam storefront briefly crashed the servers due to the demand. While initial shipments are still planned by February 2022, Valve has reported to new purchasers that wider availability will be later, with the 64 GB model and 256 GB NVMe model due in Q2 2022, and the 512 GB NVMe model by Q3 2022. Steam Deck was released on February 25, 2022."
+1567,"ARM  is a family of RISC instruction set architectures  for computer processors. Arm Ltd. develops the ISAs and licenses them to other companies, who build the physical devices that use the instruction set. It also designs and licenses cores that implement these ISAs."
+1568,"Due to their low costs, low power consumption, and low heat generation, ARM processors are useful for light, portable, battery-powered devices, including smartphones, laptops, and tablet computers, as well as embedded systems. However, ARM processors are also used for desktops and servers, including the world's fastest supercomputer  from 2020 to 2022. With over 230 billion ARM chips  produced, as of 2022, ARM is the most widely used family of instruction set architectures."
+1569,"There have been several generations of the ARM design. The original ARM1 used a 32-bit internal structure but had a 26-bit address space that limited it to 64 MB of main memory. This limitation was removed in the ARMv3 series, which has a 32-bit address space, and several additional generations up to ARMv7 remained 32-bit. Released in 2011, the ARMv8-A architecture added support for a 64-bit address space and 64-bit arithmetic with its new 32-bit fixed-length instruction set. Arm Ltd. has also released a series of additional instruction sets for different rules; the ""Thumb"" extension adds both 32- and 16-bit instructions for improved code density, while Jazelle added instructions for directly handling Java bytecode. More recent changes include the addition of simultaneous multithreading  for improved performance or fault tolerance."
+1570,"Acorn Computers' first widely successful design was the BBC Micro, introduced in December 1981. This was a relatively conventional machine based on the MOS Technology 6502 CPU but ran at roughly double the performance of competing designs like the Apple II due to its use of faster dynamic random-access memory . Typical DRAM of the era ran at about 2 MHz; Acorn arranged a deal with Hitachi for a supply of faster 4 MHz parts."
+1571,"Machines of the era generally shared memory between the processor and the framebuffer, which allowed the processor to quickly update the contents of the screen without having to perform separate input/output . As the timing of the video display is exacting, the video hardware had to have priority access to that memory. Due to a quirk of the 6502's design, the CPU left the memory untouched for half of the time. Thus by running the CPU at 1 MHz, the video system could read data during those down times, taking up the total 2 MHz bandwidth of the RAM. In the BBC Micro, the use of 4 MHz RAM allowed the same technique to be used, but running at twice the speed. This allowed it to outperform any similar machine on the market."
+1572,"1981 was also the year that the IBM Personal Computer was introduced. Using the recently introduced Intel 8088, a 16-bit CPU compared to the 6502's 8-bit design, it offered higher overall performance. Its introduction changed the desktop computer market radically: what had been largely a hobby and gaming market emerging over the prior five years began to change to a must-have business tool where the earlier 8-bit designs simply could not compete. Even newer 32-bit designs were also coming to market, such as the Motorola 68000 and National Semiconductor NS32016."
+1573,"Acorn began considering how to compete in this market and produced a new paper design named the Acorn Business Computer. They set themselves the goal of producing a machine with ten times the performance of the BBC Micro, but at the same price. This would outperform and underprice the PC. At the same time, the recent introduction of the Apple Lisa brought the graphical user interface  concept to a wider audience and suggested the future belonged to machines with a GUI. The Lisa, however, cost $9,995, as it was packed with support chips, large amounts of memory, and a hard disk drive, all very expensive then."
+1574,"The engineers then began studying all of the CPU designs available. Their conclusion about the existing 16-bit designs was that they were a lot more expensive and were still ""a bit crap"", offering only slightly higher performance than their BBC Micro design. They also almost always demanded a large number of support chips to operate even at that level, which drove up the cost of the computer as a whole. These systems would simply not hit the design goal. They also considered the new 32-bit designs, but these cost even more and had the same issues with support chips. According to Sophie Wilson, all the processors tested at that time performed about the same, with about a 4 Mbit/s bandwidth."
+1575,"Two key events led Acorn down the path to ARM. One was the publication of a series of reports from the University of California, Berkeley, which suggested that a simple chip design could nevertheless have extremely high performance, much higher than the latest 32-bit designs on the market. The second was a visit by Steve Furber and Sophie Wilson to the Western Design Center, a company run by Bill Mensch and his sister, which had become the logical successor to the MOS team and was offering new versions like the WDC 65C02. The Acorn team saw high school students producing chip layouts on Apple II machines, which suggested that anyone could do it. In contrast, a visit to another design firm working on modern 32-bit CPU revealed a team with over a dozen members which were already on revision H of their design and yet it still contained bugs. This cemented their late 1983 decision to begin their own CPU design, the Acorn RISC Machine."
+1576,"The original Berkeley RISC designs were in some sense teaching systems, not designed specifically for outright performance. To the RISC's basic register-heavy and load/store concepts, ARM added a number of the well-received design notes of the 6502. Primary among them was the ability to quickly serve interrupts, which allowed the machines to offer reasonable input/output performance with no added external hardware. To offer interrupts with similar performance as the 6502, the ARM design limited its physical address space to 64 MB of total addressable space, requiring 26 bits of address. As instructions were 4 bytes  long, and required to be aligned on 4-byte boundaries, the lower 2 bits of an instruction address were always zero. This meant the program counter  only needed to be 24 bits, allowing it to be stored along with the eight bit processor flags in a single 32-bit register. That meant that upon receiving an interrupt, the entire machine state could be saved in a single operation, whereas had the PC been a full 32-bit value, it would require separate operations to store the PC and the status flags. This decision halved the interrupt overhead."
+1577,"Another change, and among the most important in terms of practical real-world performance, was the modification of the instruction set to take advantage of page mode DRAM. Recently introduced, page mode allowed subsequent accesses of memory to run twice as fast if they were roughly in the same location, or ""page"", in the DRAM chip. Berkeley's design did not consider page mode and treated all memory equally. The ARM design added special vector-like memory access instructions, the ""S-cycles"", that could be used to fill or save multiple registers in a single page using page mode. This doubled memory performance when they could be used, and was especially important for graphics performance."
+1578,The Berkeley RISC designs used register windows to reduce the number of register saves and restores performed in procedure calls; the ARM design did not adopt this.
+1579,"Wilson developed the instruction set, writing a simulation of the processor in BBC BASIC that ran on a BBC Micro with a second 6502 processor. This convinced Acorn engineers they were on the right track. Wilson approached Acorn's CEO, Hermann Hauser, and requested more resources. Hauser gave his approval and assembled a small team to design the actual processor based on Wilson's ISA. The official Acorn RISC Machine project started in October 1983."
+1580,"Acorn chose VLSI Technology as the ""silicon partner"", as they were a source of ROMs and custom chips for Acorn. Acorn provided the design and VLSI provided the layout and production. The first samples of ARM silicon worked properly when first received and tested on 26 April 1985. Known as ARM1, these versions ran at 6 MHz."
+1581,"The first ARM application was as a second processor for the BBC Micro, where it helped in developing simulation software to finish development of the support chips , and sped up the CAD software used in ARM2 development. Wilson subsequently rewrote BBC BASIC in ARM assembly language. The in-depth knowledge gained from designing the instruction set enabled the code to be very dense, making ARM BBC BASIC an extremely good test for any ARM emulator."
+1582,"The result of the simulations on the ARM1 boards led to the late 1986 introduction of the ARM2 design running at 8 MHz, and the early 1987 speed-bumped version at 10 to 12 MHz. A significant change in the underlying architecture was the addition of a Booth multiplier, whereas formerly multiplication had to be carried out in software. Further, a new Fast Interrupt reQuest mode, FIQ for short, allowed registers 8 through 14 to be replaced as part of the interrupt itself. This meant FIQ requests did not have to save out their registers, further speeding interrupts."
+1583,"According to the Dhrystone benchmark, the ARM2 was roughly seven times the performance of a typical 7 MHz 68000-based system like the Amiga or Macintosh SE. It was twice as fast as an Intel 80386 running at 16 MHz, and about the same speed as a multi-processor VAX-11/784 superminicomputer. The only systems that beat it were the Sun SPARC and MIPS R2000 RISC-based workstations. Further, as the CPU was designed for high-speed I/O, it dispensed with many of the support chips seen in these machines; notably, it lacked any dedicated direct memory access  controller which was often found on workstations. The graphics system was also simplified based on the same set of underlying assumptions about memory and timing. The result was a dramatically simplified design, offering performance on par with expensive workstations but at a price point similar to contemporary desktops."
+1584,"The ARM2 featured a 32-bit data bus, 26-bit address space and 27 32-bit registers, of which 16 are accessible at any one time . The ARM2 had a transistor count of just 30,000, compared to Motorola's six-year-older 68000 model with around 68,000. Much of this simplicity came from the lack of microcode, which represents about one-quarter to one-third of the 68000's transistors, and the lack of  a cache. This simplicity enabled the ARM2 to have a low power consumption and simpler thermal packaging, through having fewer powered transistors, yet offering better performance than the contemporary, 1987, IBM PS/2 Model 50, which initially utilised an Intel 80286, offering 1.8 MIPS @ 10 MHz, and later in 1987, the 2 MIPS of the PS/2 70, with its Intel 386 DX @ 16 MHz."
+1585,"A successor, ARM3, was produced with a 4 KB cache, which further improved performance. The address bus was extended to 32 bits in the ARM6, but program code still had to lie within the first 64 MB of memory in 26-bit compatibility mode, due to the reserved bits for the status flags."
+1586,"In the late 1980s, Apple Computer and VLSI Technology started working with Acorn on newer versions of the ARM core. In 1990, Acorn spun off the design team into a new company named Advanced RISC Machines Ltd., which became ARM Ltd. when its parent company, Arm Holdings plc, floated on the London Stock Exchange and Nasdaq in 1998. The new Apple–ARM work would eventually evolve into the ARM6, first released in early 1992. Apple used the ARM6-based ARM610 as the basis for their Apple Newton PDA."
+1587,"In 1994, Acorn used the ARM610 as the main central processing unit  in their RiscPC computers. DEC licensed the ARMv4 architecture and produced the StrongARM. At 233 MHz, this CPU drew only one watt . This work was later passed to Intel as part of a lawsuit settlement, and Intel took the opportunity to supplement their i960 line with the StrongARM. Intel later developed its own high performance implementation named XScale, which it has since sold to Marvell. Transistor count of the ARM core remained essentially the same throughout these changes; ARM2 had 30,000 transistors, while ARM6 grew only to 35,000."
+1588,"In 2005, about 98% of all mobile phones sold used at least one ARM processor. In 2010, producers of chips based on ARM architectures reported shipments of 6.1 billion ARM-based processors, representing 95% of smartphones, 35% of digital televisions and set-top boxes, and 10% of mobile computers. In 2011, the 32-bit ARM architecture was the most widely used architecture in mobile devices and the most popular 32-bit one in embedded systems. In 2013, 10 billion were produced and ""ARM-based chips are found in nearly 60 percent of the world's mobile devices""."
+1589,"Arm Ltd.'s primary business is selling IP cores, which licensees use to create microcontrollers , CPUs, and systems-on-chips based on those cores. The original design manufacturer combines the ARM core with other parts to produce a complete device, typically one that can be built in existing semiconductor fabrication plants  at low cost and still deliver substantial performance. The most successful implementation has been the ARM7TDMI with hundreds of millions sold. Atmel has been a precursor design center in the ARM7TDMI-based embedded system."
+1590,"The ARM architectures used in smartphones, PDAs and other mobile devices range from ARMv5 to ARMv8-A."
+1591,"In 2009, some manufacturers introduced netbooks based on ARM architecture CPUs, in direct competition with netbooks based on Intel Atom."
+1592,"Arm Ltd. offers a variety of licensing terms, varying in cost and deliverables. Arm Ltd. provides to all licensees an integratable hardware description of the ARM core as well as complete software development toolset , and the right to sell manufactured silicon containing the ARM CPU."
+1593,"SoC packages integrating ARM's core designs include Nvidia Tegra's first three generations, CSR plc's Quatro family, ST-Ericsson's Nova and NovaThor, Silicon Labs's Precision32 MCU, Texas Instruments's OMAP products, Samsung's Hummingbird and Exynos products, Apple's A4, A5, and A5X, and NXP's i.MX."
+1594,"Fabless licensees, who wish to integrate an ARM core into their own chip design, are usually only interested in acquiring a ready-to-manufacture verified semiconductor intellectual property core. For these customers, Arm Ltd. delivers a gate netlist description of the chosen ARM core, along with an abstracted simulation model and test programs to aid design integration and verification. More ambitious customers, including integrated device manufacturers  and foundry operators, choose to acquire the processor IP in synthesizable RTL  form. With the synthesizable RTL, the customer has the ability to perform architectural level optimisations and extensions. This allows the designer to achieve exotic design goals not otherwise possible with an unmodified netlist . While Arm Ltd. does not grant the licensee the right to resell the ARM architecture itself, licensees may freely sell manufactured products such as chip devices, evaluation boards and complete systems. Merchant foundries can be a special case; not only are they allowed to sell finished silicon containing ARM cores, they generally hold the right to re-manufacture ARM cores for other customers."
+1595,"Arm Ltd. prices its IP based on perceived value. Lower performing ARM cores typically have lower licence costs than higher performing cores. In implementation terms, a synthesisable core costs more than a hard macro  core. Complicating price matters, a merchant foundry that holds an ARM licence, such as Samsung or Fujitsu, can offer fab customers reduced licensing costs. In exchange for acquiring the ARM core through the foundry's in-house design services, the customer can reduce or eliminate payment of ARM's upfront licence fee."
+1596,"Compared to dedicated semiconductor foundries  without in-house design services, Fujitsu/Samsung charge two- to three-times more per manufactured wafer. For low to mid volume applications, a design service foundry offers lower overall pricing . For high volume mass-produced parts, the long term cost reduction achievable through lower wafer pricing reduces the impact of ARM's NRE  costs, making the dedicated foundry a better choice."
+1597,"Companies that have developed chips with cores designed by Arm include Amazon.com's Annapurna Labs subsidiary, Analog Devices, Apple, AppliedMicro , Atmel, Broadcom, Cavium, Cypress Semiconductor, Freescale Semiconductor , Huawei, Intel, Maxim Integrated, Nvidia, NXP, Qualcomm, Renesas, Samsung Electronics, ST Microelectronics, Texas Instruments, and Xilinx."
+1598,"In February 2016, ARM announced the Built on ARM Cortex Technology licence, often shortened to Built on Cortex  licence. This licence allows companies to partner with ARM and make modifications to ARM Cortex designs. These design modifications will not be shared with other companies. These semi-custom core designs also have brand freedom, for example Kryo 280."
+1599,Companies that are current licensees of Built on ARM Cortex Technology include Qualcomm.
+1600,"Companies can also obtain an ARM architectural licence for designing their own CPU cores using the ARM instruction sets. These cores must comply fully with the ARM architecture. Companies that have designed cores that implement an ARM architecture include Apple, AppliedMicro , Broadcom, Cavium , Digital Equipment Corporation, Intel, Nvidia, Qualcomm, Samsung Electronics, Fujitsu, and NUVIA Inc. ."
+1601,"On 16 July 2019, ARM announced ARM Flexible Access. ARM Flexible Access provides unlimited access to included ARM intellectual property  for development. Per product licence fees are required once a customer reaches foundry tapeout or prototyping."
+1602,75% of ARM's most recent IP over the last two years are included in ARM Flexible Access. As of October 2019:
+1603,"Arm provides a list of vendors who implement ARM cores in their design , microprocessor and microcontrollers)."
+1604,"ARM cores are used in a number of products, particularly PDAs and smartphones. Some computing examples are Microsoft's first generation Surface, Surface 2 and Pocket PC devices , Apple's iPads, and Asus's Eee Pad Transformer tablet computers, and several Chromebook laptops. Others include Apple's iPhone smartphones and iPod portable media players, Canon PowerShot digital cameras, Nintendo Switch hybrid, the Wii security processor and 3DS handheld game consoles, and TomTom turn-by-turn navigation systems."
+1605,"In 2005, Arm took part in the development of Manchester University's computer SpiNNaker, which used ARM cores to simulate the human brain."
+1606,"ARM chips are also used in Raspberry Pi, BeagleBoard, BeagleBone, PandaBoard, and other single-board computers, because they are very small, inexpensive, and consume very little power."
+1607,"The 32-bit ARM architecture , such as ARMv7-A , was the most widely used architecture in mobile devices as of 2011."
+1608,"Since 1995, various versions of the ARM Architecture Reference Manual  have been the primary source of documentation on the ARM processor architecture and instruction set, distinguishing interfaces that all ARM processors are required to support  from implementation details that may vary. The architecture has evolved over time, and version seven of the architecture, ARMv7, defines three architecture ""profiles"":"
+1609,"Although the architecture profiles were first defined for ARMv7, ARM subsequently defined the ARMv6-M architecture  as a subset of the ARMv7-M profile with fewer instructions."
+1610,"Except in the M-profile, the 32-bit ARM architecture specifies several CPU modes, depending on the implemented architecture features. At any moment in time, the CPU can be in only one mode, but it can switch modes due to external events  or programmatically."
+1611,"The original  ARM implementation was hardwired without microcode, like the much simpler 8-bit 6502 processor used in prior Acorn microcomputers."
+1612,The 32-bit ARM architecture  includes the following RISC features:
+1613,"To compensate for the simpler design, compared with processors like the Intel 80286 and Motorola 68020, some additional design features were used:"
+1614,"ARM includes integer arithmetic operations for add, subtract, and multiply; some versions of the architecture also support divide operations."
+1615,"ARM supports 32-bit × 32-bit multiplies with either a 32-bit result or 64-bit result, though Cortex-M0 / M0+ / M1 cores do not support 64-bit results. Some ARM cores also support 16-bit × 16-bit and 32-bit × 16-bit multiplies."
+1616,The divide instructions are only included in the following ARM architectures:
+1617,Registers R0 through R7 are the same across all CPU modes; they are never banked.
+1618,Registers R8 through R12 are the same across all CPU modes except FIQ mode.  FIQ mode has its own distinct R8 through R12 registers.
+1619,"R13 and R14 are banked across all privileged CPU modes except system mode. That is, each mode that can be entered because of an exception has its own R13 and R14. These registers generally contain the stack pointer and the return address from function calls, respectively."
+1620,Aliases:
+1621,The Current Program Status Register  has the following 32 bits.
+1622,"Almost every ARM instruction has a conditional execution feature called predication, which is implemented with a 4-bit condition code selector . To allow for unconditional execution, one of the four-bit codes causes the instruction to be always executed. Most other CPU architectures only have condition codes on branch instructions."
+1623,"Though the predicate takes up four of the 32 bits in an instruction code, and thus cuts down significantly on the encoding bits available for displacements in memory access instructions, it avoids branch instructions when generating code for small if statements. Apart from eliminating the branch instructions themselves, this preserves the fetch/decode/execute pipeline at the cost of only one cycle per skipped instruction."
+1624,"An algorithm that provides a good example of conditional execution is the subtraction-based Euclidean algorithm for computing the greatest common divisor.  In the C programming language, the algorithm can be written as:"
+1625,The same algorithm can be rewritten in a way closer to target ARM instructions as:
+1626,and coded in assembly language as:
+1627,"which avoids the branches around the then and else clauses. If r0 and r1 are equal then neither of the SUB instructions will be executed, eliminating the need for a conditional branch to implement the while check at the top of the loop, for example had SUBLE  been used."
+1628,One of the ways that Thumb code provides a more dense encoding is to remove the four-bit selector from non-branch instructions.
+1629,"Another feature of the instruction set is the ability to fold shifts and rotates into the data processing  instructions, so that, for example, the statement in C language:"
+1630,"could be rendered as a one-word, one-cycle instruction:"
+1631,This results in the typical ARM program being denser than expected with fewer memory accesses; thus the pipeline is used more efficiently.
+1632,"The ARM processor also has features rarely seen in other RISC architectures, such as PC-relative addressing  and pre- and post-increment addressing modes."
+1633,"The ARM instruction set has increased over time. Some early ARM processors , for example, have no instruction to store a two-byte quantity."
+1634,"The ARM7 and earlier implementations have a three-stage pipeline; the stages being fetch, decode, and execute. Higher-performance designs, such as the ARM9, have deeper pipelines: Cortex-A8 has thirteen stages. Additional implementation changes for higher performance include a faster adder and more extensive branch prediction logic. The difference between the ARM7DI and ARM7DMI cores, for example, was an improved multiplier; hence the added ""M""."
+1635,"The ARM architecture  provides a non-intrusive way of extending the instruction set using ""coprocessors"" that can be addressed using MCR, MRC, MRRC, MCRR, and similar instructions. The coprocessor space is divided logically into 16 coprocessors with numbers from 0 to 15, coprocessor 15  being reserved for some typical control functions like managing the caches and MMU operation on processors that have one."
+1636,"In ARM-based machines, peripheral devices are usually attached to the processor by mapping their physical registers into ARM memory space, into the coprocessor space, or by connecting to another device  that in turn attaches to the processor. Coprocessor accesses have lower latency, so some peripherals—for example, an XScale interrupt controller—are accessible in both ways: through memory and through coprocessors."
+1637,"In other cases, chip designers only integrate hardware using the coprocessor mechanism. For example, an image processing engine might be a small ARM7TDMI core combined with a coprocessor that has specialised operations to support a specific set of HDTV transcoding primitives."
+1638,"All modern ARM processors include hardware debugging facilities, allowing software debuggers to perform operations such as halting, stepping, and breakpointing of code starting from reset. These facilities are built using JTAG support, though some newer cores optionally support ARM's own two-wire ""SWD"" protocol. In ARM7TDMI cores, the ""D"" represented JTAG debug support, and the ""I"" represented presence of an ""EmbeddedICE"" debug module. For ARM7 and ARM9 core generations, EmbeddedICE over JTAG was a de facto debug standard, though not architecturally guaranteed."
+1639,"The ARMv7 architecture defines basic debug facilities at an architectural level. These include breakpoints, watchpoints and instruction execution in a ""Debug Mode""; similar facilities were also available with EmbeddedICE. Both ""halt mode"" and ""monitor"" mode debugging are supported. The actual transport mechanism used to access the debug facilities is not architecturally specified, but implementations generally include JTAG support."
+1640,"There is a separate ARM ""CoreSight"" debug architecture, which is not architecturally required by ARMv7 processors."
+1641,"The Debug Access Port  is an implementation of an ARM Debug Interface.
+There are two different supported implementations, the Serial Wire JTAG Debug Port  and the Serial Wire Debug Port .
+CMSIS-DAP is a standard interface that describes how various debugging software on a host PC can communicate over USB to firmware running on a hardware debugger, which in turn talks over SWD or JTAG to a CoreSight-enabled ARM Cortex CPU."
+1642,"To improve the ARM architecture for digital signal processing and multimedia applications, DSP instructions were added to the instruction set. These are signified by an ""E"" in the name of the ARMv5TE and ARMv5TEJ architectures. E-variants also imply T, D, M, and I."
+1643,"The new instructions are common in digital signal processor  architectures. They include variations on signed multiply–accumulate, saturated add and subtract, and count leading zeros."
+1644,"First introduced in 1999, this extension of the core instruction set contrasted with ARM's earlier DSP coprocessor known as Piccolo, which employed a distinct, incompatible instruction set whose execution involved a separate program counter. Piccolo instructions employed a distinct register file of sixteen 32-bit registers, with some instructions combining registers for use as 48-bit accumulators and other instructions addressing 16-bit half-registers. Some instructions were able to operate on two such 16-bit values in parallel. Communication with the Piccolo register file involved load to Piccolo and store from Piccolo coprocessor instructions via two buffers of eight 32-bit entries. Described as reminiscent of other approaches, notably Hitachi's SH-DSP and Motorola's 68356, Piccolo did not employ dedicated local memory and relied on the bandwidth of the ARM core for DSP operand retrieval, impacting concurrent performance. Piccolo's distinct instruction set also proved not to be a ""good compiler target""."
+1645,"Introduced in the ARMv6 architecture, this was a precursor to Advanced SIMD, also named Neon."
+1646,"Jazelle DBX  is a technique that allows Java bytecode to be executed directly in the ARM architecture as a third execution state  alongside the existing ARM and Thumb-mode. Support for this state is signified by the ""J"" in the ARMv5TEJ architecture, and in ARM9EJ-S and ARM7EJ-S core names. Support for this state is required starting in ARMv6 , though newer cores only include a trivial implementation that provides no hardware acceleration."
+1647,"To improve compiled code density, processors since the ARM7TDMI  have featured the Thumb compressed instruction set, which have their own state.  When in this state, the processor executes the Thumb instruction set, a compact 16-bit encoding for a subset of the ARM instruction set. Most of the Thumb instructions are directly mapped to normal ARM instructions. The space saving comes from making some of the instruction operands implicit and limiting the number of possibilities compared to the ARM instructions executed in the ARM instruction set state."
+1648,"In Thumb, the 16-bit opcodes have less functionality. For example, only branches can be conditional, and many opcodes are restricted to accessing only half of all of the CPU's general-purpose registers. The shorter opcodes give improved code density overall, even though some operations require extra instructions. In situations where the memory port or bus width is constrained to less than 32 bits, the shorter Thumb opcodes allow increased performance compared with 32-bit ARM code, as less program code may need to be loaded into the processor over the constrained memory bandwidth."
+1649,"Unlike processor architectures with variable length  instructions, such as the Cray-1 and Hitachi SuperH, the ARM and Thumb instruction sets exist independently of each other. Embedded hardware, such as the Game Boy Advance, typically have a small amount of RAM accessible with a full 32-bit datapath; the majority is accessed via a 16-bit or narrower secondary datapath. In this situation, it usually makes sense to compile Thumb code and hand-optimise a few of the most CPU-intensive sections using full 32-bit ARM instructions, placing these wider instructions into the 32-bit bus accessible memory."
+1650,"The first processor with a Thumb instruction decoder was the ARM7TDMI. All processors supporting 32-bit instruction sets, starting with ARM9, and including XScale, have included a Thumb instruction decoder. It includes instructions adopted from the Hitachi SuperH , which was licensed by ARM. ARM's smallest processor families  implement only the 16-bit Thumb instruction set for maximum performance in lowest cost applications. ARM processors that don't support 32-bit addressing also omit Thumb."
+1651,"Thumb-2 technology was introduced in the ARM1156 core, announced in 2003. Thumb-2 extends the limited 16-bit instruction set of Thumb with additional 32-bit instructions to give the instruction set more breadth, thus producing a variable-length instruction set. A stated aim for Thumb-2 was to achieve code density similar to Thumb with performance similar to the ARM instruction set on 32-bit memory."
+1652,"Thumb-2 extends the Thumb instruction set with bit-field manipulation, table branches and conditional execution. At the same time, the ARM instruction set was extended to maintain equivalent functionality in both instruction sets. A new ""Unified Assembly Language""  supports generation of either Thumb or ARM instructions from the same source code; versions of Thumb seen on ARMv7 processors are essentially as capable as ARM code . This requires a bit of care, and use of a new ""IT""  instruction, which permits up to four successive instructions to execute based on a tested condition, or on its inverse. When compiling into ARM code, this is ignored, but when compiling into Thumb it generates an actual instruction. For example:"
+1653,"All ARMv7 chips support the Thumb instruction set. All chips in the Cortex-A series that support ARMv7, all Cortex-R series, and all ARM11 series support both ""ARM instruction set state"" and ""Thumb instruction set state"", while chips in the Cortex-M series support only the Thumb instruction set."
+1654,"ThumbEE , which was marketed as Jazelle RCT , was announced in 2005 and deprecated in 2011. It first appeared in the Cortex-A8 processor. ThumbEE is a fourth instruction set state, making small changes to the Thumb-2 extended instruction set. These changes make the instruction set particularly suited to code generated at runtime  in managed Execution Environments. ThumbEE is a target for languages such as Java, C#, Perl, and Python, and allows JIT compilers to output smaller compiled code without reducing performance."
+1655,"New features provided by ThumbEE include automatic null pointer checks on every load and store instruction, an instruction to perform an array bounds check, and special instructions that call a handler. In addition, because it utilises Thumb-2 technology, ThumbEE provides access to registers r8–r15 . Handlers are small sections of frequently called code, commonly used to implement high level languages, such as allocating memory for a new object. These changes come from repurposing a handful of opcodes, and knowing the core is in the new ThumbEE state."
+1656,"On 23 November 2011, Arm deprecated any use of the ThumbEE instruction set, and Armv8 removes support for ThumbEE."
+1657,"VFP  technology is a floating-point unit  coprocessor extension to the ARM architecture . It provides low-cost single-precision and double-precision floating-point computation fully compliant with the ANSI/IEEE Std 754-1985 Standard for Binary Floating-Point Arithmetic. VFP provides floating-point computation suitable for a wide spectrum of applications such as PDAs, smartphones, voice compression and decompression, three-dimensional graphics and digital audio, printers, set-top boxes, and automotive applications. The VFP architecture was intended to support execution of short ""vector mode"" instructions but these operated on each vector element sequentially and thus did not offer the performance of true single instruction, multiple data  vector parallelism. This vector mode was therefore removed shortly after its introduction, to be replaced with the much more powerful Advanced SIMD, also named Neon."
+1658,"Some devices such as the ARM Cortex-A8 have a cut-down VFPLite module instead of a full VFP module, and require roughly ten times more clock cycles per float operation. Pre-Armv8 architecture implemented floating-point/SIMD with the coprocessor interface. Other floating-point and/or SIMD units found in ARM-based processors using the coprocessor interface include FPA, FPE, iwMMXt, some of which were implemented in software by trapping but could have been implemented in hardware. They provide some of the same functionality as VFP but are not opcode-compatible with it. FPA10 also provides extended precision, but implements correct rounding  only in single precision."
+1659,"In Debian Linux and derivatives such as Ubuntu and Linux Mint, armhf  refers to the ARMv7 architecture including the additional VFP3-D16 floating-point hardware extension  above. Software packages and cross-compiler tools use the armhf vs. arm/armel suffixes to differentiate."
+1660,"The Advanced SIMD extension  is a combined 64- and 128-bit SIMD instruction set that provides standardised acceleration for media and signal processing applications. Neon is included in all Cortex-A8 devices, but is optional in Cortex-A9 devices. Neon can execute MP3 audio decoding on CPUs running at 10 MHz, and can run the GSM adaptive multi-rate  speech codec at 13 MHz. It features a comprehensive instruction set, separate register files, and independent execution hardware. Neon supports 8-, 16-, 32-, and 64-bit integer and single-precision  floating-point data and SIMD operations for handling audio and video processing as well as graphics and gaming processing. In Neon, the SIMD supports up to 16 operations at the same time. The Neon hardware shares the same floating-point registers as used in VFP. Devices such as the ARM Cortex-A8 and Cortex-A9 support 128-bit vectors, but will execute with 64 bits at a time, whereas newer Cortex-A15 devices can execute 128 bits at a time."
+1661,"A quirk of Neon in Armv7 devices is that it flushes all subnormal numbers to zero, and as a result the GCC compiler will not use it unless -funsafe-math-optimizations, which allows losing denormals, is turned on. ""Enhanced"" Neon defined since Armv8 does not have this quirk, but as of GCC 8.2 the same flag is still required to enable Neon instructions. On the other hand, GCC does consider Neon safe on AArch64 for Armv8."
+1662,"ProjectNe10 is ARM's first open-source project . The Ne10 library is a set of common, useful functions written in both Neon and C . The library was created to allow developers to use Neon optimisations without learning Neon, but it also serves as a set of highly optimised Neon intrinsic and assembly code examples for common DSP, arithmetic, and image processing routines. The source code is available on GitHub."
+1663,Helium is the M-Profile Vector Extension . It adds more than 150 scalar and vector instructions.
+1664,"The Security Extensions, marketed as TrustZone Technology, is in ARMv6KZ and later application profile architectures. It provides a low-cost alternative to adding another dedicated security core to an SoC, by providing two virtual processors backed by hardware based access control. This lets the application core switch between two states, referred to as worlds , to prevent information leaking from the more trusted world to the less trusted world. This world switch is generally orthogonal to all other capabilities of the processor, thus each world can operate independently of the other while using the same core. Memory and peripherals are then made aware of the operating world of the core and may use this to provide access control to secrets and code on the device."
+1665,"Typically, a rich operating system is run in the less trusted world, with smaller security-specialised code in the more trusted world, aiming to reduce the attack surface.  Typical applications include DRM functionality for controlling the use of media on ARM-based devices, and preventing any unapproved use of the device."
+1666,"In practice, since the specific implementation details of proprietary TrustZone implementations have not been publicly disclosed for review, it is unclear what level of assurance is provided for a given threat model, but they are not immune from attack."
+1667,Open Virtualization is an open source implementation of the trusted world architecture for TrustZone.
+1668,"AMD has licensed and incorporated TrustZone technology into its Secure Processor Technology. Enabled in some but not all products, AMD's APUs include a Cortex-A5 processor for handling secure processing. In fact, the Cortex-A5 TrustZone core had been included in earlier AMD products, but was not enabled due to time constraints."
+1669,"Samsung Knox uses TrustZone for purposes such as detecting modifications to the kernel, storing certificates and attestating keys."
+1670,"The Security Extension, marketed as TrustZone for Armv8-M Technology, was introduced in the Armv8-M architecture. While containing similar concepts to TrustZone for Armv8-A, it has a different architectural design, as world switching is performed using branch instructions instead of using exceptions. It also supports safe interleaved interrupt handling from either world regardless of the current security state. Together these features provide low latency calls to the secure world and responsive interrupt handling. ARM provides a reference stack of secure world code in the form of Trusted Firmware for M and PSA Certified."
+1671,"As of ARMv6, the ARM architecture supports no-execute page protection, which is referred to as XN, for eXecute Never."
+1672,"The Large Physical Address Extension , which extends the physical address size from 32 bits to 40 bits, was added to the Armv7-A architecture in 2011."
+1673,"The physical address size may be even larger in processors based on the 64-bit  architecture. For example, it is 44 bits in Cortex-A75 and Cortex-A65AE."
+1674,"The Armv8-R and Armv8-M architectures, announced after the Armv8-A architecture, share some features with Armv8-A. However, Armv8-M does not include any 64-bit AArch64 instructions, and Armv8-R originally did not include any AArch64 instructions; those instructions were added to Armv8-R later."
+1675,"The Armv8.1-M architecture, announced in February 2019, is an enhancement of the Armv8-M architecture. It brings new features including:"
+1676,"Announced in October 2011, Armv8-A  represents a fundamental change to the ARM architecture. It adds an optional 64-bit architecture named ""AArch64"" and the associated new ""A64"" instruction set. AArch64 provides user-space compatibility with Armv7-A, the 32-bit architecture, therein referred to as ""AArch32"" and the old 32-bit instruction set, now named ""A32"". The Thumb instruction set is referred to as ""T32"" and has no 64-bit counterpart. Armv8-A allows 32-bit applications to be executed in a 64-bit OS, and a 32-bit OS to be under the control of a 64-bit hypervisor. ARM announced their Cortex-A53 and Cortex-A57 cores on 30 October 2012. Apple was the first to release an Armv8-A compatible core in a consumer product  . AppliedMicro, using an FPGA, was the first to demo Armv8-A. The first Armv8-A SoC from Samsung is the Exynos 5433 used in the Galaxy Note 4, which features two clusters of four Cortex-A57 and Cortex-A53 cores in a big.LITTLE configuration; but it will run only in AArch32 mode."
+1677,"To both AArch32 and AArch64, Armv8-A makes VFPv3/v4 and advanced SIMD  standard. It also adds cryptography instructions supporting AES, SHA-1/SHA-256 and finite field arithmetic. AArch64 was introduced in Armv8-A and its subsequent revision.  AArch64 is not included in the 32-bit Armv8-R and Armv8-M architectures."
+1678,"Optional AArch64 support was added to the Armv8-R profile, with the first ARM core implementing it being the Cortex-R82. It adds the A64 instruction set."
+1679,"Announced in March 2021, the updated architecture places a focus on secure execution and compartmentalisation."
+1680,"Arm SystemReady, formerly named Arm ServerReady, is a certification program that helps land the generic off-the-shelf operating systems and hypervisors on to the Arm-based systems from datacenter servers to industrial edge and IoT devices. The key building blocks of the program are the specifications for minimum hardware and firmware requirements that the operating systems and hypervisors can rely upon. These specifications are:"
+1681,These specifications are co-developed by Arm and its partners in the System Architecture Advisory Committee .
+1682,Architecture Compliance Suite  is the test tools that help to check the compliance of these specifications. The Arm SystemReady Requirements Specification documents the requirements of the certifications.
+1683,This program was introduced by Arm in 2020 at the first DevSummit event. Its predecessor Arm ServerReady was introduced in 2018 at the Arm TechCon event. This program currently includes four bands:
+1684,"PSA Certified, formerly named Platform Security Architecture, is an architecture-agnostic security framework and evaluation scheme. It is intended to help secure Internet of Things  devices built on system-on-a-chip  processors. It was introduced to increase security where a full trusted execution environment is too large or complex."
+1685,"The architecture was introduced by Arm in 2017 at the annual TechCon event. Although the scheme is architecture agnostic, it was first implemented on Arm Cortex-M processor cores intended for microcontroller use. PSA Certified includes freely available threat models and security analyses that demonstrate the process for deciding on security features in common IoT products. It also provides freely downloadable application programming interface  packages, architectural specifications, open-source firmware implementations, and related test suites."
+1686,"Following the development of the architecture security framework in 2017, the PSA Certified assurance scheme launched two years later at Embedded World in 2019. PSA Certified offers a multi-level security evaluation scheme for chip vendors, OS providers and IoT device makers. The Embedded World presentation introduced chip vendors to Level 1 Certification. A draft of Level 2 protection was presented at the same time. Level 2 certification became a usable standard in February 2020."
+1687,"The certification was created by PSA Joint Stakeholders to enable a security-by-design approach for a diverse set of IoT products. PSA Certified specifications are implementation and architecture agnostic, as a result they can be applied to any chip, software or device. The certification also removes industry fragmentation for IoT product manufacturers and developers."
+1688,"The first 32-bit ARM-based personal computer, the Acorn Archimedes, was originally intended to run an ambitious operating system called ARX. The machines shipped with RISC OS which was also used on later ARM-based systems from Acorn and other vendors. Some early Acorn machines were also able to run a Unix port called RISC iX. "
+1689,"The 32-bit ARM architecture is supported by a large number of embedded and real-time operating systems, including:"
+1690,"As of March 2024, the 32-bit ARM architecture used to be the primary hardware environment for most mobile device operating systems such as the following but many of these platforms such as Android and Apple iOS have evolved to the 64-bit ARM architecture:"
+1691,"Formerly, but now discontinued:"
+1692,The 32-bit ARM architecture is supported by RISC OS and by multiple Unix-like operating systems including:
+1693,"Windows applications recompiled for ARM and linked with Winelib, from the Wine project, can run on 32-bit or 64-bit ARM in Linux, FreeBSD, or other compatible operating systems. x86 binaries, e.g. when not specially compiled for ARM, have been demonstrated on ARM using QEMU with Wine , but do not work at full speed or same capability as with Winelib."
+1694,"The form, design, and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of a CPU include the arithmetic–logic unit  that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching , decoding and execution  by directing the coordinated operations of the ALU, registers, and other components. Modern CPUs devote a lot of semiconductor area to caches and instruction-level parallelism to increase performance and to CPU modes to support operating systems and virtualization."
+1695,"Most modern CPUs are implemented on integrated circuit  microprocessors, with one or more CPUs on a single IC chip. Microprocessor chips with multiple CPUs are called multi-core processors. The individual physical CPUs, called processor cores, can also be multithreaded to support CPU-level multithreading."
+1696,"An IC that contains a CPU may also contain memory, peripheral interfaces, and other components of a computer; such integrated devices are variously called microcontrollers or systems on a chip ."
+1697,"Early computers such as the ENIAC had to be physically rewired to perform different tasks, which caused these machines to be called ""fixed-program computers"". The ""central processing unit"" term has been in use since as early as 1955. Since the term ""CPU"" is generally defined as a device for software  execution, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer."
+1698,"The idea of a stored-program computer had been already present in the design of J. Presper Eckert and John William Mauchly's ENIAC, but was initially omitted so that ENIAC could be finished sooner. On June 30, 1945, before ENIAC was made, mathematician John von Neumann distributed a paper entitled First Draft of a Report on the EDVAC. It was the outline of a stored-program computer that would eventually be completed in August 1949. EDVAC was designed to perform a certain number of instructions  of various types. Significantly, the programs written for EDVAC were to be stored in high-speed computer memory rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the considerable time and effort required to reconfigure the computer to perform a new task. With von Neumann's design, the program that EDVAC ran could be changed simply by changing the contents of the memory. EDVAC was not the first stored-program computer; the Manchester Baby, which was a small-scale experimental stored-program computer, ran its first program on 21 June 1948 and the Manchester Mark 1 ran its first program during the night of 16–17 June 1949."
+1699,"Early CPUs were custom designs used as part of a larger and sometimes distinctive computer. However, this method of designing custom CPUs for a particular application has largely given way to the development of multi-purpose processors produced in large quantities. This standardization began in the era of discrete transistor mainframes and minicomputers, and has rapidly accelerated with the popularization of the integrated circuit . The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers. Both the miniaturization and standardization of CPUs have increased the presence of digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in electronic devices ranging from automobiles to cellphones, and sometimes even in toys."
+1700,"While von Neumann is most often credited with the design of the stored-program computer because of his design of EDVAC, and the design became known as the von Neumann architecture, others before him, such as Konrad Zuse, had suggested and implemented similar ideas. The so-called Harvard architecture of the Harvard Mark I, which was completed before EDVAC, also used a stored-program design using punched paper tape rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but CPUs with the Harvard architecture are seen as well, especially in embedded applications; for instance, the Atmel AVR microcontrollers are Harvard-architecture processors."
+1701,"Relays and vacuum tubes  were commonly used as switching elements; a useful computer requires thousands or tens of thousands of switching devices. The overall speed of a system is dependent on the speed of the switches. Vacuum-tube computers such as EDVAC tended to average eight hours between failures, whereas relay computers—such as the slower but earlier Harvard Mark I—failed very rarely. In the end, tube-based CPUs became dominant because the significant speed advantages afforded generally outweighed the reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs. Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with."
+1702,"The design complexity of CPUs increased as various technologies facilitated the building of smaller and more reliable electronic devices. The first such improvement came with the advent of the transistor. Transistorized CPUs during the 1950s and 1960s no longer had to be built out of bulky, unreliable, and fragile switching elements, like vacuum tubes and relays. With this improvement, more complex and reliable CPUs were built onto one or several printed circuit boards containing discrete  components."
+1703,"In 1964, IBM introduced its IBM System/360 computer architecture that was used in a series of computers capable of running the same programs with different speeds and performances. This was significant at a time when most electronic computers were incompatible with one another, even those made by the same manufacturer. To facilitate this improvement, IBM used the concept of a microprogram , which still sees widespread use in modern CPUs. The System/360 architecture was so popular that it dominated the mainframe computer market for decades and left a legacy that is continued by similar modern computers like the IBM zSeries. In 1965, Digital Equipment Corporation  introduced another influential computer aimed at the scientific and research markets—the PDP-8."
+1704,"Transistor-based computers had several distinct advantages over their predecessors. Aside from facilitating increased reliability and lower power consumption, transistors also allowed CPUs to operate at much higher speeds because of the short switching time of a transistor in comparison to a tube or relay. The increased reliability and dramatically increased speed of the switching elements, which were almost exclusively transistors by this time; CPU clock rates in the tens of megahertz were easily obtained during this period. Additionally, while discrete transistor and IC CPUs were in heavy usage, new high-performance designs like single instruction, multiple data  vector processors began to appear. These early experimental designs later gave rise to the era of specialized supercomputers like those made by Cray Inc and Fujitsu Ltd."
+1705,"During this period, a method of manufacturing many interconnected transistors in a compact space was developed. The integrated circuit  allowed a large number of transistors to be manufactured on a single semiconductor-based die, or ""chip"". At first, only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs. CPUs based on these ""building block"" ICs are generally referred to as ""small-scale integration""  devices. SSI ICs, such as the ones used in the Apollo Guidance Computer, usually contained up to a few dozen transistors. To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs."
+1706,"IBM's System/370, follow-on to the System/360, used SSI ICs rather than Solid Logic Technology discrete-transistor modules. DEC's PDP-8/I and KI10 PDP-10 also switched from the individual transistors used by the PDP-8 and PDP-10 to SSI ICs, and their extremely popular PDP-11 line was originally built with SSI ICs, but was eventually implemented with LSI components once these became practical."
+1707,"Lee Boysel published influential articles, including a 1967 ""manifesto"", which described how to build the equivalent of a 32-bit mainframe computer from a relatively small number of large-scale integration circuits . The only way to build LSI chips, which are chips with a hundred or more gates, was to build them using a metal–oxide–semiconductor  semiconductor manufacturing process . However, some companies continued to build processors out of bipolar transistor–transistor logic  chips because bipolar junction transistors were faster than MOS chips up until the 1970s . In the 1960s, MOS ICs were slower and initially considered useful only in applications that required low power. Following the development of silicon-gate MOS technology by Federico Faggin at Fairchild Semiconductor in 1968, MOS ICs largely replaced bipolar TTL as the standard chip technology in the early 1970s."
+1708,"As the microelectronic technology advanced, an increasing number of transistors were placed on ICs, decreasing the number of individual ICs needed for a complete CPU. MSI and LSI ICs increased transistor counts to hundreds, and then thousands. By 1968, the number of ICs required to build a complete CPU had been reduced to 24 ICs of eight different types, with each IC containing roughly 1000 MOSFETs. In stark contrast with its SSI and MSI predecessors, the first LSI implementation of the PDP-11 contained a CPU composed of only four LSI integrated circuits."
+1709,"Since microprocessors were first introduced they have almost completely overtaken all other central processing unit implementation methods. The first commercially available microprocessor, made in 1971, was the Intel 4004, and the first widely used microprocessor, made in 1974, was the Intel 8080. Mainframe and minicomputer manufacturers of the time launched proprietary IC development programs to upgrade their older computer architectures, and eventually produced instruction set compatible microprocessors that were backward-compatible with their older hardware and software. Combined with the advent and eventual success of the ubiquitous personal computer, the term CPU is now applied almost exclusively to microprocessors. Several CPUs  can be combined in a single processing chip."
+1710,"Previous generations of CPUs were implemented as discrete components and numerous small integrated circuits  on one or more circuit boards. Microprocessors, on the other hand, are CPUs manufactured on a very small number of ICs; usually just one. The overall smaller CPU size, as a result of being implemented on a single die, means faster switching time because of physical factors like decreased gate parasitic capacitance. This has allowed synchronous microprocessors to have clock rates ranging from tens of megahertz to several gigahertz. Additionally, the ability to construct exceedingly small transistors on an IC has increased the complexity and number of transistors in a single CPU many fold. This widely observed trend is described by Moore's law, which had proven to be a fairly accurate predictor of the growth of CPU  complexity until 2016."
+1711,"While the complexity, size, construction and general form of CPUs have changed enormously since 1950, the basic design and function has not changed much at all. Almost all common CPUs today can be very accurately described as von Neumann stored-program machines. As Moore's law no longer holds, concerns have arisen about the limits of integrated circuit transistor technology. Extreme miniaturization of electronic gates is causing the effects of phenomena like electromigration and subthreshold leakage to become much more significant. These newer concerns are among the many factors causing researchers to investigate new methods of computing such as the quantum computer, as well as to expand the use of parallelism and other methods that extend the usefulness of the classical von Neumann model."
+1712,"The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions that is called a program. The instructions to be executed are kept in some kind of computer memory. Nearly all CPUs follow the fetch, decode and execute steps in their operation, which are collectively known as the instruction cycle."
+1713,"After the execution of an instruction, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter. If a jump instruction was executed, the program counter will be modified to contain the address of the instruction that was jumped to and program execution continues normally. In more complex CPUs, multiple instructions can be fetched, decoded and executed simultaneously. This section describes what is generally referred to as the ""classic RISC pipeline"", which is quite common among the simple CPUs used in many electronic devices . It largely ignores the important role of CPU cache, and therefore the access stage of the pipeline."
+1714,"Some instructions manipulate the program counter rather than producing result data directly; such instructions are generally called ""jumps"" and facilitate program behavior like loops, conditional program execution , and existence of functions. In some processors, some other instructions change the state of bits in a ""flags"" register. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, in such processors a ""compare"" instruction evaluates two values and sets or clears bits in the flags register to indicate which one is greater or whether they are equal; one of these flags could then be used by a later jump instruction to determine program flow."
+1715,"Fetch involves retrieving an instruction  from program memory. The instruction's location  in program memory is determined by the program counter , which stores a number that identifies the address of the next instruction to be fetched. After an instruction is fetched, the PC is incremented by the length of the instruction so that it will contain the address of the next instruction in the sequence. Often, the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned. This issue is largely addressed in modern processors by caches and pipeline architectures ."
+1716,"The instruction that the CPU fetches from memory determines what the CPU will do. In the decode step, performed by binary decoder circuitry known as the instruction decoder, the instruction is converted into signals that control other parts of the CPU."
+1717,"The way in which the instruction is interpreted is defined by the CPU's instruction set architecture . Often, one group of bits  within the instruction, called the opcode, indicates which operation is to be performed, while the remaining fields usually provide supplemental information required for the operation, such as the operands. Those operands may be specified as a constant value , or as the location of a value that may be a processor register or a memory address, as determined by some addressing mode."
+1718,"In some CPU designs the instruction decoder is implemented as a hardwired, unchangeable binary decoder circuit. In others, a microprogram is used to translate instructions into sets of CPU configuration signals that are applied sequentially over multiple clock pulses. In some cases the memory that stores the microprogram is rewritable, making it possible to change the way in which the CPU decodes instructions."
+1719,"After the fetch and decode steps, the execute step is performed. Depending on the CPU architecture, this may consist of a single action or a sequence of actions. During each action, control signals electrically enable or disable various parts of the CPU so they can perform all or part of the desired operation. The action is then completed, typically in response to a clock pulse. Very often the results are written to an internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but less expensive and higher capacity main memory."
+1720,"For example, if an instruction that performs addition is to be executed, registers containing operands  are activated, as are the parts of the arithmetic logic unit  that perform addition. When the clock pulse occurs, the operands flow from the source registers into the ALU, and the sum appears at its output. On subsequent clock pulses, other components are enabled  to move the output  to storage . If the resulting sum is too large , an arithmetic overflow flag will be set, influencing the next operation."
+1721,"Hardwired into a CPU's circuitry is a set of basic operations it can perform, called an instruction set. Such operations may involve, for example, adding or subtracting two numbers, comparing two numbers, or jumping to a different part of a program. Each instruction is represented by a unique combination of bits, known as the machine language opcode. While processing an instruction, the CPU decodes the opcode  into control signals, which orchestrate the behavior of the CPU. A complete machine language instruction consists of an opcode and, in many cases, additional bits that specify arguments for the operation . Going up the complexity scale, a machine language program is a collection of machine language instructions that the CPU executes."
+1722,"The actual mathematical operation for each instruction is performed by a combinational logic circuit within the CPU's processor known as the arithmetic–logic unit or ALU. In general, a CPU executes an instruction by fetching it from memory, using its ALU to perform an operation, and then storing the result to memory. Besides the instructions for integer mathematics and logic operations, various other machine instructions exist, such as those for loading data from memory and storing it back, branching operations, and mathematical operations on floating-point numbers performed by the CPU's floating-point unit ."
+1723,"The control unit  is a component of the CPU that directs the operation of the processor. It tells the computer's memory, arithmetic and logic unit and input and output devices how to respond to the instructions that have been sent to the processor."
+1724,"It directs the operation of the other units by providing timing and control signals. Most computer resources are managed by the CU. It directs the flow of data between the CPU and the other devices. John von Neumann included the control unit as part of the von Neumann architecture. In modern computer designs, the control unit is typically an internal part of the CPU with its overall role and operation unchanged since its introduction."
+1725,"The arithmetic logic unit  is a digital circuit within the processor that performs integer arithmetic and bitwise logic operations. The inputs to the ALU are the data words to be operated on , status information from previous operations, and a code from the control unit indicating which operation to perform. Depending on the instruction being executed, the operands may come from internal CPU registers, external memory, or constants generated by the ALU itself."
+1726,"When all input signals have settled and propagated through the ALU circuitry, the result of the performed operation appears at the ALU's outputs. The result consists of both a data word, which may be stored in a register or memory, and status information that is typically stored in a special, internal CPU register reserved for this purpose."
+1727,Modern CPUs typically contain more than one ALU to improve performance.
+1728,"The address generation unit , sometimes also called the address computation unit , is an execution unit inside the CPU that calculates addresses used by the CPU to access main memory. By having address calculations handled by separate circuitry that operates in parallel with the rest of the CPU, the number of CPU cycles required for executing various machine instructions can be reduced, bringing performance improvements."
+1729,"While performing various operations, CPUs need to calculate memory addresses required for fetching data from the memory; for example, in-memory positions of array elements must be calculated before the CPU can fetch the data from actual memory locations. Those address-generation calculations involve different integer arithmetic operations, such as addition, subtraction, modulo operations, or bit shifts. Often, calculating a memory address involves more than one general-purpose machine instruction, which do not necessarily decode and execute quickly. By incorporating an AGU into a CPU design, together with introducing specialized instructions that use the AGU, various address-generation calculations can be offloaded from the rest of the CPU, and can often be executed quickly in a single CPU cycle."
+1730,"Capabilities of an AGU depend on a particular CPU and its architecture. Thus, some AGUs implement and expose more address-calculation operations, while some also include more advanced specialized instructions that can operate on multiple operands at a time. Some CPU architectures include multiple AGUs so more than one address-calculation operation can be executed simultaneously, which brings further performance improvements due to the superscalar nature of advanced CPU designs. For example, Intel incorporates multiple AGUs into its Sandy Bridge and Haswell microarchitectures, which increase bandwidth of the CPU memory subsystem by allowing multiple memory-access instructions to be executed in parallel."
+1731,"Many microprocessors  have a memory management unit, translating logical addresses into physical RAM addresses, providing memory protection and paging abilities, useful for virtual memory. Simpler processors, especially microcontrollers, usually don't include an MMU."
+1732,"A CPU cache is a hardware cache used by the central processing unit  of a computer to reduce the average cost  to access data from the main memory. A cache is a smaller, faster memory, closer to a processor core, which stores copies of the data from frequently used main memory locations. Most CPUs have different independent caches, including instruction and data caches, where the data cache is usually organized as a hierarchy of more cache levels ."
+1733,"All modern  CPUs  have multiple levels of CPU caches. The first CPUs that used a cache had only one level of cache; unlike later level 1 caches, it was not split into L1d  and L1i . Almost all current CPUs with caches have a split L1 cache. They also have L2 caches and, for larger processors, L3 caches as well. The L2 cache is usually not split and acts as a common repository for the already split L1 cache. Every core of a multi-core processor has a dedicated L2 cache and is usually not shared between the cores. The L3 cache, and higher-level caches, are shared between the cores and are not split. An L4 cache is currently uncommon, and is generally on dynamic random-access memory , rather than on static random-access memory , on a separate die or chip. That was also the case historically with L1, while bigger chips have allowed integration of it and generally all cache levels, with the possible exception of the last level. Each extra level of cache tends to be bigger and is optimized differently."
+1734,"Other types of caches exist , such as the translation lookaside buffer  that is part of the memory management unit  that most CPUs have."
+1735,"Caches are generally sized in powers of two: 2, 8, 16 etc. KiB or MiB  sizes, although the IBM z13 has a 96 KiB L1 instruction cache."
+1736,"Most CPUs are synchronous circuits, which means they employ a clock signal to pace their sequential operations. The clock signal is produced by an external oscillator circuit that generates a consistent number of pulses each second in the form of a periodic square wave. The frequency of the clock pulses determines the rate at which a CPU executes instructions and, consequently, the faster the clock, the more instructions the CPU will execute each second."
+1737,"To ensure proper operation of the CPU, the clock period is longer than the maximum time needed for all signals to propagate  through the CPU. In setting the clock period to a value well above the worst-case propagation delay, it is possible to design the entire CPU and the way it moves data around the ""edges"" of the rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster. This limitation has largely been compensated for by various methods of increasing CPU parallelism ."
+1738,"However, architectural improvements alone do not solve all of the drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal. Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase  throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided to avoid delaying a single signal significantly enough to cause the CPU to malfunction. Another major issue, as clock rates increase dramatically, is the amount of heat that is dissipated by the CPU. The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does energy consumption, causing the CPU to require more heat dissipation in the form of CPU cooling solutions."
+1739,"One method of dealing with the switching of unneeded components is called clock gating, which involves turning off the clock signal to unneeded components . However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs. One notable recent CPU design that uses extensive clock gating is the IBM PowerPC-based Xenon used in the Xbox 360; this reduces the power requirements of the Xbox 360."
+1740,"Another method of addressing some of the problems with a global clock signal is the removal of the clock signal altogether. While removing the global clock signal makes the design process considerably more complex in many ways, asynchronous  designs carry marked advantages in power consumption and heat dissipation in comparison with similar synchronous designs. While somewhat uncommon, entire asynchronous CPUs have been built without using a global clock signal. Two notable examples of this are the ARM compliant AMULET and the MIPS R3000 compatible MiniMIPS."
+1741,"Rather than totally removing the clock signal, some CPU designs allow certain portions of the device to be asynchronous, such as using asynchronous ALUs in conjunction with superscalar pipelining to achieve some arithmetic performance gains. While it is not altogether clear whether totally asynchronous designs can perform at a comparable or better level than their synchronous counterparts, it is evident that they do at least excel in simpler math operations. This, combined with their excellent power consumption and heat dissipation properties, makes them very suitable for embedded computers."
+1742,Many modern CPUs have a die-integrated power managing module which regulates on-demand voltage supply to the CPU circuitry allowing it to keep balance between performance and power consumption.
+1743,"Every CPU represents numerical values in a specific way. For example, some early digital computers represented numbers as familiar decimal  numeral system values, and others have employed more unusual representations such as ternary . Nearly all modern CPUs represent numbers in binary form, with each digit being represented by some two-valued physical quantity such as a ""high"" or ""low"" voltage."
+1744,"Related to numeric representation is the size and precision of integer numbers that a CPU can represent. In the case of a binary CPU, this is measured by the number of bits  that the CPU can process in one operation, which is commonly called word size, bit width, data path width, integer precision, or integer size. A CPU's integer size determines the range of integer values on which it can it can directly operate. For example, an 8-bit CPU can directly manipulate integers represented by eight bits, which have a range of 256  discrete integer values."
+1745,"Integer range can also affect the number of memory locations the CPU can directly address . For example, if a binary CPU uses 32 bits to represent a memory address then it can directly address 232 memory locations. To circumvent this limitation and for various other reasons, some CPUs use mechanisms  that allow additional memory to be addressed."
+1746,"CPUs with larger word sizes require more circuitry and consequently are physically larger, cost more and consume more power . As a result, smaller 4- or 8-bit microcontrollers are commonly used in modern applications even though CPUs with much larger word sizes  are available. When higher performance is required, however, the benefits of a larger word size  may outweigh the disadvantages. A CPU can have internal data paths shorter than the word size to reduce size and cost. For example, even though the IBM System/360 instruction set architecture was a 32-bit instruction set, the System/360 Model 30 and Model 40 had 8-bit data paths in the arithmetic logical unit, so that a 32-bit add required four cycles, one for each 8 bits of the operands, and, even though the Motorola 68000 series instruction set was a 32-bit instruction set, the Motorola 68000 and Motorola 68010 had 16-bit data paths in the arithmetic logical unit, so that a 32-bit add required two cycles."
+1747,"To gain some of the advantages afforded by both lower and higher bit lengths, many instruction sets have different bit widths for integer and floating-point data, allowing CPUs implementing that instruction set to have different bit widths for different portions of the device. For example, the IBM System/360 instruction set was primarily 32 bit, but supported 64-bit floating-point values to facilitate greater accuracy and range in floating-point numbers. The System/360 Model 65 had an 8-bit adder for decimal and fixed-point binary arithmetic and a 60-bit adder for floating-point arithmetic. Many later CPU designs use similar mixed bit width, especially when the processor is meant for general-purpose use where a reasonable balance of integer and floating-point capability is required."
+1748,"The description of the basic operation of a CPU offered in the previous section describes the simplest form that a CPU can take. This type of CPU, usually referred to as subscalar, operates on and executes one instruction on one or two pieces of data at a time, that is less than one instruction per clock cycle ."
+1749,"This process gives rise to an inherent inefficiency in subscalar CPUs. Since only one instruction is executed at a time, the entire CPU must wait for that instruction to complete before proceeding to the next instruction. As a result, the subscalar CPU gets ""hung up"" on instructions which take more than one clock cycle to complete execution. Even adding a second execution unit  does not improve performance much; rather than one pathway being hung up, now two pathways are hung up and the number of unused transistors is increased. This design, wherein the CPU's execution resources can operate on only one instruction at a time, can only possibly reach scalar performance . However, the performance is nearly always subscalar ."
+1750,"Attempts to achieve scalar and better performance have resulted in a variety of design methodologies that cause the CPU to behave less linearly and more in parallel. When referring to parallelism in CPUs, two terms are generally used to classify these design techniques:"
+1751,"Each methodology differs both in the ways in which they are implemented, as well as the relative effectiveness they afford in increasing the CPU's performance for an application."
+1752,"One of the simplest methods for increased parallelism is to begin the first steps of instruction fetching and decoding before the prior instruction finishes executing. This is a technique known as instruction pipelining, and is used in almost all modern general-purpose CPUs. Pipelining allows multiple instruction to be executed at a time by breaking the execution pathway into discrete stages. This separation can be compared to an assembly line, in which an instruction is made more complete at each stage until it exits the execution pipeline and is retired."
+1753,"Pipelining does, however, introduce the possibility for a situation where the result of the previous operation is needed to complete the next operation; a condition often termed data dependency conflict. Therefore, pipelined processors must check for these sorts of conditions and delay a portion of the pipeline if necessary. A pipelined processor can become very nearly scalar, inhibited only by pipeline stalls ."
+1754,"Improvements in instruction pipelining led to further decreases in the idle time of CPU components. Designs that are said to be superscalar include a long instruction pipeline and multiple identical execution units, such as load–store units, arithmetic–logic units, floating-point units and address generation units. In a superscalar pipeline, instructions are read and passed to a dispatcher, which decides whether or not the instructions can be executed in parallel . If so, they are dispatched to execution units, resulting in their simultaneous execution. In general, the number of instructions that a superscalar CPU will complete in a cycle is dependent on the number of instructions it is able to dispatch simultaneously to execution units."
+1755,"Most of the difficulty in the design of a superscalar CPU architecture lies in creating an effective dispatcher. The dispatcher needs to be able to quickly determine whether instructions can be executed in parallel, as well as dispatch them in such a way as to keep as many execution units busy as possible. This requires that the instruction pipeline is filled as often as possible and requires significant amounts of CPU cache. It also makes hazard-avoiding techniques like branch prediction, speculative execution, register renaming, out-of-order execution and transactional memory crucial to maintaining high levels of performance. By attempting to predict which branch  a conditional instruction will take, the CPU can minimize the number of times that the entire pipeline must wait until a conditional instruction is completed. Speculative execution often provides modest performance increases by executing portions of code that may not be needed after a conditional operation completes. Out-of-order execution somewhat rearranges the order in which instructions are executed to reduce delays due to data dependencies. Also in case of single instruction stream, multiple data stream, a case when a lot of data from the same type has to be processed, modern processors can disable parts of the pipeline so that when a single instruction is executed many times, the CPU skips the fetch and decode phases and thus greatly increases performance on certain occasions, especially in highly monotonous program engines such as video creation software and photo processing."
+1756,"When a fraction of the CPU is superscalar, the part that is not suffers a performance penalty due to scheduling stalls. The Intel P5 Pentium had two superscalar ALUs which could accept one instruction per clock cycle each, but its FPU could not. Thus the P5 was integer superscalar but not floating point superscalar. Intel's successor to the P5 architecture, P6, added superscalar abilities to its floating-point features."
+1757,"Simple pipelining and superscalar design increase a CPU's ILP by allowing it to execute instructions at rates surpassing one instruction per clock cycle. Most modern CPU designs are at least somewhat superscalar, and nearly all general purpose CPUs designed in the last decade are superscalar. In later years some of the emphasis in designing high-ILP computers has been moved out of the CPU's hardware and into its software interface, or instruction set architecture . The strategy of the very long instruction word  causes some ILP to become implied directly by the software, reducing the CPU's work in boosting ILP and thereby reducing design complexity."
+1758,"Another strategy of achieving performance is to execute multiple threads or processes in parallel. This area of research is known as parallel computing. In Flynn's taxonomy, this strategy is known as multiple instruction stream, multiple data stream ."
+1759,"One technology used for this purpose is multiprocessing . The initial type of this technology is known as symmetric multiprocessing , where a small number of CPUs share a coherent view of their memory system. In this scheme, each CPU has additional hardware to maintain a constantly up-to-date view of memory. By avoiding stale views of memory, the CPUs can cooperate on the same program and programs can migrate from one CPU to another. To increase the number of cooperating CPUs beyond a handful, schemes such as non-uniform memory access  and directory-based coherence protocols were introduced in the 1990s. SMP systems are limited to a small number of CPUs while NUMA systems have been built with thousands of processors. Initially, multiprocessing was built using multiple discrete CPUs and boards to implement the interconnect between the processors. When the processors and their interconnect are all implemented on a single chip, the technology is known as chip-level multiprocessing  and the single chip as a multi-core processor."
+1760,"It was later recognized that finer-grain parallelism existed with a single program. A single program might have several threads  that could be executed separately or in parallel. Some of the earliest examples of this technology implemented input/output processing such as direct memory access as a separate thread from the computation thread. A more general approach to this technology was introduced in the 1970s when systems were designed to run multiple computation threads in parallel. This technology is known as multi-threading . The approach is considered more cost-effective than multiprocessing, as only a small number of components within a CPU are replicated to support MT as opposed to the entire CPU in the case of MP. In MT, the execution units and the memory system including the caches are shared among multiple threads. The downside of MT is that the hardware support for multithreading is more visible to software than that of MP and thus supervisor software like operating systems have to undergo larger changes to support MT. One type of MT that was implemented is known as temporal multithreading, where one thread is executed until it is stalled waiting for data to return from external memory. In this scheme, the CPU would then quickly context switch to another thread which is ready to run, the switch often done in one CPU clock cycle, such as the UltraSPARC T1. Another type of MT is simultaneous multithreading, where instructions from multiple threads are executed in parallel within one CPU clock cycle."
+1761,"For several decades from the 1970s to early 2000s, the focus in designing high performance general purpose CPUs was largely on achieving high ILP through technologies such as pipelining, caches, superscalar execution, out-of-order execution, etc. This trend culminated in large, power-hungry CPUs such as the Intel Pentium 4. By the early 2000s, CPU designers were thwarted from achieving higher performance from ILP techniques due to the growing disparity between CPU operating frequencies and main memory operating frequencies as well as escalating CPU power dissipation owing to more esoteric ILP techniques."
+1762,"CPU designers then borrowed ideas from commercial computing markets such as transaction processing, where the aggregate performance of multiple programs, also known as throughput computing, was more important than the performance of a single thread or process."
+1763,"This reversal of emphasis is evidenced by the proliferation of dual and more core processor designs and notably, Intel's newer designs resembling its less superscalar P6 architecture. Late designs in several processor families exhibit CMP, including the x86-64 Opteron and Athlon 64 X2, the SPARC UltraSPARC T1, IBM POWER4 and POWER5, as well as several video game console CPUs like the Xbox 360's triple-core PowerPC design, and the PlayStation 3's 7-core Cell microprocessor."
+1764,"A less common but increasingly important paradigm of processors  deals with data parallelism. The processors discussed earlier are all referred to as some type of scalar device. As the name implies, vector processors deal with multiple pieces of data in the context of one instruction. This contrasts with scalar processors, which deal with one piece of data for every instruction. Using Flynn's taxonomy, these two schemes of dealing with data are generally referred to as single instruction stream, multiple data stream  and single instruction stream, single data stream , respectively. The great utility in creating processors that deal with vectors of data lies in optimizing tasks that tend to require the same operation  to be performed on a large set of data. Some classic examples of these types of tasks include multimedia applications , as well as many types of scientific and engineering tasks. Whereas a scalar processor must complete the entire process of fetching, decoding and executing each instruction and value in a set of data, a vector processor can perform a single operation on a comparatively large set of data with one instruction. This is only possible when the application tends to require many steps which apply one operation to a large set of data."
+1765,"Most early vector processors, such as the Cray-1, were associated almost exclusively with scientific research and cryptography applications. However, as multimedia has largely shifted to digital media, the need for some form of SIMD in general-purpose processors has become significant. Shortly after inclusion of floating-point units started to become commonplace in general-purpose processors, specifications for and implementations of SIMD execution units also began to appear for general-purpose processors. Some of these early SIMD specifications – like HP's Multimedia Acceleration eXtensions  and Intel's MMX – were integer-only. This proved to be a significant impediment for some software developers, since many of the applications that benefit from SIMD primarily deal with floating-point numbers. Progressively, developers refined and remade these early designs into some of the common modern SIMD specifications, which are usually associated with one instruction set architecture . Some notable modern examples include Intel's Streaming SIMD Extensions  and the PowerPC-related AltiVec ."
+1766,"Many modern architectures  often include hardware performance counters , which enables low-level  collection, benchmarking, debugging or analysis of running software metrics. HPC may also be used to discover and analyze unusual or suspicious activity of the software, such as return-oriented programming  or sigreturn-oriented programming  exploits etc. This is usually done by software-security teams to assess and find malicious binary programs."
+1767,"Many major vendors  provide software interfaces  that can be used to collected data from CPUs registers in order to get metrics. Operating system vendors also provide software like perf  to record, benchmark, or trace CPU events running kernels and applications."
+1768,"Hardware counters provide a low-overhead method for collecting comprehensive performance metrics related to a CPU's core elements  – a significant advantage over software profilers. Additionally, they generally eliminate the need to modify the underlying source code of a program. Because hardware designs differ between architectures, the specific types and interpretations of hardware counters will also change."
+1769,Most modern CPUs have privileged modes to support operating systems and virtualization.
+1770,Cloud computing can use virtualization to provide virtual central processing units  for separate users.
+1771,"A host is the virtual equivalent of a physical machine, on which a virtual system is operating. When there are several physical machines operating in tandem and managed as a whole, the grouped computing and memory resources form a cluster. In some systems, it is possible to dynamically add and remove from a cluster. Resources available at a host and cluster level can be partitioned into resources pools with fine granularity."
+1772,"The performance or speed of a processor depends on, among many other factors, the clock rate  and the instructions per clock , which together are the factors for the instructions per second  that the CPU can perform.
+Many reported IPS values have represented ""peak"" execution rates on artificial instruction sequences with few branches, whereas realistic workloads consist of a mix of instructions and applications, some of which take longer to execute than others. The performance of the memory hierarchy also greatly affects processor performance, an issue barely considered in IPS calculations. Because of these problems, various standardized tests, often called ""benchmarks"" for this purpose‍—‌ such as SPECint‍—‌have been developed to attempt to measure the real effective performance in commonly used applications."
+1773,"Processing performance of computers is increased by using multi-core processors, which essentially is plugging two or more individual processors  into one integrated circuit. Ideally, a dual core processor would be nearly twice as powerful as a single core processor. In practice, the performance gain is far smaller, only about 50%, due to imperfect software algorithms and implementation. Increasing the number of cores in a processor  increases the workload that can be handled. This means that the processor can now handle numerous asynchronous events, interrupts, etc. which can take a toll on the CPU when overwhelmed. These cores can be thought of as different floors in a processing plant, with each floor handling a different task. Sometimes, these cores will handle the same tasks as cores adjacent to them if a single core is not enough to handle the information. Multi-core CPUs enhance a computer's ability to run several tasks simultaneously by providing additional processing power. However, the increase in speed is not directly proportional to the number of cores added. This is because the cores need to interact through specific channels, and this inter-core communication consumes a portion of the available processing speed."
+1774,"Due to specific capabilities of modern CPUs, such as simultaneous multithreading and uncore, which involve sharing of actual CPU resources while aiming at increased utilization, monitoring performance levels and hardware use gradually became a more complex task. As a response, some CPUs implement additional hardware logic that monitors actual use of various parts of a CPU and provides various counters accessible to software; an example is Intel's Performance Counter Monitor technology."
+1775,"Low-level languages can convert to machine code without a compiler or interpreter—second-generation programming languages use a simpler processor called an assembler—and the resulting code runs directly on the processor. A program written in a low-level language can be made to run very quickly, with a small memory footprint. An equivalent program in a high-level language can be less efficient and use more memory. Low-level languages are simple, but considered difficult to use, due to numerous technical details that the programmer must remember. By comparison, a high-level programming language isolates execution semantics of a computer architecture from the specification of the program, which simplifies development."
+1776,"Machine code is the only language a computer can process directly without a previous transformation. Currently, programmers almost never write programs directly in machine code, because it requires attention to numerous details that a high-level programming language handles automatically. Furthermore, unlike programming in an assembly language, it requires memorizing or looking up numerical codes for every instruction, and is extremely difficult to modify."
+1777,"True machine code is a stream of raw, usually binary, data. A programmer coding in ""machine code"" normally codes instructions and data in a more readable form such as decimal, octal, or hexadecimal which is translated to internal format by a program called a loader or toggled into the computer's memory from a front panel."
+1778,"Although few programs are written in machine languages, programmers often become adept at reading it through working with core dumps or debugging from the front panel."
+1779,"Example of a function in hexadecimal representation of x86-64 machine code to calculate the nth Fibonacci number, with each line corresponding to one instruction:"
+1780,"Second-generation languages provide one abstraction level on top of the machine code. In the early days of coding on computers like TX-0 and PDP-1, the first thing MIT hackers did was to write assemblers.
+Assembly language has little semantics or formal specification, being only a mapping of human-readable symbols, including symbolic addresses, to opcodes, addresses, numeric constants, strings and so on. Typically, one machine instruction is represented as one line of assembly code. Assemblers produce object files that can link with other object files or be loaded on their own."
+1781,Most assemblers provide macros to generate common sequences of instructions.
+1782,"Example: The same Fibonacci number calculator as above, but in x86-64 assembly language using AT&T syntax:"
+1783,"In this code example, the registers of the x86-64 processor are named and manipulated directly. The function loads its 32-bit argument from %edi in accordance to the System V application binary interface for x86-64 and performs its calculation by manipulating values in the %eax, %ecx, %esi, and %edi registers until it has finished and returns. Note that in this assembly language, there is no concept of returning a value. The result having been stored in the %eax register, again in accordance with System V application binary interface, the ret instruction simply removes the top 64-bit element on the stack and causes the next instruction to be fetched from that location , with the result of the function being stored in %eax. x86-64 assembly language imposes no standard for passing values to a function or returning values from a function ; those are defined by an application binary interface, such as the System V ABI for a particular instruction set."
+1784,Compare this with the same function in C:
+1785,This code is similar in structure to the assembly language example but there are significant differences in terms of abstraction:
+1786,These abstractions make the C code compilable without modification on any architecture for which a C compiler has been written. The x86 assembly language code is specific to the x86-64 architecture and the System V application binary interface for that architecture.
+1787,"During the late 1960s and 1970s, high-level languages that included some degree of access to low-level programming functions, such as PL/S, BLISS, BCPL, extended ALGOL and ESPOL , and C, were introduced. One method for this is inline assembly, in which assembly code is embedded in a high-level language that supports this feature. Some of these languages also allow architecture-dependent compiler optimization directives to adjust the way a compiler uses the target processor architecture."
+1788,Parse the source code and perform its behavior directly;
+1789,Translate source code into some efficient intermediate representation or object code and immediately execute that;
+1790,Explicitly execute stored precompiled bytecode made by a compiler and matched with the interpreter's Virtual Machine.
+1791,"Early versions of Lisp programming language and minicomputer and microcomputer BASIC dialects would be examples of the first type. Perl, Raku, Python, MATLAB, and Ruby are examples of the second, while UCSD Pascal is an example of the third type. Source programs are compiled ahead of time and stored as machine independent code, which is then linked at run-time and executed by an interpreter and/or compiler . Some systems, such as Smalltalk and contemporary versions of BASIC and Java, may also combine two and three types. Interpreters of various types have also been constructed for many languages traditionally associated with compilation, such as Algol, Fortran, Cobol, C and C++."
+1792,"While interpretation and compilation are the two main means by which programming languages are implemented, they are not mutually exclusive, as most interpreting systems also perform some translation work, just like compilers. The terms ""interpreted language"" or ""compiled language"" signify that the canonical implementation of that language is an interpreter or a compiler, respectively. A high-level language is ideally an abstraction independent of particular implementations."
+1793,"Interpreters were used as early as 1952 to ease programming within the limitations of computers at the time . Interpreters were also used to translate between low-level machine languages, allowing code to be written for machines that were still under construction and tested on computers that already existed. The first interpreted high-level language was Lisp. Lisp was first implemented by Steve Russell on an IBM 704 computer. Russell had read John McCarthy's paper, ""Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I"", and realized  that the Lisp eval function could be implemented in machine code. The result was a working Lisp interpreter which could be used to run Lisp programs, or more properly, ""evaluate Lisp expressions""."
+1794,"An interpreter usually consists of a set of known commands it can execute, and a list of these commands in the order a programmer wishes to execute them. Each command  contains the data the programmer wants to mutate, and information on how to mutate the data. For example, an interpreter might read ADD Books, 5 and interpret it as a request to add five to the Books variable."
+1795,"Interpreters have a wide variety of instructions which are specialized to perform different tasks, but you will commonly find interpreter instructions for basic mathematical operations, branching, and memory management, making most interpreters Turing complete. Many interpreters are also closely integrated with a garbage collector and debugger."
+1796,Programs written in a high-level language are either directly executed by some kind of interpreter or converted into machine code by a compiler  for the CPU to execute.
+1797,"While compilers  generally produce machine code directly executable by computer hardware, they can often  produce an intermediate form called object code. This is basically the same machine specific code but augmented with a symbol table with names and tags to make executable blocks  identifiable and relocatable. Compiled programs will typically use building blocks  kept in a library of such object code modules. A linker is used to combine  library files with the object file of the application to form a single executable file. The object files that are used to generate an executable file are thus often produced at different times, and sometimes even by different languages ."
+1798,"A simple interpreter written in a low-level language  may have similar machine code blocks implementing functions of the high-level language stored, and executed when a function's entry in a look up table points to that code. However, an interpreter written in a high-level language typically uses another approach, such as generating and then walking a parse tree, or by generating and executing intermediate software-defined instructions, or both."
+1799,"Thus, both compilers and interpreters generally turn source code  into tokens, both may  generate a parse tree, and both may generate immediate instructions . The basic difference is that a compiler system, including a  linker, generates a stand-alone machine code program, while an interpreter system instead performs the actions described by the high-level program."
+1800,"A compiler can thus make almost all the conversions from source code semantics to the machine level once and for all  while an interpreter has to do some of this conversion work every time a statement or function is executed. However, in an efficient interpreter, much of the translation work  is factored out and done only the first time a program, module, function, or even statement, is run, thus quite akin to how a compiler works. However, a compiled program still runs much faster, under most circumstances, in part because compilers are designed to optimize code, and may be given ample time for this. This is especially true for simpler high-level languages without  dynamic data structures, checks, or type checking."
+1801,"In traditional compilation, the executable output of the linkers  is typically relocatable when run under a general operating system, much like the object code modules are but with the difference that this relocation is done dynamically at run time, i.e. when the program is loaded for execution. On the other hand, compiled and linked programs for small embedded systems are typically statically allocated, often hard coded in a NOR flash memory, as there is often no secondary storage and no operating system in this sense."
+1802,"Historically, most interpreter systems have had a self-contained editor built in. This is becoming more common also for compilers , although some programmers prefer to use an editor of their choice and run the compiler, linker and other tools manually. Historically, compilers predate interpreters because hardware at that time could not support both the interpreter and interpreted code and the typical batch environment of the time limited the advantages of interpretation."
+1803,"During the software development cycle, programmers make frequent changes to source code. When using a compiler, each time a change is made to the source code, they must wait for the compiler to translate the altered source files and link all of the binary code files together before the program can be executed. The larger the program, the longer the wait. By contrast, a programmer using an interpreter does a lot less waiting, as the interpreter usually just needs to translate the code being worked on to an intermediate representation , thus requiring much less time before the changes can be tested. Effects are evident upon saving the source code and reloading the program. Compiled code is generally less readily debugged as editing, compiling, and linking are sequential processes that have to be conducted in the proper sequence with a proper set of commands. For this reason, many compilers also have an executive aid, known as a Makefile and program. The Makefile lists compiler and linker command lines and program source code files, but might take a simple command line menu input  which selects the third group  of instructions then issues the commands to the compiler, and linker feeding the specified source code files."
+1804,"A compiler converts source code into binary instruction for a specific processor's architecture, thus making it less portable. This conversion is made just once, on the developer's environment, and after that the same binary can be distributed to the user's machines where it can be executed without further translation. A cross compiler can generate binary code for the user machine even if it has a different processor than the machine where the code is compiled."
+1805,"An interpreted program can be distributed as source code. It needs to be translated in each final machine, which takes more time but makes the program distribution independent of the machine's architecture. However, the portability of interpreted source code is dependent on the target machine actually having a suitable interpreter. If the interpreter needs to be supplied along with the source, the overall installation process is more complex than delivery of a monolithic executable, since the interpreter itself is part of what needs to be installed."
+1806,"The fact that interpreted code can easily be read and copied by humans can be of concern from the point of view of copyright. However, various systems of encryption and obfuscation exist. Delivery of intermediate code, such as bytecode, has a similar effect to obfuscation, but bytecode could be decoded with a decompiler or disassembler."
+1807,The main disadvantage of interpreters is that an interpreted program typically runs more slowly than if it had been compiled. The difference in speeds could be tiny or great; often an order of magnitude and sometimes more. It generally takes longer to run a program under an interpreter than to run the compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle.
+1808,"Interpreting code is slower than running the compiled code because the interpreter must analyze each statement in the program each time it is executed and then perform the desired action, whereas the compiled code just performs the action within a fixed context determined by the compilation. This run-time analysis is known as ""interpretive overhead"". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time."
+1809,"There are various compromises between the development speed when using an interpreter and the execution speed when using a compiler. Some systems  allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute the source code as it stands but convert it into some more compact internal form. Many BASIC interpreters replace keywords with single byte tokens which can be used to find the instruction in a jump table. A few interpreters, such as the PBASIC interpreter, achieve even higher levels of program compaction by using a bit-oriented rather than a byte-oriented program memory structure, where commands tokens occupy perhaps 5 bits, nominally ""16-bit"" constants are stored in a variable-length code requiring 3, 6, 10, or 18 bits, and address operands include a ""bit offset"". Many BASIC interpreters can store and read back their own tokenized internal representation."
+1810,"An interpreter might well use the same lexical analyzer and parser as the compiler and then interpret the resulting abstract syntax tree. Example data type definitions for the latter, and a toy interpreter for syntax trees obtained from C expressions are shown in the box."
+1811,"Interpretation cannot be used as the sole method of execution: even though an interpreter can itself be interpreted and so on, a directly executed program is needed somewhere at the bottom of the stack because the code being interpreted is not, by definition, the same as the machine code that the CPU can execute."
+1812,"There is a spectrum of possibilities between interpreting and compiling, depending on the amount of analysis performed before the program is executed. For example, Emacs Lisp is compiled to bytecode, which is a highly compressed and optimized representation of the Lisp source, but is not machine code . This ""compiled"" code is then interpreted by a bytecode interpreter . The compiled code in this case is machine code for a virtual machine, which is implemented not in hardware, but in the bytecode interpreter. Such compiling interpreters are sometimes also called compreters. In a bytecode interpreter each instruction starts with a byte, and therefore bytecode interpreters have up to 256 instructions, although not all may be used. Some bytecodes may take multiple bytes, and may be arbitrarily complicated."
+1813,Control tables - that do not necessarily ever need to pass through a compiling phase - dictate appropriate algorithmic control flow via customized interpreters in similar fashion to bytecode interpreters.
+1814,"Threaded code interpreters are similar to bytecode interpreters but instead of bytes they use pointers. Each ""instruction"" is a word that points to a function or an instruction sequence, possibly followed by a parameter. The threaded code interpreter either loops fetching instructions and calling the functions they point to, or fetches the first instruction and jumps to it, and every instruction sequence ends with a fetch and jump to the next instruction. Unlike bytecode there is no effective limit on the number of different instructions other than available memory and address space. The classic example of threaded code is the Forth code used in Open Firmware systems: the source language is compiled into ""F code"" , which is then interpreted by a virtual machine."
+1815,"In the spectrum between interpreting and compiling, another approach is to transform the source code into an optimized abstract syntax tree , then execute the program following this tree structure, or use it to generate native code just-in-time. In this approach, each sentence needs to be parsed just once. As an advantage over bytecode, the AST keeps the global program structure and relations between statements , and when compressed provides a more compact representation. Thus, using AST has been proposed as a better intermediate format for just-in-time compilers than bytecode. Also, it allows the system to perform better analysis during runtime."
+1816,"However, for interpreters, an AST causes more overhead than a bytecode interpreter, because of nodes related to syntax performing no useful work, of a less sequential representation  and of overhead visiting the tree."
+1817,"Further blurring the distinction between interpreters, bytecode interpreters and compilation is just-in-time  compilation, a technique in which the intermediate representation is compiled to native machine code at runtime. This confers the efficiency of running native code, at the cost of startup time and increased memory use when the bytecode or AST is first compiled. The earliest published JIT compiler is generally attributed to work on LISP by John McCarthy in 1960. Adaptive optimization is a complementary technique in which the interpreter profiles the running program and compiles its most frequently executed parts into native code. The latter technique is a few decades old, appearing in languages such as Smalltalk in the 1980s."
+1818,"Just-in-time compilation has gained mainstream attention amongst language implementers in recent years, with Java, the .NET Framework, most modern JavaScript implementations, and Matlab now including JIT compilers."
+1819,"Making the distinction between compilers and interpreters yet again even more vague is a special interpreter design known as a template interpreter. Rather than implement the execution of code by virtue of a large switch statement containing every possible bytecode, while operating on a software stack or a tree walk, a template interpreter maintains a large array of bytecode  mapped directly to corresponding native machine instructions that can be executed on the host hardware as key value pairs , known as a ""Template"". When the particular code segment is executed the interpreter simply loads or jumps to the opcode mapping in the template and directly runs it on the hardware. Due to its design, the template interpreter very strongly resembles a just-in-time compiler rather than a traditional interpreter, however it is technically not a JIT due to the fact that it merely translates code from the language into native calls one opcode at a time rather than creating optimized sequences of CPU executable instructions from the entire code segment. Due to the interpreter's simple design of simply passing calls directly to the hardware rather than implementing them directly, it is much faster than every other type, even bytecode interpreters, and to an extent less prone to bugs, but as a tradeoff is more difficult to maintain due to the interpreter having to support translation to multiple different architectures instead of a platform independent virtual machine/stack. To date, the only template interpreter implementations of widely known languages to exist are the interpreter within Java's official reference implementation, the Sun HotSpot Java Virtual Machine, and the Ignition Interpreter in the Google V8 javascript execution engine."
+1820,A self-interpreter is a programming language interpreter written in a programming language which can interpret itself; an example is a BASIC interpreter written in BASIC. Self-interpreters are related to self-hosting compilers.
+1821,"If no compiler exists for the language to be interpreted, creating a self-interpreter requires the implementation of the language in a host language . By having a first interpreter such as this, the system is bootstrapped and new versions of the interpreter can be developed in the language itself. It was in this way that Donald Knuth developed the TANGLE interpreter for the language WEB of the de-facto standard TeX typesetting system."
+1822,"Defining a computer language is usually done in relation to an abstract machine  or as a mathematical function . A language may also be defined by an interpreter in which the semantics of the host language is given. The definition of a language by a self-interpreter is not well-founded , but a self-interpreter tells a reader about the expressiveness and elegance of a language. It also enables the interpreter to interpret its source code, the first step towards reflective interpreting."
+1823,"An important design dimension in the implementation of a self-interpreter is whether a feature of the interpreted language is implemented with the same feature in the interpreter's host language. An example is whether a closure in a Lisp-like language is implemented using closures in the interpreter language or implemented ""manually"" with a data structure explicitly storing the environment. The more features implemented by the same feature in the host language, the less control the programmer of the interpreter has; for example, a different behavior for dealing with number overflows cannot be realized if the arithmetic operations are delegated to corresponding operations in the host language."
+1824,"Some languages such as Lisp and Prolog have elegant self-interpreters. Much research on self-interpreters  has been conducted in the Scheme programming language, a dialect of Lisp. In general, however, any Turing-complete language allows writing of its own interpreter. Lisp is such a language, because Lisp programs are lists of symbols and other lists. XSLT is such a language, because XSLT programs are written in XML. A sub-domain of metaprogramming is the writing of domain-specific languages ."
+1825,"Clive Gifford introduced a measure quality of self-interpreter , the limit of the ratio between computer time spent running a stack of N self-interpreters and time spent to run a stack of N − 1 self-interpreters as N goes to infinity. This value does not depend on the program being run."
+1826,The book Structure and Interpretation of Computer Programs presents examples of meta-circular interpretation for Scheme and its dialects. Other examples of languages with a self-interpreter are Forth and Pascal.
+1827,"Microcode is a very commonly used technique ""that imposes an interpreter between the hardware and the architectural level of a computer"". As such, the microcode is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements.  Microcode is used in general-purpose central processing units, as well as in more specialized processors such as microcontrollers, digital signal processors, channel controllers, disk controllers, network interface controllers, network processors, graphics processing units, and in other hardware."
+1828,"Microcode typically resides in special high-speed memory and translates machine instructions, state machine data or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying electronics so that instructions can be designed and altered more freely. It also facilitates the building of complex multi-step instructions, while reducing the complexity of computer circuits. Writing microcode is often called microprogramming and the microcode in a particular processor implementation is sometimes called a microprogram."
+1829,"More extensive microcoding allows small and simple microarchitectures to emulate more powerful architectures with wider word length, more execution units and so on, which is a relatively simple way to achieve software compatibility between different products in a processor family."
+1830,Even a non microcoding computer processor itself can be considered to be a parsing immediate execution interpreter that is written in a general purpose hardware description language such as VHDL to create a system that parses the machine code instructions and immediately executes them.
+1831,"Interpreters, such as those written in Java, Perl, and Tcl, are now necessary for a wide range of computational tasks, including binary emulation and internet applications. Interpreter performance is still a worry despite their adaptability, particularly on systems with limited hardware resources. Advanced instrumentation and tracing approaches provide insights into interpreter implementations and processor resource utilization during execution through evaluations of interpreters tailored for the MIPS instruction set and programming languages such as Tcl, Perl, and Java. Performance characteristics are influenced by interpreter complexity, as demonstrated by comparisons with compiled code. It is clear that interpreter performance is more dependent on the nuances and resource needs of the interpreter than it is on the particular application that is being interpreted."
+1832,"In computer programming,  assembly language , often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence between the instructions in the language and the architecture's machine code instructions. Assembly language usually has one statement per machine instruction , but  constants, comments, assembler directives, symbolic labels of, e.g., memory locations, registers, and macros are generally also supported."
+1833,"The first assembly code in which a language is used to represent machine code instructions is found in Kathleen and Andrew Donald Booth's 1947 work, Coding for A.R.C.. Assembly code is converted into executable machine code by a utility program referred to as an assembler. The term ""assembler"" is generally attributed to Wilkes, Wheeler and Gill in their 1951 book The Preparation of Programs for an Electronic Digital Computer, who, however, used the term to mean ""a program that assembles another program consisting of several sections into a single program"". The conversion process is referred to as assembly, as in assembling the source code. The computational step when an assembler is processing a program is called assembly time."
+1834,"Because assembly depends on the machine code instructions, each assembly language is specific to a particular computer architecture."
+1835,"Sometimes there is more than one assembler for the same architecture, and sometimes an assembler is specific to an operating system or to particular operating systems. Most assembly languages do not provide specific syntax for operating system calls, and most assembly languages can be used universally with any operating system, as the language provides access to all the real capabilities of the processor, upon which all system call mechanisms ultimately rest. In contrast to assembly languages, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling, much more complicated tasks than assembling."
+1836,"In the first decades of computing, it was commonplace for both systems programming and application programming to take place entirely in assembly language.  While still irreplaceable for some purposes, the majority of programming is now conducted in higher-level interpreted and compiled languages.  In ""No Silver Bullet"", Fred Brooks summarised the effects of the switch away from assembly language programming: ""Surely the most powerful stroke for software productivity, reliability, and simplicity has been the progressive use of high-level languages for programming. Most observers credit that development with at least a factor of five in productivity, and with concomitant gains in reliability, simplicity, and comprehensibility."""
+1837,"Today, it is typical to use small amounts of assembly language code within larger systems implemented in a higher-level language, for performance reasons or to interact directly with hardware in ways unsupported by the higher-level language.  For instance, just under 2% of version 4.9 of the Linux kernel source code is written in assembly; more than 97% is written in C."
+1838,"Assembly language uses a mnemonic to represent, e.g., each low-level machine instruction or opcode, each directive, typically also each architectural register, flag, etc. Some of the mnemonics may be built in and some user defined. Many operations require one or more operands in order to form a complete instruction. Most assemblers permit named constants, registers, and labels for program and memory locations, and can calculate expressions for operands. Thus, programmers are freed from tedious repetitive calculations and assembler programs are much more readable than machine code. Depending on the architecture, these elements may also be combined for specific instructions or addressing modes using offsets or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aid debugging."
+1839,"Some are column oriented, with specific fields in specific columns; this was very common for machines using punched cards in the 1950s and early 1960s. Some assemblers have free-form syntax, with fields separated by delimiters, e.g., punctuation, white space. Some assemblers are hybrid, with, e.g., labels, in a specific column and other fields separated by delimiters; this became more common than column oriented syntax in the 1960s."
+1840,"An assembler program creates object code by translating combinations of mnemonics and syntax for operations and addressing modes into their numerical equivalents. This representation typically includes an operation code  as well as other control bits and data. The assembler also calculates constant expressions and resolves symbolic names for memory locations and other entities. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include macro facilities for performing textual substitution – e.g., to generate common short sequences of instructions as inline, instead of called subroutines."
+1841,"Some assemblers may also be able to perform some simple types of instruction set-specific optimizations. One concrete example of this may be the ubiquitous x86 assemblers from various vendors. Called jump-sizing, most of them are able to perform jump-instruction replacements  in any number of passes, on request. Others may even do simple rearrangement or insertion of instructions, such as some assemblers for RISC architectures that can help optimize a sensible instruction scheduling to exploit the CPU pipeline as efficiently as possible."
+1842,"Assemblers have been available since the 1950s, as the first step above machine language and before high-level programming languages such as Fortran, Algol, COBOL and Lisp. There have also been several classes of translators and semi-automatic code generators with properties similar to both assembly and high-level languages, with Speedcode as perhaps one of the better-known examples."
+1843,"There may be several assemblers with different syntax for a particular CPU or instruction set architecture. For instance, an instruction to add memory data to a register in a x86-family processor might be add eax,, in original Intel syntax, whereas this would be written addl ,%eax in the AT&T syntax used by the GNU Assembler. Despite different appearances, different syntactic forms generally generate the same numeric machine code. A single assembler may also have different modes in order to support variations in syntactic forms as well as their exact semantic interpretations ."
+1844,There are two types of assemblers based on how many passes through the source are needed  to produce the object file.
+1845,"In both cases, the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of subsequent symbols. This means that if the size of an operation referring to an operand defined later depends on the type or distance of the operand, the assembler will make a pessimistic estimate when first encountering the operation, and if necessary, pad it with one or more
+""no-operation"" instructions in a later pass or the errata. In an assembler with peephole optimization, addresses may be recalculated between passes to allow replacing pessimistic code with code tailored to the exact distance from the target."
+1846,"The original reason for the use of one-pass assemblers was memory size and speed of assembly – often a second pass would require storing the symbol table in memory , rewinding and rereading the program source on tape, or rereading a deck of cards or punched paper tape. Later computers with much larger memories , had the space to perform all necessary processing without such re-reading. The advantage of the multi-pass assembler is that the absence of errata makes the linking process  faster."
+1847,"Example: in the following code snippet, a one-pass assembler would be able to determine the address of the backward reference BKWD when assembling statement S2, but would not be able to determine the address of the forward reference FWD when assembling the branch statement S1; indeed, FWD may be undefined. A two-pass assembler would determine both addresses in pass 1, so they would be known when generating code in pass 2."
+1848,More sophisticated high-level assemblers provide language abstractions such as:
+1849,See Language design below for more details.
+1850,"A program written in assembly language consists of a series of mnemonic processor instructions and meta-statements , comments and data. Assembly language instructions usually consist of an opcode mnemonic followed by an operand, which might be a list of data, arguments or parameters.  Some instructions may be ""implied"", which means the data upon which the instruction operates is implicitly defined by the instruction itself—such an instruction does not take an operand.  The resulting statement is translated by an assembler into machine language instructions that can be loaded into memory and executed."
+1851,"For example, the instruction below tells an x86/IA-32 processor to move an immediate 8-bit value into a register. The binary code for this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for the AL register is 000, so the following machine code loads the AL register with the data 01100001."
+1852,This binary computer code can be made more human-readable by expressing it in hexadecimal as follows.
+1853,"Here, B0 means 'Move a copy of the following value into AL, and 61 is a hexadecimal representation of the value 01100001, which is 97 in decimal. Assembly language for the 8086 family provides the mnemonic MOV  for instructions such as this, so the machine code above can be written as follows in assembly language, complete with an explanatory comment if required, after the semicolon. This is much easier to read and to remember."
+1854,"In some assembly languages  the same mnemonic, such as MOV, may be used for a family of related instructions for loading, copying and moving data, whether these are immediate values, values in registers, or memory locations pointed to by values in registers or by immediate  addresses.  Other assemblers may use separate opcode mnemonics such as L for ""move memory to register"", ST for ""move register to memory"", LR for ""move register to register"", MVI for ""move immediate operand to memory"", etc."
+1855,"If the same mnemonic is used for different instructions, that means that the mnemonic corresponds to several different binary instruction codes, excluding data , depending on the operands that follow the mnemonic.  For example, for the x86/IA-32 CPUs, the Intel assembly language syntax MOV AL, AH represents an instruction that moves the contents of register AH into register AL. The hexadecimal form of this instruction is:"
+1856,"The first byte, 88h, identifies a move between a byte-sized register and either another register or memory, and the second byte, E0h, is encoded  to specify that both operands are registers, the source is AH, and the destination is AL."
+1857,"In a case like this where the same mnemonic can represent more than one binary instruction, the assembler determines which instruction to generate by examining the operands.  In the first example, the operand 61h is a valid hexadecimal numeric constant and is not a valid register name, so only the B0 instruction can be applicable.  In the second example, the operand AH is a valid register name and not a valid numeric constant , so only the 88 instruction can be applicable."
+1858,"Assembly languages are always designed so that this sort of lack of ambiguity is universally enforced by their syntax.  For example, in the Intel x86 assembly language, a hexadecimal constant must start with a numeral digit, so that the hexadecimal number 'A'  would be written as 0Ah or 0AH, not AH, specifically so that it cannot appear to be the name of register AH.  "
+1859,"Returning to the original example, while the x86 opcode 10110000  copies an 8-bit value into the AL register, 10110001  moves it into CL and 10110010  does so into DL. Assembly language examples for these follow."
+1860,The syntax of MOV can also be more complex as the following examples show.
+1861,"In each case, the MOV mnemonic is translated directly into one of the opcodes 88-8C, 8E, A0-A3, B0-BF, C6 or C7 by an assembler, and the programmer normally does not have to know or remember which."
+1862,"Transforming assembly language into machine code is the job of an assembler, and the reverse can at least partially be achieved by a disassembler. Unlike high-level languages, there is a one-to-one correspondence between many simple assembly statements and machine language instructions. However, in some cases, an assembler may provide pseudoinstructions  which expand into several machine language instructions to provide commonly needed functionality. For example, for a machine that lacks a ""branch if greater or equal"" instruction, an assembler may provide a pseudoinstruction that expands to the machine's ""set if less than"" and ""branch if zero "". Most full-featured assemblers also provide a rich macro language  which is used by vendors and programmers to generate more complex code and data sequences. Since the information about pseudoinstructions and macros defined in the assembler environment is not present in the object program, a disassembler cannot reconstruct the macro and pseudoinstruction invocations but can only disassemble the actual machine instructions that the assembler generated from those abstract assembly-language entities. Likewise, since comments in the assembly language source file are ignored by the assembler and have no effect on the object code it generates, a disassembler is always completely unable to recover source comments."
+1863,"Each computer architecture has its own machine language.  Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. While most general-purpose computers are able to carry out essentially the same functionality, the ways they do so differ; the corresponding assembly languages reflect these differences."
+1864,"Multiple sets of mnemonics or assembly-language syntax may exist for a single instruction set, typically instantiated in different assembler programs. In these cases, the most popular one is usually that supplied by the CPU manufacturer and used in its documentation."
+1865,"Two examples of CPUs that have two different sets of mnemonics are the Intel 8080 family and the Intel 8086/8088.  Because Intel claimed copyright on its assembly language mnemonics , some companies that independently produced CPUs compatible with Intel instruction sets invented their own mnemonics.  The Zilog Z80 CPU, an enhancement of the Intel 8080A, supports all the 8080A instructions plus many more; Zilog invented an entirely new assembly language, not only for the new instructions but also for all of the 8080A instructions.  For example, where Intel uses the mnemonics MOV, MVI, LDA, STA, LXI, LDAX, STAX, LHLD, and SHLD for various data transfer instructions, the Z80 assembly language uses the mnemonic LD for all of them.  A similar case is the NEC V20 and V30 CPUs, enhanced copies of the Intel 8086 and 8088, respectively.  Like Zilog with the Z80, NEC invented new mnemonics for all of the 8086 and 8088 instructions, to avoid accusations of infringement of Intel's copyright.    It is doubtful whether in practice many people who programmed the V20 and V30 actually wrote in NEC's assembly language rather than Intel's; since any two assembly languages for the same instruction set architecture are isomorphic , there is no requirement to use a manufacturer's own published assembly language with that manufacturer's products."
+1866,"There is a large degree of diversity in the way the authors of assemblers categorize statements and in the nomenclature that they use. In particular, some describe anything other than a machine mnemonic or extended mnemonic as a pseudo-operation . A typical assembly language consists of 3 types of instruction statements that are used to define program operations:"
+1867,"Instructions  in assembly language are generally very simple, unlike those in high-level languages. Generally, a mnemonic is a symbolic name for a single executable machine language instruction , and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of an operation or opcode plus zero or more operands. Most instructions refer to a single value or a pair of values.  Operands can be immediate , registers specified in the instruction or implied, or the addresses of data located elsewhere in storage. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. Extended mnemonics are often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers use B as an extended mnemonic for BC with a mask of 15 and NOP  for BC with a mask of 0."
+1868,"Extended mnemonics are often used to support specialized uses of instructions, often for purposes not obvious from the instruction name. For example, many CPU's do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. In 8086 CPUs the instruction xchg ax,ax is used for nop, with nop being a pseudo-opcode to encode the instruction xchg ax,ax. Some disassemblers recognize this and will decode the xchg ax,ax instruction as nop. Similarly, IBM assemblers for System/360 and System/370 use the extended mnemonics NOP and NOPR for BC and BCR with zero masks.  For the SPARC architecture, these are known as synthetic instructions."
+1869,"Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers the instruction ld hl,bc is recognized to generate ld l,c followed by ld h,b. These are sometimes known as pseudo-opcodes."
+1870,Mnemonics are arbitrary symbols; in 1985 the IEEE published Standard 694 for a uniform set of mnemonics to be used by all assemblers. The standard has since been withdrawn.
+1871,"There are instructions used to define data elements to hold data and variables.  They define the type of data, the length and the alignment of data. These instructions can also define whether the data is available to outside programs  or only to the program in which the data section is defined. Some assemblers classify these as pseudo-ops."
+1872,"Assembly directives, also called pseudo-opcodes, pseudo-operations or pseudo-ops, are commands given to an assembler ""directing it to perform operations other than assembling instructions"". Directives affect how the assembler operates and ""may affect the object code, the symbol table, the listing file, and the values of internal assembler parameters"". Sometimes the term pseudo-opcode is reserved for directives that generate object code, such as those that generate data."
+1873,"The names of pseudo-ops often start with a dot to distinguish them from machine instructions.  Pseudo-ops can make the assembly of the program dependent on parameters input by a programmer, so that one program can be assembled in different ways, perhaps for different applications. Or, a pseudo-op can be used to manipulate presentation of a program to make it easier to read and maintain. Another common use of pseudo-ops is to reserve storage areas for run-time data and optionally initialize their contents to known values."
+1874,"Symbolic assemblers let programmers associate arbitrary names  with memory locations and various constants. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promoting self-documenting code. In executable code, the name of each subroutine is associated with its entry point, so any calls to a subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support local symbols which are often lexically distinct from normal symbols ."
+1875,"Some assemblers, such as NASM, provide flexible symbol management, letting programmers manage different namespaces, automatically calculate offsets within data structures, and assign labels that refer to literal values or the result of simple computations performed by the assembler. Labels can also be used to initialize constants and variables with relocatable addresses."
+1876,"Assembly languages, like most other computer languages, allow comments to be added to program source code that will be ignored during assembly. Judicious commenting is essential in assembly language programs, as the meaning and purpose of a sequence of binary machine instructions can be difficult to determine. The ""raw""  assembly language generated by compilers or disassemblers is quite difficult to read when changes must be made."
+1877,"Many assemblers support predefined macros, and others support programmer-defined  macros involving sequences of text lines in which variables and constants are embedded. The macro definition is most commonly a mixture of assembler statements, e.g., directives, symbolic machine instructions, and templates for assembler statements. This sequence of text lines may include opcodes or directives. Once a macro has been defined its name may be used in place of a mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them as if they existed in the source code file . Macros in this sense date to IBM autocoders of the 1950s."
+1878,"Macro assemblers typically have directives to, e.g., define macros, define variables, set variables to the result of an arithmetic, logical or string expression, iterate, conditionally generate code. Some of those directives may be restricted to use within a macro definition, e.g., MEXIT in HLASM, while others may be permitted within open code , e.g., AIF and COPY in HLASM."
+1879,"In assembly language, the term ""macro"" represents a more comprehensive concept than it does in some other contexts, such as the pre-processor in the C programming language, where its #define directive typically is used to create short single line macros. Assembler macro instructions, like macros in PL/I and some other languages, can be lengthy ""programs"" by themselves, executed by interpretation by the assembler during assembly."
+1880,"Since macros can have 'short' names but expand to several or indeed many lines of code, they can be used to make assembly language programs appear to be far shorter, requiring fewer lines of source code, as with higher level languages. They can also be used to add higher levels of structure to assembly programs, optionally introduce embedded debugging code via parameters and other similar features."
+1881,"Macro assemblers often allow macros to take parameters. Some assemblers include quite sophisticated macro languages, incorporating such high-level language elements as optional parameters, symbolic variables, conditionals, string manipulation, and arithmetic operations, all usable during the execution of a given macro, and allowing macros to save context or exchange information. Thus a macro might generate numerous assembly language instructions or data definitions, based on the macro arguments. This could be used to generate record-style data structures or ""unrolled"" loops, for example, or could generate entire algorithms based on complex parameters. For instance, a ""sort"" macro could accept the specification of a complex sort key and generate code crafted for that specific key, not needing the run-time tests that would be required for a general procedure interpreting the specification. An organization using assembly language that has been heavily extended using such a macro suite can be considered to be working in a higher-level language since such programmers are not working with a computer's lowest-level conceptual elements. Underlining this point, macros were used to implement an early virtual machine in SNOBOL4 , which was written in the SNOBOL Implementation Language , an assembly language for a virtual machine. The target machine would translate this to its native code using a macro assembler. This allowed a high degree of portability for the time."
+1882,"Macros were used to customize large scale software systems for specific customers in the mainframe era and were also used by customer personnel to satisfy their employers' needs by making specific versions of manufacturer operating systems. This was done, for example, by systems programmers working with IBM's Conversational Monitor System / Virtual Machine  and with IBM's ""real time transaction processing"" add-ons, Customer Information Control System CICS, and ACP/TPF, the airline/financial system that began in the 1970s and still runs many large computer reservation systems  and credit card systems today."
+1883,"It is also possible to use solely the macro processing abilities of an assembler to generate code written in completely different languages, for example, to generate a version of a program in COBOL using a pure macro assembler program containing lines of COBOL code inside assembly time operators instructing the assembler to generate arbitrary code. IBM OS/360 uses macros to perform system generation. The user specifies options by coding a series of assembler macros.  Assembling these macros generates a job stream to build the system, including job control language and utility control statements."
+1884,"This is because, as was realized in the 1960s, the concept of ""macro processing"" is independent of the concept of ""assembly"", the former being in modern terms more word processing, text processing, than generating object code. The concept of macro processing appeared, and appears, in the C programming language, which supports ""preprocessor instructions"" to set variables, and make conditional tests on their values. Unlike certain previous macro processors inside assemblers, the C preprocessor is not Turing-complete because it lacks the ability to either loop or ""go to"", the latter allowing programs to loop."
+1885,"Despite the power of macro processing, it fell into disuse in many high level languages  while remaining a perennial for assemblers."
+1886,"Macro parameter substitution is strictly by name: at macro processing time, the value of a parameter is textually substituted for its name. The most famous class of bugs resulting was the use of a parameter that itself was an expression and not a simple name when the macro writer expected a name. In the macro:"
+1887,"the intention was that the caller would provide the name of a variable, and the ""global"" variable or constant b would be used to multiply ""a"". If foo is called with the parameter a-c, the macro expansion of load a-c*b occurs.  To avoid any possible ambiguity, users of macro processors can parenthesize formal parameters inside macro definitions, or callers can parenthesize the input parameters."
+1888,"Packages of macros have been written providing structured programming elements to encode execution flow. The earliest example of this approach was in the Concept-14 macro set, originally proposed by Harlan Mills , and implemented by Marvin Kessler at IBM's Federal Systems Division, which provided IF/ELSE/ENDIF and similar control flow blocks for OS/360 assembler programs. This was a way to reduce or eliminate the use of GOTO operations in assembly code, one of the main factors causing spaghetti code in assembly language. This approach was widely accepted in the early 1980s . IBM's High Level Assembler Toolkit includes such a macro package."
+1889,"A curious design was A-Natural, a ""stream-oriented"" assembler for 8080/Z80, processors from Whitesmiths Ltd. . The language was classified as an assembler because it worked with raw machine elements such as opcodes, registers, and memory references; but it incorporated an expression syntax to indicate execution order. Parentheses and other special symbols, along with block-oriented structured programming constructs, controlled the sequence of the generated instructions. A-natural was built as the object language of a C compiler, rather than for hand-coding, but its logical syntax won some fans."
+1890,"There has been little apparent demand for more sophisticated assemblers since the decline of large-scale assembly language development. In spite of that, they are still being developed and applied in cases where resource constraints or peculiarities in the target system's architecture prevent the effective use of higher-level languages."
+1891,"Assemblers with a strong macro engine allow structured programming via macros, such as the switch macro provided with the Masm32 package :"
+1892,"Assembly languages were not available at the time when the stored-program computer was introduced. Kathleen Booth ""is credited with inventing assembly language"" based on theoretical work she began in 1947, while working on the ARC2 at Birkbeck, University of London following consultation by Andrew Booth  with mathematician John von Neumann and physicist Herman Goldstine at the Institute for Advanced Study."
+1893,"In late 1948, the Electronic Delay Storage Automatic Calculator  had an assembler  integrated into its bootstrap program. It used one-letter mnemonics developed by David Wheeler, who is credited by the IEEE Computer Society as the creator of the first ""assembler"". Reports on the EDSAC introduced the term ""assembly"" for the process of combining fields into an instruction word. SOAP  was an assembly language for the IBM 650 computer written by Stan Poley in 1955."
+1894,"Assembly languages eliminate much of the error-prone, tedious, and time-consuming first-generation programming needed with the earliest computers, freeing programmers from tedium such as remembering numeric codes and calculating addresses. They were once widely used for all sorts of programming. However, by the late 1950s, their use had largely been supplanted by higher-level languages, in the search for improved programming productivity. Today, assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems ."
+1895,"Numerous programs have been written entirely in assembly language. The Burroughs MCP  was the first computer for which an operating system was not developed entirely in assembly language; it was written in Executive Systems Problem Oriented Language , an Algol dialect. Many commercial applications were written in assembly language as well, including a large amount of the IBM mainframe software written by large corporations. COBOL, FORTRAN and some PL/I eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 1990s."
+1896,"Assembly language has long been the primary development language for 8-bit home computers such Atari 8-bit family, Apple II, MSX, ZX Spectrum, and Commodore 64. Interpreted BASIC dialects on these systems offer insufficient execution speed and insufficient facilities to take full advantage of the available hardware. These systems have severe resource constraints,  idiosyncratic memory and display architectures, and provide limited system services. There are also few high-level language compilers suitable for microcomputer use. Similarly, assembly language is the default choice for 8-bit consoles such as the Atari 2600 and Nintendo Entertainment System."
+1897,"Key software for IBM PC compatibles was written in assembly language, such as MS-DOS, Turbo Pascal, and the Lotus 1-2-3 spreadsheet. As computer speed grew exponentially, assembly language became a tool for speeding up parts of programs, such as the rendering of Doom, rather than a dominant development language. In the 1990s, assembly language was used to get performance out of systems such as the Sega Saturn  and as the primary language for arcade hardware based on the TMS34010 integrated CPU/GPU such as Mortal Kombat and NBA Jam."
+1898,There has been debate over the usefulness and performance of assembly language relative to high-level languages.
+1899,"Although assembly language has specific niche uses where it is important , there are other tools for optimization."
+1900,"As of July 2017, the TIOBE index of programming language popularity ranks assembly language at 11, ahead of Visual Basic, for example. Assembler can be used to optimize for speed or optimize for size. In the case of speed optimization, modern optimizing compilers are claimed to render high-level languages into code that can run as fast as hand-written assembly, despite the counter-examples that can be found. The complexity of modern processors and memory sub-systems makes effective optimization increasingly difficult for compilers, as well as for assembly programmers. Moreover, increasing processor performance has meant that most CPUs sit idle most of the time, with delays caused by predictable bottlenecks such as cache misses, I/O operations and paging. This has made raw code execution speed a non-issue for many programmers."
+1901,There are some situations in which developers might choose to use assembly language:
+1902,"Assembly language is still taught in most computer science and electronic engineering programs. Although few programmers today regularly work with assembly language as a tool, the underlying concepts remain important. Such fundamental topics as binary arithmetic, memory allocation, stack processing, character set encoding, interrupt processing, and compiler design would be hard to study in detail without a grasp of how a computer operates at the hardware level. Since a computer's behavior is fundamentally defined by its instruction set, the logical way to learn such concepts is to study an assembly language. Most modern computers have similar instruction sets. Therefore, studying a single assembly language is sufficient to learn: I) the basic concepts; II) to recognize situations where the use of assembly language might be appropriate; and III) to see how efficient executable code can be created from high-level languages."
+1903,"In neuropsychology, linguistics, and philosophy of language, a natural language or ordinary language is any language that occurs naturally in a human community by a process of use, repetition, and change without conscious planning or premeditation. It can take different forms, namely either a spoken language or a sign language. Natural languages are distinguished from constructed and formal languages such as those used to program computers or to study logic."
+1904,Natural language can be broadly defined as different from
+1905,"All varieties of world languages are natural languages, including those that are associated with linguistic prescriptivism or language regulation.  An official language with a regulating academy such as Standard French, overseen by the Académie Française, is classified as a natural language , as its prescriptive aspects do not make it constructed enough to be a constructed language or controlled enough to be a controlled natural language."
+1906,"Controlled natural languages are subsets of natural languages whose grammars and dictionaries have been restricted in order to reduce ambiguity and complexity. This may be accomplished by decreasing usage of superlative or adverbial forms, or irregular verbs. Typical purposes for developing and implementing a controlled natural language are to aid understanding by non-native speakers or to ease computer processing. An example of a widely-used controlled natural language is Simplified Technical English, which was originally developed for aerospace and avionics industry manuals."
+1907,"Being constructed, International auxiliary languages such as Esperanto and Interlingua are not considered natural languages, with the possible exception of true native speakers of such languages. Natural languages evolve, through fluctuations in vocabulary and syntax, to incrementally improve human communication. In contrast, Esperanto was created by Polish ophthalmologist L. L. Zamenhof in the late 19th century."
+1908,"Some natural languages have become organically ""standardized"" through the synthesis of two or more pre-existing natural languages over a relatively short period of time through the development of a pidgin, which is not considered a language, into a stable creole language. A creole such as Haitian Creole has its own grammar, vocabulary and literature. It is spoken by over 10 million people worldwide and is one of the two official languages of the Republic of Haiti."
+1909,"As of 1996, there were 350 attested families with one or more native speakers of Esperanto. Latino sine flexione, another international auxiliary language, is no longer widely spoken."
+1910,A programming language is a system of notation for writing computer programs.
+1911,"Programming languages are described in terms of their syntax  and semantics , usually defined by a formal language. Languages usually provide features such as a type system, variables and mechanisms for error handling. An implementation of a programming language in the form of a compiler or interpreter allows programs to be executed, either directly or by producing an executable."
+1912,"Computer architecture has strongly influenced the design of programming languages, with the most common type  developed to perform well on the popular von Neumann architecture. While early programming languages were closely tied to the hardware, over time, they have developed more abstraction to hide implementation details for greater simplicity."
+1913,"Thousands of programming languages—often classified as imperative, functional, logic, or object-oriented—have been developed for a wide variety of uses. Many aspects of programming language design involve tradeoffs—for example, exception handling simplifies error handling, but at a performance cost. Programming language theory is the subfield of computer science that studies the design, implementation, analysis, characterization, and classification of programming languages."
+1914,There are a variety of criteria that may be considered when defining what constitutes a programming language.
+1915,"The term computer language is sometimes used interchangeably with programming language. However, the usage of both terms varies among authors, including the exact scope of each. One usage describes programming languages as a subset of computer languages. Similarly, languages used in computing that have a different goal than expressing computer programs are generically designated computer languages. For instance, markup languages are sometimes referred to as computer languages to emphasize that they are not meant to be used for programming.
+One way of classifying computer languages is by the computations they are capable of expressing, as described by the theory of computation. The majority of practical programming languages are Turing complete, and all Turing complete languages can implement the same set of algorithms. ANSI/ISO SQL-92 and Charity are examples of languages that are not Turing complete, yet are often called programming languages. However, some authors restrict the term ""programming language"" to Turing complete languages."
+1916,"Another usage regards programming languages as theoretical constructs for programming abstract machines and computer languages as the subset thereof that runs on physical computers, which have finite hardware resources. John C. Reynolds emphasizes that formal specification languages are just as much programming languages as are the languages intended for execution. He also argues that textual and even graphical input formats that affect the behavior of a computer are programming languages, despite the fact they are commonly not Turing-complete, and remarks that ignorance of programming language concepts is the reason for many flaws in input formats."
+1917,"In most practical contexts, a programming language involves a computer; consequently, programming languages are usually defined and studied this way. Programming languages differ from natural languages in that natural languages are only used for interaction between people, while programming languages also allow humans to communicate instructions to machines."
+1918,"The domain of the language is also worth consideration. Markup languages like XML, HTML, or troff, which define structured data, are not usually considered programming languages. Programming languages may, however, share the syntax with markup languages if a computational semantics is defined. XSLT, for example, is a Turing complete language entirely using XML syntax. Moreover, LaTeX, which is mostly used for structuring documents, also contains a Turing complete subset."
+1919,Programming languages usually contain abstractions for defining and manipulating data structures or controlling the flow of execution. The practical necessity that a programming language supports adequate abstractions is expressed by the abstraction principle. This principle is sometimes formulated as a recommendation to the programmer to make proper use of such abstractions.
+1920,"The first programmable computers were invented at the end of the 1940s, and with them, the first programming languages. The earliest computers were programmed in first-generation programming languages , machine language . This code was very difficult to debug and was not portable between different computer systems. In order to improve the ease of programming, assembly languages  were invented, diverging from the machine language to make programs easier to understand for humans, although they did not increase portability."
+1921,"Initially, hardware resources were scarce and expensive, while human resources were cheaper. Therefore, cumbersome languages that were time-consuming to use, but were closer to the hardware for higher efficiency were favored. The introduction of high-level programming languages —revolutionized programming. These languages abstracted away the details of the hardware, instead being designed to express algorithms that could be understood more easily by humans. For example, arithmetic expressions could now be written in symbolic notation and later translated into machine code that the hardware could execute. In 1957, Fortran  was invented. Often considered the first compiled high-level programming language, Fortran has remained in use into the twenty-first century."
+1922,"Around 1960, the first mainframes—general purpose computers—were developed, although they could only be operated by professionals and the cost was extreme. The data and instructions were input by punch cards, meaning that no input could be added while the program was running. The languages developed at this time therefore are designed for minimal interaction. After the invention of the microprocessor, computers in the 1970s became dramatically cheaper. New computers also allowed more user interaction, which was supported by newer programming languages."
+1923,"Lisp, implemented in 1958, was the first functional programming language. Unlike Fortran, it supports recursion and conditional expressions, and it also introduced dynamic memory management on a heap and automatic garbage collection. For the next decades, Lisp dominated artificial intelligence applications. In 1978, another functional language, ML, introduced inferred types and polymorphic parameters."
+1924,"After ALGOL  was released in 1958 and 1960, it became the standard in computing literature for describing algorithms. Although its commercial success was limited, most popular imperative languages—including C, Pascal, Ada, C++, Java, and C#—are directly or indirectly descended from ALGOL 60. Among its innovations adopted by later programming languages included greater portability and the first use of context-free, BNF grammar. Simula, the first language to support object-oriented programming , also descends from ALGOL and achieved commercial success. C, another ALGOL descendant, has sustained popularity into the twenty-first century. C allows access to lower-level machine operations more than other contemporary languages. Its power and efficiency, generated in part with flexible pointer operations, comes at the cost of making it more difficult to write correct code."
+1925,"Prolog, designed in 1972, was the first logic programming language, communicating with a computer using formal logic notation. With logic programming, the programmer specifies a desired result and allows the interpreter to decide how to achieve it."
+1926,"During the 1980s, the invention of the personal computer transformed the roles for which programming languages were used. New languages introduced in the 1980s included C++, a superset of C that can compile C programs but also supports classes and inheritance. Ada and other new languages introduced support for concurrency. The Japanese government invested heavily into the so-called fifth-generation languages that added support for concurrency to logic programming constructs, but these languages were outperformed by other concurrency-supporting languages."
+1927,"Due to the rapid growth of the Internet and the World Wide Web in the 1990s, new programming languages were introduced to support Web pages and networking. Java, based on C++ and designed for increased portability across systems and security, enjoyed large-scale success because these features are essential for many Internet applications. Another development was that of dynamically typed scripting languages—Python, JavaScript, PHP, and Ruby—designed to quickly produce small programs that coordinate existing applications. Due to their integration with HTML, they have also been used for building web pages hosted on servers."
+1928,"During the 2000s, there was a slowdown in the development of new programming languages that achieved widespread popularity. One innovation was service-oriented programming, designed to exploit distributed systems whose components are connected by a network. Services are similar to objects in object-oriented programming, but run on a separate process. C# and F# cross-pollinated ideas between imperative and functional programming. After 2010, several new languages—Rust, Go, Swift, Zig and Carbon —competed for the performance-critical software for which C had historically been used.Most of the new programming languages uses static typing while a few numbers of new languages use dynamic typing like Ring and Julia."
+1929,"Some of the new programming languages are classified as visual programming languages like Scratch, LabVIEW and PWCT. Also, some of these languages mix between textual and visual programming usage like Ballerina. Also, this trend lead to developing projects that help in developing new VPLs like Blockly by Google.Many game engines like Unreal and Unity added support for visual scripting too."
+1930,All programming languages have some primitive building blocks for the description of data and the processes or transformations applied to them . These primitives are defined by syntactic and semantic rules which describe their structure and meaning respectively.
+1931,"A programming language's surface form is known as its syntax. Most programming languages are purely textual; they use sequences of text including words, numbers, and punctuation, much like written natural languages. On the other hand, some programming languages are more graphical in nature, using visual relationships between symbols to specify a program."
+1932,"The syntax of a language describes the possible combinations of symbols that form a syntactically correct program. The meaning given to a combination of symbols is handled by semantics . Since most languages are textual, this article discusses textual syntax."
+1933,"The programming language syntax is usually defined using a combination of regular expressions  and Backus–Naur form . Below is a simple grammar, based on Lisp:"
+1934,This grammar specifies the following:
+1935,"The following are examples of well-formed token sequences in this grammar: 12345,  and )."
+1936,"Not all syntactically correct programs are semantically correct. Many syntactically correct programs are nonetheless ill-formed, per the language's rules; and may  result in an error on translation or execution. In some cases, such programs may exhibit undefined behavior. Even when a program is well-defined within a language, it may still have a meaning that is not intended by the person who wrote it."
+1937,"Using natural language as an example, it may not be possible to assign a meaning to a grammatically correct sentence or the sentence may be false:"
+1938,"The following C language fragment is syntactically correct, but performs operations that are not semantically defined :"
+1939,"If the type declaration on the first line were omitted, the program would trigger an error on the undefined variable p during compilation. However, the program would still be syntactically correct since type declarations provide only semantic information."
+1940,"The grammar needed to specify a programming language can be classified by its position in the Chomsky hierarchy. The syntax of most programming languages can be specified using a Type-2 grammar, i.e., they are context-free grammars. Some languages, including Perl and Lisp, contain constructs that allow execution during the parsing phase. Languages that have constructs that allow the programmer to alter the behavior of the parser make syntax analysis an undecidable problem, and generally blur the distinction between parsing and execution. In contrast to Lisp's macro system and Perl's BEGIN blocks, which may contain general computations, C macros are merely string replacements and do not require code execution."
+1941,"The term semantics refers to the meaning of languages, as opposed to their form ."
+1942,"Static semantics defines restrictions on the structure of valid texts that are hard or impossible to express in standard syntactic formalisms. For compiled languages, static semantics essentially include those semantic rules that can be checked at compile time. Examples include checking that every identifier is declared before it is used  or that the labels on the arms of a case statement are distinct. Many important restrictions of this type, like checking that identifiers are used in the appropriate context , or that subroutine calls have the appropriate number and type of arguments, can be enforced by defining them as rules in a logic called a type system. Other forms of static analyses like data flow analysis may also be part of static semantics. Programming languages such as Java and C# have definite assignment analysis, a form of data flow analysis, as part of their respective static semantics."
+1943,"Once data has been specified, the machine must be instructed to perform operations on the data. For example, the semantics may define the strategy by which expressions are evaluated to values, or the manner in which control structures conditionally execute statements. The dynamic semantics  of a language defines how and when the various constructs of a language should produce a program behavior. There are many ways of defining execution semantics. Natural language is often used to specify the execution semantics of languages commonly used in practice. A significant amount of academic research goes into formal semantics of programming languages, which allows execution semantics to be specified in a formal manner. Results from this field of research have seen limited application to programming language design and implementation outside academia."
+1944,"A type system defines how a programming language classifies values and expressions into types, how it can manipulate those types and how they interact. The goal of a type system is to verify and usually enforce a certain level of correctness in programs written in that language by detecting certain incorrect operations. Any decidable type system involves a trade-off: while it rejects many incorrect programs, it can also prohibit some correct, albeit unusual programs. In order to bypass this downside, a number of languages have type loopholes, usually unchecked casts that may be used by the programmer to explicitly allow a normally disallowed operation between different types. In most typed languages, the type system is used only to type check programs, but a number of languages, usually functional ones, infer types, relieving the programmer from the need to write type annotations. The formal design and study of type systems is known as type theory."
+1945,"A language is typed if the specification of every operation defines types of data to which the operation is applicable. For example, the data represented by ""this text between the quotes"" is a string, and in many programming languages, dividing a number by a string has no meaning and will not be executed. The invalid operation may be detected when the program is compiled  and will be rejected by the compiler with a compilation error message, or it may be detected while the program is running , resulting in a run-time exception. Many languages allow a function called an exception handler to handle this exception and, for example, always return ""-1"" as the result."
+1946,"A special case of typed languages is the single-typed languages. These are often scripting or markup languages, such as REXX or SGML, and have only one data type–—most commonly character strings which are used for both symbolic and numeric data."
+1947,"In contrast, an untyped language, such as most assembly languages, allows any operation to be performed on any data, generally sequences of bits of various lengths. High-level untyped languages include BCPL, Tcl, and some varieties of Forth."
+1948,"In practice, while few languages are considered typed from the type theory , most modern languages offer a degree of typing. Many production languages provide means to bypass or subvert the type system, trading type safety for finer control over the program's execution ."
+1949,"In static typing, all expressions have their types determined before a program executes, typically at compile-time. For example, 1 and  are integer expressions; they cannot be passed to a function that expects a string or stored in a variable that is defined to hold dates."
+1950,"Statically-typed languages can be either manifestly typed or type-inferred. In the first case, the programmer must explicitly write types at certain textual positions . In the second case, the compiler infers the types of expressions and declarations based on context. Most mainstream statically-typed languages, such as C++, C#, and Java, are manifestly typed. Complete type inference has traditionally been associated with functional languages such as Haskell and ML. However, many manifestly-typed languages support partial type inference; for example, C++, Java, and C# all infer types in certain limited cases. Additionally, some programming languages allow for some types to be automatically converted to other types; for example, an int can be used where the program expects a float."
+1951,"Dynamic typing, also called latent typing, determines the type-safety of operations at run time; in other words, types are associated with run-time values rather than textual expressions. As with type-inferred languages, dynamically-typed languages do not require the programmer to write explicit type annotations on expressions. Among other things, this may permit a single variable to refer to values of different types at different points in the program execution. However, type errors cannot be automatically detected until a piece of code is actually executed, potentially making debugging more difficult. Lisp, Smalltalk, Perl, Python, JavaScript, Ruby, Ring and Julia are all examples of dynamically-typed languages."
+1952,"Weak typing allows a value of one type to be treated as another, for example treating a string as a number. This can occasionally be useful, but it can also allow some kinds of program faults to go undetected at compile time and even at run time."
+1953,Strong typing prevents these program faults. An attempt to perform an operation on the wrong type of value raises an error. Strongly-typed languages are often termed type-safe or safe.
+1954,"An alternative definition for ""weakly typed"" refers to languages, such as Perl, Ring and JavaScript, which permit a large number of implicit type conversions. In JavaScript, for example, the expression 2 * x implicitly converts x to a number, and this conversion succeeds even if x is null, undefined, an Array, or a string of letters. Such implicit conversions are often useful, but they can mask programming errors. Strong and static are now generally considered orthogonal concepts, but usage in the literature differs. Some use the term strongly typed to mean strongly, statically typed, or, even more confusingly, to mean simply statically typed. Thus C has been called both strongly typed and weakly, statically typed."
+1955,"It may seem odd to some professional programmers that C could be ""weakly, statically typed"". However, the use of the generic pointer, the void* pointer, does allow casting pointers to other pointers without needing to do an explicit cast. This is extremely similar to somehow casting an array of bytes to any kind of datatype in C without using an explicit cast, such as  or ."
+1956,"Most programming languages have an associated core library , which is conventionally made available by all implementations of the language. Core libraries typically include definitions for commonly used algorithms, data structures, and mechanisms for input and output."
+1957,"The line between a language and its core library differs from language to language. In some cases, the language designers may treat the library as a separate entity from the language. However, a language's core library is often treated as part of the language by its users, and some language specifications even require that this library be made available in all implementations. Indeed, some languages are designed so that the meanings of certain syntactic constructs cannot even be described without referring to the core library. For example, in Java, a string literal is defined as an instance of the java.lang.String class; similarly, in Smalltalk, an anonymous function expression  constructs an instance of the library's BlockContext class. Conversely, Scheme contains multiple coherent subsets that suffice to construct the rest of the language as library macros, and so the language designers do not even bother to say which portions of the language must be implemented as language constructs, and which must be implemented as parts of a library."
+1958,"In computing, multiple instructions can be executed simultaneously. Many programming languages support instruction-level and subprogram-level concurrency. By the twenty-first century, additional processing power on computers was increasingly coming from the use of additional processors, which requires programmers to design software that makes use of multiple processors simultaneously to achieve improved performance. Interpreted languages such as Python and Ruby do not support the concurrent use of multiple processors. Other programming languages do support managing data shared between different threads by controlling the order of execution of key instructions via the use of semaphores, controlling access to shared data via monitor, or enabling message passing between threads."
+1959,"Many programming languages include exception handlers, a section of code triggered by runtime errors that can deal with them in two main ways:"
+1960,Some programming languages support dedicating a block of code to run regardless of whether an exception occurs before the code is reached; this is called finalization.
+1961,"There is a tradeoff between increased ability to handle exceptions and reduced performance. For example, even though array index errors are common C does not check them for performance reasons.  Although programmers can write code to catch user-defined exceptions, this can clutter a program. Standard libraries in some languages, such as C, use their return values to indicate an exception. Some languages and their compilers have the option of turning on and off error handling capability, either temporarily or permanently."
+1962,"Programming languages share properties with natural languages related to their purpose as vehicles for communication, having a syntactic form separate from its semantics, and showing language families of related languages branching one from another. But as artificial constructs, they also differ in fundamental ways from languages that have evolved through usage. A significant difference is that a programming language can be fully described and studied in its entirety since it has a precise and finite definition. By contrast, natural languages have changing meanings given by their users in different communities. While constructed languages are also artificial languages designed from the ground up with a specific purpose, they lack the precise and complete semantic definition that a programming language has."
+1963,"Many programming languages have been designed from scratch, altered to meet new needs, and combined with other languages. Many have eventually fallen into disuse. Although there have been attempts to design one ""universal"" programming language that serves all purposes, all of them have failed to be generally accepted as filling this role. The need for diverse programming languages arises from the diversity of contexts in which languages are used:"
+1964,"One common trend in the development of programming languages has been to add more ability to solve problems using a higher level of abstraction. The earliest programming languages were tied very closely to the underlying hardware of the computer. As new programming languages have developed, features have been added that let programmers express ideas that are more remote from simple translation into underlying hardware instructions. Because programmers are less tied to the complexity of the computer, their programs can do more computing with less effort from the programmer. This lets them write more functionality per time unit."
+1965,"Natural-language programming has been proposed as a way to eliminate the need for a specialized language for programming. However, this goal remains distant and its benefits are open to debate. Edsger W. Dijkstra took the position that the use of a formal language is essential to prevent the introduction of meaningless constructs, and dismissed natural-language programming as ""foolish"". Alan Perlis was similarly dismissive of the idea. Hybrid approaches have been taken in Structured English and SQL."
+1966,A language's designers and users must construct a number of artifacts that govern and enable the practice of programming. The most important of these artifacts are the language specification and implementation.
+1967,"The specification of a programming language is an artifact that the language users and the implementors can use to agree upon whether a piece of source code is a valid program in that language, and if so what its behavior shall be."
+1968,"A programming language specification can take several forms, including the following:"
+1969,"An implementation of a programming language provides a way to write programs in that language and execute them on one or more configurations of hardware and software. There are, broadly, two approaches to programming language implementation: compilation and interpretation. It is generally possible to implement a language using either technique."
+1970,"The output of a compiler may be executed by hardware or a program called an interpreter. In some implementations that make use of the interpreter approach, there is no distinct boundary between compiling and interpreting. For instance, some implementations of BASIC compile and then execute the source one line at a time."
+1971,Programs that are executed directly on the hardware usually run much faster than those that are interpreted in software.
+1972,"One technique for improving the performance of interpreted programs is just-in-time compilation. Here the virtual machine, just before execution, translates the blocks of bytecode which are going to be used to machine code, for direct execution on the hardware."
+1973,"Although most of the most commonly used programming languages have fully open specifications and implementations, many programming languages exist only as proprietary programming languages with the implementation available only from a single vendor, which may claim that such a proprietary language is their intellectual property. Proprietary programming languages are commonly domain-specific languages or internal scripting languages for a single product; some proprietary languages are used only internally within a vendor, while others are available to external users."
+1974,"Some programming languages exist on the border between proprietary and open; for example, Oracle Corporation asserts proprietary rights to some aspects of the Java programming language, and Microsoft's C# programming language, which has open implementations of most parts of the system, also has Common Language Runtime  as a closed environment."
+1975,"Many proprietary languages are widely used, in spite of their proprietary nature; examples include MATLAB, VBScript, and Wolfram Language. Some languages may make the transition from closed to open; for example, Erlang was originally Ericsson's internal programming language."
+1976,"Open source programming languages are particularly helpful for open science applications, enhancing the capacity for replication and code sharing."
+1977,"Thousands of different programming languages have been created, mainly in the computing field.
+Individual software projects commonly use five programming languages or more."
+1978,"Programming languages differ from most other forms of human expression in that they require a greater degree of precision and completeness. When using a natural language to communicate with other people, human authors and speakers can be ambiguous and make small errors, and still expect their intent to be understood. However, figuratively speaking, computers ""do exactly what they are told to do"", and cannot ""understand"" what code the programmer intended to write. The combination of the language definition, a program, and the program's inputs must fully specify the external behavior that occurs when the program is executed, within the domain of control of that program. On the other hand, ideas about an algorithm can be communicated to humans without the precision required for execution by using pseudocode, which interleaves natural language with code written in a programming language."
+1979,"A programming language provides a structured mechanism for defining pieces of data, and the operations or transformations that may be carried out automatically on that data. A programmer uses the abstractions present in the language to represent the concepts involved in a computation. These concepts are represented as a collection of the simplest elements available . Programming is the process by which programmers combine these primitives to compose new programs, or adapt existing ones to new uses or a changing environment."
+1980,"Programs for a computer might be executed in a batch process without human interaction, or a user might type commands in an interactive session of an interpreter. In this case the ""commands"" are simply programs, whose execution is chained together. When a language can run its commands through an interpreter , without compiling, it is called a scripting language."
+1981,"Determining which is the most widely used programming language is difficult since the definition of usage varies by context. One language may occupy the greater number of programmer hours, a different one has more lines of code, and a third may consume the most CPU time. Some languages are very popular for particular kinds of applications. For example, COBOL is still strong in the corporate data center, often on large mainframes; Fortran in scientific and engineering applications; Ada in aerospace, transportation, military, real-time, and embedded applications; and C in embedded applications and operating systems. Other languages are regularly used to write many different kinds of applications."
+1982,"Various methods of measuring language popularity, each subject to a different bias over what is measured, have been proposed:"
+1983,"Combining and averaging information from various internet sites, stackify.com reported the ten most popular programming languages : Java, C, C++, Python, C#, JavaScript, VB .NET, R, PHP, and MATLAB."
+1984,"As of February 2024, the top five programming languages as measured by TIOBE index are Python, C, C++, Java and C#. TIOBE provide a list of top 100 programming languages according to popularity and update this list every month."
+1985,"A dialect of a programming language or a data exchange language is a  variation or extension of the language that does not change its intrinsic nature. With languages such as Scheme and Forth, standards may be considered insufficient, inadequate, or illegitimate by implementors, so often they will deviate from the standard, making a new dialect. In other cases, a dialect is created for use in a domain-specific language, often a subset. In the Lisp world, most languages that use basic S-expression syntax and Lisp-like semantics are considered Lisp dialects, although they vary wildly as do, say, Racket and Clojure. As it is common for one language to have several dialects, it can become quite difficult for an inexperienced programmer to find the right documentation. The BASIC language has many dialects."
+1986,"Programming languages are often placed into four main categories: imperative, functional, logic, and object oriented."
+1987,"Although markup languages are not programming languages, some have extensions that support limited programming. Additionally, there are special-purpose languages that are not easily compared to other programming languages."
+1988,"In computer programming,  assembly language , often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence between the instructions in the language and the architecture's machine code instructions. Assembly language usually has one statement per machine instruction , but  constants, comments, assembler directives, symbolic labels of, e.g., memory locations, registers, and macros are generally also supported."
+1989,"The first assembly code in which a language is used to represent machine code instructions is found in Kathleen and Andrew Donald Booth's 1947 work, Coding for A.R.C.. Assembly code is converted into executable machine code by a utility program referred to as an assembler. The term ""assembler"" is generally attributed to Wilkes, Wheeler and Gill in their 1951 book The Preparation of Programs for an Electronic Digital Computer, who, however, used the term to mean ""a program that assembles another program consisting of several sections into a single program"". The conversion process is referred to as assembly, as in assembling the source code. The computational step when an assembler is processing a program is called assembly time."
+1990,"Because assembly depends on the machine code instructions, each assembly language is specific to a particular computer architecture."
+1991,"Sometimes there is more than one assembler for the same architecture, and sometimes an assembler is specific to an operating system or to particular operating systems. Most assembly languages do not provide specific syntax for operating system calls, and most assembly languages can be used universally with any operating system, as the language provides access to all the real capabilities of the processor, upon which all system call mechanisms ultimately rest. In contrast to assembly languages, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling, much more complicated tasks than assembling."
+1992,"In the first decades of computing, it was commonplace for both systems programming and application programming to take place entirely in assembly language.  While still irreplaceable for some purposes, the majority of programming is now conducted in higher-level interpreted and compiled languages.  In ""No Silver Bullet"", Fred Brooks summarised the effects of the switch away from assembly language programming: ""Surely the most powerful stroke for software productivity, reliability, and simplicity has been the progressive use of high-level languages for programming. Most observers credit that development with at least a factor of five in productivity, and with concomitant gains in reliability, simplicity, and comprehensibility."""
+1993,"Today, it is typical to use small amounts of assembly language code within larger systems implemented in a higher-level language, for performance reasons or to interact directly with hardware in ways unsupported by the higher-level language.  For instance, just under 2% of version 4.9 of the Linux kernel source code is written in assembly; more than 97% is written in C."
+1994,"Assembly language uses a mnemonic to represent, e.g., each low-level machine instruction or opcode, each directive, typically also each architectural register, flag, etc. Some of the mnemonics may be built in and some user defined. Many operations require one or more operands in order to form a complete instruction. Most assemblers permit named constants, registers, and labels for program and memory locations, and can calculate expressions for operands. Thus, programmers are freed from tedious repetitive calculations and assembler programs are much more readable than machine code. Depending on the architecture, these elements may also be combined for specific instructions or addressing modes using offsets or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate program development, to control the assembly process, and to aid debugging."
+1995,"Some are column oriented, with specific fields in specific columns; this was very common for machines using punched cards in the 1950s and early 1960s. Some assemblers have free-form syntax, with fields separated by delimiters, e.g., punctuation, white space. Some assemblers are hybrid, with, e.g., labels, in a specific column and other fields separated by delimiters; this became more common than column oriented syntax in the 1960s."
+1996,"An assembler program creates object code by translating combinations of mnemonics and syntax for operations and addressing modes into their numerical equivalents. This representation typically includes an operation code  as well as other control bits and data. The assembler also calculates constant expressions and resolves symbolic names for memory locations and other entities. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include macro facilities for performing textual substitution – e.g., to generate common short sequences of instructions as inline, instead of called subroutines."
+1997,"Some assemblers may also be able to perform some simple types of instruction set-specific optimizations. One concrete example of this may be the ubiquitous x86 assemblers from various vendors. Called jump-sizing, most of them are able to perform jump-instruction replacements  in any number of passes, on request. Others may even do simple rearrangement or insertion of instructions, such as some assemblers for RISC architectures that can help optimize a sensible instruction scheduling to exploit the CPU pipeline as efficiently as possible."
+1998,"Assemblers have been available since the 1950s, as the first step above machine language and before high-level programming languages such as Fortran, Algol, COBOL and Lisp. There have also been several classes of translators and semi-automatic code generators with properties similar to both assembly and high-level languages, with Speedcode as perhaps one of the better-known examples."
+1999,"There may be several assemblers with different syntax for a particular CPU or instruction set architecture. For instance, an instruction to add memory data to a register in a x86-family processor might be add eax,, in original Intel syntax, whereas this would be written addl ,%eax in the AT&T syntax used by the GNU Assembler. Despite different appearances, different syntactic forms generally generate the same numeric machine code. A single assembler may also have different modes in order to support variations in syntactic forms as well as their exact semantic interpretations ."
+2000,There are two types of assemblers based on how many passes through the source are needed  to produce the object file.
+2001,"In both cases, the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of subsequent symbols. This means that if the size of an operation referring to an operand defined later depends on the type or distance of the operand, the assembler will make a pessimistic estimate when first encountering the operation, and if necessary, pad it with one or more
+""no-operation"" instructions in a later pass or the errata. In an assembler with peephole optimization, addresses may be recalculated between passes to allow replacing pessimistic code with code tailored to the exact distance from the target."
+2002,"The original reason for the use of one-pass assemblers was memory size and speed of assembly – often a second pass would require storing the symbol table in memory , rewinding and rereading the program source on tape, or rereading a deck of cards or punched paper tape. Later computers with much larger memories , had the space to perform all necessary processing without such re-reading. The advantage of the multi-pass assembler is that the absence of errata makes the linking process  faster."
+2003,"Example: in the following code snippet, a one-pass assembler would be able to determine the address of the backward reference BKWD when assembling statement S2, but would not be able to determine the address of the forward reference FWD when assembling the branch statement S1; indeed, FWD may be undefined. A two-pass assembler would determine both addresses in pass 1, so they would be known when generating code in pass 2."
+2004,More sophisticated high-level assemblers provide language abstractions such as:
+2005,See Language design below for more details.
+2006,"A program written in assembly language consists of a series of mnemonic processor instructions and meta-statements , comments and data. Assembly language instructions usually consist of an opcode mnemonic followed by an operand, which might be a list of data, arguments or parameters.  Some instructions may be ""implied"", which means the data upon which the instruction operates is implicitly defined by the instruction itself—such an instruction does not take an operand.  The resulting statement is translated by an assembler into machine language instructions that can be loaded into memory and executed."
+2007,"For example, the instruction below tells an x86/IA-32 processor to move an immediate 8-bit value into a register. The binary code for this instruction is 10110 followed by a 3-bit identifier for which register to use. The identifier for the AL register is 000, so the following machine code loads the AL register with the data 01100001."
+2008,This binary computer code can be made more human-readable by expressing it in hexadecimal as follows.
+2009,"Here, B0 means 'Move a copy of the following value into AL, and 61 is a hexadecimal representation of the value 01100001, which is 97 in decimal. Assembly language for the 8086 family provides the mnemonic MOV  for instructions such as this, so the machine code above can be written as follows in assembly language, complete with an explanatory comment if required, after the semicolon. This is much easier to read and to remember."
+2010,"In some assembly languages  the same mnemonic, such as MOV, may be used for a family of related instructions for loading, copying and moving data, whether these are immediate values, values in registers, or memory locations pointed to by values in registers or by immediate  addresses.  Other assemblers may use separate opcode mnemonics such as L for ""move memory to register"", ST for ""move register to memory"", LR for ""move register to register"", MVI for ""move immediate operand to memory"", etc."
+2011,"If the same mnemonic is used for different instructions, that means that the mnemonic corresponds to several different binary instruction codes, excluding data , depending on the operands that follow the mnemonic.  For example, for the x86/IA-32 CPUs, the Intel assembly language syntax MOV AL, AH represents an instruction that moves the contents of register AH into register AL. The hexadecimal form of this instruction is:"
+2012,"The first byte, 88h, identifies a move between a byte-sized register and either another register or memory, and the second byte, E0h, is encoded  to specify that both operands are registers, the source is AH, and the destination is AL."
+2013,"In a case like this where the same mnemonic can represent more than one binary instruction, the assembler determines which instruction to generate by examining the operands.  In the first example, the operand 61h is a valid hexadecimal numeric constant and is not a valid register name, so only the B0 instruction can be applicable.  In the second example, the operand AH is a valid register name and not a valid numeric constant , so only the 88 instruction can be applicable."
+2014,"Assembly languages are always designed so that this sort of lack of ambiguity is universally enforced by their syntax.  For example, in the Intel x86 assembly language, a hexadecimal constant must start with a numeral digit, so that the hexadecimal number 'A'  would be written as 0Ah or 0AH, not AH, specifically so that it cannot appear to be the name of register AH.  "
+2015,"Returning to the original example, while the x86 opcode 10110000  copies an 8-bit value into the AL register, 10110001  moves it into CL and 10110010  does so into DL. Assembly language examples for these follow."
+2016,The syntax of MOV can also be more complex as the following examples show.
+2017,"In each case, the MOV mnemonic is translated directly into one of the opcodes 88-8C, 8E, A0-A3, B0-BF, C6 or C7 by an assembler, and the programmer normally does not have to know or remember which."
+2018,"Transforming assembly language into machine code is the job of an assembler, and the reverse can at least partially be achieved by a disassembler. Unlike high-level languages, there is a one-to-one correspondence between many simple assembly statements and machine language instructions. However, in some cases, an assembler may provide pseudoinstructions  which expand into several machine language instructions to provide commonly needed functionality. For example, for a machine that lacks a ""branch if greater or equal"" instruction, an assembler may provide a pseudoinstruction that expands to the machine's ""set if less than"" and ""branch if zero "". Most full-featured assemblers also provide a rich macro language  which is used by vendors and programmers to generate more complex code and data sequences. Since the information about pseudoinstructions and macros defined in the assembler environment is not present in the object program, a disassembler cannot reconstruct the macro and pseudoinstruction invocations but can only disassemble the actual machine instructions that the assembler generated from those abstract assembly-language entities. Likewise, since comments in the assembly language source file are ignored by the assembler and have no effect on the object code it generates, a disassembler is always completely unable to recover source comments."
+2019,"Each computer architecture has its own machine language.  Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. While most general-purpose computers are able to carry out essentially the same functionality, the ways they do so differ; the corresponding assembly languages reflect these differences."
+2020,"Multiple sets of mnemonics or assembly-language syntax may exist for a single instruction set, typically instantiated in different assembler programs. In these cases, the most popular one is usually that supplied by the CPU manufacturer and used in its documentation."
+2021,"Two examples of CPUs that have two different sets of mnemonics are the Intel 8080 family and the Intel 8086/8088.  Because Intel claimed copyright on its assembly language mnemonics , some companies that independently produced CPUs compatible with Intel instruction sets invented their own mnemonics.  The Zilog Z80 CPU, an enhancement of the Intel 8080A, supports all the 8080A instructions plus many more; Zilog invented an entirely new assembly language, not only for the new instructions but also for all of the 8080A instructions.  For example, where Intel uses the mnemonics MOV, MVI, LDA, STA, LXI, LDAX, STAX, LHLD, and SHLD for various data transfer instructions, the Z80 assembly language uses the mnemonic LD for all of them.  A similar case is the NEC V20 and V30 CPUs, enhanced copies of the Intel 8086 and 8088, respectively.  Like Zilog with the Z80, NEC invented new mnemonics for all of the 8086 and 8088 instructions, to avoid accusations of infringement of Intel's copyright.    It is doubtful whether in practice many people who programmed the V20 and V30 actually wrote in NEC's assembly language rather than Intel's; since any two assembly languages for the same instruction set architecture are isomorphic , there is no requirement to use a manufacturer's own published assembly language with that manufacturer's products."
+2022,"There is a large degree of diversity in the way the authors of assemblers categorize statements and in the nomenclature that they use. In particular, some describe anything other than a machine mnemonic or extended mnemonic as a pseudo-operation . A typical assembly language consists of 3 types of instruction statements that are used to define program operations:"
+2023,"Instructions  in assembly language are generally very simple, unlike those in high-level languages. Generally, a mnemonic is a symbolic name for a single executable machine language instruction , and there is at least one opcode mnemonic defined for each machine language instruction. Each instruction typically consists of an operation or opcode plus zero or more operands. Most instructions refer to a single value or a pair of values.  Operands can be immediate , registers specified in the instruction or implied, or the addresses of data located elsewhere in storage. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. Extended mnemonics are often used to specify a combination of an opcode with a specific operand, e.g., the System/360 assemblers use B as an extended mnemonic for BC with a mask of 15 and NOP  for BC with a mask of 0."
+2024,"Extended mnemonics are often used to support specialized uses of instructions, often for purposes not obvious from the instruction name. For example, many CPU's do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. In 8086 CPUs the instruction xchg ax,ax is used for nop, with nop being a pseudo-opcode to encode the instruction xchg ax,ax. Some disassemblers recognize this and will decode the xchg ax,ax instruction as nop. Similarly, IBM assemblers for System/360 and System/370 use the extended mnemonics NOP and NOPR for BC and BCR with zero masks.  For the SPARC architecture, these are known as synthetic instructions."
+2025,"Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. For instance, with some Z80 assemblers the instruction ld hl,bc is recognized to generate ld l,c followed by ld h,b. These are sometimes known as pseudo-opcodes."
+2026,Mnemonics are arbitrary symbols; in 1985 the IEEE published Standard 694 for a uniform set of mnemonics to be used by all assemblers. The standard has since been withdrawn.
+2027,"There are instructions used to define data elements to hold data and variables.  They define the type of data, the length and the alignment of data. These instructions can also define whether the data is available to outside programs  or only to the program in which the data section is defined. Some assemblers classify these as pseudo-ops."
+2028,"Assembly directives, also called pseudo-opcodes, pseudo-operations or pseudo-ops, are commands given to an assembler ""directing it to perform operations other than assembling instructions"". Directives affect how the assembler operates and ""may affect the object code, the symbol table, the listing file, and the values of internal assembler parameters"". Sometimes the term pseudo-opcode is reserved for directives that generate object code, such as those that generate data."
+2029,"The names of pseudo-ops often start with a dot to distinguish them from machine instructions.  Pseudo-ops can make the assembly of the program dependent on parameters input by a programmer, so that one program can be assembled in different ways, perhaps for different applications. Or, a pseudo-op can be used to manipulate presentation of a program to make it easier to read and maintain. Another common use of pseudo-ops is to reserve storage areas for run-time data and optionally initialize their contents to known values."
+2030,"Symbolic assemblers let programmers associate arbitrary names  with memory locations and various constants. Usually, every constant and variable is given a name so instructions can reference those locations by name, thus promoting self-documenting code. In executable code, the name of each subroutine is associated with its entry point, so any calls to a subroutine can use its name. Inside subroutines, GOTO destinations are given labels. Some assemblers support local symbols which are often lexically distinct from normal symbols ."
+2031,"Some assemblers, such as NASM, provide flexible symbol management, letting programmers manage different namespaces, automatically calculate offsets within data structures, and assign labels that refer to literal values or the result of simple computations performed by the assembler. Labels can also be used to initialize constants and variables with relocatable addresses."
+2032,"Assembly languages, like most other computer languages, allow comments to be added to program source code that will be ignored during assembly. Judicious commenting is essential in assembly language programs, as the meaning and purpose of a sequence of binary machine instructions can be difficult to determine. The ""raw""  assembly language generated by compilers or disassemblers is quite difficult to read when changes must be made."
+2033,"Many assemblers support predefined macros, and others support programmer-defined  macros involving sequences of text lines in which variables and constants are embedded. The macro definition is most commonly a mixture of assembler statements, e.g., directives, symbolic machine instructions, and templates for assembler statements. This sequence of text lines may include opcodes or directives. Once a macro has been defined its name may be used in place of a mnemonic. When the assembler processes such a statement, it replaces the statement with the text lines associated with that macro, then processes them as if they existed in the source code file . Macros in this sense date to IBM autocoders of the 1950s."
+2034,"Macro assemblers typically have directives to, e.g., define macros, define variables, set variables to the result of an arithmetic, logical or string expression, iterate, conditionally generate code. Some of those directives may be restricted to use within a macro definition, e.g., MEXIT in HLASM, while others may be permitted within open code , e.g., AIF and COPY in HLASM."
+2035,"In assembly language, the term ""macro"" represents a more comprehensive concept than it does in some other contexts, such as the pre-processor in the C programming language, where its #define directive typically is used to create short single line macros. Assembler macro instructions, like macros in PL/I and some other languages, can be lengthy ""programs"" by themselves, executed by interpretation by the assembler during assembly."
+2036,"Since macros can have 'short' names but expand to several or indeed many lines of code, they can be used to make assembly language programs appear to be far shorter, requiring fewer lines of source code, as with higher level languages. They can also be used to add higher levels of structure to assembly programs, optionally introduce embedded debugging code via parameters and other similar features."
+2037,"Macro assemblers often allow macros to take parameters. Some assemblers include quite sophisticated macro languages, incorporating such high-level language elements as optional parameters, symbolic variables, conditionals, string manipulation, and arithmetic operations, all usable during the execution of a given macro, and allowing macros to save context or exchange information. Thus a macro might generate numerous assembly language instructions or data definitions, based on the macro arguments. This could be used to generate record-style data structures or ""unrolled"" loops, for example, or could generate entire algorithms based on complex parameters. For instance, a ""sort"" macro could accept the specification of a complex sort key and generate code crafted for that specific key, not needing the run-time tests that would be required for a general procedure interpreting the specification. An organization using assembly language that has been heavily extended using such a macro suite can be considered to be working in a higher-level language since such programmers are not working with a computer's lowest-level conceptual elements. Underlining this point, macros were used to implement an early virtual machine in SNOBOL4 , which was written in the SNOBOL Implementation Language , an assembly language for a virtual machine. The target machine would translate this to its native code using a macro assembler. This allowed a high degree of portability for the time."
+2038,"Macros were used to customize large scale software systems for specific customers in the mainframe era and were also used by customer personnel to satisfy their employers' needs by making specific versions of manufacturer operating systems. This was done, for example, by systems programmers working with IBM's Conversational Monitor System / Virtual Machine  and with IBM's ""real time transaction processing"" add-ons, Customer Information Control System CICS, and ACP/TPF, the airline/financial system that began in the 1970s and still runs many large computer reservation systems  and credit card systems today."
+2039,"It is also possible to use solely the macro processing abilities of an assembler to generate code written in completely different languages, for example, to generate a version of a program in COBOL using a pure macro assembler program containing lines of COBOL code inside assembly time operators instructing the assembler to generate arbitrary code. IBM OS/360 uses macros to perform system generation. The user specifies options by coding a series of assembler macros.  Assembling these macros generates a job stream to build the system, including job control language and utility control statements."
+2040,"This is because, as was realized in the 1960s, the concept of ""macro processing"" is independent of the concept of ""assembly"", the former being in modern terms more word processing, text processing, than generating object code. The concept of macro processing appeared, and appears, in the C programming language, which supports ""preprocessor instructions"" to set variables, and make conditional tests on their values. Unlike certain previous macro processors inside assemblers, the C preprocessor is not Turing-complete because it lacks the ability to either loop or ""go to"", the latter allowing programs to loop."
+2041,"Despite the power of macro processing, it fell into disuse in many high level languages  while remaining a perennial for assemblers."
+2042,"Macro parameter substitution is strictly by name: at macro processing time, the value of a parameter is textually substituted for its name. The most famous class of bugs resulting was the use of a parameter that itself was an expression and not a simple name when the macro writer expected a name. In the macro:"
+2043,"the intention was that the caller would provide the name of a variable, and the ""global"" variable or constant b would be used to multiply ""a"". If foo is called with the parameter a-c, the macro expansion of load a-c*b occurs.  To avoid any possible ambiguity, users of macro processors can parenthesize formal parameters inside macro definitions, or callers can parenthesize the input parameters."
+2044,"Packages of macros have been written providing structured programming elements to encode execution flow. The earliest example of this approach was in the Concept-14 macro set, originally proposed by Harlan Mills , and implemented by Marvin Kessler at IBM's Federal Systems Division, which provided IF/ELSE/ENDIF and similar control flow blocks for OS/360 assembler programs. This was a way to reduce or eliminate the use of GOTO operations in assembly code, one of the main factors causing spaghetti code in assembly language. This approach was widely accepted in the early 1980s . IBM's High Level Assembler Toolkit includes such a macro package."
+2045,"A curious design was A-Natural, a ""stream-oriented"" assembler for 8080/Z80, processors from Whitesmiths Ltd. . The language was classified as an assembler because it worked with raw machine elements such as opcodes, registers, and memory references; but it incorporated an expression syntax to indicate execution order. Parentheses and other special symbols, along with block-oriented structured programming constructs, controlled the sequence of the generated instructions. A-natural was built as the object language of a C compiler, rather than for hand-coding, but its logical syntax won some fans."
+2046,"There has been little apparent demand for more sophisticated assemblers since the decline of large-scale assembly language development. In spite of that, they are still being developed and applied in cases where resource constraints or peculiarities in the target system's architecture prevent the effective use of higher-level languages."
+2047,"Assemblers with a strong macro engine allow structured programming via macros, such as the switch macro provided with the Masm32 package :"
+2048,"Assembly languages were not available at the time when the stored-program computer was introduced. Kathleen Booth ""is credited with inventing assembly language"" based on theoretical work she began in 1947, while working on the ARC2 at Birkbeck, University of London following consultation by Andrew Booth  with mathematician John von Neumann and physicist Herman Goldstine at the Institute for Advanced Study."
+2049,"In late 1948, the Electronic Delay Storage Automatic Calculator  had an assembler  integrated into its bootstrap program. It used one-letter mnemonics developed by David Wheeler, who is credited by the IEEE Computer Society as the creator of the first ""assembler"". Reports on the EDSAC introduced the term ""assembly"" for the process of combining fields into an instruction word. SOAP  was an assembly language for the IBM 650 computer written by Stan Poley in 1955."
+2050,"Assembly languages eliminate much of the error-prone, tedious, and time-consuming first-generation programming needed with the earliest computers, freeing programmers from tedium such as remembering numeric codes and calculating addresses. They were once widely used for all sorts of programming. However, by the late 1950s, their use had largely been supplanted by higher-level languages, in the search for improved programming productivity. Today, assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device drivers, low-level embedded systems, and real-time systems ."
+2051,"Numerous programs have been written entirely in assembly language. The Burroughs MCP  was the first computer for which an operating system was not developed entirely in assembly language; it was written in Executive Systems Problem Oriented Language , an Algol dialect. Many commercial applications were written in assembly language as well, including a large amount of the IBM mainframe software written by large corporations. COBOL, FORTRAN and some PL/I eventually displaced much of this work, although a number of large organizations retained assembly-language application infrastructures well into the 1990s."
+2052,"Assembly language has long been the primary development language for 8-bit home computers such Atari 8-bit family, Apple II, MSX, ZX Spectrum, and Commodore 64. Interpreted BASIC dialects on these systems offer insufficient execution speed and insufficient facilities to take full advantage of the available hardware. These systems have severe resource constraints,  idiosyncratic memory and display architectures, and provide limited system services. There are also few high-level language compilers suitable for microcomputer use. Similarly, assembly language is the default choice for 8-bit consoles such as the Atari 2600 and Nintendo Entertainment System."
+2053,"Key software for IBM PC compatibles was written in assembly language, such as MS-DOS, Turbo Pascal, and the Lotus 1-2-3 spreadsheet. As computer speed grew exponentially, assembly language became a tool for speeding up parts of programs, such as the rendering of Doom, rather than a dominant development language. In the 1990s, assembly language was used to get performance out of systems such as the Sega Saturn  and as the primary language for arcade hardware based on the TMS34010 integrated CPU/GPU such as Mortal Kombat and NBA Jam."
+2054,There has been debate over the usefulness and performance of assembly language relative to high-level languages.
+2055,"Although assembly language has specific niche uses where it is important , there are other tools for optimization."
+2056,"As of July 2017, the TIOBE index of programming language popularity ranks assembly language at 11, ahead of Visual Basic, for example. Assembler can be used to optimize for speed or optimize for size. In the case of speed optimization, modern optimizing compilers are claimed to render high-level languages into code that can run as fast as hand-written assembly, despite the counter-examples that can be found. The complexity of modern processors and memory sub-systems makes effective optimization increasingly difficult for compilers, as well as for assembly programmers. Moreover, increasing processor performance has meant that most CPUs sit idle most of the time, with delays caused by predictable bottlenecks such as cache misses, I/O operations and paging. This has made raw code execution speed a non-issue for many programmers."
+2057,There are some situations in which developers might choose to use assembly language:
+2058,"Assembly language is still taught in most computer science and electronic engineering programs. Although few programmers today regularly work with assembly language as a tool, the underlying concepts remain important. Such fundamental topics as binary arithmetic, memory allocation, stack processing, character set encoding, interrupt processing, and compiler design would be hard to study in detail without a grasp of how a computer operates at the hardware level. Since a computer's behavior is fundamentally defined by its instruction set, the logical way to learn such concepts is to study an assembly language. Most modern computers have similar instruction sets. Therefore, studying a single assembly language is sufficient to learn: I) the basic concepts; II) to recognize situations where the use of assembly language might be appropriate; and III) to see how efficient executable code can be created from high-level languages."
+2059,"It makes use of elaborative encoding, retrieval cues and imagery as specific tools to encode information in a way that allows for efficient storage and retrieval. It aids original information in becoming associated with something more accessible or meaningful—which in turn provides better retention of the information."
+2060,"Commonly encountered mnemonics are often used for lists and in auditory form such as short poems, acronyms, initialisms or memorable phrases. They can also be used for other types of information and in visual or kinesthetic forms. Their use is based on the observation that the human mind more easily remembers spatial, personal, surprising, physical, sexual, humorous and otherwise ""relatable"" information rather than more abstract or impersonal forms of information."
+2061,"Ancient Greeks and Romans distinguished between two types of memory: the ""natural"" memory and the ""artificial"" memory. The former is inborn and is the one that everyone uses instinctively. The latter in contrast has to be trained and developed through the learning and practice of a variety of mnemonic techniques."
+2062,Mnemonic systems are techniques or strategies consciously used to improve memory. They help use information already stored in long-term memory to make memorization an easier task.
+2063,"Mnemonic is derived from the Ancient Greek word μνημονικός  which means 'of memory' or 'relating to memory'. It is related to Mnemosyne, the name of the goddess of memory in Greek mythology. Both of these words are derived from μνήμη , 'remembrance, memory'. Mnemonics in antiquity were most often considered in the context of what is today known as the art of memory."
+2064,"The general name of mnemonics, or memoria technica, was the name applied to devices for aiding the memory, to enable the mind to reproduce a relatively unfamiliar idea, and especially a series of dissociated ideas, by connecting it, or them, in some artificial whole, the parts of which are mutually suggestive. Mnemonic devices were much cultivated by Greek sophists and philosophers and are frequently referred to by Plato and Aristotle."
+2065,Philosopher Charmadas was famous for his outstanding memory and for his ability to memorize whole books and then recite them.
+2066,"In later times, the poet Simonides was credited for development of these techniques, perhaps for no reason other than that the power of his memory was famous. Cicero, who attaches considerable importance to the art, but more to the principle of order as the best help to memory, speaks of Carneades  of Athens and Metrodorus of Scepsis as distinguished examples of people who used well-ordered images to aid the memory. The Romans valued such helps in order to support facility in public speaking."
+2067,"The Greek and the Roman system of mnemonics was founded on the use of mental places and signs or pictures, known as ""topical"" mnemonics. The most usual method was to choose a large house, of which the apartments, walls, windows, statues, furniture, etc., were each associated with certain names, phrases, events or ideas, by means of symbolic pictures. To recall these, an individual had only to search over the apartments of the house until discovering the places where images had been placed by the imagination."
+2068,"In accordance with this system, if it were desired to fix a historic date in memory, it was localised in an imaginary town divided into a certain number of districts, each with ten houses, each house with ten rooms, and each room with a hundred quadrates or memory-places, partly on the floor, partly on the four walls, partly on the ceiling. Therefore, if it were desired to fix in the memory the date of the invention of printing , an imaginary book, or some other symbol of printing, would be placed in the thirty-sixth quadrate or memory-place of the fourth room of the first house of the historic district of the town. Except that the rules of mnemonics are referred to by Martianus Capella, nothing further is known regarding the practice until the 13th century."
+2069,"Among the voluminous writings of Roger Bacon is a tractate De arte memorativa. Ramon Llull devoted special attention to mnemonics in connection with his ars generalis. The first important modification of the method of the Romans was that invented by the German poet Conrad Celtes, who, in his Epitoma in utramque Ciceronis rhetoricam cum arte memorativa nova , used letters of the alphabet for associations, rather than places. About the end of the 15th century, Peter of Ravenna  provoked such astonishment in Italy by his mnemonic feats that he was believed by many to be a necromancer. His Phoenix artis memoriae  went through as many as nine editions, the seventh being published at Cologne in 1608."
+2070,"About the end of the 16th century, Lambert Schenkel , who taught mnemonics in France, Italy and Germany, similarly surprised people with his memory. He was denounced as a sorcerer by the University of Louvain, but in 1593 he published his tractate De memoria at Douai with the sanction of that celebrated theological faculty. The most complete account of his system is given in two works by his pupil Martin Sommer, published in Venice in 1619. In 1618 John Willis  published Mnemonica; sive ars reminiscendi, containing a clear statement of the principles of topical or local mnemonics. Giordano Bruno included a memoria technica in his treatise De umbris idearum, as part of his study of the ars generalis of Llull. Other writers of this period are the Florentine Publicius ; Johannes Romberch ; Hieronimo Morafiot, Ars memoriae ;and B. Porta, Ars reminiscendi ."
+2071,"In 1648 Stanislaus Mink von Wennsshein revealed what he called the ""most fertile secret"" in mnemonics—using consonants for figures, thus expressing numbers by words , in order to create associations more readily remembered. The philosopher Gottfried Wilhelm Leibniz adopted an alphabet very similar to that of Wennsshein for his scheme of a form of writing common to all languages."
+2072,"Wennsshein's method was adopted with slight changes afterward by the majority of subsequent ""original"" systems. It was modified and supplemented by Richard Grey , a priest who published a Memoria technica in 1730. The principal part of Grey's method is briefly this:"
+2073,"Wennsshein's method is comparable to a Hebrew system by which letters also stand for numerals, and therefore words for dates."
+2074,"To assist in retaining the mnemonical words in the memory, they were formed into memorial lines. Such strange words in difficult hexameter scansion, are by no means easy to memorise. The vowel or consonant, which Grey connected with a particular figure, was chosen arbitrarily."
+2075,"A later modification was made in 1806 Gregor von Feinaigle, a German monk from Salem near Constance. While living and working in Paris, he expounded a system of mnemonics in which  the numerical figures are represented by letters chosen due to some similarity to the figure or an accidental connection with it. This alphabet was supplemented by a complicated system of localities and signs. Feinaigle, who apparently did not publish any written documentation of this method, travelled to England in 1811. The following year one of his pupils published The New Art of Memory , giving Feinaigle's system. In addition, it contains valuable historical material about previous systems."
+2076,"Other mnemonists later published simplified forms, as the more complicated mnemonics were generally abandoned. Methods founded chiefly on the so-called laws of association  were taught with some success in Germany."
+2077,"A wide range of mnemonics are used for several purposes. The most commonly used mnemonics are those for lists, numerical sequences, foreign-language acquisition, and medical treatment for patients with memory deficits."
+2078,A common mnemonic technique for remembering a list is to create an easily remembered acronym. Another is to create a memorable phrase with words which share the same first letter  as the list members. Mnemonic techniques can be applied to most memorization of novel materials.
+2079,Some common examples for first-letter mnemonics:
+2080,"Mnemonic phrases or poems can be used to encode numeric sequences by various methods, one common one is to create a new phrase in which the number of letters in each word represents the according digit of pi. For example, the first 15 digits of the mathematical constant pi  can be encoded as ""Now I need a drink, alcoholic of course, after the heavy lectures involving quantum mechanics""; ""Now"", having 3 letters, represents the first number, 3. Piphilology is the practice dedicated to creating mnemonics for pi."
+2081,"Another is used for ""calculating"" the multiples of 9 up to 9 × 10 using one's fingers. Begin by holding out both hands with all fingers stretched out. Now count left to right the number of fingers that indicates the multiple. For example, to figure 9 × 4, count four fingers from the left, ending at your left-hand index finger. Bend this finger down and count the remaining fingers. Fingers to the left of the bent finger represent tens, fingers to the right are ones. There are three fingers to the left and six to the right, which indicates 9 × 4 = 36. This works for 9 × 1 up through 9 × 10."
+2082,"For remembering the rules in adding and multiplying two signed numbers, Balbuena and Buayan  made the letter strategies LAUS  and LPUN , respectively."
+2083,"PUIMURI  is a Finnish mnemonic regarding electricity: the first and last three letters can be arranged into the equations 
+  
+    
+      
+        P
+        =
+        U
+        ×
+        I
+      
+    
+    {\displaystyle P=U\times I}
+  
+ and 
+  
+    
+      
+        U
+        =
+        R
+        ×
+        I
+      
+    
+    {\displaystyle U=R\times I}
+  
+. "
+2084,"Mnemonics may be helpful in learning foreign languages, for example by transposing difficult foreign words with words in a language the learner knows already, also called ""cognates"" which are very common in Romance languages and other Germanic languages. A useful such technique is to find linkwords, words that have the same pronunciation in a known language as the target word, and associate them visually or auditorially with the target word."
+2085,"For example, in trying to assist the learner to remember ohel , the Hebrew word for tent, the linguist Ghil'ad Zuckermann proposes the memorable sentence ""Oh hell, there's a raccoon in my tent"". The memorable sentence ""There's a fork in Ma's leg"" helps the learner remember that the Hebrew word for fork is mazleg . Similarly, to remember the Hebrew word bayit , meaning house, one can use the sentence ""that's a lovely house, I'd like to buy it."" The linguist Michel Thomas taught students to remember that estar is the Spanish word for to be by using the phrase ""to be a star""."
+2086,"Another Spanish example is by using the mnemonic ""Vin Diesel Has Ten Weapons"" to teach irregular command verbs in the you  form. Spanish verb forms and tenses are regularly seen as the hardest part of learning the language. With a high number of verb tenses, and many verb forms that are not found in English, Spanish verbs can be hard to remember and then conjugate. The use of mnemonics has been proven to help students better learn foreign languages, and this holds true for Spanish verbs. A particularly hard verb tense to remember is command verbs. Command verbs in Spanish are conjugated differently depending on who the command is being given to. The phrase, when pronounced with a Spanish accent, is used to remember ""Ven Di Sal Haz Ten Ve Pon Sé"", all of the irregular Spanish command verbs in the you  form. This mnemonic helps students attempting to memorize different verb tenses.
+Another technique is for learners of gendered languages to associate their mental images of words with a colour that matches the gender in the target language. An example here is to remember the Spanish word for ""foot"", pie,  with the image of a foot stepping on a pie, which then spills blue filling ."
+2087,"For French verbs which use être as an auxiliary verb for compound tenses: DR and MRS VANDERTRAMPP: descendre, rester, monter, revenir, sortir, venir, arriver, naître, devenir, entrer, rentrer, tomber, retourner, aller, mourir, partir, passer."
+2088,"Masculine countries in French : ""Neither can a breeze make a sane Japanese chilly in the USA.""  Netherlands , Canada, Brazil , Mexico , Senegal, Japan , Chile , &  USA ."
+2089,"Mnemonics can be used in aiding patients with memory deficits that could be caused by head injuries, strokes, epilepsy, multiple sclerosis and other neurological conditions."
+2090,"In a study conducted by Doornhein and De Haan, the patients were treated with six different memory strategies including the mnemonics technique. The results concluded that there were significant improvements on the immediate and delayed subtest of the RBMT, delayed recall on the Appointments test, and relatives rating on the MAC from the patients that received mnemonics treatment. However, in the case of stroke patients, the results did not reach statistical significance."
+2091,"Academic study of the use of mnemonics has shown their effectiveness. In one such experiment, subjects of different ages who applied mnemonic techniques to learn novel vocabulary outperformed control groups that applied contextual learning and free-learning styles."
+2092,"Mnemonics were seen to be more effective for groups of people who struggled with or had weak long-term memory, like the elderly. Five years after a mnemonic training study, a research team followed-up 112 community-dwelling older adults, 60 years of age and over. Delayed recall of a word list was assessed prior to, and immediately following mnemonic training, and at the 5-year follow-up. Overall, there was no significant difference between word recall prior to training and that exhibited at follow-up. However, pre-training performance gains scores in performance immediately post-training and use of the mnemonic predicted performance at follow-up. Individuals who self-reported using the mnemonic exhibited the highest performance overall, with scores significantly higher than at pre-training. The findings suggest that mnemonic training has long-term benefits for some older adults, particularly those who continue to employ the mnemonic."
+2093,This contrasts with a study from surveys of medical students that approximately only 20% frequently used mnemonic acronyms.
+2094,"In humans, the process of aging particularly affects the medial temporal lobe and hippocampus, in which the episodic memory is synthesized. The episodic memory stores information about items, objects, or features with spatiotemporal contexts. Since mnemonics aid better in remembering spatial or physical information rather than more abstract forms, its effect may vary according to a subject's age and how well the subject's medial temporal lobe and hippocampus function."
+2095,"This could be further explained by one recent study which indicates a general deficit in the memory for spatial locations in aged adults  compared to young adults . At first, the difference in target recognition was not significant."
+2096,"The researchers then divided the aged adults into two groups, aged unimpaired and aged impaired, according to a neuropsychological testing. With the aged groups split, there was an apparent deficit in target recognition in aged impaired adults compared to both young adults and aged unimpaired adults. This further supports the varying effectiveness of mnemonics in different age groups."
+2097,"Moreover, different research was done previously with the same notion, which presented with similar results to that of Reagh et al. in a verbal mnemonics discrimination task."
+2098,"Studies  have suggested that the short-term memory of adult humans can hold only a limited number of items; grouping items into larger chunks such as in a mnemonic might be part of what permits the retention of a larger total amount of information in short-term memory, which in turn can aid in the creation of long-term memories."
+2099,The dictionary definition of mnemonic at Wiktionary
+2100,"The Harvard Mark I, or IBM Automatic Sequence Controlled Calculator , was one of the earliest general-purpose electromechanical computers used in the war effort during the last part of World War II."
+2101,"One of the first programs to run on the Mark I was initiated on 29 March 1944 by John von Neumann. At that time, von Neumann was working on the Manhattan Project, and needed to determine whether implosion was a viable choice to detonate the atomic bomb that would be used a year later. The Mark I also computed and printed mathematical tables, which had been the initial goal of British inventor Charles Babbage for his ""analytical engine"" in 1837."
+2102,"According to Edmund Berkeley, the operators of the Mark I often called the machine “Bessy, the Bessel engine,” after Bessel functions."
+2103,"The Mark I was disassembled in 1959; part of it was given to IBM, part went to the Smithsonian Institution, and part entered the Harvard Collection of Historical Scientific Instruments. For decades, Harvard's portion was on display in the lobby of the Aiken Computation Lab. About 1997, it was moved to the Harvard Science Center. In 2021, it was moved again, to the lobby of Harvard's new Science and Engineering Complex in Allston, Massachusetts."
+2104,"The original concept was presented to IBM by Howard Aiken in November 1937.  After a feasibility study by IBM engineers, the company chairman Thomas Watson Sr. personally approved the project and its funding in February 1939."
+2105,"Howard Aiken had started to look for a company to design and build his calculator in early 1937.  After two rejections, he was shown a demonstration set that Charles Babbage’s son had given to Harvard University 70 years earlier.  This led him to study Babbage and to add references to the Analytical Engine to his proposal; the resulting machine ""brought Babbage’s principles of the Analytical Engine almost to full realization, while adding important new features."""
+2106,"The ASCC was developed and built by IBM at their Endicott plant and shipped to Harvard in February 1944. It began computations for the US Navy Bureau of Ships in May and was officially presented to the university on August 7, 1944."
+2107,"Although not the first working computer, the machine was the first to automate the execution of complex calculations, making it a significant step forward for computing."
+2108,"The ASCC was built from switches, relays, rotating shafts, and clutches. It used 765,000 electromechanical components and hundreds of miles of wire, comprising a volume of 816 cubic feet  – 51 feet  in length, 8 feet  in height, and 2 feet  deep. It weighed about 9,445 pounds . The basic calculating units had to be synchronized and  powered mechanically, so they were operated by a 50-foot  drive shaft coupled to a 5 horsepower  electric motor, which served as the main power source and system clock. From the IBM Archives:"
+2109,The enclosure for the Mark I was designed by futuristic American industrial designer Norman Bel Geddes at IBM's expense.  Aiken was annoyed that the cost  was not used to build additional computer equipment.
+2110,"The Mark I had 60 sets of 24 switches for manual data entry and could store 72 numbers, each 23 decimal digits long.  It could do 3 additions or subtractions in a second. A multiplication took 6 seconds, a division took 15.3 seconds, and a logarithm or a trigonometric function took over one minute."
+2111,"The Mark I read its instructions from a 24-channel punched paper tape. It executed the current instruction and then read the next one. A separate tape could contain numbers for input, but the tape formats were not interchangeable. Instructions could not be executed from the storage registers. Because instructions were not stored in working memory, it is widely claimed that the Harvard Mark I was the origin of the Harvard architecture. However, this is disputed in The Myth of the Harvard Architecture published in the IEEE Annals of History of Computing, which shows  the term 'Harvard architecture' did not come into use until the 1970s  and was only retrospectively applied to the Harvard machines, and that the term could only be applied to the Mark III and IV, not to the Mark I or II."
+2112,"The main sequence mechanism was unidirectional. This meant that complex programs had to be physically lengthy. A program loop was accomplished by loop unrolling or by joining the end of the paper tape containing the program back to the beginning of the tape . At first, conditional branching in Mark I was performed manually. Later modifications in 1946 introduced automatic program branching . The first programmers of the Mark I were computing pioneers Richard Milton Bloch, Robert Campbell, and Grace Hopper. There was also a small technical team whose assignment was to actually operate the machine; some had been IBM employees before being required to join the Navy to work on the machine. This technical team was not informed of the overall purpose of their work while at Harvard."
+2113,"The 24 channels of the input tape were divided into three fields of eight channels. Each storage location, each set of switches, and the registers associated with the input, output, and arithmetic units were assigned a unique identifying index number. These numbers were represented in binary on the control tape. The first field was the binary index of the result of the operation, the second was the source datum for the operation and the third field was a code for the operation to be performed."
+2114,"In 1928 L.J. Comrie was the first to turn IBM ""punched-card equipment to scientific use: computation of astronomical tables by the method of finite differences, as envisioned by Babbage 100 years earlier for his Difference Engine"". Very soon after, IBM  started to modify its tabulators to facilitate this kind of computation.  One of these tabulators, built in 1931, was The Columbia Difference Tabulator."
+2115,"John von Neumann had a team at Los Alamos that used ""modified IBM punched-card machines"" to determine the effects of the implosion. In March 1944, he proposed to run certain problems regarding implosion of the Mark I, and in 1944 he arrived with two mathematicians to write a simulation program to study the implosion of the first atomic bomb."
+2116,"""Von Neumann joined the Manhattan Project in 1943, working on the immense number of calculations needed to build the atomic bomb. He showed that the implosion design, which would later be used in the Trinity and Fat Man bombs, was likely faster and more efficient than the gun design."""
+2117,"Aiken published a press release announcing the Mark I listing himself as the sole “inventor”. James W. Bryce was the only IBM person mentioned, even though several IBM engineers including Clair Lake and Frank Hamilton had helped to build various elements. IBM chairman Thomas J. Watson was enraged, and only reluctantly attended the dedication ceremony on August 7, 1944. Aiken, in turn, decided to build further machines without IBM's help, and the ASCC came to be generally known as the ""Harvard Mark I"". IBM went on to build its Selective Sequence Electronic Calculator  to both test new technology and provide more publicity for the company's efforts."
+2118,"The Mark I was followed by the Harvard Mark II , Mark III/ADEC , and Harvard Mark IV  – all the work of Aiken. The Mark II was an improvement over the Mark I, although it still was based on electromechanical relays. The Mark III used mostly electronic components—vacuum tubes and crystal diodes—but also included mechanical components: rotating magnetic drums for storage, plus relays for transferring data between drums. The Mark IV was all-electronic, replacing the remaining mechanical components with magnetic core memory. The Mark II and Mark III were delivered to the US Navy base at Dahlgren, Virginia. The Mark IV was built for the US Air Force, but it stayed at Harvard."
+2119,"The Mark I was disassembled in 1959, and portions of it went on display  in the Science Center, as part of the Harvard Collection of Historical Scientific Instruments.  It was relocated to the new Science and Engineering Complex in Allston in July 2021. Other sections of the original machine had much earlier been transferred to IBM and the Smithsonian Institution."
+2120,"In computing, an opcode  is the portion of a machine language instruction that specifies the operation to be performed. Beside the opcode itself, most instructions also specify the data they will process, in the form of operands. In addition to opcodes used in the instruction set architectures of various CPUs, which are hardware devices, they can also be used in abstract computing machines as part of their byte code specifications."
+2121,"Specifications and format of the opcodes are laid out in the instruction set architecture  of the processor in question, which may be a general CPU or a more specialized processing unit. Opcodes for a given instruction set can be described through the use of an opcode table detailing all possible opcodes. Apart from the opcode itself, an instruction normally also has one or more specifiers for operands  on which the operation should act, although some operations may have implicit operands, or none at all. There are instruction sets with nearly uniform fields for opcode and operand specifiers, as well as others  with a more complicated, variable-length structure. Instruction sets can be extended through the use of opcode prefixes which add a subset of new instructions made up of existing opcodes following reserved byte sequences."
+2122,"Depending on architecture, the operands may be register values, values in the stack, other memory values, I/O ports , etc., specified and accessed using more or less complex addressing modes. The types of operations include arithmetic, data copying, logical operations, and program control, as well as special instructions ."
+2123,"Assembly language, or just assembly, is a low-level programming language, which uses mnemonic instructions and operands to represent machine code. This enhances the readability while still giving precise control over the machine instructions. Most programming is currently done using high-level programming languages, which are typically easier for humans to understand and write. These languages need to be compiled  by a system-specific compiler, or run through other compiled programs."
+2124,"Opcodes can also be found in so-called byte codes and other representations intended for a software interpreter rather than a hardware device. These software-based instruction sets often employ slightly higher-level data types and operations than most hardware counterparts, but are nevertheless constructed along similar lines. Examples include the byte code found in Java class files which are then interpreted by the Java Virtual Machine , the byte code used in GNU Emacs for compiled Lisp code, .NET Common Intermediate Language , and many others."
+2125,"In computer programming, machine code is computer code consisting of machine language instructions, which are used to control a computer's central processing unit . Although decimal computers were once common, the contemporary marketplace is dominated by binary computers; for those computers, machine code is ""the binary representation of a computer program which is actually read and interpreted by the computer. A program in machine code consists of a sequence of machine instructions ."""
+2126,"Each instruction causes the CPU to perform a very specific task, such as a load, a store, a jump, or an arithmetic logic unit  operation on one or more units of data in the CPU's registers or memory."
+2127,"Early CPUs had specific machine code that might break backward compatibility with each new CPU released. The notion of an instruction set architecture  defines and specifies the behavior and encoding in memory of the instruction set of the system, without specifying its exact implementation. This acts as an abstraction layer, enabling compatibility within the same family of CPUs, so that machine code written or generated according to the ISA for the family will run on all CPUs in the family, including future CPUs."
+2128,"In general, each architecture family  has its own ISA, and hence its own specific machine code language. There are exceptions, such as the VAX architecture, which included optional support of the PDP-11 instruction set and IA-64, which included optional support of the IA-32 instruction set. Another example is the PowerPC 615, a processor designed to natively process both PowerPC and x86 instructions."
+2129,"Machine code is a strictly numerical language, and is the lowest-level interface to the CPU intended for a programmer. Assembly language provides a direct mapping between the numerical machine code and a human-readable version where numerical opcodes and operands are replaced by readable strings . While it is possible to write programs directly in machine code, managing individual bits and calculating numerical addresses and constants manually is tedious and error-prone. For this reason, programs are very rarely written directly in machine code in modern contexts, but may be done for low-level debugging, program patching  and assembly language disassembly."
+2130,"The majority of practical programs today are written in higher-level languages. Those programs are either translated into machine code by a compiler, or are interpreted by an interpreter, usually after being translated into an intermediate code, such as a bytecode, that is then interpreted."
+2131,"Machine code is by definition the lowest level of programming detail visible to the programmer, but internally many processors use microcode or optimize and transform machine code instructions into sequences of micro-ops. Microcode and micro-ops are not generally considered to be machine code; except on some machines, the user cannot write microcode or micro-ops, and the operation of microcode and the transformation of machine-code instructions into micro-ops happens transparently to the programmer except for performance related side effects."
+2132,"Every processor or processor family has its own instruction set. Instructions are patterns of bits, digits, or characters that correspond to machine commands. Thus, the instruction set is specific to a class of processors using  the same architecture. Successor or derivative processor designs often include instructions of a predecessor and may add new additional instructions. Occasionally, a successor design will discontinue or alter the meaning of some instruction code , affecting code compatibility to some extent; even compatible processors may show slightly different behavior for some instructions, but this is rarely a problem. Systems may also differ in other details, such as memory arrangement, operating systems, or peripheral devices. Because a program normally relies on such factors, different systems will typically not run the same machine code, even when the same type of processor is used."
+2133,"A processor's instruction set may have fixed-length or variable-length instructions. How the patterns are organized varies with the particular architecture and type of instruction. Most instructions have one or more opcode fields that specify the basic instruction type , the operation , and other fields that may give the type of the operand, the addressing mode, the addressing offset or index, or the operand value itself ."
+2134,"Not all machines or individual instructions have explicit operands. On a machine with a single accumulator, the accumulator is implicitly both the left operand and result of most arithmetic instructions. Some other architectures, such as the x86 architecture, have accumulator versions of common instructions, with the accumulator regarded as one of the general registers by longer instructions. A stack machine has most or all of its operands on an implicit stack. Special purpose instructions also often lack explicit operands; for example, CPUID in the x86 architecture writes values into four implicit destination registers. This distinction between explicit and implicit operands is important in code generators, especially in the register allocation and live range tracking parts. A good code optimizer can track implicit as well as explicit operands which may allow more frequent constant propagation, constant folding of registers  and other code enhancements."
+2135,"A computer program is a list of instructions that can be executed by a central processing unit . A program's execution is done in order for the CPU that is executing it to solve a problem and thus accomplish a result. While simple processors are able to execute instructions one after another, superscalar processors are able under certain circumstances  of executing two or more instructions simultaneously. As an example, the original Intel Pentium from 1993 can execute at most two instructions per clock cycle when its pipeline is full."
+2136,"Program flow may be influenced by special 'jump' instructions that transfer execution to an address  other than the next numerically sequential address. Whether these conditional jumps occur is dependent upon a condition such as a value being greater than, less than, or equal to another value."
+2137,"A much more human friendly rendition of machine language, called assembly language, uses mnemonic codes to refer to machine code instructions, rather than using the instructions' numeric values directly, and uses symbolic names to refer to storage locations and sometimes registers. For example, on the Zilog Z80 processor, the machine code 00000101, which causes the CPU to decrement the B general-purpose register, would be represented in assembly language as DEC B."
+2138,"The MIPS architecture provides a specific example for a machine code whose instructions are always 32 bits long.: 299  The general type of instruction is given by the op  field, the highest 6 bits. J-type  and I-type  instructions are fully specified by op. R-type  instructions include an additional field funct to determine the exact operation. The fields used in these types are:"
+2139,"rs, rt, and rd indicate register operands; shamt gives a shift amount; and the address or immediate fields contain an operand directly.: 299–301"
+2140,"For example, adding the registers 1 and 2 and placing the result in register 6 is encoded:: 554"
+2141,"Load a value into register 8, taken from the memory cell 68 cells after the location listed in register 3:: 552"
+2142,Jumping to the address 1024:: 552
+2143,"On processor architectures with variable-length instruction sets  it is, within the limits of the control-flow resynchronizing phenomenon known as the Kruskal count, sometimes possible through opcode-level programming to deliberately arrange the resulting code so that two code paths share a common fragment of opcode sequences. These are called overlapping instructions, overlapping opcodes, overlapping code, overlapped code, instruction scission, or jump into the middle of an instruction, and represent a form of superposition."
+2144,"In the 1970s and 1980s, overlapping instructions were sometimes used to preserve memory space. One example were in the implementation of error tables in Microsoft's Altair BASIC, where interleaved instructions mutually shared their instruction bytes. The technique is rarely used today, but might still be necessary to resort to in areas where extreme optimization for size is necessary on byte-level such as in the implementation of boot loaders which have to fit into boot sectors."
+2145,It is also sometimes used as a code obfuscation technique as a measure against disassembly and tampering.
+2146,The principle is also utilized in shared code sequences of fat binaries which must run on multiple instruction-set-incompatible processor platforms.
+2147,This property is also used to find unintended instructions called gadgets in existing code repositories and is utilized in return-oriented programming as alternative to code injection for exploits such as return-to-libc attacks.
+2148,"In some computers, the machine code of the  architecture is implemented by an even more fundamental underlying layer called microcode, providing a common machine language interface across a line or family of different models of computer with widely different underlying dataflows. This is done to facilitate porting of machine language programs between different models. An example of this use is the IBM System/360 family of computers and their successors. With dataflow path widths of 8 bits to 64 bits and beyond, they nevertheless present a common architecture at the machine language level across the entire line."
+2149,"Using microcode to implement an emulator enables the computer to present the architecture of an entirely different computer. The System/360 line used this to allow porting programs from earlier IBM machines to the new family of computers, e.g. an IBM 1401/1440/1460 emulator on the IBM S/360 model 40."
+2150,"Machine code is generally different from bytecode , which is either executed by an interpreter or itself compiled into machine code for faster  execution. An exception is when a processor is designed to use a particular bytecode directly as its machine code, such as is the case with Java processors."
+2151,Machine code and assembly code are sometimes called native code when referring to platform-dependent parts of language features or libraries.
+2152,"From the point of view of the CPU, machine code is stored in RAM, but is typically also kept in a set of caches for performance reasons. There may be different caches for instructions and data, depending on the architecture."
+2153,"The CPU knows what machine code to execute, based on its internal program counter. The program counter points to a memory address and is changed based on special instructions which may cause programmatic branches. The program counter is typically set to a hard coded value when the CPU is first powered on, and will hence execute whatever machine code happens to be at this address."
+2154,"Similarly, the program counter can be set to execute whatever machine code is at some arbitrary address, even if this is not valid machine code. This will typically trigger an architecture specific protection fault."
+2155,"The CPU is oftentimes told, by page permissions in a paging based system, if the current page actually holds machine code by an execute bit — pages have multiple such permission bits  for various housekeeping functionality. E.g. on Unix-like systems memory pages can be toggled to be executable with the mprotect system call, and on Windows, VirtualProtect can be used to achieve a similar result. If an attempt is made to execute machine code on a non-executable page, an architecture specific fault will typically occur. Treating data as machine code, or finding new ways to use existing machine code, by various techniques, is the basis of some security vulnerabilities."
+2156,"Similarly, in a segment based system, segment descriptors can indicate whether a segment can contain executable code and in what rings that code can run."
+2157,"From the point of view of a process, the code space is the part of its address space where the code in execution is stored. In multitasking systems this comprises the program's code segment and usually shared libraries. In multi-threading environment, different threads of one process share code space along with data space, which reduces the overhead of context switching considerably as compared to process switching."
+2158,"Pamela Samuelson wrote that machine code is so unreadable that the United States Copyright Office cannot identify whether a particular encoded program is an original work of authorship; however, the US Copyright Office does allow for copyright registration of computer programs and a program's machine code can sometimes be decompiled in order to make its functioning more easily understandable to humans. However, the output of a decompiler or disassembler will be missing the comments and symbolic references, so while the output may be easier to read than the object code, it will still be more difficult than the original source code. This problem does not exist for object-code formats like SQUOZE, where the source code is included in the file."
+2159,"Cognitive science professor Douglas Hofstadter has compared machine code to genetic code, saying that ""Looking at a program written in machine language is vaguely comparable to looking at a DNA molecule atom by atom."""
+2160,"MIPS  is a family of reduced instruction set computer  instruction set architectures : A-1 : 19  developed by MIPS Computer Systems, now MIPS Technologies, based in the United States."
+2161,"There are multiple versions of MIPS: including MIPS I, II, III, IV, and V; as well as five releases of MIPS32/64 . The early MIPS architectures were 32-bit; 64-bit versions were developed later. As of April 2017, the current version of MIPS is MIPS32/64 Release 6. MIPS32/64 primarily differs from MIPS I–V by defining the privileged kernel mode System Control Coprocessor in addition to the user mode architecture."
+2162,"The MIPS architecture has several optional extensions. MIPS-3D which is a simple set of floating-point SIMD instructions dedicated to common 3D tasks, MDMX  which is a more extensive integer SIMD instruction set using the 64-bit floating-point registers, MIPS16e which adds compression to the instruction stream to make programs take up less room, and MIPS MT, which adds multithreading capability."
+2163,"Computer architecture courses in universities and technical schools often study the MIPS architecture. The architecture greatly influenced later RISC architectures such as Alpha. In March 2021, MIPS announced that the development of the MIPS architecture had ended as the company is making the transition to RISC-V."
+2164,"The first version of the MIPS architecture was designed by MIPS Computer Systems for its R2000 microprocessor, the first MIPS implementation. Both MIPS and the R2000 were introduced together in 1985. When MIPS II was introduced, MIPS was renamed MIPS I to distinguish it from the new version.: 32"
+2165,"MIPS Computer Systems' R6000 microprocessor  was the first MIPS II implementation.: 8  Designed for servers, the R6000 was fabricated and sold by Bipolar Integrated Technology, but was a commercial failure. During the mid-1990s, many new 32-bit MIPS processors for embedded systems were MIPS II implementations because the introduction of the 64-bit MIPS III architecture in 1991 left MIPS II as the newest 32-bit MIPS architecture until MIPS32 was introduced in 1999.: 19"
+2166,"MIPS Computer Systems' R4000 microprocessor  was the first MIPS III implementation. It was designed for use in personal, workstation, and server computers. MIPS Computer Systems aggressively promoted the MIPS architecture and R4000, establishing the Advanced Computing Environment  consortium to advance its Advanced RISC Computing  standard, which aimed to establish MIPS as the dominant personal computing platform. ARC found little success in personal computers, but the R4000  were widely used in workstation and server computers, especially by its largest user, Silicon Graphics. Other uses of the R4000 included high-end embedded systems and supercomputers. MIPS III was eventually implemented by a number of embedded microprocessors. Quantum Effect Design's R4600  and its derivatives was widely used in high-end embedded systems and low-end workstations and servers. MIPS Technologies' R4200 , was designed for embedded systems, laptop, and personal computers. A derivative, the R4300i, fabricated by NEC Electronics, was used in the Nintendo 64 game console. The Nintendo 64, along with the PlayStation, were among the highest volume users of MIPS architecture processors in the mid-1990s."
+2167,"The first MIPS IV implementation was the MIPS Technologies R8000 microprocessor chipset . The design of the R8000 began at Silicon Graphics, Inc. and it was only used in high-end workstations and servers for scientific and technical applications where high performance on large floating-point workloads was important. Later implementations were the MIPS Technologies R10000  and the Quantum Effect Devices R5000  and RM7000 . The R10000, fabricated and sold by NEC Electronics and Toshiba, and its derivatives were used by NEC, Pyramid Technology, Silicon Graphics, and Tandem Computers  in workstations, servers, and supercomputers. The R5000 and R7000 found use in high-end embedded systems, personal computers, and low-end workstations and servers. A derivative of the R5000 from Toshiba, the R5900, was used in Sony Computer Entertainment's Emotion Engine, which powered its PlayStation 2 game console."
+2168,"Announced on October 21, 1996, at the Microprocessor Forum 1996 alongside the MIPS Digital Media Extensions  extension, MIPS V was designed to improve the performance of 3D graphics transformations. In the mid-1990s, a major use of non-embedded MIPS microprocessors were graphics workstations from Silicon Graphics. MIPS V was completed by the integer-only MDMX extension to provide a complete system for improving the performance of 3D graphics applications. MIPS V implementations were never introduced. On May 12, 1997, Silicon Graphics announced the H1  and H2  microprocessors. The former was to have been the first MIPS V implementation, and was due to be introduced in the first half of 1999. The H1 and H2 projects were later combined and eventually canceled in 1998. While there have not been any MIPS V implementations, MIPS64 Release 1  was based on MIPS V and retains all of its features as an optional Coprocessor 1  feature called Paired-Single."
+2169,"When MIPS Technologies was spun-out of Silicon Graphics in 1998, it refocused on the embedded market. Through MIPS V, each successive version was a strict superset of the previous version, but this property was found to be a problem, and the architecture definition was changed to define a 32-bit and a 64-bit architecture: MIPS32 and MIPS64. Both were introduced in 1999. MIPS32 is based on MIPS II with some additional features from MIPS III, MIPS IV, and MIPS V; MIPS64 is based on MIPS V. NEC, Toshiba and SiByte  each obtained licenses for MIPS64 as soon as it was announced. Philips, LSI Logic, IDT, Raza Microelectronics, Inc., Cavium, Loongson Technology and Ingenic Semiconductor have since joined them. MIPS32/MIPS64 Release 5 was announced on December 6, 2012. According to the Product Marketing Director at MIPS, Release 4 was skipped because the number four is perceived as unlucky in many Asian cultures."
+2170,"In December 2018, Wave Computing, the new owner of the MIPS architecture, announced that MIPS ISA would be open-sourced in a program dubbed the MIPS Open initiative. The program was intended to open up access to the most recent versions of both the 32-bit and 64-bit designs making them available without any licensing or royalty fees as well as granting participants licenses to existing MIPS patents."
+2171,"In March 2019, one version of the architecture was made available under a royalty-free license, but later that year the program was shut down again."
+2172,"In March 2021, Wave Computing announced that the development of the MIPS architecture has ceased. The company has joined the RISC-V foundation and future processor designs will be based on the RISC-V architecture. In spite of this, some licensees such as Loongson continue with new extension of MIPS-compatible ISAs on their own."
+2173,"In January 2024, Loongson won a case over rights to use MIPS architecture."
+2174,"MIPS is a modular architecture supporting up to four coprocessors . In MIPS terminology, CP0 is the System Control Coprocessor , CP1 is an optional floating-point unit  and CP2/3 are optional implementation-defined coprocessors . For example, in the PlayStation video game console, CP2 is the Geometry Transformation Engine , which accelerates the processing of geometry in 3D computer graphics."
+2175,"MIPS is a load/store architecture ; except for the load/store instructions used to access memory, all instructions operate on the registers."
+2176,"MIPS I has thirty-two 32-bit general-purpose registers . Register $0 is hardwired to zero and writes to it are discarded. Register $31 is the link register. For integer multiplication and division instructions, which run asynchronously from other instructions, a pair of 32-bit registers, HI and LO, are provided. There is a small set of instructions for copying data between the general-purpose registers and the HI/LO registers."
+2177,The program counter has 32 bits. The two low-order bits always contain zero since MIPS I instructions are 32 bits long and are aligned to their natural word boundaries.
+2178,"Instructions are divided into three types: R , I , and J . Every instruction starts with a 6-bit opcode. In addition to the opcode, R-type instructions specify three registers, a shift amount field, and a function field; I-type instructions specify two registers and a 16-bit immediate value; J-type instructions follow the opcode with a 26-bit jump target.: A-174"
+2179,The following are the three formats used for the core instruction set:
+2180,"MIPS I has instructions that load and store 8-bit bytes, 16-bit halfwords, and 32-bit words. Only one addressing mode is supported: base + displacement. Since MIPS I is a 32-bit architecture, loading quantities fewer than 32 bits requires the datum to be either sign-extended or zero-extended to 32 bits. The load instructions suffixed by ""unsigned"" perform zero extension; otherwise sign extension is performed. Load instructions source the base from the contents of a GPR  and write the result to another GPR . Store instructions source the base from the contents of a GPR  and the store data from another GPR . All load and store instructions compute the memory address by summing the base with the sign-extended 16-bit immediate. MIPS I requires all memory accesses to be aligned to their natural word boundaries, otherwise an exception is signaled. To support efficient unaligned memory accesses, there are load/store word instructions suffixed by ""left"" or ""right"". All load instructions are followed by a load delay slot. The instruction in the load delay slot cannot use the data loaded by the load instruction. The load delay slot can be filled with an instruction that is not dependent on the load; a nop is substituted if such an instruction cannot be found."
+2181,"MIPS I has instructions to perform addition and subtraction. These instructions source their operands from two GPRs , and write the result to a third GPR . Alternatively, addition can source one of the operands from a 16-bit immediate . The instructions for addition and subtraction have two variants: by default, an exception is signaled if the result overflows; instructions with the ""unsigned"" suffix do not signal an exception. The overflow check interprets the result as a 32-bit two's complement integer. MIPS I has instructions to perform bitwise logical AND, OR, XOR, and NOR. These instructions source their operands from two GPRs and write the result to a third GPR. The AND, OR, and XOR instructions can alternatively source one of the operands from a 16-bit immediate . The Set on relation instructions write one or zero to the destination register if the specified relation is true or false. These instructions source their operands from two GPRs or one GPR and a 16-bit immediate , and write the result to a third GPR. By default, the operands are interpreted as signed integers. The variants of these instructions that are suffixed with ""unsigned"" interpret the operands as unsigned integers ."
+2182,The Load Immediate Upper instruction copies the 16-bit immediate into the high-order 16 bits of a GPR. It is used in conjunction with the Or Immediate instruction to load a 32-bit immediate into a register.
+2183,"MIPS I has instructions to perform left and right logical shifts and right arithmetic shifts. The operand is obtained from a GPR , and the result is written to another GPR . The shift distance is obtained from either a GPR  or a 5-bit ""shift amount"" ."
+2184,"MIPS I has instructions for signed and unsigned integer multiplication and division. These instructions source their operands from two GPRs and write their results to a pair of 32-bit registers called HI and LO, since they may execute separately from  the other CPU instructions. For multiplication, the high- and low-order halves of the 64-bit product is written to HI and LO . For division, the quotient is written to LO and the remainder to HI. To access the results, a pair of instructions  is provided to copy the contents of HI or LO to a GPR. These instructions are interlocked: reads of HI and LO do not proceed past an unfinished arithmetic instruction that will write to HI and LO. Another pair of instructions  copies the contents of a GPR to HI and LO. These instructions are used to restore HI and LO to their original state after exception handling. Instructions that read HI or LO must be separated by two instructions that do not write to HI or LO."
+2185,"All MIPS I control flow instructions are followed by a branch delay slot. Unless the branch delay slot is filled by an instruction performing useful work, an nop is substituted. MIPS I branch instructions compare the contents of a GPR  against zero or another GPR  as signed integers and branch if the specified condition is true. Control is transferred to the address computed by shifting the 16-bit offset left by two bits, sign-extending the 18-bit result, and adding the 32-bit sign-extended result to the sum of the program counter  and 810. Jumps have two versions: absolute and register-indirect. Absolute jumps  compute the address to which control is transferred by shifting the 26-bit instr_index left by two bits and concatenating the 28-bit result with the four high-order bits of the address of the instruction in the branch delay slot. Register-indirect jumps transfer control to the instruction at the address sourced from a GPR . The address sourced from the GPR must be word-aligned, else an exception is signaled after the instruction in the branch delay slot is executed. Branch and jump instructions that link  save the return address to GPR 31. The ""Jump and Link Register"" instruction permits the return address to be saved to any writable GPR."
+2186,MIPS I has two instructions for software to signal an exception: System Call and Breakpoint. System Call is used by user mode software to make kernel calls; and Breakpoint is used to transfer control to a debugger via the kernel's exception handler. Both instructions have a 20-bit Code field that can contain operating environment-specific information for the exception handler.
+2187,"MIPS has 32 floating-point registers. Two registers are paired for double precision numbers. Odd numbered registers cannot be used for arithmetic or branching, just as part of a double precision register pair, resulting in 16 usable registers for most instructions ."
+2188,"Single precision is denoted by the .s suffix, while double precision is denoted by the .d suffix."
+2189,"MIPS II removed the load delay slot: 41  and added several sets of instructions. For shared-memory multiprocessing, the Synchronize Shared Memory, Load Linked Word, and Store Conditional Word instructions were added. A set of Trap-on-Condition instructions were added. These instructions caused an exception if the evaluated condition is true. All existing branch instructions were given branch-likely versions that executed the instruction in the branch delay slot only if the branch is taken.: 40  These instructions improve performance in certain cases by allowing useful instructions to fill the branch delay slot.: 212  Doubleword load and store instructions for COP1–3 were added. Consistent with other memory access instructions, these loads and stores required the doubleword to be naturally aligned."
+2190,The instruction set for the floating point coprocessor also had several instructions added to it. An IEEE 754-compliant floating-point square root instruction was added. It supported both single- and double-precision operands. A set of instructions that converted single- and double-precision floating-point numbers to 32-bit words were added. These complemented the existing conversion instructions by allowing the IEEE rounding mode to be specified by the instruction instead of the Floating Point Control and Status Register.
+2191,"MIPS III is a backwards-compatible extension of MIPS II that added support for 64-bit memory addressing and integer operations. The 64-bit data type is called a doubleword, and MIPS III extended the general-purpose registers, HI/LO registers, and program counter to 64 bits to support it. New instructions were added to load and store doublewords, to perform integer addition, subtraction, multiplication, division, and shift operations on them, and to move doubleword between the GPRs and HI/LO registers. For shared-memory multiprocessing, the Load Linked Double Word, and Store Conditional Double Word instructions were added. Existing instructions originally defined to operate on 32-bit words were redefined, where necessary, to sign-extend the 32-bit results to permit words and doublewords to be treated identically by most instructions. Among those instructions redefined was Load Word. In MIPS III it sign-extends words to 64 bits. To complement Load Word, a version that zero-extends was added."
+2192,"The R instruction format's inability to specify the full shift distance for 64-bit shifts  required MIPS III to provide three 64-bit versions of each MIPS I shift instruction. The first version is a 64-bit version of the original shift instructions, used to specify constant shift distances of 0–31 bits. The second version is similar to the first, but adds 3210 the shift amount field's value so that constant shift distances of 32–63 bits can be specified. The third version obtains the shift distance from the six low-order bits of a GPR."
+2193,MIPS III added a supervisor privilege level in between the existing kernel and user privilege levels. This feature only affected the implementation-defined System Control Processor .
+2194,"MIPS III removed the Coprocessor 3  support instructions, and reused its opcodes for the new doubleword instructions. The remaining coprocessors gained instructions to move doublewords between coprocessor registers and the GPRs. The floating general registers  were extended to 64 bits and the requirement for instructions to use even-numbered register only was removed. This is incompatible with earlier versions of the architecture; a bit in the floating-point control/status register is used to operate the MIPS III floating-point unit  in a MIPS I- and II-compatible mode. The floating-point control registers were not extended for compatibility. The only new floating-point instructions added were those to copy doublewords between the CPU and FPU convert single- and double-precision floating-point numbers into doubleword integers and vice versa."
+2195,"MIPS IV is the fourth version of the architecture. It is a superset of MIPS III and is compatible with all existing versions of MIPS.: A-1  MIPS IV was designed to mainly improve floating-point  performance. To improve access to operands, an indexed addressing mode  for FP loads and stores was added, as were prefetch instructions for performing memory prefetching and specifying cache hints ."
+2196,"MIPS IV added several features to improve instruction-level parallelism. To alleviate the bottleneck caused by a single condition bit, seven condition code bits were added to the floating-point control and status register, bringing the total to eight. FP comparison and branch instructions were redefined so they could specify which condition bit was written or read ; and the delay slot in between an FP branch that read the condition bit written to by a prior FP comparison was removed. Support for partial predication was added in the form of conditional move instructions for both GPRs and FPRs; and an implementation could choose between having precise or imprecise exceptions for IEEE 754 traps."
+2197,"MIPS IV added several new FP arithmetic instructions for both single- and double-precision FPNs: fused-multiply add or subtract, reciprocal, and reciprocal square-root. The FP fused-multiply add or subtract instructions perform either one or two roundings , to exceed or meet IEEE 754 accuracy requirements . The FP reciprocal and reciprocal square-root instructions do not comply with IEEE 754 accuracy requirements, and produce results that differ from the required accuracy by one or two units of last place . These instructions serve applications where instruction latency is more important than accuracy."
+2198,"MIPS V added a new data type, the Paired Single , which consisted of two single-precision  floating-point numbers stored in the existing 64-bit floating-point registers. Variants of existing floating-point instructions for arithmetic, compare and conditional move were added to operate on this data type in a SIMD fashion. New instructions were added for loading, rearranging and converting PS data.: 426–429  It was the first instruction set to exploit floating-point SIMD with existing resources."
+2199,"The first release of MIPS32, based on MIPS II, added conditional moves, prefetch instructions, and other features from the R4000 and R5000 families of 64-bit processors.  The first release of MIPS64 adds a MIPS32 mode to run 32-bit code. The MUL and MADD  instructions, previously available in some implementations, were added to the MIPS32 and MIPS64 specifications, as were cache control instructions. For the purpose of cache control, both SYNC and SYNCI instructions were prepared."
+2200,MIPS32/MIPS64 Release 6 in 2014 added the following:
+2201,Removed infrequently used instructions:
+2202,"Reorganized the instruction encoding, freeing space for future expansions."
+2203,"The microMIPS32/64 architectures are supersets of the MIPS32 and MIPS64 architectures  designed to replace the MIPS16e ASE. A disadvantage of MIPS16e is that it requires a mode switch before any of its 16-bit instructions can be processed. microMIPS adds versions of the most-frequently used 32-bit instructions that are encoded as 16-bit instructions. This allows programs to intermix 16- and 32-bit instructions without having to switch modes. microMIPS was introduced alongside of MIPS32/64 Release 3, and each subsequent release of MIPS32/64 has a corresponding microMIPS32/64 version. A processor may implement microMIPS32/64 or both microMIPS32/64 and its corresponding MIPS32/64 subset. Starting with MIPS32/64 Release 6, support for MIPS16e ended, and microMIPS is the only form of code compression in MIPS."
+2204,"The base MIPS32 and MIPS64 architectures can be supplemented with a number of optional architectural extensions, which are collectively referred to as application-specific extensions . These ASEs provide features that improve the efficiency and performance of certain workloads, such as digital signal processing."
+2205,"MIPS has had several calling conventions, especially on the 32-bit platform."
+2206,"The O32 ABI is the most commonly-used ABI, owing to its status as the original System V ABI for MIPS. It is strictly stack-based, with only four registers $a0-$a3 available to pass arguments. Space on the stack is reserved in case the callee needs to save its arguments, but the registers are not stored there by the caller. The return value is stored in register $v0; a second return value may be stored in $v1. The ABI took shape in 1990 and was last updated in 1994. This perceived slowness, along with an antique floating-point model with only 16 registers, has encouraged the proliferation of many other calling conventions. It is only defined for 32-bit MIPS, but GCC has created a 64-bit variation called O64."
+2207,"For 64-bit, the N64 ABI by Silicon Graphics is most commonly used. The most important improvement is that eight registers are now available for argument passing; it also increases the number of floating-point registers to 32. There is also an ILP32 version called N32, which uses 32-bit pointers for smaller code, analogous to the x32 ABI. Both run under the 64-bit mode of the CPU. The N32 and N64 ABIs pass the first eight arguments to a function in the registers $a0-$a7; subsequent arguments are passed on the stack. The return value  is stored in the registers $v0; a second return value may be stored in $v1. In both the N32 and N64 ABIs all registers are considered to be 64-bits wide."
+2208,"A few attempts have been made to replace O32 with a 32-bit ABI that resembles N32 more. A 1995 conference came up with MIPS EABI, for which the 32-bit version was quite similar. EABI inspired MIPS Technologies to propose a more radical ""NUBI"" ABI additionally reuse argument registers for the return value. MIPS EABI is supported by GCC but not LLVM, and neither supports NUBI."
+2209,"For all of O32 and N32/N64, the return address is stored in a $ra register. This is automatically set with the use of the JAL  or JALR  instructions. The function prologue of a  MIPS subroutine pushes the return address  to the stack."
+2210,"On both O32 and N32/N64 the stack grows downwards, but the N32/N64 ABIs require 64-bit alignment for all stack entries. The frame pointer  is optional and in practice rarely used except when the stack allocation in a function is determined at runtime, for example, by calling alloca."
+2211,"For N32 and N64, the return address is typically stored 8 bytes before the stack pointer although this may be optional."
+2212,"For the N32 and N64 ABIs, a function must preserve the $s0-$s7 registers, the global pointer , the stack pointer  and the frame pointer . The O32 ABI is the same except the calling function is required to save the $gp register instead of the called function."
+2213,"For multi-threaded code, the thread local storage pointer is typically stored in special hardware register $29 and is accessed by using the mfhw  instruction. At least one vendor is known to store this information in the $k0 register which is normally reserved for kernel use, but this is not standard."
+2214,"The $k0 and $k1 registers  are reserved for kernel use and should not be used by applications since these registers can be changed at any time by the kernel due to interrupts, context switches or other events."
+2215,"Registers that are preserved across a call are registers that  will not be changed by a system call or procedure  call. For example, $s-registers must be saved to the stack by a procedure that needs to use them, and $sp and $fp are always incremented by constants, and decremented back after the procedure is done with them . By contrast, $ra is changed automatically by any normal function call , and $t-registers must be saved by the program before any procedure call ."
+2216,The userspace calling convention of position-independent code on Linux additionally requires that when a function is called the $t9 register must contain the address of that function. This convention dates back to the System V ABI supplement for MIPS.
+2217,"MIPS processors are used in embedded systems such as residential gateways and routers. Originally, MIPS was designed for general-purpose computing. During the 1980s and 1990s, MIPS processors for personal, workstation, and server computers were used by many companies such as Digital Equipment Corporation, MIPS Computer Systems, NEC, Pyramid Technology, SiCortex, Siemens Nixdorf, Silicon Graphics, and Tandem Computers."
+2218,"Historically, video game consoles such as the Nintendo 64, Sony PlayStation, PlayStation 2, and PlayStation Portable used MIPS processors. MIPS processors also used to be popular in supercomputers during the 1990s, but all such systems have dropped off the TOP500 list. These uses were complemented by embedded applications at first, but during the 1990s, MIPS became a major presence in the embedded processor market, and by the 2000s, most MIPS processors were for these applications."
+2219,"In the mid- to late-1990s, it was estimated that one in three RISC microprocessors produced was a MIPS processor."
+2220,"By the late 2010s, MIPS machines were still commonly used in embedded markets, including automotive, wireless router, LTE modems , and microcontrollers . They have mostly faded out of the personal, server, and application space."
+2221,"Open Virtual Platforms  includes the freely available for non-commercial use simulator OVPsim, a library of models of processors, peripherals and platforms, and APIs which enable users to develop their own models. The models in the library are open source, written in C, and include the MIPS 4K, 24K, 34K, 74K, 1004K, 1074K, M14K, microAptiv, interAptiv, proAptiv 32-bit cores and the MIPS 64-bit 5K range of cores. These models are created and maintained by Imperas and in partnership with MIPS Technologies have been tested and assigned the MIPS-Verified  mark. Sample MIPS-based platforms include both bare metal environments and platforms for booting unmodified Linux binary images. These platforms–emulators are available as source or binaries and are fast, free for non-commercial usage, and are easy to use. OVPsim is developed and maintained by Imperas and is very fast , and built to handle multicore homogeneous and heterogeneous architectures and systems."
+2222,"There is a freely available MIPS32 simulator  called SPIM for use in education. EduMIPS64 is a GPL graphical cross-platform MIPS64 CPU simulator, written in Java/Swing. It supports a wide subset of the MIPS64 ISA and allows the user to graphically see what happens in the pipeline when an assembly program is run by the CPU."
+2223,"MARS is another GUI-based MIPS emulator designed for use in education, specifically for use with Hennessy's Computer Organization and Design."
+2224,"WebMIPS is a browser-based MIPS simulator with visual representation of a generic, pipelined processor.  This simulator is quite useful for register tracking during step by step execution."
+2225,"QtMips provides simple 5-stages pipeline visualization as well as cache principle visualization for basic computer architectures courses. Windows, Linux, macOS and online version is available."
+2226,More advanced free emulators are available from the GXemul  and QEMU projects. These emulate the various MIPS III and IV microprocessors in addition to entire computer systems which use them.
+2227,"Commercial simulators are available especially for the embedded use of MIPS processors, for example Wind River Simics , Imperas , VaST Systems , and CoWare ."
+2228,The Creator simulator is portable and allows the user to learn various assembly languages of different processors .
+2229,"An electronic calculator is typically a portable electronic device used to perform calculations, ranging from basic arithmetic to complex mathematics."
+2230,"The first solid-state electronic calculator was created in the early 1960s. Pocket-sized devices became available in the 1970s, especially after the Intel 4004, the first microprocessor, was developed by Intel for the Japanese calculator company Busicom."
+2231,"Modern electronic calculators vary from cheap, give-away, credit-card-sized models to sturdy desktop models with built-in printers. They became popular in the mid-1970s as the incorporation of integrated circuits reduced their size and cost. By the end of that decade, prices had dropped to the point where a basic calculator was affordable to most and they became common in schools."
+2232,"Computer operating systems as far back as early Unix have included interactive calculator programs such as dc and hoc, and interactive BASIC could be used to do calculations on most 1970s and 1980s home computers. Calculator functions are included in most smartphones, tablets and personal digital assistant  type devices."
+2233,"In addition to general purpose calculators, there are those designed for specific markets. For example, there are scientific calculators which include trigonometric and statistical calculations. Some calculators even have the ability to do computer algebra. Graphing calculators can be used to graph functions defined on the real line, or higher-dimensional Euclidean space. As of 2016, basic calculators cost little, but scientific and graphing models tend to cost more."
+2234,"With the very wide availability of smartphones and the like, dedicated hardware calculators, while still widely used, are less common than they once were.  In 1986, calculators still represented an estimated 41% of the world's general-purpose hardware capacity to compute information. By 2007, this had diminished to less than 0.05%."
+2235,"Electronic calculators contain a keyboard with buttons for digits and arithmetical operations; some even contain ""00"" and ""000"" buttons to make larger or smaller numbers easier to enter. Most basic calculators assign only one digit or operation on each button; however, in more specific calculators, a button can perform multi-function working with key combinations."
+2236,Calculators usually have liquid-crystal displays  as output in place of historical light-emitting diode  displays and vacuum fluorescent displays ; details are provided in the section Technical improvements.
+2237,"Large-sized figures are often used to improve readability; while using decimal separator  instead of or in addition to vulgar fractions. Various symbols for function commands may also be shown on the display. Fractions such as 1⁄3 are displayed as decimal approximations, for example rounded to 0.33333333. Also, some fractions  can be difficult to recognize in decimal form; as a result, many scientific calculators are able to work in vulgar fractions or mixed numbers."
+2238,Calculators also have the ability to save numbers into computer memory. Basic calculators usually store only one number at a time; more specific types are able to store many numbers represented in variables. Usually these variables are named ans or ans. The variables can also be used for constructing formulas. Some models have the ability to extend memory capacity to store more numbers; the extended memory address is termed an array index.
+2239,"Power sources of calculators are batteries, solar cells or mains electricity , turning on with a switch or button. Some models even have no turn-off button but they provide some way to put off . Crank-powered calculators were also common in the early computer era."
+2240,"The following keys are common to most pocket calculators.  While the arrangement of the digits is standard, the positions of other keys vary from model to model; the illustration is an example."
+2241,The arrangement of digits on calculator and other numeric keypads with the 7-8-9 keys two rows above the 1-2-3 keys is  derived from calculators and cash registers. It is notably different from the layout of telephone Touch-Tone keypads which have the 1-2-3 keys on top and 7-8-9 keys on the third row.
+2242,"In general, a basic electronic calculator consists of the following components:"
+2243,"Clock rate of a processor chip refers to the frequency at which the central processing unit  is running. It is used as an indicator of the processor's speed, and is measured in clock cycles per second or hertz . For basic calculators, the speed can vary from a few hundred hertz to the kilohertz range."
+2244,A basic explanation as to how calculations are performed in a simple four-function calculator:
+2245,"To perform the calculation 25 + 9, one presses keys in the following sequence on most calculators: 2 5 + 9 =."
+2246,Other functions are usually performed using repeated additions or subtractions.
+2247,"Most pocket calculators do all their calculations in binary-coded decimal  rather than binary.  BCD is common in electronic systems where a numeric value is to be displayed, especially in systems consisting solely of digital logic, and not containing a microprocessor. By employing BCD, the manipulation of numerical data for display can be greatly simplified by treating each digit as a separate single sub-circuit. This matches much more closely the physical reality of display hardware—a designer might choose to use a series of separate identical seven-segment displays to build a metering circuit, for example. If the numeric quantity were stored and manipulated as pure binary, interfacing to such a display would require complex circuitry. Therefore, in cases where the calculations are relatively simple, working throughout with BCD can lead to a simpler overall system than converting to and from binary. "
+2248,"The same argument applies when hardware of this type uses an embedded microcontroller or other small processor. Often, smaller code results when representing numbers internally in BCD format, since a conversion from or to binary representation can be expensive on such limited processors. For these applications, some small processors feature BCD arithmetic modes, which assist when writing routines that manipulate BCD quantities."
+2249,"Where calculators have added functions , software algorithms are required to produce high precision results. Sometimes significant design effort is needed to fit all the desired functions in the limited memory space available in the calculator chip, with acceptable calculation time."
+2250,"The first known tools used to aid arithmetic calculations were: bones , pebbles, and counting boards, and the abacus, known to have been used by Sumerians and Egyptians before 2000 BC. Except for the Antikythera mechanism , development of computing tools arrived near the start of the 17th century: the geometric-military compass , logarithms and Napier bones , and the slide rule ."
+2251,"The Renaissance saw the invention of the mechanical calculator by Wilhelm Schickard in 1623, and later by Blaise Pascal in 1642. A device that was at times somewhat over-promoted as being able to perform all four arithmetic operations with minimal human intervention. Pascal's calculator could add and subtract two numbers directly and thus, if the tedium could be borne, multiply and divide by repetition. Schickard's machine, constructed several decades earlier, used a clever set of mechanised multiplication tables to ease the process of multiplication and division with the adding machine as a means of completing this operation. There is a debate about whether Pascal or Shickard should be credited as the known inventor of a calculating machine due to the differences  of both inventions. Schickard and Pascal were followed by Gottfried Leibniz who spent forty years designing a four-operation mechanical calculator, the stepped reckoner, inventing in the process his leibniz wheel, but who couldn't design a fully operational machine. There were also five unsuccessful attempts to design a calculating clock in the 17th century."
+2252,"The 18th century saw the arrival of some notable improvements, first by Poleni with the first fully functional calculating clock and four-operation machine, but these machines were almost always one of a kind. Luigi Torchi invented the first direct multiplication machine in 1834: this was also the second key-driven machine in the world, following that of James White . It was not until the 19th century and the Industrial Revolution that real developments began to occur. Although machines capable of performing all four arithmetic functions existed prior to the 19th century, the refinement of manufacturing and fabrication processes during the eve of the industrial revolution made large scale production of more compact and modern units possible. The Arithmometer, invented in 1820 as a four-operation mechanical calculator, was released to production in 1851 as an adding machine and became the first commercially successful unit; forty years later, by 1890, about 2,500 arithmometers had been sold plus a few hundreds more from two arithmometer clone makers  and Felt and Tarrant, the only other competitor in true commercial production, had sold 100 comptometers."
+2253,"It wasn't until 1902 that the familiar push-button user interface was developed, with the introduction of the Dalton Adding Machine, developed by James L. Dalton in the United States."
+2254,"In 1921, Edith Clarke invented the ""Clarke calculator"", a simple graph-based calculator for solving line equations involving hyperbolic functions. This allowed electrical engineers to simplify calculations for inductance and capacitance in power transmission lines."
+2255,"The Curta calculator was developed in 1948 and, although costly, became popular for its portability. This purely mechanical hand-held device could do addition, subtraction, multiplication and division. By the early 1970s electronic pocket calculators ended manufacture of mechanical calculators, although the Curta remains a popular collectable item."
+2256,"The first mainframe computers, initially using vacuum tubes and later transistors in the logic circuits, appeared in the 1940s and 1950s. Electronic circuits developed for computers also had application to electronic calculators."
+2257,"The Casio Computer Company, in Japan, released the Model 14-A calculator in 1957, which was the world's first all-electric  compact calculator. It did not use electronic logic but was based on relay technology, and was built into a desk. The IBM 608 plugboard programmable calculator was IBM's first all-transistor product, released in 1957; this was a console type system, with input and output on punched cards, and replaced the earlier, larger, vacuum-tube IBM 603."
+2258,"In October 1961, the world's first all-electronic desktop calculator, the British Bell Punch/Sumlock Comptometer ANITA  was announced. This machine used vacuum tubes, cold-cathode tubes and Dekatrons in its circuits, with 12 cold-cathode ""Nixie"" tubes for its display. Two models were displayed, the Mk VII for continental Europe and the Mk VIII for Britain and the rest of the world, both for delivery from early 1962. The Mk VII was a slightly earlier design with a more complicated mode of multiplication, and was soon dropped in favour of the simpler Mark VIII. The ANITA had a full keyboard, similar to mechanical comptometers of the time, a feature that was unique to it and the later Sharp CS-10A among electronic calculators. The ANITA weighed roughly 33 pounds  due to its large tube system. Bell Punch had been producing key-driven mechanical calculators of the comptometer type under the names ""Plus"" and ""Sumlock"", and had realised in the mid-1950s that the future of calculators lay in electronics. They employed the young graduate Norbert Kitz, who had worked on the early British Pilot ACE computer project, to lead the development. The ANITA sold well since it was the only electronic desktop calculator available, and was silent and quick."
+2259,"The tube technology of the ANITA was superseded in June 1963 by the U.S. manufactured Friden EC-130, which had an all-transistor design, a stack of four 13-digit numbers displayed on a 5-inch  cathode ray tube , and introduced Reverse Polish Notation  to the calculator market for a price of $2200, which was about three times the cost of an electromechanical calculator of the time. Like Bell Punch, Friden was a manufacturer of mechanical calculators that had decided that the future lay in electronics. In 1964 more all-transistor electronic calculators were introduced: Sharp introduced the CS-10A, which weighed 25 kilograms  and cost 500,000 yen , and Industria Macchine Elettroniche of Italy introduced the IME 84, to which several extra keyboard and display units could be connected so that several people could make use of it . The Victor 3900 was the first to use integrated circuits in place of individual transistors, but production problems delayed sales until 1966."
+2260,"There followed a series of electronic calculator models from these and other manufacturers, including Canon, Mathatronics, Olivetti, SCM , Sony, Toshiba, and Wang. The early calculators used hundreds of germanium transistors, which were cheaper than silicon transistors, on multiple circuit boards. Display types used were CRT, cold-cathode Nixie tubes, and filament lamps. Memory technology was usually based on the delay-line memory or the magnetic-core memory, though the Toshiba ""Toscal"" BC-1411 appears to have used an early form of dynamic RAM built from discrete components. Already there was a desire for smaller and less power-hungry machines."
+2261,"Bulgaria's ELKA 6521, introduced in 1965, was developed by the Central Institute for Calculation Technologies and built at the Elektronika factory in Sofia. The name derives from ELektronen KAlkulator, and it weighed around 8 kg . It is the first calculator in the world which includes the square root function. Later that same year were released the ELKA 22  and the ELKA 25, with an built-in printer. Several other models were developed until the first pocket model, the ELKA 101, was released in 1974. The writing on it was in Roman script, and it was exported to western countries."
+2262,"The first desktop programmable calculators were produced in the mid-1960s. They included the Mathatronics Mathatron  and the Olivetti Programma 101  which were solid-state, desktop, printing, floating point, algebraic entry, programmable, stored-program electronic calculators. Both could be programmed by the end user and print out their results. The Programma 101 saw much wider distribution and had the added feature of offline storage of programs via magnetic cards."
+2263,Another early programmable desktop calculator  was the Casio  produced in 1967. It featured a nixie tubes display and had transistor electronics and ferrite core memory.
+2264,"The Monroe Epic programmable calculator came on the market in 1967. A large, printing, desk-top unit, with an attached floor-standing logic tower, it could be programmed to perform many computer-like functions. However, the only branch instruction was an implied unconditional branch  at the end of the operation stack, returning the program to its starting instruction. Thus, it was not possible to include any conditional branch  logic. During this era, the absence of the conditional branch was sometimes used to distinguish a programmable calculator from a computer."
+2265,"The first Soviet programmable desktop calculator ISKRA 123, powered by the power grid, was released at the start of the 1970s."
+2266,"The electronic calculators of the mid-1960s were large and heavy desktop machines due to their use of hundreds of transistors on several circuit boards with a large power consumption that required an AC power supply. There were great efforts to put the logic required for a calculator into fewer and fewer integrated circuits  and calculator electronics was one of the leading edges of semiconductor development. U.S. semiconductor manufacturers led the world in large scale integration  semiconductor development, squeezing more and more functions into individual integrated circuits. This led to alliances between Japanese calculator manufacturers and U.S. semiconductor companies: Canon Inc. with Texas Instruments, Hayakawa Electric  with North-American Rockwell Microelectronics , Busicom with Mostek and Intel, and General Instrument with Sanyo."
+2267,"By 1970, a calculator could be made using just a few chips of low power consumption, allowing portable models powered from rechargeable batteries. The first handheld calculator was a 1967 prototype called Cal Tech, whose development was led by Jack Kilby at Texas Instruments in a research project to produce a portable calculator. It could add, multiply, subtract, and divide, and its output device was a paper tape. As a result of the ""Cal-Tech"" project, Texas Instruments was granted master patents on portable calculators."
+2268,"The first commercially produced portable calculators appeared in Japan in 1970, and were soon marketed around the world. These included the Sanyo ICC-0081 ""Mini Calculator"", the Canon Pocketronic, and the Sharp QT-8B ""micro Compet"". The Canon Pocketronic was a development from the ""Cal-Tech"" project. It had no traditional display; numerical output was on thermal paper tape."
+2269,"Sharp put in great efforts in size and power reduction and introduced in January 1971 the Sharp EL-8, also marketed as the Facit 1111, which was close to being a pocket calculator. It weighed 1.59 pounds , had a vacuum fluorescent display, rechargeable NiCad batteries, and initially sold for US$395."
+2270,"However, integrated circuit development efforts culminated in early 1971 with the introduction of the first ""calculator on a chip"", the MK6010 by Mostek, followed by Texas Instruments later in the year. Although these early hand-held calculators were very costly, these advances in electronics, together with developments in display technology , led within a few years to the cheap pocket calculator available to all."
+2271,"In 1971, Pico Electronics and General Instrument also introduced their first collaboration in ICs, a full single chip calculator IC for the Monroe Royal Digital III calculator. Pico was a spinout by five GI design engineers whose vision was to create single chip calculator ICs. Pico and GI went on to have significant success in the burgeoning handheld calculator market."
+2272,"The first truly pocket-sized electronic calculator was the Busicom LE-120A ""HANDY"", which was marketed early in 1971. Made in Japan, this was also the first calculator to use an LED display, the first hand-held calculator to use a single integrated circuit , the Mostek MK6010, and the first electronic calculator to run off replaceable batteries. Using four AA-size cells the LE-120A measures 4.9 by 2.8 by 0.9 inches ."
+2273,"The first European-made pocket-sized calculator, DB 800 was made in May 1971 by Digitron in Buje, Croatia  with four functions and an eight-digit display and special characters for a negative number and a warning that the calculation has too many digits to display."
+2274,"The first American-made pocket-sized calculator, the Bowmar 901B , measuring 5.2 by 3.0 by 1.5 inches , came out in the Autumn of 1971, with four functions and an eight-digit red LED display, for US$240, while in August 1972 the four-function Sinclair Executive became the first slimline pocket calculator measuring 5.4 by 2.2 by 0.35 inches  and weighing 2.5 ounces . It retailed for around £79 . By the end of the decade, similar calculators were priced less than £5 . Following protracted development over the course of two years including a botched partnership with Texas Instruments, Eldorado Electrodata released five pocket calculators in 1972. One called the Touch Magic was ""no bigger than a pack of cigarettes"" according to Administrative Management."
+2275,"The first Soviet Union made pocket-sized calculator, the Elektronika B3-04 was developed by the end of 1973 and sold at the start of 1974."
+2276,"One of the first low-cost calculators was the Sinclair Cambridge, launched in August 1973. It retailed for £29.95 , or £5  less in kit form, and later models included some scientific functions. The Sinclair calculators were successful because they were far cheaper than the competition; however, their design led to slow and less accurate computations of transcendental functions ."
+2277,"Meanwhile, Hewlett-Packard  had been developing a pocket calculator. Launched in early 1972, it was unlike the other basic four-function pocket calculators then available in that it was the first pocket calculator with scientific functions that could replace a slide rule. The $395 HP-35, along with nearly all later HP engineering calculators, uses reverse Polish notation , also called postfix notation. A calculation like ""8 plus 5"" is, using RPN, performed by pressing 8, Enter↑, 5, and +; instead of the algebraic infix notation: 8, +, 5, =. It had 35 buttons and was based on Mostek Mk6020 chip."
+2278,"The first Soviet scientific pocket-sized calculator the ""B3-18"" was completed by the end of 1975."
+2279,"In 1973, Texas Instruments  introduced the SR-10,  an algebraic entry pocket calculator using scientific notation for $150. Shortly after the SR-11 featured an added key for entering pi . It was followed the next year by the SR-50 which added log and trig functions to compete with the HP-35, and in 1977 the mass-marketed TI-30 line which is still produced."
+2280,"In 1978, a new company, Calculated Industries arose which focused on specialized markets. Their first calculator, the Loan Arranger  was a pocket calculator marketed to the Real Estate industry with preprogrammed functions to simplify the process of calculating payments and future values. In 1985, CI launched a calculator for the construction industry called the Construction Master which came preprogrammed with common construction calculations . This would be the first in a line of construction related calculators."
+2281,"The first programmable pocket calculator was the HP-65, in 1974; it had a capacity of 100 instructions, and could store and retrieve programs with a built-in magnetic card reader. Two years later the HP-25C introduced continuous memory, i.e., programs and data were retained in CMOS memory during power-off. In 1979, HP released the first alphanumeric, programmable, expandable calculator, the HP-41C. It could be expanded with random-access memory  and read-only memory  modules, and peripherals like bar code readers, microcassette and floppy disk drives, paper-roll thermal printers, and miscellaneous communication interfaces ."
+2282,"The first Soviet pocket battery-powered programmable calculator, Elektronika B3-21, was developed by the end of 1976 and released at the start of 1977. The successor of B3-21, the Elektronika B3-34 wasn't backward compatible with B3-21, even if it kept the reverse Polish notation . Thus B3-34 defined a new command set, which later was used in a series of later programmable Soviet calculators. Despite very limited abilities , people managed to write all kinds of programs for them, including adventure games and libraries of calculus-related functions for engineers. Hundreds, perhaps thousands, of programs were written for these machines, from practical scientific and business software, which were used in real-life offices and labs, to fun games for children. The Elektronika MK-52 calculator  was used in Soviet spacecraft program  as a backup of the board computer."
+2283,"This series of calculators was also noted for a large number of highly counter-intuitive mysterious undocumented features, somewhat similar to ""synthetic programming"" of the American HP-41, which were exploited by applying normal arithmetic operations to error messages, jumping to nonexistent addresses and other methods. A number of respected monthly publications, including the popular science magazine Nauka i Zhizn , featured special columns, dedicated to optimization methods for calculator programmers and updates on undocumented features for hackers, which grew into a whole esoteric science with many branches, named ""yeggogology"" . The error messages on those calculators appear as a Russian word ""YEGGOG""  which, unsurprisingly, is translated to ""Error""."
+2284,"A similar hacker culture in the US revolved around the HP-41, which was also noted for a large number of undocumented features and was much more powerful than B3-34."
+2285,"Through the 1970s the hand-held electronic calculator underwent rapid development. The red LED and blue/green vacuum fluorescent displays consumed a lot of power and the calculators either had a short battery life  or were large so that they could take larger, higher capacity batteries. In the early 1970s liquid-crystal displays  were in their infancy and there was a great deal of concern that they only had a short operating lifetime. Busicom introduced the Busicom LE-120A ""HANDY"" calculator, the first pocket-sized calculator and the first with an LED display, and announced the Busicom LC with LCD. However, there were problems with this display and the calculator never went on sale. The first successful calculators with LCDs were manufactured by Rockwell International and sold from 1972 by other companies under such names as: Dataking LC-800, Harden DT/12, Ibico 086, Lloyds 40, Lloyds 100, Prismatic 500 , Rapid Data Rapidman 1208LC. The LCDs were an early form using the Dynamic Scattering Mode DSM with the numbers appearing as bright against a dark background. To present a high-contrast display these models illuminated the LCD using a filament lamp and solid plastic light guide, which negated the low power consumption of the display. These models appear to have been sold only for a year or two."
+2286,"A more successful series of calculators using a reflective DSM-LCD was launched in 1972 by Sharp Inc with the Sharp EL-805, which was a slim pocket calculator. This, and another few similar models, used Sharp's Calculator On Substrate  technology. An extension of one glass plate needed for the liquid crystal display was used as a substrate to mount the needed chips based on a new hybrid technology. The COS technology may have been too costly since it was only used in a few models before Sharp reverted to conventional circuit boards."
+2287,"In the mid-1970s the first calculators appeared with field-effect, twisted nematic  LCDs with dark numerals against a grey background, though the early ones often had a yellow filter over them to cut out damaging ultraviolet rays. The advantage of LCDs is that they are passive light modulators reflecting light, which require much less power than light-emitting displays such as LEDs or VFDs. This led the way to the first credit-card-sized calculators, such as the Casio Mini Card LC-78 of 1978, which could run for months of normal use on button cells."
+2288,"There were also improvements to the electronics inside the calculators. All of the logic functions of a calculator had been squeezed into the first ""calculator on a chip"" integrated circuits  in 1971, but this was leading edge technology of the time and yields were low and costs were high. Many calculators continued to use two or more ICs, especially the scientific and the programmable ones, into the late 1970s."
+2289,"The power consumption of the integrated circuits was also reduced, especially with the introduction of CMOS technology. Appearing in the Sharp ""EL-801"" in 1972, the transistors in the logic cells of CMOS ICs only used any appreciable power when they changed state. The LED and VFD displays often required added driver transistors or ICs, whereas the LCDs were more amenable to being driven directly by the calculator IC itself."
+2290,"With this low power consumption came the possibility of using solar cells as the power source, realised around 1978 by calculators such as the Royal Solar 1, Sharp EL-8026, and Teal Photon."
+2291,"At the start of the 1970s, hand-held electronic calculators were very costly, at two or three weeks' wages, and so were a luxury item. The high price was due to their construction requiring many mechanical and electronic components which were costly to produce, and production runs that were too small to exploit economies of scale. Many firms saw that there were good profits to be made in the calculator business with the margin on such high prices. However, the cost of calculators fell as components and their production methods improved, and the effect of economies of scale was felt."
+2292,"By 1976, the cost of the cheapest four-function pocket calculator had dropped to a few dollars, about 1/20 of the cost five years before. The results of this were that the pocket calculator was affordable, and that it was now difficult for the manufacturers to make a profit from calculators, leading to many firms dropping out of the business or closing. The firms that survived making calculators tended to be those with high outputs of higher quality calculators, or producing high-specification scientific and programmable calculators."
+2293,"The first calculator capable of symbolic computing was the HP-28C, released in 1987. It could, for example, solve quadratic equations symbolically. The first graphing calculator was the Casio fx-7000G released in 1985."
+2294,"The two leading manufacturers, HP and TI, released increasingly feature-laden calculators during the 1980s and 1990s. At the turn of the millennium, the line between a graphing calculator and a handheld computer was not always clear, as some very advanced calculators such as the TI-89, the Voyage 200 and HP-49G could differentiate and integrate functions, solve differential equations, run word processing and PIM software, and connect by wire or IR to other calculators/computers."
+2295,"The HP 12c financial calculator is still produced. It was introduced in 1981 and is still being made with few changes. The HP 12c featured the reverse Polish notation mode of data entry. In 2003 several new models were released, including an improved version of the HP 12c, the ""HP 12c platinum edition"" which added more memory, more built-in functions, and the addition of the algebraic mode of data entry."
+2296,"Calculated Industries competed with the HP 12c in the mortgage and real estate markets by differentiating the key labeling; changing the ""I"", ""PV"", ""FV"" to easier labeling terms such as ""Int"", ""Term"", ""Pmt"", and not using the reverse Polish notation. However, CI's more successful calculators involved a line of construction calculators, which evolved and expanded in the 1990s to present. According to Mark Bollman, a mathematics and calculator historian and associate professor of mathematics at Albion College, the ""Construction Master is the first in a long and profitable line of CI construction calculators"" which carried them through the 1980s, 1990s, and to the present."
+2297,"In most countries, students use calculators for schoolwork. There was some initial resistance to the idea out of fear that basic or elementary arithmetic skills would suffer. There remains disagreement about the importance of the ability to perform calculations in the head, with some curricula restricting calculator use until a certain level of proficiency has been obtained, while others concentrate more on teaching estimation methods and problem-solving. Research suggests that inadequate guidance in the use of calculating tools can restrict the kind of mathematical thinking that students engage in. Others have argued that calculator use can even cause core mathematical skills to atrophy, or that such use can prevent understanding of advanced algebraic concepts. In December 2011 the UK's Minister of State for Schools, Nick Gibb, voiced concern that children can become ""too dependent"" on the use of calculators. As a result, the use of calculators is to be included as part of a review of the Curriculum. In the United States, many math educators and boards of education have enthusiastically endorsed the National Council of Teachers of Mathematics  standards and actively promoted the use of classroom calculators from kindergarten through high school."
+2298,"Personal computers often come with a calculator utility program that emulates the appearance and functions of a calculator, using the graphical user interface to portray a calculator. Examples include the Windows Calculator, Apple's Calculator, and KDE's KCalc. Most personal data assistants  and smartphones also have such a feature."
+2299,"The fundamental difference between a calculator and computer is that a computer can be programmed in a way that allows the program to take different branches according to intermediate results, while calculators are pre-designed with specific functions  built in. The distinction is not clear-cut: some devices classed as programmable calculators have programming functions, sometimes with support for programming languages ."
+2300,"For instance, instead of a hardware multiplier, a calculator might implement floating point mathematics with code in read-only memory , and compute trigonometric functions with the CORDIC algorithm because CORDIC does not require much multiplication. Bit serial logic designs are more common in calculators whereas bit parallel designs dominate general-purpose computers, because a bit serial design minimizes chip complexity, but takes many more clock cycles. This distinction blurs with high-end calculators, which use processor chips associated with computer and embedded systems design, more so the Z80, MC68000, and ARM architectures, and some custom designs specialized for the calculator market."
+2301,"In computer programming, a function, subprogram, procedure, method, routine or subroutine is a callable unit that has a well-defined behavior and can be invoked by other software units to exhibit that behavior."
+2302,"Callable units provide a powerful programming tool. The primary purpose is to allow for the decomposition of a large and/or complicated problem into chunks that have relatively low cognitive load and to assign the chunks meaningful names . Judicious application can reduce the cost of developing and maintaining software, while increasing its quality and reliability."
+2303,"Callable units are present at multiple levels of abstraction in the programming environment. For example, a programmer may write a function in source code that is compiled to machine code that implements similar semantics. There is a callable unit in the source code and an associated one in the machine code, but they are different kinds of callable units – with different implications and features."
+2304,"The idea of a callable unit was initially conceived by John Mauchly and Kathleen Antonelli during their work on ENIAC and recorded in a January 1947 Harvard symposium on ""Preparation of Problems for EDVAC-type Machines"". Maurice Wilkes, David Wheeler, and Stanley Gill are generally credited with the formal invention of this concept, which they termed a closed sub-routine, contrasted with an open subroutine or macro. However, Alan Turing had discussed subroutines in a paper of 1945 on design proposals for the NPL ACE, going so far as to invent the concept of a return address stack."
+2305,"The idea of a subroutine was worked out after computing machines had already existed for some time. The arithmetic and conditional jump instructions were planned ahead of time and have changed relatively little, but the special instructions used for procedure calls have changed greatly over the years. The earliest computers and microprocessors, such as the Manchester Baby and the RCA 1802, did not have a single subroutine call instruction. Subroutines could be implemented, but they required programmers to use the call sequence—a series of instructions—at each call site."
+2306,Subroutines were implemented in Konrad Zuse's Z4 in 1945.
+2307,"In 1945, Alan M. Turing used the terms ""bury"" and ""unbury"" as a means of calling and returning from subroutines."
+2308,"In January 1947 John Mauchly presented general notes at 'A Symposium of Large Scale Digital Calculating Machinery'
+under the joint sponsorship of Harvard University and the Bureau of Ordnance, United States Navy. Here he discusses serial and parallel operation suggesting"
+2309,"In other words, one can designate subroutine A as division and subroutine B as complex multiplication and subroutine C as the evaluation of a standard error of a sequence of numbers, and so on through the list of subroutines needed for a particular problem. ... All these subroutines will then be stored in the machine, and all one needs to do is make a brief reference to them by number, as they are indicated in the coding."
+2310,Kay McNulty had worked closely with John Mauchly on the ENIAC team and developed an idea for subroutines for the ENIAC computer she was programming during World War II. She and the other ENIAC programmers used the subroutines to help calculate missile trajectories.
+2311,Goldstine and von Neumann wrote a paper dated 16 August 1948 discussing the use of subroutines.
+2312,"Some very early computers and microprocessors, such as the IBM 1620, the Intel 4004 and Intel 8008, and the PIC microcontrollers, have a single-instruction subroutine call that uses a dedicated hardware stack to store return addresses—such hardware supports only a few levels of subroutine nesting, but can support recursive subroutines. Machines before the mid-1960s—such as the UNIVAC I, the PDP-1, and the IBM 1130—typically use a calling convention which saved the instruction counter in the first memory location of the called subroutine. This allows arbitrarily deep levels of subroutine nesting but does not support recursive subroutines. The IBM System/360 had a subroutine call instruction that placed the saved instruction counter value into a general-purpose register; this can be used to support arbitrarily deep subroutine nesting and recursive subroutines. The Burroughs B5000  is one of the first computers to store subroutine return data on a stack."
+2313,"The DEC PDP-6  is one of the first accumulator-based machines to have a subroutine call instruction that saved the return address in a stack addressed by an accumulator or index register. The later PDP-10 , PDP-11  and VAX-11  lines followed suit; this feature also supports both arbitrarily deep subroutine nesting and recursive subroutines."
+2314,"In the very early assemblers, subroutine support was limited. Subroutines were not explicitly separated from each other or from the main program, and indeed the source code of a subroutine could be interspersed with that of other subprograms. Some assemblers would offer predefined macros to generate the call and return sequences. By the 1960s, assemblers usually had much more sophisticated support for both inline and separately assembled subroutines that could be linked together."
+2315,One of the first programming languages to support user-written subroutines and functions was FORTRAN II.  The IBM FORTRAN II compiler was released in 1958.  ALGOL 58 and other early programming languages also supported procedural programming.
+2316,"Even with this cumbersome approach, subroutines proved very useful. They allowed the use of the same code in many different programs. Memory was a very scarce resource on early computers, and subroutines allowed significant savings in the size of programs."
+2317,"Many early computers loaded the program instructions into memory from a punched paper tape. Each subroutine could then be provided by a separate piece of tape, loaded or spliced before or after the main program ; and the same subroutine tape could then be used by many different programs. A similar approach was used in computers that loaded program instructions from punched cards. The name subroutine library originally meant a library, in the literal sense, which kept indexed collections of tapes or decks of cards for collective use."
+2318,"To remove the need for self-modifying code, computer designers eventually provided an indirect jump instruction, whose operand, instead of being the return address itself, was the location of a variable or processor register containing the return address."
+2319,"On those computers, instead of modifying the function's return jump, the calling program would store the return address in a variable so that when the function completed, it would execute an indirect jump that would direct execution to the location given by the predefined variable."
+2320,"Another advance was the jump to subroutine instruction, which combined the saving of the return address with the calling jump, thereby minimizing overhead significantly."
+2321,"In the IBM System/360, for example, the branch instructions BAL or BALR, designed for procedure calling, would save the return address in a processor register specified in the instruction, by convention register 14. To return, the subroutine had only to execute an indirect branch instruction  through that register. If the subroutine needed that register for some other purpose , it would save the register's contents to a private memory location or a register stack."
+2322,"In systems such as the HP 2100, the JSB instruction would perform a similar task, except that the return address was stored in the memory location that was the target of the branch. Execution of the procedure would actually begin at the next memory location. In the HP 2100 assembly language, one would write, for example"
+2323,to call a subroutine called MYSUB from the main program.  The subroutine would be coded as
+2324,"The JSB instruction placed the address of the NEXT instruction  into the location specified as its operand , and then branched to the NEXT location after that . The subroutine could then return to the main program by executing the indirect jump JMP MYSUB, I which branched to the location stored at location MYSUB."
+2325,"Compilers for Fortran and other languages could easily make use of these instructions when available. This approach supported multiple levels of calls; however, since the return address, parameters, and return values of a subroutine were assigned fixed memory locations, it did not allow for recursive calls."
+2326,"Incidentally, a similar method was used by Lotus 1-2-3, in the early 1980s, to discover the recalculation dependencies in a spreadsheet. Namely, a location was reserved in each cell to store the return address. Since circular references are not allowed for natural recalculation order, this allows a tree walk without reserving space for a stack in memory, which was very limited on small computers such as the IBM PC."
+2327,"Most modern implementations of a function call use a call stack, a special case of the stack data structure, to implement function calls and returns. Each procedure call creates a new entry, called a stack frame, at the top of the stack; when the procedure returns, its stack frame is deleted from the stack, and its space may be used for other procedure calls. Each stack frame contains the private data of the corresponding call, which typically includes the procedure's parameters and internal variables, and the return address."
+2328,"The call sequence can be implemented by a sequence of ordinary instructions  and very long instruction word  architectures), but many traditional machines designed since the late 1960s have included special instructions for that purpose."
+2329,"The call stack is usually implemented as a contiguous area of memory. It is an arbitrary design choice whether the bottom of the stack is the lowest or highest address within this area, so that the stack may grow forwards or backwards in memory; however, many architectures chose the latter."
+2330,"Some designs, notably some Forth implementations, used two separate stacks, one mainly for control information  and the other for data. The former was, or worked like, a call stack and was only indirectly accessible to the programmer through other language constructs while the latter was more directly accessible."
+2331,"When stack-based procedure calls were first introduced, an important motivation was to save precious memory. With this scheme, the compiler does not have to reserve separate space in memory for the private data  of each procedure. At any moment, the stack contains only the private data of the calls that are currently active . Because of the ways in which programs were usually assembled from libraries, it was  not uncommon to find programs that include thousands of functions, of which only a handful are active at any given moment. For such programs, the call stack mechanism could save significant amounts of memory. Indeed, the call stack mechanism can be viewed as the earliest and simplest method for automatic memory management."
+2332,"However, another advantage of the call stack method is that it allows recursive function calls, since each nested call to the same procedure gets a separate instance of its private data."
+2333,"In a multi-threaded environment, there is generally more than one stack. An environment that fully supports coroutines or lazy evaluation may use data structures other than stacks to store their activation records."
+2334,"One disadvantage of the call stack mechanism is the increased cost of a procedure call and its matching return. The extra cost includes incrementing and decrementing the stack pointer , and accessing the local variables and parameters by frame-relative addresses, instead of absolute addresses. The cost may be realized in increased execution time, or increased processor complexity, or both."
+2335,"This overhead is most obvious and objectionable in leaf procedures or leaf functions, which return without making any procedure calls themselves. To reduce that overhead, many modern compilers try to delay the use of a call stack until it is really needed. For example, the call of a procedure P may store the return address and parameters of the called procedure in certain processor registers, and transfer control to the procedure's body by a simple jump. If the procedure P returns without making any other call, the call stack is not used at all. If P needs to call another procedure Q, it will then use the call stack to save the contents of any registers  that will be needed after Q returns."
+2336,"In general, a callable unit is a list of instructions that, starting at the first instruction, executes sequentially except as directed via its internal logic. It can be invoked  many times during the execution of a program. Execution continues at the next instruction after the call instruction when it returns control."
+2337,"The features of implementations of callable units evolved over time and varies by context.
+This section describes features of the various common implementations."
+2338,"Most modern programming languages provide features to define and call functions, including syntax for accessing such features, including:"
+2339,"Some languages, such as Pascal, Fortran, Ada and many dialects of BASIC, use a different name for a callable unit that returns a value  vs. one that does not . 
+Other languages, such as C, C++, C# and Lisp, use only one name for a callable unit, function. The C-family languages use the keyword void to indicate no return value."
+2340,"If declared to return a value, a call can be embedded in an expression in order to consume the return value. For example, a square root callable unit might be called like y = sqrt."
+2341,"A callable unit that does not return a value is called as a stand-alone statement like print. This syntax can also be used for a callable unit that returns a value, but the return value will be ignored."
+2342,"Some older languages require a keyword for calls that do not consume a return value, like CALL print."
+2343,"Most implementations, especially in modern languages, support parameters which the callable declares as formal parameters. A caller passes actual parameters, a.k.a. arguments, to match. Different programming languages provide different conventions for passing arguments."
+2344,"In some languages, such as BASIC, a callable has different syntax  for a callable that returns a value vs. one that does not. 
+In other languages, the syntax is the same regardless.
+In some of these languages an extra keyword is used to declare no return value; for example void in C, C++ and C#.
+In some languages, such as Python, the difference is whether the body contains a return statement with a value, and a particular callable may return with or without a value based on control flow."
+2345,"In many contexts, a callable may have side effect behavior such as modifying passed or global data, reading from or writing to a peripheral device, accessing a file, halting the program or the machine, or temporarily pausing program execution."
+2346,"In strictly functional programming languages such as Haskell, a function can have no side effects, which means it cannot change the state of the program. Functions always return the same result for the same input. Such languages typically only support functions that return a value, since there is no value in a function that has neither return value nor side effect."
+2347,Most contexts support local variables – memory owned by a callable to hold intermediate values. These variables are typically stored in the call's activation record on the call stack along with other information such as the return address.
+2348,"If supported by the language, a callable may call itself, causing its execution to suspend while another nested execution of the same callable executes. Recursion is a useful means to simplify some complex algorithms and break down complex problems. Recursive languages provide a new copy of local variables on each call. If the programmer desires the recursive callable to use the same variables instead of using locals, they typically declare them in a shared context such static or global."
+2349,"Languages going back to ALGOL, PL/I and C and modern languages, almost invariably use a call stack, usually supported by the instruction sets to provide an activation record for each call. That way, a nested call can modify its local variables without effecting any of the suspended calls variables."
+2350,Recursion allows direct implementation of functionality defined by mathematical induction and recursive divide and conquer algorithms. Here is an example of a recursive function in C/C++ to find Fibonacci numbers:
+2351,"Early languages like Fortran did not initially support recursion because only one set of variables and return address were allocated for each callable. Early computer instruction sets made storing return addresses and variables on a stack difficult.  Machines with index registers or general-purpose registers, e.g., CDC 6000 series, PDP-6, GE 635, System/360, UNIVAC 1100 series, could use one of those registers as a stack pointer."
+2352,"Some languages, e.g., Ada, Pascal, PL/I, Python, support declaring and defining a function inside, e.g., a function body, such that the name of the inner is only visible within the body of the outer."
+2353,"If a callable can be executed properly even when another execution of the same callable is already in progress, that callable is said to be reentrant. A reentrant callable is also useful in multi-threaded situations since multiple threads can call the same callable without fear of interfering with each other. In the IBM CICS transaction processing system, quasi-reentrant was a slightly less restrictive, but similar, requirement for application programs that were shared by many threads."
+2354,"Some languages support overloading – allow multiple callables with the same name in the same scope, but operating on different types of input. Consider the square root function applied to real number, complex number and matrix input. The algorithm for each type of input is different, and the return value may have a different type. By writing three separate callables with the same name. i.e. sqrt, the resulting code may be easier to write and to maintain since each one has a name that is relatively easy to understand and to remember instead of giving longer and more complicated names like sqrt_real, sqrt_complex, qrt_matrix."
+2355,Overloading is supported in many languages that support strong typing. Often the compiler selects the overload to call based on the type of the input arguments or it fails if the input arguments do not select an overload. Older and weakly-typed languages generally do not support overloading.
+2356,"Here is an example of overloading in C++, two functions Area that accept different types:"
+2357,PL/I has the GENERIC attribute to define a generic name for a set of entry references called with different types of arguments. Example:
+2358,"Multiple argument definitions may be specified for each entry. A call to ""gen_name"" will result in a call to ""name"" when the argument is FIXED BINARY, ""flame"" when FLOAT"", etc. If the argument matches none of the choices ""pathname"" will be called."
+2359,"A closure is a callable plus values of some of its variables captured from the environment in which it was created. Closures were a notable feature of the Lisp programming language, introduced by John McCarthy. Depending on the implementation, closures can serve as a mechanism for side-effects."
+2360,"Besides its happy path behavior, a callable may need to inform the caller about an exceptional condition that occurred during its execution."
+2361,Most modern languages support exceptions which allows for exceptional control flow that pops the call stack until an exception handler is found to handle the condition.
+2362,Languages that do not support exceptions can use the return value to indicate success or failure of a call. Another approach is to use a well-known location like a global variable for success indication. A callable writes the value and the caller reads it after a call.
+2363,"In the IBM System/360, where return code was expected from a subroutine, the return value was often designed to be a multiple of 4—so that it could be used as a direct branch table index into a branch table often located immediately after the call instruction to avoid extra conditional tests, further improving efficiency. In the System/360 assembly language, one would write, for example:"
+2364,"A call has runtime overhead, which may include but is not limited to:"
+2365,Various techniques are employed to minimize the runtime cost of calls.
+2366,"Some optimizations for minimizing call overhead may seem straight forward, but cannot be used if the callable has side effects. For example, in the expression -1)/+1), the function f cannot be called only once with its value used two times since the two calls may return different results. Moreover, in the few languages which define the order of evaluation of the division operator's operands, the value of x must be fetched again before the second call, since the first call may have changed it. Determining whether a callable has a side effect is difficult – indeed, undecidable by virtue of Rice's theorem. So, while this optimization is safe in a purely functional programming language, a compiler for an language not limited to functional typically assumes the worst case, that every callable may have side effects."
+2367,"Inlining eliminates calls for particular callables. The compiler replaces each call with the compiled code of the callable. Not only does this avoid the call overhead, but it also allows the compiler to optimize code of the caller more effectively by taking into account the context and arguments at that call. Inlining, however, usually increases the compiled code size, except when only called once or the body is very short, like one line."
+2368,"Callables can be defined within a program, or separately in a library that can be used by multiple programs."
+2369,"A compiler translates call and return statements into machine instructions according to a well-defined calling convention. For code compiled by the same or a compatible compiler, functions can be compiled separately from the programs that call them. The instruction sequences corresponding to call and return statements are called the procedure's prologue and epilogue."
+2370,"A built-in function, or builtin function, or intrinsic function, is a function for which the compiler generates code at compile time or provides in a way other than for other functions. A built-in function does not need to be defined like other functions since it is built in to the programming language."
+2371,Advantages of breaking a program into functions include:
+2372,"Compared to using in-line code, invoking a function imposes some computational overhead in the call mechanism."
+2373,"A function typically requires standard housekeeping code – both at the entry to, and exit from, the function ."
+2374,Many programming conventions have been developed regarding callables.
+2375,"With respect to naming, many developers name a callable with a phrase starting with a verb when it does a certain task, with an adjective when it makes an inquiry, and with a noun when it is used to substitute variables."
+2376,"Some programmers suggest that a callable should perform exactly one task, and if it performs more than one task, it should be split up into multiple callables. They argue that callables are key components in software maintenance, and their roles in the program must remain distinct."
+2377,"Proponents of modular programming advocate that each callable should have minimal dependency on the rest of the codebase. For example, the use of global variables is generally deemed unwise, because it adds coupling between all callables that use the global variables. If such coupling is not necessary, they advise to refactor callables to accept passed parameters instead."
+2378,"Early BASIC variants require each line to have a unique number  that orders the lines for execution, provides no separation of the code that is callable, no mechanism for passing arguments or to return a value and all variables are global. It provides the command GOSUB where sub is short for sub procedure, subprocedure or subroutine. Control jumps to the specified line number and then continues on the next line on return."
+2379,This code repeatedly asks the user to enter a number and reports the square root of the value. Lines 100-130 are the callable.
+2380,"In Microsoft Small Basic, targeted to the student first leaning how to program in a text-based language, a callable unit is called a subroutine. 
+The Sub keyword denotes the start of a subroutine and is followed by a name identifier. Subsequent lines are the body which ends with the EndSub keyword. 
+"
+2381,"This can be called as SayHello.
+"
+2382,"In later versions of Visual Basic , including the latest product line and VB6, the term procedure is used for the callable unit concept. The keyword Sub is used to return no value and Function to return a value. When used in the context of a class, a procedure is a method.
+"
+2383,"Each parameter has a data type that can be specified, but if not, defaults to Object for later versions based on .NET and variant for VB6."
+2384,"VB supports parameter passing conventions by value and by reference via the keywords ByVal and ByRef, respectively. 
+Unless ByRef is specified, an argument is passed ByVal. Therefore, ByVal is rarely explicitly specified."
+2385,"For a simple type like a number these conventions are relatively clear. Passing ByRef allows the procedure to modify the passed variable whereas passing ByVal does not. For an object, semantics can confuse programmers since an object is always treated as a reference. Passing an object ByVal copies the reference; not the state of the object. The called procedure can modify the state of the object via its methods yet cannot modify the object reference of the actual parameter."
+2386,"The does not return a value and has to be called stand-alone, like DoSomething"
+2387,"This returns the value 5, and a call can be part of an expression like y = x + GiveMeFive"
+2388,"This has a side-effect – modifies the variable passed by reference and could be called for variable v like AddTwo. Giving v is 5 before the call, it will be 7 after."
+2389,"In C and C++, a callable unit is called a function. 
+A function definition starts with the name of the type of value that it returns or void to indicate that it does not return a value. This is followed by the function name, formal arguments in parentheses, and body lines in braces."
+2390,"In C++, a function declared in a class  is called a member function or method. A function outside of a class can be called a free function to distinguish it from a member function.
+"
+2391,"This function does not return a value and is always called stand-alone, like doSomething"
+2392,This function returns the integer value 5. The call can be stand-alone or in an expression like y = x + giveMeFive
+2393,"This function has a side-effect – modifies the value passed by address to the input value plus 2. It could be called for variable v as addTwo where the ampersand  tells the compiler to pass the address of a variable. Giving v is 5 before the call, it will be 7 after."
+2394,This function requires C++ – would not compile as C. It has the same behavior as the preceding example but passes the actual parameter by reference rather than passing its address. A call such as addTwo does not include an ampersand since the compiler handles passing by reference without syntax in the call.
+2395,"In PL/I a called procedure may be passed a descriptor providing information about the argument, such as string lengths and array bounds.  This allows the procedure to be more general and eliminates the need for the programmer to pass such information.  By default PL/I passes arguments by reference.  A  function to change the sign of each element of a two-dimensional array might look like:"
+2396,This could be called with various arrays as follows:
+2397,"In Python, the keyword def denotes the start of a function definition. The statements of the function body follow as indented on subsequent lines and end at the line that is indented the same as the first line or end of file."
+2398,"The first function returns greeting text that includes the name passed by the caller. The second function calls the first and is called like greet_martin to write ""Welcome Martin"" to the console."
+2399,"In the procedural interpretation of logic programs, logical implications behave as goal-reduction procedures. A rule  of the form:"
+2400,which has the logical reading:
+2401,behaves as a procedure that reduces goals that unify with  A to subgoals that are instances ofB.
+2402,"Consider, for example, the Prolog program:"
+2403,"Notice that the motherhood function, X = mother is represented by a relation, as in a relational database. However, relations in Prolog function as callable units."
+2404,"For example, the procedure call  ?- parent_child produces the output X = elizabeth. But the same procedure can be called with other input-output patterns. For example:"
+2405,"A branch instruction can be either an unconditional branch, which always results in branching, or a conditional branch, which may or may not cause branching depending on some condition. Also, depending on how it specifies the address of the new instruction sequence , a branch instruction is generally classified as direct, indirect or relative, meaning that the instruction contains the target address, or it specifies where the target address is to be found , or it specifies the difference between the current and target addresses."
+2406,"Branch instructions can alter the contents of the CPU's Program Counter  . The PC maintains the memory address of the next machine instruction to be fetched and executed. Therefore, a branch, if executed, causes the CPU to execute code from a new memory address, changing the program logic according to the algorithm planned by the programmer."
+2407,"One type of machine level branch is the jump instruction. These may or may not result in the PC being loaded or modified with some new, different value other than what it ordinarily would have been . Jumps typically have unconditional and conditional forms where the latter may be taken or not taken  depending on some condition."
+2408,"The second type of machine level branch is the call instruction which is used to implement subroutines. Like jump instructions, calls may or may not modify the PC according to condition codes, however, additionally a return address is saved in a secure place in memory . Upon completion of the subroutine, this return address is restored to the PC, and so program execution resumes with the instruction following the call instruction."
+2409,"The third type of machine level branch is the return instruction. This ""pops"" a return address off the stack and loads it into the PC register, thus returning control to the calling routine. Return instructions may also be conditionally executed. This description pertains to ordinary practice; however, the machine programmer has considerable powers to manipulate the return address on the stack, and so redirect program execution in any number of different ways."
+2410,"Depending on the processor, jump and call instructions may alter the contents of the PC register in different ways. An absolute address may be loaded, or the current contents of the PC may have some value  added or subtracted from its current value, making the destination address relative to the current place in the program. The source of the displacement value may vary, such as an immediate value embedded within the instruction, or the contents of a processor register or memory location, or the contents of some location added to an index value."
+2411,"The term branch can also be used when referring to programs in high-level programming languages. In these branches usually take the form of conditional statements of various forms that encapsulate the instruction sequence that will be executed if the conditions are satisfied. Unconditional branch instructions such as GOTO are used to unconditionally jump to a different instruction sequence. If the algorithm requires a conditional branch, the GOTO  is preceded by an IF-THEN statement specifying the condition. All high level languages support algorithms that can re-use code as a loop, a control structure that repeats a sequence of instructions until some condition is satisfied that causes the loop to terminate. Loops also qualify as branch instructions. At the machine level, loops are implemented as ordinary conditional jumps that redirect execution to repeating code."
+2412,"In CPUs with flag registers, an earlier instruction sets a condition in the flag register. The earlier instruction may be arithmetic, or a logic instruction.  It is often close to the branch, though not necessarily the instruction immediately before the branch.  The stored condition is then used in a branch such as jump if overflow-flag set. This temporary information is often stored in a flag register but may also be located elsewhere.  A flag register design is simple in slower, simple computers. In fast computers a flag register can place a bottleneck on speed, because instructions that could otherwise operate in parallel  need to set the flag bits in a particular sequence."
+2413,"There are also machines  where the condition may be checked by the jump instruction itself, such as branch <label> if register X negative. In simple computer designs, comparison branches execute more arithmetic and can use more power than flag register branches. In fast computer designs comparison branches can run faster than flag register branches, because comparison branches can access the registers with more parallelism, using the same CPU mechanisms as a calculation."
+2414,"Some early and simple CPU architectures, still found in microcontrollers, may not implement a conditional jump, but rather only a conditional ""skip the next instruction"" operation. A conditional jump or call is thus implemented as a conditional skip of an unconditional jump or call instruction."
+2415,"Depending on the computer architecture, the assembly language mnemonic for a jump instruction is typically some shortened form of the word jump or the word branch, often along with other informative letters  representing the condition. Sometimes other details are included as well, such as the range of the jump  or a special addressing mode that should be used to locate the actual effective offset."
+2416,This table lists the machine level branch or jump instructions found in several well-known architectures:
+2417,"* x86, the PDP-11, VAX, and some others, set the carry-flag to signal borrow and clear the carry-flag to signal no borrow. ARM, 6502, the PIC, and some others, do the opposite for subtractive operations. This inverted function of the carry flag for certain instructions is marked by , that is, borrow=not carry in some parts of the table, but if not otherwise noted, borrow≡carry. However, carry on additive operations are handled the same way by most architectures."
+2418,"To achieve high performance, modern processors are pipelined. They consist of multiple parts that each partially process an instruction, feed their results to the next stage in the pipeline, and start working on the next instruction in the program. This design expects instructions to execute in a particular unchanging sequence. Conditional branch instructions make it impossible to know this sequence.  So conditional branches can cause ""stalls"" in which the pipeline has to be restarted on a different part of the program."
+2419,Several techniques improve speed by reducing stalls from conditional branches.
+2420,"Historically, branch prediction took statistics, and used the result to optimize code.  A programmer would compile a test version of a program, and run it with test data.  The test code counted how the branches were actually taken. The statistics from the test code were then used by the compiler to optimize the branches of released code. The optimization would arrange that the fastest branch direction  would always be the most frequently taken control flow path. To permit this, CPUs must be designed with  predictable branch timing. Some CPUs have instruction sets  that were designed with ""branch hints"" so that a compiler can tell a CPU how each branch is to be taken."
+2421,The problem with software branch prediction is that it requires a complex software development process.
+2422,"To run any software, hardware branch predictors moved the statistics into the electronics. Branch predictors are parts of a processor that guess the outcome of a conditional branch. Then the processor's logic gambles on the guess by beginning to execute the expected instruction flow. An example of a simple hardware branch prediction scheme is to assume that all backward branches  are taken , and all forward branches  are not taken . Better branch predictors are developed and validated statistically by running them in simulation on a variety of test programs.  Good predictors usually count the outcomes of previous executions of a branch. Faster, more expensive computers can then run faster by investing in better branch prediction electronics. In a CPU with hardware branch prediction, branch hints let the compiler's presumably superior branch prediction override the hardware's more simplistic branch prediction."
+2423,"Some logic can be written without branches or with fewer branches. It is often possible to use bitwise operations, conditional moves or other predication instead of branches. In fact, branch-free code is a must for cryptography due to timing attacks."
+2424,"Another technique is a branch delay slot. In this approach, at least one instruction following a branch is always executed, with some exceptions such like the legacy MIPS architecture likely/unlikely branch instruction. Therefore, the computer can use this instruction to do useful work whether or not its pipeline stalls. This approach was historically popular in RISC computers. In a family of compatible CPUs, it complicates multicycle CPUs , faster CPUs with longer-than-expected pipelines, and superscalar CPUs "
+2425,The RA-machine's equivalent of the universal Turing machine – with its program in the registers as well as its data – is called the random-access stored-program machine or RASP-machine. It is an example of the so-called von Neumann architecture and is closest to the common notion of a computer.
+2426,"Together with the Turing machine and counter-machine models, the RA-machine and RASP-machine models are used for computational complexity analysis. Van Emde Boas  calls these three together with the pointer machine, ""sequential machine"" models, to distinguish them from ""parallel random-access machine"" models."
+2427,An RA-machine consists of the following:
+2428,"For a description of a similar concept, but humorous, see the ""programming language"" Branflakes."
+2429,"The concept of a random-access machine  starts with the simplest model of all, the so-called counter machine model. Two additions move it away from the counter machine, however. The first enhances the machine with the convenience of indirect addressing; the second moves the model toward the more conventional accumulator-based computer with the addition of one or more auxiliary  registers, the most common of which is called ""the accumulator""."
+2430,"A random-access machine  is an abstract computational-machine model identical to a multiple-register counter machine with the addition of indirect addressing. At the discretion of instruction from its finite state machine's TABLE, the machine derives a ""target"" register's address either  directly from the instruction itself, or  indirectly from the contents  of the ""pointer"" register specified in the instruction."
+2431,"By definition: A register is a location with both an address  and a content – a single natural number. For precision we will use the quasi-formal symbolism from Boolos-Burgess-Jeffrey  to specify a register, its contents, and an operation on a register:"
+2432,"Definition: A direct instruction is one that specifies in the instruction itself the address of the source or destination register whose contents will be the subject of the instruction.
+Definition: An indirect instruction is one that specifies a ""pointer register"", the contents of which is the address of a ""target"" register. The target register can be either a source or a destination . A register can address itself indirectly."
+2433,"Definition: The contents of source register is used by the instruction. The source register's address can be specified either  directly by the instruction, or  indirectly by the pointer register specified by the instruction."
+2434,"Definition: The contents of the pointer register is the address of the ""target"" register."
+2435,"Definition: The contents of the pointer register points to the target register – the ""target"" may be either a source or a destination register."
+2436,"Definition: The destination register is where the instruction deposits its result. The source register's address can be specified either  directly by the instruction, or  indirectly by the pointer register specified by the instruction. The source and destination registers can be one."
+2437,"The register machine has, for a memory external to its finite-state machine – an unbounded  collection of discrete and uniquely labelled locations with unbounded capacity, called ""registers"". These registers hold only natural numbers . Per a list of sequential instructions in the finite state machine's TABLE, a few  types of primitive operations operate on the contents of these ""registers"". Finally, a conditional-expression in the form of an IF-THEN-ELSE is available to test the contents of one or two registers and ""branch/jump"" the finite state machine out of the default instruction-sequence."
+2438,Base model 1: The model closest to Minsky's  visualization and to Lambek :
+2439,"Base model 2: The ""successor"" model :"
+2440,"Base model 3: Used by Elgot-Robinson  in their investigation of bounded and unbounded RASPs – the ""successor"" model with COPY in the place of CLEAR:"
+2441,"The three base sets 1, 2, or 3 above are equivalent in the sense that one can create the instructions of one set using the instructions of another set  – declare a reserved register e.g. call it ""0""  to contain the number 0). The choice of model will depend on which an author finds easiest to use in a demonstration, or a proof, etc."
+2442,"Moreover, from base sets 1, 2, or 3 we can create any of the primitive recursive functions , Boolos-Burgess-Jeffrey  ). . However, building the primitive recursive functions is difficult because the instruction sets are so ... primitive . One solution is to expand a particular set with ""convenience instructions"" from another set:"
+2443,"Again, all of this is for convenience only; none of this increases the model's intrinsic power."
+2444,"For example: the most expanded set would include each unique instruction from the three sets, plus unconditional jump J  i.e.:"
+2445,"Most authors pick one or the other of the conditional jumps, e.g. Shepherdson-Sturgis  use the above set minus JE ."
+2446,"In our daily lives the notion of an ""indirect operation"" is not unusual."
+2447,"Indirection specifies a location identified as the pirate chest in ""Tom_&_Becky's_cave..."" that acts as a pointer to any other location : its contents  provides the ""address"" of the target location
+""under_Thatcher's_front_porch"" where the real action is occurring."
+2448,In the following one must remember that these models are abstract models with two fundamental differences from anything physically real: unbounded numbers of registers each with unbounded capacities. The problem appears most dramatically when one tries to use a counter-machine model to build a RASP that is Turing equivalent and thus compute any partial mu recursive function:
+2449,"So how do we address a register beyond the bounds of the finite state machine? One approach would be to modify the program-instructions  so that they contain more than one command. But this too can be exhausted unless an instruction is of  unbounded size. So why not use just one ""über-instruction"" – one really really big number – that contains all the program instructions encoded into it! This is how Minsky solves the problem, but the Gödel numbering he uses represents a great inconvenience to the model, and the result is nothing at all like our intuitive notion of a ""stored program computer""."
+2450,"Elgot and Robinson  come to a similar conclusion with respect to a RASP that is ""finitely determined"". Indeed it can access an unbounded number of registers  but only if the RASP allows ""self modification"" of its program instructions, and has encoded its ""data"" in a Gödel number ."
+2451,In the context of a more computer-like model using his RPT  instruction Minsky  tantalizes us with a solution to the problem  but offers no firm resolution. He asserts:
+2452,"He offers us a bounded RPT that together with CLR  and INC  can compute any primitive recursive function, and he offers the unbounded RPT quoted above that as playing the role of μ operator; it together with CLR  and INC  can compute the mu recursive functions. But he does not discuss ""indirection"" or the RAM model per se."
+2453,"From the references in Hartmanis  it appears that Cook  has firmed up the notion of indirect addressing. This becomes clearer in the paper of Cook and Reckhow  – Cook is Reckhow's Master's thesis advisor. Hartmanis' model – quite similar to Melzak's  model – uses two and three-register adds and subtracts and two parameter copies; Cook and Reckhow's model reduce the number of parameters  to one call-out by use of an accumulator ""AC""."
+2454,"The solution in a nutshell: Design our machine/model with unbounded indirection – provide an unbounded ""address"" register that can potentially name  any register no matter how many there are. For this to work, in general, the unbounded register requires an ability to be cleared and then incremented  by a potentially infinite loop. In this sense the solution represents the unbounded μ operator that can, if necessary, hunt ad infinitum along the unbounded string of registers until it finds what it is looking for. The pointer register is exactly like any other register with one exception: under the circumstances called ""indirect addressing"" it provides its contents, rather than the address-operand in the state machine's TABLE, to be the address of the target register ."
+2455,"If we eschew the Minsky approach of one monster number in one register, and specify that our machine model will be ""like a computer"" we have to confront this problem of indirection if we are to compute the recursive functions  – both total and partial varieties."
+2456,"Our simpler counter-machine model can do a ""bounded"" form of indirection – and thereby compute the sub-class of primitive recursive functions – by using a primitive recursive ""operator"" called ""definition by cases""  p. 229 and Boolos-Burgess-Jeffrey p. 74). Such a ""bounded indirection"" is a laborious, tedious affair. ""Definition by cases"" requires the machine to determine/distinguish the contents of the pointer register by attempting, time after time until success, to match this contents against a number/name that the case operator explicitly declares. Thus the definition by cases starts from e.g. the lower bound address and continues ad nauseam toward the upper bound address attempting to make a match:"
+2457,"""Bounded"" indirection will not allow us to compute the partial recursive functions – for those we need unbounded indirection aka the μ operator."
+2458,"To be Turing equivalent the counter machine needs to either use the unfortunate single-register Minsky Gödel number method, or be augmented with an ability to explore the ends of its register string, ad infinitum if necessary.  pp. 316ff Chapter XII Partial Recursive Functions, in particular p. 323-325.) See more on this in the example below."
+2459,"For unbounded indirection we require a ""hardware"" change in our machine model. Once we make this change the model is no longer a counter machine, but rather a random-access machine."
+2460,"Now when e.g. INC  is specified, the finite state machine's instruction will have to specify where the address of the register of interest will come from. This where can be either  the state machine's instruction that provides an explicit label, or  the pointer-register whose contents is the address of interest. Whenever an instruction specifies a register address it now will also need to specify an additional parameter ""i/d"" – ""indirect/direct"". In a sense this new ""i/d"" parameter is a ""switch"" that flips one way to get the direct address as specified in the instruction or the other way to get the indirect address from the pointer register . This ""mutually exclusive but exhaustive choice"" is yet another example of ""definition by cases"", and the arithmetic equivalent shown in the example below is derived from the definition in Kleene  p. 229."
+2461,"Probably the most useful of the added instructions is COPY. Indeed, Elgot-Robinson  provide their models P0 and P'0 with the COPY instructions, and Cook-Reckhow  provide their accumulator-based model with only two indirect instructions – COPY to accumulator indirectly, COPY from accumulator indirectly."
+2462,"A plethora of instructions: Because any instruction acting on a single register can be augmented with its indirect ""dual"" , the inclusion of indirect instructions will double the number of single parameter/register instructions , INC ). Worse, every two parameter/register instruction will have 4 possible varieties, e.g.:"
+2463,"In a similar manner every three-register instruction that involves two source registers rs1 rs2 and a destination register rd will result in 8 varieties, for example the addition:"
+2464,"If we designate one register to be the ""accumulator""  and place strong restrictions on the various instructions allowed then we can greatly reduce the plethora of direct and indirect operations. However, one must be sure that the resulting reduced instruction-set is sufficient, and we must be aware that the reduction will come at the expense of more instructions per ""significant"" operation."
+2465,"Historical convention dedicates a register to the accumulator,  an ""arithmetic organ"" that literally accumulates its number during a sequence of arithmetic operations:"
+2466,"However, the accumulator comes at the expense of more instructions per arithmetic ""operation"", in particular with respect to what are called 'read-modify-write' instructions such as ""Increment indirectly the contents of the register pointed to by register r2 "". ""A"" designates the ""accumulator"" register A:"
+2467,"If we stick with a specific name for the accumulator, e.g. ""A"", we can imply the accumulator in the instructions, for example,"
+2468,"However, when we write the CPY instructions without the accumulator called out the instructions are ambiguous or they must have empty parameters:"
+2469,"Historically what has happened is these two CPY instructions have received distinctive names; however, no convention exists. Tradition  imaginary MIX computer) uses two names called LOAD and STORE. Here we are adding the ""i/d"" parameter:"
+2470,"The typical accumulator-based model will have all its two-variable arithmetic and constant operations , SUB  ) use  the accumulator's contents, together with  a specified register's contents.  The one-variable operations , DEC  and CLR  ) require only the accumulator. Both instruction-types deposit the result  in the accumulator."
+2471,"If we so choose, we can abbreviate the mnemonics because at least one source-register and the destination register is always the accumulator A. Thus we have :"
+2472,If our model has an unbounded accumulator can we bound all the other registers? Not until we provide for at least one unbounded register from which we derive our indirect addresses.
+2473,The minimalist approach is to use itself .
+2474,"Another approach  is to declare a specific register the ""indirect address register"" and confine indirection relative to this register . Again our new register has no conventional name – perhaps ""N"" from ""iNdex"", or ""iNdirect"" or ""address Number""."
+2475,"For maximum flexibility, as we have done for the accumulator A – we will consider N just another register subject to increment, decrement, clear, test, direct copy, etc. Again we can shrink the instruction to a single-parameter that provides for direction and indirection, for example."
+2476,Why is this such an interesting approach? At least two reasons:
+2477, An instruction set with no parameters:
+2478,Schönhage does this to produce his RAM0 instruction set. See section below.
+2479, Reduce a RAM to a Post-Turing machine:
+2480,"Posing as minimalists, we reduce all the registers excepting the accumulator A and indirection register N e.g. r = { r0, r1, r2, ... } to an unbounded string of  bounded-capacity pigeon-holes. These will do nothing but hold  bounded numbers e.g. a lone bit with value { 0, 1 }. Likewise we shrink the accumulator to a single bit. We restrict any arithmetic to the registers { A, N }, use indirect operations to pull the contents of registers into the accumulator and write 0 or 1 from the accumulator to a register:"
+2481,"We push further and eliminate A altogether by the use of two ""constant"" registers called ""ERASE"" and ""PRINT"": =0, =1."
+2482,"Rename the COPY instructions and call INC  = RIGHT, DEC  = LEFT and we have the same instructions as the Post-Turing machine, plus an extra CLRN :"
+2483,"In the section above we informally showed that a RAM with an unbounded indirection capability produces a Post–Turing machine. The Post–Turing machine is Turing equivalent, so we have shown that the RAM with indirection is Turing equivalent."
+2484,"We give here a slightly more formal demonstration. Begin by designing our model with three reserved registers ""E"", ""P"", and ""N"", plus an unbounded set of registers 1, 2, ..., n to the right. The registers 1, 2, ..., n will be considered ""the squares of the tape"". Register ""N"" points to ""the scanned square"" that ""the head"" is currently observing. The ""head"" can be thought of as being in the conditional jump – observe that it uses indirect addressing . As we decrement or increment ""N"" the  head will ""move left"" or ""right"" along the squares. We will move the contents of ""E""=0 or ""P""=1 to the ""scanned square"" as pointed to by N, using the indirect CPY."
+2485,"The fact that our tape is left-ended presents us with a minor problem: Whenever LEFT occurs our instructions will have to test to determine whether or not the contents of ""N"" is zero; if so we should leave its count at ""0"" ."
+2486,The following table both defines the Post-Turing instructions in terms of their RAM equivalent instructions and gives an example of their functioning. The location of the head along the tape of registers r0-r5 . . . is shown shaded:
+2487,"Throughout this demonstration we have to keep in mind that the instructions in the finite state machine's TABLE is bounded, i.e. finite:"
+2488,"We will build the indirect CPY  with the CASE operator. The address of the target register will be specified by the contents of register ""q""; once the CASE operator has determined what this number is, CPY will directly deposit the contents of the register with that number into register ""φ"". We will need an additional register that we will call ""y"" – it serves as an up-counter."
+2489,"The CASE ""operator"" is described in Kleene   and in Boolos-Burgess-Jeffrey  ; the latter authors emphasize its utility. The following definition is per Kleene but modified to reflect the familiar ""IF-THEN-ELSE"" construction."
+2490,"The CASE operator ""returns"" a natural number into φ depending on which ""case"" is satisfied, starting with ""case_0"" and going successively through ""case_last""; if no case is satisfied then the number called ""default""  is returned into φ ):"
+2491,Definition by cases φ :
+2492,"Kleene require that the ""predicates"" Qn that doing the testing are all mutually exclusive – ""predicates"" are functions that produce only { true, false } for output; Boolos-Burgess-Jeffrey add the requirement that the cases are ""exhaustive""."
+2493,"We begin with a number in register q that represents the address of the target register. But what is this number? The ""predicates"" will test it to find out, one trial after another: JE  followed by INC . Once the number is identified explicitly, the CASE operator directly/explicitly copies the contents of this register to φ:"
+2494,Case_0  looks like this:
+2495,"Case_n  looks like this; remember, each instance of ""n"", ""n+1"", ..., ""last"" must be an explicit natural number:"
+2496,Case_last stops the induction and bounds the CASE operator :
+2497,"If the CASE could continue ad infinitum it would be the mu operator. But it can't – its finite state machine's ""state register"" has reached its maximum count  or its table has run out of instructions; it is a finite machine, after all."
+2498,The commonly encountered Cook and Rechkow model is a bit like the ternary-register Malzek model .
+2499,"Schönhage  describes a very primitive, atomized model chosen for his proof of the equivalence of his SMM pointer machine model:"
+2500,"RAM1 model: Schönhage demonstrates how his construction can be used to form the more common, usable form of ""successor""-like RAM :"
+2501,"RAM0 model: Schönhage's RAM0 machine has 6 instructions indicated by a single letter (the 6th ""C xxx"" seems to involve 'skip over next parameter'. Schönhage designated the accumulator with ""z"", ""N"" with ""n"", etc. Rather than Schönhage's mnemonics we will use the mnemonics developed above."
+2502,"Indirection comes  from CPYAN  working with store_A_via_N STAN, and from  the peculiar indirection instruction LDAA ."
+2503,"The definitional fact that any sort of counter machine without an unbounded register-""address"" register must specify a register ""r"" by name indicates that the model requires ""r"" to be finite, although it is ""unbounded"" in the sense that the model implies no upper limit to the number of registers necessary to do its job. For example, we do not require r < 83,617,563,821,029,283,746 nor r < 2^1,000,001, etc."
+2504,We can escape this restriction by providing an unbounded register to provide the address of the register that specifies an indirect address.
+2505,"With a few exceptions, these references are the same as those at Register machine."
+2506,"A programmer, computer programmer or coder is an author of computer source code – someone with skill in computer programming."
+2507,The professional titles software developer and software engineer are used for jobs that require a programmer.
+2508,"Generally, a programmer writes code in a computer language and with an intent to build software that achieves some goal."
+2509,"Sometimes a programmer or job position is identified by the language used or target platform. For example, assembly programmer, web developer."
+2510,The job titles that include programming tasks have differing connotations across the computer industry and to different individuals. The following are notable descriptions.
+2511,"A software developer primarily implements software based on specifications and fixes bugs. Other duties may include reviewing code changes and testing. 
+To achieve the required skills for the job, they might obtain a computer science or associate degree, attend a programming boot camp or be self-taught."
+2512,"A software engineer usually is responsible for the same tasks as a developer
+plus broader responsibilities of software engineering including architecting and designing new features and applications, targeting new platforms, managing the software development lifecycle , leading a team of programmers, communicating with customers, managers and other engineers, considering system stability and quality, and exploring software development methodologies."
+2513,"Sometimes, a software engineer is required to have a degree in software engineering, computer engineering, or computer science. Some countries legally require an engineering degree to be called engineer. 
+"
+2514,"British countess and mathematician Ada Lovelace is often considered to be the first computer programmer.
+She authored an algorithm, which was published in October 1842, for calculating Bernoulli numbers on the Charles Babbage analytical engine. 
+Because the machine was not completed in her lifetime, she never experienced the algorithm in action."
+2515,"In 1941, German civil engineer Konrad Zuse was the first person to execute a program on a working, program-controlled, electronic computer. From 1943 to 1945, per computer scientist Wolfgang K. Giloi and AI professor Raúl Rojas et al., Zuse created the first, high-level programming language, Plankalkül."
+2516,"Members of the 1945 ENIAC programming team of Kay McNulty, Betty Jennings, Betty Snyder, Marlyn Wescoff, Fran Bilas and Ruth Lichterman have since been credited as the first professional computer programmers."
+2517,"The first company founded specifically to provide software products and services was the Computer Usage Company in 1955. Before that time, computers were programmed either by customers or the few commercial computer manufacturers of the time, such as Sperry Rand and IBM."
+2518,"The software industry expanded in the early 1960s, almost immediately after computers were first sold in mass-produced quantities. Universities, governments, and businesses created a demand for software. Many of these programs were written in-house by full-time staff programmers; some were distributed between users of a particular machine for no charge, while others were sold on a commercial basis. Other firms, such as Computer Sciences Corporation , also started to grow. Computer manufacturers soon started bundling operating systems, system software and programming environments with their machines; the IBM 1620 came with the 1620 Symbolic Programming System and FORTRAN."
+2519,"The industry expanded greatly with the rise of the personal computer  in the mid-1970s, which brought computing to the average office worker. In the following years, the PC also helped create a constantly growing market for games, applications and utility software. This resulted in increased demand for software developers for that period of time."
+2520,"Computer programmers write, test, debug, and maintain the detailed instructions, called computer programs, that computers must follow to perform their functions. Programmers also conceive, design, and test logical structures for solving problems by computer. Many technical innovations in programming — advanced computing technologies and sophisticated new languages and programming tools — have redefined the role of a programmer and elevated much of the programming work done today. Job titles and descriptions may vary, depending on the organization."
+2521,"Programmers work in many settings, including corporate information technology  departments, big software companies, small service firms and government entities of all sizes. Many professional programmers also work for consulting companies at client sites as contractors. Licensing is not typically required to work as a programmer, although professional certifications are commonly held by programmers. Programming is considered a profession."
+2522,"Programmers' work varies widely depending on the type of business for which they are writing programs. For example, the instructions involved in updating financial records are very different from those required to duplicate conditions on an aircraft for pilots training in a flight simulator. Simple programs can be written in a few hours. More complex ones may require more than a year of work, while others are never considered 'complete' but rather are continuously improved as long as they stay in use. In most cases, several programmers work together as a team under a senior programmer's supervision."
+2523,"Programming editors, also known as source code editors, are text editors that are specifically designed for programmers or developers to write the source code of an application or a program. Most of these editors include features useful for programmers, which may include color syntax highlighting, auto indentation, auto-complete, bracket matching, syntax check, and allows plug-ins. These features aid the users during coding, debugging and testing."
+2524,"According to BBC News, 17% of computer science students could not find work in their field six months after graduation in 2009 which was the highest rate of the university subjects surveyed while 0% of medical students were unemployed in the same survey."
+2525,"After the crash of the dot-com bubble  and the Great Recession , many U.S. programmers were left without work or with lower wages. In addition, enrollment in computer-related degrees and other STEM degrees  in the US has been dropping for years, especially for women, which, according to Beaubouef and Mason, could be attributed to a lack of general interest in science and mathematics and also out of an apparent fear that programming will be subject to the same pressures as manufacturing and agriculture careers. For programmers, the U.S. Bureau of Labor Statistics  Occupational Outlook originally predicted a growth for programmers of 12 percent from 2010 to 2020 and thereafter a decline of -7 percent from 2016 to 2026, a further decline of -9 percent from 2019 to 2029, a decline of -10 percent from 2021 to 2031. and then a decline of -11 percent from 2022 to 2032. Since computer programming can be done from anywhere in the world, companies sometimes hire programmers in countries where wages are lower. However, for software developers BLS projects for 2019 to 2029 a 22% increase in employment, from 1,469,200 to 1,785,200 jobs with a median base salary of $110,000 per year. This prediction is lower than the earlier 2010 to 2020 predicted increase of 30% for software developers. Though the distinction is somewhat ambiguous, software developers engage in a wider array of aspects of application development and are generally higher skilled than programmers, making outsourcing less of a risk. Another reason for the decline for programmers is their skills are being merged with other professions, such as developers, as employers increase the requirements for a position over time. Then there is the additional concern that recent advances in artificial intelligence might impact the demand for future generations of Software professions."
+2526,"For such cases, it is a more accurate measure than measuring instructions per second."
+2527,"Floating-point arithmetic is needed for very large or very small real numbers, or computations that require a large dynamic range. Floating-point representation is similar to scientific notation, except everything is carried out in base two, rather than base ten. The encoding scheme stores the sign, the exponent  and the significand . While several similar formats are in use, the most common is ANSI/IEEE Std. 754-1985. This standard defines the format for 32-bit numbers called single precision, as well as 64-bit numbers called double precision and longer numbers called extended precision . Floating-point representations can support a much wider range of values than fixed-point, with the ability to represent very small numbers and very large numbers."
+2528,"The exponentiation inherent in floating-point computation assures a much larger dynamic range – the largest and smallest numbers that can be represented – which is especially important when processing data sets where some of the data may have extremely large range of numerical values or where the range may be unpredictable. As such, floating-point processors are ideally suited for computationally intensive applications."
+2529,"FLOPS and MIPS are units of measure for the numerical computing performance of a computer. Floating-point operations are typically used in fields such as scientific computational research, as well as in machine learning. However, before the late 1980s floating-point hardware  was typically an optional feature, and computers that had it were said to be ""scientific computers"", or to have ""scientific computation"" capability. Thus the unit MIPS was useful to measure integer performance of any computer, including those without such a capability, and to account for architecture differences, similar MOPS  was used as early as 1970 as well. Note that besides integer  arithmetics, examples of integer operation include data movement  or value testing . That's why MIPS as a performance benchmark is adequate when a computer is used in database queries, word processing, spreadsheets, or to run multiple virtual operating systems. In 1974 David Kuck coined the terms flops and megaflops for the description of supercomputer performance of the day by the number of floating-point calculations they performed per second. This was much better than using the prevalent MIPS to compare computers as this statistic usually had little bearing on the arithmetic capability of the machine on scientific tasks."
+2530,FLOPS on an HPC-system can be calculated using this equation:
+2531,This can be simplified to the most common case: a computer that has exactly 1 CPU:
+2532,"FLOPS can be recorded in different measures of precision, for example, the TOP500 supercomputer list ranks computers by 64 bit  operations per second, abbreviated to FP64. Similar measures are available for 32-bit  and 16-bit  operations."
+2533,"In June 1997, Intel's ASCI Red was the world's first computer to achieve one teraFLOPS and beyond. Sandia director Bill Camp said that ASCI Red had the best reliability of any supercomputer ever built, and ""was supercomputing's high-water mark in longevity, price, and performance""."
+2534,NEC's SX-9 supercomputer was the world's first vector processor to exceed 100 gigaFLOPS per single core.
+2535,"In June 2006, a new computer was announced by Japanese research institute RIKEN, the MDGRAPE-3. The computer's performance tops out at one petaFLOPS, almost two times faster than the Blue Gene/L, but MDGRAPE-3 is not a general purpose computer, which is why it does not appear in the Top500.org list. It has special-purpose pipelines for simulating molecular dynamics."
+2536,"By 2007, Intel Corporation unveiled the experimental multi-core POLARIS chip, which achieves 1 teraFLOPS at 3.13 GHz. The 80-core chip can raise this result to 2 teraFLOPS at 6.26 GHz, although the thermal dissipation at this frequency exceeds 190 watts."
+2537,"In June 2007, Top500.org reported the fastest computer in the world to be the IBM Blue Gene/L supercomputer, measuring a peak of 596 teraFLOPS. The Cray XT4 hit second place with 101.7 teraFLOPS."
+2538,"On June 26, 2007, IBM announced the second generation of its top supercomputer, dubbed Blue Gene/P and designed to continuously operate at speeds exceeding one petaFLOPS, faster than the Blue Gene/L. When configured to do so, it can reach speeds in excess of three petaFLOPS."
+2539,"On October 25, 2007, NEC Corporation of Japan issued a press release announcing its SX series model SX-9, claiming it to be the world's fastest vector supercomputer. The SX-9 features the first CPU capable of a peak vector performance of 102.4 gigaFLOPS per single core."
+2540,"On February 4, 2008, the NSF and the University of Texas at Austin opened full scale research runs on an AMD, Sun supercomputer named Ranger,
+the most powerful supercomputing system in the world for open science research, which operates at sustained speed of 0.5 petaFLOPS."
+2541,"On May 25, 2008, an American supercomputer built by IBM, named 'Roadrunner', reached the computing milestone of one petaFLOPS. It headed the June 2008 and November 2008 TOP500 list of the most powerful supercomputers . The computer is located at Los Alamos National Laboratory in New Mexico. The computer's name refers to the New Mexico state bird, the greater roadrunner ."
+2542,"In June 2008, AMD released ATI Radeon HD 4800 series, which are reported to be the first GPUs to achieve one teraFLOPS. On August 12, 2008, AMD released the ATI Radeon HD 4870X2 graphics card with two Radeon R770 GPUs totaling 2.4 teraFLOPS."
+2543,"In November 2008, an upgrade to the Cray Jaguar supercomputer at the Department of Energy's  Oak Ridge National Laboratory  raised the system's computing power to a peak 1.64 petaFLOPS, making Jaguar the world's first petaFLOPS system dedicated to open research. In early 2009 the supercomputer was named after a mythical creature, Kraken. Kraken was declared the world's fastest university-managed supercomputer and sixth fastest overall in the 2009 TOP500 list. In 2010 Kraken was upgraded and can operate faster and is more powerful."
+2544,"In 2009, the Cray Jaguar performed at 1.75 petaFLOPS, beating the IBM Roadrunner for the number one spot on the TOP500 list."
+2545,"In October 2010, China unveiled the Tianhe-1, a supercomputer that operates at a peak computing rate of 2.5 petaFLOPS."
+2546,"As of 2010 the fastest PC processor reached 109 gigaFLOPS  in double precision calculations. GPUs are considerably more powerful. For example, Nvidia Tesla C2050 GPU computing processors perform around 515 gigaFLOPS in double precision calculations, and the AMD FireStream 9270 peaks at 240 gigaFLOPS."
+2547,"In November 2011, it was announced that Japan had achieved 10.51 petaFLOPS with its K computer. It has 88,128 SPARC64 VIIIfx processors in 864 racks, with theoretical performance of 11.28 petaFLOPS. It is named after the Japanese word ""kei"", which stands for 10 quadrillion, corresponding to the target speed of 10 petaFLOPS."
+2548,"On November 15, 2011, Intel demonstrated a single x86-based processor, code-named ""Knights Corner"", sustaining more than a teraFLOPS on a wide range of DGEMM operations. Intel emphasized during the demonstration that this was a sustained teraFLOPS , and that it was the first general purpose processor to ever cross a teraFLOPS."
+2549,"On June 18, 2012, IBM's Sequoia supercomputer system, based at the U.S. Lawrence Livermore National Laboratory , reached 16 petaFLOPS, setting the world record and claiming first place in the latest TOP500 list."
+2550,"On November 12, 2012, the TOP500 list certified Titan as the world's fastest supercomputer per the LINPACK benchmark, at 17.59 petaFLOPS. It was developed by Cray Inc. at the Oak Ridge National Laboratory and combines AMD Opteron processors with ""Kepler"" NVIDIA Tesla graphics processing unit  technologies."
+2551,"On June 10, 2013, China's Tianhe-2 was ranked the world's fastest with 33.86 petaFLOPS."
+2552,"On June 20, 2016, China's Sunway TaihuLight was ranked the world's fastest with 93 petaFLOPS on the LINPACK benchmark . The system, which is almost exclusively based on technology developed in China, is installed at the National Supercomputing Center in Wuxi, and represents more performance than the next five most powerful systems on the TOP500 list combined."
+2553,"In June 2019, Summit, an IBM-built supercomputer now running at the Department of Energy's  Oak Ridge National Laboratory , captured the number one spot with a performance of 148.6 petaFLOPS on High Performance Linpack , the benchmark used to rank the TOP500 list. Summit has 4,356 nodes, each one equipped with two 22-core Power9 CPUs, and six NVIDIA Tesla V100 GPUs."
+2554,Distributed computing uses the Internet to link personal computers to achieve more FLOPS:
+2555,"Total system GFLOPS = 89,794 / TFLOPS= 89.2794"
+2556,Total system cost incl. realistic but low cost parts; matched with other example = $2839
+2557,US$/GFLOP = $0.0314
+2558,"A web browser is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used on a range of devices, including desktops, laptops, tablets, and smartphones. In 2020, an estimated 4.9 billion people have used a browser. The most-used browser is Google Chrome, with a 64% global market share on all devices, followed by Safari with 19%."
+2559,"A web browser is not the same thing as a search engine, though the two are often confused. A search engine is a website that provides links to other websites. However, to connect to a website's server and display its web pages, a user must have a web browser installed. In some technical contexts, browsers are referred to as user agents."
+2560,"The purpose of a web browser is to fetch content from the Web or local storage and display it on the user's device. This process begins when the user inputs a Uniform Resource Locator , such as https://en.wikipedia.org/, into the browser. Virtually all URLs on the Web start with either http: or https: which means they are retrieved with the Hypertext Transfer Protocol . For secure mode , the connection between the browser and web server is encrypted, providing a secure and private data transfer."
+2561,"Web pages usually contain hyperlinks to other pages and resources. Each link contains a URL, and when it is clicked or tapped, the browser navigates to the new resource. Most browsers use an internal cache of web page resources to improve loading times for subsequent visits to the same page. The cache can store many items, such as large images, so they do not need to be downloaded from the server again. Cached items are usually only stored for as long as the web server stipulates in its HTTP response messages."
+2562,"During the course of browsing, cookies received from various websites are stored by the browser. Some of them contain login credentials or site preferences. However, others are used for tracking user behavior over long periods of time, so browsers typically provide a section in the menu for deleting cookies. Finer-grained management of cookies usually requires a browser extension."
+2563,"The first web browser, called WorldWideWeb, was created in 1990 by Sir Tim Berners-Lee. He then recruited Nicola Pellow to write the Line Mode Browser, which displayed web pages on dumb terminals. The Mosaic web browser was released in April 1993, and was later credited as the first web browser to find mainstream popularity. Its innovative graphical user interface made the World Wide Web easy to navigate and thus more accessible to the average person. This, in turn, sparked the Internet boom of the 1990s, when the Web grew at a very rapid rate. The lead developers of Mosaic then founded the Netscape corporation, which released the Mosaic-influenced Netscape Navigator in 1994. Navigator quickly became the most popular browser."
+2564,"Microsoft debuted Internet Explorer in 1995, leading to a browser war with Netscape. Within a few years, Microsoft gained a dominant position in the browser market for two reasons: it bundled Internet Explorer with its popular Windows operating system and did so as freeware with no restrictions on usage. The market share of Internet Explorer peaked at over 95% in the early 2000s. In 1998, Netscape launched what would become the Mozilla Foundation to create a new browser using the open-source software model. This work evolved into the Firefox browser, first released by Mozilla in 2004. Firefox's market share peaked at 32% in 2010. Apple released its Safari browser in 2003; it remains the dominant browser on Apple devices, though it did not become popular elsewhere."
+2565,"Google debuted its Chrome browser in 2008, which steadily took market share from Internet Explorer and became the most popular browser in 2012. Chrome has remained dominant ever since. By 2015, Microsoft replaced Internet Explorer with Edge for the Windows 10 release."
+2566,"Since the early 2000s, browsers have greatly expanded their HTML, CSS, JavaScript, and multimedia capabilities. One reason has been to enable more sophisticated websites, such as web apps. Another factor is the significant increase of broadband connectivity in many parts of the world, enabling people to access data-intensive content, such as streaming HD video on YouTube, that was not possible during the era of dial-up modems."
+2567,"Google Chrome has been the dominant browser since the mid-2010s and currently has a 64% global market share on all devices. The vast majority of its source code comes from Google's open-source Chromium project; this code is also the basis for many other browsers, including Microsoft Edge, currently in third place with about a 5% share, and Opera and Samsung Internet in fifth and sixth place with over 2% each."
+2568,"The other two browsers in the top four are made from different codebases. Safari, based on Apple's WebKit code, is dominant on Apple devices, resulting in a 19% global share. Firefox, with about a 3% share, is based on Mozilla's code. Both of these codebases are open-source, so a number of small niche browsers are also made from them."
+2569,"The most popular browsers share many features in common. They automatically log users' browsing history, unless the users turn off their browsing history or use the non-logging private mode. They also allow users to set bookmarks, customize the browser with extensions, and can manage user passwords. Some provide a sync service and web accessibility features."
+2570,Common user interface  features:
+2571,"While mobile browsers have similar UI features as desktop versions, the limitations of touch screens require mobile UIs to be simpler. The difference is significant for users accustomed to keyboard shortcuts. The most popular desktop browsers also have sophisticated web development tools."
+2572,"Web browsers are popular targets for hackers, who exploit security holes to steal information, destroy files, and other malicious activities. Browser vendors regularly patch these security holes, so users are strongly encouraged to keep their browser software updated. Other protection measures are antivirus software and being aware of scams."
+2573,"Early word processors were stand-alone devices dedicated to the function, but current word processors are word processor programs running on general purpose computers."
+2574,"The functions of a word processor program fall somewhere between those of a simple text editor and a fully functioned desktop publishing program. However, the distinctions between these three have changed over time and were unclear after 2010."
+2575,"Word processors developed from mechanical machines, later merging with computer technology. The history of word processing is the story of the gradual automation of the physical aspects of writing and editing, and then to the refinement of the technology to make it available to corporations and Individuals."
+2576,"The term word processing appeared in American offices in the early 1970s centered on the idea of streamlining the work to typists, but the meaning soon shifted toward the automation of the whole editing cycle."
+2577,"At first, the designers of word processing systems combined existing technologies with emerging ones to develop stand-alone equipment, creating a new business distinct from the emerging world of the personal computer.  The concept of word processing arose from the more general data processing, which since the 1950s had been the application of computers to business administration."
+2578,"Through history, there have been three types of word processors: mechanical, electronic and software."
+2579,"The first word processing device  was patented in 1714 by Henry Mill for a machine that was capable of ""writing so clearly and accurately you could not distinguish it from a printing press"". More than a century later, another patent appeared in the name of William Austin Burt for the typographer. In the late 19th century, Christopher Latham Sholes created the first recognizable typewriter, which was described as a ""literary piano""."
+2580,"The only ""word processing"" these mechanical systems could perform was to change where letters appeared on the page, to fill in spaces that were previously left on the page, or to skip over lines. It was not until decades later that the introduction of electricity and electronics into typewriters began to help the writer with the mechanical part. The term “word processing”  itself was possibly created in the 1950s by Ulrich Steinhilper, a German IBM typewriter sales executive, or by an American electro-mechanical typewriter executive, George M. Ryan, who obtained a trademark registration in the USPTO for the phrase.  However, it did not make its appearance in 1960s office management or computing literature , though many of the ideas, products, and technologies to which it would later be applied were already well known. Nonetheless, by 1971, the term was recognized by the New York Times as a business ""buzz word"". Word processing paralleled the more general ""data processing"", or the application of computers to business administration."
+2581,"Thus, by 1972, the discussion of word processing was common in publications devoted to business office management and technology; by the mid-1970s, the term would have been familiar to any office manager who consulted business periodicals."
+2582,"By the late 1960s, IBM had developed the IBM MT/ST .  It was a model of the IBM Selectric typewriter from earlier in 1961, but it came built into its own desk, integrated with magnetic tape recording and playback facilities along with controls and a bank of electrical relays. The MT/ST automated word wrap, but it had no screen.  This device allowed a user to rewrite text that had been written on another tape, and it also allowed limited collaboration in the sense that a user could send the tape to another person to let them edit the document or make a copy.  It was a revolution for the word processing industry.  In 1969, the tapes were replaced by magnetic cards. These memory cards were inserted into an extra device that accompanied the MT/ST, able to read and record users' work."
+2583,"Throughout the 1960s and 70s, word processing began to slowly shift from glorified typewriters augmented with electronic features to become fully computer-based  with the development of several innovations.  Just before the arrival of the personal computer , IBM developed the floppy disk.  In the 1970s, the first proper word-processing systems appeared, which allowed display and editing of documents on CRT screens."
+2584,"During this era, these early stand-alone word processing systems were designed, built, and marketed by several pioneering companies. Linolex Systems was founded in 1970 by James Lincoln and Robert Oleksiak. Linolex based its technology on microprocessors, floppy drives and software.  It was a computer-based system for application in the word processing businesses and it sold systems through its own sales force. With a base of installed systems in over 500 sites, Linolex Systems sold 3 million units in 1975 — a year before the Apple computer was released."
+2585,"At that time, the Lexitron Corporation also produced a series of dedicated word-processing microcomputers. Lexitron was the first to use a full-sized video display screen  in its models by 1978. Lexitron also used 51⁄4 inch floppy diskettes, which became the standard in the personal computer field. The program disk was inserted in one drive, and the system booted up. The data diskette was then put in the second drive. The operating system and the word processing program were combined in one file."
+2586,"Another of the early word processing adopters was Vydec, which created in 1973 the first modern text processor, the ""Vydec Word Processing System"". It had built-in multiple functions like the ability to share content by diskette and print it. The Vydec Word Processing System sold for $12,000 at the time, ."
+2587,"The Redactron Corporation  designed and manufactured editing systems, including correcting/editing typewriters, cassette and card units, and eventually a word processor called the Data Secretary. The Burroughs Corporation acquired Redactron in 1976."
+2588,"A CRT-based system by Wang Laboratories became one of the most popular systems of the 1970s and early 1980s. The Wang system displayed text on a CRT screen, and incorporated virtually every fundamental characteristic of word processors as they are known today. While early computerized word processor system were often expensive and hard to use , the Wang system was a true office machine, affordable to organizations such as medium-sized law firms, and easily mastered and operated by secretarial staff."
+2589,"The phrase ""word processor"" rapidly came to refer to CRT-based machines similar to Wang's. Numerous machines of this kind emerged, typically marketed by traditional office-equipment companies such as IBM, Lanier , CPT, and NBI. All were specialized, dedicated, proprietary systems, with prices in the $10,000 range. Cheap general-purpose personal computers were still the domain of hobbyists."
+2590,"In Japan, even though typewriters with Japanese writing system had widely been used for businesses and  governments, they were limited to specialists and required special skills due to the wide variety of letters, until computer-based devices came onto the market.   In 1977, Sharp showcased a prototype of a computer-based word processing dedicated device with Japanese writing system in Business Show in Tokyo."
+2591,"Toshiba released the first Japanese word processor JW-10  in February 1979.  The price was 6,300,000 JPY, equivalent to US$45,000.  This is selected as one of the milestones of IEEE."
+2592,"The Japanese writing system uses a large number of kanji  which require 2 bytes to store, so having one key per each symbol is infeasible. Japanese word processing became possible with the development of the Japanese input method  -- now widely used in personal computers. Oki launched OKI WORD EDITOR-200 in March 1979 with this kana-based keyboard input system.  In 1980 several electronics and office equipment brands including entered this rapidly growing market with more compact and affordable devices.  For instance, NEC introduced the NWP-20 , and Fujitsu launched the Fujitsu OASYS . While the average unit price in 1980 was 2,000,000 JPY , it was dropped to 164,000 JPY  in 1985. Even after personal computers became widely available, Japanese word processors remained popular as they tended to be more portable , and become commonplace  for business and academics, even for private individuals in the second half of the 1980s.  The phrase ""word processor"" has been abbreviated as ""Wa-pro"" or ""wapuro"" in Japanese."
+2593,"The final step in word processing came with the advent of the personal computer in the late 1970s and 1980s and with the subsequent creation of word processing software. Word processing software that would create much more complex and capable output was developed and prices began to fall, making them more accessible to the public.  By the late 1970s, computerized word processors were still primarily used by employees composing documents for large and midsized businesses .  Within a few years, the falling prices of PCs made word processing available for the first time to all writers in the convenience of their homes."
+2594,"The first word processing program for personal computers  was Electric Pencil, from Michael Shrayer Software, which went on sale in December 1976. In 1978 WordStar appeared and because of its many new features soon dominated the market. WordStar was written for the early CP/M  operating system, ported to CP/M-86, then to MS-DOS, and was the most popular word processing program until 1985 when WordPerfect sales first exceeded WordStar sales."
+2595,"Early word processing software was not as intuitive as word processor devices. Most early word processing software required users to memorize semi-mnemonic key combinations rather than pressing keys such as ""copy"" or ""bold"". Moreover, CP/M lacked cursor keys; for example WordStar used the E-S-D-X-centered ""diamond"" for cursor navigation. A notable exception was the software Lexitype for MS-DOS that took inspiration from the Lexitron dedicated word processor's user interface and which mapped individual functions to particular keyboard function keys, and a set of stick-on ""keycaps"" describing the function were provided with the software. Lexitype was popular with large organizations that had previously used the Lexitron. Eventually, the price differences between dedicated word processors and general-purpose PCs, and the value added to the latter by software such as “killer app” spreadsheet applications, e.g. VisiCalc and Lotus 1-2-3, were so compelling that personal computers and word processing software became serious competition for the dedicated machines and soon dominated the market."
+2596,"In the late 1980s, innovations such as the advent of laser printers, a ""typographic"" approach to word processing , using bitmap displays with multiple fonts , and graphical user interfaces such as “copy and paste” . These were popularized by MacWrite on the Apple Macintosh in 1983, and Microsoft Word on the IBM PC in 1984. These were probably the first true WYSIWYG word processors to become known to many people.
+Of particular interest also is the standardization of TrueType fonts used in both Macintosh and Windows PCs. While the publishers of the operating systems provide TrueType typefaces, they are largely gathered from traditional typefaces converted by smaller font publishing houses to replicate standard fonts. Demand for new and interesting fonts, which can be found free of copyright restrictions, or commissioned from font designers, developed."
+2597,"The growing popularity of the Windows operating system in the 1990s later took Microsoft Word along with it. Originally called ""Microsoft Multi-Tool Word"", this program quickly became a synonym for “word processor”."
+2598,"Early in the 21st century, Google Docs popularized the transition to online or offline web browser based word processing, this was enabled by the widespread adoption of suitable internet connectivity in businesses and domestic households and later the popularity of smartphones. Google Docs enabled word processing from within any vendor's web browser, which could run on any vendor's operating system on any physical device type including tablets and smartphones, although offline editing is limited to a few Chromium based web browsers. Google Docs also enabled the significant growth of use of information technology such as remote access to files and collaborative real-time editing, both becoming simple to do with little or no need for costly software and specialist IT support."
+2599,"The term is often used in contrast to declarative programming, which focuses on what the program should accomplish without specifying all the details of how the program should achieve the result."
+2600,"Procedural programming is a type of imperative programming in which the program is built from one or more procedures . The terms are often used as synonyms, but the use of procedures has a dramatic effect on how imperative programs appear and how they are constructed. Heavy procedural programming, in which state changes are localized to procedures or restricted to explicit arguments and returns from procedures, is a form of structured programming. Since the 1960s, structured programming and modular programming in general have been promoted as techniques to improve the maintainability and overall quality of imperative programs. The concepts behind object-oriented programming attempt to extend this approach."
+2601,"Procedural programming could be considered a step toward declarative programming. A programmer can often tell, simply by looking at the names, arguments, and return types of procedures , what a particular procedure is supposed to do, without necessarily looking at the details of how it achieves its result. At the same time, a complete program is still imperative since it fixes the statements to be executed and their order of execution to a large extent."
+2602,"The programming paradigm used to build programs for almost all computers typically follows an imperative model. Digital computer hardware is designed to execute machine code, which is native to the computer and is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as lisp machines."
+2603,"From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer. Higher-level imperative languages use variables and more complex statements, but still follow the same paradigm. Recipes and process checklists, while not computer programs, are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state. Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style."
+2604,"Assignment statements, in imperative paradigm, perform an operation on information located in memory and store the results in memory for later use. High-level imperative languages, in addition, permit the evaluation of complex expressions, which may consist of a combination of arithmetic operations and function evaluations, and the assignment of the resulting value to memory. Looping statements  allow a sequence of statements to be executed multiple times. Loops can either execute the statements they contain a predefined number of times, or they can execute them repeatedly until some condition is met. Conditional branching statements allow a sequence of statements to be executed only if some condition is met. Otherwise, the statements are skipped and the execution sequence continues from the statement following them. Unconditional branching statements allow an execution sequence to be transferred to another part of a program. These include the jump , switch, and the subprogram, subroutine, or procedure call ."
+2605,"Early in the development of high-level programming languages, the introduction of the block enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside the introduction of subroutines, enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures."
+2606,Many imperative programming languages  are abstractions of assembly language.
+2607,"The earliest imperative languages were the machine languages of the original computers. In these languages, instructions were very simple, which made hardware implementation easier but hindered the creation of complex programs. FORTRAN, developed by John Backus at International Business Machines  starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs. FORTRAN was a compiled language that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages. The next two decades saw the development of many other major high-level imperative programming languages. In the late 1950s and 1960s, ALGOL was developed in order to allow mathematical algorithms to be more easily expressed and even served as the operating system's target language for some computers. MUMPS  carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to the extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $TEST. COBOL  and BASIC  were both attempts to make programming syntax look more like English. In the 1970s, Pascal was developed by Niklaus Wirth, and C was created by Dennis Ritchie while he was working at Bell Laboratories. Wirth went on to design Modula-2 and Oberon. For the needs of the United States Department of Defense, Jean Ichbiah and a team at Honeywell began designing Ada in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995, 2005, and 2012."
+2608,"The 1980s saw a rapid growth in interest in object-oriented programming. These languages were imperative in style, but added features to support objects. The last two decades of the 20th century saw the development of many such languages. Smalltalk-80, originally conceived by Alan Kay in 1969, was released in 1980, by the Xerox Palo Alto Research Center . Drawing from concepts in another object-oriented language—Simula —Bjarne Stroustrup designed C++, an object-oriented language based on C. Design of C++ began in 1979 and the first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were Perl, released by Larry Wall in 1987; Python, released by Guido van Rossum in 1990; Visual Basic and Visual C++  2.0), released by Microsoft in 1991 and 1993 respectively; PHP, released by Rasmus Lerdorf in 1994; Java, by James Gosling  in 1995, JavaScript, by Brendan Eich , and Ruby, by Yukihiro ""Matz"" Matsumoto, both released in 1995. Microsoft's .NET Framework  is imperative at its core, as are its main target languages, VB.NET and C# that run on it; however Microsoft's F#, a functional language, also runs on it."
+2609,"FORTRAN  was unveiled as ""The IBM Mathematical FORmula TRANslating system."" It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and statements, it supported:"
+2610,It succeeded because:
+2611,"However, non IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute  developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:"
+2612,"COBOL  stands for ""COmmon Business Oriented Language."" Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced. The US Department of Defense influenced COBOL's development, with Grace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal."
+2613,"COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming."
+2614,"ALGOL  stands for ""ALGOrithmic Language."" It had a profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like:"
+2615,"Algol's direct descendants include Pascal, Modula-2, Ada, Delphi and Oberon on one branch. On another branch there's C, C++ and Java."
+2616,"BASIC  stands for ""Beginner's All Purpose Symbolic Instruction Code."" It was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language."
+2617,Basic pioneered the interactive session. It offered operating system commands within its environment:
+2618,"However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface."
+2619,"C programming language  got its name because the language BCPL was replaced with B, and AT&T Bell Labs called the next version ""C."" Its purpose was to write the UNIX operating system. C is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but uses a high-level syntax. It added advanced features like:"
+2620,C allows the programmer to control in which region of memory data is to be stored. Global variables and static variables require the fewest clock cycles to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc function.
+2621,"In the 1970s, software engineers needed language support to break large projects down into modules. One obvious feature was to decompose large projects physically into separate files. A less obvious feature was to decompose large projects logically into abstract datatypes. At the time, languages supported concrete  datatypes like integer numbers, floating-point numbers, and strings of characters. Concrete datatypes have their representation as part of their name. Abstract datatypes are structures of concrete datatypes — with a new name assigned. For example, a list of integers could be called integer_list."
+2622,"In object-oriented jargon, abstract datatypes are called classes. However, a class is only a definition; no memory is allocated. When memory is allocated to a class, it's called an object."
+2623,"Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming. A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method, member function, or operation. Object-oriented programming is executing operations on objects."
+2624,"Object-oriented languages support a syntax to model subset/superset relationships. In set theory, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. Object-oriented languages model subset/superset relationships using inheritance. Object-oriented programming became the dominant language paradigm by the late 1990s."
+2625,"C++  was originally called ""C with Classes."" It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula."
+2626,An object-oriented module is composed of two files. The definitions file is called the header file. Here is a C++ header file for the GRADE class in a simple school application:
+2627,A constructor operation is a function with the same name as the class name. It is executed when the calling operation executes the new statement.
+2628,A module's other file is the source file. Here is a C++ source file for the GRADE class in a simple school application:
+2629,Here is a C++ header file for the PERSON class in a simple school application:
+2630,Here is a C++ source file for the PERSON class in a simple school application:
+2631,Here is a C++ header file for the STUDENT class in a simple school application:
+2632,Here is a C++ source file for the STUDENT class in a simple school application:
+2633,Here is a driver program for demonstration:
+2634,Here is a makefile to compile everything:
+2635,"In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, fundamental features , and the input/output model of a family of implementations of the ISA."
+2636,"An ISA specifies the behavior of machine code running on implementations of that ISA in a fashion that does not depend on the characteristics of that implementation, providing binary compatibility between implementations. This enables multiple implementations of an ISA that differ in characteristics such as performance, physical size, and monetary cost , but that are capable of running the same machine code, so that a lower-performance, lower-cost machine can be replaced with a higher-cost, higher-performance machine without having to replace software.  It also enables the evolution of the microarchitectures of the implementations of that ISA, so that a newer, higher-performance implementation of an ISA can run software that runs on previous generations of implementations."
+2637,"If an operating system maintains a standard and compatible application binary interface  for a particular ISA, machine code will run on future implementations of that ISA and operating system.  However, if an ISA supports running multiple operating systems, it does not guarantee that machine code for one operating system will run on another operating system, unless the first operating system supports running machine code built for the other operating system."
+2638,"An ISA can be extended by adding instructions or other capabilities, or adding support for larger addresses and data values; an implementation of the extended ISA will still be able to execute machine code for versions of the ISA without those extensions.  Machine code using those extensions will only run on implementations that support those extensions."
+2639,The binary compatibility that they provide makes ISAs one of the most fundamental abstractions in computing.
+2640,"An instruction set architecture is distinguished from a microarchitecture, which is the set of processor design techniques used, in a particular processor, to implement the instruction set. Processors with different microarchitectures can share a common instruction set. For example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but they have radically different internal designs."
+2641,"The concept of an architecture, distinct from the design of a specific machine, was developed by Fred Brooks at IBM during the design phase of System/360."
+2642,"Some virtual machines that support bytecode as their ISA such as Smalltalk, the Java virtual machine, and Microsoft's Common Language Runtime, implement this by translating the bytecode for commonly used code paths into native machine code. In addition, these virtual machines execute less frequently used code paths by interpretation . Transmeta implemented the x86 instruction set atop VLIW processors in this fashion."
+2643,"An ISA may be classified in a number of different ways. A common classification is by architectural complexity. A complex instruction set computer  has many specialized instructions, some of which may only be rarely used in practical programs. A reduced instruction set computer  simplifies the processor by efficiently implementing only the instructions that are frequently used in programs, while the less common operations are implemented as subroutines, having their resulting additional processor execution time offset by infrequent use."
+2644,"Other types include very long instruction word  architectures, and the closely related long instruction word  and explicitly parallel instruction computing  architectures. These architectures seek to exploit instruction-level parallelism with less hardware than RISC and CISC by making the compiler responsible for instruction issue and scheduling."
+2645,"Architectures with even less complexity have been studied, such as the minimal instruction set computer  and one-instruction set computer . These are theoretically important types, but have not been commercialized."
+2646,"Machine language is built up from discrete statements or instructions. On the processing architecture, a given instruction may specify:"
+2647,"More complex operations are built up by combining these simple instructions, which are executed sequentially, or as otherwise directed by control flow instructions."
+2648,Examples of operations common to many instruction sets include:
+2649,"Processors may include ""complex"" instructions in their instruction set. A single ""complex"" instruction does something that may take many instructions on other computers. Such instructions are typified by instructions that take multiple steps, control multiple functional units, or otherwise appear on a larger scale than the bulk of simple instructions implemented by the given processor. Some examples of ""complex"" instructions include:"
+2650,"Complex instructions are more common in CISC instruction sets than in RISC instruction sets, but RISC instruction sets may include them as well. RISC instruction sets generally do not include ALU operations with memory operands, or instructions to move large blocks of memory, but most RISC instruction sets include SIMD or vector instructions that perform the same arithmetic operation on multiple pieces of data at the same time. SIMD instructions have the ability of manipulating large vectors and matrices in minimal time. SIMD instructions allow easy parallelization of algorithms commonly involved in sound, image, and video processing. Various SIMD implementations have been brought to market under trade names such as MMX, 3DNow!, and AltiVec."
+2651,"On traditional architectures, an instruction includes an opcode that specifies the operation to perform, such as add contents of memory to register—and zero or more operand specifiers, which may specify registers, memory locations, or literal data. The operand specifiers may have addressing modes determining their meaning or may be in fixed fields. In very long instruction word  architectures, which include many microcode architectures, multiple simultaneous opcodes and operands are specified in a single instruction."
+2652,"Some exotic instruction sets do not have an opcode field, such as transport triggered architectures , only operand."
+2653,"Most stack machines have ""0-operand"" instruction sets in which arithmetic and logical operations lack any operand specifier fields; only instructions that push operands onto the evaluation stack or that pop operands from the stack into variables have operand specifiers. The instruction set carries out most ALU actions with postfix  operations that work only on the expression stack, not on data registers or arbitrary main memory cells. This can be very convenient for compiling high-level languages, because most arithmetic expressions can be easily translated into postfix notation."
+2654,"Conditional instructions often have a predicate field—a few bits that encode the specific condition to cause an operation to be performed rather than not performed. For example, a conditional branch instruction will transfer control if the condition is true, so that execution proceeds to a different part of the program, and not transfer control if the condition is false, so that execution continues sequentially. Some instruction sets also have conditional moves, so that the move will be executed, and the data stored in the target location, if the condition is true, and not executed, and the target location not modified, if the condition is false. Similarly, IBM z/Architecture has a conditional store instruction. A few instruction sets include a predicate field in every instruction; this is called branch predication."
+2655,Instruction sets may be categorized by the maximum number of operands explicitly specified in instructions.
+2656," addresses referring to memory cells, while reg1 and so on refer to machine registers.)"
+2657,"Due to the large number of bits needed to encode the three registers of a 3-operand instruction, RISC architectures that have 16-bit instructions are invariably 2-operand designs, such as the Atmel AVR, TI MSP430, and some versions of ARM Thumb. RISC architectures that have 32-bit instructions are usually 3-operand designs, such as the ARM, AVR32, MIPS, Power ISA, and SPARC architectures."
+2658,"Each instruction specifies some number of operands  explicitly. Some instructions give one or both operands implicitly, such as by being stored on top of the stack or in an implicit register. If some of the operands are given implicitly, fewer operands need be specified in the instruction. When a ""destination operand"" explicitly specifies the destination, an additional operand must be supplied. Consequently, the number of operands encoded in an instruction may differ from the mathematically necessary number of arguments for a logical or arithmetic operation . Operands are either encoded in the ""opcode"" representation of the instruction, or else are given as values or addresses following the opcode."
+2659,"Register pressure measures the availability of free registers at any point in time during the program execution. Register pressure is high when a large number of the available registers are in use; thus, the higher the register pressure, the more often the register contents must be spilled into memory. Increasing the number of registers in an architecture decreases register pressure but increases the cost."
+2660,"While embedded instruction sets such as Thumb suffer from extremely high register pressure because they have small register sets, general-purpose RISC ISAs like MIPS and Alpha enjoy low register pressure. CISC ISAs like x86-64 offer low register pressure despite having smaller register sets. This is due to the many addressing modes and optimizations  that CISC ISAs offer."
+2661,"The size or length of an instruction varies widely, from as little as four bits in some microcontrollers to many hundreds of bits in some VLIW systems. Processors used in personal computers, mainframes, and supercomputers have minimum instruction sizes between 8 and 64 bits. The longest possible instruction on x86 is 15 bytes . Within an instruction set, different instructions may have different lengths. In some architectures, notably most reduced instruction set computers , instructions are a fixed length, typically corresponding with that architecture's word size. In other architectures, instructions have variable length, typically integral multiples of a byte or a halfword. Some, such as the ARM with Thumb-extension have mixed variable encoding, that is two fixed, usually 32-bit and 16-bit encodings, where instructions cannot be mixed freely but must be switched between on a branch ."
+2662,"Fixed-length instructions are less complicated to handle than variable-length instructions for several reasons , and are therefore somewhat easier to optimize for speed."
+2663,"In early 1960s computers, main memory was expensive and very limited, even on mainframes. Minimizing the size of a program to make sure it would fit in the limited memory was often central. Thus the size of the instructions needed to perform a particular task, the code density, was an important characteristic of any instruction set. It remained important on the initially-tiny memories of minicomputers and then microprocessors. Density remains important today, for smartphone applications, applications downloaded into browsers over slow Internet connections, and in ROMs for embedded applications. A more general advantage of increased density is improved effectiveness of caches and instruction prefetch."
+2664,"Computers with high code density often have complex instructions for procedure entry, parameterized returns, loops, etc. . However, more typical, or frequent, ""CISC"" instructions merely combine a basic ALU operation, such as ""add"", with the access of one or more operands in memory . Certain architectures may allow two or three operands  directly in memory or may be able to perform functions such as automatic pointer increment, etc. Software-implemented instruction sets may have even more complex and powerful instructions."
+2665,"Reduced instruction-set computers, RISC, were first widely implemented during a period of rapidly growing memory subsystems. They sacrifice code density to simplify implementation circuitry, and try to increase performance via higher clock frequencies and more registers. A single RISC instruction typically performs only a single operation, such as an ""add"" of registers or a ""load"" from a memory location into a register. A RISC instruction set normally has a fixed instruction length, whereas a typical CISC instruction set has instructions of widely varying length. However, as RISC computers normally require more and often longer instructions to implement a given task, they inherently make less optimal use of bus bandwidth and cache memories."
+2666,"Certain embedded RISC ISAs like Thumb and AVR32 typically exhibit very high density owing to a technique called code compression. This technique packs two 16-bit instructions into one 32-bit word, which is then unpacked at the decode stage and executed as two instructions."
+2667,"Minimal instruction set computers  are commonly a form of stack machine, where there are few separate instructions , so that multiple instructions can be fit into a single machine word. These types of cores often take little silicon to implement, so they can be easily realized in an FPGA or in a multi-core form. The code density of MISC is similar to the code density of RISC; the increased instruction density is offset by requiring more of the primitive instructions to do a task."
+2668,There has been research into executable compression as a mechanism for improving code density. The mathematics of Kolmogorov complexity describes the challenges and limits of this.
+2669,"In practice, code density is also dependent on the compiler. Most optimizing compilers have options that control whether to optimize code generation for execution speed or for code density. For instance GCC has the option -Os to optimize for small machine code size, and -O3 to optimize for execution speed at the cost of larger machine code."
+2670,"The instructions constituting a program are rarely specified using their internal, numeric form ; they may be specified by programmers using an assembly language or, more commonly, may be generated from high-level programming languages by compilers."
+2671,"The design of instruction sets is a complex issue. There were two stages in history for the microprocessor. The first was the CISC , which had many different instructions. In the 1970s, however, places like IBM did research and found that many instructions in the set could be eliminated. The result was the RISC , an architecture that uses a smaller set of instructions. A simpler instruction set may offer the potential for higher speeds, reduced processor size, and reduced power consumption. However, a more complex set may optimize common operations, improve memory and cache efficiency, or simplify programming."
+2672,"Some instruction set designers reserve one or more opcodes for some kind of system call or software interrupt. For example, MOS Technology 6502 uses 00H, Zilog Z80 uses the eight codes C7,CF,D7,DF,E7,EF,F7,FFH while Motorola 68000 use codes in the range A000..AFFFH."
+2673,Fast virtual machines are much easier to implement if an instruction set meets the Popek and Goldberg virtualization requirements.
+2674,"The NOP slide used in immunity-aware programming is much easier to implement if the ""unprogrammed"" state of the memory is interpreted as a NOP."
+2675,"On systems with multiple processors, non-blocking synchronization algorithms are much easier to implement if the instruction set includes support for something such as ""fetch-and-add"", ""load-link/store-conditional"" , or ""atomic compare-and-swap""."
+2676,"A given instruction set can be implemented in a variety of ways. All ways of implementing a particular instruction set provide the same programming model, and all implementations of that instruction set are able to run the same executables. The various ways of implementing an instruction set give different tradeoffs between cost, performance, power consumption, size, etc."
+2677,"When designing the microarchitecture of a processor, engineers use blocks of ""hard-wired"" electronic circuitry  such as adders, multiplexers, counters, registers, ALUs, etc. Some kind of register transfer language is then often used to describe the decoding and sequencing of each instruction of an ISA using this physical microarchitecture.
+There are two basic ways to build a control unit to implement this description :"
+2678,"Some computer designs ""hardwire"" the complete instruction set decoding and sequencing ."
+2679,"Other designs employ microcode routines or tables  to do this, using ROMs or writable RAMs , PLAs, or both."
+2680,Some microcoded CPU designs with a writable control store use it to allow the instruction set to be changed .
+2681,CPUs designed for reconfigurable computing may use field-programmable gate arrays .
+2682,"An ISA can also be emulated in software by an interpreter. Naturally, due to the interpretation overhead, this is slower than directly running programs on the emulated hardware, unless the hardware running the emulator is an order of magnitude faster. Today, it is common practice for vendors of new ISAs or microarchitectures to make software emulators available to software developers before the hardware implementation is ready."
+2683,"Often the details of the implementation have a strong influence on the particular instructions selected for the instruction set. For example, many implementations of the instruction pipeline only allow a single memory load or memory store per instruction, leading to a load–store architecture . For another example, some early ways of implementing the instruction pipeline led to a delay slot."
+2684,"The demands of high-speed digital signal processing have pushed in the opposite direction—forcing instructions to be implemented in a particular way. For example, to perform digital filters fast enough, the MAC instruction in a typical digital signal processor  must use a kind of Harvard architecture that can fetch an instruction and two data words simultaneously, and it requires a single-cycle multiply–accumulate multiplier."
+2685,"IBM PC compatible computers are similar to the original IBM PC, XT, and AT, all from computer giant IBM, that are able to use the same software and expansion cards.  Such computers were referred to as PC clones, IBM clones or IBM PC  clones. The term ""IBM PC compatible"" is now a historical description only, since IBM no longer sells personal computers after it sold its personal computer division in 2005 to Chinese technology company Lenovo. The designation ""PC"", as used in much of personal computer history, has not meant ""personal computer"" generally, but rather an x86 computer capable of running the same software that a contemporary IBM PC could. The term was initially in contrast to the variety of home computer systems available in the early 1980s, such as the Apple II, TRS-80, and Commodore 64. Later, the term was primarily used in contrast to Apple's Macintosh computers."
+2686,"These ""clones"" duplicated almost all the significant features of the original IBM PC architectures. This was facilitated by IBM's choice of commodity hardware components, which were cheap, and by various manufacturers' ability to reverse-engineer the BIOS firmware using a ""clean room design"" technique. Columbia Data Products built the first clone of the IBM personal computer, the MPC 1600 by a clean-room reverse-engineered implementation of its BIOS. Other rival companies, Corona Data Systems, Eagle Computer, and the Handwell Corporation were threatened with legal action by IBM, who settled with them. Soon after in 1982, Compaq released the very successful Compaq Portable, also with a clean-room reverse-engineered BIOS, and also not challenged legally by IBM."
+2687,"Early IBM PC compatibles used the same computer buses as their IBM counterparts, switching from the 8-bit IBM PC and XT bus to the 16-bit IBM AT bus with the release of the AT. IBM's introduction of the proprietary Micro Channel architecture  in its PS/2 series resulted in the establishment of the Extended Industry Standard Architecture bus open standard by a consortium of IBM PC compatible vendors, redefining the 16-bit IBM AT bus as the Industry Standard Architecture  bus. Additional bus standards were subsequently adopted to improve compatibility between IBM PC compatibles, including the VESA Local Bus , Peripheral Component Interconnect , and the Accelerated Graphics Port ."
+2688,"Descendants of the x86 IBM PC compatibles, namely 64-bit computers based on ""x86-64/AMD64"" chips comprise the majority of desktop computers on the market as of 2021, with the dominant operating system being Microsoft Windows. Interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent. Many modern computers are unable to use old software or hardware that depends on portions of the IBM PC compatible architecture which are missing or do not have equivalents in modern computers. For example, computers which boot using Unified Extensible Firmware Interface-based firmware that lack a Compatibility Support Module, or CSM, required to emulate the old BIOS-based firmware interface, or have their CSMs disabled, cannot natively run MS-DOS since MS-DOS depends on a BIOS interface to boot."
+2689,"Only the Macintosh had kept significant market share without having compatibility with the IBM PC, although that changed during the Intel Macs era running Mac OS X, often dual-booting Windows with Boot Camp."
+2690,"IBM decided in 1980 to market a low-cost single-user computer as quickly as possible. On August 12, 1981, the first IBM PC went on sale. There were three operating systems  available for it. The least expensive and most popular was PC DOS made by Microsoft. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM computers. The only component of the original PC architecture exclusive to IBM was the BIOS ."
+2691,"IBM at first asked developers to avoid writing software that addressed the computer's hardware directly and to instead make standard calls to BIOS functions that carried out hardware-dependent operations. This software would run on any machine using MS-DOS or PC DOS. Software that directly addressed the hardware instead of making standard calls was faster, however; this was particularly relevant to games. Software addressing IBM PC hardware in this way would not run on MS-DOS machines with different hardware . The IBM PC was sold in high enough volumes to justify writing software specifically for it, and this encouraged other manufacturers to produce machines that could use the same programs, expansion cards, and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not hardware- and software-compatible with the PC. The 640 KB barrier on ""conventional"" system memory available to MS-DOS is a legacy of that period; other non-clone machines, while subject to a limit, could exceed 640 KB."
+2692,"Rumors of ""lookalike,"" compatible computers, created without IBM's approval, began almost immediately after the IBM PC's release. InfoWorld wrote on the first anniversary of the IBM PC that"
+2693,"By June 1983 PC Magazine defined ""PC 'clone'"" as ""a computer  accommodate the user who takes a disk home from an IBM PC, walks across the room, and plugs it into the 'foreign' machine"". Because of a shortage of IBM PCs that year, many customers purchased clones instead. Columbia Data Products produced the first computer more or less compatible with the IBM PC standard during June 1982, soon followed by Eagle Computer. Compaq announced its first product, an IBM PC compatible in November 1982, the Compaq Portable. The Compaq was the first sewing machine-sized portable computer that was essentially 100% PC-compatible. The court decision in Apple v. Franklin, was that BIOS code was protected by copyright law, but it could reverse-engineer the IBM BIOS and then write its own BIOS using clean room design. Note this was over a year after Compaq released the Portable. The money and research put into reverse-engineering the BIOS was a calculated risk."
+2694,"At the same time, many manufacturers such as Tandy/RadioShack, Xerox, Hewlett-Packard, Digital Equipment Corporation, Sanyo, Texas Instruments, Tulip, Wang and Olivetti introduced personal computers that supported MS-DOS, but were not completely software- or hardware-compatible with the IBM PC."
+2695,"Tandy described the Tandy 2000, for example, as having a ""'next generation' true 16-bit CPU"", and with ""More speed. More disk storage. More expansion"" than the IBM PC or ""other MS-DOS computers"". While admitting in 1984 that many PC DOS programs did not work on the computer, the company stated that ""the most popular, sophisticated software on the market"" was available, either immediately or ""over the next six months""."
+2696,"Like IBM, Microsoft's apparent intention was that application writers would write to the application programming interfaces in MS-DOS or the firmware BIOS, and that this would form what would now be termed a hardware abstraction layer. Each computer would have its own Original Equipment Manufacturer  version of MS-DOS, customized to its hardware. Any software written for MS-DOS would operate on any MS-DOS computer, despite variations in hardware design."
+2697,"This expectation seemed reasonable in the computer marketplace of the time. Until then Microsoft's business was based primarily on computer languages such as BASIC. The established small system operating software was CP/M from Digital Research which was in use both at the hobbyist level and by the more professional of those using microcomputers. To achieve such widespread use, and thus make the product viable economically, the OS had to operate across a range of machines from different vendors that had widely varying hardware. Those customers who needed other applications than the starter programs could reasonably expect publishers to offer their products for a variety of computers, on suitable media for each."
+2698,"Microsoft's competing OS was intended initially to operate on a similar varied spectrum of hardware, although all based on the 8086 processor. Thus, MS-DOS was for several years sold only as an OEM product. There was no Microsoft-branded MS-DOS: MS-DOS could not be purchased directly from Microsoft, and each OEM release was packaged with the trade dress of the given PC vendor. Malfunctions were to be reported to the OEM, not to Microsoft. However, as machines that were compatible with IBM hardware—thus supporting direct calls to the hardware—became widespread, it soon became clear that the OEM versions of MS-DOS were virtually identical, except perhaps for the provision of a few utility programs."
+2699,"MS-DOS provided adequate functionality for character-oriented applications such as those that could have been implemented on a text-only terminal. Had the bulk of commercially important software been of this nature, low-level hardware compatibility might not have mattered. However, in order to provide maximum performance and leverage hardware features , PC applications quickly developed beyond the simple terminal applications that MS-DOS supported directly. Spreadsheets, WYSIWYG word processors, presentation software and remote communication software established new markets that exploited the PC's strengths, but required capabilities beyond what MS-DOS provided. Thus, from very early in the development of the MS-DOS software environment, many significant commercial software products were written directly to the hardware, for a variety of reasons:"
+2700,"In May 1983, Future Computing defined four levels of compatibility:"
+2701,"During development, Compaq engineers found that Microsoft Flight Simulator would not run because of what subLOGIC's Bruce Artwick described as ""a bug in one of Intel's chips"", forcing them to make their new computer bug compatible with the IBM PC. At first, few clones other than Compaq's offered truly full compatibility. Jerry Pournelle purchased an IBM PC in mid-1983, ""rotten keyboard and all"", because he had ""four cubic feet of unevaluated software, much of which won't run on anything but an IBM PC. Although a lot of machines claim to be 100 percent IBM PC compatible, I've yet to have one arrive ... Alas, a lot of stuff doesn't run with Eagle, Z-100, Compupro, or anything else we have around here"". Columbia Data Products's November 1983 sales brochure stated that during tests with retail-purchased computers in October 1983, its own and Compaq's products were compatible with all tested PC software, while Corona and Eagle's were less compatible. Columbia University reported in January 1984 that Kermit ran without modification on Compaq and Columbia Data Products clones, but not on those from Eagle or Seequa. Other MS-DOS computers also required custom code."
+2702,"By December 1983 Future Computing stated that companies like Compaq, Columbia Data Products, and Corona that emphasized IBM PC compatibility had been successful, while non-compatible computers had hurt the reputations of others like TI and DEC despite superior technology. At a San Francisco meeting it warned 200 attendees, from many American and foreign computer companies as well as IBM itself, to ""Jump on the IBM PC-compatible bandwagon—quickly, and as compatibly as possible"". Future Computing said in February 1984 that some computers were ""press-release compatible"", exaggerating their actual compatibility with the IBM PC. Many companies were reluctant to have their products' PC compatibility tested. When PC Magazine requested samples from computer manufacturers that claimed to produce compatibles for an April 1984 review, 14 of 31 declined. Corona Data Systems specified that ""Our systems run all software that conforms to IBM PC programming standards. And the most popular software does."" When a BYTE journalist asked to test Peachtext at the Spring 1983 COMDEX, Corona representatives ""hemmed and hawed a bit, but they finally led me ... off in the corner where no one would see it should it fail"". The magazine reported that ""Their hesitancy was unnecessary. The disk booted up without a problem"". Zenith Data Systems was bolder, bragging that its Z-150 ran all applications people brought to test with at the 1984 West Coast Computer Faire."
+2703,"Creative Computing in 1985 stated, ""we reiterate our standard line regarding the IBM PC compatibles: try the package you want to use before you buy the computer."" Companies modified their computers' BIOS to work with newly discovered incompatible applications, and reviewers and users developed stress tests to measure compatibility; by 1984 the ability to operate Lotus 1-2-3 and Flight Simulator became the standard, with compatibles specifically designed to run them."
+2704,"IBM believed that some companies such as Eagle, Corona, and Handwell infringed on its copyright, and after Apple Computer, Inc. v. Franklin Computer Corp. successfully forced the clone makers to stop using the BIOS. The Phoenix BIOS in 1984, however, and similar products such as AMI BIOS, permitted computer makers to legally build essentially 100%-compatible clones without having to reverse-engineer the PC BIOS themselves. A September 1985 InfoWorld chart listed seven compatibles with 256 KB RAM, two disk drives, and monochrome monitors for $1,495 to $2,320, while the equivalent IBM PC cost $2,820. The inexpensive Leading Edge Model D is even compatible with IBM proprietary diagnostic software, unlike the Compaq Portable. By 1986 Compute! stated that ""clones are generally reliable and about 99 percent compatible"", and a 1987 survey in the magazine of the clone industry did not mention software compatibility, stating that ""PC by now has come to stand for a computer capable of running programs that are managed by MS-DOS""."
+2705,"In February 1984 Byte wrote that ""IBM's burgeoning influence in the PC community is stifling innovation because so many other companies are mimicking Big Blue"", but The Economist stated in November 1983, ""The main reason why an IBM standard is not worrying is that it can help competition to flourish""."
+2706,"By 1983, IBM had about 25% of sales of personal computers between $1,000 and $10,000, and computers with some PC compatibility were another 25%. As the market and competition grew IBM's influence diminished. In November 1985 PC Magazine stated ""Now that it has created the  market, the market doesn't necessarily need IBM for the machines. It may depend on IBM to set standards and to develop higher-performance machines, but IBM had better conform to existing standards so as to not hurt users"". In January 1987, Bruce Webster wrote in Byte of rumors that IBM would introduce proprietary personal computers with a proprietary operating system: ""Who cares? If IBM does it, they will most likely just isolate themselves from the largest marketplace, in which they really can't compete anymore anyway"". The magazine predicted that in 1987 the market ""will complete its transition from an IBM standard to an Intel/MS-DOS/expansion bus standard ... Folks aren't so much concerned about IBM compatibility as they are about Lotus 1-2-3 compatibility"". By 1992, Macworld stated that because of clones, ""IBM lost control of its own market and became a minor player with its own technology""."
+2707,"The Economist predicted in 1983 that ""IBM will soon be as much a prisoner of its standards as its competitors are"", because ""Once enough IBM machines have been bought, IBM cannot make sudden changes in their basic design; what might be useful for shedding competitors would shake off even more customers"". After IBM announced the OS/2-oriented PS/2 line in early 1987, sales of existing DOS-compatible PC compatibles rose, in part because the proprietary operating system was not available. In 1988, Gartner Group estimated that the public purchased 1.5 clones for every IBM PC. By 1989 Compaq was so influential that industry executives spoke of ""Compaq compatible"", with observers stating that customers saw the company as IBM's equal or superior."
+2708,"After 1987, IBM PC compatibles dominated both the home and business markets of commodity computers, with other notable alternative architectures being used in niche markets, like the Macintosh computers offered by Apple Inc. and used mainly for desktop publishing at the time, the aging 8-bit Commodore 64 which was selling for $150 by this time and became the world's bestselling computer, the 32-bit Commodore Amiga line used for television and video production and the 32-bit Atari ST used by the music industry. However, IBM itself lost the main role in the market for IBM PC compatibles by 1990. A few events in retrospect are important:"
+2709,"Despite popularity of its ThinkPad set of laptop PC's, IBM finally relinquished its role as a consumer PC manufacturer during April 2005, when it sold its laptop and desktop PC divisions  to Lenovo for US$1.75 billion."
+2710,"As of October 2007, Hewlett-Packard and Dell had the largest shares of the PC market in North America. They were also successful overseas, with Acer, Lenovo, and Toshiba also notable. Worldwide, a huge number of PCs are ""white box"" systems assembled by myriad local systems builders. Despite advances of computer technology, the IBM PC compatibles remained very much compatible with the original IBM PC computers, although most of the components implement the compatibility in special backward compatibility modes used only during a system boot. It was often more practical to run old software on a modern system using an emulator rather than relying on these features."
+2711,In 2014 Lenovo acquired IBM's x86-based server  business for US$2.1 billion.
+2712,"One of the strengths of the PC compatible design is its modular hardware design. End-users could readily upgrade peripherals and, to some degree, processor and memory without modifying the computer's motherboard or replacing the whole computer, as was the case with many of the microcomputers of the time. However, as processor speed and memory width increased, the limits of the original XT/AT bus design were soon reached, particularly when driving graphics video cards. IBM did introduce an upgraded bus in the IBM PS/2 computer that overcame many of the technical limits of the XT/AT bus, but this was rarely used as the basis for IBM compatible computers since it required license payments to IBM both for the PS/2 bus and any prior AT-bus designs produced by the company seeking a license. This was unpopular with hardware manufacturers and several competing bus standards were developed by consortiums, with more agreeable license terms. Various attempts to standardize the interfaces were made, but in practice, many of these attempts were either flawed or ignored. Even so, there were many expansion options, and despite the confusion of its users, the PC compatible design advanced much faster than other competing designs of the time, even if only because of its market dominance."
+2713,"During the 1990s, IBM's influence on PC architecture started to decline. ""IBM PC compatible"" becomes ""Standard PC"" in 1990s, and later ""ACPI PC"" in 2000s. An IBM-brand PC became the exception rather than the rule. Instead of placing importance on compatibility with the IBM PC, vendors began to emphasize compatibility with Windows. In 1993, a version of Windows NT was released that could operate on processors other than the x86 set. While it required that applications be recompiled, which most developers did not do, its hardware independence was used for Silicon Graphics  x86 workstations–thanks to NT's Hardware abstraction layer , they could operate NT ."
+2714,"No mass-market personal computer hardware vendor dared to be incompatible with the latest version of Windows, and Microsoft's annual WinHEC conferences provided a setting in which Microsoft could lobby for—and in some cases dictate—the pace and direction of the hardware of the PC industry. Microsoft and Intel had become so important to the ongoing development of PC hardware that industry writers began using the word Wintel to refer to the combined hardware-software system."
+2715,"This terminology itself is becoming a misnomer, as Intel has lost absolute control over the direction of x86 hardware development with AMD's AMD64. Additionally, non-Windows operating systems like macOS and Linux have established a presence on the x86 architecture."
+2716,"Although the IBM PC was designed for expandability, the designers could not anticipate the hardware developments of the 1980s, nor the size of the industry they would engender. To make things worse, IBM's choice of the Intel 8088 for the CPU introduced several limitations for developing software for the PC compatible platform. For example, the 8088 processor only had a 20-bit memory addressing space. To expand PCs beyond one megabyte, Lotus, Intel, and Microsoft jointly created expanded memory , a bank-switching scheme to allow more memory provided by add-in hardware, and accessed by a set of four 16-kilobyte ""windows"" inside the 20-bit addressing. Later, Intel CPUs had larger address spaces and could directly address 16 MB  or more, causing Microsoft to develop extended memory  which did not require additional hardware."
+2717,"""Expanded"" and ""extended"" memory have incompatible interfaces, so anyone writing software that used more than one megabyte had to provide for both systems for the greatest compatibility until MS-DOS began including EMM386, which simulated EMS memory using XMS memory. A protected mode OS can also be written for the 80286, but DOS application compatibility was more difficult than expected, not only because most DOS applications accessed the hardware directly, bypassing BIOS routines intended to ensure compatibility, but also that most BIOS requests were made by the first 32 interrupt vectors, which were marked as ""reserved"" for protected mode processor exceptions by Intel."
+2718,"Video cards suffered from their own incompatibilities. There was no standard interface for using higher-resolution SVGA graphics modes supported by later video cards. Each manufacturer developed their own methods of accessing the screen memory, including different mode numberings and different bank switching arrangements. The latter were used to address large images within a single 64 KB segment of memory. Previously, the VGA standard had used planar video memory arrangements to the same effect, but this did not easily extend to the greater color depths and higher resolutions offered by SVGA adapters. An attempt at creating a standard named VESA BIOS Extensions  was made, but not all manufacturers used it."
+2719,"When the 386 was introduced, again a protected mode OS could be written for it. This time, DOS compatibility was much easier because of virtual 8086 mode. Unfortunately programs could not switch directly between them, so eventually, some new memory-model APIs were developed, VCPI and DPMI, the latter becoming the most popular."
+2720,"Because of the great number of third-party adapters and no standard for them, programming the PC could be difficult. Professional developers would operate a large test-suite of various known-to-be-popular hardware combinations."
+2721,"Meanwhile, consumers were overwhelmed by the competing, incompatible standards and many different combinations of hardware on offer. To give them some idea of what sort of PC they would need to operate their software, the Multimedia PC  standard was set during 1990. A PC that met the minimum MPC standard could be marketed with the MPC logo, giving consumers an easy-to-understand specification to look for. Software that could operate on the most minimally MPC-compliant PC would be guaranteed to operate on any MPC. The MPC level 2 and MPC level 3 standards were set later, but the term ""MPC compliant"" never became popular. After MPC level 3 during 1996, no further MPC standards were established."
+2722,"By the late 1990s, the success of Microsoft Windows had driven rival commercial operating systems into near-extinction, and had ensured that the ""IBM PC compatible"" computer was the dominant computing platform. This meant that if a developer made their software only for the Wintel platform, they would still be able to reach the vast majority of computer users. The only major competitor to Windows with more than a few percentage points of market share was Apple Inc.'s Macintosh. The Mac started out billed as ""the computer for the rest of us"", but high prices and closed architecture drove the Macintosh into an education and desktop publishing niche, from which it only emerged in the mid-2000s. By the mid-1990s the Mac's market share had dwindled to around 5% and introducing a new rival operating system had become too risky a commercial venture. Experience had shown that even if an operating system was technically superior to Windows, it would be a failure in the market . In 1989, Steve Jobs said of his new NeXT system, ""It will either be the last new hardware platform to succeed, or the first to fail."" Four years later in 1993, NeXT announced it was ending production of the NeXTcube and porting NeXTSTEP to Intel processors."
+2723,"Very early on in PC history, some companies introduced their own XT-compatible chipsets. For example, Chips and Technologies introduced their 82C100 XT Controller which integrated and replaced six of the original XT circuits: one 8237 DMA controller, one 8253 interrupt timer, one 8255 parallel interface controller, one 8259 interrupt controller, one 8284 clock generator, and one 8288 bus controller. Similar non-Intel chipsets appeared for the AT-compatibles, for example OPTi's 82C206 or 82C495XLC which were found in many 486 and early Pentium systems. The x86 chipset market was very volatile though. In 1993, VLSI Technology had become the dominant market player only to be virtually wiped out by Intel a year later. Intel has been the uncontested leader ever since. As the ""Wintel"" platform gained dominance Intel gradually abandoned the practice of licensing its technologies to other chipset makers; in 2010 Intel was involved in litigation related to their refusal to license their processor bus and related technologies to other companies like Nvidia."
+2724,"Companies such as AMD and Cyrix developed alternative x86 CPUs that were functionally compatible with Intel's. Towards the end of the 1990s, AMD was taking an increasing share of the CPU market for PCs. AMD even ended up playing a significant role in directing the development of the x86 platform when its Athlon line of processors continued to develop the classic x86 architecture as Intel deviated with its NetBurst architecture for the Pentium 4 CPUs and the IA-64 architecture for the Itanium set of server CPUs. AMD developed AMD64, the first major extension not created by Intel, which Intel later adopted as x86-64. During 2006 Intel began abandoning NetBurst with the release of their set of ""Core"" processors that represented a development of the earlier Pentium III."
+2725,"A major alternative to Wintel domination is the rise of alternative operating systems since the early 2000s, which marked as the start of the post-PC era. This would include both the rapid growth of the smart phone  as an alternative to the personal computer; and the increasing prevalence of Linux and Unix-like operating systems in the server farms of large corporations such as Google or Amazon."
+2726,"The term ""IBM PC compatible"" is not commonly used presently because many current mainstream desktop and laptop computers are based on the PC architecture, and IBM no longer makes PCs. The competing hardware architectures have either been discontinued or, like the Amiga, have been relegated to niche, enthusiast markets. In the past, the most successful exception was Apple's Macintosh platform, which used non-Intel processors from its inception. Although Macintosh was initially based on the Motorola 68000 series, then transitioned to the PowerPC architecture, Macintosh computers transitioned to Intel processors beginning in 2006. Until 2020 Macintosh computers shared the same system architecture as their Wintel counterparts and could boot Microsoft Windows without a DOS Compatibility Card. However, with the announcement of the internally developed ARM-based M1 CPU, they are again the exception to IBM compatibility."
+2727,"The processor speed and memory capacity of modern PCs are many orders of magnitude greater than they were for the original IBM PC and yet backwards compatibility has been largely maintained –  a 32-bit operating system released during the 2000s can still operate many of the simpler programs written for the OS of the early 1980s without needing an emulator, though an emulator like DOSBox now has near-native functionality at full speed . Additionally, many modern PCs can still run DOS directly, although special options such as USB legacy mode and SATA-to-PATA emulation may need to be set in the BIOS setup utility. Computers using the UEFI might need to be set at legacy BIOS mode to be able to boot DOS. However, the BIOS/UEFI options in most mass-produced consumer-grade computers are very limited and cannot be configured to truly handle OSes such as the original variants of DOS."
+2728,"The spread of the x86-64 architecture has further distanced current computers' and operating systems' internal similarity with the original IBM PC by introducing yet another processor mode with an instruction set modified for 64-bit addressing, but x86-64 capable processors also retain standard x86 compatibility."
+2729,"Applications may be bundled with the computer and its system software or published separately and may be coded as proprietary, open-source, or projects. The term ""app"" usually refers to applications for mobile devices such as phones."
+2730,"In information technology, an application , an application program, or application software is a computer program designed to help people perform an activity. Depending on the activity for which it was designed, an application can manipulate text, numbers, audio, graphics, and a combination of these elements. Some application packages focus on a single task, such as word processing; others called integrated software include several applications."
+2731,"User-written software tailors systems to meet the user's specific needs. User-written software includes spreadsheet templates, word processor macros, scientific simulations, audio, graphics, and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is."
+2732,"The delineation between system software such as operating systems and application software is not exact, however, and is occasionally the object of controversy. For example, one of the key questions in the United States v. Microsoft Corp. antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or a separate piece of application software.  As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the operating systems built over this kernel. In some types of embedded systems, the application software and the operating system software may be indistinguishable from the user, as in the case of software used to control a VCR, DVD player, or microwave oven. The above definitions may exclude some applications that may exist on some computers in large organizations. For an alternative definition of an app:  see Application Portfolio Management."
+2733,"The word ""application"" used as an adjective is not restricted to the ""of or on application software"" meaning. For example, concepts such as application programming interface , application server, application virtualization, application lifecycle management and portable application apply to all computer programs alike, not just application software."
+2734,"Some applications are available in versions for several different platforms; others only work on one and are thus called, for example, a geography application for Microsoft Windows, or an Android application for education, or a Linux game.  Sometimes a new and popular application arises that only runs on one platform, increasing the desirability of that platform.  This is called a killer application or killer app, coined in the late 1980s. For example, VisiCalc was the first modern spreadsheet software for the Apple II and helped sell the then-new personal computers into offices. For Blackberry it was their email software."
+2735,"In recent years, the shortened term ""app""  has become popular to refer to applications for mobile devices such as smartphones and tablets, the shortened form matching their typically smaller scope compared to applications on PCs. Even more recently, the shortened version is used for desktop application software as well."
+2736,There are many different and alternative ways to classify application software.
+2737,"From the legal point of view, application software is mainly classified with a black-box approach, about the rights of its end-users or subscribers ."
+2738,"Software applications are also classified in respect of the programming language in which the source code is written or executed, and concerning their purpose and outputs."
+2739,"Application software is usually distinguished into two main classes: closed source vs open source software applications, and free or proprietary software applications."
+2740,"Proprietary software is placed under the exclusive copyright, and a software license grants limited usage rights. The open-closed principle states that software may be ""open only for extension, but not for modification"". Such applications can only get add-on by third parties."
+2741,"Free and open-source software shall be run, distributed, sold, or extended for any purpose, and -being open- shall be modified or reversed in the same way."
+2742,"FOSS software applications released under a free license may be perpetual and also royalty-free. Perhaps, the owner, the holder or third-party enforcer of any right  are entitled to add exceptions, limitations, time decays or expiring dates to the license terms of use."
+2743,"Public-domain software is a type of FOSS, which is royalty-free and - openly or reservedly- can be run, distributed, modified, reversed, republished, or created in derivative works without any copyright attribution and therefore revocation. It can even be sold, but without transferring the public domain property to other single subjects. Public-domain SW can be released under a licensing legal statement, which enforces those terms and conditions for an indefinite duration ."
+2744,"Since the development and near-universal adoption of the web, an important distinction that has emerged, has been between web applications — written with HTML, JavaScript and other web-native technologies and typically requiring one to be online and running a web browser — and the more traditional native applications written in whatever languages are available for one's particular type of computer. There has been a contentious debate in the computing community regarding web applications replacing native applications for many purposes, especially on mobile devices such as smartphones and tablets. Web apps have indeed greatly increased in popularity for some uses, but the advantages of applications make them unlikely to disappear soon, if ever. Furthermore, the two can be complementary, and even integrated."
+2745,"Application software can also be seen as being either horizontal or vertical.  Horizontal applications are more popular and widespread, because they are general purpose, for example word processors or databases. Vertical applications are niche products, designed for a particular type of industry or business, or department within an organization. Integrated suites of software will try to handle every specific aspect possible of, for example, manufacturing or banking worker, accounting, or customer service."
+2746,There are many types of application software:
+2747,"Applications can also be classified by computing platforms such as a desktop application for a particular operating system, delivery network such as in cloud computing and Web 2.0 applications, or delivery devices such as mobile apps for mobile devices."
+2748,"The operating system itself can be considered application software when performing simple calculating, measuring, rendering, and word processing tasks not used to control hardware via a command-line interface or graphical user interface. This does not include application software bundled within operating systems such as a software calculator or text editor."
+2749,"Application software is software that allows users to do user-oriented tasks such as create text documents, play or develop games, create presentations, listen to music, draw pictures, or browse the web. Examples are: computational science software, game engines, search engines, industrial automation, and software as a service applications."
+2750,"In the late 1940s, application software was custom-written by computer users to fit their specific hardware and requirements. System software was usually supplied by the manufacturer of the computer hardware and was intended to be used by most or all users of that system."
+2751,"Many operating systems come pre-packaged with basic application software. Such software is not considered system software when it can be uninstalled without affecting the functioning of other software. Examples of such software are games and simple editing tools supplied with Microsoft Windows, or software development toolchains supplied with many Linux distributions."
+2752,"Some of the grayer areas between system and application software are web browsers integrated deeply into the operating system such as Internet Explorer in some versions of Microsoft Windows, or ChromeOS where the browser functions as the only user interface and the only way to run programs ."
+2753,"Cloud-based software is another example of systems software, providing services to a software client , not to the user directly. It is developed using system programming methodologies and systems programming languages."
+2754,"The operating system , allows the parts of a computer to work together by performing tasks like transferring data between memory and disks or rendering output onto a display device. It provides a platform  to run high-level system software and application software."
+2755,A kernel is the core part of the operating system that defines an Application programming interface for applications programs  and an interface to device drivers.
+2756,"Device drivers and devices firmware, including computer BIOS, provide basic functionality to operate and control the hardware connected to or built into the computer."
+2757,"A user interface interact with a computer. It can either be a command-line interface  or, since the 1980s, a graphical user interface . This is the part of the operating system the user directly interacts with, it is considered an application and not system software."
+2758,"Some organizations use the term systems programmer to describe a job function that is more accurately termed systems administrator. Software tools these employees use are then called system software. This utility software helps to analyze, configure, optimize and maintain the computer, such as virus protection. The term system software can also include software development tools ."
+2759,System software of video game consoles
+2760,"Digital media platforms, such as YouTube, Vimeo, and Twitch, accounted for viewership rates of 27.9 billion hours in 2020. A contributing factor to its part in what is commonly referred to as the digital revolution can be attributed to the use of interconnectivity."
+2761,"Examples of digital media include software, digital images, digital video, video games, web pages and websites, social media, digital data and databases, digital audio such as MP3, electronic documents and electronic books. Digital media often contrasts with print media, such as printed books, newspapers and magazines, and other traditional or analog media, such as photographic film, audio tapes or video tapes."
+2762,"Digital media has had a significantly broad and complex impact on society and culture. Combined with the Internet and personal computing, digital media has caused disruptive innovation in publishing, journalism, public relations, entertainment, education, commerce and politics. Digital media has also posed new challenges to copyright and intellectual property laws, fostering an open content movement in which content creators voluntarily give up some or all of their legal rights to their work. The ubiquity of digital media and its effects on society suggest that we are at the start of a new era in industrial history, called the Information Age, perhaps leading to a paperless society in which all media are produced and consumed on computers. However, challenges to a digital transition remain, including outdated copyright laws, censorship, the digital divide, and the spectre of a digital dark age, in which older media becomes inaccessible to new or upgraded information systems. Digital media has a significant, wide-ranging and complex impact on society and culture."
+2763,"Digital media platforms like YouTube work through a triple-product business model in which platforms provide information and entertainment  to the public often at no cost, while simultaneously capturing their attention, and also collecting user data to sell to advertisers. This business model aims to maximize consumer engagement on the platform."
+2764,"Paid media refers to promotional channels that marketers pay to use, including traditional media , online and digital media . This model compels businesses to develop sponsored media then pay social media platforms like Instagram for the right to show such media to customers in the platforms' newsfeeds. These customers become exposed to paid media, sometimes referred to as promoted or sponsored posts."
+2765,"refers to digital assets and channels that a company or individual controls and manages. This includes websites, social media profiles for example Facebook etc., blogs, and any other content platforms own and operated by the entity. Entity means the owner or controller of the channel such as business or person managing their online presence"
+2766,"Earned Media denotes public relations media channels like television, newspapers, blogs, or video sites that do not require direct payment or control by marketers but are included because viewers, readers, or users are interested in them. Free media is essentially online word of mouth, typically in ""viral"" trends, mentions, shares, retweets, reviews, recommendations, or content from third-party websites. When one's product or service is so good that users cannot help but post it on their social media, they get a lot of ""earned media"". They win the credibility of the media compared to other forms of credibility, becoming more transparent."
+2767,"Codes and information by machines were first conceptualized by Charles Babbage in the early 1800s. Babbage imagined that these codes would give him instructions for his Motor of Difference and Analytical Engine, machines that Babbage had designed to solve the problem of error in calculations. Between 1822 and 1823, the mathematician Ada Lovelace wrote the first instructions for calculating numbers on Babbage engines. Lovelace's instructions are now believed to be the first computer program. Although the machines were designed to perform analysis tasks, Lovelace anticipated the possible social impact of computers and program writing. ""For in the distribution and combination of truths and formulas of analysis, which may become easier and more quickly subjected to the mechanical combinations of the engine, the relationships and the nature of many subjects in which science necessarily relates in new subjects, and more deeply researched  there are in all extensions of human power or additions to human knowledge, various collateral influences, in addition to the primary and primary object reached."" Other old machine readable media include instructions for pianolas and weaving machines."
+2768,It is estimated that in the year 1986 less than 1% of the world's media storage capacity was digital and in 2007 it was already 94%. The year 2002 is assumed to be the year when human kind was able to store more information in digital than in analog media .
+2769,"Though they used machine-readable media, Babbage's engines, player pianos, jacquard looms and many other early calculating machines were themselves analog computers, with physical, mechanical parts. The first truly digital media came into existence with the rise of digital computers.  Digital computers use binary code and Boolean logic to store and process information, allowing one machine in one configuration to perform many different tasks. The first modern, programmable, digital computers, the Manchester Mark 1 and the EDSAC, were independently invented between 1948 and 1949. Though different in many ways from modern computers, these machines had digital software controlling their logical operations. They were encoded in binary, a system of ones and zeroes that are combined to make hundreds of characters. The 1s and 0s of binary are the ""digits"" of digital media."
+2770,"While digital media did not come into common use until the late 20th century, the conceptual foundation of digital media is traced to the work of scientist and engineer Vannevar Bush and his celebrated essay ""As We May Think"", published in The Atlantic Monthly in 1945. Bush envisioned a system of devices that could be used to help scientists, doctors, and historians, among others, to store, analyze and communicate information. Calling this then-imaginary device a ""memex"", Bush wrote:"
+2771,"Bush hoped that the creation of this memex would be the work of scientists after World War II. Though the essay predated digital computers by several years, ""As We May Think"" anticipated the potential social and intellectual benefits of digital media and provided the conceptual framework for digital scholarship, the World Wide Web, wikis and even social media. It was recognized as a significant work even at the time of its publication."
+2772,"Since the 1960s, computing power and storage capacity have increased exponentially, largely as a result of MOSFET scaling which enables MOS transistor counts to increase at a rapid pace predicted by Moore's law. Personal computers and smartphones put the ability to access, modify, store and share digital media in the hands of billions of people. Many electronic devices, from digital cameras to drones have the ability to create, transmit and view digital media. Combined with the World Wide Web and the Internet, digital media has transformed 21st century society in a way that is frequently compared to the cultural, economic and social impact of the printing press. The change has been so rapid and so widespread that it has launched an economic transition from an industrial economy to an information-based economy, creating a new period in human history known as the Information Age or the digital revolution."
+2773,"The transition has created some uncertainty about definitions. Digital media, new media, multimedia, and similar terms all have a relationship to both the engineering innovations and cultural impact of digital media.  The blending of digital media with other media, and with cultural and social factors, is sometimes known as new media or ""the new media."" Similarly, digital media seems to demand a new set of communications skills, called transliteracy, media literacy, or digital literacy. These skills include not only the ability to read and write—traditional literacy—but the ability to navigate the Internet, evaluate sources, and create digital content. The idea that we are moving toward a fully digital, paperless society is accompanied by the fear that we may soon—or currently—be facing a digital dark age, in which older media are no longer accessible on modern devices or using modern methods of scholarship. Digital media has a significant, wide-ranging and complex effect on society and culture."
+2774,"A senior engineer at Motorola named Martin Cooper was the first person to make a phone call on April 3, 1973. He decided the first phone call should be to a rival telecommunications company saying ""I'm speaking via a mobile phone"". Ten years later, Motorola released the Motorola DynaTAC, the first commercially available mobile phone. In the early 1990s Nokia released the Nokia 1011, the first mass-produced mobile phone. The number of smartphone users has increased dramatically, as has the commercial landscape. While Android and iOS both dominate the smartphone market. A study By Gartner found that in 2016 about 88% of the worldwide smartphones were Android while iOS had a market share of about 12%. About 85% of the mobile market revenue came from mobile games."
+2775,"The impact of the digital revolution can also be assessed by exploring the amount of worldwide mobile smart device users there are. This can be split into 2 categories; smart phone users and smart tablet users. Worldwide there are currently 2.32 billion smartphone users across the world. This figure is to exceed 2.87 billion by 2020. Smart tablet users reached a total of 1 billion in 2015, 15% of the world's population."
+2776,The statistics evidence the impact of digital media communications today. What is also of relevance is the fact that the number of smart device users is rising rapidly yet the amount of functional uses increase daily. A smartphone or tablet can be used for hundreds of daily needs. There are currently over 1 million apps on the Apple App store. These are all opportunities for digital marketing efforts. A smartphone user is impacted with digital advertising every second they open their Apple or Android device. This further evidences the digital revolution and the impact of revolution. This has resulted in a total of 13 billion dollars being paid out to the various app developers over the years. This growth has fueled the development of millions of software applications. Most of these apps are able to generate income via in app advertising. Gross revenue for 2020 is projected to be about $189 million.
+2777,"Compared with print media, the mass media, and other analog technologies, digital media are easy to copy, store, share and modify. This quality of digital media has led to significant changes in many industries, especially journalism, publishing, education, entertainment, and the music business. The overall effect of these changes is so far-reaching that it is difficult to quantify. For example, in movie-making, the transition from analog film cameras to digital cameras is nearly complete. The transition has economic benefits to Hollywood, making distribution easier and making it possible to add high-quality digital effects to films. At the same time, it has affected the analog special effects, stunt, and animation industries in Hollywood.  It has imposed painful costs on small movie theaters, some of which did not or will not survive the transition to digital. The effect of digital media on other media industries is similarly sweeping and complex."
+2778,"Between 2000 and 2015, the print newspaper advertising revenue has fallen from $60 billion to a nearly $20 billion. Even one of the most popular days for papers, Sunday, has seen a 9% circulation decrease the lowest since 1945."
+2779,"In journalism, digital media and citizen journalism have led to the loss of thousands of jobs in print media and the bankruptcy of many major newspapers. But the rise of digital journalism has also created thousands of new jobs and specializations. E-books and self-publishing are changing the book industry, and digital textbooks and other media-inclusive curricula are changing primary and secondary education."
+2780,"In academia, digital media has led to a new form of scholarship, also called digital scholarship, making open access and open science possible thanks to the low cost of distribution. New fields of study have grown, such as digital humanities and digital history. It has changed the way libraries are used and their role in society. Every major media, communications and academic endeavor is facing a period of transition and uncertainty related to digital media."
+2781,"Often time the magazine or publisher have a Digital edition which can be referred to an electronic formatted version identical to the print version. There is a huge benefit to the publisher and cost, as half of traditional publishers' costs come from production, including raw materials, technical processing, and distribution."
+2782,"Since 2004, there has been a decrease in newspaper industry employment, with only about 40,000 people working in the workforce currently. Alliance of Audited Media & Publishers information during the 2008 recession, over 10% of print sales are diminished for certain magazines, with a hardship coming from only 75% of the sales advertisements as before. However, in 2018, major newspapers advertising revenue was 35% from digital ads."
+2783,"In contrast, mobile versions of newspapers and magazines came in second with a huge growth of 135%. The New York Times has noted a 47% year of year rise in their digital subscriptions. 43% of adults get news often from news websites or social media, compared with 49% for television. Pew Research also asked respondents if they got news from a streaming device on their TV – 9% of U.S. adults said that they do so often."
+2784,"Digital media has also allowed individuals to be much more active in content creation. Anyone with access to computers and the Internet can participate in social media and contribute their own writing, art, videos, photography and commentary to the Internet, as well as conduct business online. The dramatic reduction in the costs required to create and share content have led to a democratization of content creation as well as the creation of new types of content, like blogs, memes, and video essays. Some of these activities have also been labelled citizen journalism. This spike in user-created content is due to the development of the internet as well as the way in which users interact with media today. As more users join and use social media sites, the relevance of content creation increases. The release of technologies such mobile devices allow for easier and quicker access to all things media. Many media creation tools that were once available to only a few are now free and easy to use. The cost of devices that can access the Internet is steadily falling, and personal ownership of multiple digital devices is now becoming the standard. These elements have significantly affected political participation. Digital media is seen by many scholars as having a role in Arab Spring, and crackdowns on the use of digital and social media by embattled governments are increasingly common. Many governments restrict access to digital media in some way, either to prevent obscenity or in a broader form of political censorship."
+2785,"Over the years YouTube has grown to become a website with user generated media. This content is oftentimes not mediated by any company or agency, leading to a wide array of personalities and opinions online. Over the years, YouTube and other platforms have also shown their monetary gains. In 2020, the top 10 highest earning YouTube content creators each generated over 15 million dollars. Many of these YouTube profiles over the years have a multi camera set up as we would see on TV. Many of these creators also creating their own digital companies as their personalities grow. Personal devices have also seen an increase over the years. Over 1.5 billion users of tablets exist in this world right now and that is expected to slowly grow  About 20% of people in the world regularly watch their content using tablets in 2018"
+2786,"User-generated content raises issues of privacy, credibility, civility and compensation for cultural, intellectual and artistic contributions. The spread of digital media, and the wide range of literacy and communications skills necessary to use it effectively, have deepened the digital divide between those who have access to digital media and those who do not."
+2787,The rising of digital media has made the consumer's audio collection more precise and personalized. It is no longer necessary to purchase an entire album if the consumer is ultimately interested in only a few audio files.
+2788,"The rise of streaming services has led to a decrease of cable TV services to about 59%, while streaming services are growing at around 29%, and 9% are still users of the digital antenna. TV Controllers now incorporate designated buttons for streaming platforms. Users are spending an average of 1:55 with digital video each day, and only 1:44 on social networks. 6 out of 10 people report viewing their television shows and news via a streaming service. Platforms such as Netflix have gained attraction due to their adorability, accessibility, and for its original content. Companies such as Netflix have even bought previously cancelled shows such as Designated Survivor, Lucifer, and Arrested Development. As the internet becomes more and more prevalent, more companies are beginning to distribute content through internet only means. Indeed, young people today are increasingly likely to use TikTok over Google, television or newspapers for their news. With the loss of viewers, there is a loss of revenue but not as bad as what would be expected."
+2789,As of 2024 there has also been a wave of those considered too controversial by main-stream media moving over to online platforms such as X  to keep spreading their messages. One instance is Tucker Carlson leaving Fox News due to his controversial opinions and moving over to X. This has sparked debate surrounding topics such as free speech and hate speech.
+2790,"Digital media encompasses numerical networks of interactive systems that link databases, allowing users to navigate from one bit of content or webpage to another. Because of this ease, digital media poses several challenges to the current copyright and intellectual property laws. The ease of creating, modifying, and sharing digital media can influence copyright enforcement challenging and many copyright laws are widely seen as outdated. Under current copyright law, common Internet memes are generally illegal to share in many countries. Legal rights can be unclear for many common Internet activities. These include posting pictures from someone else's social media account, writing fanfiction, or covering and/or using popular songs in content such as YouTube videos. During the last decade, the concepts of fair use and copyright have been applied to different types of online media."
+2791,"Copyright challenges are spreading to all parts of digital media. Content creators on platforms such as YouTube follow guidelines set by copyright, IP laws, and the platform's copyright requirements. If these guidelines are not followed, the content may get demonetized, deleted, or sued. The situation can also occur when creators accidentally use audio tracks or background scenes that are under copyright. To avoid or resolve some of these issues, content creators can voluntarily adopt open, or copyleft licenses or they can release their work to the public domain. By doing this, creators are giving up certain legal rights regarding their content. Fair use is a doctrine of the US Copyright Law that allows limited use of copyrighted materials without the need to obtain permission. There are four factors that make up fair use. The first, Purpose, refers to what the content is being used for. The second factor is what copyrighted content is being used. If the content is non-fiction, it is more likely to fall under fair use than if the content is fiction. The third factor is how much of the copyrighted content is in use. Small amounts of copyrighted content are more likely to be considered fair. The last factor is, whether the use of copyrighted content earns money or affect the value of the content."
+2792,"Wikipedia uses some of the most common open licenses, Creative Commons licenses, and the GNU Free Documentation License. Open licenses are one aspect of a broad open content movement that advocates for the reduction or removal of copyright restrictions from software, data, and other digital media. To facilitate the collection and consumption of such licensing information and availability status, tools like the Creative Commons Search engine are used mostly for web images, and Unpaywall, or used for scholarly communication."
+2793,"Additional software has been developed to restrict access to digital media. Digital rights management  is used to lock material. This allows users to apply the media content to specific cases. DRM allows movie producers to rent at a lower price. This restricts the movie rental license length, rather than only selling the movie at full price. Additionally, DRM can prevent unauthorized modification or sharing of media."
+2794,"Digital media copyright protection technologies fall under intellectual property protection technology. This is because a series of computer technologies protect the digital content being created and transmitted. The Digital Millennium Copyright Act  provides safety to intermediaries that host user content, such as YouTube, from being held liable for copyright infringement so long as they meet all required conditions. The most notable of which is the ""notice and take down"" policy. The policy requires online intermediaries to remove and/or disable access to the content in question when there are court orders and/or allegations of illegal use of the content on their site. As a result, YouTube has and continues to develop more policies and standards that go far past what the DMCA requires. YouTube has also created an algorithm which continuously scans their cite to make sure all content follows all policies."
+2795,"One digital media platform known to have copyright concerns is the short video-sharing app TikTok. TikTok is a social media app that allows users to share short videos up to one minute in length, using a variety of visual effects and audio.  According to Loyola University's Chicago School of Law, around 50% of the music used on TikTok is unlicensed. TikTok has several music licensing agreements with various artists and labels, creating a library of fair and legal use of music. However, this does not cover all content for its users. A user could still commit a copyright violation on TikTok. One example is, accidentally having music playing on a stereo in the background or recording a laptop screen playing a song."
+2796,"Online magazines or digital magazines are one of the largest targets for copyright issues. According to the Audit Bureau of Circulations report from March 2011, the definition of this medium is when a digital magazine involves the distribution of magazine content by electronic means; it may be a replica. This definition can be considered outdated now that PDF replicas of print magazines are no longer common practice. These days digital magazines refer to magazines specifically created to be interactive digital platforms such as the internet, mobile phones, private networks, iPad, or other devices. The barriers to digital magazine distribution are thus decreasing. However, these platforms are also broadening the scope of where digital magazines can be published; smartphones are an example. Thanks to the improvements in tablets and other personal electronic devices, digital magazines have become much more readable and enticing through the use of graphic art. The evolution of online magazines began to focus on becoming more of a social media and entertainment platform."
+2797,"Online piracy has become one of the larger issues that have occurred concerning digital media copyright. The piracy of digital media, such as film and television, directly impacts the copyright party . This action can impact the ""health"" of the digital media industry. Piracy directly breaks the laws and morals of copyright. Along with piracy, digital media has contributed to the ability to spread false information or fake news. Due to the widespread use of digital media, fake news can receive more notoriety. This notoriety enhances the negative effects fake news creates. As a result, people's health and well-being can directly be affected."
+2798,Documentation is an important part of software engineering. Types of documentation include:
+2799,"Requirements documentation is the description of what a particular software does or shall do. It is used throughout development to communicate how the software functions or how it is intended to operate. It is also used as an agreement or as the foundation for agreement on what the software will do. Requirements are produced and consumed by everyone involved in the production of software, including: end users, customers, project managers, sales, marketing, software architects, usability engineers, interaction designers, developers, and testers."
+2800,"Requirements come in a variety of styles, notations and formality. Requirements can be goal-like , close to design , and anything in between. They can be specified as statements in natural language, as drawn figures, as detailed mathematical formulas, or as a combination of them all."
+2801,"The variation and complexity of requirement documentation make it a proven challenge. Requirements may be implicit and hard to uncover. It is difficult to know exactly how much and what kind of documentation is needed and how much can be left to the architecture and design documentation, and it is difficult to know how to document requirements considering the variety of people who shall read and use the documentation. Thus, requirements documentation is often incomplete . Without proper requirements documentation, software changes become more difficult — and therefore more error prone  and time-consuming ."
+2802,"The need for requirements documentation is typically related to the complexity of the product, the impact of the product, and the life expectancy of the software. If the software is very complex or developed by many people , requirements can help better communicate what to achieve. If the software is safety-critical and can have a negative impact on human life , more formal requirements documentation is often required. If the software is expected to live for only a month or two  very little requirements documentation may be needed. If the software is a first release that is later built upon, requirements documentation is very helpful when managing the change of the software and verifying that nothing has been broken in the software when it is modified."
+2803,"Traditionally, requirements are specified in requirements documents . To manage the increased complexity and changing nature of requirements documentation , database-centric systems and special-purpose requirements management tools are advocated."
+2804,"In Agile software development, requirements are often expressed as User Stories with accompanying acceptance criteria."
+2805,"Architecture documentation  is a special type of design document. In a way, architecture documents are third derivative from the code . Very little in the architecture documents is specific to the code itself. These documents do not describe how to program a particular routine, or even why that particular routine exists in the form that it does, but instead merely lays out the general requirements that would motivate the existence of such a routine. A good architecture document is short on details but thick on explanation. It may suggest approaches for lower level design, but leave the actual exploration trade studies to other documents."
+2806,"Another type of design document is the comparison document, or trade study. This would often take the form of a whitepaper. It focuses on one specific aspect of the system and suggests alternate approaches. It could be at the user interface, code, design, or even architectural level. It will outline what the situation is, describe one or more alternatives, and enumerate the pros and cons of each. A good trade study document is heavy on research, expresses its idea clearly , and most importantly is impartial. It should honestly and clearly explain the costs of whatever solution it offers as best. The objective of a trade study is to devise the best solution, rather than to push a particular point of view. It is perfectly acceptable to state no conclusion, or to conclude that none of the alternatives are sufficiently better than the baseline to warrant a change. It should be approached as a scientific endeavor, not as a marketing technique."
+2807,"A very important part of the design document in enterprise software development is the Database Design Document . It contains Conceptual, Logical, and Physical Design Elements. The DDD includes the formal information that the people who interact with the database need. The purpose of preparing it is to create a common source to be used by all players within the scene. The potential users are:"
+2808,"When talking about Relational Database Systems, the document should include following parts:"
+2809,It is very important to include all information that is to be used by all actors in the scene. It is also very important to update the documents as any change occurs in the database as well.
+2810,"It is important for the code documents associated with the source code  to be thorough, but not so verbose that it becomes overly time-consuming or difficult to maintain them. Various how-to and overview documentation guides are commonly found specific to the software application or software product being documented by API writers. This documentation may be used by developers, testers, and also end-users. Today, a lot of high-end applications are seen in the fields of power, energy, transportation, networks, aerospace, safety, security, industry automation, and a variety of other domains. Technical documentation has become important within such organizations as the basic and advanced level of information may change over a period of time with architecture changes.  There is evidence that the existence of good code documentation actually reduces maintenance costs for software."
+2811,"Code documents are often organized into a reference guide style, allowing a programmer to quickly look up an arbitrary function or class."
+2812,"Often, tools such as Doxygen, NDoc, Visual Expert, Javadoc, JSDoc, EiffelStudio, Sandcastle, ROBODoc, POD, TwinText, or Universal Report can be used to auto-generate the code documents—that is, they extract the comments and software contracts, where available, from the source code and create reference manuals in such forms as text or HTML files."
+2813,"The idea of auto-generating documentation is attractive to programmers for various reasons.  For example, because it is extracted from the source code itself , the programmer can write it while referring to the code, and use the same tools used to create the source code to make the documentation.  This makes it much easier to keep the documentation up-to-date."
+2814,"A possible downside is that only programmers can edit this kind of documentation, and it depends on them to refresh the output . Some would characterize this as a pro rather than a con."
+2815,"Respected computer scientist Donald Knuth has noted that documentation can be a very difficult afterthought process and has advocated literate programming, written at the same time and location as the source code and extracted by automatic means. The programming languages Haskell and CoffeeScript have built-in support for a simple form of literate programming, but this support is not widely used."
+2816,Elucidative Programming is the result of practical applications of Literate Programming in real programming contexts. The Elucidative paradigm proposes that source code and documentation be stored separately.
+2817,"Often, software developers need to be able to create and access information that is not going to be part of the source file itself. Such annotations are usually part of several software development activities, such as code walks and porting, where third-party source code is analysed in a functional way. Annotations can therefore help the developer during any stage of software development where a formal documentation system would hinder progress."
+2818,"Unlike code documents, user documents simply describe how a program is used."
+2819,"In the case of a software library, the code documents and user documents could in some cases be effectively equivalent and worth conjoining, but for a general application this is not often true."
+2820,"Typically, the user documentation describes each feature of the program, and assists the user in realizing these features. It is very important for user documents to not be confusing, and for them to be up to date.  User documents do not need to be organized in any particular way, but it is very important for them to have a thorough index. Consistency and simplicity are also very valuable. User documentation is considered to constitute a contract specifying what the software will do. API Writers are very well accomplished towards writing good user documents as they would be well aware of the software architecture and programming techniques used. See also technical writing."
+2821,"User documentation can be produced in a variety of online and print formats. However, there are three broad ways in which user documentation can be organized."
+2822,"Tutorial: A tutorial approach is considered the most useful for a new user, in which they are guided through each step of accomplishing particular tasks."
+2823,"Thematic: A thematic approach, where chapters or sections concentrate on one particular area of interest, is of more general use to an intermediate user.  Some authors prefer to convey their ideas through a knowledge based article to facilitate the user needs.  This approach is usually practiced by a dynamic industry, such as Information technology."
+2824,"List or Reference: The final type of organizing principle is one in which commands or tasks are simply listed alphabetically or logically grouped, often via cross-referenced indexes. This latter approach is of greater use to advanced users who know exactly what sort of information they are looking for."
+2825,"A common complaint among users regarding software documentation is that only one of these three approaches was taken to the near-exclusion of the other two. It is common to limit provided software documentation for personal computers to online help that give only reference information on commands or menu items.  The job of tutoring new users or helping more experienced users get the most out of a program is left to private publishers, who are often given significant assistance by the software developer."
+2826,"Like other forms of technical documentation, good user documentation benefits from an organized process of development. In the case of user documentation, the process as it commonly occurs in industry consists of five steps:"
+2827,"User analysis, the basic research phase of the process."
+2828,"Planning, or the actual documentation phase."
+2829,"Draft review, a self-explanatory phase where feedback is sought on the draft composed in the previous step."
+2830,"Usability testing, whereby the usability of the document is tested empirically."
+2831,"Editing, the final step in which the information collected in steps three and four is used to produce the final draft."
+2832,For many applications it is necessary to have some promotional materials to encourage casual observers to spend more time learning about the product.  This form of documentation has three purposes:
+2833,To excite the potential user about the product and instill in them a desire for becoming more involved with it.
+2834,"To inform them about what exactly the product does, so that their expectations are in line with what they will be receiving."
+2835,To explain the position of this product with respect to other alternatives.
+2836,"""The resistance to documentation among developers is well known and needs no emphasis."" 
+This situation is particularly prevalent in agile software development because these methodologies try to avoid any unnecessary activities that do not directly bring value.
+Specifically, the Agile Manifesto advocates valuing ""working software over comprehensive documentation"", which could be interpreted cynically as ""We want to spend all our time coding. Remember, real programmers don't write documentation."""
+2837,"A survey among software engineering experts revealed, however, that documentation is by no means considered unnecessary in agile development.
+Yet it is acknowledged that there are motivational problems in development, and that documentation methods tailored to agile development  may be needed."
+2838,"Data exists in three states: data at rest, data in transit and data in use. Data within a computer, in most cases, moves as parallel data. Data moving to or from a computer, in most cases, moves as serial data. Data sourced from an analog device, such as a temperature sensor, may be converted to digital using an analog-to-digital converter. Data representing quantities, characters, or symbols on which operations are performed by a computer are stored and recorded on magnetic, optical, electronic, or mechanical recording media, and transmitted in the form of digital electrical or optical signals. Data pass in and out of computers via peripheral devices."
+2839,"Physical computer memory elements consist of an address and a byte/word of data storage. Digital data are often stored in relational databases, like tables or SQL databases, and can generally be represented as abstract key/value pairs. Data can be organized in many different types of data structures, including arrays, graphs, and objects. Data structures can store data of many different types, including numbers, strings and even other data structures."
+2840,"Metadata helps translate data to information. Metadata is data about the data. Metadata may be implied, specified or given."
+2841,"Data relating to physical events or processes will have a temporal component. This temporal component may be implied. This is the case when a device such as a temperature logger receives data from a temperature sensor. When the temperature is received it is assumed that the data has a temporal reference of now. So the device records the date, time and temperature together. When the data logger communicates temperatures, it must also report the date and time as metadata for each temperature reading."
+2842,"Fundamentally, computers follow a sequence of instructions they are given in the form of data. A set of instructions to perform a given task  is called a program. A program is data in the form of coded instructions to control the operation of a computer or other machine. In the nominal case, the program, as executed by the computer, will consist of machine code. The elements of storage manipulated by the program, but not actually executed by the central processing unit , are also data. At its most essential, a single datum is a value stored at a specific location. Therefore, it is possible for computer programs to operate on other computer programs, by manipulating their programmatic data."
+2843,"To store data bytes in a file, they have to be serialized in a file format. Typically, programs are stored in special file types, different from those used for other data. Executable files contain programs; all other files are also data files. However, executable files may also contain data used by the program which is built into the program. In particular, some executable files have a data segment, which nominally contains constants and initial values for variables, both of which can be considered data."
+2844,"The line between program and data can become blurry. An interpreter, for example, is a program. The input data to an interpreter is itself a program, just not one expressed in native machine language. In many cases, the interpreted program will be a human-readable text file, which is manipulated with a text editor program. Metaprogramming similarly involves programs manipulating other programs as data. Programs like compilers, linkers, debuggers, program updaters, virus scanners and such use other programs as their data."
+2845,"For example, a user might first instruct the operating system to load a word processor program from one file, and then use the running program to open and edit a document stored in another file. In this example, the document would be considered data. If the word processor also features a spell checker, then the dictionary  for the spell checker would also be considered data. The algorithms used by the spell checker to suggest corrections would be either machine code data or text in some interpretable programming language."
+2846,"In an alternate usage, binary files  are sometimes called data as distinguished from human-readable text."
+2847,The total amount of digital data in 2007 was estimated to be 281 billion gigabytes .
+2848,"Keys in data provide the context for values. Regardless of the structure of data, there is always a key component present. Keys in data and data-structures are essential for giving meaning to data values. Without a key that is directly or indirectly associated with a value, or collection of values in a structure, the values become meaningless and cease to be data. That is to say, there has to be a key component linked to a value component in order for it to be considered data."
+2849,"Data can be represented in computers in multiple ways, as per the following examples:"
+2850,"The taxonomic rank-structure of classes, which is an example of a hierarchical data structure; and"
+2851,"at run time, the creation of references to in-memory data-structures of objects that have been instantiated from a class library."
+2852,"It is only after instantiation that an object of a specified class exists. After an object's reference is cleared, the object also ceases to exist. The memory locations where the object's data was stored are garbage and are reclassified as unused memory available for reuse."
+2853,"In computer science, a library is a collection of read-only resources that is leveraged during software development to implement a computer program."
+2854,"Historically, a library consisted of subroutines . The concept now includes other forms of executable code including classes and non-executable data including images and text. It can also refer to a collection of source code."
+2855,"For example, a program could use a library to indirectly make system calls instead of making those system calls directly in the program."
+2856,"A library can be used by multiple, independent consumers . This differs from resources defined in a program which can usually only be used by that program."
+2857,"When a consumer uses a library resource, it gains the value of the library without having to implement it itself. Libraries encourage code reuse in a modular fashion."
+2858,"When writing code that uses a library, a programmer only needs to know high-level information such as what items it contains at and how to use the items – not all of the internal details of the library."
+2859,Libraries can use other libraries resulting in a hierarchy of libraries in a program.
+2860,"A library of executable code has a well-defined interface by which the functionality is invoked. 
+For example, in C, a library function is invoked via C's normal function call capability. The linker generates code to call a function via the library mechanism if the function is available from a library instead of from the program itself."
+2861,"The functions of a library can be connected to the invoking program at different program lifecycle phases. If the code of the library is accessed during the build of the invoking program, then the library is called a static library. An alternative is to build the program executable to be separate from the library file. The library functions are connected after the executable is started, either at load-time or runtime. In this case, the library is called a dynamic library."
+2862,"Most compiled languages have a standard library, although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement the majority of the system services. Such libraries have organized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries."
+2863,"The idea of a computer library dates back to the first computers created by Charles Babbage. An 1888 paper on his Analytical Engine suggested that computer operations could be punched on separate cards from numerical input. If these operation punch cards were saved for reuse then ""by degrees the engine would have a library of its own."""
+2864,"In 1947 Goldstine and von Neumann speculated that it would be useful to create a ""library"" of subroutines for their work on the IAS machine, an early computer that was not yet operational at that time. They envisioned a physical library of magnetic wire recordings, with each wire storing reusable computer code."
+2865,"Inspired by von Neumann, Wilkes and his team constructed EDSAC. A filing cabinet of punched tape held the subroutine library for this computer. Programs for EDSAC consisted of a main program and a sequence of subroutines copied from the subroutine library. In 1951 the team published the first textbook on programming, The Preparation of Programs for an Electronic Digital Computer, which detailed the creation and the purpose of the library."
+2866,"COBOL included ""primitive capabilities for a library system"" in 1959, but Jean Sammet described them as ""inadequate library facilities"" in retrospect."
+2867,"JOVIAL has a Communication Pool , roughly a library of header files."
+2868,"Another major contributor to the modern library concept came in the form of the subprogram innovation of FORTRAN. FORTRAN subprograms can be compiled independently of each other, but the compiler lacked a linker. So prior to the introduction of modules in Fortran-90, type checking between FORTRAN subprograms was impossible."
+2869,"By the mid 1960s, copy and macro libraries for assemblers were common. Starting with the popularity of the IBM System/360, libraries containing other types of text elements, e.g., system parameters, also became common."
+2870,In IBM's OS/360 and its successors this is called a partitioned data set.
+2871,"The first object-oriented programming language, Simula, developed in 1965, supported adding classes to libraries via its compiler.
+
+"
+2872,"Libraries are important in the program linking or binding process, which resolves references known as links or symbols to library modules. The linking process is usually automatically done by a linker or binder program that searches a set of libraries and other modules in a given order. Usually it is not considered an error if a link target can be found multiple times in a given set of libraries. Linking may be done when an executable file is created , or whenever the program is used at runtime ."
+2873,"The references being resolved may be addresses for jumps and other routine calls. They may be in the main program, or in one module depending upon another. They are resolved into fixed or relocatable addresses  by allocating runtime memory for the memory segments of each module referenced."
+2874,"Some programming languages use a feature called smart linking whereby the linker is aware of or integrated with the compiler, such that the linker knows how external references are used, and code in a library that is never actually used, even though internally referenced, can be discarded from the compiled application. For example, a program that only uses integers for arithmetic, or does no arithmetic operations at all, can exclude floating-point library routines. This smart-linking feature can lead to smaller application file sizes and reduced memory usage."
+2875,"Some references in a program or library module are stored in a relative or symbolic form which cannot be resolved until all code and libraries are assigned final static addresses. Relocation is the process of adjusting these references, and is done either by the linker or the loader. In general, relocation cannot be done to individual libraries themselves because the addresses in memory may vary depending on the program using them and other libraries they are combined with. Position-independent code avoids references to absolute addresses and therefore does not require relocation."
+2876,"When linking is performed during the creation of an executable or another object file, it is known as static linking or early binding. In this case, the linking is usually done by a linker, but may also be done by the compiler. A static library, also known as an archive, is one intended to be statically linked. Originally, only static libraries existed. Static linking must be performed when any modules are recompiled."
+2877,"All of the modules required by a program are sometimes statically linked and copied into the executable file. This process, and the resulting stand-alone file, is known as a static build of the program. A static build may not need any further relocation if virtual memory is used and no address space layout randomization is desired."
+2878,"A shared library or shared object is a file that is intended to be shared by executable files and further shared object files. Modules used by a program are loaded from individual shared objects into memory at load time or runtime, rather than being copied by a linker when it creates a single monolithic executable file for the program."
+2879,"Shared libraries can be statically linked during compile-time, meaning that references to the library modules are resolved and the modules are allocated memory when the executable file is created. But often linking of shared libraries is postponed until they are loaded."
+2880,"Although originally pioneered in the 1960s, dynamic linking did not reach the most commonly-used operating systems until the late 1980s. It was generally available in some form in most operating systems by the early 1990s. During this same period, object-oriented programming  was becoming a significant part of the programming landscape. OOP with runtime binding requires additional information that traditional libraries do not supply. In addition to the names and entry points of the code located within, they also require a list of the objects they depend on. This is a side-effect of one of OOP's core concepts, inheritance, which means that parts of the complete definition of any method may be in different places. This is more than simply listing that one library requires the services of another: in a true OOP system, the libraries themselves may not be known at compile time, and vary from system to system."
+2881,"At the same time many developers worked on the idea of multi-tier programs, in which a ""display"" running on a desktop computer would use the services of a mainframe or minicomputer for data storage or processing. For instance, a program on a GUI-based computer would send messages to a minicomputer to return small samples of a huge dataset for display. Remote procedure calls  already handled these tasks, but there was no standard RPC system."
+2882,"Soon the majority of the minicomputer and mainframe vendors instigated projects to combine the two, producing an OOP library format that could be used anywhere. Such systems were known as object libraries, or distributed objects, if they supported remote access . Microsoft's COM is an example of such a system for local use. DCOM, a modified version of COM, supports remote access."
+2883,"For some time object libraries held the status of the ""next big thing"" in the programming world. There were a number of efforts to create systems that would run across platforms, and companies competed to try to get developers locked into their own system. Examples include IBM's System Object Model , Sun Microsystems' Distributed Objects Everywhere , NeXT's Portable Distributed Objects , Digital's ObjectBroker, Microsoft's Component Object Model , and any number of CORBA-based systems."
+2884,"Class libraries are the rough OOP equivalent of older types of code libraries. They contain classes, which describe characteristics and define actions  that involve objects. Class libraries are used to create instances, or objects with their characteristics set to specific values. In some OOP languages, like Java, the distinction is clear, with the classes often contained in library files  and the instantiated objects residing only in memory . In others, like Smalltalk, the class libraries are merely the starting point for a system image that includes the entire state of the environment, classes and all instantiated objects."
+2885,Today most class libraries are stored in a package repository . Client code explicitly declare the dependencies to external libraries in build configuration files .
+2886,"Another library technique uses completely separate executables  and calls them using a remote procedure call  over a network to another computer. This maximizes operating system re-use: the code needed to support the library is the same code being used to provide application support and security for every other program. Additionally, such systems do not require the library to exist on the same machine, but can forward the requests over the network."
+2887,"However, such an approach means that every library call requires a considerable amount of overhead. RPC calls are much more expensive than calling a shared library that has already been loaded on the same machine. This approach is commonly used in a distributed architecture that makes heavy use of such remote calls, notably client-server systems and application servers such as Enterprise JavaBeans."
+2888,"Code generation libraries are high-level APIs that can generate or transform byte code for Java. They are used by aspect-oriented programming, some data access frameworks, and for testing to generate dynamic proxy objects. They also are used to intercept field access."
+2889,"The system stores libfoo.a and libfoo.so files in directories such as /lib, /usr/lib or /usr/local/lib. The filenames always start with lib, and end with a suffix of .a  or of .so . Some systems might have multiple names for a dynamically linked library. These names typically share the same prefix and have different suffixes indicating the version number. Most of the names are names for symbolic links to the latest version. For example, on some systems libfoo.so.2 would be the filename for the second major interface revision of the dynamically linked library libfoo. The .la files sometimes found in the library directories are libtool archives, not usable by the system as such."
+2890,"The system inherits static library conventions from BSD, with the library stored in a .a file, and can use .so-style dynamically linked libraries . Most libraries in macOS, however, consist of ""frameworks"", placed inside special directories called ""bundles"" which wrap the library's required files and metadata. For example, a framework called MyFramework would be implemented in a bundle called MyFramework.framework, with MyFramework.framework/MyFramework being either the dynamically linked library file or being a symlink to the dynamically linked library file in MyFramework.framework/Versions/Current/MyFramework."
+2891,"Dynamic-link libraries usually have the suffix *.DLL, although other file name extensions may identify specific-purpose dynamically linked libraries, e.g. *.OCX for OLE libraries. The interface revisions are either encoded in the file names, or abstracted away using COM-object interfaces. Depending on how they are compiled, *.LIB files can be either static libraries or representations of dynamically linkable libraries needed only during compilation, known as ""import libraries"". Unlike in the UNIX world, which uses different file extensions, when linking against .LIB file in Windows one must first know if it is a regular static library or an import library. In the latter case, a .DLL file must be present at runtime."
+2892,"A computer-aided design library or CAD library is a cloud based repository of 3D models or parts for computer-aided design , computer-aided engineering , computer-aided manufacturing , or Building information modeling . Examples of CAD libraries are GrabCAD, Sketchup 3D Warehouse, Sketchfab, McMaster-Carr, TurboSquid, Chaos Cosmos, and Thingiverse.  The models can be free and open source or proprietary and have to pay a subscription to have access to the CAD library 3D models.  Generative Ai CAD libraries are being developed using linked open data of schematics  and diagrams."
+2893,"CAD libraries can have assets such as 3D models, materials/textures, bump maps, trees/plants, HDRIs, and different Computer graphics lighting sources to be rendered."
+2894,"Thus, these are different from distributed computing problems that need communication between tasks, especially communication of intermediate results. They are easy to perform on server farms which lack the special infrastructure used in a true supercomputer cluster. They are thus well suited to large, Internet-based volunteer computing platforms such as BOINC, and do not suffer from parallel slowdown. The opposite of embarrassingly parallel problems are inherently serial problems, which cannot be parallelized at all."
+2895,"A common example of an embarrassingly parallel problem is 3D video rendering handled by a graphics processing unit, where each frame  or pixel  can be handled with no interdependency. Some forms of password cracking are another embarrassingly parallel task that is easily distributed on central processing units, CPU cores, or clusters."
+2896,"""Embarrassingly"" is used here to refer to parallelization problems which are ""embarrassingly easy"". The term may imply embarrassment on the part of developers or compilers: ""Because so many important problems remain unsolved mainly due to their intrinsic computational complexity, it would be embarrassing not to develop parallel implementations of polynomial homotopy continuation methods."" The term is first found in the literature in a 1986 book on multiprocessors by MATLAB's creator Cleve Moler, who claims to have invented the term."
+2897,"An alternative term, pleasingly parallel, has gained some use, perhaps to avoid the negative connotations of embarrassment in favor of a positive reflection on the parallelizability of the problems: ""Of course, there is nothing embarrassing about these programs at all."""
+2898,Some examples of embarrassingly parallel problems include:
+2899,"Cryptography, or cryptology , is the practice and study of techniques for secure communication in the presence of adversarial behavior. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, information security, electrical engineering, digital signal processing, physics, and others. Core concepts related to  information security  are also central to cryptography. Practical applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications."
+2900,"Cryptography prior to the modern age was effectively synonymous with encryption, converting readable information  to unintelligible nonsense text , which can only be read by reversing the process . The sender of an encrypted  message shares the decryption  technique only with the intended recipients to preclude access from adversaries. The cryptography literature often uses the names ""Alice""  for the sender, ""Bob""  for the intended recipient, and ""Eve""  for the eavesdropping adversary. Since the development of rotor cipher machines in World War I and the advent of computers in World War II, cryptography methods have become increasingly complex and their applications more varied."
+2901,"Modern cryptography is heavily based on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in actual practice by any adversary. While it is theoretically possible to break into a well-designed system, it is infeasible in actual practice to do so. Such schemes, if well designed, are therefore termed ""computationally secure"". Theoretical advances  and faster computing technology require these designs to be continually reevaluated and, if necessary, adapted. Information-theoretically secure schemes that provably cannot be broken even with unlimited computing power, such as the one-time pad, are much more difficult to use in practice than the best theoretically breakable but computationally secure schemes."
+2902,"The growth of cryptographic technology has raised a number of legal issues in the Information Age. Cryptography's potential for use as a tool for espionage and sedition has led many governments to classify it as a weapon and to limit or even prohibit its use and export. In some jurisdictions where the use of cryptography is legal, laws permit investigators to compel the disclosure of encryption keys for documents relevant to an investigation. Cryptography also plays a major role in digital rights management and copyright infringement disputes with regard to digital media."
+2903,"The first use of the term ""cryptograph""  dates back to the 19th century—originating from ""The Gold-Bug"", a story by Edgar Allan Poe."
+2904,"Until modern times, cryptography referred almost exclusively to ""encryption"", which is the process of converting ordinary information  into an unintelligible form . Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher  is a pair of algorithms that carry out the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a ""key"". The key is a secret , usually a string of characters , which is needed to decrypt the ciphertext.  In formal mathematical terms, a ""cryptosystem"" is the ordered list of elements of finite possible plaintexts, finite possible cyphertexts, finite possible keys, and the encryption and decryption algorithms that correspond to each key.  Keys are important both formally and in actual practice, as ciphers without variable keys can be trivially broken with only the knowledge of the cipher used and are therefore useless  for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks."
+2905,"There are two main types of cryptosystems: symmetric and asymmetric. In symmetric systems, the only ones known until the 1970s, the same secret key encrypts and decrypts a message. Data manipulation in symmetric systems is significantly faster than in asymmetric systems. Asymmetric systems use a ""public key"" to encrypt a message and a related ""private key"" to decrypt it. The advantage of asymmetric systems is that the public key can be freely published, allowing parties to establish secure communication without having a shared secret key.  In practice, asymmetric systems are used to first exchange a secret key, and then secure communication proceeds via a more efficient symmetric system using that key. Examples of asymmetric systems include Diffie–Hellman key exchange, RSA , ECC , and Post-quantum cryptography. Secure symmetric algorithms include the commonly used AES  which replaced the older DES .  Insecure symmetric algorithms include children's language tangling schemes such as Pig Latin or other cant, and all historical cryptographic schemes, however seriously intended, prior to the invention of the one-time pad early in the 20th century."
+2906,"In colloquial use, the term ""code"" is often used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning: the replacement of a unit of plaintext  with a code word .  A cypher, in contrast, is a scheme for changing or substituting an element below such a level  in order to produce a cyphertext."
+2907,"Cryptanalysis is the term used for the study of methods for obtaining the meaning of encrypted information without access to the key normally required to do so; i.e., it is the study of how to ""crack"" encryption algorithms or their implementations."
+2908,"Some use the terms ""cryptography"" and ""cryptology"" interchangeably in English, while others  use ""cryptography"" to refer specifically to the use and practice of cryptographic techniques and ""cryptology"" to refer to the combined study of cryptography and cryptanalysis. English is more flexible than several other languages in which ""cryptology""  is always used in the second sense above. RFC 2828 advises that steganography is sometimes included in cryptology."
+2909,The study of characteristics of languages that have some application in cryptography or cryptology  is called cryptolinguistics. Cryptolingusitics is especially used in military intelligence applications for deciphering foreign communications.
+2910,"Before the modern era, cryptography focused on message confidentiality —conversion of messages from a comprehensible form into an incomprehensible one and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge . Encryption attempted to ensure secrecy in communications, such as those of spies, military leaders, and diplomats. In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs and secure computation, among others."
+2911,"The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message , and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters . Simple versions of either have never offered much confidentiality from enterprising opponents. An early substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. Suetonius reports that Julius Caesar used it with a shift of three to communicate with his generals. Atbash is an example of an early Hebrew cipher. The earliest known use of cryptography is some carved ciphertext on stone in Egypt , but this may have been done for the amusement of literate observers rather than as a way of concealing information."
+2912,"The Greeks of Classical times are said to have known of ciphers . Steganography  was also first developed in ancient times. An early example, from Herodotus, was a message tattooed on a slave's shaved head and concealed under the regrown hair. Other steganography methods involve 'hiding in plain sight,' such as using a music cipher to disguise an encrypted message within a regular piece of sheet music. More modern examples of steganography include the use of invisible ink, microdots, and digital watermarks to conceal information."
+2913,"In India, the 2000-year-old Kamasutra of Vātsyāyana speaks of two different kinds of ciphers called Kautiliyam and Mulavediya. In the Kautiliyam, the cipher letter substitutions are based on phonetic relations, such as vowels becoming consonants. In the Mulavediya, the cipher alphabet consists of pairing letters and using the reciprocal ones."
+2914,"In Sassanid Persia, there were two secret scripts, according to the Muslim author Ibn al-Nadim: the šāh-dabīrīya  which was used for official correspondence, and the rāz-saharīya which was used to communicate secret messages with other countries."
+2915,"David Kahn notes in The Codebreakers that modern cryptology originated among the Arabs, the first people to systematically document cryptanalytic methods. Al-Khalil  wrote the Book of Cryptographic Messages, which contains the first use of permutations and combinations to list all possible Arabic words with and without vowels."
+2916,"Ciphertexts produced by a classical cipher  will reveal statistical information about the plaintext, and that information can often be used to break the cipher. After the discovery of frequency analysis, perhaps by the Arab mathematician and polymath Al-Kindi  in the 9th century, nearly all such ciphers could be broken by an informed attacker. Such classical ciphers still enjoy popularity today, though mostly as puzzles . Al-Kindi wrote a book on cryptography entitled Risalah fi Istikhraj al-Mu'amma , which described the first known use of frequency analysis cryptanalysis techniques."
+2917,"Language letter frequencies may offer little help for some extended historical encryption techniques such as homophonic cipher that tend to flatten the frequency distribution. For those ciphers, language letter group  frequencies may provide an attack."
+2918,"Essentially all ciphers remained vulnerable to cryptanalysis using the frequency analysis technique until the development of the polyalphabetic cipher, most clearly by Leon Battista Alberti around the year 1467, though there is some indication that it was already known to Al-Kindi. Alberti's innovation was to use different ciphers  for various parts of a message . He also invented what was probably the first automatic cipher device, a wheel that implemented a partial realization of his invention. In the Vigenère cipher, a polyalphabetic cipher, encryption uses a key word, which controls letter substitution depending on which letter of the key word is used. In the mid-19th century Charles Babbage showed that the Vigenère cipher was vulnerable to Kasiski examination, but this was first published about ten years later by Friedrich Kasiski."
+2919,"Although frequency analysis can be a powerful and general technique against many ciphers, encryption has still often been effective in practice, as many a would-be cryptanalyst was unaware of the technique. Breaking a message without using frequency analysis essentially required knowledge of the cipher used and perhaps of the key involved, thus making espionage, bribery, burglary, defection, etc., more attractive approaches to the cryptanalytically uninformed. It was finally explicitly recognized in the 19th century that secrecy of a cipher's algorithm is not a sensible nor practical safeguard of message security; in fact, it was further realized that any adequate cryptographic scheme  should remain secure even if the adversary fully understands the cipher algorithm itself. Security of the key used should alone be sufficient for a good cipher to maintain confidentiality under an attack. This fundamental principle was first explicitly stated in 1883 by Auguste Kerckhoffs and is generally called Kerckhoffs's Principle; alternatively and more bluntly, it was restated by Claude Shannon, the inventor of information theory and the fundamentals of theoretical cryptography, as Shannon's Maxim—'the enemy knows the system'."
+2920,"Different physical devices and aids have been used to assist with ciphers. One of the earliest may have been the scytale of ancient Greece, a rod supposedly used by the Spartans as an aid for a transposition cipher. In medieval times, other aids were invented such as the cipher grille, which was also used for a kind of steganography. With the invention of polyalphabetic ciphers came more sophisticated aids such as Alberti's own cipher disk, Johannes Trithemius' tabula recta scheme, and Thomas Jefferson's wheel cypher . Many mechanical encryption/decryption devices were invented early in the 20th century, and several patented, among them rotor machines—famously including the Enigma machine used by the German government and military from the late 1920s and during World War II. The ciphers implemented by better quality examples of these machine designs brought about a substantial increase in cryptanalytic difficulty after WWI."
+2921,"Cryptanalysis of the new mechanical ciphering devices proved to be both difficult and laborious. In the United Kingdom, cryptanalytic efforts at Bletchley Park during WWII spurred the development of more efficient means for carrying out repetitive tasks, such as military code breaking . This culminated in the development of the Colossus, the world's first fully electronic, digital, programmable computer, which assisted in the decryption of ciphers generated by the German Army's Lorenz SZ40/42 machine."
+2922,"Extensive open academic research into cryptography is relatively recent, beginning in the mid-1970s. In the early 1970s IBM personnel designed the Data Encryption Standard  algorithm that became the first federal government cryptography standard in the United States. In 1976 Whitfield Diffie and Martin Hellman published the Diffie–Hellman key exchange algorithm. In 1977 the RSA algorithm was published in Martin Gardner's Scientific American column. Since then, cryptography has become a widely used tool in communications, computer networks, and computer security generally."
+2923,"Some modern cryptographic techniques can only keep their keys secret if certain mathematical problems are intractable, such as the integer factorization or the discrete logarithm problems, so there are deep connections with abstract mathematics. There are very few cryptosystems that are proven to be unconditionally secure. The one-time pad is one, and was proven to be so by Claude Shannon. There are a few important algorithms that have been proven secure under certain assumptions. For example, the infeasibility of factoring extremely large integers is the basis for believing that RSA is secure, and some other systems, but even so, proof of unbreakability is unavailable since the underlying mathematical problem remains open. In practice, these are widely used, and are believed unbreakable in practice by most competent observers.  There are systems similar to RSA, such as one by Michael O. Rabin that are provably secure provided factoring  n = pq is impossible;  it is quite unusable in practice. The discrete logarithm problem is the basis for believing some other cryptosystems are secure, and again, there are related, less practical systems that are provably secure relative to the solvability or insolvability discrete log problem."
+2924,"As well as being aware of cryptographic history, cryptographic algorithm and system designers must also sensibly consider probable future developments while working on their designs. For instance, continuous improvements in computer processing power have increased the scope of brute-force attacks, so when specifying key lengths, the required key lengths are similarly advancing. The potential impact of quantum computing are already being considered by some cryptographic system designers developing post-quantum cryptography. The announced imminence of small implementations of these machines may be making the need for preemptive caution rather more than merely speculative."
+2925,"Prior to the early 20th century, cryptography was mainly concerned with linguistic and lexicographic patterns. Since then cryptography has broadened in scope, and now makes extensive use of mathematical subdisciplines, including information theory, computational complexity, statistics, combinatorics, abstract algebra, number theory, and finite mathematics. Cryptography is also a branch of engineering, but an unusual one since it deals with active, intelligent, and malevolent opposition; other kinds of engineering  need deal only with neutral natural forces. There is also active research examining the relationship between cryptographic problems and quantum physics."
+2926,"Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data representable in any binary format, unlike classical ciphers which only encrypted written language texts; this was new and significant. Computer use has thus supplanted linguistic cryptography, both for cipher design and cryptanalysis. Many computer ciphers can be characterized by their operation on binary bit sequences , unlike classical and mechanical schemes, which generally manipulate traditional characters  directly. However, computers have also assisted cryptanalysis, which has compensated to some extent for increased cipher complexity. Nonetheless, good modern ciphers have stayed ahead of cryptanalysis; it is typically the case that use of a quality cipher is very efficient , while breaking it requires an effort many orders of magnitude larger, and vastly larger than that required for any classical cipher, making cryptanalysis so inefficient and impractical as to be effectively impossible."
+2927,Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key . This was the only kind of encryption publicly known until June 1976.
+2928,"Symmetric key ciphers are implemented as either block ciphers or stream ciphers. A block cipher enciphers input in blocks of plaintext as opposed to individual characters, the input form used by a stream cipher."
+2929,"The Data Encryption Standard  and the Advanced Encryption Standard  are block cipher designs that have been designated cryptography standards by the US government . Despite its deprecation as an official standard, DES  remains quite popular; it is used across a wide range of applications, from ATM encryption to e-mail privacy and secure remote access. Many other block ciphers have been designed and released, with considerable variation in quality. Many, even some designed by capable practitioners, have been thoroughly broken, such as FEAL."
+2930,"Stream ciphers, in contrast to the 'block' type, create an arbitrarily long stream of key material, which is combined with the plaintext bit-by-bit or character-by-character, somewhat like the one-time pad. In a stream cipher, the output stream is created based on a hidden internal state that changes as the cipher operates. That internal state is initially set up using the secret key material. RC4 is a widely used stream cipher. Block ciphers can be used as stream ciphers by generating blocks of a keystream  and applying an XOR operation to each bit of the plaintext with each bit of the keystream."
+2931,"Message authentication codes  are much like cryptographic hash functions, except that a secret key can be used to authenticate the hash value upon receipt; this additional complication blocks an attack scheme against bare digest algorithms, and so has been thought worth the effort. Cryptographic hash functions are a third type of cryptographic algorithm. They take a message of any length as input, and output a short, fixed-length hash, which can be used in  a digital signature. For good hash functions, an attacker cannot find two messages that produce the same hash. MD4 is a long-used hash function that is now broken; MD5, a strengthened variant of MD4, is also widely used but broken in practice. The US National Security Agency developed the Secure Hash Algorithm series of MD5-like hash functions: SHA-0 was a flawed algorithm that the agency withdrew; SHA-1 is widely deployed and more secure than MD5, but cryptanalysts have identified attacks against it; the SHA-2 family improves on SHA-1, but is vulnerable to clashes as of 2011; and the US standards authority thought it ""prudent"" from a security perspective to develop a new standard to ""significantly improve the robustness of NIST's overall hash algorithm toolkit."" Thus, a hash function design competition was meant to select a new U.S. national standard, to be called SHA-3, by 2012. The competition ended on October 2, 2012, when the NIST announced that Keccak would be the new SHA-3 hash algorithm. Unlike block and stream ciphers that are invertible, cryptographic hash functions produce a hashed output that cannot be used to retrieve the original input data. Cryptographic hash functions are used to verify the authenticity of data retrieved from an untrusted source or to add a layer of security."
+2932,"Symmetric-key cryptosystems use the same key for encryption and decryption of a message, although a message or group of messages can have a different key than others. A significant disadvantage of symmetric ciphers is the key management necessary to use them securely. Each distinct pair of communicating parties must, ideally, share a different key, and perhaps for each ciphertext exchanged as well. The number of keys required increases as the square of the number of network members, which very quickly requires complex key management schemes to keep them all consistent and secret."
+2933,"In a groundbreaking 1976 paper, Whitfield Diffie and Martin Hellman proposed the notion of public-key  cryptography in which two different but mathematically related keys are used—a public key and a private key. A public key system is so constructed that calculation of one key  is computationally infeasible from the other , even though they are necessarily related. Instead, both keys are generated secretly, as an interrelated pair. The historian David Kahn described public-key cryptography as ""the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance""."
+2934,"In public-key cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. In a public-key encryption system, the public key is used for encryption, while the private or secret key is used for decryption. While Diffie and Hellman could not find such a system, they showed that public-key cryptography was indeed possible by presenting the Diffie–Hellman key exchange protocol, a solution that is now widely used in secure communications to allow two parties to secretly agree on a shared encryption key.
+The X.509 standard defines the most commonly used format for public key certificates."
+2935,"Diffie and Hellman's publication sparked widespread academic efforts in finding a practical public-key encryption system. This race was finally won in 1978 by Ronald Rivest, Adi Shamir, and Len Adleman, whose solution has since become known as the RSA algorithm."
+2936,"The Diffie–Hellman and RSA algorithms, in addition to being the first publicly known examples of high-quality public-key algorithms, have been among the most widely used. Other asymmetric-key algorithms include the Cramer–Shoup cryptosystem, ElGamal encryption, and various elliptic curve techniques."
+2937,"A document published in 1997 by the Government Communications Headquarters , a British intelligence organization, revealed that cryptographers at GCHQ had anticipated several academic developments. Reportedly, around 1970, James H. Ellis had conceived the principles of asymmetric key cryptography. In 1973, Clifford Cocks invented a solution that was very similar in design rationale to RSA. In 1974, Malcolm J. Williamson is claimed to have developed the Diffie–Hellman key exchange."
+2938,"Public-key cryptography is also used for implementing digital signature schemes. A digital signature is reminiscent of an ordinary signature; they both have the characteristic of being easy for a user to produce, but difficult for anyone else to forge. Digital signatures can also be permanently tied to the content of the message being signed; they cannot then be 'moved' from one document to another, for any attempt will be detectable. In digital signature schemes, there are two algorithms: one for signing, in which a secret key is used to process the message , and one for verification, in which the matching public key is used with the message to check the validity of the signature. RSA and DSA are two of the most popular digital signature schemes. Digital signatures are central to the operation of public key infrastructures and many network security schemes ."
+2939,"Public-key algorithms are most often based on the computational complexity of ""hard"" problems, often from number theory. For example, the hardness of RSA is related to the integer factorization problem, while Diffie–Hellman and DSA are related to the discrete logarithm problem. The security of elliptic curve cryptography is based on number theoretic problems involving elliptic curves. Because of the difficulty of the underlying problems, most public-key algorithms involve operations such as modular multiplication and exponentiation, which are much more computationally expensive than the techniques used in most block ciphers, especially with typical key sizes. As a result, public-key cryptosystems are commonly hybrid cryptosystems, in which a fast high-quality symmetric-key encryption algorithm is used for the message itself, while the relevant symmetric key is sent with the message, but encrypted using a public-key algorithm. Similarly, hybrid signature schemes are often used, in which a cryptographic hash function is computed, and only the resulting hash is digitally signed."
+2940,"Cryptographic hash functions are functions that take a variable-length input and return a fixed-length output, which can be used in, for example, a digital signature. For a hash function to be secure, it must be difficult to compute two inputs that hash to the same value  and to compute an input that hashes to a given output . MD4 is a long-used hash function that is now broken; MD5, a strengthened variant of MD4, is also widely used but broken in practice. The US National Security Agency developed the Secure Hash Algorithm series of MD5-like hash functions: SHA-0 was a flawed algorithm that the agency withdrew; SHA-1 is widely deployed and more secure than MD5, but cryptanalysts have identified attacks against it; the SHA-2 family improves on SHA-1, but is vulnerable to clashes as of 2011; and the US standards authority thought it ""prudent"" from a security perspective to develop a new standard to ""significantly improve the robustness of NIST's overall hash algorithm toolkit."" Thus, a hash function design competition was meant to select a new U.S. national standard, to be called SHA-3, by 2012. The competition ended on October 2, 2012, when the NIST announced that Keccak would be the new SHA-3 hash algorithm. Unlike block and stream ciphers that are invertible, cryptographic hash functions produce a hashed output that cannot be used to retrieve the original input data. Cryptographic hash functions are used to verify the authenticity of data retrieved from an untrusted source or to add a layer of security."
+2941,"The goal of cryptanalysis is to find some weakness or insecurity in a cryptographic scheme, thus permitting its subversion or evasion."
+2942,"It is a common misconception that every encryption method can be broken. In connection with his WWII work at Bell Labs, Claude Shannon proved that the one-time pad cipher is unbreakable, provided the key material is truly random, never reused, kept secret from all possible attackers, and of equal or greater length than the message. Most ciphers, apart from the one-time pad, can be broken with enough computational effort by brute force attack, but the amount of effort needed may be exponentially dependent on the key size, as compared to the effort needed to make use of the cipher. In such cases, effective security could be achieved if it is proven that the effort required  is beyond the ability of any adversary. This means it must be shown that no efficient method  can be found to break the cipher. Since no such proof has been found to date, the one-time-pad remains the only theoretically unbreakable cipher. Although well-implemented one-time-pad encryption cannot be broken, traffic analysis is still possible."
+2943,"There are a wide variety of cryptanalytic attacks, and they can be classified in any of several ways. A common distinction turns on what Eve  knows and what capabilities are available. In a ciphertext-only attack, Eve has access only to the ciphertext . In a known-plaintext attack, Eve has access to a ciphertext and its corresponding plaintext . In a chosen-plaintext attack, Eve may choose a plaintext and learn its corresponding ciphertext ; an example is gardening, used by the British during WWII. In a chosen-ciphertext attack, Eve may be able to choose ciphertexts and learn their corresponding plaintexts. Finally in a man-in-the-middle attack Eve gets in between Alice  and Bob , accesses and modifies the traffic and then forwards it to the recipient. Also important, often overwhelmingly so, are mistakes ."
+2944,"Cryptanalysis of symmetric-key ciphers typically involves looking for attacks against the block ciphers or stream ciphers that are more efficient than any attack that could be against a perfect cipher. For example, a simple brute force attack against DES requires one known plaintext and 255 decryptions, trying approximately half of the possible keys, to reach a point at which chances are better than even that the key sought will have been found. But this may not be enough assurance; a linear cryptanalysis attack against DES requires 243 known plaintexts  and approximately 243 DES operations. This is a considerable improvement over brute force attacks."
+2945,"Public-key algorithms are based on the computational difficulty of various problems. The most famous of these are the difficulty of integer factorization of semiprimes and the difficulty of calculating discrete logarithms, both of which are not yet proven to be solvable in polynomial time  using only a classical Turing-complete computer. Much public-key cryptanalysis concerns designing algorithms in P that can solve these problems, or using other technologies, such as quantum computers. For instance, the best-known algorithms for solving the elliptic curve-based version of discrete logarithm are much more time-consuming than the best-known algorithms for factoring, at least for problems of more or less equivalent size. Thus, to achieve an equivalent strength of encryption, techniques that depend upon the difficulty of factoring large composite numbers, such as the RSA cryptosystem, require larger keys than elliptic curve techniques. For this reason, public-key cryptosystems based on elliptic curves have become popular since their invention in the mid-1990s."
+2946,"While pure cryptanalysis uses weaknesses in the algorithms themselves, other attacks on cryptosystems are based on actual use of the algorithms in real devices, and are called side-channel attacks. If a cryptanalyst has access to, for example, the amount of time the device took to encrypt a number of plaintexts or report an error in a password or PIN character, they may be able to use a timing attack to break a cipher that is otherwise resistant to analysis. An attacker might also study the pattern and length of messages to derive valuable information; this is known as traffic analysis and can be quite useful to an alert adversary. Poor administration of a cryptosystem, such as permitting too short keys, will make any system vulnerable, regardless of other virtues. Social engineering and other attacks against humans  are usually employed due to being more cost-effective and feasible to perform in a reasonable amount of time compared to pure cryptanalysis by a high margin."
+2947,"Much of the theoretical work in cryptography concerns cryptographic primitives—algorithms with basic cryptographic properties—and their relationship to other cryptographic problems. More complicated cryptographic tools are then built from these basic primitives. These primitives provide fundamental properties, which are used to develop more complex tools called cryptosystems or cryptographic protocols, which guarantee one or more high-level security properties. Note, however, that the distinction between cryptographic primitives and cryptosystems, is quite arbitrary; for example, the RSA algorithm is sometimes considered a cryptosystem, and sometimes a primitive. Typical examples of cryptographic primitives include pseudorandom functions, one-way functions, etc."
+2948,"One or more cryptographic primitives are often used to develop a more complex algorithm, called a cryptographic system, or cryptosystem. Cryptosystems  are designed to provide particular functionality  while guaranteeing certain security properties  security in the random oracle model). Cryptosystems use the properties of the underlying cryptographic primitives to support the system's security properties. As the distinction between primitives and cryptosystems is somewhat arbitrary, a sophisticated cryptosystem can be derived from a combination of several more primitive cryptosystems. In many cases, the cryptosystem's structure involves back and forth communication among two or more parties in space  or across time . Such cryptosystems are sometimes called cryptographic protocols."
+2949,"Some widely known cryptosystems include RSA, Schnorr signature, ElGamal encryption, and Pretty Good Privacy . More complex cryptosystems include electronic cash systems, signcryption systems, etc. Some more 'theoretical' cryptosystems include interactive proof systems,  and systems for secret sharing,."
+2950,"Lightweight cryptography  concerns cryptographic algorithms developed for a strictly constrained environment. The growth of Internet of Things  has spiked research into the development of lightweight algorithms that are better suited for the environment. An IoT environment requires strict constraints on power consumption, processing power, and security. Algorithms such as PRESENT, AES, and SPECK are examples of the many LWC algorithms that have been developed to achieve the standard set by the National Institute of Standards and Technology."
+2951,"Cryptography is widely used on the internet to help protect user-data and prevent eavesdropping. To ensure secrecy during transmission, many systems use private key cryptography to protect transmitted information. With public-key systems, one can maintain secrecy without a master key or a large number of keys. But, some algorithms like Bitlocker and Veracrypt are generally not private-public key cryptography. For example, Veracrypt uses a password hash to generate the single private key. However, it can be configured to run in public-private key systems. The C++ opensource encryption library OpenSSL provides free and opensource encryption software and tools. The most commonly used encryption cipher suit is AES, as it has hardware acceleration for all x86 based processors that has AES-NI. A close contender is ChaCha20-Poly1305, which is a stream cipher, however it is commonly used for mobile devices as they are ARM based which does not feature AES-NI instruction set extension."
+2952,"Cryptography can be used to secure communications by encrypting them. Websites use encryption via HTTPS. ""End-to-end"" encryption, where only sender and receiver can read messages, is implemented for email in Pretty Good Privacy and for secure messaging in general in WhatsApp, Signal and Telegram."
+2953,"Operating systems use encryption to keep passwords secret, conceal parts of the system, and ensure that software updates are truly from the system maker. Instead of storing plaintext passwords, computer systems store hashes thereof; then, when a user logs in, the system passes the given password through a cryptographic hash function and compares it to the hashed value on file. In this manner, neither the system nor an attacker has at any point access to the password in plaintext."
+2954,"Encryption is sometimes used to encrypt one's entire drive. For example, University College London has implemented BitLocker  to render drive data opaque without users logging in."
+2955,"Cryptographic techniques enable cryptocurrency technologies, such as distributed ledger technologies , which finance cryptoeconomics applications such as decentralized finance . Key cryptographic techniques that enable cryptocurrencies and cryptoeconomics include, but are not limited to: cryptographic keys, cryptographic hash function, asymmetric  encryption, Multi-Factor Authentication , End-to-End Encryption , and Zero Knowledge Proofs ."
+2956,"Cryptography has long been of interest to intelligence gathering and law enforcement agencies. Secret communications may be criminal or even treasonous. Because of its facilitation of privacy, and the diminution of privacy attendant on its prohibition, cryptography is also of considerable interest to civil rights supporters. Accordingly, there has been a history of controversial legal issues surrounding cryptography, especially since the advent of inexpensive computers has made widespread access to high-quality cryptography possible."
+2957,"In some countries, even the domestic use of cryptography is, or has been, restricted. Until 1999, France significantly restricted the use of cryptography domestically, though it has since relaxed many of these rules. In China and Iran, a license is still required to use cryptography. Many countries have tight restrictions on the use of cryptography. Among the more restrictive are laws in Belarus, Kazakhstan, Mongolia, Pakistan, Singapore, Tunisia, and Vietnam."
+2958,"In the United States, cryptography is legal for domestic use, but there has been much conflict over legal issues related to cryptography. One particularly important issue has been the export of cryptography and cryptographic software and hardware. Probably because of the importance of cryptanalysis in World War II and an expectation that cryptography would continue to be important for national security, many Western governments have, at some point, strictly regulated export of cryptography. After World War II, it was illegal in the US to sell or distribute encryption technology overseas; in fact, encryption was designated as auxiliary military equipment and put on the United States Munitions List. Until the development of the personal computer, asymmetric key algorithms , and the Internet, this was not especially problematic. However, as the Internet grew and computers became more widely available, high-quality encryption techniques became well known around the globe."
+2959,"In the 1990s, there were several challenges to US export regulation of cryptography. After the source code for Philip Zimmermann's Pretty Good Privacy  encryption program found its way onto the Internet in June 1991, a complaint by RSA Security  resulted in a lengthy criminal investigation of Zimmermann by the US Customs Service and the FBI, though no charges were ever filed. Daniel J. Bernstein, then a graduate student at UC Berkeley, brought a lawsuit against the US government challenging some aspects of the restrictions based on free speech grounds. The 1995 case Bernstein v. United States ultimately resulted in a 1999 decision that printed source code for cryptographic algorithms and systems was protected as free speech by the United States Constitution."
+2960,"In 1996, thirty-nine countries signed the Wassenaar Arrangement, an arms control treaty that deals with the export of arms and ""dual-use"" technologies such as cryptography. The treaty stipulated that the use of cryptography with short key-lengths  would no longer be export-controlled. Cryptography exports from the US became less strictly regulated as a consequence of a major relaxation in 2000; there are no longer very many restrictions on key sizes in US-exported mass-market software. Since this relaxation in US export restrictions, and because most personal computers connected to the Internet include US-sourced web browsers such as Firefox or Internet Explorer, almost every Internet user worldwide has potential access to quality cryptography via their browsers . The Mozilla Thunderbird and Microsoft Outlook E-mail client programs similarly can transmit and receive emails via TLS, and can send and receive email encrypted with S/MIME. Many Internet users do not realize that their basic application software contains such extensive cryptosystems. These browsers and email programs are so ubiquitous that even governments whose intent is to regulate civilian use of cryptography generally do not find it practical to do much to control distribution or use of cryptography of this quality, so even when such laws are in force, actual enforcement is often effectively impossible."
+2961,"Another contentious issue connected to cryptography in the United States is the influence of the National Security Agency on cipher development and policy. The NSA was involved with the design of DES during its development at IBM and its consideration by the National Bureau of Standards as a possible Federal Standard for cryptography. DES was designed to be resistant to differential cryptanalysis, a powerful and general cryptanalytic technique known to the NSA and IBM, that became publicly known only when it was rediscovered in the late 1980s. According to Steven Levy, IBM discovered differential cryptanalysis, but kept the technique secret at the NSA's request. The technique became publicly known only when Biham and Shamir re-discovered and announced it some years later. The entire affair illustrates the difficulty of determining what resources and knowledge an attacker might actually have."
+2962,"Another instance of the NSA's involvement was the 1993 Clipper chip affair, an encryption microchip intended to be part of the Capstone cryptography-control initiative. Clipper was widely criticized by cryptographers for two reasons. The cipher algorithm  was then classified . The classified cipher caused concerns that the NSA had deliberately made the cipher weak in order to assist its intelligence efforts. The whole initiative was also criticized based on its violation of Kerckhoffs's Principle, as the scheme included a special escrow key held by the government for use by law enforcement ."
+2963,"Cryptography is central to digital rights management , a group of techniques for technologically controlling use of copyrighted material, being widely implemented and deployed at the behest of some copyright holders. In 1998, U.S. President Bill Clinton signed the Digital Millennium Copyright Act , which criminalized all production, dissemination, and use of certain cryptanalytic techniques and technology ; specifically, those that could be used to circumvent DRM technological schemes. This had a noticeable impact on the cryptography research community since an argument can be made that any cryptanalytic research violated the DMCA. Similar statutes have since been enacted in several countries and regions, including the implementation in the EU Copyright Directive. Similar restrictions are called for by treaties signed by World Intellectual Property Organization member-states."
+2964,"The United States Department of Justice and FBI have not enforced the DMCA as rigorously as had been feared by some, but the law, nonetheless, remains a controversial one. Niels Ferguson, a well-respected cryptography researcher, has publicly stated that he will not release some of his research into an Intel security design for fear of prosecution under the DMCA. Cryptologist Bruce Schneier has argued that the DMCA encourages vendor lock-in, while inhibiting actual measures toward cyber-security. Both Alan Cox  and Edward Felten  have encountered problems related to the Act. Dmitry Sklyarov was arrested during a visit to the US from Russia, and jailed for five months pending trial for alleged violations of the DMCA arising from work he had done in Russia, where the work was legal. In 2007, the cryptographic keys responsible for Blu-ray and HD DVD content scrambling were discovered and released onto the Internet. In both cases, the Motion Picture Association of America sent out numerous DMCA takedown notices, and there was a massive Internet backlash triggered by the perceived impact of such notices on fair use and free speech."
+2965,"In the United Kingdom, the Regulation of Investigatory Powers Act gives UK police the powers to force suspects to decrypt files or hand over passwords that protect encryption keys. Failure to comply is an offense in its own right, punishable on conviction by a two-year jail sentence or up to five years in cases involving national security. Successful prosecutions have occurred under the Act; the first, in 2009, resulted in a term of 13 months' imprisonment. Similar forced disclosure laws in Australia, Finland, France, and India compel individual suspects under investigation to hand over encryption keys or passwords during a criminal investigation."
+2966,"In the United States, the federal criminal case of United States v. Fricosu addressed whether a search warrant can compel a person to reveal an encryption passphrase or password. The Electronic Frontier Foundation  argued that this is a violation of the protection from self-incrimination given by the Fifth Amendment. In 2012, the court ruled that under the All Writs Act, the defendant was required to produce an unencrypted hard drive for the court."
+2967,"In many jurisdictions, the legal status of forced disclosure remains unclear."
+2968,The 2016 FBI–Apple encryption dispute concerns the ability of courts in the United States to compel manufacturers' assistance in unlocking cell phones whose contents are cryptographically protected.
+2969,"As a potential counter-measure to forced disclosure some cryptographic software supports plausible deniability, where the encrypted data is indistinguishable from unused random data ."
+2970,"A software application or component that performs rendering is called a rendering engine, render engine, rendering system, graphics engine, or simply a renderer."
+2971,"Rendering is one of the major sub-topics of 3D computer graphics, and in practice it is always connected to the others. It is the last major step in the graphics pipeline, giving models and animation their final appearance. With the increasing sophistication of computer graphics since the 1970s, it has become a more distinct subject."
+2972,"Rendering has uses in architecture, video games, simulators, movie and TV visual effects, and design visualization, each employing a different balance of features and techniques. A wide variety of renderers are available for use. Some are integrated into larger modeling and animation packages, some are stand-alone, and some are free open-source projects. On the inside, a renderer is a carefully engineered program based on multiple disciplines, including light physics, visual perception, mathematics, and software development."
+2973,"Though the technical details of rendering methods vary, the general challenges to overcome in producing a 2D image on a screen from a 3D representation stored in a scene file are handled by the graphics pipeline in a rendering device such as a GPU. A GPU is a purpose-built device that assists a CPU in performing complex rendering calculations. If a scene is to look relatively realistic and predictable under virtual lighting, the rendering software must solve the rendering equation. The rendering equation does not account for all lighting phenomena, but instead acts as a general lighting model for computer-generated imagery."
+2974,"In the case of 3D graphics, scenes can be pre-rendered or generated in realtime.  Pre-rendering is a slow, computationally intensive process that is typically used for movie creation, where scenes can be generated ahead of time, while real-time rendering is often done for 3D video games and other applications that must dynamically create scenes.  3D hardware accelerators can improve realtime rendering performance."
+2975,"When the pre-image  is complete, rendering is used, which adds in bitmap textures or procedural textures, lights, bump mapping and relative position to other objects. The result is a completed image the consumer or intended viewer sees."
+2976,"For movie animations, several images  must be rendered, and stitched together in a program capable of making an animation of this sort. Most 3D image editing programs can do this."
+2977,"A rendered image can be understood in terms of a number of visible features. Rendering research and development has been largely motivated by finding ways to simulate these efficiently. Some relate directly to particular algorithms and techniques, while others are produced together."
+2978,"CAD libraries can have assets such as 3D models, textures, bump maps, HDRIs, and different Computer graphics lighting sources to be rendered."
+2979,"Many rendering algorithms have been researched, and software used for rendering may employ a number of different techniques to obtain a final image."
+2980,"Choosing how to render a scene usually involves a trade-off between speed and realism . The techniques developed over the years follow a loose progression, with more advanced methods becoming practical as computing power and memory capacity increased."
+2981,"An important distinction is between image order algorithms, which iterate over pixels of the image plane, and object order algorithms, which iterate over objects in the scene. For simple scenes, object order is usually more efficient, as there are fewer objects than pixels."
+2982,"Each of the above approaches has many variations, and there is some overlap. Path tracing may be considered either a distinct technique or a particular type of ray tracing.: 846, 1021  Note that the usage of terminology related to ray tracing and path tracing has changed significantly over time.: 7"
+2983,"Ray marching is a family of algorithms, used by ray casting, for finding intersections between a ray and a complex object, such as a volumetric dataset or a surface defined by a signed distance function. It is not, by itself, a rendering method, but it can be incorporated into ray tracing and path tracing, and is used by rasterization to implement screen-space reflection and other effects.: 13"
+2984,"A technique called photon mapping or photon tracing uses forward ray tracing , tracing paths of photons from a light source to an object, rather than backward from the camera. The additional data collected by this process is used together with conventional backward ray tracing or path tracing.: 1037-1039  Rendering a scene using only forward ray tracing is impractical, even though it corresponds more closely to reality, because a huge number of photons would need to be simulated, only a tiny fraction of which actually hit the camera.: 7-9"
+2985,"Real-time rendering, including video game graphics, typically uses rasterization, but increasingly combines it with ray tracing and path tracing.: 2  To enable realistic global illumination, real-time rendering often relies on pre-rendered  lighting for stationary objects. For moving objects, it may use a technique called light probes, in which lighting is recorded by rendering omnidirectional views of the scene at chosen points in space . These are similar to environment maps, but typically use a very low resolution or an approximation such as spherical harmonics. "
+2986,"A high-level representation of an image necessarily contains elements in a different domain from pixels.  These elements are referred to as primitives.  In a schematic drawing, for instance, line segments and curves might be primitives.  In a graphical user interface, windows and buttons might be the primitives.  In rendering of 3D models, triangles and polygons in space might be primitives."
+2987,"If a pixel-by-pixel  approach to rendering is impractical or too slow for some task, then a primitive-by-primitive  approach to rendering may prove useful.  Here, one loop through each of the primitives, determines which pixels in the image it affects, and modifies those pixels accordingly.  This is called rasterization, and is the rendering method used by all current graphics cards."
+2988,"Rasterization is frequently faster than pixel-by-pixel rendering.  First, large areas of the image may be empty of primitives; rasterization will ignore these areas, but pixel-by-pixel rendering must pass through them.  Second, rasterization can improve cache coherency and reduce redundant work by taking advantage of the fact that the pixels occupied by a single primitive tend to be contiguous in the image.  For these reasons, rasterization is usually the approach of choice when interactive rendering is required; however, the pixel-by-pixel approach can often produce higher-quality images and is more versatile because it does not depend on as many assumptions about the image as rasterization."
+2989,"The older form of rasterization is characterized by rendering an entire face  as a single color.  Alternatively, rasterization can be done in a more complicated manner by first rendering the vertices of a face and then rendering the pixels of that face as a blending of the vertex colors. This version of rasterization has overtaken the old method as it allows the graphics to flow without complicated textures .  This newer method of rasterization utilizes the graphics card's more taxing shading functions and still achieves better performance because the simpler textures stored in memory use less space. Sometimes designers will use one rasterization method on some faces and the other method on others based on the angle at which that face meets other joined faces, thus increasing speed and not hurting the overall effect."
+2990,"In ray casting the geometry which has been modeled is parsed pixel by pixel, line by line, from the point of view outward, as if casting rays out from the point of view. Where an object is intersected, the color value at the point may be evaluated using several methods. In the simplest, the color value of the object at the point of intersection becomes the value of that pixel. The color may be determined from a texture-map. A more sophisticated method is to modify the color value by an illumination factor, but without calculating the relationship to a simulated light source. To reduce artifacts, a number of rays in slightly different directions may be averaged."
+2991,"Ray casting involves calculating the ""view direction"" , and incrementally following along that ""ray cast"" through ""solid 3d objects"" in the scene, while accumulating the resulting value from each point in 3D space.  This is related and similar to ""ray tracing"" except that the raycast is usually not ""bounced"" off surfaces . ""Ray casting"" implies that the light ray is following a straight path .  The ray cast is a vector that can originate from the camera or from the scene endpoint .  Sometimes the final light value is derived from a ""transfer function"" and sometimes it's used directly."
+2992,"Rough simulations of optical properties may be additionally employed: a simple calculation of the ray from the object to the point of view is made. Another calculation is made of the angle of incidence of light rays from the light source, and from these as well as the specified intensities of the light sources, the value of the pixel is calculated. Another simulation uses illumination plotted from a radiosity algorithm, or a combination of these two."
+2993,"Ray tracing aims to simulate the natural flow of light, interpreted as particles. Often, ray tracing methods are utilized to approximate the solution to the rendering equation by applying Monte Carlo methods to it. Some of the most used methods are path tracing, bidirectional path tracing, or Metropolis light transport, but also semi realistic methods are in use, like Whitted Style Ray Tracing, or hybrids. While most implementations let light propagate on straight lines, applications exist to simulate relativistic spacetime effects."
+2994,"In a final, production quality rendering of a ray traced work, multiple rays are generally shot for each pixel, and traced not just to the first object of intersection, but rather, through a number of sequential 'bounces', using the known laws of optics such as ""angle of incidence equals angle of reflection"" and more advanced laws that deal with refraction and surface roughness."
+2995,"Once the ray either encounters a light source, or more probably once a set limiting number of bounces has been evaluated, then the surface illumination at that final point is evaluated using techniques described above, and the changes along the way through the various bounces evaluated to estimate a value observed at the point of view. This is all repeated for each sample, for each pixel."
+2996,"In distribution ray tracing, at each point of intersection, multiple rays may be spawned. In path tracing, however, only a single ray or none is fired at each intersection, utilizing the statistical nature of Monte Carlo experiments."
+2997,"As part of the approach known as physically based rendering, path tracing has become the dominant technique for rendering realistic scenes, including effects for movies. For example, the popular open source 3D software Blender uses path tracing in its Cycles renderer. Images produced using path tracing for global illumination are generally noisier than when using radiosity , but radiosity can be difficult to apply to complex scenes and is prone to artifacts that arise from using a tessellated representation of irradiance.: 975-976, 1045"
+2998,"Path tracing's relative simplicity and its nature as a Monte Carlo method  make it attractive to implement on a GPU, especially on recent GPUs that support ray tracing acceleration technology such as Nvidia's RTX and OptiX. Many techniques have been developed to denoise the output of path tracing, reducing the number of paths required to achieve acceptable quality, at the risk of losing some detail or introducing small-scale artifacts that are more objectionable than noise; neural networks are now widely used for this purpose."
+2999,"Advances in GPU technology have made real-time ray tracing possible in games, although it is currently almost always used in combination with rasterization.: 2  This enables visual effects that are difficult with only rasterization, including reflection from curved surfaces and interreflective objects,: 305  and shadows that are accurate over a wide range of distances and surface orientations.: 159-160  Ray tracing support is included in recent versions of the graphics APIs used by games, such as DirectX, Metal, and Vulkan."
+3000,"Neural rendering is a rendering method using artificial neural networks. Neural rendering includes image-based rendering methods that are used to reconstruct 3D models from 2-dimensional images.One of these methods are photogrammetry, which is a method in which a collection of images from multiple angles of an object are turned into a 3D model. There have also been recent developments in generating and rendering 3D models from text and coarse paintings by notably Nvidia, Google and various other companies."
+3001,Radiosity is a method which attempts to simulate the way in which directly illuminated surfaces act as indirect light sources that illuminate other surfaces.  This produces more realistic shading and seems to better capture the 'ambience' of an indoor scene. A classic example is a way that shadows 'hug' the corners of rooms.
+3002,The optical basis of the simulation is that some diffused light from a given point on a given surface is reflected in a large spectrum of directions and illuminates the area around it.
+3003,"The simulation technique may vary in complexity. Many renderings have a very rough estimate of radiosity, simply illuminating an entire scene very slightly with a factor known as ambiance. However, when advanced radiosity estimation is coupled with a high quality ray tracing algorithm, images may exhibit convincing realism, particularly for indoor scenes."
+3004,"In advanced radiosity simulation, recursive, finite-element algorithms 'bounce' light back and forth between surfaces in the model, until some recursion limit is reached. The colouring of one surface in this way influences the colouring of a neighbouring surface, and vice versa. The resulting values of illumination throughout the model  are stored and used as additional inputs when performing calculations in a ray-casting or ray-tracing model."
+3005,"Due to the iterative/recursive nature of the technique, complex objects are particularly slow to emulate. Prior to the standardization of rapid radiosity calculation, some digital artists used a technique referred to loosely as false radiosity by darkening areas of texture maps corresponding to corners, joints and recesses, and applying them via self-illumination or diffuse mapping for scanline rendering. Even now, advanced radiosity calculations may be reserved for calculating the ambiance of the room, from the light reflecting off walls, floor and ceiling, without examining the contribution that complex objects make to the radiosity –  or complex objects may be replaced in the radiosity calculation with simpler objects of similar size and texture."
+3006,"Radiosity calculations are viewpoint independent which increases the computations involved, but makes them useful for all viewpoints. If there is little rearrangement of radiosity objects in the scene, the same radiosity data may be reused for a number of frames, making radiosity an effective way to improve on the flatness of ray casting, without seriously impacting the overall rendering time-per-frame."
+3007,"Because of this, radiosity is a prime component of leading real-time rendering methods, and has been used from beginning-to-end to create a large number of well-known recent feature-length animated 3D-cartoon films."
+3008,"One problem that any rendering system must deal with, no matter which approach it takes, is the sampling problem.  Essentially, the rendering process tries to depict a continuous function from image space to colors by using a finite number of pixels.  As a consequence of the Nyquist–Shannon sampling theorem , any spatial waveform that can be displayed must consist of at least two pixels, which is proportional to image resolution.  In simpler terms, this expresses the idea that an image cannot display details, peaks or troughs in color or intensity, that are smaller than one pixel."
+3009,"If a naive rendering algorithm is used without any filtering, high frequencies in the image function will cause ugly aliasing to be present in the final image.  Aliasing typically manifests itself as jaggies, or jagged edges on objects where the pixel grid is visible.  In order to remove aliasing, all rendering algorithms  must use some kind of low-pass filter on the image function to remove high frequencies, a process called antialiasing."
+3010,"Due to the large number of calculations, a work in progress is usually only rendered in detail appropriate to the portion of the work being developed at a given time, so in the initial stages of modeling, wireframe and ray casting may be used, even where the target output is ray tracing with radiosity. It is also common to render only parts of the scene at high detail, and to remove objects that are not important to what is currently being developed."
+3011,"For real-time, it is appropriate to simplify one or more common approximations, and tune to the exact parameters of the scenery in question, which is also tuned to the agreed parameters to get the most 'bang for the buck'."
+3012,The implementation of a realistic renderer always has some basic element of physical simulation or emulation –  some computation which resembles or abstracts a real physical process.
+3013,"The term ""physically based"" indicates the use of physical models and approximations that are more general and widely accepted outside rendering. A particular set of related techniques have gradually become established in the rendering community."
+3014,"The basic concepts are moderately straightforward, but intractable to calculate; and a single elegant algorithm or approach has been elusive for more general purpose renderers. In order to meet demands of robustness, accuracy and practicality, an implementation will be a complex combination of different techniques."
+3015,Rendering research is concerned with both the adaptation of scientific models and their efficient application.
+3016,This is the key academic/theoretical concept in rendering. It serves as the most abstract formal expression of the non-perceptual aspect of rendering. All more complete algorithms can be seen as solutions to particular formulations of this equation.
+3017,"Meaning: at a particular position and direction, the outgoing light  is the sum of the emitted light  and the reflected light. The reflected light being the sum of the incoming light  from all directions, multiplied by the surface reflection and incoming angle. By connecting outward light to inward light, via an interaction point, this equation stands for the whole 'light transport' –  all the movement of light –  in a scene."
+3018,The bidirectional reflectance distribution function  expresses a simple model of light interaction with a surface as follows:
+3019,"Light interaction is often approximated by the even simpler models: diffuse reflection and specular reflection, although both can ALSO be BRDFs."
+3020,"Rendering is practically exclusively concerned with the particle aspect of light physics –  known as geometrical optics. Treating light, at its basic level, as particles bouncing around is a simplification, but appropriate: the wave aspects of light are negligible in most scenes, and are significantly more difficult to simulate. Notable wave aspect phenomena include diffraction  and polarisation . Both types of effect, if needed, are made by appearance-oriented adjustment of the reflection model."
+3021,"Though it receives less attention, an understanding of human visual perception is valuable to rendering. This is mainly because image displays and human perception have restricted ranges. A renderer can simulate a wide range of light brightness and color, but current displays –  movie screen, computer monitor, etc. –  cannot handle so much, and something must be discarded or compressed. Human perception also has limits, and so does not need to be given large-range images to create realism. This can help solve the problem of fitting images into displays, and, furthermore, suggest what short-cuts could be used in the rendering simulation, since certain subtleties will not be noticeable. This related subject is tone mapping."
+3022,"Mathematics used in rendering includes: linear algebra, calculus, numerical mathematics, signal processing, and Monte Carlo methods."
+3023,Rendering for movies often takes place on a network of tightly connected computers known as a render farm.
+3024,The current state of the art in 3-D image description for movie creation is the Mental Ray scene description language designed at Mental Images and RenderMan Shading Language designed at Pixar .
+3025,"Other renderers  can and are sometimes used, but most other renderers tend to miss one or more of the often needed features like good texture filtering, texture caching, programmable shaders, highend geometry types like hair, subdivision or nurbs surfaces with tesselation on demand, geometry caching, raytracing with geometry caching, high quality shadow mapping, speed or patent-free implementations.  Other highly sought features these days may include interactive photorealistic rendering  and hardware rendering/shading."
+3026,"Computer simulations are realized by running computer programs that can be either small, running almost instantly on small devices, or large-scale programs that run for hours or days on network-based groups of computers. The scale of events being simulated by computer simulations has far exceeded anything possible  using traditional paper-and-pencil mathematical modeling. In 1997, a desert-battle simulation of one force invading another involved the modeling of 66,239 tanks, trucks and other vehicles on simulated terrain around Kuwait, using multiple supercomputers in the DoD High Performance Computer Modernization Program.
+Other examples include a 1-billion-atom model of material deformation; a 2.64-million-atom model of the complex protein-producing organelle of all living organisms, the ribosome, in 2005;
+a complete simulation of the life cycle of Mycoplasma genitalium in 2012; and the Blue Brain project at EPFL , begun in May 2005 to create the first computer simulation of the entire human brain, right down to the molecular level."
+3027,"Because of the computational cost of simulation, computer experiments are used to perform inference such as uncertainty quantification."
+3028,"A model consists of the equations used to capture the behavior of a system. By contrast, computer simulation is the actual running of the program that perform algorithms which solve those equations, often in an approximate manner. Simulation, therefore, is the process of running a model. Thus one would not ""build a simulation""; instead, one would ""build a model "", and then either ""run the model"" or equivalently ""run a simulation""."
+3029,"Computer simulation developed hand-in-hand with the rapid growth of the computer, following its first large-scale deployment during the Manhattan Project in World War II to model the process of nuclear detonation. It was a simulation of 12 hard spheres using a Monte Carlo algorithm. Computer simulation is often used as an adjunct to, or substitute for, modeling systems for which simple closed form analytic solutions are not possible. There are many types of computer simulations; their common feature is the attempt to generate a sample of representative scenarios for a model in which a complete enumeration of all possible states of the model would be prohibitive or impossible."
+3030,"The external data requirements of simulations and models vary widely. For some, the input might be just a few numbers , while others might require terabytes of information ."
+3031,Input sources also vary widely:
+3032,"Lastly, the time at which data is available varies:"
+3033,"Because of this variety, and because diverse simulation systems have many common elements, there are a large number of specialized simulation languages. The best-known may be Simula. There are now many others."
+3034,"Systems that accept data from external sources must be very careful in knowing what they are receiving. While it is easy for computers to read in values from text or binary files, what is much harder is knowing what the accuracy  of the values are. Often they are expressed as ""error bars"", a minimum and maximum deviation from the value range within which the true value  lie. Because digital computer mathematics is not perfect, rounding and truncation errors multiply this error, so it is useful to perform an ""error analysis"" to confirm that values output by the simulation will still be usefully accurate."
+3035,"Models used for computer simulations can be classified according to several independent pairs of attributes, including:"
+3036,"Another way of categorizing models is to look at the underlying data structures. For time-stepped simulations, there are two main classes:"
+3037,"For steady-state simulations, equations define the relationships between elements of the modeled system and attempt to find a state in which the system is in equilibrium. Such models are often used in simulating physical systems, as a simpler modeling case before dynamic simulation is attempted."
+3038,"Formerly, the output data from a computer simulation was sometimes presented in a table or a matrix showing how data were affected by numerous changes in the simulation parameters. The use of the matrix format was related to traditional use of the matrix concept in mathematical models. However, psychologists and others noted that humans could quickly perceive trends by looking at graphs or even moving-images or motion-pictures generated from the data, as displayed by computer-generated-imagery  animation. Although observers could not necessarily read out numbers or quote math formulas, from observing a moving weather chart they might be able to predict events  much faster than by scanning tables of rain-cloud coordinates. Such intense graphical displays, which transcended the world of numbers and formulae, sometimes also led to output that lacked a coordinate grid or omitted timestamps, as if straying too far from numeric data displays. Today, weather forecasting models tend to balance the view of moving rain/snow clouds against a map that uses numeric coordinates and numeric timestamps of events."
+3039,"Similarly, CGI computer simulations of CAT scans can simulate how a tumor might shrink or change during an extended period of medical treatment, presenting the passage of time as a spinning view of the visible human head, as the tumor changes."
+3040,"Other applications of CGI computer simulations are being developed to graphically display large amounts of data, in motion, as changes occur during a simulation run."
+3041,"Generic examples of types of computer simulations in science, which are derived from an underlying mathematical description:"
+3042,Specific examples of computer simulations include:
+3043,"Notable, and sometimes controversial, computer simulations used in science include: Donella Meadows' World3 used in the Limits to Growth, James Lovelock's Daisyworld and Thomas Ray's Tierra."
+3044,"In social sciences, computer simulation is an integral component of the five angles of analysis fostered by the data percolation methodology, which also includes qualitative and quantitative methods, reviews of the literature , and interviews with experts, and which forms an extension of data triangulation. Of course, similar to any other scientific method, replication is an important part of computational modeling "
+3045,"Computer simulations are used in a wide variety of practical contexts, such as:"
+3046,"The reliability and the trust people put in computer simulations depends on the validity of the simulation model, therefore verification and validation are of crucial importance in the development of computer simulations. Another important aspect of computer simulations is that of reproducibility of the results, meaning that a simulation model should not provide a different answer for each execution. Although this might seem obvious, this is a special point of attention in stochastic simulations, where random numbers should actually be semi-random numbers. An exception to reproducibility are human-in-the-loop simulations such as flight simulations and computer games. Here a human is part of the simulation and thus influences the outcome in a way that is hard, if not impossible, to reproduce exactly."
+3047,"Vehicle manufacturers make use of computer simulation to test safety features in new designs. By building a copy of the car in a physics simulation environment, they can save the hundreds of thousands of dollars that would otherwise be required to build and test a unique prototype. Engineers can step through the simulation milliseconds at a time to determine the exact stresses being put upon each section of the prototype."
+3048,"Computer graphics can be used to display the results of a computer simulation. Animations can be used to experience a simulation in real-time, e.g., in training simulations. In some cases animations may also be useful in faster than real-time or even slower than real-time modes. For example, faster than real-time animations can be useful in visualizing the buildup of queues in the simulation of humans evacuating a building. Furthermore, simulation results are often aggregated into static images using various ways of scientific visualization."
+3049,"In debugging, simulating a program execution under test  can detect far more errors than the hardware itself can detect and, at the same time, log useful debugging information such as instruction trace, memory alterations and instruction counts. This technique can also detect buffer overflow and similar ""hard to detect"" errors as well as produce performance information and tuning data."
+3050,"Although sometimes ignored in computer simulations, it is very important to perform a sensitivity analysis to ensure that the accuracy of the results is properly understood. For example, the probabilistic risk analysis of factors determining the success of an oilfield exploration program involves combining samples from a variety of statistical distributions using the Monte Carlo method. If, for instance, one of the key parameters  is known to only one significant figure, then the result of the simulation might not be more precise than one significant figure, although it might  be presented as having four significant figures."
+3051,"A multi-core processor implements multiprocessing in a single physical package. Designers may couple cores in a multi-core device tightly or loosely. For example, cores may or may not share caches, and they may implement message passing or shared-memory inter-core communication methods. Common network topologies used to interconnect cores include bus, ring, two-dimensional mesh, and crossbar. Homogeneous multi-core systems include only identical cores; heterogeneous multi-core systems have cores that are not identical . Just as with single-processor systems, cores in multi-core systems may implement architectures such as VLIW, superscalar, vector, or multithreading."
+3052,"Multi-core processors are widely used across many application domains, including general-purpose, embedded, network, digital signal processing , and graphics . Core count goes up to even dozens, and for specialized chips over 10,000, and in supercomputers  the count can go over 10 million ."
+3053,"The improvement in performance gained by the use of a multi-core processor depends very much on the software algorithms used and their implementation. In particular, possible gains are limited by the fraction of the software that can run in parallel simultaneously on multiple cores; this effect is described by Amdahl's law. In the best case, so-called embarrassingly parallel problems may realize speedup factors near the number of cores, or even more if the problem is split up enough to fit within each core's cache, avoiding use of much slower main-system memory. Most applications, however, are not accelerated as much unless programmers invest effort in refactoring."
+3054,The parallelization of software is a significant ongoing topic of research. Cointegration of multiprocessor applications provides flexibility in network architecture design. Adaptability within parallel models is an additional feature of systems utilizing these protocols.
+3055,"In the consumer market, dual-core processors  started becoming commonplace in the late 2000s. Quad-core processors were also being adopted for higher-end systems. In the late 2010s, hexa-core  started entering the mainstream."
+3056,"The terms multi-core and dual-core most commonly refer to some sort of central processing unit , but are sometimes also applied to digital signal processors  and system on a chip . The terms are generally used only to refer to multi-core microprocessors that are manufactured on the same integrated circuit die; separate microprocessor dies in the same package are generally referred to by another name, such as multi-chip module. This article uses the terms ""multi-core"" and ""dual-core"" for CPUs manufactured on the same integrated circuit, unless otherwise noted."
+3057,"In contrast to multi-core systems, the term multi-CPU refers to multiple physically separate processing-units ."
+3058,The terms many-core and massively multi-core are sometimes used to describe multi-core architectures with an especially high number of cores .
+3059,"Some systems use many soft microprocessor cores placed on a single FPGA. Each ""core"" can be considered a ""semiconductor intellectual property core"" as well as a CPU core."
+3060,"While manufacturing technology improves, reducing the size of individual gates, physical limits of semiconductor-based microelectronics have become a major design concern. These physical limitations can cause significant heat dissipation and data synchronization problems. Various other methods are used to improve CPU performance. Some instruction-level parallelism  methods such as superscalar pipelining are suitable for many applications, but are inefficient for others that contain difficult-to-predict code. Many applications are better suited to thread-level parallelism  methods, and multiple independent CPUs are commonly used to increase a system's overall TLP. A combination of increased available space  and the demand for increased TLP led to the development of multi-core CPUs."
+3061,"Several business motives drive the development of multi-core architectures. For decades, it was possible to improve performance of a CPU by shrinking the area of the integrated circuit , which reduced the cost per device on the IC. Alternatively, for the same circuit area, more transistors could be used in the design, which increased functionality, especially for complex instruction set computing  architectures. Clock rates also increased by orders of magnitude in the decades of the late 20th century, from several megahertz in the 1980s to several gigahertz in the early 2000s."
+3062,"As the rate of clock speed improvements slowed, increased use of parallel computing in the form of multi-core processors has been pursued to improve overall processing performance. Multiple cores were used on the same CPU chip, which could then lead to better sales of CPU chips with two or more cores. For example, Intel has produced a 48-core processor for research in cloud computing; each core has an x86 architecture."
+3063,"Since computer manufacturers have long implemented symmetric multiprocessing  designs using discrete CPUs, the issues regarding implementing multi-core processor architecture and supporting it with software are well known."
+3064,Additionally:
+3065,"In order to continue delivering regular performance improvements for general-purpose processors, manufacturers such as Intel and AMD have turned to multi-core designs, sacrificing lower manufacturing-costs for higher performance in some applications and systems. Multi-core architectures are being developed, but so are the alternatives. An especially strong contender for established markets is the further integration of peripheral functions into the chip."
+3066,"The proximity of multiple CPU cores on the same die allows the cache coherency circuitry to operate at a much higher clock rate than what is possible if the signals have to travel off-chip. Combining equivalent CPUs on a single die significantly improves the performance of cache snoop  operations. Put simply, this means that signals between different CPUs travel shorter distances, and therefore those signals degrade less. These higher-quality signals allow more data to be sent in a given time period, since individual signals can be shorter and do not need to be repeated as often."
+3067,"Assuming that the die can physically fit into the package, multi-core CPU designs require much less printed circuit board  space than do multi-chip SMP designs. Also, a dual-core processor uses slightly less power than two coupled single-core processors, principally because of the decreased power required to drive signals external to the chip. Furthermore, the cores share some circuitry, like the L2 cache and the interface to the front-side bus . In terms of competing technologies for the available silicon die area, multi-core design can make use of proven CPU core library designs and produce a product with lower risk of design error than devising a new wider-core design. Also, adding more cache suffers from diminishing returns."
+3068,"Multi-core chips also allow higher performance at lower energy. This can be a big factor in mobile devices that operate on batteries. Since each core in a multi-core CPU is generally more energy-efficient, the chip becomes more efficient than having a single large monolithic core. This allows higher performance with less energy. A challenge in this, however, is the additional overhead of writing parallel code."
+3069,"Maximizing the usage of the computing resources provided by multi-core processors requires adjustments both to the operating system  support and to existing application software. Also, the ability of multi-core processors to increase application performance depends on the use of multiple threads within applications."
+3070,"Integration of a multi-core chip can lower the chip production yields. They are also more difficult to manage thermally than lower-density single-core designs. Intel has partially countered this first problem by creating its quad-core designs by combining two dual-core ones on a single die with a unified cache, hence any two working dual-core dies can be used, as opposed to producing four cores on a single die and requiring all four to work to produce a quad-core CPU. From an architectural point of view, ultimately, single CPU designs may make better use of the silicon surface area than multiprocessing cores, so a development commitment to this architecture may carry the risk of obsolescence. Finally, raw processing power is not the only constraint on system performance. Two processing cores sharing the same system bus and memory bandwidth limits the real-world performance advantage. In a 2009 report, Dr Jun Ni showed that if a single core is close to being memory-bandwidth limited, then going to dual-core might give 30% to 70% improvement; if memory bandwidth is not a problem, then a 90% improvement can be expected; however, Amdahl's law makes this claim dubious. It would be possible for an application that used two CPUs to end up running faster on a single-core one if communication between the CPUs was the limiting factor, which would count as more than 100% improvement."
+3071,"The trend in processor development has been towards an ever-increasing number of cores, as processors with hundreds or even thousands of cores become theoretically possible. In addition, multi-core chips mixed with simultaneous multithreading, memory-on-chip, and special-purpose ""heterogeneous""  cores promise further performance and efficiency gains, especially in processing multimedia, recognition and networking applications. For example, a big.LITTLE core includes a high-performance core  and a low-power core . There is also a trend towards improving energy-efficiency by focusing on performance-per-watt with advanced fine-grain or ultra fine-grain power management and dynamic voltage and frequency scaling ."
+3072,"Chips designed from the outset for a large number of cores  are sometimes referred to as manycore designs, emphasising qualitative differences."
+3073,"The composition and balance of the cores in multi-core architecture show great variety. Some architectures use one core design repeated consistently , while others use a mixture of different cores, each optimized for a different, ""heterogeneous"" role."
+3074,"How multiple cores are implemented and integrated significantly affects both the developer's programming skills and the consumer's expectations of apps and interactivity versus the device. A device advertised as being octa-core will only have independent cores if advertised as True Octa-core, or similar styling, as opposed to being merely two sets of quad-cores each with fixed clock speeds."
+3075,"The article ""CPU designers debate multi-core future"" by Rick Merritt, EE Times 2008, includes these comments:"
+3076," Atsushi Hasegawa, a senior chief engineer at Renesas, generally agreed. He suggested the cellphone's use of many specialty cores working in concert is a good model for future multi-core designs."
+3077," Anant Agarwal, founder and chief executive of startup Tilera, took the opposing view. He said multi-core chips need to be homogeneous collections of general-purpose cores to keep the software model simple."
+3078,"An outdated version of an anti-virus application may create a new thread for a scan process, while its GUI thread waits for commands from the user . In such cases, a multi-core architecture is of little benefit for the application itself due to the single thread doing all the heavy lifting and the inability to balance the work evenly across multiple cores. Programming truly multithreaded code often requires complex co-ordination of threads and can easily introduce subtle and difficult-to-find bugs due to the interweaving of processing on data shared between threads . Consequently, such code is much more difficult to debug than single-threaded code when it breaks. There has been a perceived lack of motivation for writing consumer-level threaded applications because of the relative rarity of consumer-level demand for maximum use of computer hardware. Also, serial tasks like decoding the entropy encoding algorithms used in video codecs are impossible to parallelize because each result generated is used to help create the next result of the entropy decoding algorithm."
+3079,"Given the increasing emphasis on multi-core chip design, stemming from the grave thermal and power consumption problems posed by any further significant increase in processor clock speeds, the extent to which software can be multithreaded to take advantage of these new chips is likely to be the single greatest constraint on computer performance in the future. If developers are unable to design software to fully exploit the resources provided by multiple cores, then they will ultimately reach an insurmountable performance ceiling."
+3080,The telecommunications market had been one of the first that needed a new design of parallel datapath packet processing because there was a very quick adoption of these multiple-core processors for the datapath and the control plane. These MPUs are going to replace the traditional Network Processors that were based on proprietary microcode or picocode.
+3081,"Parallel programming techniques can benefit from multiple cores directly. Some existing parallel programming models such as Cilk Plus, OpenMP, OpenHMPP, FastFlow, Skandium, MPI, and Erlang can be used on multi-core platforms. Intel introduced a new abstraction for C++ parallelism called TBB. Other research efforts include the Codeplay Sieve System, Cray's Chapel, Sun's Fortress, and IBM's X10."
+3082,"Multi-core processing has also affected the ability of modern computational software development. Developers programming in newer languages might find that their modern languages do not support multi-core functionality. This then requires the use of numerical libraries to access code written in languages like C and Fortran, which perform math computations faster than newer languages like C#. Intel's MKL and AMD's ACML are written in these native languages and take advantage of multi-core processing. Balancing the application workload across processors can be problematic, especially if they have different performance characteristics. There are different conceptual models to deal with the problem, for example using a coordination language and program building blocks . Each block can have a different native implementation for each processor type. Users simply program using these abstractions and an intelligent compiler chooses the best implementation based on the context."
+3083,Managing concurrency acquires a central role in developing parallel applications. The basic steps in designing parallel applications are:
+3084,"On the other hand, on the server side, multi-core processors are ideal because they allow many users to connect to a site simultaneously and have independent threads of execution. This allows for Web servers and application servers that have much better throughput."
+3085,"Vendors may license some software ""per processor"". This can give rise to ambiguity, because a ""processor"" may consist either of a single core or of a combination of cores."
+3086,"Embedded computing operates in an area of processor technology distinct from that of ""mainstream"" PCs. The same technological drives towards multi-core apply here too. Indeed, in many cases the application is a ""natural"" fit for multi-core technologies, if the task can easily be partitioned between the different processors."
+3087,"In addition, embedded software is typically developed for a specific hardware release, making issues of software portability, legacy code or supporting independent developers less critical than is the case for PC or enterprise computing. As a result, it is easier for developers to adopt new technologies and as a result there is a greater variety of multi-core processing architectures and suppliers."
+3088,"As of 2010, multi-core network processors have become mainstream, with companies such as Freescale Semiconductor, Cavium Networks, Wintegra and Broadcom all manufacturing products with eight processors. For the system developer, a key challenge is how to exploit all the cores in these devices to achieve maximum networking performance at the system level, despite the performance limitations inherent in a symmetric multiprocessing  operating system. Companies such as 6WIND provide portable packet processing software designed so that the networking data plane runs in a fast path environment outside the operating system of the network device."
+3089,"In digital signal processing the same trend applies: Texas Instruments has the three-core TMS320C6488 and four-core TMS320C5441, Freescale the four-core MSC8144 and six-core MSC8156 . Newer entries include the Storm-1 family from Stream Processors, Inc with 40 and 80 general purpose ALUs per chip, all programmable in C as a SIMD engine and Picochip with 300 processors on a single die, focused on communication applications."
+3090,"In heterogeneous computing, where a system uses more than one kind of processor or cores, multi-core solutions are becoming more common: Xilinx Zynq UltraScale+ MPSoC has a quad-core ARM Cortex-A53 and dual-core ARM Cortex-R5. Software solutions such as OpenAMP are being used to help with inter-processor communication."
+3091,Mobile devices may use the ARM big.LITTLE architecture.
+3092,"The research and development of multicore processors often compares many options, and benchmarks are developed to help such evaluations. Existing benchmarks include SPLASH-2, PARSEC, and COSMIC for heterogeneous systems."
+3093,"^  Digital signal processors  have used multi-core architectures for much longer than high-end general-purpose processors. A typical example of a DSP-specific implementation would be a combination of a RISC CPU and a DSP MPU. This allows for the design of products that require a general-purpose processor for user interfaces and a DSP for real-time data processing; this type of design is common in mobile phones. In other applications, a growing number of companies have developed multi-core DSPs with very large numbers of processors."
+3094,"^  Two types of operating systems are able to use a dual-CPU multiprocessor: partitioned multiprocessing and symmetric multiprocessing . In a partitioned architecture, each CPU boots into separate segments of physical memory and operate independently; in an SMP OS, processors work in a shared space, executing threads within the OS independently."
+3095,"Multiprocessing is the use of two or more central processing units  within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There are many variations on this basic theme, and the definition of multiprocessing can vary with context, mostly as a function of how CPUs are defined ."
+3096,"According to some on-line dictionaries, a multiprocessor is a computer system having two or more processing units  each sharing main memory and peripherals, in order to simultaneously process programs. A 2009 textbook defined multiprocessor system similarly, but noting that the processors may share ""some or all of the system’s memory and I/O facilities""; it also gave tightly coupled system as a synonymous term."
+3097,"At the operating system level, multiprocessing is sometimes used to refer to the execution of multiple concurrent processes in a system, with each process running on a separate CPU or core, as opposed to a single process at any one instant. When used with this definition, multiprocessing is sometimes contrasted with multitasking, which may use just a single processor but switch it in time slices between tasks . Multiprocessing however means true parallel execution of multiple processes using more than one processor. Multiprocessing doesn't necessarily mean that a single process or task uses more than one processor simultaneously; the term parallel processing is generally used to denote that scenario. Other authors prefer to refer to the operating system techniques as multiprogramming and reserve the term multiprocessing for the hardware aspect of having more than one processor. The remainder of this article discusses multiprocessing only in this hardware sense."
+3098,"In Flynn's taxonomy, multiprocessors as defined above are MIMD machines. As the term ""multiprocessor"" normally refers to tightly coupled systems in which all processors share memory, multiprocessors are not the entire class of MIMD machines, which also contains message passing multicomputer systems."
+3099,"In a multiprocessing system, all CPUs may be equal, or some may be reserved for special purposes.  A combination of hardware and operating system software design considerations determine the symmetry  in a given system.  For example, hardware or software considerations may require that only one particular CPU respond to all hardware interrupts, whereas all other work in the system may be distributed equally among CPUs; or execution of kernel-mode code may be restricted to only one particular CPU, whereas user-mode code may be executed in any combination of processors.  Multiprocessing systems are often easier to design if such restrictions are imposed, but they tend to be less efficient than systems in which all CPUs are utilized."
+3100,"Systems that treat all CPUs equally are called symmetric multiprocessing  systems.  In systems where all CPUs are not equal, system resources may be divided in a number of ways, including asymmetric multiprocessing , non-uniform memory access  multiprocessing, and clustered multiprocessing."
+3101,"In a master/slave multiprocessor system, the master CPU is in control of the computer and the slave CPU performs assigned tasks.  The CPUs can be completely different in terms of speed and architecture.  Some  of the CPUs can share a common bus, each can also have a private bus , or they may be isolated except for a common communications pathway.  Likewise, the CPUs can share common RAM and/or have private RAM that the other processor cannot access.  The roles of master and slave can change from one CPU to another."
+3102,Two early examples of a mainframe master/slave multiprocessor are the Bull Gamma 60 and the Burroughs B5000.
+3103,"An early example of a master/slave multiprocessor system of microprocessors is the Tandy/Radio Shack TRS-80 Model 16 desktop computer which came out in February 1982 and ran the multi-user/multi-tasking Xenix operating system, Microsoft's version of UNIX .  The Model 16 has two microprocessors: an 8-bit Zilog Z80 CPU running at 4 MHz, and  a 16-bit Motorola 68000 CPU running at 6 MHz. When the system is booted, the Z-80 is the master and the Xenix boot process initializes the slave 68000, and then transfers control to the 68000, whereupon the CPUs change roles and the Z-80 becomes a slave processor responsible for all I/O operations including disk, communications, printer and network, as well as the keyboard and integrated monitor, while the operating system and applications run on the 68000 CPU. The Z-80 can be used to do other tasks."
+3104,"The earlier TRS-80 Model II, which was released in 1979, could also be considered a multiprocessor system as it had both a Z-80 CPU and an Intel 8021 microcontroller in the keyboard.  The 8021 made the Model II the first desktop computer system with a separate detachable lightweight keyboard connected with by a single thin flexible wire, and likely the first keyboard to use a dedicated microcontroller, both attributes that would later be copied years later by Apple and IBM."
+3105,"In multiprocessing, the processors can be used to execute a single sequence of instructions in multiple contexts , multiple sequences of instructions in a single context , or multiple sequences of instructions in multiple contexts ."
+3106,"Tightly coupled multiprocessor systems contain multiple CPUs that are connected at the bus level.  These CPUs may have access to a central shared memory , or may participate in a memory hierarchy with both local and shared memory . The IBM p690 Regatta is an example of a high end SMP system. Intel Xeon processors dominated the multiprocessor market for business PCs and were the only major x86 option until the release of AMD's Opteron range of processors in 2004. Both ranges of processors had their own onboard cache but provided access to shared memory; the Xeon processors via a common pipe and the Opteron processors via independent pathways to the system RAM."
+3107,"Chip multiprocessors, also known as multi-core computing, involves more than one processor placed on a single chip and can be thought of the most extreme form of tightly coupled multiprocessing. Mainframe systems with multiple processors are often tightly coupled."
+3108,Loosely coupled multiprocessor systems  are based on multiple standalone relatively low processor count commodity computers interconnected via a high speed communication system . A Linux Beowulf cluster is an example of a loosely coupled system.
+3109,"Tightly coupled systems perform better and are physically smaller than loosely coupled systems, but have historically required greater initial investments and may depreciate rapidly; nodes in a loosely coupled system are usually inexpensive commodity computers and can be recycled as independent machines upon retirement from the cluster."
+3110,"Power consumption is also a consideration. Tightly coupled systems tend to be much more energy-efficient than clusters. This is because a considerable reduction in power consumption can be realized by designing components to work together from the beginning in tightly coupled systems, whereas loosely coupled systems use components that were not necessarily intended specifically for use in such systems."
+3111,Loosely coupled systems have the ability to run different operating systems or OS versions on different systems.
+3112,"The user can be the source of an event. The user may interact with the software through the computer's peripherals - for example, by typing on a akeyboard or clicking with a mouse. Another source is a hardware device such as a timer. Software can also trigger its own set of events into the event loop, e.g. to communicate the completion of a task. Software that changes its behavior in response to events is said to be event-driven, often with the goal of being interactive."
+3113,"Event driven systems are typically used when there is some asynchronous external activity that needs to be handled by a program; for example, a user who presses a button on their mouse. An event driven system typically runs an event loop, that keeps waiting for such activities, e.g. input from devices or internal alarms. When one of these occurs, it collects data about the event and dispatches the event to the event handler software that will deal with it."
+3114,"A program can choose to ignore events, and there may be libraries to dispatch an event to multiple handlers that may be programmed to listen for a particular event. The data associated with an event at a minimum specifies what type of event it is, but may include other information such as when it occurred, who or what caused it to occur, and extra data provided by the event source to the handler about how the event should be processed."
+3115,"Events are typically used in user interfaces, where actions in the outside world  are handled by the program as a series of events. Programs written for many windowing environments consist predominantly of event handlers."
+3116,"Events can also be used at instruction set level, where they complement interrupts. Compared to interrupts, events are normally implemented synchronously: the program explicitly waits for an event to be generated and handled , whereas an interrupt can demand immediate service."
+3117,There are a large number of situations or events that a program or system may generate or respond to. Some common user generated events include:
+3118,"A pointing device can generate a number of software recognisable pointing device gestures. A mouse can generate a number of mouse events, such as mouse move , mouse left/right button up/down and mouse wheel motion, or a combination of these gestures. For example, double-clicks commonly select words and characters within boundary, and triple-clicks entire paragraphs."
+3119,"Pressing a key on a keyboard or a combination of keys generates a keyboard event, enabling the program currently running to respond to the introduced data such as which key/s the user pressed."
+3120,Moving a joystick generates an X-Y analogue signal. They often have multiple buttons to trigger events. Some gamepads for popular game boxes use joysticks.
+3121,The events generated using a touchscreen are commonly referred to as touch events or gestures.
+3122,"Device events include action by or to a device, such as a shake, tilt, rotation, move etc."
+3123,"A common variant in object-oriented programming is the delegate event model, which is provided by some graphic user interfaces. This model is based on three entities:"
+3124,"Furthermore, the model requires that:"
+3125,"C# uses events as special delegates that can only be fired by the class that declares it. This allows for better abstraction, for example:"
+3126,"In computer programming, an event handler may be implemented using a callback subroutine that handles inputs received in a program . Each event is a piece of application-level information from the underlying framework, typically the GUI toolkit. GUI events include key presses, mouse movement, action selections, and timers expiring. On a lower level, events can represent availability of new data for reading a file or network stream. Event handlers are a central concept in event-driven programming."
+3127,"The events are created by the framework based on interpreting lower-level inputs, which may be lower-level events themselves. For example, mouse movements and clicks are interpreted as menu selections. The events initially originate from actions on the operating system level, such as interrupts generated by hardware devices, software interrupt instructions, or state changes in polling. On this level, interrupt handlers and signal handlers correspond to event handlers."
+3128,"Created events are first processed by an event dispatcher within the framework. It typically manages the associations between events and event handlers, and may queue event handlers or events for later processing. Event dispatchers may call event handlers directly, or wait for events to be dequeued with information about the handler to be executed."
+3129,Event notification is a term used in conjunction with communications software for linking applications that generate small messages  to applications that monitor the associated conditions and may take actions triggered by events.
+3130,"Event notification is an important feature in modern database systems , modern operating systems , and modern distributed systems, where the producer of an event might be on a different machine than the consumer, or consumers. Event notification platforms are normally designed so that the application producing events does not need to know which applications will consume them, or even how many applications will monitor the event stream."
+3131,"It is sometimes used as a synonym for publish-subscribe, a term that relates to one class of products supporting event notification in networked settings. The virtual synchrony model is sometimes used to endow event notification systems, and publish-subscribe systems, with stronger fault-tolerance and consistency guarantees."
+3132,"Interrupts are commonly used by hardware devices to indicate electronic or physical state changes that require time-sensitive attention. Interrupts are also commonly used to implement computer multitasking and system calls, especially in real-time computing. Systems that use interrupts in these ways are said to be interrupt-driven."
+3133,"Hardware interrupts were introduced as an optimization, eliminating unproductive waiting time in polling loops, waiting for external events. The first system to use this approach was the DYSEAC, completed in 1954, although earlier systems provided error trap functions."
+3134,"The UNIVAC 1103A computer is generally credited with the earliest use of interrupts in 1953. Earlier, on the UNIVAC I  ""Arithmetic overflow either triggered the execution of a two-instruction fix-up routine at address 0, or, at the programmer's option, caused the computer to stop."" The IBM 650  incorporated the first occurrence of interrupt masking. The National Bureau of Standards DYSEAC  was the first to use interrupts for I/O. The IBM 704 was the first to use interrupts for debugging, with a ""transfer trap"", which could invoke a special routine when a branch instruction was encountered. The MIT Lincoln Laboratory TX-2 system  was the first to provide multiple levels of priority interrupts."
+3135,"Interrupt signals may be issued in response to hardware or software events. These are classified as hardware interrupts or software interrupts, respectively. For any particular processor, the number of interrupt types is limited by the architecture."
+3136,"A hardware interrupt is a condition related to the state of the hardware that may be signaled by an external hardware device, e.g., an interrupt request  line on a PC, or detected by devices embedded in processor logic , to communicate that the device needs attention from the operating system  or, if there is no OS, from the bare metal program running on the CPU. Such external devices may be part of the computer  or they may be external peripherals. For example, pressing a keyboard key or moving a mouse plugged into a PS/2 port triggers hardware interrupts that cause the processor to read the keystroke or mouse position."
+3137,"Hardware interrupts can arrive asynchronously with respect to the processor clock, and at any time during instruction execution. Consequently, all incoming hardware interrupt signals are conditioned by synchronizing them to the processor clock, and acted upon only at instruction execution boundaries."
+3138,"In many systems, each device is associated with a particular IRQ signal. This makes it possible to quickly determine which hardware device is requesting service, and to expedite servicing of that device."
+3139,"On some older systems, such as the 1964 CDC 3600, all interrupts went to the same location, and the OS used a specialized instruction to determine the highest-priority outstanding unmasked interrupt. On contemporary systems, there is generally a distinct interrupt routine for each type of interrupt , often implemented as one or more interrupt vector tables."
+3140,"To mask an interrupt is to disable it, so it is deferred or ignored by the processor, while to unmask an interrupt is to enable it."
+3141,"Processors typically have an internal interrupt mask register, which allows selective enabling  of hardware interrupts. Each interrupt signal is associated with a bit in the mask register. On some systems, the interrupt is enabled when the bit is set, and disabled when the bit is clear. On others, the reverse is true, and a set bit disables the interrupt. When the interrupt is disabled, the associated interrupt signal may be ignored by the processor, or it may remain pending. Signals which are affected by the mask are called maskable interrupts."
+3142,"Some interrupt signals are not affected by the interrupt mask and therefore cannot be disabled; these are called non-maskable interrupts . These indicate high-priority events which cannot be ignored under any circumstances, such as the timeout signal from a watchdog timer. With regard to SPARC, the Non-Maskable Interrupt , despite having the highest priority among interrupts, can be prevented from occurring through the use of an interrupt mask."
+3143,"One failure mode is when the hardware does not generate the expected interrupt for a change in state, causing the operating system to wait indefinitely. Depending on the details, the failure might affect only a single process or might have global impact. Some operating systems have code specifically to deal with this."
+3144,"As an example, IBM Operating System/360  relies on a not-ready to ready device-end interrupt when a tape has been mounted on a tape drive, and will not read the tape label until that interrupt occurs or is simulated. IBM added code in OS/360 so that the VARY ONLINE command will simulate a device end interrupt on the target device."
+3145,"A spurious interrupt is a hardware interrupt for which no source can be found.  The term ""phantom interrupt"" or ""ghost interrupt"" may also be used to describe this phenomenon.  Spurious interrupts tend to be a problem with a wired-OR interrupt circuit attached to a level-sensitive processor input.  Such interrupts may be difficult to identify when a system misbehaves."
+3146,"In a wired-OR circuit, parasitic capacitance charging/discharging through the interrupt line's bias resistor will cause a small delay before the processor recognizes that the interrupt source has been cleared.  If the interrupting device is cleared too late in the interrupt service routine , there will not be enough time for the interrupt circuit to return to the quiescent state before the current instance of the ISR terminates.  The result is the processor will think another interrupt is pending, since the voltage at its interrupt request input will be not high or low enough to establish an unambiguous internal logic 1 or logic 0.  The apparent interrupt will have no identifiable source, hence the ""spurious"" moniker."
+3147,"A spurious interrupt may also be the result of electrical anomalies due to faulty circuit design, high noise levels, crosstalk, timing issues, or more rarely, device errata."
+3148,"A spurious interrupt may result in system deadlock or other undefined operation if the ISR does not account for the possibility of such an interrupt occurring.  As spurious interrupts are mostly a problem with wired-OR interrupt circuits, good programming practice in such systems is for the ISR to check all interrupt sources for activity and take no action  if none of the sources is interrupting. They may even lead to crashing of the computer in adverse scenarios."
+3149,A software interrupt is requested by the processor itself upon executing particular instructions or when certain conditions are met. Every software interrupt signal is associated with a particular interrupt handler.
+3150,"A software interrupt may be intentionally caused by executing a special instruction which, by design, invokes an interrupt when executed. Such instructions function similarly to subroutine calls and are used for a variety of purposes, such as requesting operating system services and interacting with device drivers . Software interrupts may also be triggered by program execution errors or by the virtual memory system."
+3151,"Typically, the operating system kernel will catch and handle such interrupts. Some interrupts are handled transparently to the program - for example, the normal resolution of a page fault is to make the required page accessible in physical memory. But in other cases such as a segmentation fault the operating system executes a process callback. On Unix-like operating systems this involves sending a signal such as SIGSEGV, SIGBUS, SIGILL or SIGFPE, which may either call a signal handler or execute a default action . On Windows the callback is made using Structured Exception Handling with an exception code such as STATUS_ACCESS_VIOLATION or STATUS_INTEGER_DIVIDE_BY_ZERO."
+3152,"In a kernel process, it is often the case that some types of software interrupts are not supposed to happen. If they occur nonetheless, an operating system crash may result."
+3153,"The terms interrupt, trap, exception, fault, and abort are used to distinguish types of interrupts, although ""there is no clear consensus as to the exact meaning of these terms"". The term trap may refer to any interrupt, to any software interrupt, to any synchronous software interrupt, or only to interrupts caused by instructions with trap in their names. In some usages, the term trap refers specifically to a breakpoint intended to initiate a context switch to a monitor program or debugger. It may also refer to a synchronous interrupt caused by an exceptional condition , although the term exception is more common for this."
+3154,"x86 divides interrupts into  interrupts and software exceptions, and identifies three types of exceptions: faults, traps, and aborts.  interrupts are interrupts triggered asynchronously by an I/O device, and allow the program to be restarted with no loss of continuity. A fault is restartable as well but is tied to the synchronous execution of an instruction - the return address points to the faulting instruction. A trap is similar to a fault except that the return address points to the instruction to be executed after the trapping instruction; one prominent use is to implement system calls. An abort is used for severe errors, such as hardware errors and illegal values in system tables, and often does not allow a restart of the program."
+3155,"Arm uses the term exception to refer to all types of interrupts, and divides exceptions into  interrupts, aborts, reset, and exception-generating instructions. Aborts correspond to x86 exceptions and may be prefetch aborts  or data aborts , and may be synchronous or asynchronous. Asynchronous aborts may be precise or imprecise. MMU aborts  are synchronous."
+3156,RISC-V uses interrupt as the overall term as well as for the external subset; internal interrupts are called exceptions.
+3157,Each interrupt signal input is designed to be triggered by either a logic signal level or a particular signal edge . Level-sensitive inputs continuously request processor service so long as a particular  logic level is applied to the input. Edge-sensitive inputs react to signal edges: a particular  edge will cause a service request to be latched; the processor resets the latch when the interrupt handler executes.
+3158,"A level-triggered interrupt is requested by holding the interrupt signal at its particular  active logic level. A device invokes a level-triggered interrupt by driving the signal to and holding it at the active level. It negates the signal when the processor commands it to do so, typically after the device has been serviced."
+3159,The processor samples the interrupt input signal during each instruction cycle. The processor will recognize the interrupt request if the signal is asserted when sampling occurs.
+3160,"Level-triggered inputs allow multiple devices to share a common interrupt signal via wired-OR connections. The processor polls to determine which devices are requesting service. After servicing a device, the processor may again poll and, if necessary, service other devices before exiting the ISR."
+3161,"An edge-triggered interrupt is an interrupt signaled by a level transition on the interrupt line, either a falling edge  or a rising edge . A device wishing to signal an interrupt drives a pulse onto the line and then releases the line to its inactive state. If the pulse is too short to be detected by polled I/O then special hardware may be required to detect it. The important part of edge triggering is that the signal must transition to trigger the interrupt; for example, if the signal was high-low-low, there would only be one falling edge interrupt triggered, and the continued low level would not trigger a further interrupt. The signal must return to the high level and fall again in order to trigger a further interrupt. This contrasts with a level trigger where the low level would continue to create interrupts  until the signal returns to its high level."
+3162,Computers with edge-triggered interrupts may include an interrupt register that retains the status of pending interrupts. Systems with interrupt registers generally have interrupt mask registers as well.
+3163,"The processor samples the interrupt trigger signals or interrupt register during each instruction cycle, and will process the highest priority enabled interrupt found.
+Regardless of the triggering method, the processor will begin interrupt processing at the next instruction boundary following a detected trigger, thus ensuring:"
+3164,"There are several different architectures for handling interrupts. In some, there is a single interrupt handler that must scan for the highest priority enabled interrupt. In others, there are separate interrupt handlers for separate interrupt types, separate I/O channels or devices, or both.  Several interrupt causes may have the same interrupt type and thus the same interrupt handler, requiring the interrupt handler to determine the cause."
+3165,"Interrupts may be fully handled in hardware by the CPU, or may be handled by both the CPU and another component such as a programmable interrupt controller or a southbridge."
+3166,"If an additional component is used, that component would be connected between the interrupting device and the processor's interrupt pin to multiplex several sources of interrupt onto the one or two CPU lines typically available. If implemented as part of the memory controller, interrupts are mapped into the system's memory address space."
+3167,"In systems on a chip  implementations, interrupts come from different blocks of the chip and are usually aggregated in an interrupt controller attached to one or several processors ."
+3168,"Multiple devices may share an edge-triggered interrupt line if they are designed to.  The interrupt line must have a pull-down or pull-up resistor so that when not actively driven it settles to its inactive state, which is the default state of it. Devices signal an interrupt by briefly driving the line to its non-default state, and let the line float  when not signaling an interrupt. This type of connection is also referred to as open collector. The line then carries all the pulses generated by all the devices.  However, interrupt pulses from different devices may merge if they occur close in time. To avoid losing interrupts the CPU must trigger on the trailing edge of the pulse .  After detecting an interrupt the CPU must check all the devices for service requirements."
+3169,"Edge-triggered interrupts do not suffer the problems that level-triggered interrupts have with sharing.  Service of a low-priority device can be postponed arbitrarily, while interrupts from high-priority devices continue to be received and get serviced. If there is a device that the CPU does not know how to service, which may raise spurious interrupts, it will not interfere with interrupt signaling of other devices. However, it is easy for an edge-triggered interrupt to be missed - for example, when interrupts are masked for a period - and unless there is some type of hardware latch that records the event it is impossible to recover. This problem caused many ""lockups"" in early computer hardware because the processor did not know it was expected to do something. More modern hardware often has one or more interrupt status registers that latch interrupts requests; well-written edge-driven interrupt handling code can check these registers to ensure no events are missed."
+3170,"The elderly Industry Standard Architecture  bus uses edge-triggered interrupts, without mandating that devices be able to share IRQ lines, but all mainstream ISA motherboards include pull-up resistors on their IRQ lines, so well-behaved ISA devices sharing IRQ lines should just work fine. The parallel port also uses edge-triggered interrupts. Many older devices assume that they have exclusive use of IRQ lines, making it electrically unsafe to share them."
+3171,"There are 3 ways multiple devices ""sharing the same line"" can be raised.  First is by exclusive conduction  or exclusive connection .  Next is by bus : cards on a bus must know when they are to talk and not talk .  Talking can be triggered in two ways: by accumulation latch or by logic gates.  Logic gates expect a continual data flow that is monitored for key signals.  Accumulators only trigger when the remote side excites the gate beyond a threshold, thus no negotiated speed is required.  Each has its speed versus distance advantages.  A trigger, generally, is the method in which excitation is detected: rising edge, falling edge, threshold ."
+3172,"Triggering for software interrupts must be built into the software .  A 'C' app has a trigger table  in its header, which both the app and OS know of and use appropriately that is not related to hardware.  However do not confuse this with hardware interrupts which signal the CPU ."
+3173,"Multiple devices sharing an interrupt line  all act as spurious interrupt sources with respect to each other.  With many devices on one line, the workload in servicing interrupts grows in proportion to the square of the number of devices.  It is therefore preferred to spread devices evenly across the available interrupt lines.  Shortage of interrupt lines is a problem in older system designs where the interrupt lines are distinct physical conductors.  Message-signaled interrupts, where the interrupt line is virtual, are favored in new system architectures  and relieve this problem to a considerable extent."
+3174,"Some devices with a poorly designed programming interface provide no way to determine whether they have requested service. They may lock up or otherwise misbehave if serviced when they do not want it.  Such devices cannot tolerate spurious interrupts, and so also cannot tolerate sharing an interrupt line. ISA cards, due to often cheap design and construction, are notorious for this problem.  Such devices are becoming much rarer, as hardware logic becomes cheaper and new system architectures mandate shareable interrupts."
+3175,"Some systems use a hybrid of level-triggered and edge-triggered signaling. The hardware not only looks for an edge, but it also verifies that the interrupt signal stays active for a certain period of time."
+3176,"A common use of a hybrid interrupt is for the NMI  input. Because NMIs generally signal major – or even catastrophic – system events, a good implementation of this signal tries to ensure that the interrupt is valid by verifying that it remains active for a period of time. This 2-step approach helps to eliminate false interrupts from affecting the system."
+3177,"A message-signaled interrupt does not use a physical interrupt line.  Instead, a device signals its request for service by sending a short message over some communications medium, typically a computer bus. The message might be of a type reserved for interrupts, or it might be of some pre-existing type such as a memory write."
+3178,"Message-signalled interrupts behave very much like edge-triggered interrupts, in that the interrupt is a momentary signal rather than a continuous condition.  Interrupt-handling software treats the two in much the same manner. Typically, multiple pending message-signaled interrupts with the same message  are allowed to merge, just as closely spaced edge-triggered interrupts can merge."
+3179,"Message-signalled interrupt vectors can be shared, to the extent that the underlying communication medium can be shared. No additional effort is required."
+3180,"Because the identity of the interrupt is indicated by a pattern of data bits, not requiring a separate physical conductor, many more distinct interrupts can be efficiently handled.  This reduces the need for sharing. Interrupt messages can also be passed over a serial bus, not requiring any additional lines."
+3181,"PCI Express, a serial computer bus, uses message-signaled interrupts exclusively."
+3182,"In a push button analogy applied to computer systems, the term doorbell or doorbell interrupt is often used to describe a mechanism whereby a software system can signal or notify a computer hardware device that there is some work to be done.  Typically, the software system will place data in some well-known and mutually agreed upon memory locations, and ""ring the doorbell"" by writing to a different memory location.  This different memory location is often called the doorbell region, and there may even be multiple doorbells serving different purposes in this region. It is this act of writing to the doorbell region of memory that ""rings the bell"" and notifies the hardware device that the data are ready and waiting.  The hardware device would now know that the data are valid and can be acted upon.  It would typically write the data to a  hard disk drive, or send them over a network, or encrypt them, etc."
+3183,"The term doorbell interrupt is usually a misnomer.  It is similar to an interrupt, because it causes some work to be done by the device; however, the doorbell region is sometimes implemented as a polled region, sometimes the doorbell region writes through to physical device registers, and sometimes the doorbell region is hardwired directly to physical device registers. When either writing through or directly to physical device registers, this may cause a real interrupt to occur at the device's central processor unit , if it has one."
+3184,"Doorbell interrupts can be compared to Message Signaled Interrupts, as they have some similarities."
+3185,"In multiprocessor systems, a processor may send an interrupt request to another processor via inter-processor interrupts ."
+3186,"Interrupts provide low overhead and good latency at low load, but degrade significantly at high interrupt rate unless care is taken to prevent several pathologies. The phenomenon where the overall system performance is severely hindered by excessive amounts of processing time spent handling interrupts is called an interrupt storm."
+3187,"There are various forms of livelocks, when the system spends all of its time processing interrupts to the exclusion of other required tasks.
+Under extreme conditions, a large number of interrupts  may completely stall the system. To avoid such problems, an operating system must schedule network interrupt handling as carefully as it schedules process execution."
+3188,"With multi-core processors, additional performance improvements in interrupt handling can be achieved through receive-side scaling  when multiqueue NICs are used.  Such NICs provide multiple receive queues associated to separate interrupts; by routing each of those interrupts to different cores, processing of the interrupt requests triggered by the network traffic received by a single NIC can be distributed among multiple cores.  Distribution of the interrupts among cores can be performed automatically by the operating system, or the routing of interrupts  can be manually configured."
+3189,"A purely software-based implementation of the receiving traffic distribution, known as receive packet steering , distributes received traffic among cores later in the data path, as part of the interrupt handler functionality.  Advantages of RPS over RSS include no requirements for specific hardware, more advanced traffic distribution filters, and reduced rate of interrupts produced by a NIC.  As a downside, RPS increases the rate of inter-processor interrupts .  Receive flow steering  takes the software-based approach further by accounting for application locality; further performance improvements are achieved by processing interrupt requests by the same cores on which particular network packets will be consumed by the targeted application."
+3190,"Interrupts are commonly used to service hardware timers, transfer data to and from storage  and communication interfaces , handle keyboard and mouse events, and to respond to any other time-sensitive events as required by the application system. Non-maskable interrupts are typically used to respond to high-priority requests such as watchdog timer timeouts, power-down signals and traps."
+3191,"Hardware timers are often used to generate periodic interrupts. In some applications, such interrupts are counted by the interrupt handler to keep track of absolute or elapsed time, or used by the OS task scheduler to manage execution of running processes, or both. Periodic interrupts are also commonly used to invoke sampling from input devices such as analog-to-digital converters, incremental encoder interfaces, and GPIO inputs, and to program output devices such as digital-to-analog converters, motor controllers, and GPIO outputs."
+3192,"A disk interrupt signals the completion of a data transfer from or to the disk peripheral; this may cause a process to run which is waiting to read or write. A power-off interrupt predicts imminent loss of power, allowing the computer to perform an orderly shut-down while there still remains enough power to do so. Keyboard interrupts typically cause keystrokes to be buffered so as to implement typeahead."
+3193,"Interrupts are sometimes used to emulate instructions which are unimplemented on some computers in a product family. For example floating point instructions may be implemented in hardware on some systems and emulated on lower-cost systems. In the latter case, execution of an unimplemented floating point instruction will cause an ""illegal instruction"" exception interrupt. The interrupt handler will implement the floating point function in software and then return to the interrupted program as if the hardware-implemented instruction had been executed. This provides application software portability across the entire line."
+3194,"Interrupts are similar to signals, the difference being that signals are used for inter-process communication , mediated by the kernel  and handled by processes, while interrupts are mediated by the processor and handled by the kernel. The kernel may pass an interrupt as a signal to the process that caused it ."
+3195,"Multitasking does not require parallel execution of multiple tasks at exactly the same time; instead, it allows more than one task to advance over a given period of time.  Even on multiprocessor computers, multitasking allows many more tasks to be run than there are CPUs."
+3196,"Multitasking is a common feature of computer operating systems since at least the 1960s.  It allows more efficient use of the computer hardware; when a program is waiting for some external event such as a user input or an input/output transfer with a peripheral to complete, the central processor can still be used with another program. In a time-sharing system, multiple human operators use the same processor as if it was dedicated to their use, while behind the scenes the computer is serving many users by multitasking their individual programs. In multiprogramming systems, a task runs until it must wait for an external event or until the operating system's scheduler forcibly swaps the running task out of the CPU. Real-time systems such as those designed to control industrial robots, require timely processing; a single processor might be shared between calculations of machine movement, communications, and user interface."
+3197,"Often multitasking operating systems include measures to change the priority of individual tasks, so that important jobs receive more processor time than those considered less significant. Depending on the operating system, a task might be as large as an entire application program, or might be made up of smaller threads that carry out portions of the overall program."
+3198,"A processor intended for use with multitasking operating systems may include special hardware to securely support multiple tasks, such as memory protection, and protection rings that ensure the supervisory software cannot be damaged or subverted by user-mode program errors."
+3199,"The term ""multitasking"" has become an international term, as the same word is used in many other languages such as German, Italian, Dutch, Romanian, Czech, Danish and Norwegian."
+3200,"In the early days of computing, CPU time was expensive, and peripherals were very slow. When the computer ran a program that needed access to a peripheral, the central processing unit  would have to stop executing program instructions while the peripheral processed the data. This was usually very inefficient. Multiprogramming is a computing technique that enables multiple programs to be concurrently loaded and executed into a computer's memory, allowing the CPU to switch between them swiftly. This optimizes CPU utilization by keeping it engaged with the execution of tasks, particularly useful when one program is waiting for I/O operations to complete."
+3201,"The Bull Gamma 60, initially designed in 1957 and first released in 1960, was the first computer designed with multiprogramming in mind. Its architecture featured a central memory and a Program Distributor feeding up to twenty-five autonomous processing units with code and data, and allowing concurrent operation of multiple clusters."
+3202,"Another such computer was the LEO III, first released in 1961. During batch processing, several different programs were loaded in the computer memory, and the first one began to run. When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running."
+3203,"The use of multiprogramming was enhanced by the arrival of virtual memory and virtual machine technology, which enabled individual programs to make use of memory and operating system resources as if other concurrently running programs were, for all practical purposes, nonexistent."
+3204,"Multiprogramming gives no guarantee that a program will run in a timely manner. Indeed, the first program may very well run for hours without needing access to a peripheral. As there were no users waiting at an interactive terminal, this was no problem: users handed in a deck of punched cards to an operator, and came back a few hours later for printed results. Multiprogramming greatly reduced wait times when multiple batches were being processed."
+3205,"Early multitasking systems used applications that voluntarily ceded time to one another. This approach, which was eventually supported by many computer operating systems, is known today as cooperative multitasking. Although it is now rarely used in larger systems except for specific applications such as CICS or the JES2 subsystem, cooperative multitasking was once the only scheduling scheme employed by Microsoft Windows and classic Mac OS to enable multiple applications to run simultaneously.  Cooperative multitasking is still used today on RISC OS systems."
+3206,"As a cooperatively multitasked system relies on each process regularly giving up time to other processes on the system, one poorly designed program can consume all of the CPU time for itself, either by performing extensive calculations or by busy waiting; both would cause the whole system to hang. In a server environment, this is a hazard that makes the entire environment unacceptably fragile."
+3207,"Preemptive multitasking allows the computer system to more reliably guarantee to each process a regular ""slice"" of operating time. It also allows the system to deal rapidly with important external events like incoming data, which might require the immediate attention of one or another process.  Operating systems were developed to take advantage of these hardware capabilities and run multiple processes preemptively. Preemptive multitasking was implemented in the PDP-6 Monitor and Multics in 1964, in OS/360 MFT in 1967, and in Unix in 1969, and was available in some operating systems for computers as small as DEC's PDP-8; it is a core feature of all Unix-like operating systems, such as Linux, Solaris and BSD with its derivatives, as well as modern versions of Windows."
+3208,"At any specific time, processes can be grouped into two categories: those that are waiting for input or output , and those that are fully utilizing the CPU . In primitive systems, the software would often ""poll"", or ""busywait"" while waiting for requested input . During this time, the system was not performing useful work. With the advent of interrupts and preemptive multitasking, I/O bound processes could be ""blocked"", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution."
+3209,"The earliest preemptive multitasking OS available to home users was Sinclair QDOS on the Sinclair QL, released in 1984, but it was not a big success. Commodore's Amiga, released the following year, was the first commercially successful home computer to use the technology, and its multimedia abilities make it a clear ancestor of contemporary multitasking personal computers. Microsoft made preemptive multitasking a core feature of their flagship operating system in the early 1990s when developing Windows NT 3.1 and then Windows 95. In 1988 Apple offered A/UX as a UNIX System V-based alternative to the Classic Mac OS. In 2001 Apple switched to the NeXTSTEP-influenced Mac OS X."
+3210,"A similar model is used in Windows 9x and the Windows NT family, where native 32-bit applications are multitasked preemptively. 64-bit editions of Windows, both for the x86-64 and Itanium architectures, no longer support legacy 16-bit applications, and thus provide preemptive multitasking for all supported applications."
+3211,"Another reason for multitasking was in the design of real-time computing systems, where there are a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system is coupled with process prioritization to ensure that key activities were given a greater share of available process time."
+3212,"As multitasking greatly improved the throughput of computers, programmers started to implement applications as sets of cooperating processes . This, however, required some tools to allow processes to efficiently exchange data."
+3213,"Threads were born from the idea that the most efficient way for cooperating processes to exchange data would be to share their entire memory space. Thus, threads are effectively processes that run in the same memory context and share other resources with their parent processes, such as open files. Threads are described as lightweight processes because switching between threads does not involve changing the memory context."
+3214,"While threads are scheduled preemptively, some operating systems provide a variant to threads, named fibers, that are scheduled cooperatively. On operating systems that do not provide fibers, an application may implement its own fibers using repeated calls to worker functions. Fibers are even more lightweight than threads, and somewhat easier to program with, although they tend to lose some or all of the benefits of threads on machines with multiple processors."
+3215,Some systems directly support multithreading in hardware.
+3216,"Essential to any multitasking system is to safely and effectively share access to system resources. Access to memory must be strictly managed to ensure that no process can inadvertently or deliberately read or write to memory locations outside the process's address space. This is done for the purpose of general system stability and data integrity, as well as data security."
+3217,"In general, memory access management is a responsibility of the operating system kernel, in combination with hardware mechanisms that provide supporting functionalities, such as a memory management unit . If a process attempts to access a memory location outside its memory space, the MMU denies the request and signals the kernel to take appropriate actions; this usually results in forcibly terminating the offending process. Depending on the software and kernel design and the specific error in question, the user may receive an access violation error message such as ""segmentation fault""."
+3218,"In a well designed and correctly implemented multitasking system, a given process can never directly access memory that belongs to another process. An exception to this rule is in the case of shared memory; for example, in the System V inter-process communication mechanism the kernel allocates memory to be mutually shared by multiple processes. Such features are often used by database management software such as PostgreSQL."
+3219,"Inadequate memory protection mechanisms, either due to flaws in their design or poor implementations, allow for security vulnerabilities that may be potentially exploited by malicious software."
+3220,"Use of a swap file or swap partition is a way for the operating system to provide more memory than is physically available by keeping portions of the primary memory in secondary storage. While multitasking and memory swapping are two completely unrelated techniques, they are very often used together, as swapping memory allows more tasks to be loaded at the same time.  Typically, a multitasking system allows another process to run when the running process hits a point where it has to wait for some portion of memory to be reloaded from secondary storage."
+3221,Processes that are entirely independent are not much trouble to program in a multitasking environment. Most of the complexity in multitasking systems comes from the need to share computer resources between tasks and to synchronize the operation of co-operating tasks.
+3222,Various concurrent computing techniques are used to avoid potential problems caused by multiple tasks attempting to access the same resource.
+3223,"Bigger systems were sometimes built with a central processor and some number of I/O processors, a kind of asymmetric multiprocessing."
+3224,"Over the years, multitasking systems have been refined. Modern operating systems generally include detailed mechanisms for prioritizing processes, while symmetric multiprocessing has introduced new complexities and capabilities."
+3225,"3-D computer graphics, contrary to what the name suggests, are most often displayed on two-dimensional displays. Unlike 3-D film and similar techniques, the result is two-dimensional, without visual depth. More often, 3-D graphics are being displayed on 3-D displays, like in virtual reality systems."
+3226,3-D graphics stand in contrast to 2-D computer graphics which typically use completely different methods and formats for creation and rendering.
+3227,"3-D computer graphics rely on many of the same algorithms as 2-D computer vector graphics in the wire-frame model and 2-D computer raster graphics in the final rendered display. In computer graphics software, 2-D applications may use 3-D techniques to achieve effects such as lighting, and similarly, 3-D may use some 2-D rendering techniques."
+3228,"The objects in 3-D computer graphics are often referred to as 3-D models. Unlike the rendered image, a model's data is contained within a graphical data file. A 3-D model is a mathematical representation of any three-dimensional object; a model is not technically a graphic until it is displayed. A model can be displayed visually as a two-dimensional image through a process called 3-D rendering, or it can be used in non-graphical computer simulations and calculations. With 3-D printing, models are rendered into an actual 3-D physical representation of themselves, with some limitations as to how accurately the physical model can match the virtual model."
+3229,"William Fetter was credited with coining the term computer graphics in 1961 to describe his work at Boeing. An early example of interactive 3-D computer graphics was explored in 1963 by the Sketchpad program at Massachusetts Institute of Technology's Lincoln Laboratory. One of the first displays of computer animation was Futureworld , which included an animation of a human face and a hand that had originally appeared in the 1971 experimental short A Computer Animated Hand, created by University of Utah students Edwin Catmull and Fred Parke."
+3230,"3-D computer graphics software began appearing for home computers in the late 1970s. The earliest known example is 3D Art Graphics, a set of 3-D computer graphics effects, written by Kazumasa Mitazawa and released in June 1978 for the Apple II."
+3231,3-D computer graphics production workflow falls into three basic phases:
+3232,3-D modeling – the process of forming a computer model of an object's shape
+3233,Layout and CGI animation – the placement and movement of objects  within a scene
+3234,"3-D rendering – the computer calculations that, based on light placement, surface types, and other qualities, generate  an image"
+3235,"The model describes the process of forming the shape of an object. The two most common sources of 3-D models are those that an artist or engineer originates on the computer with some kind of 3D modeling tool, and models scanned into a computer from real-world objects . Models can also be produced procedurally or via physical simulation. Basically, a 3-D model is formed from points called vertices that define the shape and form polygons. A polygon is an area formed from at least three vertices . A polygon of n points is an n-gon. The overall integrity of the model and its suitability to use in animation depend on the structure of the polygons."
+3236,"Before rendering into an image, objects must be laid out in a 3D scene. This defines spatial relationships between objects, including location and size. Animation refers to the temporal description of an object . These techniques are often used in combination. As with animation, physical simulation also specifies motion."
+3237,"Materials and textures are properties that the render engine uses to render the model. One can give the model materials to tell the render engine how to treat light when it hits the surface. Textures are used to give the material color using a color or albedo map, or give the surface features using a bump map or normal map. It can be also used to deform the model itself using a displacement map."
+3238,"Rendering converts a model into an image either by simulating light transport to get photo-realistic images, or by applying an art style as in non-photorealistic rendering. The two basic operations in realistic rendering are transport  and scattering . This step is usually performed using 3-D computer graphics software or a 3-D graphics API. Altering the scene into a suitable form for rendering also involves 3-D projection, which displays a three-dimensional image in two dimensions. Although 3-D modeling and CAD software may perform 3-D rendering as well , exclusive 3-D rendering software also exists "
+3239,"3-D computer graphics software produces computer-generated imagery  through 3-D modeling and 3-D rendering or produces 3-D models for analytic, scientific and industrial purposes."
+3240,"There are many varieties of files supporting 3-D graphics, for example, Wavefront .obj files and .x DirectX files. Each file type generally tends to have its own unique data structure."
+3241,"Each file format can be accessed through their respective applications, such as DirectX files, and Quake. Alternatively, files can be accessed through third-party standalone programs, or via manual decompilation."
+3242,3-D modeling software is a class of 3-D computer graphics software used to produce 3-D models. Individual programs of this class are called modeling applications or modelers.
+3243,"3-D modeling starts by describing 3 display models : Drawing Points, Drawing Lines and Drawing triangles and other Polygonal patches."
+3244,"3-D modelers allow users to create and alter models via their 3-D mesh. Users can add, subtract, stretch and otherwise change the mesh to their desire. Models can be viewed from a variety of angles, usually simultaneously. Models can be rotated and the view can be zoomed in and out."
+3245,"3-D modelers can export their models to files, which can then be imported into other applications as long as the metadata are compatible. Many modelers allow importers and exporters to be plugged-in, so they can read and write data in the native formats of other applications."
+3246,"Most 3-D modelers contain a number of related features, such as ray tracers and other rendering alternatives and texture mapping facilities. Some also contain features that support or allow animation of models. Some may be able to generate full-motion video of a series of rendered scenes ."
+3247,"Computer aided design software may employ the same fundamental 3-D modeling techniques that 3-D modeling software use but their goal differs. They are used in computer-aided engineering, computer-aided manufacturing, Finite element analysis, product lifecycle management, 3D printing and computer-aided architectural design."
+3248,"After producing video, studios then edit or composite the video using programs such as Adobe Premiere Pro or Final Cut Pro at the mid-level, or Autodesk Combustion, Digital Fusion, Shake at the high-end. Match moving software is commonly used to match live video with computer-generated video, keeping the two in sync as the camera moves."
+3249,Use of real-time computer graphics engines to create a cinematic production is called machinima.
+3250,Not all computer graphics that appear 3D are based on a wireframe model. 2D computer graphics with 3D photorealistic effects are often achieved without wireframe modeling and are sometimes indistinguishable in the final form. Some graphic art software includes filters that can be applied to 2D vector graphics or 2D raster graphics on transparent layers. Visual artists may also copy or visualize 3D effects and manually render photorealistic effects without the use of filters.
+3251,"Some video games use 2.5D graphics, involving restricted projections of three-dimensional environments, such as isometric graphics or virtual cameras with fixed angles, either as a way to improve performance of the game engine or for stylistic and gameplay concerns. By contrast, games using 3D computer graphics without such restrictions are said to use true 3D."
+3252,"Although most laptop manufacturers no longer have optical drives bundled with their products, external drives are still available for purchase separately."
+3253,"As of 2021, most of the optical disc drives on the market are DVD-ROM drives and BD-ROM drives which read and record from those formats, along with having backward compatibility with CD, CD-R and CD-ROM discs; compact disc drives are no longer manufactured outside of audio devices. Read-only DVD and Blu-ray drives are also manufactured, but are less commonly found in the consumer market and mainly limited to media devices such as game consoles and disc media players. Over the last ten years, laptop computers no longer come with optical disc drives in order to reduce costs and make devices lighter, requiring consumers to purchase external optical drives."
+3254,"Optical disc drives are an integral part of standalone appliances such as CD players, DVD players, Blu-ray Disc players, DVD recorders, and video game consoles. As of 2017, the PlayStation and Xbox consoles are the only home video game consoles that are currently using optical discs as its primary storage format, as the Wii U's successor, the Nintendo Switch, began using game cartridges, while the PlayStation Portable is the only handheld console to use optical discs, using Sony's proprietary UMD format. They are also very commonly used in computers to read software and media distributed on disc and to record discs for archival and data exchange purposes. Floppy disk drives, with capacity of 1.44 MB, have been made obsolete: optical media are cheap and have vastly higher capacity to handle the large files used since the days of floppy discs, and the vast majority of computers and much consumer entertainment hardware have optical writers. USB flash drives, high-capacity, small, and inexpensive, are suitable where read/write capability is required."
+3255,"Disc recording is restricted to storing files playable on consumer appliances , relatively small volumes of data  for local use, and data for distribution, but only on a small scale; mass-producing large numbers of identical discs by pressing  is cheaper and faster than individual recording ."
+3256,"To support 8 centimetre diameter discs, drives with mechanical tray loading  have an indentation in the tray. It can however only be used in horizontal operation. 
+Slot loading drives, frequently used in game consoles and car radios, might be able to accept 8 centimetre discs and center the disc automatically."
+3257,"Optical discs are used to back up relatively small volumes of data, but backing up of entire hard drives, which as of 2015 typically contain many hundreds of gigabytes or even multiple terabytes, is less practical. Large backups are often instead made on external hard drives, as their price has dropped to a level making this viable; in professional environments magnetic tape drives are also used."
+3258,Some optical drives also allow predictively scanning the surface of discs for errors and detecting poor recording quality.
+3259,"With an option in the optical disc authoring software, optical disc writers are able to simulate the writing process on CD-R, CD-RW, DVD-R and DVD-RW, which allows for testing such as observing the writing speeds and patterns  with different writing speed settings and testing the highest capacity of an individual disc that would be achievable using overburning, without writing any data to the disc."
+3260,"Few optical drives allow simulating a FAT32 flash drive from optical discs containing ISO9660/Joliet and UDF file systems or audio tracks , for compatibility with most USB multimedia appliances."
+3261,Optical drives for computers come in two main form factors: half-height  and slim type . They exist as both internal and external variants.
+3262,"Half-height optical drives are around 4 centimetres tall, while slim type optical drives are around 1 cm tall."
+3263,"Half-height optical drives operate upwards of twice the speeds as slim type optical drives, because speeds on slim type optical drives are constrained to the physical limitations of the drive motor's rotation speed  rather than the performance of the optical pickup system."
+3264,"Because half-height demand much more electrical power and a voltage of 12 V DC, while slim optical drives run on 5 volts, external half height optical drives require separate external power input, while external slim type are usually able to operate entirely on power delivered through a computer's USB port.  Half height drives are also faster than Slim drives due to this, since more power is required to spin the disc at higher speeds."
+3265,Half-height optical drives hold discs in place from both sides while slim type optical drives fasten the disc from the bottom.
+3266,"Half height drives fasten the disc using 2 spindles containing a magnet each, one under and one above the disc tray. The spindles may be lined with flocking or a texturized silicone material to exert friction on the disc, to keep it from slipping. The upper spindle is left slightly loose and is attracted to the lower spindle because of the magnets they have. When the tray is opened, a mechanism driven by the movement of the tray pulls the lower spindle away from the upper spindle and vice versa when the tray is closed. When the tray is closed, the lower spindle touches the inner circumference of the disc, and slightly raises the disc from the tray to the upper spindle, which is attracted to the magnet on the lower disc, clamping the disc in place. Only the lower spindle is motorized. Trays in half height drives often fully open and close using a motorized mechanism that can be pushed to close, controlled by the computer, or controlled using a button on the drive. Trays on half height and slim drives can also be locked by whatever program is using it, however it can still be ejected by inserting the end of a paper clip into an emergency eject hole on the front of the drive. Early CD players such as the Sony CDP-101 used a separate motorized mechanism to clamp the disc to the motorized spindle."
+3267,"Slim drives use a special spindle with spring loaded specially shaped studs that radiate outwards, pressing against the inner edge of the disc. The user has to put uniform pressure onto the inner circumference of the disc to clamp it to the spindle and pull from the outer circumference while placing the thumb on the spindle to remove the disc, flexing it slightly in the process and returning to its normal shape after removal. The outer rim of the spindle may have a texturized silicone surface to exert friction keeping the disc from slipping. In slim drives most if not all components are on the disc tray, which pops out using a spring mechanism that can be controlled by the computer. These trays cannot close on their own; they have to be pushed until the tray reaches a stop. 
+"
+3268,"The most important part of an optical disc drive is an optical path, which is inside a pickup head . The PUH is also known as a laser pickup, optical pickup, pickup, pickup assembly, laser assembly, laser optical assembly, optical pickup head/unit or optical assembly. It usually consists of a semiconductor laser diode, a lens for focusing the laser beam, and photodiodes for detecting the light reflected from the disc's surface."
+3269,"Initially, CD-type lasers with a wavelength of 780 nm  were used. For DVDs, the wavelength was reduced to 650 nm , and for Blu-ray Disc this was reduced even further to 405 nm ."
+3270,"Two main servomechanisms are used, the first to maintain the proper distance between lens and disc, to ensure the laser beam is focused as a small laser spot on the disc. The second servo moves the pickup head along the disc's radius, keeping the beam on the track, a continuous spiral data path. Optical disc media are 'read' beginning at the inner radius to the outer edge."
+3271,"Near the laser lens, optical drives are usually equipped with one to three tiny potentiometers  that can be turned using a fine screwdriver. The potentiometer is in a series circuit with the laser lens."
+3272,"The laser diode used in DVD writers can have powers of up to 100 milliwatts, such high powers are used during writing. Some CD players have automatic gain control  to vary the power of the laser to ensure reliable playback of CD-RW discs."
+3273,"Readability  may vary among optical drives due to differences in optical pickup systems, firmwares, and damage patterns."
+3274,"On factory-pressed read only media , during the manufacturing process the tracks are formed by pressing a thermoplastic resin into a nickel stamper that was made by plating a glass 'master' with raised 'bumps' on a flat surface, thus creating pits and lands in the plastic disk. Because the depth of the pits is approximately one-quarter to one-sixth of the laser's wavelength, the reflected beam's phase is shifted in relation to the incoming beam, causing mutual destructive interference and reducing the reflected beam's intensity. This is detected by photodiodes that create corresponding electrical signals."
+3275,"An optical disk recorder encodes  data onto a recordable CD-R, DVD-R, DVD+R, or BD-R disc  by selectively heating  parts of an organic dye layer with a laser."
+3276,"This changes the reflectivity of the dye, thereby creating marks that can be read like the pits and lands on pressed discs. For recordable discs, the process is permanent and the media can be written to only once. While the reading laser is usually not stronger than 5 mW, the writing laser is considerably more powerful. DVD lasers operate at voltages of around 2.5 volts."
+3277,"The higher the writing speed, the less time a laser has to heat a point on the media, thus its power has to increase proportionally. DVD burners' lasers often peak at about 200 mW, either in continuous wave and pulses, although some have been driven up to 400 mW before the diode fails."
+3278,"For rewritable CD-RW, DVD-RW, DVD+RW, DVD-RAM, or BD-RE media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc.  Depending on the amount of power applied, the substance may be allowed to melt back  into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created."
+3279,"Double-sided media may be used, but they are not easily accessed with a standard drive, as they must be physically turned over to access the data on the other side."
+3280,"Double layer or dual layer  media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to change the laser's focus. Traditional single layer  writable media are produced with a spiral groove molded in the protective polycarbonate layer , to lead and synchronize the speed of recording head. Double-layered writable media have: a first polycarbonate layer with a  groove, a first data layer, a semi-reflective layer, a second  polycarbonate layer with another  groove, and a second data layer. The first groove spiral usually starts on the inner edge and extends outwards, while the second groove start on the outer edge and extends inwards."
+3281,"Some drives support Hewlett-Packard's LightScribe, or the alternative LabelFlash photothermal printing technology for labeling specially coated discs."
+3282,"Zen Technology and Sony have developed drives that use several laser beams simultaneously to read discs and write to them at higher speeds than what would be possible with a single laser beam. The limitation with a single laser beam comes from wobbling of the disc that may occur at high rotational speeds; at 25,000 RPMs CDs become unreadable while Blu-rays cannot be written to beyond 5,000 RPMs. With a single laser beam, the only way to increase read and write speeds without reducing the pit length of the disc  is by increasing the rotational speed of the disc which reads more pits in less time, increasing data rate; hence why faster drives spin the disc at higher speeds. In addition, CDs at 27,500 RPMs  may explode causing extensive damage to the disc's surroundings, and poor quality or damaged discs may explode at lower speeds."
+3283,"In Zen's system , a diffraction grating is used to split a laser beam into 7 beams, which are then focused into the disc; a central beam is used for focusing and tracking the groove of the disc leaving 6 remaining beams  that are spaced evenly to read 6 separate portions of the groove of the disc in parallel, effectively increasing read speeds at lower RPMs, reducing drive noise and 
+stress on the disc. The beams then reflect back from the disc, and are collimated and projected into a special photodiode array to be read. The first drives using the technology could read at 40x, later increasing to 52x and finally 72x. It uses a single optical pickup."
+3284,"In Sony's system  the drive has 4 optical pickups, two on each side of the disc, with each pickup having two lenses for a total of 8 lenses and laser beams. This allows for both sides of the disc to be read and written to at the same time, and for the contents of the disc to be verified during writing."
+3285,"The rotational mechanism in an optical drive differs considerably from that of a hard disk drive's, in that the latter keeps a constant angular velocity , in other words a constant number of revolutions per minute . With CAV, a higher throughput is generally achievable at the outer disc compared to the inner."
+3286,"On the other hand, optical drives were developed with an assumption of achieving a constant throughput, in CD drives initially equal to 150 KiB/s. It was a feature important for streaming audio data that always tend to require a constant bit rate. But to ensure no disc capacity was wasted, a head had to transfer data at a maximum linear rate at all times too, without slowing on the outer rim of the disc. This led to optical drives—until recently—operating with a constant linear velocity . The spiral groove of the disc passed under its head at a constant speed. The implication of CLV, as opposed to CAV, is that disc angular velocity is no longer constant, and the spindle motor needed to be designed to vary its speed from between 200 RPM on the outer rim and 500 RPM on the inner, keeping the data rate constant."
+3287,"Later CD drives kept the CLV paradigm, but evolved to achieve higher rotational speeds, popularly described in multiples of a base speed. As a result, a 4× CLV drive, for instance, would rotate at 800-2000 RPM, while transferring data steadily at 600 KiB/s, which is equal to 4 × 150 KiB/s."
+3288,"For DVDs, base or 1× speed is 1.385 MB/s, equal to 1.32 MiB/s, approximately nine times faster than the CD base speed. For Blu-ray drives, base speed is 6.74 MB/s, equal to 6.43 MiB/s."
+3289,"Because keeping a constant transfer rate for the whole disc is not so important in most contemporary CD uses, a pure CLV approach had to be abandoned to keep the rotational speed of the disc safely low while maximizing data rate. Some drives work in a partial CLV  scheme, by switching from CLV to CAV only when a rotational limit is reached. But switching to CAV requires considerable changes in hardware design, so instead most drives use the zoned constant linear velocity  scheme. This divides the disc into several zones, each having its own constant linear velocity. A Z-CLV recorder rated at ""52×"", for example, would write at 20× on the innermost zone and then progressively increase the speed in several discrete steps up to 52× at the outer rim. Without higher rotational speeds, increased read performance may be attainable by simultaneously reading more than one point of a data groove, also known as multi-beam, but drives with such mechanisms are more expensive, less compatible, and very uncommon."
+3290,Both DVDs and CDs have been known to explode when damaged or spun at excessive speeds. This imposes a constraint on the maximum safe speeds  at which drives can operate.
+3291,"The reading speeds of most half-height optical disc drives released since c. 2007 are limited to ×48 for CDs, ×16 for DVDs and ×12  for Blu-ray Discs. Writing speeds on selected write-once media are higher."
+3292,"Some optical drives additionally throttle the reading speed based on the contents of optical discs, such as max. 40× CAV  for the Digital Audio Extraction  of Audio CD tracks, 16× CAV for Video CD contents and even lower limitations on earlier models such as 4× CLV  for Video CDs."
+3293,"Current optical drives use either a tray-loading mechanism, where the disc is loaded onto a motorized tray , a manually operated tray , or a slot-loading mechanism, where the disc is slid into a slot and drawn in by motorized rollers. Slot-loading optical drives exist in both half-height  and slim type  form factors."
+3294,"With both types of mechanisms, if a CD or DVD is left in the drive after the computer is turned off, the disc cannot be ejected using the normal eject mechanism of the drive. However, tray-loading drives account for this situation by providing a small hole where one can insert a paperclip to manually open the drive tray to retrieve the disc."
+3295,"Slot-loading optical disc drives are prominently used in game consoles and vehicle audio units. Although allowing more convenient insertion, those have the disadvantages that they cannot usually accept the smaller 80 mm diameter discs  or any non-standard sizes, usually have no emergency eject hole or eject button, and therefore have to be disassembled if the optical disc cannot be ejected normally.  However, some slot-loading optical drives have been engineered to support miniature discs. The Nintendo Wii, because of backward compatibility with GameCube games, and PlayStation 3 video game consoles are able to load both standard size DVDs and 80 mm discs in the same slot-loading drive. Its successor's slot drive however, the Wii U, lacks miniature disc compatibility."
+3296,"There were also some early CD-ROM drives for desktop PCs in which its tray-loading mechanism will eject slightly and user has to pull out the tray manually to load a CD, similar to the tray ejecting method used in internal optical disc drives of modern laptops and modern external slim portable optical disc drives. Like the top-loading mechanism, they have spring-loaded ball bearings on the spindle."
+3297,"A small number of drive models, mostly compact portable units, have a top-loading mechanism where the drive lid is manually opened upwards and the disc is placed directly onto the spindle . These sometimes have the advantage of using spring-loaded ball bearings to hold the disc in place, minimizing damage to the disc if the drive is moved while it is spun up."
+3298,"Unlike tray and slot loading mechanisms by default, top-load optical drives can be opened without being connected to power."
+3299,"Some early CD-ROM drives used a mechanism where CDs had to be inserted into special cartridges or caddies, somewhat similar in appearance to a 3.5 inch micro floppy diskette. This was intended to protect the disc from accidental damage by enclosing it in a tougher plastic casing, but did not gain wide acceptance due to the additional cost and compatibility concerns—such drives would also inconveniently require ""bare"" discs to be manually inserted into an openable caddy before use. Ultra Density Optical , Magneto-optical drives, Universal Media Disc , DataPlay, Professional Disc, MiniDisc, Optical Disc Archive as well as early DVD-RAM and Blu-ray discs use optical disc cartridges."
+3300,"All optical disc-drives use the SCSI-protocol on a command bus level, and initial systems used either a fully featured SCSI bus or as these were somewhat cost-prohibitive to sell to consumer applications, a proprietary cost-reduced version of the bus. This is because conventional ATA-standards at the time did not support, or have any provisions for any sort of removable media or hot-plugging of disk drives. Most modern internal drives for personal computers, servers, and workstations are designed to fit in a standard 5+1⁄4-inch  drive bay and connect to their host via an ATA or SATA bus interface, but communicate using the SCSI protocol commands on software level as per the ATA Package Interface standard developed for making Parallel ATA/IDE interfaces compatible with removable media. Some devices may support vendor-specific commands such as recording density , laser power setting , ability to manually hard-limit rotation speed in a way that overrides the universal speed setting , and adjusting the lens and tray movement speeds where a lower setting reduces noise, as implmenented on some Plextor drives, as well as the ability to force overspeed burning, meaning beyond speed recommended for the media type, for testing purposes, as implemented on some Lite-On drives. Additionally, there may be digital and analog outputs for audio. The outputs may be connected via a header cable to the sound card or the motherboard or to headphones or an external speaker with a 3.5mm AUX plug cable that many early optical drives are equipped with. At one time, computer software resembling CD players controlled playback of the CD. Today the information is extracted from the disc as digital data, to be played back or converted to other file formats."
+3301,"Some early optical drives have dedicated buttons for CD playback controls on their front panel, allowing them to act as a standalone compact disc player."
+3302,"External drives were popular in the beginning, because the drives often required complex electronics to institute, rivaling in complexity the Host computer system itself. External drives using SCSI, Parallel port, USB and FireWire interfaces exist, most modern drives being USB. Some portable versions for laptops power themselves from batteries or directly from their interface bus."
+3303,"Drives with a SCSI interface were originally the only system interface available, but they never became popular in the price sensitive low-end consumer market which constituted majority of the demand. They were less common and tended to be more expensive, because of the cost of their interface chipsets, more complex SCSI connectors, and small volume of sales in comparison to proprietary cost-reduced applications, but most importantly because most consumer market computer systems did not have any sort of SCSI interface in them the market for them was small. However, support for multitude of various cost-reduced proprietary optical drive bus standards were usually embedded with sound cards which were often bundled with the optical drives themselves in the early years. Some sound card and optical drive bundles even featured a full SCSI bus. Modern IDE/ATAPI compliant Parallel ATA and Serial ATA drive control chipsets and their interface technology is more complex to manufacture than a traditional 8bit 50Mhz SCSI drive interface, because they feature properties of both the SCSI and ATA bus, but are cheaper to make overall due to economies of scale."
+3304,"When the optical disc drive was first developed, it was not easy to add to computer systems. Some computers such as the IBM PS/2 were standardizing on the 3+1⁄2-inch floppy and 3+1⁄2-inch hard disk and did not include a place for a large internal device. Also IBM PCs and clones at first only included a single  ATA drive interface, which by the time the CD-ROM was introduced, was already being used to support two hard drives and were completely incapable of supporting removable media, a drive falling off or being removed from the bus while the system was live, would cause an unrecoverable error and crash the entire system. Early consumer grade laptops simply had no built-in high-speed interface for supporting an external storage device. High-end workstation systems and laptops featured a SCSI interface which had a standard for externally connected devices."
+3305,This was solved through several techniques:
+3306,"Due to lack of asynchrony in existing implementations, an optical drive encountering damaged sectors may cause computer programs trying to access the drives, such as Windows Explorer, to lock up."
+3307,"Drive models may support adjustment of behavioural parameters for performance optimization and testing purposes, such as the read retry count , write retry count , and the option to deactivate error correction . For example, the read retry count specifies how often the drive should attempt reading a damaged sector. A higher value may increase the chance of successfully reading individual damaged sectors, but at the expense of responsiveness, since it adds delays during which the device seems unresponsive to the computer."
+3308,"The sdparm command-line utility allows manually controlling such parameters. For example, sdparm --set RRC=10 /dev/sr0 sets the read retry count to 10 for the optical drive device file ""sr0"", and sdparm --all /dev/sr0 lists all code pages. The values may be interpreted varyingly among drive models or vendors."
+3309,The optical drives in the photos are shown right side up; the disc would sit on top of them. The laser and optical system scans the underside of the disc.
+3310,"With reference to the top photo, just to the right of image center is the disc motor, a metal cylinder, with a gray centering hub and black rubber drive ring on top. There is a disc-shaped round clamp, loosely held inside the cover and free to rotate; it's not in the photo. After the disc tray stops moving inward, as the motor and its attached parts rise, a magnet near the top of the rotating assembly contacts and strongly attracts the clamp to hold and center the disc. This motor is an ""outrunner""-style brushless DC motor which has an external rotor – every visible part of it spins."
+3311,"Two parallel guide rods that run between upper left and lower right in the photo carry the ""sled"", the moving optical read-write head. As shown, this ""sled"" is close to, or at the position where it reads or writes at the edge of the disc. To move the ""sled"" during continuous read or write operations, a stepper motor rotates a leadscrew to move the ""sled"" throughout its total travel range. The motor, itself, is the short gray cylinder just to the left of the most-distant shock mount; its shaft is parallel to the support rods. The leadscrew is the rod with evenly-spaced darker details; these are the helical grooves that engage a pin on the ""sled""."
+3312,"In contrast, the mechanism shown in the second photo, which comes from a cheaply made DVD player, uses less accurate and less efficient brushed DC motors to both move the sled and spin the disc. Some older drives use a DC motor to move the sled, but also have a magnetic rotary encoder to keep track of the position. Most drives in computers use stepper motors."
+3313,"The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise from residual imbalance when running fast. The soft shock mount grommets are just below the brass-colored screws at the four corners ."
+3314,"In the third photo, the components under the cover of the lens mechanism are visible. The two permanent magnets on either side of the lens holder as well as the coils that move the lens can be seen. This allows the lens to be moved up, down, forwards, and backwards to stabilize the focus of the beam."
+3315,"In the fourth photo, the inside of the optics package can be seen. Note that since this is a CD-ROM drive, there is only one laser, which is the black component mounted to the bottom left of the assembly. Just above the laser are the first focusing lens and prism that direct the beam at the disc. The tall, thin object in the center is a half-silvered mirror that splits the laser beam in multiple directions. To the bottom right of the mirror is the main photodiode that senses the beam reflected off the disc. Above the main photodiode is a second photodiode that is used to sense and regulate the power of the laser."
+3316,"The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are ""flexible circuits"" that connect everything to the electronics ."
+3317,"The first laser disc, demonstrated in 1972, was the Laservision 12-inch video disc. The video signal was stored as an analog format like a video cassette. The first digitally recorded optical disc was a 5-inch audio compact disc  in a read-only format created by Sony and Philips in 1975."
+3318,"The first erasable optical disc drives were announced in 1983, by Matsushita , Sony, and Kokusai Denshin Denwa . Sony eventually released the first commercial erasable and rewritable 5+1⁄4-inch optical disc drive in 1987, with dual-sided discs capable of holding 325 MB per side."
+3319,"The CD-ROM format was developed by Sony and Denon, introduced in 1984, as an extension of Compact Disc Digital Audio and adapted to hold any form of digital data. The CD-ROM format has a storage capacity of 650 MB. Also in 1984, Sony introduced a LaserDisc data storage format, with a larger data capacity of 3.28 GB."
+3320,"In September 1992, Sony announced the MiniDisc format, which was supposed to combine the audio clarity of CD's and the convenience of a cassette size. The standard capacity holds 80 minutes of audio. In January 2004, Sony revealed an upgraded Hi-MD format, which increased the capacity to 1 GB ."
+3321,"The DVD format, developed by Panasonic, Sony, and Toshiba, was released in 1995, and was capable of holding 4.7 GB per layer; with the first DVD players shipping on November 1, 1996, by Panasonic and Toshiba in Japan and the first DVD-ROM compatible computers being shipped on November 6 of that year by Fujitsu. Sales of DVD-ROM drives for computers in the U.S. began on March 24, 1997, when Creative Labs released their PC-DVD kit to the market."
+3322,"In 1999, Kenwood released a multi-beam optical drive that achieved burning speeds as high as 72×, which would require dangerous spinning speeds to attain with single-beam burning. However, it suffered from reliability issues."
+3323,"The first Blu-ray prototype was unveiled by Sony in October 2000, and the first commercial recording device was released to market on April 10, 2003. In January 2005, TDK announced that they had developed an ultra-hard yet very thin polymer coating  for Blu-ray Discs; this was a significant technical advance because better protection was desired for the consumer market to protect bare discs against scratching and damage compared to DVD. Technically Blu-ray Disc also required a thinner layer for the narrower beam and shorter wavelength 'blue' laser. The first BD-ROM players  were shipped in mid-June 2006. The first Blu-ray Disc titles were released by Sony and MGM on June 20, 2006. The first mass-market Blu-ray Disc rewritable drive for the PC was the BWU-100A, released by Sony on July 18, 2006."
+3324,"Starting in the mid-2010s, computer manufacturers began to stop including built-in optical disc drives on their products, with the advent of cheap, rugged , fast and high capacity USB drives and video on demand over the internet. Excluding an optical drive allows for circuit boards in laptops to be larger and less dense, requiring less layers, reducing production costs while also reducing weight and thickness, or for batteries to be larger. Computer case manufacturers also began to stop including 5+1⁄4-inch bays for installing optical disc drives. However, new optical disc drives are still  available for purchase. Notable optical disc drive OEMs include Hitachi, LG Electronics , Toshiba, Samsung Electronics , Sony, NEC , Lite-On, Philips , Pioneer Corporation, Plextor, Panasonic, Yamaha Corporation and Kenwood."
+3325,"Most optical drives are backward compatible with their ancestors up to CD, although this is not required by standards."
+3326,"Compared to a CD's 1.2 mm layer of polycarbonate, a DVD's laser beam only has to penetrate 0.6 mm in order to reach the recording surface. This allows a DVD drive to focus the beam on a smaller spot size and to read smaller pits. DVD lens supports a different focus for CD or DVD media with same laser.  With the newer Blu-ray Disc drives, the laser only has to penetrate 0.1 mm of material.  Thus the optical assembly would normally have to have an even greater focus range.  In practice, the Blu-ray optical system is separate from the DVD/CD system."
+3327,"During the times of CD writer drives, they are often marked with three different speed ratings. In these cases, the first speed is for write-once  operations, the second speed for re-write  operations, and the last speed for read-only  operations. For example, a 40×/16×/48× CD writer drive is capable of writing to CD-R media at speed of 40× , writing to CD-RW media at speed of 16× , and reading from a CD-ROM media at speed of 48× ."
+3328,"During the times of combo  drives, an additional speed rating  is designated for DVD-ROM media reading operations."
+3329,"For DVD writer drives, Blu-ray Disc combo drives, and Blu-ray Disc writer drives, the writing and reading speed of their respective optical media are specified in its retail box, user's manual, or bundled brochures or pamphlets."
+3330,"In the late 1990s, buffer underruns became a very common problem as high-speed CD recorders began to appear in home and office computers, which—for a variety of reasons—often could not muster the I/O performance to keep the data stream to the recorder steadily fed. The recorder, should it run short, would be forced to halt the recording process, leaving a truncated track that usually renders the disc useless."
+3331,"In response, manufacturers of CD recorders began shipping drives with ""buffer underrun protection"" . These can suspend and resume the recording process in such a way that the gap the stoppage produces can be dealt with by the error-correcting logic built into CD players and CD-ROM drives.  The first of these drives were rated at 12× and 16×."
+3332,"The first optical drive to support recording DVDs at 16× speed was the Pioneer DVR-108, released in the second half of 2004. At that time however, no recordable DVD media supported that high recording speed yet."
+3333,"While drives are burning DVD+R, DVD+RW and all Blu-ray formats, they do not require any such error correcting recovery as the recorder is able to place the new data exactly on the end of the suspended write effectively producing a continuous track .  Although later interfaces were able to stream data at the required speed, many drives now write in a 'zoned constant linear velocity' .  This means that the drive has to temporarily suspend the write operation while it changes speed and then recommence it once the new speed is attained.  This is handled in the same manner as a buffer underrun."
+3334,"The internal buffer of optical disc writer drives is: 8 MiB or 4 MiB when recording BD-R, BD-R DL, BD-RE, or BD-RE DL media; 2 MiB when recording DVD-R, DVD-RW, DVD-R DL, DVD+R, DVD+RW, DVD+RW DL, DVD-RAM, CD-R, or CD-RW media."
+3335,"CD recording on personal computers was originally a batch-oriented task in that it required specialised authoring software to create an ""image"" of the data to record and to record it to disc in the one session. This was acceptable for archival purposes, but limited the general convenience of CD-R and CD-RW discs as a removable storage medium."
+3336,"Packet writing is a scheme in which the recorder writes incrementally to disc in short bursts, or packets. Sequential packet writing fills the disc with packets from bottom up. To make it readable in CD-ROM and DVD-ROM drives, the disc can be closed at any time by writing a final table-of-contents to the start of the disc; thereafter, the disc cannot be packet-written any further. Packet writing, together with support from the operating system and a file system like UDF, can be used to mimic random write-access as in media like flash memory and magnetic disks."
+3337,"Fixed-length packet writing  divides up the disc into padded, fixed-size packets. The padding reduces the capacity of the disc, but allows the recorder to start and stop recording on an individual packet without affecting its neighbours.  These resemble the block-writable access offered by magnetic media closely enough that many conventional file systems will work as-is.  Such discs, however, are not readable in most CD-ROM and DVD-ROM drives or on most operating systems without additional third-party drivers.  The division into packets is not as reliable as it may seem as CD-R and DVD-R drives can only locate data to within a data block.  Although generous gaps  are left between blocks, the drive nevertheless can occasionally miss and either destroy some existing data or even render the disc unreadable."
+3338,The DVD+RW disc format eliminates this unreliability by embedding more accurate timing hints in the data groove of the disc and allowing individual data blocks  to be replaced without affecting backward compatibility . The format itself was designed to deal with discontinuous recording because it was expected to be widely used in digital video recorders.  Many such DVRs use variable-rate video compression schemes which require them to record in short bursts; some allow simultaneous playback and recording by alternating quickly between recording to the tail of the disc whilst reading from elsewhere.  The Blu-ray Disc system also encompasses this technology.
+3339,"Mount Rainier aims to make packet-written CD-RW and DVD+RW discs as convenient to use as that of removable magnetic media by having the firmware format new discs in the background and manage media defects .  As of February 2007, support for Mount Rainier is natively supported in Windows Vista.  All previous versions of Windows require a third-party solution, as does Mac OS X."
+3340,"Owing to pressure from the music industry, as represented by the IFPI and RIAA, Philips developed the Recorder Identification Code  to allow media to be uniquely associated with the recorder that has written it. This standard is contained in the Rainbow Books. The RID-Code consists of a supplier code , a model number and the unique ID of the recorder. Quoting Philips, the RID ""enables a trace for each disc back to the exact machine on which it was made using coded information in the recording itself. The use of the RID code is mandatory."""
+3341,"Although the RID was introduced for music and video industry purposes, the RID is included on every disc written by every drive, including data and backup discs.  The value of the RID is questionable as it is  impossible to locate any individual recorder due to there being no database."
+3342,"The Source Identification Code  is an eight character supplier code that is placed on optical discs by the manufacturer. The SID identifies not only manufacturer, but also the individual factory and machine that produced the disc."
+3343,"According to Phillips, the administrator of the SID codes, the SID code provides an optical disc production facility with the means to identify all discs mastered or replicated in its plant, including the specific Laser Beam Recorder  signal processor or mould that produced a particular stamper or disc."
+3344,"The standard use of RID and SID mean that each disc written contains a record of the machine that produced a disc , and which drive wrote it . This combined knowledge may be very useful to law enforcement, to investigative agencies, and to private or corporate investigators."
+3345,"A significant motivation for introducing the SID code was to identify disc manufacturing plants producing unauthorised copies of commercial CDs. By the 1990s, the production process for CDs had evolved from requiring a ""clean-room"" environment involving multiple processes, this demanding a substantial investment and likely to be confined to ""responsible"" organisations, into an activity that could be undertaken with ""mono-liner"" equipment, this having been developed in the late 1980s and capable of packaging ""the whole process into a single box"" that could occupy ""no more space than a couple of office desks"". Consequently, the CD manufacturing industry had grown to include less reputable organisations and, by 1994, could produce a volume of discs twice that of the estimated demand for ""legitimate CDs"", with music industry organisations claiming that illicit copies were outselling legitimate copies by significant margins in some markets. Philips and the IFPI envisaged that combinations of codes, each identifying a disc mastering establishment and the manufacturing plant used to make a particular disc, would assist in identifying those responsible for illicit CD production. However, the scheme relied on existing manufacturing plants upgrading their equipment to support the introduction of this measure, and the accompanying challenge of convincing such facilities was perceived as ""a little difficult"" in cases where those facilities were already involved in making considerable numbers of illicit discs."
+3346,A floppy disk or floppy diskette  is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk. Floppy disks store digital data which can be read and written when the disk is inserted into a floppy disk drive  connected to or inside a computer or other device.
+3347,"The first floppy disks, invented and made by IBM, had a disk diameter of 8 inches . Subsequently, the 5¼-inch  and then the 3½-inch  became a ubiquitous form of data storage and transfer into the first years of the 21st century. 3½-inch floppy disks can still be used with an external USB floppy disk drive. USB drives for 5¼-inch, 8-inch, and other-size floppy disks are rare to non-existent. Some individuals and organizations continue to use older equipment to read or transfer data from floppy disks."
+3348,"Floppy disks were so common in late 20th-century culture that many electronic and software programs continue to use save icons that look like floppy disks well into the 21st century, as a form of skeuomorphic design. While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have been superseded by data storage methods with much greater data storage capacity and data transfer speed, such as USB flash drives, memory cards, optical discs, and storage available through local computer networks and cloud storage."
+3349,"The first commercial floppy disks, developed in the late 1960s, were 8 inches  in diameter; they became commercially available in 1971 as a component of IBM products and both drives and disks were then sold separately starting in 1972 by Memorex and others. These disks and associated drives were produced and improved upon by IBM and other companies such as Memorex, Shugart Associates, and Burroughs Corporation. The term ""floppy disk"" appeared in print as early as 1970, and although IBM announced its first media as the Type 1 Diskette in 1973, the industry continued to use the terms ""floppy disk"" or ""floppy""."
+3350,"In 1976, Shugart Associates introduced the 5¼-inch FDD. By 1978, there were more than ten manufacturers producing such FDDs. There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as differential Manchester encoding , modified frequency modulation , M2FM and group coded recording . The 5¼-inch format displaced the 8-inch one for most uses, and the hard-sectored disk format disappeared. The most common capacity of the 5¼-inch format in DOS-based PCs was 360 KB  for the Double-Sided Double-Density  format using MFM encoding. In 1984, IBM introduced with its PC/AT the 1.2 MB  dual-sided 5¼-inch floppy disk, but it never became very popular. IBM started using the 720 KB double density 3½-inch microfloppy disk on its Convertible laptop computer in 1986 and the 1.44 MB high-density version with the IBM Personal System/2  line in 1987. These disk drives could be added to older PC models. In 1988, Y-E Data introduced a drive for 2.88 MB Double-Sided Extended-Density  diskettes which was used by IBM in its top-of-the-line PS/2 and some RS/6000 models and in the second-generation NeXTcube and NeXTstation; however, this format had limited market success due to lack of standards and movement to 1.44 MB drives."
+3351,"Throughout the early 1980s, limits of the 5¼-inch format became clear. Originally designed to be more practical than the 8-inch format, it was becoming considered too large; as the quality of recording media grew, data could be stored in a smaller area. Several solutions were developed, with drives at 2-, 2½-, 3-, 3¼-, 3½- and 4-inches  disk) offered by various companies. They all had several advantages over the old format, including a rigid case with a sliding metal  shutter over the head slot, which helped protect the delicate magnetic medium from dust and damage, and a sliding write protection tab, which was far more convenient than the adhesive tabs used with earlier disks. The large market share of the well-established 5¼-inch format made it difficult for these diverse mutually-incompatible new formats to gain significant market share. A variant on the Sony design, introduced in 1983 by many manufacturers, was then rapidly adopted. By 1988, the 3½-inch was outselling the 5¼-inch."
+3352,"Generally, the term floppy disk persisted, even though later style floppy disks have a rigid case around an internal floppy disk."
+3353,"By the end of the 1980s, 5¼-inch disks had been superseded by 3½-inch disks. During this time, PCs frequently came equipped with drives of both sizes. By the mid-1990s, 5¼-inch drives had virtually disappeared, as the 3½-inch disk became the predominant floppy disk. The advantages of the 3½-inch disk were its higher capacity, its smaller physical size, and its rigid case which provided better protection from dirt and other environmental risks."
+3354,"Floppy disks became commonplace during the 1980s and 1990s in their use with personal computers to distribute software, transfer data, and create backups. Before hard disks became affordable to the general population, floppy disks were often used to store a computer's operating system . Most home computers from that time have an elementary OS and BASIC stored in read-only memory , with the option of loading a more advanced OS from a floppy disk."
+3355,"By the early 1990s, the increasing software size meant large packages like Windows or Adobe Photoshop required a dozen disks or more. In 1996, there were an estimated five billion standard floppy disks in use."
+3356,"An attempt to enhance the existing 3½-inch designs was the SuperDisk in the late 1990s, using very narrow data tracks and a high precision head guidance mechanism with a capacity of 120 MB and backward-compatibility with standard 3½-inch floppies; a format war briefly occurred between SuperDisk and other high-density floppy-disk products, although ultimately recordable CDs/DVDs, solid-state flash storage, and eventually cloud-based online storage would render all these removable disk formats obsolete. External USB-based floppy disk drives are still available, and many modern systems provide firmware support for booting from such drives."
+3357,"In the mid-1990s, mechanically incompatible higher-density floppy disks were introduced, like the Iomega Zip disk. Adoption was limited by the competition between proprietary formats and the need to buy expensive drives for computers where the disks would be used. In some cases, failure in market penetration was exacerbated by the release of higher-capacity versions of the drive and media being not backward-compatible with the original drives, dividing the users between new and old adopters. Consumers were wary of making costly investments into unproven and rapidly changing technologies, so none of the technologies became the established standard."
+3358,"Apple introduced the iMac G3 in 1998 with a CD-ROM drive but no floppy drive; this made USB-connected floppy drives popular accessories, as the iMac came without any writable removable media device."
+3359,"Recordable CDs were touted as an alternative, because of the greater capacity, compatibility with existing CD-ROM drives, and—with the advent of re-writeable CDs and packet writing—a similar reusability as floppy disks. However, CD-R/RWs remained mostly an archival medium, not a medium for exchanging data or editing files on the medium itself, because there was no common standard for packet writing which allowed for small updates. Other formats, such as magneto-optical discs, had the flexibility of floppy disks combined with greater capacity, but remained niche due to costs. High-capacity backward compatible floppy technologies became popular for a while and were sold as an option or even included in standard PCs, but in the long run, their use was limited to professionals and enthusiasts."
+3360,"Flash-based USB-thumb drives finally were a practical and popular replacement, that supported traditional file systems and all common usage scenarios of floppy disks. As opposed to other solutions, no new drive type or special software was required that impeded adoption, since all that was necessary was an already common USB port."
+3361,"By 2002, most manufacturers still provided floppy disk drives as standard equipment to meet user demand for file-transfer and an emergency boot device, as well as for the general secure feeling of having the familiar device. By this time, the retail cost of a floppy drive had fallen to around $20 , so there was little financial incentive to omit the device from a system. Subsequently, enabled by the widespread support for USB flash drives and BIOS boot, manufacturers and retailers progressively reduced the availability of floppy disk drives as standard equipment. In February 2003, Dell, one of the leading personal computer vendors, announced that floppy drives would no longer be pre-installed on Dell Dimension home computers, although they were still available as a selectable option and purchasable as an aftermarket OEM add-on. By January 2007, only 2% of computers sold in stores contained built-in floppy disk drives."
+3362,"Floppy disks are used for emergency boots in aging systems lacking support for other bootable media and for BIOS updates, since most BIOS and firmware programs can still be executed from bootable floppy disks. If BIOS updates fail or become corrupt, floppy drives can sometimes be used to perform a recovery. The music and theatre industries still use equipment requiring standard floppy disks . Industrial automation equipment such as programmable machinery and industrial robots may not have a USB interface; data and programs are then loaded from disks, damageable in industrial environments. This equipment may not be replaced due to cost or requirement for continuous availability; existing software emulation and virtualization do not solve this problem because a customized operating system is used that has no drivers for USB devices. Hardware floppy disk emulators can be made to interface floppy-disk controllers to a USB port that can be used for flash drives."
+3363,"In May 2016, the United States Government Accountability Office released a report that covered the need to upgrade or replace legacy computer systems within federal agencies. According to this document, old IBM Series/1 minicomputers running on 8-inch floppy disks are still used to coordinate ""the operational functions of the United States' nuclear forces"". The government planned to update some of the technology by the end of the 2017 fiscal year."
+3364,"Windows 10 and Windows 11 no longer come with drivers for floppy disk drives . However, they will still support them with a separate device driver provided by Microsoft."
+3365,"The British Airways Boeing 747-400 fleet, up to its retirement in 2020, used 3½-inch floppy disks to load avionics software."
+3366,"Sony, who had been in the floppy disk business since 1983, ended domestic sales of all six 3½-inch floppy disk models as of March 2011. This has been viewed by some as the end of the floppy disk. While production of new floppy disk media has ceased, sales and uses of this media from inventories is expected to continue until at least 2026."
+3367,"For more than two decades, the floppy disk was the primary external writable storage device used. Most computing environments before the 1990s were non-networked, and floppy disks were the primary means to transfer data between computers, a method known informally as sneakernet. Unlike hard disks, floppy disks are handled and seen; even a novice user can identify a floppy disk. Because of these factors, a picture of a 3½-inch floppy disk became an interface metaphor for saving data. The floppy disk symbol is still used by software on user-interface elements related to saving files  even though physical floppy disks are largely obsolete."
+3368,The 8-inch and 5¼-inch floppy disks contain a magnetically coated round plastic medium with a large circular hole in the center for a drive's spindle. The medium is contained in a square plastic cover that has a small oblong opening in both sides to allow the drive's heads to read and write data and a large hole in the center to allow the magnetic medium to spin by rotating it from its middle hole.
+3369,"Inside the cover are two layers of fabric with the magnetic medium sandwiched in the middle. The fabric is designed to reduce friction between the medium and the outer cover, and catch particles of debris abraded off the disk to keep them from accumulating on the heads. The cover is usually a one-part sheet, double-folded with flaps glued or spot-welded together."
+3370,"A small notch on the side of the disk identifies that it is writable, detected by a mechanical switch or phototransistor above it; if it is not present, the disk can be written; in the 8-inch disk the notch is covered to enable writing while in the 5¼-inch disk the notch is open to enable writing. Tape may be used over the notch to change the mode of the disk. Punch devices were sold to convert read-only disks to writable ones and enable writing on the unused side of single sided disks; such modified disks became known as flippy disks."
+3371,"Another LED/photo-transistor pair located near the center of the disk detects the index hole once per rotation in the magnetic disk; it is used to detect the angular start of each track and whether or not the disk is rotating at the correct speed. Early 8‑inch and 5¼‑inch disks had physical holes for each sector and were termed hard sectored disks. Later soft-sectored disks have only one index hole, and sector position is determined by the disk controller or low-level software from patterns marking the start of a sector. Generally, the same drives are used to read and write both types of disks, with only the disks and controllers differing. Some operating systems using soft sectors, such as Apple DOS, do not use the index hole, and the drives designed for such systems often lack the corresponding sensor; this was mainly a hardware cost-saving measure."
+3372,"The core of the 3½-inch disk is the same as the other two disks, but the front has only a label and a small opening for reading and writing data, protected by the shutter—a spring-loaded metal or plastic cover, pushed to the side on entry into the drive. Rather than having a hole in the center, it has a metal hub which mates to the spindle of the drive. Typical 3½-inch disk magnetic coating materials are:"
+3373,"Two holes at the bottom left and right indicate whether the disk is write-protected and whether it is high-density; these holes are spaced as far apart as the holes in punched A4 paper, allowing write-protected high-density floppies to be clipped into standard ring binders. The dimensions of the disk shell are not quite square: its width is slightly less than its depth, so that it is impossible to insert the disk into a drive slot sideways . A diagonal notch at top right ensures that the disk is inserted into the drive in the correct orientation—not upside down or label-end first—and an arrow at top left indicates direction of insertion. The drive usually has a button that, when pressed, ejects the disk with varying degrees of force, the discrepancy due to the ejection force provided by the spring of the shutter. In IBM PC compatibles, Commodores, Apple II/IIIs, and other non-Apple-Macintosh machines with standard floppy disk drives, a disk may be ejected manually at any time. The drive has a disk-change switch that detects when a disk is ejected or inserted. Failure of this mechanical switch is a common source of disk corruption if a disk is changed and the drive  fails to notice."
+3374,"One of the chief usability problems of the floppy disk is its vulnerability; even inside a closed plastic housing, the disk medium is highly sensitive to dust, condensation and temperature extremes. As with all magnetic storage, it is vulnerable to magnetic fields. Blank disks have been distributed with an extensive set of warnings, cautioning the user not to expose it to dangerous conditions. Rough treatment or removing the disk from the drive while the magnetic media is still spinning is likely to cause damage to the disk, drive head, or stored data. On the other hand, the 3½‑inch floppy has been lauded for its mechanical usability by human–computer interaction expert Donald Norman:"
+3375,"A spindle motor in the drive rotates the magnetic medium at a certain speed, while a stepper motor-operated mechanism moves the magnetic read/write heads radially along the surface of the disk. Both read and write operations require the media to be rotating and the head to contact the disk media, an action originally accomplished by a disk-load solenoid. Later drives held the heads out of contact until a front-panel lever was rotated  or disk insertion was complete . To write data, current is sent through a coil in the head as the media rotates. The head's magnetic field aligns the magnetization of the particles directly below the head on the media. When the current is reversed the magnetization aligns in the opposite direction, encoding one bit of data. To read data, the magnetization of the particles in the media induce a tiny voltage in the head coil as they pass under it. This small signal is amplified and sent to the floppy disk controller, which converts the streams of pulses from the media into data, checks it for errors, and sends it to the host computer system."
+3376,"A blank unformatted diskette has a coating of magnetic oxide with no magnetic order to the particles. During formatting, the magnetizations of the particles are aligned forming tracks, each broken up into sectors, enabling the controller to properly read and write data. The tracks are concentric rings around the center, with spaces between tracks where no data is written; gaps with padding bytes are provided between the sectors and at the end of the track to allow for slight speed variations in the disk drive, and to permit better interoperability with disk drives connected to other similar systems."
+3377,Each sector of data has a header that identifies the sector location on the disk. A cyclic redundancy check  is written into the sector headers and at the end of the user data so that the disk controller can detect potential errors.
+3378,Some errors are soft and can be resolved by automatically re-trying the read operation; other errors are permanent and the disk controller will signal a failure to the operating system if multiple attempts to read the data still fail.
+3379,"After a disk is inserted, a catch or lever at the front of the drive is manually lowered to prevent the disk from accidentally emerging, engage the spindle clamping hub, and in two-sided drives, engage the second read/write head with the media."
+3380,"In some 5¼-inch drives, insertion of the disk compresses and locks an ejection spring which partially ejects the disk upon opening the catch or lever. This enables a smaller concave area for the thumb and fingers to grasp the disk during removal."
+3381,"Newer 5¼-inch drives and all 3½-inch drives automatically engage the spindle and heads when a disk is inserted, doing the opposite with the press of the eject button."
+3382,"On Apple Macintosh computers with built-in 3½-inch disk drives, the ejection button is replaced by software controlling an ejection motor which only does so when the operating system no longer needs to access the drive. The user could drag the image of the floppy drive to the trash can on the desktop to eject the disk. In the case of a power failure or drive malfunction, a loaded disk can be removed manually by inserting a straightened paper clip into a small hole at the drive's front panel, just as one would do with a CD-ROM drive in a similar situation. The Sharp X68000 featured soft-eject 5¼-inch drives. Some late-generation IBM PS/2 machines had soft-eject 3½-inch disk drives as well for which some issues of DOS  offered an EJECT command."
+3383,"Before a disk can be accessed, the drive needs to synchronize its head position with the disk tracks. In some drives, this is accomplished with a Track Zero Sensor, while for others it involves the drive head striking an immobile reference surface."
+3384,"In either case, the head is moved so that it is approaching track zero position of the disk. When a drive with the sensor has reached track zero, the head stops moving immediately and is correctly aligned. For a drive without the sensor, the mechanism attempts to move the head the maximum possible number of positions needed to reach track zero, knowing that once this motion is complete, the head will be positioned over track zero."
+3385,"Some drive mechanisms such as the Apple II 5¼-inch drive without a track zero sensor, produce characteristic mechanical noises when trying to move the heads past the reference surface. This physical striking is responsible for the 5¼-inch drive clicking during the boot of an Apple II, and the loud rattles of its DOS and ProDOS when disk errors occurred and track zero synchronization was attempted."
+3386,"All 8-inch and some 5¼-inch drives used a mechanical method to locate sectors, known as either hard sectors or soft sectors, and is the purpose of the small hole in the jacket, off to the side of the spindle hole. A light beam sensor detects when a punched hole in the disk is visible through the hole in the jacket."
+3387,"For a soft-sectored disk, there is only a single hole, which is used to locate the first sector of each track. Clock timing is then used to find the other sectors behind it, which requires precise speed regulation of the drive motor."
+3388,"For a hard-sectored disk, there are many holes, one for each sector row, plus an additional hole in a half-sector position, that is used to indicate sector zero."
+3389,"The Apple II computer system is notable in that it did not have an index hole sensor and ignored the presence of hard or soft sectoring. Instead, it used special repeating data synchronization patterns written to the disk between each sector, to assist the computer in finding and synchronizing with the data in each track."
+3390,"The later 3½-inch drives of the mid-1980s did not use sector index holes, but instead also used synchronization patterns."
+3391,"Most 3½-inch drives used a constant speed drive motor and contain the same number of sectors across all tracks. This is sometimes referred to as Constant Angular Velocity . In order to fit more data onto a disk, some 3½-inch drives  instead use Constant Linear Velocity , which uses a variable speed drive motor that spins more slowly as the head moves away from the center of the disk, maintaining the same speed of the head relative to the surface of the disk. This allows more sectors to be written to the longer middle and outer tracks as the track length increases."
+3392,"While the original IBM 8-inch disk was actually so defined, the other sizes are defined in the metric system, their usual names being but rough approximations."
+3393,"Different sizes of floppy disks are mechanically incompatible, and disks can fit only one size of drive. Drive assemblies with both 3½-inch and 5¼-inch slots were available during the transition period between the sizes, but they contained two separate drive mechanisms. In addition, there are many subtle, usually software-driven incompatibilities between the two. 5¼-inch disks formatted for use with Apple II computers would be unreadable and treated as unformatted on a Commodore. As computer platforms began to form, attempts were made at interchangeability. For example, the ""SuperDrive"" included from the Macintosh SE to the Power Macintosh G3 could read, write and format IBM PC format 3½-inch disks, but few IBM-compatible computers had drives that did the reverse. 8-inch, 5¼-inch and 3½-inch drives were manufactured in a variety of sizes, most to fit standardized drive bays. Alongside the common disk sizes were non-classical sizes for specialized systems."
+3394,"Floppy disks of the first standard are 8 inches in diameter, protected by a flexible plastic jacket. It was a read-only device used by IBM as a way of loading microcode. Read/write floppy disks and their drives became available in 1972, but it was IBM's 1973 introduction of the 3740 data entry system that began the establishment of floppy disks, called by IBM the Diskette 1, as an industry standard for information interchange. Formatted diskette for this system store 242,944 bytes. Early microcomputers used for engineering, business, or word processing often used one or more 8-inch disk drives for removable storage; the CP/M operating system was developed for microcomputers with 8-inch drives."
+3395,"The family of 8-inch disks and drives increased over time and later versions could store up to 1.2 MB; many microcomputer applications did not need that much capacity on one disk, so a smaller size disk with lower-cost media and drives was feasible. The 5¼-inch drive succeeded the 8-inch size in many applications, and developed to about the same storage capacity as the original 8-inch size, using higher-density media and recording techniques."
+3396,"The head gap of an 80‑track high-density  5¼‑inch drive  is smaller than that of a 40‑track double-density  drive but can also format, read and write 40‑track disks provided the controller supports double stepping or has a switch to do so. 5¼-inch 80-track drives were also called hyper drives. A blank 40‑track disk formatted and written on an 80‑track drive can be taken to its native drive without problems, and a disk formatted on a 40‑track drive can be used on an 80‑track drive. Disks written on a 40‑track drive and then updated on an 80 track drive become unreadable on any 40‑track drives due to track width incompatibility."
+3397,"Single-sided disks were coated on both sides, despite the availability of more expensive double sided disks. The reason usually given for the higher price was that double sided disks were certified error-free on both sides of the media. Double-sided disks could be used in some drives for single-sided disks, as long as an index signal was not needed. This was done one side at a time, by turning them over ; more expensive dual-head drives which could read both sides without turning over were later produced, and eventually became used universally."
+3398,"In the early 1980s, many manufacturers introduced smaller floppy drives and media in various formats. A consortium of 21 companies eventually settled on a 3½-inch design known as the Micro diskette, Micro disk, or Micro floppy, similar to a Sony design but improved to support both single-sided and double-sided media, with formatted capacities generally of 360 KB and 720 KB respectively. Single-sided drives of the consortium design first shipped in 1983, and double-sided in 1984. The double-sided, high-density 1.44 MB  disk drive, which would become the most popular, first shipped in 1986. The first Macintosh computers used single-sided 3½-inch floppy disks, but with 400 KB formatted capacity. These were followed in 1986 by double-sided 800 KB floppies. The higher capacity was achieved at the same recording density by varying the disk-rotation speed with head position so that the linear speed of the disk was closer to constant. Later Macs could also read and write 1.44 MB HD disks in PC format with fixed rotation speed. Higher capacities were similarly achieved by Acorn's RISC OS  and AmigaOS ."
+3399,"All 3½-inch disks have a rectangular hole in one corner which, if obstructed, write-enables the disk. A sliding detented piece can be moved to block or reveal the part of the rectangular hole that is sensed by the drive. The HD 1.44 MB disks have a second, unobstructed hole in the opposite corner that identifies them as being of that capacity."
+3400,"In IBM-compatible PCs, the three densities of 3½-inch floppy disks are backwards-compatible; higher-density drives can read, write and format lower-density media. It is also possible to format a disk at a lower density than that for which it was intended, but only if the disk is first thoroughly demagnetized with a bulk eraser, as the high-density format is magnetically stronger and will prevent the disk from working in lower-density modes."
+3401,"Writing at different densities than those at which disks were intended, sometimes by altering or drilling holes, was possible but not supported by manufacturers. A hole on one side of a 3½-inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons. Some computers, such as the PS/2 and Acorn Archimedes, ignored these holes altogether."
+3402,"Other smaller floppy sizes were proposed, especially for portable or pocket-sized devices that needed a smaller storage device."
+3403,None of these sizes achieved much market success.
+3404,"Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3½-inch disks is entitled in part ""90 mm "" though 90 mm is closer to 3.54 inches. Formatted capacities are generally set in terms of kilobytes and megabytes."
+3405,"Data is generally written to floppy disks in sectors  and tracks . For example, the HD format of 3½-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. Some disk controllers can vary these parameters at the user's request, increasing storage on the disk, although they may not be able to be read on machines with other controllers. For example, Microsoft applications were often distributed on 3½-inch 1.68 MB DMF disks formatted with 21 sectors instead of 18; they could still be recognized by a standard controller. On the IBM PC, MSX and most other microcomputer platforms, disks were written using a constant angular velocity  format, with the disk spinning at a constant speed and the sectors holding the same amount of information on each track regardless of radial location."
+3406,"Because the sectors have constant angular size, the 512 bytes in each sector are compressed more near the disk's center. A more space-efficient technique would be to increase the number of sectors per track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping nearly constant the amount of physical disk space used for storing each sector; an example is zone bit recording. Apple implemented this in early Macintosh computers by spinning the disk more slowly when the head was at the edge, while maintaining the data rate, allowing 400 KB of storage per side and an extra 80 KB on a double-sided disk. This higher capacity came with a disadvantage: the format used a unique drive mechanism and control circuitry, meaning that Mac disks could not be read on other computers. Apple eventually reverted to constant angular velocity on HD floppy disks with their later machines, still unique to Apple as they supported the older variable-speed formats."
+3407,"Disk formatting is usually done by a utility program supplied by the computer OS manufacturer; generally, it sets up a file storage directory system on the disk, and initializes its sectors and tracks. Areas of the disk unusable for storage due to flaws can be locked  so that the operating system does not attempt to use them. This was time-consuming so many environments had quick formatting which skipped the error checking process. When floppy disks were often used, disks pre-formatted for popular computers were sold. The unformatted capacity of a floppy disk does not include the sector and track headings of a formatted disk; the difference in storage between them depends on the drive's application. Floppy disk drive and media manufacturers specify the unformatted capacity . It is implied that this should not be exceeded, since doing so will most likely result in performance problems. DMF was introduced permitting 1.68 MB to fit onto an otherwise standard 3½-inch disk; utilities then appeared allowing disks to be formatted as such."
+3408,"Mixtures of decimal prefixes and binary sector sizes require care to properly calculate total capacity. Whereas semiconductor memory naturally favors powers of two , the capacity of a disk drive is the product of sector size, sectors per track, tracks per side and sides . Although other sector sizes have been known in the past, formatted sector sizes are now almost always set to powers of two , and, in some cases, disk capacity is calculated as multiples of the sector size rather than only in bytes, leading to a combination of decimal multiples of sectors and binary sector sizes. For example, 1.44 MB 3½-inch HD disks have the ""M"" prefix peculiar to their context, coming from their capacity of 2,880 512-byte sectors , consistent with neither a decimal megabyte nor a binary mebibyte . Hence, these disks hold 1.47 MB or 1.41 MiB. Usable data capacity is a function of the disk format used, which in turn is determined by the FDD controller and its settings. Differences between such formats can result in capacities ranging from approximately 1,300 to 1,760 KiB  on a standard 3½-inch high-density floppy . The highest capacity techniques require much tighter matching of drive head geometry between drives, something not always possible and unreliable. For example, the LS-240 drive supports a 32 MB capacity on standard 3½-inch HD disks, but this is a write-once technique, and requires its own drive."
+3409,"The raw maximum transfer rate of 3½-inch ED floppy drives  is nominally 1,000 kilobits/s, or approximately 83% that of single-speed CD‑ROM . This represents the speed of raw data bits moving under the read head; however, the effective speed is somewhat less due to space used for headers, gaps and other format fields and can be even further reduced by delays to seek between tracks."
+3410,"A hard disk drive , hard disk, hard drive, or fixed disk, is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box."
+3411,"Hard disk drives were introduced by IBM in 1956, and were the dominant secondary storage device for general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern era of servers and personal computers, though personal computing devices produced in large volume, like mobile phones and tablets, rely on flash memory storage devices. More than 224 companies have produced HDDs historically, though after extensive industry consolidation, most units are manufactured by Seagate, Toshiba, and Western Digital. HDDs dominate the volume of storage produced  for servers. Though production is growing slowly , sales revenues and unit shipments are declining, because solid-state drives  have higher data-transfer rates, higher areal storage density, somewhat better reliability, and much lower latency and access times."
+3412,"The revenues for SSDs, most of which use NAND flash memory, slightly exceeded those for HDDs in 2018. Flash storage products had more than twice the revenue of hard disk drives as of 2017. Though SSDs have four to nine times higher cost per bit, they are replacing HDDs in applications where speed, power consumption, small size, high capacity and durability are important. As of 2019, the cost per bit of SSDs is falling, and the price premium over HDDs has narrowed."
+3413,"The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte  drive has a capacity of 1,000 gigabytes, where 1 gigabyte = 1 000 megabytes = 1 000 000 kilobytes  = 1 000 000 000 bytes . Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. There can be confusion regarding storage capacity, since capacities are stated in decimal gigabytes  by HDD manufacturers, whereas the most commonly used operating systems report capacities in powers of 1024, which results in a smaller number than advertised. Performance is specified as the time required to move the heads to a track or cylinder , the time it takes for the desired sector to move under the head , and finally, the speed at which the data is transmitted ."
+3414,"The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as SATA , USB, SAS , or PATA  cables."
+3415,"The first production IBM hard disk drive, the 350 disk storage, shipped in 1957 as a component of the IBM 305 RAMAC system. It was approximately the size of two large refrigerators and stored five million six-bit characters  on a stack of 52 disks . The 350 had a single arm with two read/write heads, one facing up and the other down, that moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set.  Variants of the IBM 350 were the IBM 355, IBM 7300 and IBM 1405."
+3416,"In 1961, IBM announced, and in 1962 shipped, the IBM 1301 disk storage unit, which superseded
+the IBM 350 and similar drives. The 1301 consisted of one  or two  modules, each containing 25 platters, each platter about 1⁄8-inch  thick and 24 inches  in diameter. While the earlier IBM disk drives used only two read/write heads per arm, the 1301 used an array of 48 heads , each array moving horizontally as a single unit, one head per surface used. Cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches  above the platter surface. Motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three large refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes per module. Access time was about a quarter of a second."
+3417,"Also in 1962, IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called ""fixed disk"" drives."
+3418,"In 1963, IBM introduced the 1302, with twice the track capacity and twice as many tracks per cylinder as the 1301. The 1302 had one  or two  modules, each containing a separate comb for the first 250 tracks and the last 250 tracks."
+3419,"Some high-performance HDDs were manufactured with one head per track, e.g., Burroughs B-475 in 1964, IBM 2305 in 1970, so that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head. Known as fixed-head or head-per-track disk drives, they were very expensive and are no longer in production."
+3420,"In 1973, IBM introduced a new type of HDD code-named ""Winchester"". Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to ""land"" on a special area of the disk surface upon spin-down, ""taking off"" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of ""Winchester technology"" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later ""Winchester"" drives abandoned the removable media concept and returned to non-removable platters."
+3421,"In 1974, IBM introduced the swinging arm actuator, made feasible because the Winchester recording heads function well when skewed to the recorded tracks. The simple design of the IBM GV  drive, invented at IBM's UK Hursley Labs, became IBM's most licensed electro-mechanical invention of all time, the actuator and filtration system being adopted in the 1980s eventually for all HDDs, and still universal nearly 40 years and 10 billion arms later."
+3422,"Like the first removable pack drive, the first ""Winchester"" drives used platters 14 inches  in diameter. In 1978, IBM introduced a swing arm drive, the IBM 0680 , with eight inch platters, exploring the possibility that smaller platters might offer advantages. Other eight inch drives followed, then 5+1⁄4 in  drives, sized to replace the contemporary floppy disk drives. The latter were primarily intended for the then fledgling personal computer  market."
+3423,"Over time, as recording densities were greatly increased, further reductions in disk diameter to 3.5"" and 2.5"" were found to be optimum. Powerful rare earth magnet materials became affordable during this period, and were complementary to the swing arm actuator design to make possible the compact form factors of modern HDDs."
+3424,"As the 1980s began, HDDs were a rare and very expensive additional feature in PCs, but by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest computers."
+3425,"Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter, internal HDDs proliferated on personal computers."
+3426,"External HDDs remained popular for much longer on the Apple Macintosh. Many Macintosh computers made between 1986 and 1998 featured a SCSI port on the back, making external expansion simple. Older compact Macintosh computers did not have user-accessible hard drive bays , so on those models, external SCSI disks were the only reasonable option for expanding upon any internal storage."
+3427,"HDD improvements have been driven by increasing areal density, listed in the table above. Applications expanded through the 2000s, from the mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content."
+3428,"In the 2000s and 2010s, NAND began supplanting HDDs in applications requiring portability or high performance. NAND performance is improving faster than HDDs, and applications for HDDs are eroding. In 2018, the largest hard drive had a capacity of 15 TB, while the largest capacity SSD had a capacity of 100 TB. As of 2018, HDDs were forecast to reach 100 TB capacities around 2025, but as of 2019, the expected pace of improvement was pared back to 50 TB by 2026. Smaller form factors, 1.8-inches and below, were discontinued around 2010. The cost of solid-state storage , represented by Moore's law, is improving faster than HDDs. NAND has a higher price elasticity of demand than HDDs, and this drives market growth. During the late 2000s and 2010s, the product life cycle of HDDs entered a mature phase, and slowing sales may indicate the onset of the declining phase."
+3429,The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013.
+3430,"In 2019, Western Digital closed its last Malaysian HDD factory due to decreasing demand, to focus on SSD production. All three remaining HDD manufacturers have had decreasing demand for their HDDs since 2014."
+3431,"A modern HDD records data by magnetizing a thin film of ferromagnetic material on both sides of a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization.  User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions."
+3432,"A typical HDD design consists of a spindle that holds flat circular disks, called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminum alloy, glass, or ceramic. They are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is 0.07–0.18 mm  thick."
+3433,"The platters in contemporary HDDs are spun at speeds varying from 4,200 RPM in energy-efficient portable devices, to 15,000 rpm for high-performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. As of November 2019, the platters in most consumer-grade HDDs spin at 5,400 or 7,200 RPM."
+3434,"Information is written to and read from a platter as it rotates past devices called read-and-write heads that are positioned to operate very close to the magnetic surface, with their flying height often in the range of tens of nanometers. The read-and-write head is used to detect and modify the magnetization of the material passing immediately under it."
+3435,"In modern drives, there is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm  moves the heads on an arc  across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or, in some older designs, a stepper motor. Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track, but modern drives  use zone bit recording, increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones."
+3436,"In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects⁠ ⁠— thermally induced magnetic instability which is commonly known as the ""superparamagnetic limit"". To counter this, the platters are coated with two parallel magnetic layers, separated by a three-atom layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other. Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording , first shipped in 2005, and as of 2007, used in certain HDDs. Perpendicular recording may be accompanied by changes in the manufacturing of the read/write heads to increase the strength of the magnetic field created by the heads."
+3437,"In 2004, a higher-density recording media was introduced, consisting of coupled soft and hard magnetic layers. So-called exchange spring media magnetic storage technology, also known as exchange coupled composite media, allows good writability due to the write-assist nature of the soft layer. However, the thermal stability is determined only by the hardest layer and not influenced by the soft layer."
+3438,"Flux control MAMR  allows a hard drive to have increased recording capacity without the need for new hard disk drive platter materials. MAMR hard drives have a microwave generating spin torque generator  on the read/write heads which allows physically smaller bits to be recorded to the platters, increasing areal density. Normally hard drive recording heads have a pole called a main pole that is used for writing to the platters, and adjacent to this pole is an air gap and a shield. The write coil of the head surrounds the pole. The STO device is placed in the air gap between the pole and the shield to increase the strength of the magnetic field created by the pole; FC-MAMR technically doesn’t use microwaves, but uses technology employed in MAMR. The STO has a Field Generation Layer  and a Spin Injection Layer , and the FGL produces a magnetic field using spin-polarised electrons originating in the SIL, which is a form of spin torque energy.
+"
+3439,"A typical HDD has two electric motors: a spindle motor that spins the disks and an actuator  that positions the read/write head assembly across the spinning disks. The disk motor has an external rotor attached to the disks; the stator windings are fixed in place. Opposite the actuator at the end of the head support arm is the read-write head; thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 g."
+3440,"The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron  high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor ."
+3441,"The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it is wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead  then interact with the magnetic field of the fixed magnet. Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force. If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore, the surface of the magnet is half north pole and half south pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head."
+3442,The HDD's electronics controls the movement of the actuator and the rotation of the disk and transfers data to/from a disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles  or segments interspersed with real data . The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil motor to rotate the arm. A more modern servo system also employs milli and/or micro actuators to more accurately position the read/write heads. The spinning of the disks uses fluid-bearing spindle motors. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.
+3443,"Modern drives make extensive use of error correction codes , particularly Reed–Solomon error correction. These techniques store extra bits, determined by mathematical formulas, for each block of data; the extra bits allow many errors to be corrected invisibly. The extra bits themselves take up space on the HDD, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity. For example, a typical 1 TB hard disk with 512-byte sectors provides additional capacity of about 93 GB for the ECC data."
+3444,"In the newest drives, as of 2009, low-density parity-check codes  were supplanting Reed–Solomon; LDPC codes enable performance close to the Shannon limit and thus provide the highest storage density available."
+3445,"Typical hard disk drives attempt to ""remap"" the data in a physical sector that is failing to a spare physical sector provided by the drive's ""spare sector pool"" , while relying on the ECC to recover stored data while the number of errors in a bad sector is still low enough. The S.M.A.R.T  feature counts the total number of errors in the entire HDD fixed by ECC , and the total number of performed sector remappings, as the occurrence of many such errors may predict an HDD failure."
+3446,"The ""No-ID Format"", developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located."
+3447,Only a tiny fraction of the detected errors end up as not correctable. Examples of specified uncorrected bit read error rates include:
+3448,Within a given manufacturers model the uncorrected bit error rate is typically the same regardless of capacity of the drive.
+3449,The worst type of errors are silent data corruptions which are errors undetected by the disk firmware or the host operating system; some of these errors may be caused by hard disk drive malfunctions while others originate elsewhere in the connection between the drive and the host.
+3450,"The rate of areal density advancement was similar to Moore's law  through 2010: 60% per year during 1988–1996, 100% during 1996–2003 and 30% during 2003–2010. Speaking in 1997, Gordon Moore called the increase ""flabbergasting"", while observing later that growth cannot continue forever. Price improvement decelerated to −12% per year during 2010–2017, as the growth of areal density slowed. The rate of advancement for areal density slowed to 10% per year during 2010–2016, and there was difficulty in migrating from perpendicular recording to newer technologies."
+3451,"As bit cell size decreases, more data can be put onto a single drive platter. In 2013, a production desktop 3 TB HDD  would have had an areal density of about 500 Gbit/in2 which would have amounted to a bit cell comprising about 18 magnetic grains . Since the mid-2000s, areal density progress has been challenged by a superparamagnetic trilemma involving grain size, grain magnetic strength and ability of the head to write. In order to maintain acceptable signal-to-noise, smaller grains are required; smaller grains may self-reverse  unless their magnetic strength is increased, but known write head materials are unable to generate a strong enough magnetic field sufficient to write the medium in the increasingly smaller space taken by grains."
+3452,"Magnetic storage technologies are being developed to address this trilemma, and compete with flash memory–based solid-state drives . In 2013, Seagate introduced shingled magnetic recording , intended as something of a ""stopgap"" technology between PMR and Seagate's intended successor heat-assisted magnetic recording . SMR utilises overlapping tracks for increased data density, at the cost of design complexity and lower data access speeds ."
+3453,"By contrast, HGST  focused on developing ways to seal helium-filled drives instead of the usual filtered air. Since  turbulence and friction are reduced, higher areal densities can be achieved due to using a smaller track width, and the energy dissipated due to friction is lower as well, resulting in a lower power draw. Furthermore, more platters can be fit into the same enclosure space, although helium gas is notoriously difficult to prevent escaping. Thus, helium drives are completely sealed and do not have a breather port, unlike their air-filled counterparts."
+3454,"Other recording technologies are either under research or have been commercially implemented to increase areal density, including Seagate's heat-assisted magnetic recording . HAMR requires a different architecture with redesigned media and read/write heads, new lasers, and new near-field optical transducers. HAMR is expected to ship commercially in late 2020 or 2021. Technical issues delayed the introduction of HAMR by a decade, from earlier projections of 2009, 2015, 2016, and the first half of 2019. Some drives have adopted dual independent actuator arms to increase read/write speeds and compete with SSDs. HAMR's planned successor, bit-patterned recording , has been removed from the roadmaps of Western Digital and Seagate. Western Digital's microwave-assisted magnetic recording , also referred to as energy-assisted magnetic recording , was sampled in 2020, with the first EAMR drive, the Ultrastar HC550, shipping in late 2020. Two-dimensional magnetic recording  and ""current perpendicular to plane"" giant magnetoresistance  heads have appeared in research papers.  A 3D-actuated vacuum drive  concept and 3D magnetic recording have been proposed."
+3455,"Depending upon assumptions on feasibility and timing of these technologies, Seagate forecasts that areal density will grow 20% per year during 2020–2034."
+3456,"The highest-capacity HDDs shipping commercially in 2024 are 32 TB. The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons, e.g. the operating system using some space, use of some space for data redundancy, space use for file system structures. Confusion of decimal prefixes and binary prefixes can also lead to errors."
+3457,"Modern hard disk drives appear to their host controller as a contiguous set of logical blocks, and the gross drive capacity is calculated by multiplying the number of blocks by the block size. This information is available from the manufacturer's product specification, and from the drive itself through use of operating system functions that invoke low-level drive commands. Older IBM and compatible drives, e.g. IBM 3390 using the CKD record format, have variable length records; such drive capacity calculations must take into account the characteristics of the records. Some newer DASD simulate CKD, and the same capacity formulae apply."
+3458,"The gross capacity of older sector-oriented HDDs is calculated as the product of the number of cylinders per recording zone, the number of bytes per sector , and the count of zones of the drive. Some modern SATA drives also report cylinder-head-sector  capacities, but these are not physical parameters because the reported values are constrained by historic operating system interfaces. The C/H/S scheme has been replaced by logical block addressing , a simple linear addressing scheme that locates blocks by an integer index, which starts at LBA 0 for the first block and increments thereafter. When using the C/H/S method to describe modern large drives, the number of heads is often set to 64, although a typical modern hard disk drive has between one and four platters. In modern HDDs, spare capacity for defect management is not included in the published capacity; however, in many early HDDs, a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system. Furthermore, many HDDs store their firmware in a reserved service zone, which is typically not accessible by the user, and is not included in the capacity calculation."
+3459,"For RAID subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID 1 array has about half the total capacity as a result of data mirroring, while a RAID 5 array with n drives loses 1/n of capacity  due to storing parity information. RAID subsystems are multiple drives that appear to be one drive or more drives to the user, but provide fault tolerance. Most RAID vendors use checksums to improve data integrity at the block level. Some vendors design systems using HDDs with sectors of 520 bytes to contain 512 bytes of user data and eight checksum bytes, or by using separate 512-byte sectors for the checksum data."
+3460,"Some systems may use hidden partitions for system recovery, reducing the capacity available to the end user without knowledge of special disk partitioning utilities like diskpart in Windows."
+3461,"Data is stored on a hard drive in a series of logical blocks.  Each block is delimited by markers identifying its start and end, error detecting and correcting information, and space between blocks to allow for minor timing variations.  These blocks often contained 512 bytes of usable data, but other sizes have been used.  As drive density increased, an initiative known as Advanced Format extended the block size to 4096 bytes of usable data, with a resulting significant reduction in the amount of disk space used for block headers, error checking data, and spacing."
+3462,"The process of initializing these logical blocks on the physical disk platters is called low-level formatting, which is usually performed at the factory and is not normally changed in the field. High-level formatting writes data structures used by the operating system to organize data files on the disk. This includes writing partition and file system structures into selected logical blocks. For example, some of the disk space will be used to hold a directory of disk file names and a list of logical blocks associated with a particular file."
+3463,"Examples of partition mapping scheme include Master boot record  and GUID Partition Table . Examples of data structures stored on disk to retrieve files include the File Allocation Table  in the DOS file system and inodes in many UNIX file systems, as well as other operating system data structures . As a consequence, not all the space on an HDD is available for user files, but this system overhead is usually small compared with user data."
+3464,"In the early days of computing, the total capacity of HDDs was specified in seven to nine decimal digits frequently truncated with the idiom millions.
+By the 1970s, the total capacity of HDDs was given by manufacturers using SI decimal prefixes such as megabytes , gigabytes  and terabytes . However, capacities of memory are usually quoted using a binary interpretation of the prefixes, i.e. using powers of 1024 instead of 1000."
+3465,"Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes. The Microsoft Windows family of operating systems uses the binary convention when reporting storage capacity, so an HDD offered by its manufacturer as a 1 TB drive is reported by these operating systems as a 931 GB HDD. Mac OS X 10.6  uses decimal convention when reporting HDD capacity. The default behavior of the df command-line utility on Linux is to report the HDD capacity as a number of 1024-byte units."
+3466,"The difference between the decimal and binary prefix interpretation caused some consumer confusion and led to class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal prefixes effectively misled consumers, while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.  In 2020, a California court ruled that use of the decimal prefixes with a decimal meaning was not misleading."
+3467,"IBM's first hard disk drive, the IBM 350, used a stack of fifty 24-inch platters, stored 3.75 MB of data , and was of a size comparable to two large refrigerators. In 1962, IBM introduced its model 1311 disk, which used six 14-inch  platters in a removable pack and was roughly the size of a washing machine. This became a standard platter size for many years, used also by other manufacturers. The IBM 2314 used platters of the same size in an eleven-high pack and introduced the ""drive in a drawer"" layout, sometimes called the ""pizza oven"", although the ""drawer"" was not the complete drive. Into the 1970s, HDDs were offered in standalone cabinets of varying dimensions containing from one to four HDDs."
+3468,"Beginning in the late 1960s, drives were offered that fit entirely into a chassis that would mount in a 19-inch rack. Digital's RK05 and RL01 were early examples using single 14-inch platters in removable packs, the entire drive fitting in a 10.5-inch-high rack space . In the mid-to-late 1980s, the similarly sized Fujitsu Eagle, which used  10.5-inch platters, was a popular product."
+3469,"With increasing sales of microcomputers having built-in floppy-disk drives , HDDs that would fit to the FDD mountings became desirable. Starting with the Shugart Associates SA1000, HDD form factors initially followed those of 8-inch, 5¼-inch, and 3½-inch floppy disk drives. Although referred to by these nominal sizes, the actual sizes for those three drives respectively are 9.5"", 5.75"" and 4"" wide. Because there were no smaller floppy disk drives, smaller HDD form factors such as 2½-inch drives  developed from product offerings or industry standards."
+3470,"As of 2019, 2½-inch and 3½-inch hard disks are the most popular sizes.  By 2009, all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory, which has no moving parts. While nominal sizes are in inches, actual dimensions are specified in millimeters."
+3471,"The factors that limit the time to access the data on an HDD are mostly related to the mechanical nature of the rotating disks and moving heads, including:"
+3472,Delay may also occur if the drive disks are stopped to save energy.
+3473,"Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk. Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the procedure is in progress."
+3474,"Time to access data can be improved by increasing rotational speed  or by reducing the time spent seeking. Increasing areal density increases throughput by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. The time to access data has not kept up with throughput increases, which themselves have not kept up with growth in bit density and storage capacity."
+3475,"As of 2010, a typical 7,200-rpm desktop HDD has a sustained ""disk-to-buffer"" data transfer rate up to 1,030 Mbit/s. This rate depends on the track location; the rate is higher for data on the outer tracks  and lower toward the inner tracks ; and is generally somewhat higher for 10,000-rpm drives. A current, widely-used standard for the ""buffer-to-computer"" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s  from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate  can be measured by writing a large file to disk using special file-generator tools, then reading back the file. Transfer rate can be influenced by file system fragmentation and the layout of the files."
+3476,"HDD data transfer rate depends upon the rotational speed of the platters and the data recording density. Because heat and vibration limit rotational speed, advancing density becomes the main method to improve sequential transfer rates. Higher speeds require a more powerful spindle motor, which creates more heat. While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, only the latter increases the data transfer rate for a given rpm. Since data transfer rate performance tracks only one of the two components of areal density, its performance improves at a lower rate."
+3477,"Other performance considerations include quality-adjusted price, power consumption, audible noise, and both operating and non-operating shock resistance."
+3478,"Current hard drives connect to a computer over one of several bus types, including parallel ATA, Serial ATA, SCSI, Serial Attached SCSI , and Fibre Channel. Some drives, especially external portable drives, use IEEE 1394, or USB. All of these interfaces are digital; electronics on the drive process the analog signals from the read/write heads. Current drives present a consistent interface to the rest of the computer, independent of the data encoding scheme used internally, and independent of the physical number of disks and heads within the drive."
+3479,"Typically, a DSP in the electronics inside the drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error correction to decode the data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks."
+3480,"Modern interfaces connect the drive to the host interface with a single data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit. Older interfaces had separate cables for data signals and for drive control signals."
+3481,"Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash – a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads."
+3482,"The HDD's spindle system relies on air density inside the disk enclosure to support the heads at their proper flying height while the disk rotates. HDDs require a certain range of air densities to operate properly. The connection to the external environment and density occurs through a small hole in the enclosure , usually with a filter on the inside . If the air density is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m . Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives – they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation  filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity present for extended periods of time can corrode the heads and platters. An exception to this are hermetically sealed, helium filled HDDs that largely eliminate environmental issues that can arise due to humidity or atmospheric pressure changes. Such HDDs were introduced by HGST in their first successful high volume implementation in 2013."
+3483,"For giant magnetoresistive  heads in particular, a minor head crash from contamination  still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes ."
+3484,"When the logic board of a hard disk fails, the drive can often be restored to functioning order and the data recovered by replacing the circuit board with one of an identical hard disk. In the case of read-write head faults, they can be replaced using specialized tools in a dust-free environment. If the disk platters are undamaged, they can be transferred into an identical enclosure and the data can be copied or cloned onto a new drive. In the event of disk-platter failures, disassembly and imaging of the disk platters may be required. For logical damage to file systems, a variety of tools, including fsck on UNIX-like systems and CHKDSK on Windows, can be used for data recovery. Recovery from logical damage can require file carving."
+3485,"A common expectation is that hard disk drives designed and marketed for server use will fail less frequently than consumer-grade drives usually used in desktop computers. However, two independent studies by Carnegie Mellon University and Google found that the ""grade"" of a drive does not relate to the drive's failure rate."
+3486,"A 2011 summary of research, into SSD and magnetic disk failure patterns by Tom's Hardware summarized research findings as follows:"
+3487,"As of 2019, Backblaze, a storage provider, reported an annualized failure rate of two percent per year for a storage farm with 110,000 off-the-shelf HDDs with the reliability varying widely between models and manufacturers. Backblaze subsequently reported that the failure rate for HDDs and SSD of equivalent age was similar."
+3488,"To minimize cost and overcome failures of individual HDDs, storage systems providers rely on redundant HDD arrays. HDDs that fail are replaced on an ongoing basis."
+3489,These drives typically spin at 5400 RPM and include:
+3490,"HDD price per byte decreased at the rate of 40% per year during 1988–1996, 51% per year during 1996–2003 and 34% per year during 2003–2010. The price decrease slowed down to 13% per year during 2011–2014, as areal density increase slowed and the 2011 Thailand floods damaged manufacturing facilities and have held at 11% per year during 2010–2017."
+3491,"The Federal Reserve Board has published a quality-adjusted price index for large-scale enterprise storage systems including three or more enterprise HDDs and associated controllers, racks and cables. Prices for these large-scale storage systems decreased at the rate of 30% per year during 2004–2009 and 22% per year during 2009–2014."
+3492,"More than 200 companies have manufactured HDDs over time, but consolidations have concentrated production to just three manufacturers today: Western Digital, Seagate, and Toshiba.  Production is mainly in the Pacific rim."
+3493,HDD unit shipments peaked at 651 million units in 2010 and have been declining since then to 166 million units in 2022. Seagate at 43% of units had the largest market share.
+3494,"HDDs are being superseded by solid-state drives  in markets where the higher speed  NVMe drives and 2.5 gigabytes per second for PCIe expansion card drives)), ruggedness, and lower power of SSDs are more important than price, since the bit cost of SSDs is four to nine times higher than HDDs. As of 2016, HDDs are reported to have a failure rate of 2–9% per year, while SSDs have fewer failures: 1–3% per year. However, SSDs have more un-correctable data errors than HDDs."
+3495,"SSDs offer larger capacities  than the largest HDD and/or higher storage densities , although their cost remains prohibitive."
+3496,"A laboratory demonstration of a 1.33-Tb 3D NAND chip with 96 layers ) had 5.5 Tbit/in2 as of 2019, while the maximum areal density for HDDs is 1.5 Tbit/in2. The areal density of flash memory is doubling every two years, similar to Moore's law  and faster than the 10–20% per year for HDDs. As of 2018, the maximum capacity was 16 terabytes for an HDD, and 100 terabytes for an SSD. HDDs were used in 70% of the desktop and notebook computers produced in 2016, and SSDs were used in 30%. The usage share of HDDs is declining and could drop below 50% in 2018–2019 according to one forecast, because SSDs are replacing smaller-capacity  HDDs in desktop and notebook computers and MP3 players."
+3497,"The market for silicon-based flash memory  chips, used in SSDs and other applications, is growing faster than for HDDs. Worldwide NAND revenue grew 16% per year from $22 billion to $57 billion during 2011–2017, while production grew 45% per year from 19 exabytes to 175 exabytes."
+3498,"I/O devices are the pieces of hardware used by a human  to communicate with a computer. For instance, a keyboard or computer mouse is an input device for a computer, while monitors and printers are output devices. Devices for communication between computers, such as modems and network cards, typically perform both input and output operations. Any interaction with the system by an interactor is an input and the reaction the system responds is called the output."
+3499,"The designation of a device as either input or output depends on perspective. Mice and keyboards take physical movements that the human user outputs and convert them into input signals that a computer can understand; the output from these devices is the computer's input. Similarly, printers and monitors take signals that computers output as input, and they convert these signals into a representation that human users can understand. From the human user's perspective, the process of reading or seeing these representations is receiving output; this type of interaction between computers and humans is studied in the field of human–computer interaction. A further complication is that a device traditionally considered an input device, e.g., card reader, keyboard, may accept control commands to, e.g., select stacker, display keyboard lights, while a device traditionally considered as an output device may provide status data ."
+3500,"In computer architecture, the combination of the CPU and main memory, to which the CPU can read or write directly using individual instructions, is considered the brain of a computer. Any transfer of information to or from the CPU/memory combo, for example by reading data from a disk drive, is considered I/O. The CPU and its supporting circuitry may provide memory-mapped I/O that is used in low-level computer programming, such as in the implementation of device drivers, or may provide access to I/O channels. An I/O algorithm is one designed to exploit locality and perform efficiently when exchanging data with a secondary storage device, such as a disk drive."
+3501,"An I/O interface is required whenever the I/O device is driven by a processor. Typically a CPU communicates with devices via a bus. The interface must have the necessary logic to interpret the device address generated by the processor. Handshaking should be implemented by the interface using appropriate commands , and the processor can communicate with an I/O device through the interface. If different data formats are being exchanged, the interface must be able to convert serial data to parallel form and vice versa. Because it would be a waste for a processor to be idle while it waits for data from an input device there must be provision for generating interrupts and the corresponding type numbers for further processing by the processor if required."
+3502,"A computer that uses memory-mapped I/O accesses hardware by reading and writing to specific memory locations, using the same assembly language instructions that computer would normally use to access memory. An alternative method is via instruction-based I/O which requires that a CPU have specialized instructions for I/O. Both input and output devices have a data processing rate that can vary greatly. With some devices able to exchange data at very high speeds direct access to memory  without the continuous aid of a CPU is required."
+3503,"Higher-level operating system and programming facilities employ separate, more abstract I/O concepts and primitives. For example, most operating systems provide application programs with the concept of files. The C and C++ programming languages, and operating systems in the Unix family, traditionally abstract files and devices as streams, which can be read or written, or sometimes both. The C standard library provides functions for manipulating streams for input and output."
+3504,"In the context of the ALGOL 68 programming language, the input and output facilities are collectively referred to as transput. The ALGOL 68 transput library recognizes the following standard files/devices: stand in, stand out, stand errors and stand back."
+3505,"An alternative to special primitive functions is the I/O monad, which permits programs to just describe I/O, and the actions are carried out outside the program. This is notable because the I/O functions would introduce side-effects to any programming language, but this allows purely functional programming to be practical."
+3506,"Channel I/O requires the use of instructions that are specifically designed to perform I/O operations. The I/O instructions address the channel or the channel and device; the channel asynchronously accesses all other required addressing and control information. This is similar to DMA, but more flexible."
+3507,"Port-mapped I/O also requires the use of special I/O instructions. Typically one or more ports are assigned to the device, each with a special purpose. The port numbers are in a separate address space from that used by normal instructions."
+3508,Direct memory access  is a means for devices to transfer large chunks of data to and from memory independently of the CPU.
+3509,"Other types of caches exist , such as the translation lookaside buffer  which is part of the memory management unit  which most CPUs have."
+3510,"When trying to read from or write to a location in the main memory, the processor checks whether the data from that location is already in the cache. If so, the processor will read from or write to the cache instead of the much slower main memory."
+3511,"Many modern desktop, server, and industrial CPUs have at least three independent caches:"
+3512,"Early examples of CPU caches include the Atlas 2 and the IBM System/360 Model 85 in the 1960s. The first CPUs that used a cache had only one level of cache; unlike later level 1 cache, it was not split into L1d  and L1i . Split L1 cache started in 1976 with the IBM 801 CPU, became mainstream in the late 1980s, and in 1997 entered the embedded CPU market with the ARMv5TE. In 2015, even sub-dollar SoCs split the L1 cache. They also have L2 caches and, for larger processors, L3 caches as well. The L2 cache is usually not split, and acts as a common repository for the already split L1 cache. Every core of a multi-core processor has a dedicated L1 cache and is usually not shared between the cores. The L2 cache, and higher-level caches, may be shared between the cores. L4 cache is currently uncommon, and is generally dynamic random-access memory  on a separate die or chip, rather than static random-access memory . An exception to this is when eDRAM is used for all levels of cache, down to L1. Historically L1 was also on a separate die, however bigger die sizes have allowed integration of it as well as other cache levels, with the possible exception of the last level. Each extra level of cache tends to be bigger and optimized differently."
+3513,"Caches  have generally been sized in powers of: 2, 4, 8, 16 etc. KiB; when up to MiB sizes , very early on the pattern broke down, to allow for larger caches without being forced into the doubling-in-size paradigm, with e.g. Intel Core 2 Duo with 3 MiB L2 cache in April 2008. This happened much later for L1 caches, as their size is generally still a small number of KiB. The IBM zEC12 from 2012 is an exception however, to gain unusually large 96 KiB L1 data cache for its time, and e.g. the IBM z13 having a 96 KiB L1 instruction cache , and Intel Ice Lake-based processors from 2018, having 48 KiB L1 data cache and 48 KiB L1 instruction cache. In 2020, some Intel Atom CPUs  have  4.5 MiB and 15 MiB cache sizes."
+3514,"Data is transferred between memory and cache in blocks of fixed size, called cache lines or cache blocks. When a cache line is copied from memory into the cache, a cache entry is created. The cache entry will include the copied data as well as the requested memory location ."
+3515,"When the processor needs to read or write a location in memory, it first checks for a corresponding entry in the cache. The cache checks for the contents of the requested memory location in any cache lines that might contain that address. If the processor finds that the memory location is in the cache, a cache hit has occurred. However, if the processor does not find the memory location in the cache, a cache miss has occurred. In the case of a cache hit, the processor immediately reads or writes the data in the cache line. For a cache miss, the cache allocates a new entry and copies data from main memory, then the request is fulfilled from the contents of the cache."
+3516,"To make room for the new entry on a cache miss, the cache may have to evict one of the existing entries. The heuristic it uses to choose the entry to evict is called the replacement policy. The fundamental problem with any replacement policy is that it must predict which existing cache entry is least likely to be used in the future. Predicting the future is difficult, so there is no perfect method to choose among the variety of replacement policies available. One popular replacement policy, least-recently used , replaces the least recently accessed entry."
+3517,"Marking some memory ranges as non-cacheable can improve performance, by avoiding caching of memory regions that are rarely re-accessed. This avoids the overhead of loading something into the cache without having any reuse. Cache entries may also be disabled or locked depending on the context."
+3518,"If data is written to the cache, at some point it must also be written to main memory; the timing of this write is known as the write policy. In a write-through cache, every write to the cache causes a write to main memory. Alternatively, in a write-back or copy-back cache, writes are not immediately mirrored to the main memory, and the cache instead tracks which locations have been written over, marking them as dirty. The data in these locations is written back to the main memory only when that data is evicted from the cache. For this reason, a read miss in a write-back cache may sometimes require two memory accesses to service: one to first write the dirty location to main memory, and then another to read the new location from memory. Also, a write to a main memory location that is not yet mapped in a write-back cache may evict an already dirty location, thereby freeing that cache space for the new memory location."
+3519,"There are intermediate policies as well. The cache may be write-through, but the writes may be held in a store data queue temporarily, usually so multiple stores can be processed together ."
+3520,"Cached data from the main memory may be changed by other entities  or another core in a multi-core processor), in which case the copy in the cache may become out-of-date or stale. Alternatively, when a CPU in a multiprocessor system updates data in the cache, copies of data in caches associated with other CPUs become stale. Communication protocols between the cache managers that keep the data consistent are known as cache coherence protocols."
+3521,"Cache performance measurement has become important in recent times where the speed gap between the memory performance and the processor performance is increasing exponentially. The cache was introduced to reduce this speed gap. Thus knowing how well the cache is able to bridge the gap in the speed of processor and memory becomes important, especially in high-performance systems. The cache hit rate and the cache miss rate play an important role in determining this performance. To improve the cache performance, reducing the miss rate becomes one of the necessary steps among other steps. Decreasing the access time to the cache also gives a boost to its performance and helps with optimization."
+3522,"The time taken to fetch one cache line from memory  matters because the CPU will run out of work while waiting for the cache line. When a CPU reaches this state, it is called a stall. As CPUs become faster compared to main memory, stalls due to cache misses displace more potential computation; modern CPUs can execute hundreds of instructions in the time taken to fetch a single cache line from main memory."
+3523,"Various techniques have been employed to keep the CPU busy during this time, including out-of-order execution in which the CPU attempts to execute independent instructions after the instruction that is waiting for the cache miss data. Another technology, used by many processors, is simultaneous multithreading , which allows an alternate thread to use the CPU core while the first thread waits for required CPU resources to become available."
+3524,"The placement policy decides where in the cache a copy of a particular entry of main memory will go. If the placement policy is free to choose any entry in the cache to hold the copy, the cache is called fully associative. At the other extreme, if each entry in the main memory can go in just one place in the cache, the cache is direct-mapped. Many caches implement a compromise in which each entry in the main memory can go to any one of N places in the cache, and are described as N-way set associative. For example, the level-1 data cache in an AMD Athlon is two-way set associative, which means that any particular location in main memory can be cached in either of two locations in the level-1 data cache."
+3525,"Choosing the right value of associativity involves a trade-off. If there are ten places to which the placement policy could have mapped a memory location, then to check if that location is in the cache, ten cache entries must be searched. Checking more places takes more power and chip area, and potentially more time. On the other hand, caches with more associativity suffer fewer misses , so that the CPU wastes less time reading from the slow main memory. The general guideline is that doubling the associativity, from direct mapped to two-way, or from two-way to four-way, has about the same effect on raising the hit rate as doubling the cache size. However, increasing associativity more than four does not improve hit rate as much, and are generally done for other reasons . Some CPUs can dynamically reduce the associativity of their caches in low-power states, which acts as a power-saving measure."
+3526,In order of worse but simple to better but complex:
+3527,"In this cache organization, each location in the main memory can go in only one entry in the cache. Therefore, a direct-mapped cache can also be called a ""one-way set associative"" cache. It does not have a placement policy as such, since there is no choice of which cache entry's contents to evict. This means that if two locations map to the same entry, they may continually knock each other out. Although simpler, a direct-mapped cache needs to be much larger than an associative one to give comparable performance, and it is more unpredictable. Let x be block number in cache, y be block number of memory, and n be number of blocks in cache, then mapping is done with the help of the equation x = y mod n."
+3528,"If each location in the main memory can be cached in either of two locations in the cache, one logical question is: which one of the two? The simplest and most commonly used scheme, shown in the right-hand diagram above, is to use the least significant bits of the memory location's index as the index for the cache memory, and to have two entries for each index. One benefit of this scheme is that the tags stored in the cache do not have to include that part of the main memory address which is implied by the cache memory's index. Since the cache tags have fewer bits, they require fewer transistors, take less space on the processor circuit board or on the microprocessor chip, and can be read and compared faster. Also LRU algorithm is especially simple since only one bit needs to be stored for each pair."
+3529,"One of the advantages of a direct-mapped cache is that it allows simple and fast speculation. Once the address has been computed, the one cache index which might have a copy of that location in memory is known. That cache entry can be read, and the processor can continue to work with that data before it finishes checking that the tag actually matches the requested address."
+3530,"The idea of having the processor use the cached data before the tag match completes can be applied to associative caches as well. A subset of the tag, called a hint, can be used to pick just one of the possible cache entries mapping to the requested address. The entry selected by the hint can then be used in parallel with checking the full tag. The hint technique works best when used in the context of address translation, as explained below."
+3531,"Other schemes have been suggested, such as the skewed cache, where the index for way 0 is direct, as above, but the index for way 1 is formed with a hash function. A good hash function has the property that addresses which conflict with the direct mapping tend not to conflict when mapped with the hash function, and so it is less likely that a program will suffer from an unexpectedly large number of conflict misses due to a pathological access pattern. The downside is extra latency from computing the hash function. Additionally, when it comes time to load a new line and evict an old line, it may be difficult to determine which existing line was least recently used, because the new line conflicts with data at different indexes in each way; LRU tracking for non-skewed caches is usually done on a per-set basis. Nevertheless, skewed-associative caches have major advantages over conventional set-associative ones."
+3532,"A true set-associative cache tests all the possible ways simultaneously, using something like a content-addressable memory. A pseudo-associative cache tests each possible way one at a time. A hash-rehash cache and a column-associative cache are examples of a pseudo-associative cache."
+3533,"In the common case of finding a hit in the first way tested, a pseudo-associative cache is as fast as a direct-mapped cache, but it has a much lower conflict miss rate than a direct-mapped cache, closer to the miss rate of a fully associative cache."
+3534,"Comparing with a direct-mapped cache, a set associative cache has a reduced number of bits for its cache set index that maps to a cache set, where multiple ways or blocks stays, such as 2 blocks for a 2-way set associative cache and 4 blocks for a 4-way set associative cache. Comparing with a direct mapped cache, the unused cache index bits become a part of the tag bits. For example, a 2-way set associative cache contributes 1 bit to the tag and a 4-way set associative cache contributes 2 bits to the tag. The basic idea of the multicolumn cache is to use the set index to map to a cache set as a conventional set associative cache does, and to use the added tag bits to index a way in the set. For example, in a 4-way set associative cache, the two bits are used to index way 00, way 01, way 10, and way 11, respectively. This double cache indexing is called a “major location mapping”, and its latency is equivalent to a direct-mapped access. Extensive experiments in multicolumn cache design shows that the hit ratio to major locations is as high as 90%. If cache mapping conflicts with a cache block in the major location, the existing cache block will be moved to another cache way in the same set, which is called “selected location”. Because the newly indexed cache block is a most recently used  block, it is placed in the major location in multicolumn cache with a consideration of temporal locality. Since multicolumn cache is designed for a cache with a high associativity, the number of ways in each set is high; thus, it is easy find a selected location in the set. A selected location index by an additional hardware is maintained for the major location in a cache block."
+3535,"Multicolumn cache remains a high hit ratio due to its high associativity, and has a comparable low latency to a direct-mapped cache due to its high percentage of hits in major locations. The concepts of major locations and selected locations in multicolumn cache have been used in several cache designs in ARM Cortex R chip, Intel's way-predicting cache memory, IBM's reconfigurable multi-way associative cache memory and Oracle's dynamic cache replacement way selection based on address tab bits."
+3536,Cache row entries usually have the following structure:
+3537,The data block  contains the actual data fetched from the main memory. The tag contains  the address of the actual data fetched from the main memory. The flag bits are discussed below.
+3538,"The ""size"" of the cache is the amount of main memory data it can hold. This size can be calculated as the number of bytes stored in each data block times the number of blocks stored in the cache. "
+3539,"An effective memory address which goes along with the cache line  is split  into the tag, the index and the block offset."
+3540,"Some authors refer to the block offset as simply the ""offset"" or the ""displacement""."
+3541,"The original Pentium 4 processor had a four-way set associative L1 data cache of 8 KiB in size, with 64-byte cache blocks. Hence, there are 8 KiB / 64 = 128 cache blocks. The number of sets is equal to the number of cache blocks divided by the number of ways of associativity, what leads to 128 / 4 = 32 sets, and hence 25 = 32 different indices. There are 26 = 64 possible offsets. Since the CPU address is 32 bits wide, this implies 32 - 5 - 6 = 21 bits for the tag field."
+3542,"The original Pentium 4 processor also had an eight-way set associative L2 integrated cache 256 KiB in size, with 128-byte cache blocks. This implies 32 - 8 - 7 = 17 bits for the tag field."
+3543,An instruction cache requires only one flag bit per cache row entry: a valid bit. The valid bit indicates whether or not a cache block has been loaded with valid data.
+3544,"On power-up, the hardware sets all the valid bits in all the caches to ""invalid"". Some systems also set a valid bit to ""invalid"" at other times, such as when multi-master bus snooping hardware in the cache of one processor hears an address broadcast from some other processor, and realizes that certain data blocks in the local cache are now stale and should be marked invalid."
+3545,"A data cache typically requires two flag bits per cache line – a valid bit and a dirty bit. Having a dirty bit set indicates that the associated cache line has been changed since it was read from main memory , meaning that the processor has written data to that line and the new value has not propagated all the way to main memory."
+3546,"A cache miss is a failed attempt to read or write a piece of data in the cache, which results in a main memory access with much longer latency. There are three kinds of cache misses: instruction read miss, data read miss, and data write miss."
+3547,"Cache read misses from an instruction cache generally cause the largest delay, because the processor, or at least the thread of execution, has to wait  until the instruction is fetched from main memory. Cache read misses from a data cache usually cause a smaller delay, because instructions not dependent on the cache read can be issued and continue execution until the data is returned from main memory, and the dependent instructions can resume execution. Cache write misses to a data cache generally cause the shortest delay, because the write can be queued and there are few limitations on the execution of subsequent instructions; the processor can continue until the queue is full. For a detailed introduction to the types of misses, see cache performance measurement and metric."
+3548,"Most general purpose CPUs implement some form of virtual memory. To summarize, either each program running on the machine sees its own simplified address space, which contains code and data for that program only, or all programs run in a common virtual address space. A program executes by calculating, comparing, reading and writing to addresses of its virtual address space, rather than addresses of physical address space, making programs simpler and thus easier to write."
+3549,"Virtual memory requires the processor to translate virtual addresses generated by the program into physical addresses in main memory. The portion of the processor that does this translation is known as the memory management unit . The fast path through the MMU can perform those translations stored in the translation lookaside buffer , which is a cache of mappings from the operating system's page table, segment table, or both."
+3550,"For the purposes of the present discussion, there are three important features of address translation:"
+3551,"One early virtual memory system, the IBM M44/44X, required an access to a mapping table held in core memory before every programmed access to main memory. With no caches, and with the mapping table memory running at the same speed as main memory this effectively cut the speed of memory access in half. Two early machines that used a page table in main memory for mapping, the IBM System/360 Model 67 and the GE 645, both had a small associative memory as a cache for accesses to the in-memory page table. Both machines predated the first machine with a cache for main memory, the IBM System/360 Model 85, so the first hardware cache used in a computer system was not a data or instruction cache, but rather a TLB."
+3552,"Caches can be divided into four types, based on whether the index or tag correspond to physical or virtual addresses:"
+3553,"The speed of this recurrence  is crucial to CPU performance, and so most modern level-1 caches are virtually indexed, which at least allows the MMU's TLB lookup to proceed in parallel with fetching the data from the cache RAM."
+3554,"But virtual indexing is not the best choice for all cache levels. The cost of dealing with virtual aliases grows with cache size, and as a result most level-2 and larger caches are physically indexed."
+3555,"Caches have historically used both virtual and physical addresses for the cache tags, although virtual tagging is now uncommon. If the TLB lookup can finish before the cache RAM lookup, then the physical address is available in time for tag compare, and there is no need for virtual tagging. Large caches, then, tend to be physically tagged, and only small, very low latency caches are virtually tagged. In recent general-purpose CPUs, virtual tagging has been superseded by vhints, as described below."
+3556,"A cache that relies on virtual indexing and tagging becomes inconsistent after the same virtual address is mapped into different physical addresses , which can be solved by using physical address for tagging, or by storing the address space identifier in the cache line. However, the latter approach does not help against the synonym problem, in which several cache lines end up storing data for the same physical address. Writing to such locations may update only one location in the cache, leaving the others with inconsistent data. This issue may be solved by using non-overlapping memory layouts for different address spaces, or otherwise the cache  must be flushed when the mapping changes."
+3557,"The great advantage of virtual tags is that, for associative caches, they allow the tag match to proceed before the virtual to physical translation is done. However, coherence probes and evictions present a physical address for action. The hardware must have some means of converting the physical addresses into a cache index, generally by storing physical tags as well as virtual tags. For comparison, a physically tagged cache does not need to keep virtual tags, which is simpler. When a virtual to physical mapping is deleted from the TLB, cache entries with those virtual addresses will have to be flushed somehow. Alternatively, if cache entries are allowed on pages not mapped by the TLB, then those entries will have to be flushed when the access rights on those pages are changed in the page table."
+3558,"It is also possible for the operating system to ensure that no virtual aliases are simultaneously resident in the cache. The operating system makes this guarantee by enforcing page coloring, which is described below. Some early RISC processors  took this approach. It has not been used recently, as the hardware cost of detecting and evicting virtual aliases has fallen and the software complexity and performance penalty of perfect page coloring has risen."
+3559,"It can be useful to distinguish the two functions of tags in an associative cache: they are used to determine which way of the entry set to select, and they are used to determine if the cache hit or missed. The second function must always be correct, but it is permissible for the first function to guess, and get the wrong answer occasionally."
+3560,"Some processors  have caches with both virtual and physical tags. The virtual tags are used for way selection, and the physical tags are used for determining hit or miss. This kind of cache enjoys the latency advantage of a virtually tagged cache, and the simple software interface of a physically tagged cache. It bears the added cost of duplicated tags, however. Also, during miss processing, the alternate ways of the cache line indexed have to be probed for virtual aliases and any matches evicted."
+3561,"The extra area  can be mitigated by keeping virtual hints with each cache entry instead of virtual tags. These hints are a subset or hash of the virtual tag, and are used for selecting the way of the cache from which to get data and a physical tag. Like a virtually tagged cache, there may be a virtual hint match but physical tag mismatch, in which case the cache entry with the matching hint must be evicted so that cache accesses after the cache fill at this address will have just one hint match. Since virtual hints have fewer bits than virtual tags distinguishing them from one another, a virtually hinted cache suffers more conflict misses than a virtually tagged cache."
+3562,"Perhaps the ultimate reduction of virtual hints can be found in the Pentium 4 . In these processors the virtual hint is effectively two bits, and the cache is four-way set associative. Effectively, the hardware maintains a simple permutation from virtual address to cache index, so that no content-addressable memory  is necessary to select the right one of the four ways fetched."
+3563,"Large physically indexed caches  run into a problem: the operating system rather than the application controls which pages collide with one another in the cache. Differences in page allocation from one program run to the next lead to differences in the cache collision patterns, which can lead to very large differences in program performance. These differences can make it very difficult to get a consistent and repeatable timing for a benchmark run."
+3564,"To understand the problem, consider a CPU with a 1 MiB physically indexed direct-mapped level-2 cache and 4 KiB virtual memory pages. Sequential physical pages map to sequential locations in the cache until after 256 pages the pattern wraps around. We can label each physical page with a color of 0–255 to denote where in the cache it can go. Locations within physical pages with different colors cannot conflict in the cache."
+3565,"Programmers attempting to make maximum use of the cache may arrange their programs' access patterns so that only 1 MiB of data need be cached at any given time, thus avoiding capacity misses. But they should also ensure that the access patterns do not have conflict misses. One way to think about this problem is to divide up the virtual pages the program uses and assign them virtual colors in the same way as physical colors were assigned to physical pages before. Programmers can then arrange the access patterns of their code so that no two pages with the same virtual color are in use at the same time. There is a wide literature on such optimizations , largely coming from the High Performance Computing  community."
+3566,"The snag is that while all the pages in use at any given moment may have different virtual colors, some may have the same physical colors. In fact, if the operating system assigns physical pages to virtual pages randomly and uniformly, it is extremely likely that some pages will have the same physical color, and then locations from those pages will collide in the cache ."
+3567,"The solution is to have the operating system attempt to assign different physical color pages to different virtual colors, a technique called page coloring. Although the actual mapping from virtual to physical color is irrelevant to system performance, odd mappings are difficult to keep track of and have little benefit, so most approaches to page coloring simply try to keep physical and virtual page colors the same."
+3568,"If the operating system can guarantee that each physical page maps to only one virtual color, then there are no virtual aliases, and the processor can use virtually indexed caches with no need for extra virtual alias probes during miss handling. Alternatively, the OS can flush a page from the cache whenever it changes from one virtual color to another. As mentioned above, this approach was used for some early SPARC and RS/6000 designs."
+3569,The software page coloring technique has been used to effectively partition the shared Last level Cache  in multicore processors. This operating system-based LLC management in multicore processors has been adopted by Intel.
+3570,"Modern processors have multiple interacting on-chip caches. The operation of a particular cache can be completely specified by the cache size, the cache block size, the number of blocks in a set, the cache set replacement policy, and the cache write policy ."
+3571,"While all of the cache blocks in a particular cache are the same size and have the same associativity, typically the ""lower-level"" caches  have a smaller number of blocks, smaller block size, and fewer blocks in a set, but have very short access times. ""Higher-level"" caches  have progressively larger numbers of blocks, larger block size, more blocks in a set, and relatively longer access times, but are still much faster than main memory."
+3572,"Cache entry replacement policy is determined by a cache algorithm selected to be implemented by the processor designers. In some cases, multiple algorithms are provided for different kinds of work loads."
+3573,"Pipelined CPUs access memory from multiple points in the pipeline: instruction fetch, virtual-to-physical address translation, and data fetch . The natural design is to use different physical caches for each of these points, so that no one physical resource has to be scheduled to service two points in the pipeline. Thus the pipeline naturally ends up with at least three separate caches , each specialized to its particular role."
+3574,"A victim cache is a cache used to hold blocks evicted from a CPU cache upon replacement. The victim cache lies between the main cache and its refill path, and holds only those blocks of data that were evicted from the main cache. The victim cache is usually fully associative, and is intended to reduce the number of conflict misses. Many commonly used programs do not require an associative mapping for all the accesses. In fact, only a small fraction of the memory accesses of the program require high associativity. The victim cache exploits this property by providing high associativity to only these accesses. It was introduced by Norman Jouppi from DEC in 1990."
+3575,Intel's Crystalwell variant of its Haswell processors introduced an on-package 128 MiB eDRAM Level 4 cache which serves as a victim cache to the processors' Level 3 cache. In the Skylake microarchitecture the Level 4 cache no longer works as a victim cache.
+3576,One of the more extreme examples of cache specialization is the trace cache  found in the Intel Pentium 4 microprocessors. A trace cache is a mechanism for increasing the instruction fetch bandwidth and decreasing power consumption  by storing traces of instructions that have already been fetched and decoded.
+3577,"A trace cache stores instructions either after they have been decoded, or as they are retired. Generally, instructions are added to trace caches in groups representing either individual basic blocks or dynamic instruction traces. The Pentium 4's trace cache stores micro-operations resulting from decoding x86 instructions, providing also the functionality of a micro-operation cache. Having this, the next time an instruction is needed, it does not have to be decoded into micro-ops again.: 63–68"
+3578,"Write Coalescing Cache is a special cache that is part of L2 cache in AMD's Bulldozer microarchitecture. Stores from both L1D caches in the module go through the WCC, where they are buffered and coalesced.
+The WCC's task is reducing number of writes to the L2 cache."
+3579,"A micro-operation cache  is a specialized cache that stores micro-operations of decoded instructions, as received directly from the instruction decoders or from the instruction cache. When an instruction needs to be decoded, the μop cache is checked for its decoded form which is re-used if cached; if it is not available, the instruction is decoded and then cached."
+3580,"One of the early works describing μop cache as an alternative frontend for the Intel P6 processor family is the 2001 paper ""Micro-Operation Cache: A Power Aware Frontend for Variable Instruction Length ISA"". Later, Intel included μop caches in its Sandy Bridge processors and in successive microarchitectures like Ivy Bridge and Haswell.: 121–123  AMD implemented a μop cache in their Zen microarchitecture."
+3581,"Fetching complete pre-decoded instructions eliminates the need to repeatedly decode variable length complex instructions into simpler fixed-length micro-operations, and simplifies the process of predicting, fetching, rotating and aligning fetched instructions. A μop cache effectively offloads the fetch and decode hardware, thus decreasing power consumption and improving the frontend supply of decoded micro-operations. The μop cache also increases performance by more consistently delivering decoded micro-operations to the backend and eliminating various bottlenecks in the CPU's fetch and decode logic."
+3582,"A μop cache has many similarities with a trace cache, although a μop cache is much simpler thus providing better power efficiency; this makes it better suited for implementations on battery-powered devices. The main disadvantage of the trace cache, leading to its power inefficiency, is the hardware complexity required for its heuristic deciding on caching and reusing dynamically created instruction traces."
+3583,"A branch target cache or branch target instruction cache, the name used on ARM microprocessors, is a specialized cache which holds the first few instructions at the destination of a taken branch. This is used by low-powered processors which do not need a normal instruction cache because the memory system is capable of delivering instructions fast enough to satisfy the CPU without one. However, this only applies to consecutive instructions in sequence; it still takes several cycles of latency to restart instruction fetch at a new address, causing a few cycles of pipeline bubble after a control transfer. A branch target cache provides instructions for those few cycles avoiding a delay after most taken branches."
+3584,This allows full-speed operation with a much smaller cache than a traditional full-time instruction cache.
+3585,"Smart cache is a level 2 or level 3 caching method for multiple execution cores, developed by Intel."
+3586,"Smart Cache shares the actual cache memory between the cores of a multi-core processor. In comparison to a dedicated per-core cache, the overall cache miss rate decreases when cores do not require equal parts of the cache space. Consequently, a single core can use the full level 2 or level 3 cache while the other cores are inactive. Furthermore, the shared cache makes it faster to share memory among different execution cores."
+3587,"Another issue is the fundamental tradeoff between cache latency and hit rate. Larger caches have better hit rates but longer latency. To address this tradeoff, many computers use multiple levels of cache, with small fast caches backed up by larger, slower caches. Multi-level caches generally operate by checking the fastest cache, level 1 , first; if it hits, the processor proceeds at high speed. If that smaller cache misses, the next fastest cache, level 2 , is checked, and so on, before accessing external memory."
+3588,"As the latency difference between main memory and the fastest cache has become larger, some processors have begun to utilize as many as three levels of on-chip cache. Price-sensitive designs used this to pull the entire cache hierarchy on-chip, but by the 2010s some of the highest-performance designs returned to having large off-chip caches, which is often implemented in eDRAM and mounted on a multi-chip module, as a fourth cache level. In rare cases, such as in the mainframe CPU IBM z15 , all levels down to L1 are implemented by eDRAM, replacing SRAM entirely . The ARM-based Apple M1 has a 192 KiB L1 cache for each of the four high-performance cores, an unusually large amount; however the four high-efficiency cores only have 128 KiB."
+3589,The benefits of L3 and L4 caches depend on the application's access patterns. Examples of products incorporating L3 and L4 caches include the following:
+3590,"Finally, at the other end of the memory hierarchy, the CPU register file itself can be considered the smallest, fastest cache in the system, with the special characteristic that it is scheduled in software—typically by a compiler, as it allocates registers to hold values retrieved from main memory for, as an example, loop nest optimization. However, with register renaming most compiler register assignments are reallocated dynamically by hardware at runtime into a register bank, allowing the CPU to break false data dependencies and thus easing pipeline hazards."
+3591,"Register files sometimes also have hierarchy: The Cray-1  had eight address ""A"" and eight scalar data ""S"" registers that were generally usable. There was also a set of 64 address ""B"" and 64 scalar data ""T"" registers that took longer to access, but were faster than main memory. The ""B"" and ""T"" registers were provided because the Cray-1 did not have a data cache. "
+3592,"When considering a chip with multiple cores, there is a question of whether the caches should be shared or local to each core. Implementing shared cache inevitably introduces more wiring and complexity. But then, having one cache per chip, rather than core, greatly reduces the amount of space needed, and thus one can include a larger cache."
+3593,"Typically, sharing the L1 cache is undesirable because the resulting increase in latency would make each core run considerably slower than a single-core chip. However, for the highest-level cache, the last one called before accessing memory, having a global cache is desirable for several reasons, such as allowing a single core to use the whole cache, reducing data redundancy by making it possible for different processes or threads to share cached data, and reducing the complexity of utilized cache coherency protocols. For example, an eight-core chip with three levels may include an L1 cache for each core, one intermediate L2 cache for each pair of cores, and one L3 cache shared between all cores."
+3594,"A shared highest-level cache, which is called before accessing memory, is usually referred to as a last level cache . Additional techniques are used for increasing the level of parallelism when LLC is shared between multiple cores, including slicing it into multiple pieces which are addressing certain ranges of memory addresses, and can be accessed independently."
+3595,"In a separate cache structure, instructions and data are cached separately, meaning that a cache line is used to cache either instructions or data, but not both; various benefits have been demonstrated with separate data and instruction translation lookaside buffers. In a unified structure, this constraint is not present, and cache lines can be used to cache both instructions and data."
+3596,"Multi-level caches introduce new design decisions. For instance, in some processors, all data in the L1 cache must also be somewhere in the L2 cache. These caches are called strictly inclusive. Other processors  have exclusive caches: data is guaranteed to be in at most one of the L1 and L2 caches, never in both. Still other processors  do not require that data in the L1 cache also reside in the L2 cache, although it may often do so. There is no universally accepted name for this intermediate policy; two common names are ""non-exclusive"" and ""partially-inclusive""."
+3597,"The advantage of exclusive caches is that they store more data. This advantage is larger when the exclusive L1 cache is comparable to the L2 cache, and diminishes if the L2 cache is many times larger than the L1 cache. When the L1 misses and the L2 hits on an access, the hitting cache line in the L2 is exchanged with a line in the L1. This exchange is quite a bit more work than just copying a line from L2 to L1, which is what an inclusive cache does."
+3598,"One advantage of strictly inclusive caches is that when external devices or other processors in a multiprocessor system wish to remove a cache line from the processor, they need only have the processor check the L2 cache. In cache hierarchies which do not enforce inclusion, the L1 cache must be checked as well. As a drawback, there is a correlation between the associativities of L1 and L2 caches: if the L2 cache does not have at least as many ways as all L1 caches together, the effective associativity of the L1 caches is restricted. Another disadvantage of inclusive cache is that whenever there is an eviction in L2 cache, the  corresponding lines in L1 also have to get evicted in order to maintain inclusiveness. This is quite a bit of work, and would result in a higher L1 miss rate."
+3599,"Another advantage of inclusive caches is that the larger cache can use larger cache lines, which reduces the size of the secondary cache tags.  If the secondary cache is an order of magnitude larger than the primary, and the cache data is an order of magnitude larger than the cache tags, this tag area saved can be comparable to the incremental area needed to store the L1 cache data in the L2."
+3600,"Scratchpad memory , also known as scratchpad, scratchpad RAM or local store in computer terminology, is a high-speed internal memory used for temporary storage of calculations, data, and other work in progress."
+3601,"To illustrate both specialization and multi-level caching, here is the cache hierarchy of the K8 core in the AMD Athlon 64 CPU."
+3602,"The K8 has four specialized caches: an instruction cache, an instruction TLB, a data TLB, and a data cache. Each of these caches is specialized:"
+3603,"The K8 also has multiple-level caches. There are second-level instruction and data TLBs, which store only PTEs mapping 4 KiB. Both instruction and data caches, and the various TLBs, can fill from the large unified L2 cache. This cache is exclusive to both the L1 instruction and data caches, which means that any 8-byte line can only be in one of the L1 instruction cache, the L1 data cache, or the L2 cache. It is, however, possible for a line in the data cache to have a PTE which is also in one of the TLBs—the operating system is responsible for keeping the TLBs coherent by flushing portions of them when the page tables in memory are updated."
+3604,"The K8 also caches information that is never stored in memory—prediction information. These caches are not shown in the above diagram. As is usual for this class of CPU, the K8 has fairly complex
+branch prediction, with tables that help predict whether branches are taken and other tables which predict the targets of branches and jumps. Some of this information is associated with instructions, in both the level 1 instruction cache and the unified secondary cache."
+3605,"The K8 uses an interesting trick to store prediction information with instructions in the secondary cache. Lines in the secondary cache are protected from accidental data corruption  by either ECC or parity, depending on whether those lines were evicted from the data or instruction primary caches. Since the parity code takes fewer bits than the ECC code, lines from the instruction cache have a few spare bits. These bits are used to cache branch prediction information associated with those instructions. The net result is that the branch predictor has a larger effective history table, and so has better accuracy."
+3606,"Other processors have other kinds of predictors , and various specialized predictors are likely to flourish in future processors."
+3607,"These predictors are caches in that they store information that is costly to compute. Some of the terminology used when discussing predictors is the same as that for caches , but predictors are not generally thought of as part of the cache hierarchy."
+3608,"The K8 keeps the instruction and data caches coherent in hardware, which means that a store into an instruction closely following the store instruction will change that following instruction. Other processors, like those in the Alpha and MIPS family, have relied on software to keep the instruction cache coherent. Stores are not guaranteed to show up in the instruction stream until a program calls an operating system facility to ensure coherency."
+3609,"In computer engineering, a tag RAM is used to specify which of the possible memory locations is currently stored in a CPU cache. For a simple, direct-mapped design fast SRAM can be used. Higher associative caches usually employ content-addressable memory."
+3610,"Cache reads are the most common CPU operation that takes more than a single cycle. Program execution time tends to be very sensitive to the latency of a level-1 data cache hit. A great deal of design effort, and often power and silicon area are expended making the caches as fast as possible."
+3611,"The simplest cache is a virtually indexed direct-mapped cache. The virtual address is calculated with an adder, the relevant portion of the address extracted and used to index an SRAM, which returns the loaded data. The data is byte aligned in a byte shifter, and from there is bypassed to the next operation. There is no need for any tag checking in the inner loop – in fact, the tags need not even be read. Later in the pipeline, but before the load instruction is retired, the tag for the loaded data must be read, and checked against the virtual address to make sure there was a cache hit. On a miss, the cache is updated with the requested cache line and the pipeline is restarted."
+3612,"An associative cache is more complicated, because some form of tag must be read to determine which entry of the cache to select. An N-way set-associative level-1 cache usually reads all N possible tags and N data in parallel, and then chooses the data associated with the matching tag. Level-2 caches sometimes save power by reading the tags first, so that only one data element is read from the data SRAM."
+3613,"The adjacent diagram is intended to clarify the manner in which the various fields of the address are used. Address bit 31 is most significant, bit 0 is least significant. The diagram shows the SRAMs, indexing, and multiplexing for a 4 KiB, 2-way set-associative, virtually indexed and virtually tagged cache with 64 byte  lines, a 32-bit read width and 32-bit virtual address."
+3614,"Because the cache is 4 KiB and has 64 B lines, there are just 64 lines in the cache, and we read two at a time from a Tag SRAM which has 32 rows, each with a pair of 21 bit tags. Although any function of virtual address bits 31 through 6 could be used to index the tag and data SRAMs, it is simplest to use the least significant bits."
+3615,"Similarly, because the cache is 4 KiB and has a 4 B read path, and reads two ways for each access, the Data SRAM is 512 rows by 8 bytes wide."
+3616,"A more modern cache might be 16 KiB, 4-way set-associative, virtually indexed, virtually hinted, and physically tagged, with 32 B lines, 32-bit read width and 36-bit physical addresses. The read path recurrence for such a cache looks very similar to the path above. Instead of tags, vhints are read, and matched against a subset of the virtual address. Later on in the pipeline, the virtual address is translated into a physical address by the TLB, and the physical tag is read . Finally the physical address is compared to the physical tag to determine if a hit has occurred."
+3617,Some SPARC designs have improved the speed of their L1 caches by a few gate delays by collapsing the virtual address adder into the SRAM decoders. See sum-addressed decoder.
+3618,"The early history of cache technology is closely tied to the invention and use of virtual memory.  Because of scarcity and cost of semi-conductor memories, early mainframe computers in the 1960s used a complex hierarchy of physical memory, mapped onto a flat virtual memory space used by programs. The memory technologies would span semi-conductor, magnetic core, drum and disc. Virtual memory seen and used by programs would be flat and caching would be used to fetch data and instructions into the fastest memory ahead of processor access. Extensive studies were done to optimize the cache sizes. Optimal values were found to depend greatly on the programming language used with Algol needing the smallest and Fortran and Cobol needing the largest cache sizes."
+3619,"In the early days of microcomputer technology, memory access was only slightly slower than register access. But since the 1980s the performance gap between processor and memory has been growing. Microprocessors have advanced much faster than memory, especially in terms of their operating frequency, so memory became a performance bottleneck. While it was technically possible to have all the main memory as fast as the CPU, a more economically viable path has been taken: use plenty of low-speed memory, but also introduce a small high-speed cache memory to alleviate the performance gap. This provided an order of magnitude more capacity—for the same price—with only a slightly reduced combined performance."
+3620,"The first documented uses of a TLB were on the GE 645 and the IBM 360/67, both of which used an associative memory as a TLB."
+3621,The first documented use of an instruction cache was on the CDC 6600.
+3622,The first documented use of a data cache was on the IBM System/360 Model 85.
+3623,"The 68010, released in 1982, has a ""loop mode"" which can be considered a tiny and special-case instruction cache that accelerates loops that consist of only two instructions. The 68020, released in 1984, replaced that with a typical instruction cache of 256 bytes, being the first 68k series processor to feature true on-chip cache memory."
+3624,"The 68030, released in 1987, is basically a 68020 core with an additional 256-byte data cache, an on-chip memory management unit , a process shrink, and added burst mode for the caches. The 68040, released in 1990, has split instruction and data caches of four kilobytes each. The 68060, released in 1994, has the following: 8 KiB data cache , 8 KiB instruction cache , 96-byte FIFO instruction buffer, 256-entry branch cache, and 64-entry address translation cache MMU buffer ."
+3625,"As the x86 microprocessors reached clock rates of 20 MHz and above in the 386, small amounts of fast cache memory began to be featured in systems to improve performance. This was because the DRAM used for main memory had significant latency, up to 120 ns, as well as refresh cycles. The cache was constructed from more expensive, but significantly faster, SRAM memory cells, which at the time had latencies around 10–25 ns. The early caches were external to the processor and typically located on the motherboard in the form of eight or nine DIP devices placed in sockets to enable the cache as an optional extra or upgrade feature."
+3626,Some versions of the Intel 386 processor could support 16 to 256 KiB of external cache.
+3627,"With the 486 processor, an 8 KiB cache was integrated directly into the CPU die. This cache was termed Level 1 or L1 cache to differentiate it from the slower on-motherboard, or Level 2  cache. These on-motherboard caches were much larger, with the most common size being 256 KiB. There were some system boards that contained sockets for the Intel 485Turbocache daughtercard which had either 64 or 128 Kbyte of cache memory. The popularity of on-motherboard cache continued through the Pentium MMX era but was made obsolete by the introduction of SDRAM and the growing disparity between bus clock rates and CPU clock rates, which caused on-motherboard cache to be only slightly faster than main memory."
+3628,"The next development in cache implementation in the x86 microprocessors began with the Pentium Pro, which brought the secondary cache onto the same package as the microprocessor, clocked at the same frequency as the microprocessor."
+3629,"On-motherboard caches enjoyed prolonged popularity thanks to the AMD K6-2 and AMD K6-III processors that still used Socket 7, which was previously used by Intel with on-motherboard caches. K6-III included 256 KiB on-die L2 cache and took advantage of the on-board cache as a third level cache, named L3 . After the Socket 7 became obsolete, on-motherboard cache disappeared from the x86 systems."
+3630,"The three-level caches were used again first with the introduction of multiple processor cores, where the L3 cache was added to the CPU die. It became common for the total cache sizes to be increasingly larger in newer processor generations, and recently  it is not uncommon to find Level 3 cache sizes of tens of megabytes."
+3631,"Intel introduced a Level 4 on-package cache with the Haswell microarchitecture. Crystalwell Haswell CPUs, equipped with the GT3e variant of Intel's integrated Iris Pro graphics, effectively feature 128 MiB of embedded DRAM  on the same package. This L4 cache is shared dynamically between the on-die GPU and CPU, and serves as a victim cache to the CPU's L3 cache."
+3632,"Apple M1 CPU has 128 or 192 KiB instruction L1 cache for each core , depending on core type. This is an unusually large L1 cache for any CPU type ; the total cache memory size is not unusually large  for a laptop, and much larger total  sizes are available in IBM's mainframes."
+3633,"Early cache designs focused entirely on the direct cost of cache and RAM and average execution speed.
+More recent cache designs also consider energy efficiency, fault tolerance, and other goals."
+3634,"There are several tools available to computer architects to help explore tradeoffs between the cache cycle time, energy, and area; the CACTI cache simulator and the SimpleScalar instruction set simulator are two open-source options."
+3635,"A multi-ported cache is a cache which can serve more than one request at a time. When accessing a traditional cache we normally use a single memory address, whereas in a multi-ported cache we may request N addresses at a time – where N is the number of ports that connected through the processor and the cache. The benefit of this is that a pipelined processor may access memory from different phases in its pipeline. Another benefit is that it allows the concept of super-scalar processors through different cache levels."
+3636,"Firmware is found in a wide-range of computing devices including personal computers, phones, home appliances, vehicles, computer peripherals and in many of the digital chips inside each of these larger systems."
+3637,"Firmware is stored in non-volatile memory – either read-only memory  or programmable memory such as EPROM, EEPROM, or flash. Changing a device's firmware stored in ROM requires physically replacing the memory chip – although some chips are not designed to be removed after manufacture. Programmable firmware memory can be reprogrammed via a procedure sometimes called flashing."
+3638,Common reasons for changing firmware include fixing bugs and adding features.
+3639,"Ascher Opler used the term firmware in a 1967 Datamation article, as an intermediary term between ""hardware"" and ""software"". In this article, Opler was referring to a new kind of computer program that had a different practical and psychological purpose from traditional programs from the user's perspective."
+3640,"As computers began to increase in complexity, it became clear that various programs needed to first be initiated and run to provide a consistent environment necessary for running more complex programs at the user's discretion. This required programming the computer to run those programs automatically. Furthermore, as companies, universities, and marketers wanted to sell computers to laypeople with little technical knowledge, greater automation became necessary to allow a lay-user to easily run programs for practical purposes. This gave rise to a kind of software that a user would not consciously run, and it led to software that a lay user wouldn't even know about."
+3641,"Originally, it meant the contents of a writable control store , containing microcode that defined and implemented the computer's instruction set, and that could be reloaded to specialize or modify the instructions that the central processing unit  could execute. As originally used, firmware contrasted with hardware  and software . It was not composed of CPU machine instructions, but of lower-level microcode involved in the implementation of machine instructions. It existed on the boundary between hardware and software; thus the name firmware. Over time, popular usage extended the word firmware to denote any computer program that is tightly linked to hardware, including BIOS on PCs, boot firmware on smartphones, computer peripherals, or the control systems on simple consumer electronic devices such as microwave ovens, remote controls."
+3642,"In some respects, the various firmware components are as important as the operating system in a working computer. However, unlike most modern operating systems, firmware rarely has a well-evolved automatic mechanism of updating itself to fix any functionality issues detected after shipping the unit."
+3643,"A computer's firmware may be manually updated by a user via a small utility program. In contrast, firmware in mass storage devices  is less frequently updated, even when flash memory  storage is used for the firmware."
+3644,"Most computer peripherals are themselves special-purpose computers. Devices such as printers, scanners, webcams, and USB flash drives have internally-stored firmware; some devices may also permit field upgrading of their firmware."
+3645,Examples of computer firmware include:
+3646,"Consumer appliances like gaming consoles, digital cameras and portable music players support firmware upgrades. Some companies use firmware updates to add new playable file formats . Other features that may change with firmware updates include the GUI or even the battery life. Smartphones have a firmware over the air upgrade capability for adding new features and patching security issues."
+3647,"Since 1996, most automobiles have employed an on-board computer and various sensors to detect mechanical problems. As of 2010, modern vehicles also employ computer-controlled anti-lock braking systems  and computer-operated transmission control units . The driver can also get in-dash information while driving in this manner, such as real-time fuel economy and tire pressure readings. Local dealers can update most vehicle firmware."
+3648,Other firmware applications include:
+3649,"Flashing involves the overwriting of existing firmware or data, contained in EEPROM or flash memory module present in an electronic device, with new data. This can be done to upgrade a device or to change the provider of a service associated with the function of the device, such as changing from one mobile phone service provider to another or installing a new operating system. If firmware is upgradable, it is often done via a program from the provider, and will often allow the old firmware to be saved before upgrading so it can be reverted to if the process fails, or if the newer version performs worse. Free software replacements for vendor flashing tools have been developed, such as Flashrom."
+3650,"Sometimes, third parties develop an unofficial new or modified  version of firmware to provide new features or to unlock hidden functionality; this is referred to as custom firmware. An example is Rockbox as a firmware replacement for portable media players. There are many homebrew projects for various devices, which often unlock general-purpose computing functionality in previously limited devices ."
+3651,"Firmware hacks usually take advantage of the firmware update facility on many devices to install or run themselves. Some, however, must resort to exploits to run, because the manufacturer has attempted to lock the hardware to stop it from running unlicensed code."
+3652,Most firmware hacks are free software.
+3653,"The Moscow-based Kaspersky Lab discovered that a group of developers it refers to as the ""Equation Group"" has developed hard disk drive firmware modifications for various drive models, containing a trojan horse that allows data to be stored on the drive in locations that will not be erased even if the drive is formatted or wiped.  Although the Kaspersky Lab report did not explicitly claim that this group is part of the United States National Security Agency , evidence obtained from the code of various Equation Group software suggests that they are part of the NSA."
+3654,"Researchers from the Kaspersky Lab categorized the undertakings by Equation Group as the most advanced hacking operation ever uncovered, also documenting around 500 infections caused by the Equation Group in at least 42 countries."
+3655,"Mark Shuttleworth, the founder of the company Canonical, which created the Ubuntu Linux distribution, has described proprietary firmware as a security risk, saying that ""firmware on your device is the NSA's best friend"" and calling firmware ""a trojan horse of monumental proportions"". He has asserted that low-quality, closed source firmware is a major threat to system security: ""Your biggest mistake is to assume that the NSA is the only institution abusing this position of trust –  in fact, it's reasonable to assume that all firmware is a cesspool of insecurity, courtesy of incompetence of the highest degree from manufacturers, and competence of the highest degree from a very wide range of such agencies"".  As a potential solution to this problem, he has called for declarative firmware, which would describe ""hardware linkage and dependencies"" and ""should not include executable code"". Firmware should be open-source so that the code can be checked and verified."
+3656,"Custom firmware hacks have also focused on injecting malware into devices such as smartphones or USB devices. One such smartphone injection was demonstrated on the Symbian OS at MalCon, a hacker convention. A USB device firmware hack called BadUSB was presented at the Black Hat USA 2014 conference, demonstrating how a USB flash drive microcontroller can be reprogrammed to spoof various other device types to take control of a computer, exfiltrate data, or spy on the user. Other security researchers have worked further on how to exploit the principles behind BadUSB, releasing at the same time the source code of hacking tools that can be used to modify the behavior of different USB devices."
+3657,"An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is embedded as part of a complete device often including electrical or electronic hardware and mechanical parts. 
+Because an embedded system typically controls physical operations of the machine that it is embedded within, it often has real-time computing constraints.  Embedded systems control many devices in common use. In 2009, it was estimated that ninety-eight percent of all microprocessors manufactured were used in embedded systems."
+3658,"Modern embedded systems are often based on microcontrollers , but ordinary microprocessors  are also common, especially in more complex systems. In either case, the processor used may be types ranging from general purpose to those specialized in a certain class of computations, or even custom designed for the application at hand. A common standard class of dedicated processors is the digital signal processor ."
+3659,"Since the embedded system is dedicated to specific tasks, design engineers can optimize it to reduce the size and cost of the product and increase its reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale."
+3660,"Embedded systems range in size from portable personal devices such as digital watches and MP3 players to bigger machines like home appliances, industrial assembly lines, robots, transport vehicles, traffic light controllers, and medical imaging systems. Often they constitute subsystems of other machines like avionics in aircraft and astrionics in spacecraft. Large installations like factories, pipelines, and electrical grids rely on multiple embedded systems networked together. Generalized through software customization, embedded systems such as programmable logic controllers frequently comprise their functional units."
+3661,"Embedded systems range from those low in complexity, with a single microcontroller chip, to very high with multiple units, peripherals and networks, which may reside in equipment racks or across large geographical areas connected via long-distance communications lines."
+3662,"The origins of the microprocessor and the microcontroller can be traced back to the MOS integrated circuit, which is an integrated circuit chip fabricated from MOSFETs  and was developed in the early 1960s. By 1964, MOS chips had reached higher transistor density and lower manufacturing costs than bipolar chips. MOS chips further increased in complexity at a rate predicted by Moore's law, leading to large-scale integration  with hundreds of transistors on a single MOS chip by the late 1960s. The application of MOS LSI chips to computing was the basis for the first microprocessors, as engineers began recognizing that a complete computer processor system could be contained on several MOS LSI chips."
+3663,"The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was the Intel 4004, released in 1971. It was developed by Federico Faggin, using his silicon-gate MOS technology, along with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima."
+3664,"One of the first recognizably modern embedded systems was the Apollo Guidance Computer, developed ca. 1965 by Charles Stark Draper at the MIT Instrumentation Laboratory. At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project as it employed the then newly developed monolithic integrated circuits to reduce the computer's size and weight."
+3665,"An early mass-produced embedded system was the Autonetics D-17 guidance computer for the Minuteman missile, released in 1961. When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that represented the first high-volume use of integrated circuits."
+3666,"Since these early applications in the 1960s, embedded systems have come down in price and there has been a dramatic rise in processing power and functionality. An early microprocessor, the Intel 4004 , was designed for calculators and other small systems but still required external memory and support chips. By the early 1980s, memory, input and output system components had been integrated into the same chip as the processor forming a microcontroller. Microcontrollers find applications where a general-purpose computer would be too costly. As the cost of microprocessors and microcontrollers fell, the prevalence of embedded systems increased."
+3667,"A comparatively low-cost microcontroller may be programmed to fulfill the same role as a large number of separate components. With microcontrollers, it became feasible to replace, even in consumer products, expensive knob-based analog components such as potentiometers and variable capacitors with up/down buttons or knobs read out by a microprocessor. Although in this context an embedded system is usually more complex than a traditional solution, most of the complexity is contained within the microcontroller itself. Very few additional components may be needed and most of the design effort is in the software. Software prototype and test can be quicker compared with the design and construction of a new circuit not using an embedded processor."
+3668,"Embedded systems are commonly found in consumer, industrial, automotive, home appliances, medical, telecommunication, commercial, aerospace and military applications."
+3669,Telecommunications systems employ numerous embedded systems from telephone switches for the network to cell phones at the end user. Computer networking uses dedicated routers and network bridges to route data.
+3670,"Consumer electronics include MP3 players, television sets, mobile phones, video game consoles, digital cameras, GPS receivers, and printers. Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility, efficiency and features. Advanced heating, ventilation, and air conditioning  systems use networked thermostats to more accurately and efficiently control temperature that can change by time of day and season. Home automation uses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, surveillance, etc., all of which use embedded devices for sensing and controlling."
+3671,"Transportation systems from flight to automobiles increasingly use embedded systems. New airplanes contain advanced avionics such as inertial guidance systems and GPS receivers that also have considerable safety requirements. Spacecraft rely on astrionics systems for trajectory correction. Various electric motors — brushless DC motors, induction motors and DC motors — use electronic motor controllers. Automobiles, electric vehicles, and hybrid vehicles increasingly use embedded systems to maximize efficiency and reduce pollution. Other automotive safety systems using embedded systems include anti-lock braking system , electronic stability control , traction control  and automatic four-wheel drive."
+3672,"Medical equipment uses embedded systems for monitoring, and various medical imaging , single-photon emission computed tomography , computed tomography , and magnetic resonance imaging  for non-invasive internal inspections. Embedded systems within medical equipment are often powered by industrial computers."
+3673,"Embedded systems are used for safety-critical systems in aerospace and defense industries. Unless connected to wired or wireless networks via on-chip 3G cellular or other methods for IoT monitoring and control purposes, these systems can be isolated from hacking and thus be more secure. For fire safety, the systems can be designed to have a greater ability to handle higher temperatures and continue to operate. In dealing with security, the embedded systems can be self-sufficient and be able to deal with cut electrical and communication systems."
+3674,"Miniature wireless devices called motes are networked wireless sensors. Wireless sensor networking makes use of miniaturization made possible by advanced integrated circuit  design to couple full wireless subsystems to sophisticated sensors, enabling people and companies to measure a myriad of things in the physical world and act on this information through monitoring and control systems. These motes are completely self-contained and will typically run off a battery source for years before the batteries need to be changed or charged."
+3675,"Embedded systems are designed to perform a specific task, in contrast with general-purpose computers designed for multiple tasks. Some have real-time performance constraints that must be met, for reasons such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs."
+3676,"Embedded systems are not always standalone devices. Many embedded systems are a small part within a larger device that serves a more general purpose. For example, the Gibson Robot Guitar features an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is to play music. Similarly, an embedded system in an automobile provides a specific function as a subsystem of the car itself."
+3677,"The program instructions written for embedded systems are referred to as firmware, and are stored in read-only memory or flash memory chips. They run with limited computer hardware resources: little memory, small or non-existent keyboard or screen."
+3678,"Embedded systems range from no user interface at all, in systems dedicated to one task, to complex graphical user interfaces that resemble modern computer desktop operating systems. Simple embedded devices use buttons, light-emitting diodes , graphic or character liquid-crystal displays  with a simple menu system. More sophisticated devices that use a graphical screen with touch sensing or screen-edge soft keys provide flexibility while minimizing space used: the meaning of the buttons can change with the screen, and selection involves the natural behavior of pointing at what is desired."
+3679,"Some systems provide user interface remotely with the help of a serial  or network  connection. This approach extends the capabilities of the embedded system, avoids the cost of a display, simplifies the board support package  and allows designers to build a rich user interface on the PC. A good example of this is the combination of an embedded HTTP server running on an embedded device . The user interface is displayed in a web browser on a PC connected to the device."
+3680,"Examples of properties of typical embedded computers when compared with general-purpose counterparts, are low power consumption, small size, rugged operating ranges, and low per-unit cost. This comes at the expense of limited processing resources."
+3681,"Numerous microcontrollers have been developed for embedded systems use. General-purpose microprocessors are also used in embedded systems, but generally, require more support circuitry than microcontrollers."
+3682,"PC/104 and PC/104+ are examples of standards for ready-made computer boards intended for small, low-volume embedded and ruggedized systems. These are mostly x86-based and often physically small compared to a standard PC, although still quite large compared to most simple  embedded systems. They may use DOS, FreeBSD, Linux, NetBSD, OpenHarmony or an embedded real-time operating system  such as MicroC/OS-II, QNX or VxWorks."
+3683,"In certain applications, where small size or power efficiency are not primary concerns, the components used may be compatible with those used in general-purpose x86 personal computers. Boards such as the VIA EPIA range help to bridge the gap by being PC-compatible but highly integrated, physically smaller or have other attributes making them attractive to embedded engineers. The advantage of this approach is that low-cost commodity components may be used along with the same software development tools used for general software development. Systems built in this way are still regarded as embedded since they are integrated into larger devices and fulfill a single role. Examples of devices that may adopt this approach are automated teller machines  and arcade machines, which contain code specific to the application."
+3684,"However, most ready-made embedded systems boards are not PC-centered and do not use the ISA or PCI busses. When a system-on-a-chip processor is involved, there may be little benefit to having a standardized bus connecting discrete components, and the environment for both hardware and software tools may be very different."
+3685,"One common design style uses a small system module, perhaps the size of a business card, holding high density BGA chips such as an ARM-based system-on-a-chip processor and peripherals, external flash memory for storage, and DRAM for runtime memory. The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually including Linux and some real-time choices. These modules can be manufactured in high volume, by organizations familiar with their specialized testing issues, and combined with much lower volume custom mainboards with application-specific external peripherals. Prominent examples of this approach include Arduino and Raspberry Pi."
+3686,"A system on a chip  contains a complete system - consisting of multiple processors, multipliers, caches, even different types of memory and commonly various peripherals like interfaces for wired or wireless communication on a single chip. Often graphics processing units  and DSPs are included such chips. SoCs can be implemented as an application-specific integrated circuit  or using a field-programmable gate array  which typically can be reconfigured."
+3687,"ASIC implementations are common for very-high-volume embedded systems like mobile phones and smartphones. ASIC or FPGA implementations may be used for not-so-high-volume embedded systems with special needs in kind of signal processing performance, interfaces and reliability, like in avionics."
+3688,"Embedded systems talk with the outside world via peripherals, such as:"
+3689,"As with other software, embedded system designers use compilers, assemblers, and debuggers to develop embedded system software. However, they may also use more specific tools:"
+3690,Software tools can come from several sources:
+3691,"As the complexity of embedded systems grows, higher-level tools and operating systems are migrating into machinery where it makes sense. For example, cellphones, personal digital assistants and other consumer computers often need significant software that is purchased or provided by a person other than the manufacturer of the electronics. In these systems, an open programming environment such as Linux, NetBSD, FreeBSD, OSGi or Embedded Java is required so that the third-party software provider can sell to a large market."
+3692,"Embedded debugging may be performed at different levels, depending on the facilities available. Considerations include: does it slow down the main application, how close is the debugged system or application to the actual system or application, how expressive are the triggers that can be set for debugging , and what can be inspected in the debugging process ."
+3693,From simplest to most sophisticated debugging techniques and systems be roughly grouped into the following areas:
+3694,"Unless restricted to external debugging, the programmer can typically load and run software through the tools, view the code running in the processor, and start or stop its operation. The view of the code may be as high-level programming language, assembly code or mixture of both."
+3695,"Real-time operating systems often support tracing of operating system events. A graphical view is presented by a host PC tool, based on a recording of the system behavior. The trace recording can be performed in software, by the RTOS, or by special tracing hardware. RTOS tracing allows developers to understand timing and performance issues of the software system and gives a good understanding of the high-level system behaviors. Trace recording in embedded systems can be achieved using hardware or software solutions. Software-based trace recording does not require specialized debugging hardware and can be used to record traces in deployed devices, but it can have an impact on CPU and RAM usage. One example of a software-based tracing method used in RTOS environments is the use of empty macros which are invoked by the operating system at strategic places in the code, and can be implemented to serve as hooks."
+3696,"Embedded systems often reside in machines that are expected to run continuously for years without error, and in some cases recover by themselves if an error occurs. Therefore, the software is usually developed and tested more carefully than that for personal computers, and unreliable mechanical moving parts such as disk drives, switches or buttons are avoided."
+3697,Specific reliability issues may include:
+3698,"A variety of techniques are used, sometimes in combination, to recover from errors—both software bugs such as memory leaks, and also soft errors in the hardware:"
+3699,"For high-volume systems such as mobile phones, minimizing cost is usually the primary design consideration. Engineers typically select hardware that is just good enough to implement the necessary functions."
+3700,"For low-volume or prototype embedded systems, general-purpose computers may be adapted by limiting the programs or by replacing the operating system with an RTOS."
+3701,"In 1978 National Electrical Manufacturers Association released ICS 3-1978, a standard for programmable microcontrollers, including almost any computer-based controllers, such as single-board computers, numerical, and event-based controllers."
+3702,There are several different types of software architecture in common use.
+3703,"In this design, the software simply has a loop which monitors the input devices. The loop calls subroutines, each of which manages a part of the hardware or software. Hence it is called a simple control loop or programmed input-output."
+3704,"Some embedded systems are predominantly controlled by interrupts. This means that tasks performed by the system are triggered by different kinds of events; an interrupt could be generated, for example, by a timer at a predefined interval, or by a serial port controller receiving data."
+3705,"This architecture is used if event handlers need low latency, and the event handlers are short and simple. These systems run a simple task in a main loop also, but this task is not very sensitive to unexpected delays. Sometimes the interrupt handler will add longer tasks to a queue structure. Later, after the interrupt handler has finished, these tasks are executed by the main loop. This method brings the system close to a multitasking kernel with discrete processes."
+3706,"Cooperative multitasking is very similar to the simple control loop scheme, except that the loop is hidden in an API. The programmer defines a series of tasks, and each task gets its own environment to run in. When a task is idle, it calls an idle routine which passes control to another task."
+3707,"The advantages and disadvantages are similar to that of the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue."
+3708,"In this type of system, a low-level piece of code switches between tasks or threads based on a timer invoking an interrupt. This is the level at which the system is generally considered to have an operating system kernel. Depending on how much functionality is required, it introduces more or less of the complexities of managing multiple tasks running conceptually in parallel."
+3709,"As any code can potentially damage the data of another task  programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy such as message queues, semaphores or a non-blocking synchronization scheme."
+3710,"Because of these complexities, it is common for organizations to use an off-the-shelf RTOS, allowing the application programmers to concentrate on device functionality rather than operating system services. The choice to include an RTOS brings in its own issues, however, as the selection must be made prior to starting the application development process. This timing forces developers to choose the embedded operating system for their device based on current requirements and so restricts future options to a large extent."
+3711,"The level of complexity in embedded systems is continuously growing as devices are required to manage peripherals and tasks such as serial, USB, TCP/IP, Bluetooth, Wireless LAN, trunk radio, multiple channels, data and voice, enhanced graphics, multiple states, multiple threads, numerous wait states and so on. These trends are leading to the uptake of embedded middleware in addition to an RTOS."
+3712,"A microkernel allocates memory and switches the CPU to different threads of execution. User-mode processes implement major functions such as file systems, network interfaces, etc."
+3713,Exokernels communicate efficiently by normal subroutine calls. The hardware and all the software in the system are available to and extensible by application programmers.
+3714,"A monolithic kernel is a relatively large kernel with sophisticated capabilities adapted to suit an embedded environment. This gives programmers an environment similar to a desktop operating system like Linux or Microsoft Windows, and is therefore very productive for development. On the downside, it requires considerably more hardware resources, is often more expensive, and, because of the complexity of these kernels, can be less predictable and reliable."
+3715,"Common examples of embedded monolithic kernels are embedded Linux, VXWorks and Windows CE."
+3716,"Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such as wireless routers and GPS navigation systems."
+3717,"In addition to the core operating system, many embedded systems have additional upper-layer software components. These components include networking protocol stacks like CAN, TCP/IP, FTP, HTTP, and HTTPS, and storage capabilities like FAT and flash memory management systems. If the embedded device has audio and video capabilities, then the appropriate drivers and codecs will be present in the system. In the case of the monolithic kernels, many of these software layers may be included in the kernel. In the RTOS category, the availability of additional software components depends upon the commercial offering."
+3718,"In the automotive sector, AUTOSAR is a standard architecture for embedded software."
+3719,"In computing, BIOS  is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the booting process . The BIOS firmware comes pre-installed on an IBM PC or IBM PC compatible's system board and exists in some UEFI-based systems to maintain compatibility with operating systems that do not support UEFI native operation. The name originates from the Basic Input/Output System used in the CP/M operating system in 1975. The BIOS originally proprietary to the IBM PC has been reverse engineered by some companies  looking to create compatible systems. The interface of that original system serves as a de facto standard."
+3720,"The BIOS in modern PCs initializes and tests the system hardware components , and loads a boot loader from a mass storage device which then initializes a kernel. In the era of DOS, the BIOS provided BIOS interrupt calls for the keyboard, display, storage, and other input/output  devices that standardized an interface to application programs and the operating system. More recent operating systems do not use the BIOS interrupt calls after startup."
+3721,"Most BIOS implementations are specifically designed to work with a particular computer or motherboard model, by interfacing with various devices especially system chipset. Originally, BIOS firmware was stored in a ROM chip on the PC motherboard. In later computer systems, the BIOS contents are stored on flash memory so it can be rewritten without removing the chip from the motherboard. This allows easy, end-user updates to the BIOS firmware so new features can be added or bugs can be fixed, but it also creates a possibility for the computer to become infected with BIOS rootkits. Furthermore, a BIOS upgrade that fails could brick the motherboard. The last version of Microsoft Windows to officially support running on PCs which use legacy BIOS firmware is Windows 10 as Windows 11 requires a UEFI-compliant system."
+3722,"Unified Extensible Firmware Interface  is a successor to the legacy PC BIOS, aiming to address its technical limitations."
+3723,"The term BIOS  was created by Gary Kildall and first appeared in the CP/M operating system in 1975, describing the machine-specific part of CP/M loaded during boot time that interfaces directly with the hardware. "
+3724,"Versions of MS-DOS, PC DOS or DR-DOS contain a file called variously ""IO.SYS"", ""IBMBIO.COM"", ""IBMBIO.SYS"", or ""DRBIOS.SYS""; this file is known as the ""DOS BIOS""  and contains the lower-level hardware-specific part of the operating system. Together with the underlying hardware-specific but operating system-independent ""System BIOS"", which resides in ROM, it represents the analogue to the ""CP/M BIOS""."
+3725,The BIOS originally proprietary to the IBM PC has been reverse engineered by some companies  looking to create compatible systems.
+3726,"With the introduction of PS/2 machines, IBM divided the System BIOS into real- and protected-mode portions. The real-mode portion was meant to provide backward compatibility with existing operating systems such as DOS, and therefore was named ""CBIOS"" , whereas the ""ABIOS""  provided new interfaces specifically suited for multitasking operating systems such as OS/2."
+3727,"The BIOS of the original IBM PC and XT had no interactive user interface. Error codes or messages were displayed on the screen, or coded series of sounds were generated to signal errors when the power-on self-test  had not proceeded to the point of successfully initializing a video display adapter. Options on the IBM PC and XT were set by switches and jumpers on the main board and on expansion cards. Starting around the mid-1990s, it became typical for the BIOS ROM to include a ""BIOS configuration utility""  or ""BIOS setup utility"", accessed at system power-up by a particular key sequence. This program allowed the user to set system configuration options, of the type formerly set using DIP switches, through an interactive menu system controlled through the keyboard. In the interim period, IBM-compatible PCs‍—‌including the IBM AT‍—‌held configuration settings in battery-backed RAM and used a bootable configuration program on floppy disk, not in the ROM, to set the configuration options contained in this memory. The floppy disk was supplied with the computer, and if it was lost the system settings could not be changed. The same applied in general to computers with an EISA bus, for which the configuration program was called an EISA Configuration Utility ."
+3728,"A modern Wintel-compatible computer provides a setup routine essentially unchanged in nature from the ROM-resident BIOS setup utilities of the late 1990s; the user can configure hardware options using the keyboard and video display. The modern Wintel machine may store the BIOS configuration settings in flash ROM, perhaps the same flash ROM that holds the BIOS itself."
+3729,"Early Intel processors started at physical address 000FFFF0h. Systems with later processors provide logic to start running the BIOS from the system ROM.
+"
+3730,"If the system has just been powered up or the reset button was pressed , the full power-on self-test  is run. If Ctrl+Alt+Delete was pressed , a special flag value stored in nonvolatile BIOS memory  tested by the BIOS allows bypass of the lengthy POST and memory detection."
+3731,"The POST identifies, tests and initializes system devices such as the CPU, chipset, RAM, motherboard, video card, keyboard, mouse, hard disk drive, optical disc drive and other hardware, including integrated peripherals."
+3732,Early IBM PCs had a routine in the POST that would download a program into RAM through the keyboard port and run it. This feature was intended for factory test or diagnostic purposes.
+3733,"After the option ROM scan is completed and all detected ROM modules with valid checksums have been called, or immediately after POST in a BIOS version that does not scan for option ROMs, the BIOS calls INT 19h to start boot processing. Post-boot, programs loaded can also call INT 19h to reboot the system, but they must be careful to disable interrupts and other asynchronous hardware processes that may interfere with the BIOS rebooting process, or else the system may hang or crash while it is rebooting."
+3734,"When INT 19h is called, the BIOS attempts to locate boot loader software on a ""boot device"", such as a hard disk, a floppy disk, CD, or DVD. It loads and executes the first boot software it finds, giving it control of the PC."
+3735,"The BIOS uses the boot devices set in Nonvolatile BIOS memory , or, in the earliest PCs, DIP switches. The BIOS checks each device in order to see if it is bootable by attempting to load the first sector . If the sector cannot be read, the BIOS proceeds to the next device. If the sector is read successfully, some BIOSes will also check for the boot sector signature 0x55 0xAA in the last two bytes of the sector , before accepting a boot sector and considering the device bootable."
+3736,"When a bootable device is found, the BIOS transfers control to the loaded sector. The BIOS does not interpret the contents of the boot sector other than to possibly check for the boot sector signature in the last two bytes. Interpretation of data structures like partition tables and BIOS Parameter Blocks is done by the boot program in the boot sector itself or by other programs loaded through the boot process."
+3737,"A non-disk device such as a network adapter attempts booting by a procedure that is defined by its option ROM or the equivalent integrated into the motherboard BIOS ROM. As such, option ROMs may also influence or supplant the boot process defined by the motherboard BIOS ROM."
+3738,"With the El Torito optical media boot standard, the optical drive actually emulates a 3.5"" high-density floppy disk to the BIOS for boot purposes. Reading the ""first sector"" of a CD-ROM or DVD-ROM is not a simply defined operation like it is on a floppy disk or a hard disk. Furthermore, the complexity of the medium makes it difficult to write a useful boot program in one sector. The bootable virtual floppy disk can contain software that provides access to the optical medium in its native format."
+3739,"The user can select the boot priority implemented by the BIOS. For example, most computers have a hard disk that is bootable, but sometimes there is a removable-media drive that has higher boot priority, so the user can cause a removable disk to be booted."
+3740,"In most modern BIOSes, the boot priority order can be configured by the user. In older BIOSes, limited boot priority options are selectable; in the earliest BIOSes, a fixed priority scheme was implemented, with floppy disk drives first, fixed disks  second, and typically no other boot devices supported, subject to modification of these rules by installed option ROMs. The BIOS in an early PC also usually would only boot from the first floppy disk drive or the first hard disk drive, even if there were two drives installed."
+3741,"On the original IBM PC and XT, if no bootable disk was found, ROM BASIC was started by calling INT 18h. Since few programs used BASIC in ROM, clone PC makers left it out; then a computer that failed to boot from a disk would display ""No ROM BASIC"" and halt ."
+3742,"Later computers would display a message like ""No bootable disk found""; some would prompt for a disk to be inserted and a key to be pressed to retry the boot process. A modern BIOS may display nothing or may automatically enter the BIOS configuration utility when the boot process fails."
+3743,"The environment for the boot program is very simple: the CPU is in real mode and the general-purpose and segment registers are undefined, except SS, SP, CS, and DL. CS:IP always points to physical address 0x07C00. What values CS and IP actually have is not well defined. Some BIOSes use a CS:IP of 0x0000:0x7C00 while others may use 0x07C0:0x0000. Because boot programs are always loaded at this fixed address, there is no need for a boot program to be relocatable. DL may contain the drive number, as used with INT 13h, of the boot device. SS:SP points to a valid stack that is presumably large enough to support hardware interrupts, but otherwise SS and SP are undefined.  The boot program must set up its own stack, because the size of the stack set up by BIOS is unknown and its location is likewise variable; although the boot program can investigate the default stack by examining SS:SP, it is easier and shorter to just unconditionally set up a new stack."
+3744,"At boot time, all BIOS services are available, and the memory below address 0x00400 contains the interrupt vector table. BIOS POST has initialized the system timers, interrupt controller, DMA controller, and other motherboard/chipset hardware as necessary to bring all BIOS services to ready status. DRAM refresh for all system DRAM in conventional memory and extended memory, but not necessarily expanded memory, has been set up and is running. The interrupt vectors corresponding to the BIOS interrupts have been set to point at the appropriate entry points in the BIOS, hardware interrupt vectors for devices initialized by the BIOS have been set to point to the BIOS-provided ISRs, and some other interrupts, including ones that BIOS generates for programs to hook, have been set to a default dummy ISR that immediately returns. The BIOS maintains a reserved block of system RAM at addresses 0x00400–0x004FF with various parameters initialized during the POST. All memory at and above address 0x00500 can be used by the boot program; it may even overwrite itself."
+3745,"Peripheral cards such as hard disk drive host bus adapters and video cards have their own firmware, and BIOS extension option ROM may be a part of the expansion card firmware, which provide additional functionality to BIOS. Code in option ROMs runs before the BIOS boots the operating system from mass storage. These ROMs typically test and initialize hardware, add new BIOS services, or replace existing BIOS services with their own services. For example, a SCSI controller usually has a BIOS extension ROM that adds support for hard drives connected through that controller. An extension ROM could in principle contain operating system, or it could implement an entirely different boot process such as network booting. Operation of an IBM-compatible computer system can be completely changed by removing or inserting an adapter card  that contains a BIOS extension ROM."
+3746,"The motherboard BIOS typically contains code for initializing and bootstrapping integrated display and integrated storage. In addition, plug-in adapter cards such as SCSI, RAID, network interface cards, and video cards often include their own BIOS , complementing or replacing the system BIOS code for the given component. Even devices built into the motherboard can behave in this way; their option ROMs can be a part of the motherboard BIOS."
+3747,An add-in card requires an option ROM if the card is not supported by the motherboard BIOS and the card needs to be initialized or made accessible through BIOS services before the operating system can be loaded . An additional advantage of ROM on some early PC systems  was that ROM was faster than main system RAM.  into RAM so it will run faster.)
+3748,"If an expansion ROM wishes to change the way the system boots  in a cooperative way, it can use the BIOS Boot Specification  API to register its ability to do so. Once the expansion ROMs have registered using the BBS APIs, the user can select among the available boot options from within the BIOS's user interface. This is why most BBS compliant PC BIOS implementations will not allow the user to enter the BIOS's user interface until the expansion ROMs have finished executing and registering themselves with the BBS API."
+3749,"Also, if an expansion ROM wishes to change the way the system boots unilaterally, it can simply hook INT 19h or other interrupts normally called from interrupt 19h, such as INT 13h, the BIOS disk service, to intercept the BIOS boot process. Then it can replace the BIOS boot process with one of its own, or it can merely modify the boot sequence by inserting its own boot actions into it, by preventing the BIOS from detecting certain devices as bootable, or both. Before the BIOS Boot Specification was promulgated, this was the only way for expansion ROMs to implement boot capability for devices not supported for booting by the native BIOS of the motherboard."
+3750,"After the motherboard BIOS completes its POST, most BIOS versions search for option ROM modules, also called BIOS extension ROMs, and execute them. The motherboard BIOS scans for extension ROMs in a portion of the ""upper memory area""  and runs each ROM found, in order. To discover memory-mapped option ROMs, a BIOS implementation scans the real-mode address space from 0x0C0000 to 0x0F0000 on 2 KB  boundaries, looking for a two-byte ROM signature: 0x55 followed by 0xAA. In a valid expansion ROM, this signature is followed by a single byte indicating the number of 512-byte blocks the expansion ROM occupies in real memory, and the next byte is the option ROM's entry point . If the ROM has a valid checksum, the BIOS transfers control to the entry address, which in a normal BIOS extension ROM should be the beginning of the extension's initialization routine."
+3751,"At this point, the extension ROM code takes over, typically testing and initializing the hardware it controls and registering interrupt vectors for use by post-boot applications. It may use BIOS services  to provide a user configuration interface, to display diagnostic information, or to do anything else that it requires. It is possible that an option ROM will not return to BIOS, pre-empting the BIOS's boot sequence altogether."
+3752,"An option ROM should normally return to the BIOS after completing its initialization process. Once  an option ROM returns, the BIOS continues searching for more option ROMs, calling each as it is found, until the entire option ROM area in the memory space has been scanned."
+3753,"Option ROMs normally reside on adapter cards. However, the original PC, and perhaps also the PC XT, have a spare ROM socket on the motherboard  into which an option ROM can be inserted, and the four ROMs that contain the BASIC interpreter can also be removed and replaced with custom ROMs which can be option ROMs. The IBM PCjr is unique among PCs in having two ROM cartridge slots on the front. Cartridges in these slots map into the same region of the upper memory area used for option ROMs, and the cartridges can contain option ROM modules that the BIOS would recognize. The cartridges can also contain other types of ROM modules, such as BASIC programs, that are handled differently. One PCjr cartridge can contain several ROM modules of different types, possibly stored together in one ROM chip."
+3754,"The BIOS ROM is customized to the particular manufacturer's hardware, allowing low-level services  to be provided in a standardized way to programs, including operating systems. For example, an IBM PC might have either a monochrome or a color display adapter , but a single, standard, BIOS system call may be invoked to display a character at a specified position on the screen in text mode or graphics mode."
+3755,"The BIOS provides a small library of basic input/output functions to operate peripherals . When using MS-DOS, BIOS services could be accessed by an application program  by executing an INT 13h interrupt instruction to access disk functions, or by executing one of a number of other documented BIOS interrupt calls to access video display, keyboard, cassette, and other device functions."
+3756,"Operating systems and executive software that are designed to supersede this basic firmware functionality provide replacement software interfaces to application software. Applications can also provide these services to themselves. This began even in the 1980s under MS-DOS, when programmers observed that using the BIOS video services for graphics display were very slow. To increase the speed of screen output, many programs bypassed the BIOS and programmed the video display hardware directly. Other graphics programmers, particularly but not exclusively in the demoscene, observed that there were technical capabilities of the PC display adapters that were not supported by the IBM BIOS and could not be taken advantage of without circumventing it. Since the AT-compatible BIOS ran in Intel real mode, operating systems that ran in protected mode on 286 and later processors required hardware device drivers compatible with protected mode operation to replace BIOS services."
+3757,"In modern PCs running modern operating systems  the BIOS interrupt calls is used only during booting and initial loading of operating systems. Before the operating system's first graphical screen is displayed, input and output are typically handled through BIOS. A boot menu such as the textual menu of Windows, which allows users to choose an operating system to boot, to boot into the safe mode, or to use the last known good configuration, is displayed through BIOS and receives keyboard input through BIOS."
+3758,"Many modern PCs can still boot and run legacy operating systems such as MS-DOS or DR-DOS that rely heavily on BIOS for their console and disk I/O, providing that the system has a BIOS, or a CSM-capable UEFI firmware."
+3759,"Intel processors have reprogrammable microcode since the P6 microarchitecture. AMD processors have reprogrammable microcode since the K7 microarchitecture. The BIOS contain patches to the processor microcode that fix errors in the initial processor microcode; microcode is loaded into processor's SRAM so reprogramming is not persistent, thus loading of microcode updates is performed each time the system is powered up. Without reprogrammable microcode, an expensive processor swap would be required; for example, the Pentium FDIV bug became an expensive fiasco for Intel as it required a product recall because the original Pentium processor's defective microcode could not be reprogrammed. Operating systems can update main processor microcode also."
+3760,"Some BIOSes contain a software licensing description table , a digital signature placed inside the BIOS by the original equipment manufacturer , for example Dell. The SLIC is inserted into the ACPI data table and contains no active code."
+3761,"Computer manufacturers that distribute OEM versions of Microsoft Windows and Microsoft application software can use the SLIC to authenticate licensing to the OEM Windows Installation disk and system recovery disc containing Windows software. Systems with a SLIC can be preactivated with an OEM product key, and they verify an XML formatted OEM certificate against the SLIC in the BIOS as a means of self-activating . If a user performs a fresh install of Windows, they will need to have possession of both the OEM key  and the digital certificate for their SLIC in order to bypass activation. This can be achieved if the user performs a restore using a pre-customised image provided by the OEM. Power users can copy the necessary certificate files from the OEM image, decode the SLP product key, then perform SLP activation manually."
+3762,"Some BIOS implementations allow overclocking, an action in which the CPU is adjusted to a higher clock rate than its manufacturer rating for guaranteed capability. Overclocking may, however, seriously compromise system reliability in insufficiently cooled computers and generally shorten component lifespan. Overclocking, when incorrectly performed, may also cause components to overheat so quickly that they mechanically destroy themselves."
+3763,"Some older operating systems, for example MS-DOS, rely on the BIOS to carry out most input/output tasks within the PC."
+3764,Calling real mode BIOS services directly is inefficient for protected mode  operating systems. BIOS interrupt calls are not used by modern multitasking operating systems after they initially load.
+3765,"In the 1990s, BIOS provided some protected mode interfaces for Microsoft Windows and Unix-like operating systems, such as Advanced Power Management , Plug and Play BIOS, Desktop Management Interface , VESA BIOS Extensions , e820 and MultiProcessor Specification . Starting from the 2000, most BIOSes provide ACPI, SMBIOS, VBE and e820 interfaces for modern operating systems."
+3766,"After operating systems load, the System Management Mode code is still running in SMRAM. Since 2010, BIOS technology is in a transitional process toward UEFI."
+3767,"Historically, the BIOS in the IBM PC and XT had no built-in user interface. The BIOS versions in earlier PCs  were not software configurable; instead, users set the options via DIP switches on the motherboard. Later computers, including all IBM-compatibles with 80286 CPUs, had a battery-backed nonvolatile BIOS memory  that held BIOS settings. These settings, such as video-adapter type, memory size, and hard-disk parameters, could only be configured by running a configuration program from a disk, not built into the ROM. A special ""reference diskette"" was inserted in an IBM AT to configure settings such as memory size."
+3768,"Early BIOS versions did not have passwords or boot-device selection options. The BIOS was hard-coded to boot from the first floppy drive, or, if that failed, the first hard disk. Access control in early AT-class machines was by a physical keylock switch . Anyone who could switch on the computer could boot it."
+3769,"Later, 386-class computers started integrating the BIOS setup utility in the ROM itself, alongside the BIOS code; these computers usually boot into the BIOS setup utility if a certain key or key combination is pressed, otherwise the BIOS POST and boot process are executed."
+3770,"A modern BIOS setup utility has a text user interface  or graphical user interface  accessed by pressing a certain key on the keyboard when the PC starts. Usually, the key is advertised for short time during the early startup, for example ""Press DEL to enter Setup"". The actual key depends on specific hardware. Features present in the BIOS setup utility typically include:"
+3771,"A modern BIOS setup screen often features a PC Health Status or a Hardware Monitoring tab, which directly interfaces with a Hardware Monitor chip of the mainboard. This makes it possible to monitor CPU and chassis temperature, the voltage provided by the power supply unit, as well as monitor and control the speed of the fans connected to the motherboard."
+3772,"Once the system is booted, hardware monitoring and computer fan control is normally done directly by the Hardware Monitor chip itself, which can be a separate chip, interfaced through I²C or SMBus, or come as a part of a Super I/O solution, interfaced through Industry Standard Architecture  or Low Pin Count . Some operating systems, like NetBSD with envsys and OpenBSD with sysctl hw.sensors, feature integrated interfacing with hardware monitors."
+3773,"However, in some circumstances, the BIOS also provides the underlying information about hardware monitoring through ACPI, in which case, the operating system may be using ACPI to perform hardware monitoring."
+3774,"In modern PCs the BIOS is stored in rewritable EEPROM or NOR flash memory, allowing the contents to be replaced and modified. This rewriting of the contents is sometimes termed flashing. It can be done by a special program, usually provided by the system's manufacturer, or at POST, with a BIOS image in a hard drive or USB flash drive. A file containing such contents is sometimes termed ""a BIOS image"". A BIOS might be reflashed in order to upgrade to a newer version to fix bugs or provide improved performance or to support newer hardware."
+3775,"The original IBM PC BIOS  was stored on mask-programmed read-only memory  chips in sockets on the motherboard. ROMs could be replaced, but not altered, by users. To allow for updates, many compatible computers used re-programmable BIOS memory devices such as EPROM, EEPROM and later flash memory  devices. According to Robert Braver, the president of the BIOS manufacturer Micro Firmware, Flash BIOS chips became common around 1995 because the electrically erasable PROM  chips are cheaper and easier to program than standard ultraviolet erasable PROM  chips. Flash chips are programmed  in-circuit, while EPROM chips need to be removed from the motherboard for re-programming. BIOS versions are upgraded to take advantage of newer versions of hardware and to correct bugs in previous revisions of BIOSes."
+3776,"Beginning with the IBM AT, PCs supported a hardware clock settable through BIOS. It had a century bit which allowed for manually changing the century when the year 2000 happened. Most BIOS revisions created in 1995 and nearly all BIOS revisions in 1997 supported the year 2000 by setting the century bit automatically when the clock rolled past midnight, 31 December 1999."
+3777,"The first flash chips were attached to the ISA bus. Starting in 1998, the BIOS flash moved to the LPC bus, following a new standard implementation known as ""firmware hub"" . In 2005, the BIOS flash memory moved to the SPI bus."
+3778,"The size of the BIOS, and the capacity of the ROM, EEPROM, or other media it may be stored on, has increased over time as new features have been added to the code; BIOS versions now exist with sizes up to 32 megabytes. For contrast, the original IBM PC BIOS was contained in an 8 KB mask ROM. Some modern motherboards are including even bigger NAND flash memory ICs on board which are capable of storing whole compact operating systems, such as some Linux distributions. For example, some ASUS notebooks included Splashtop OS embedded into their NAND flash memory ICs. However, the idea of including an operating system along with BIOS in the ROM of a PC is not new; in the 1980s, Microsoft offered a ROM option for MS-DOS, and it was included in the ROMs of some PC clones such as the Tandy 1000 HX."
+3779,"Another type of firmware chip was found on the IBM PC AT and early compatibles. In the AT, the keyboard interface was controlled by a microcontroller with its own programmable memory. On the IBM AT, that was a 40-pin socketed device, while some manufacturers used an EPROM version of this chip which resembled an EPROM. This controller was also assigned the A20 gate function to manage memory above the one-megabyte range; occasionally an upgrade of this ""keyboard BIOS"" was necessary to take advantage of software that could use upper memory."
+3780,"The BIOS may contain components such as the Memory Reference Code , which is responsible for the memory initialization .: 8 "
+3781,"Modern BIOS includes
+Intel Management Engine or AMD Platform Security Processor firmware."
+3782,"IBM published the entire listings of the BIOS for its original PC, PC XT, PC AT, and other contemporary PC models, in an appendix of the IBM PC Technical Reference Manual for each machine type. The effect of the publication of the BIOS listings is that anyone can see exactly what a definitive BIOS does and how it does it."
+3783,"In May 1984 Phoenix Software Associates released its first ROM-BIOS, which enabled OEMs to build essentially fully compatible clones without having to reverse-engineer the IBM PC BIOS themselves, as Compaq had done for the Portable, helping fuel the growth in the PC-compatibles industry and sales of non-IBM versions of DOS. And the first American Megatrends  BIOS was released in 1986."
+3784,"New standards grafted onto the BIOS are usually without complete public documentation or any BIOS listings. As a result, it is not as easy to learn the intimate details about the many non-IBM additions to BIOS as about the core BIOS services."
+3785,"Many PC motherboard suppliers licensed the BIOS ""core"" and toolkit from a commercial third party, known as an ""independent BIOS vendor"" or IBV. The motherboard manufacturer then customized this BIOS to suit its own hardware. For this reason, updated BIOSes are normally obtained directly from the motherboard manufacturer. Major IBV included American Megatrends , Insyde Software, Phoenix Technologies, and Byosoft. Microid Research and Award Software were acquired by Phoenix Technologies in 1998; Phoenix later phased out the Award brand name. General Software, which was also acquired by Phoenix in 2007, sold BIOS for embedded systems based on Intel processors."
+3786,"The open-source community increased their effort to develop a replacement for proprietary BIOSes and their future incarnations with an open-sourced counterparts. Open Firmware was an early attempt to make open source standard for booting firmware. It was initially endorsed by IEEE in its IEEE 1275-1994 standard but was withdrawn in 2005. Later examples include the libreboot, coreboot and OpenBIOS/Open Firmware projects. AMD provided product specifications for some chipsets, and Google is sponsoring the project. Motherboard manufacturer Tyan offers coreboot next to the standard BIOS with their Opteron line of motherboards."
+3787,"EEPROM and Flash memory chips are advantageous because they can be easily updated by the user; it is customary for hardware manufacturers to issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, this advantage had the risk that an improperly executed or aborted BIOS update could render the computer or device unusable. To avoid these situations, more recent BIOSes use a ""boot block""; a portion of the BIOS which runs first and must be updated separately. This code verifies if the rest of the BIOS is intact  before transferring control to it. If the boot block detects any corruption in the main BIOS, it will typically warn the user that a recovery process must be initiated by booting from removable media  so the user can try flashing the BIOS again. Some motherboards have a backup BIOS  to recover from BIOS corruptions."
+3788,"There are at least five known viruses that attack the BIOS. Two of which were for demonstration purposes. The first one found in the wild was Mebromi, targeting Chinese users."
+3789,"The first BIOS virus was BIOS Meningitis, which instead of erasing BIOS chips it infected them. BIOS Meningitis was relatively harmless, compared to a virus like CIH."
+3790,"The second BIOS virus was CIH, also known as the ""Chernobyl Virus"", which was able to erase flash ROM BIOS content on compatible chipsets. CIH appeared in mid-1998 and became active in April 1999. Often, infected computers could no longer boot, and people had to remove the flash ROM IC from the motherboard and reprogram it. CIH targeted the then-widespread Intel i430TX motherboard chipset and took advantage of the fact that the Windows 9x operating systems, also widespread at the time, allowed direct hardware access to all programs."
+3791,"Modern systems are not vulnerable to CIH because of a variety of chipsets being used which are incompatible with the Intel i430TX chipset, and also other flash ROM IC types. There is also extra protection from accidental BIOS rewrites in the form of boot blocks which are protected from accidental overwrite or dual and quad BIOS equipped systems which may, in the event of a crash, use a backup BIOS. Also, all modern operating systems such as FreeBSD, Linux, macOS, Windows NT-based Windows OS like Windows 2000, Windows XP and newer, do not allow user-mode programs to have direct hardware access using a hardware abstraction layer."
+3792,"As a result, as of 2008, CIH has become essentially harmless, at worst causing annoyance by infecting executable files and triggering antivirus software. Other BIOS viruses remain possible, however; since most Windows home users without Windows Vista/7's UAC run all applications with administrative privileges, a modern CIH-like virus could in principle still gain access to hardware without first using an exploit. The operating system OpenBSD prevents all users from having this access and the grsecurity patch for the Linux kernel also prevents this direct hardware access by default, the difference being an attacker requiring a much more difficult kernel level exploit or reboot of the machine."
+3793,"The third BIOS virus was a technique presented by John Heasman, principal security consultant for UK-based Next-Generation Security Software. In 2006, at the Black Hat Security Conference, he showed how to elevate privileges and read physical memory, using malicious procedures that replaced normal ACPI functions stored in flash memory."
+3794,"The fourth BIOS virus was a technique called ""Persistent BIOS infection."" It appeared in 2009 at the CanSecWest Security Conference in Vancouver, and at the SyScan Security Conference in Singapore. Researchers Anibal Sacco and Alfredo Ortega, from Core Security Technologies, demonstrated how to insert malicious code into the decompression routines in the BIOS, allowing for nearly full control of the PC at start-up, even before the operating system is booted. The proof-of-concept does not exploit a flaw in the BIOS implementation, but only involves the normal BIOS flashing procedures. Thus, it requires physical access to the machine, or for the user to be root. Despite these requirements, Ortega underlined the profound implications of his and Sacco's discovery: ""We can patch a driver to drop a fully working rootkit. We even have a little code that can remove or disable antivirus."""
+3795,"Mebromi is a trojan which targets computers with AwardBIOS, Microsoft Windows, and antivirus software from two Chinese companies: Rising Antivirus and Jiangmin KV Antivirus. Mebromi installs a rootkit which infects the Master boot record."
+3796,"In a December 2013 interview with 60 Minutes, Deborah Plunkett, Information Assurance Director for the US National Security Agency claimed the NSA had uncovered and thwarted a possible BIOS attack by a foreign nation state, targeting the US financial system. The program cited anonymous sources alleging it was a Chinese plot. However follow-up articles in The Guardian, The Atlantic, Wired and The Register refuted the NSA's claims."
+3797,"Newer Intel platforms have Intel Boot Guard  technology enabled, this technology will check the BIOS digital signature at startup, and the IBG public key is fused into the PCH. End users can't disable this function."
+3798,"Unified Extensible Firmware Interface  supplements the BIOS in many new machines. Initially written for the Intel Itanium architecture, UEFI is now available for x86 and Arm platforms; the specification development is driven by the Unified EFI Forum, an industry Special Interest Group. EFI booting has been supported in only Microsoft Windows versions supporting GPT, the Linux kernel 2.6.1 and later, and macOS on Intel-based Macs. As of 2014, new PC hardware predominantly ships with UEFI firmware. The architecture of the rootkit safeguard can also prevent the system from running the user's own software changes, which makes UEFI controversial as a legacy BIOS replacement in the open hardware community. Also, Windows 11 requires UEFI to boot."
+3799,"Other alternatives to the functionality of the ""Legacy BIOS"" in the x86 world include coreboot and libreboot."
+3800,"Some servers and workstations use a platform-independent Open Firmware  based on the Forth programming language; it is included with Sun's SPARC computers, IBM's RS/6000 line, and other PowerPC systems such as the CHRP motherboards, along with the x86-based OLPC XO-1."
+3801,"As of at least 2015, Apple has removed legacy BIOS support from MacBook Pro computers. As such the BIOS utility no longer supports the legacy option, and prints ""Legacy mode not supported on this system"". In 2017, Intel announced that it would remove legacy BIOS support by 2020. Since 2019, new Intel platform OEM PCs no longer support the legacy option."
+3802,"Unlike the ones' complement scheme, the two's complement scheme has only one representation for zero. Furthermore, arithmetic implementations can be used on signed as well as unsigned integers
+and differ only in the integer overflow situations."
+3803,The two's complement of an integer is computed by:
+3804,"For example, to calculate the decimal number −6 in binary from the number 6:"
+3805,"To verify that 1010 indeed has a value of −6, add the place values together, but subtract the sign value from the final calculation. Because the most significant value is the sign value, it must be subtracted to produce the correct result: 1010 = − +  +  +  = 1×−8 + 0 + 1×2 + 0 = −6."
+3806,"Two's complement is an example of a radix complement.
+The 'two' in the name refers to the term which, expanded fully in an N-bit system, is actually ""two to the power of N"" - 2N , and it is only this full term in respect to which the complement is calculated. As such, the precise definition of the Two's complement of an N-bit number is the complement of that number with respect to 2N."
+3807,"The defining property of being a complement to a number with respect to 2N is simply that the summation of this number with the original produce 2N. For example, using binary with numbers up to three-bits , a two's complement for the number 3  is 5 , because summed to the original it gives 23 = 10002 = 0112 + 1012. Where this correspondence is employed for representing negative numbers, it effectively means, using an analogy with decimal digits and a number-space only allowing eight non-negative numbers 0 through 7, dividing the number-space in two sets: the first four of the numbers 0 1 2 3 remain the same, while the remaining four encode negative numbers, maintaining their growing order, so making 4 encode -4, 5 encode -3, 6 encode -2 and 7 encode -1. A binary representation has an additional utility however, because the most significant bit also indicates the group : it is 0 for the first group of non-negatives, and 1 for the second group of negatives. The tables at right illustrate this property."
+3808,"Calculation of the binary two's complement of a positive number essentially means subtracting the number from the 2N. But as can be seen for the three-bit example and the four-bit 10002 , the number 2N will not itself be representable in a system limited to N bits, as it is just outside the N bits space . Because of this, systems with maximally N-bits must break the subtraction into two operations: first subtract from the maximum number in the N-bit system, that is 2N-1  and then adding the one. Coincidentally, that intermediate number before adding the one is also used in computer science as another method of signed number representation and is called a Ones' complement ."
+3809,"Compared to other systems for representing signed numbers , the two's complement has the advantage that the fundamental arithmetic operations of addition, subtraction, and multiplication are identical to those for unsigned binary numbers . This property makes the system simpler to implement, especially for higher-precision arithmetic. Additionally, unlike ones' complement systems, two's complement has no representation for negative zero, and thus does not suffer from its associated difficulties. Otherwise, both schemes have the desired property that the sign of integers can be reversed by taking the complement of its binary representation, but two's complement has an exception - the lowest negative, as can be seen in the tables."
+3810,"The method of complements had long been used to perform subtraction in decimal adding machines and mechanical calculators. John von Neumann suggested use of two's complement binary representation in his 1945 First Draft of a Report on the EDVAC proposal for an electronic stored-program digital computer. The 1949 EDSAC, which was inspired by the First Draft, used two's complement representation of negative binary integers."
+3811,"Many early computers, including the CDC 6600, the LINC, the PDP-1, and the UNIVAC 1107, use ones' complement notation; the descendants of the UNIVAC 1107, the UNIVAC 1100/2200 series, continued to do so. The IBM 700/7000 series scientific machines use sign/magnitude notation, except for the index registers which are two's complement. Early commercial computers storing negative values in two's complement form include the English Electric DEUCE  and the Digital Equipment Corporation PDP-5  and PDP-6 . The System/360, introduced in 1964 by IBM, then the dominant player in the computer industry, made two's complement the most widely used binary representation in the computer industry. The first minicomputer, the PDP-8 introduced in 1965, uses two's complement arithmetic, as do the 1969 Data General Nova, the 1970 PDP-11, and almost all subsequent minicomputers and microcomputers."
+3812,"A two's-complement number system encodes positive and negative numbers in a binary number representation. The weight of each bit is a power of two, except for the most significant bit, whose weight is the negative of the corresponding power of two."
+3813,"The most significant bit determines the sign of the number and is sometimes called the sign bit. Unlike in sign-and-magnitude representation, the sign bit also has the weight − shown above. Using N bits, all integers from − to 2N − 1 − 1 can be represented."
+3814,"In two's complement notation, a non-negative number is represented by its ordinary binary representation; in this case, the most significant bit is 0. Though, the range of numbers represented is not the same as with unsigned binary numbers.  For example, an 8-bit unsigned number can represent the values 0 to 255 .  However a two's complement 8-bit number can only represent non-negative integers from 0 to 127 , because the rest of the bit combinations with the most significant bit as '1' represent the negative integers −1 to −128."
+3815,"The two's complement operation is the additive inverse operation, so negative numbers are represented by the two's complement of the absolute value."
+3816,"To get the two's complement of a negative binary number, all bits are inverted, or ""flipped"", by using the bitwise NOT operation; the value of 1 is then added to the resulting value, ignoring the overflow which occurs when taking the two's complement of 0."
+3817,"For example, using 1 byte , the decimal number 5 is represented by"
+3818,"The most significant bit  is 0, so the pattern represents a non-negative value. To convert to −5 in two's-complement notation, first, all bits are inverted, that is: 0 becomes 1 and 1 becomes 0:"
+3819,"At this point, the representation is the ones' complement of the decimal value −5. To obtain the two's complement, 1 is added to the result, giving:"
+3820,"The result is a signed binary number representing the decimal value −5 in two's-complement form. The most significant bit is 1, so the value represented is negative."
+3821,"The two's complement of a negative number is the corresponding positive value, except in the special case of the most negative number. For example, inverting the bits of −5  gives:"
+3822,And adding one gives the final value:
+3823,"Likewise, the two's complement of zero is zero: inverting gives all ones, and adding one changes the ones back to zeros ."
+3824,"The two's complement of the most negative number representable  is itself. Hence, there is an 'extra' negative number for which two's complement does not give the negation, see § Most negative number below."
+3825,"The sum of a number and its ones' complement is an N-bit word with all 1 bits, which is  2N − 1. Then adding a number to its two's complement results in the N lowest bits set to 0 and the carry bit 1, where the latter has the weight  of 2N. Hence, in the unsigned binary arithmetic the value of two's-complement negative number x* of a positive x satisfies the equality x* = 2N − x."
+3826,"For example, to find the four-bit representation of −5 :"
+3827,"Hence, with N = 4:"
+3828,"The calculation can be done entirely in base 10, converting to base 2 at the end:"
+3829,"A shortcut to manually convert a binary number into its two's complement is to start at the least significant bit , and copy all the zeros, working from LSB toward the most significant bit  until the first 1 is reached; then copy that 1, and flip all the remaining bits . This shortcut allows a person to convert a number to its two's complement without first forming its ones' complement. For example: in two's complement representation, the negation of ""0011 1100"" is ""1100 0100"", where the underlined digits were unchanged by the copying operation ."
+3830,"In computer circuitry, this method is no faster than the ""complement and add one"" method; both methods require working sequentially from right to left, propagating logic changes. The method of complementing and adding one can be sped up by a standard carry look-ahead adder circuit; the LSB towards MSB method can be sped up by a similar logic transformation."
+3831,"When turning a two's-complement number with a certain number of bits into one with more bits , the most-significant bit must be repeated in all the extra bits.  Some processors do this in a single instruction; on other processors, a conditional must be used followed by code to set the relevant bits or bytes."
+3832,"Similarly, when a number is shifted to the right, the most-significant bit, which contains the sign information, must be maintained. However, when shifted to the left, a bit is shifted out. These rules preserve the common semantics that left shifts multiply the number by two and right shifts divide the number by two.  However, if the most-significant bit changes from 0 to 1 , overflow is said to occur in the case that the value represents a signed integer."
+3833,"Both shifting and doubling the precision are important for some multiplication algorithms. Note that unlike addition and subtraction, width extension and right shifting are done differently for signed and unsigned numbers."
+3834,"With only one exception, starting with any number in two's-complement representation, if all the bits are flipped and 1 added, the two's-complement representation of the negative of that number is obtained. Positive 12 becomes negative 12, positive 5 becomes negative 5, zero becomes zero, etc."
+3835,"Taking the two's complement  of the minimum number in the range will not have the desired effect of negating the number. For example, the two's complement of  −128  in an eight-bit system is  −128 , as shown in the table to the right. Although the expected result from negating  −128  is  +128 , there is no representation of  +128  with an eight bit two's complement system and thus it is in fact impossible to represent the negation. Note that the two's complement being the same number is detected as an overflow condition since there was a carry into but not out of the most-significant bit."
+3836,"Having a nonzero number equal to its own negation is forced by the fact that zero is its own negation, and that the total number of numbers is even. Proof: there are  2^n - 1 nonzero numbers . Negation would partition the nonzero numbers into sets of size 2, but this would result in the set of nonzero numbers having even cardinality. So at least one of the sets has size 1, i.e., a nonzero number is its own negation."
+3837,"The presence of the most negative number can lead to unexpected programming bugs where the result has an unexpected sign, or leads to an unexpected overflow exception, or leads to completely strange behaviors. For example,"
+3838,"In the C and C++ programming languages, the above behaviours are undefined and not only may they return strange results, but the compiler is free to assume that the programmer has ensured that undefined numerical operations never happen, and make inferences from that assumption. This enables a number of optimizations, but also leads to a number of strange bugs in programs with these undefined calculations."
+3839,"This most negative number in two's complement is sometimes called ""the weird number"", because it is the only exception.
+Although the number is an exception, it is a valid number in regular two's complement systems. All arithmetic operations work with it both as an operand and  a result."
+3840,"Given a set of all possible N-bit values, we can assign the lower  half to be the integers from 0 to  inclusive and the upper half to be −2N − 1 to −1 inclusive.  The upper half  can be used to represent negative integers from −2N − 1 to −1 because, under addition modulo 2N they behave the same way as those negative integers.  That is to say that, because i + j mod 2N = i +  mod 2N, any value in the set { j + k 2N | k is an integer }  can be used in place of j."
+3841,"For example, with eight bits, the unsigned bytes are 0 to 255. Subtracting 256 from the top half  yields the signed bytes −128 to −1."
+3842,"The relationship to two's complement is realised by noting that 256 = 255 + 1, and  is the ones' complement of x."
+3843,"For example, an 8 bit number can only represent every integer from −128. to 127., inclusive, since .  −95. modulo 256. is equivalent to 161. since"
+3844,"Fundamentally, the system represents negative integers by counting backward and wrapping around. The boundary between positive and negative numbers is arbitrary, but by convention all negative numbers have a left-most bit  of one. Therefore, the most positive four-bit number is 0111  and the most negative is 1000 . Because of the use of the left-most bit as the sign bit, the absolute value of the most negative number  is too large to represent. Negating a two's complement number is simple: Invert all the bits and add one to the result. For example, negating 1111, we get 0000 + 1 = 1. Therefore, 1111 in binary must represent −1 in decimal."
+3845,"The system is useful in simplifying the implementation of arithmetic on computer hardware. Adding 0011  to 1111  at first seems to give the incorrect answer of 10010. However, the hardware can simply ignore the left-most bit to give the correct answer of 0010 . Overflow checks still must exist to catch operations such as summing 0100 and 0100."
+3846,"The system therefore allows addition of negative operands without a subtraction circuit or a circuit that detects the sign of a number. Moreover, that addition circuit can also perform subtraction by taking the two's complement of a number , which only requires an additional cycle or its own adder circuit. To perform this, the circuit merely operates as if there were an extra left-most bit of 1."
+3847,"Adding two's complement numbers requires no special processing even if the operands have opposite signs; the sign of the result is determined automatically.  For example, adding 15 and −5:"
+3848,Or the computation of 5 − 15 = 5 + :
+3849,"This process depends upon restricting to 8 bits of precision; a carry to the  9th most significant bit is ignored, resulting in the arithmetically correct result of 1010."
+3850,"The last two bits of the carry row  contain vital information: whether the calculation resulted in an arithmetic overflow, a number too large for the binary system to represent .  An overflow condition exists when these last two bits are different from one another.  As mentioned above, the sign of the number is encoded in the MSB of the result."
+3851,"In other terms, if the left two carry bits  are both 1s or both 0s, the result is valid; if the left two carry bits are ""1 0"" or ""0 1"", a sign overflow has occurred.  Conveniently, an XOR operation on these two bits can quickly determine if an overflow condition exists.  As an example, consider the signed 4-bit addition of 7 and 3:"
+3852,"In this case, the far left two  carry bits are ""01"", which means there was a two's-complement addition overflow.  That is, 10102 = 1010 is outside the permitted range of −8 to 7. The result would be correct if treated as unsigned integer."
+3853,"In general, any two N-bit numbers may be added without overflow, by first sign-extending both of them to N + 1 bits, and then adding as above. The N + 1 bits result is large enough to represent any possible sum  so overflow will never occur. It is then possible, if desired, to 'truncate' the result back to N bits while preserving the value if and only if the discarded bit is a proper sign extension of the retained result bits. This provides another method of detecting overflow—which is equivalent to the method of comparing the carry bits—but which may be easier to implement in some situations, because it does not require access to the internals of the addition."
+3854,"Computers usually use the method of complements to implement subtraction. Using complements for subtraction is closely related to using complements for representing negative numbers, since the combination allows all signs of operands and results; direct subtraction works with two's-complement numbers as well. Like addition, the advantage of using two's complement is the elimination of examining the signs of the operands to determine whether addition or subtraction is needed. For example, subtracting −5 from 15 is really adding 5 to 15, but this is hidden by the two's-complement representation:"
+3855,"Overflow is detected the same way as for addition, by examining the two leftmost  bits of the borrows; overflow has occurred if they are different."
+3856,Another example is a subtraction operation where the result is negative: 15 − 35 = −20:
+3857,"As for addition, overflow in subtraction may be avoided  by first sign-extending both inputs by an extra bit."
+3858,The product of two N-bit numbers requires 2N bits to contain all possible values.
+3859,"If the precision of the two operands using two's complement is doubled before the multiplication, direct multiplication  will provide the correct result. For example, take 6 ×  = −30. First, the precision is extended from four bits to eight. Then the numbers are multiplied, discarding the bits beyond the eighth bit :"
+3860,"This is very inefficient; by doubling the precision ahead of time, all additions must be double-precision and at least twice as many partial products are needed than for the more efficient algorithms actually implemented in computers. Some multiplication algorithms are designed for two's complement, notably Booth's multiplication algorithm. Methods for multiplying sign-magnitude numbers do not work with two's-complement numbers without adaptation. There is not usually a problem when the multiplicand  is negative; the issue is setting the initial bits of the product correctly when the multiplier is negative. Two methods for adapting algorithms to handle two's-complement numbers are common:"
+3861,"As an example of the second method, take the common add-and-shift algorithm for multiplication. Instead of shifting partial products to the left as is done with pencil and paper, the accumulated product is shifted right, into a second register that will eventually hold the least significant half of the product. Since the least significant bits are not changed once they are calculated, the additions can be single precision, accumulating in the register that will eventually hold the most significant half of the product. In the following example, again multiplying 6 by −5, the two registers and the extended sign bit are separated by ""|"":"
+3862,"Comparison is often implemented with a dummy subtraction, where the flags in the computer's status register are checked, but the main result is ignored. The zero flag indicates if two values compared equal. If the exclusive-or of the sign and overflow flags is 1, the subtraction result was less than zero, otherwise the result was zero or greater. These checks are often implemented in computers in conditional branch instructions."
+3863,"Unsigned binary numbers can be ordered by a simple lexicographic ordering, where the bit value 0 is defined as less than the bit value 1.  For two's complement values, the meaning of the most significant bit is reversed ."
+3864,"The following algorithm  sets the result register R to −1 if A < B, to +1 if A > B, and to 0 if A and B are equal:"
+3865,"In a classic HAKMEM published by the MIT AI Lab in 1972, Bill Gosper noted that whether or not a machine's internal representation was two's-complement could be determined by summing the successive powers of two. In a flight of fancy, he noted that the result of doing this algebraically indicated that ""algebra is run on a machine  which is two's-complement."""
+3866,"Gosper's end conclusion is not necessarily meant to be taken seriously, and it is akin to a mathematical joke. The critical step is ""...110 = ...111 − 1"", i.e., ""2X = X − 1"", and thus X = ...111 = −1. This presupposes a method by which an infinite string of 1s is considered a number, which requires an extension of the finite place-value concepts in elementary arithmetic. It is meaningful either as part of a two's-complement notation for all integers, as a typical 2-adic number, or even as one of the generalized sums defined for the divergent series of real numbers 1 + 2 + 4 + 8 + ···. Digital arithmetic circuits, idealized to operate with infinite  bit strings, produce 2-adic addition and multiplication compatible with two's complement representation. Continuity of binary arithmetical and bitwise operations in 2-adic metric also has some use in cryptography."
+3867,"To convert a number with a fractional part, such as .0101, one must convert starting from right to left the 1s to decimal as in a normal conversion. In this example 0101 is equal to 5 in decimal. Each digit after the floating point represents a fraction where the denominator is a multiplier of 2. So, the first is 1/2, the second is 1/4 and so on. Having already calculated the decimal value as mentioned above, only the denominator of the LSB  is used. The final result of this conversion is 5/16."
+3868,"For instance, having the floating value of .0110 for this method to work, one should not consider the last 0 from the right. Hence, instead of calculating the decimal value for 0110, we calculate the value 011, which is 3 in decimal . The denominator is 8, giving a final result of 3/8."
+3869,"The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures. To disambiguate arbitrarily sized bytes from the common 8-bit definition, network protocol documents such as the Internet Protocol  refer to an 8-bit byte as an octet. Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness."
+3870,"The size of the byte has historically been hardware-dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used. The six-bit character code was an often-used implementation in early encoding systems, and computers using six-bit and nine-bit bytes were common in the 1960s. These systems often had memory words of 12, 18, 24, 30, 36, 48, or 60 bits, corresponding to 2, 3, 4, 5, 6, 8, or 10 six-bit bytes. In this era, bit groupings in the instruction stream were often referred to as syllables or slab, before the term byte became common."
+3871,"The modern de facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenient power of two permitting the binary-encoded values 0 through 255 for one byte, as 2 to the power of 8 is 256. The international standard IEC 80000-13 codified this common meaning. Many types of applications use information representable in eight or fewer bits and processor designers commonly optimize for this usage. The popularity of major commercial computing architectures has aided in the ubiquitous acceptance of the 8-bit byte. Modern architectures typically use 32- or 64-bit words, built of four or eight bytes, respectively."
+3872,"The unit symbol for the byte was designated as the upper-case letter B by the International Electrotechnical Commission  and Institute of Electrical and Electronics Engineers . Internationally, the unit octet, symbol o, explicitly defines a sequence of eight bits, eliminating the potential ambiguity of the term ""byte""."
+3873,"The term byte was coined by Werner Buchholz in June 1956, during the early design phase for the IBM Stretch computer, which had addressing to the bit and variable field length  instructions with a byte size encoded in the instruction. It is a deliberate respelling of bite to avoid accidental mutation to bit."
+3874,"Another origin of byte for bit groups smaller than a computer's word size, and in particular groups of four bits, is on record by Louis G. Dooley, who claimed he coined the term while working with Jules Schwartz and Dick Beeler on an air defense system called SAGE at MIT Lincoln Laboratory in 1956 or 1957, which was jointly developed by Rand, MIT, and IBM. Later on, Schwartz's language JOVIAL actually used the term, but the author recalled vaguely that it was derived from AN/FSQ-31."
+3875,"Early computers used a variety of four-bit binary-coded decimal  representations and the six-bit codes for printable graphic patterns common in the U.S. Army  and Navy. These representations included alphanumeric characters and special graphical symbols. These sets were expanded in 1963 to seven bits of coding, called the American Standard Code for Information Interchange  as the Federal Information Processing Standard, which replaced the incompatible teleprinter codes in use by different branches of the U.S. government and universities during the 1960s. ASCII included the distinction of upper- and lowercase alphabets and a set of control characters to facilitate the transmission of written language as well as printing device functions, such as page advance and line feed, and the physical or logical control of data flow over the transmission media. During the early 1960s, while also active in ASCII standardization, IBM simultaneously introduced in its product line of System/360 the eight-bit Extended Binary Coded Decimal Interchange Code , an expansion of their six-bit binary-coded decimal  representations used in earlier card punches.
+The prominence of the System/360 led to the ubiquitous adoption of the eight-bit storage size, while in detail the EBCDIC and ASCII encoding schemes are different."
+3876,"In the early 1960s, AT&T introduced digital telephony on long-distance trunk lines. These used the eight-bit μ-law encoding. This large investment promised to reduce transmission costs for eight-bit data."
+3877,"In Volume 1 of The Art of Computer Programming , Donald Knuth uses byte in his hypothetical MIX computer to denote a unit which ""contains an unspecified amount of information ... capable of holding at least 64 distinct values ... at most 100 distinct values. On a binary computer a byte must therefore be composed of six bits"". He notes that ""Since 1975 or so, the word byte has come to mean a sequence of precisely eight binary digits...When we speak of bytes in connection with MIX we shall confine ourselves to the former sense of the word, harking back to the days when bytes were not yet standardized."""
+3878,"The development of eight-bit microprocessors in the 1970s popularized this storage size. Microprocessors such as the Intel 8008, the direct predecessor of the 8080 and the 8086, used in early personal computers, could also perform a small number of operations on the four-bit pairs in a byte, such as the decimal-add-adjust  instruction. A four-bit quantity is often called a nibble, also nybble, which is conveniently represented by a single hexadecimal digit."
+3879,The term octet is used to unambiguously specify a size of eight bits. It is used extensively in protocol definitions.
+3880,"Historically, the term octad or octade was used to denote eight bits as well at least in Western Europe; however, this usage is no longer common. The exact origin of the term is unclear, but it can be found in British, Dutch, and German sources of the 1960s and 1970s, and throughout the documentation of Philips mainframe computers."
+3881,"The unit symbol for the byte is specified in IEC 80000-13, IEEE 1541 and the Metric Interchange Format as the upper-case character B."
+3882,"In the International System of Quantities , B is also the symbol of the bel, a unit of logarithmic power ratio named after Alexander Graham Bell, creating a conflict with the IEC specification. However, little danger of confusion exists, because the bel is a rarely used unit. It is used primarily in its decadic fraction, the decibel , for signal strength and sound pressure level measurements, while a unit for one-tenth of a byte, the decibyte, and other fractions, are only used in derived units, such as transmission rates."
+3883,"The lowercase letter o for octet is defined as the symbol for octet in IEC 80000-13 and is commonly used in languages such as French and Romanian, and is also combined with metric prefixes for multiples, for example ko and Mo."
+3884,"More than one system exists to define unit multiples based on the byte. Some systems are based on powers of 10, following the International System of Units , which defines for example the prefix kilo as 1000 ; other systems are based on powers of 2. Nomenclature for these systems has  confusion. Systems based on powers of 10 use standard SI prefixes  and their corresponding symbols . Systems based on powers of 2, however, might use binary prefixes  and their corresponding symbols  or they might use the prefixes K, M, and G, creating ambiguity when the prefixes M or G are used."
+3885,"While the difference between the decimal and binary interpretations is relatively small for the kilobyte , the systems deviate increasingly as units grow larger . For example, a power-of-10-based terabyte is about 9% smaller than power-of-2-based tebibyte."
+3886,"Definition of prefixes using powers of 10—in which 1 kilobyte  is defined to equal 1,000 bytes—is recommended by the International Electrotechnical Commission . The IEC standard defines eight such multiples, up to 1 yottabyte , equal to 10008 bytes. The additional prefixes ronna- for 10009 and quetta- for 100010 were adopted by the International Bureau of Weights and Measures  in 2022."
+3887,"This definition is most commonly used for data-rate units in computer networks, internal bus, hard drive and flash media transfer speeds, and for the capacities of most storage media, particularly hard drives, flash-based storage, and DVDs. Operating systems that use this definition include macOS, iOS, Ubuntu, and Debian. It is also consistent with the other uses of the SI prefixes in computing, such as CPU clock speeds or measures of performance."
+3888,"A system of units based on powers of 2 in which 1 kibibyte  is equal to 1,024  bytes is defined by international standard IEC 80000-13 and is supported by national and international standards bodies . The IEC standard defines eight such multiples, up to 1 yobibyte , equal to 10248 bytes. The natural binary counterparts to ronna- and quetta- were given in a consultation paper of the International Committee for Weights and Measures' Consultative Committee for Units  as robi-  and quebi- , but have not yet been adopted by the IEC and ISO."
+3889,"An alternative system of nomenclature for the same units , in which 1 kilobyte  is equal to 1,024 bytes, 1 megabyte  is equal to 10242 bytes and  1 gigabyte  is equal to 10243 bytes is mentioned by a 1990s JEDEC standard. Only the first three multiples  are mentioned by the JEDEC standard, which makes no mention of TB and larger. While confusing and incorrect, the customary convention is used by the Microsoft Windows operating system and random-access memory capacity, such as main memory and CPU cache size, and in marketing and billing by telecommunication companies, such as Vodafone, AT&T, Orange and Telstra."
+3890,"For storage capacity, the customary convention was used by macOS and iOS through Mac OS X 10.6 Snow Leopard and iOS 10, after which they switched to units based on powers of 10."
+3891,"Various computer vendors have coined terms for data of various sizes, sometimes with different sizes for the same term even within a single vendor. These terms include double word, half word, long word, quad word, slab, superword and syllable. There are also informal terms. e.g., half byte and nybble for 4 bits, octal K for 10008."
+3892,"Contemporary computer memory has a binary architecture making a definition of memory units based on powers of 2 most practical. The use of the metric prefix kilo for binary multiples arose as a convenience, because 1,024 is approximately 1,000. This definition was popular in early decades of personal computing, with products like the Tandon 51⁄4-inch DD floppy format  being advertised as ""360 KB"", following the 1,024-byte convention. It was not universal, however. The Shugart SA-400 51⁄4-inch floppy disk held 109,375 bytes unformatted, and was advertised as ""110 Kbyte"", using the 1000 convention. Likewise, the 8-inch DEC RX01 floppy  held 256,256 bytes formatted, and was advertised as ""256k"". Other disks were advertised using a mixture of the two definitions: notably, 3+1⁄2-inch HD disks advertised as ""1.44 MB"" in fact have a capacity of 1,440 KiB, the equivalent of 1.47 MB or 1.41 MiB."
+3893,"In 1995, the International Union of Pure and Applied Chemistry's  Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set of binary prefixes for the powers of 1024, including kibi , mebi , and gibi ."
+3894,"In December 1998, the IEC addressed such multiple usages and definitions by adopting the IUPAC's proposed prefixes  to unambiguously denote powers of 1024. Thus one kibibyte  is 10241  bytes = 1024 bytes, one mebibyte  is 10242  bytes = 1,048,576 bytes, and so on."
+3895,"In 1999, Donald Knuth suggested calling the kibibyte a ""large kilobyte"" ."
+3896,"The IEC adopted the IUPAC proposal and published the standard in January 1999. The IEC prefixes are part of the International System of Quantities. The IEC further specified that the kilobyte should only be used to refer to 1,000 bytes."
+3897,"Lawsuits arising from alleged consumer confusion over the binary and decimal definitions of multiples of the byte have generally ended in favor of the manufacturers, with courts holding that the legal definition of gigabyte or GB is 1 GB = 1,000,000,000  bytes , rather than the binary definition . Specifically, the United States District Court for the Northern District of California held that ""the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce'  The California Legislature has likewise adopted the decimal system for all 'transactions in this state.'"""
+3898,"Earlier lawsuits had ended in settlement with no court ruling on the question, such as a lawsuit against drive manufacturer Western Digital. Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity. Seagate was sued on similar grounds and also settled."
+3899,Many programming languages define the data type byte.
+3900,"The C and C++ programming languages define byte as an ""addressable unit of data storage large enough to hold any member of the basic character set of the execution environment"" . The C standard requires that the integral data type unsigned char must hold at least 256 different values, and is represented by at least eight bits . Various implementations of C and C++ reserve 8, 9, 16, 32, or 36 bits for the storage of a byte. In addition, the C and C++ standards require that there are no gaps between two bytes. This means every bit in memory is part of a byte."
+3901,"Java's primitive data type byte is defined as eight bits. It is a signed data type, holding values from −128 to 127."
+3902,".NET programming languages, such as C#, define byte as an unsigned type, and the sbyte as a signed data type, holding values from 0 to 255, and −128 to 127, respectively."
+3903,"In data transmission systems, the byte is used as a contiguous sequence of bits in a serial data stream, representing the smallest distinguished unit of data. For asynchronous communication a full transmission unit usually additionally includes a start bit, 1 or 2 stop bits, and possibly a parity bit, and thus its size may vary from seven to twelve bits for five to eight bits of actual data. For synchronous communication the error checking usually uses bytes at the end of a frame."
+3904,"For example,"
+3905,"Matrices are used to represent linear maps and allow explicit computations in linear algebra. Therefore, the study of matrices is a large part of linear algebra, and most properties and operations of abstract linear algebra can be expressed in terms of matrices. For example, matrix multiplication represents the composition of linear maps."
+3906,"Not all matrices are related to linear algebra. This is, in particular, the case in graph theory, of incidence matrices, and adjacency matrices. This article focuses on matrices related to linear algebra, and, unless otherwise specified, all matrices represent linear maps or may be viewed as such."
+3907,"Square matrices, matrices with the same number of rows and columns, play a major role in matrix theory. Square matrices of a given dimension form a noncommutative ring, which is one of the most common examples of a noncommutative ring. The determinant of a square matrix is a number associated to the matrix, which is fundamental for the study of a square matrix; for example, a square matrix is invertible if and only if it has a nonzero determinant, and the eigenvalues of a square matrix are the roots of a polynomial determinant."
+3908,"In geometry, matrices are widely used for specifying and representing geometric transformations  and coordinate changes. In numerical analysis, many computational problems are solved by reducing them to a matrix computation, and this often involves computing with matrices of huge dimension. Matrices are used in most areas of mathematics and most scientific fields, either directly, or through their use in geometry and numerical analysis."
+3909,"Matrix theory is the branch of mathematics that focuses on the study of matrices. It was initially a sub-branch of linear algebra, but soon grew to include subjects related to graph theory, algebra, combinatorics and statistics."
+3910,"A matrix is a rectangular array of numbers , called the entries of the matrix. Matrices are subject to standard operations such as addition and multiplication. Most commonly, a matrix over a field F is a rectangular array of elements of F. A real matrix and a complex matrix are matrices whose entries are respectively real numbers or complex numbers. More general types of entries are discussed below. For instance, this is a real matrix:"
+3911,"The numbers, symbols, or expressions in the matrix are called its entries or its elements. The horizontal and vertical lines of entries in a matrix are called rows and columns, respectively."
+3912,"Matrices with a single row are called row vectors, and those with a single column are called column vectors. A matrix with the same number of rows and columns is called a square matrix. A matrix with an infinite number of rows or columns  is called an infinite matrix. In some contexts, such as computer algebra programs, it is useful to consider a matrix with no rows or no columns, called an empty matrix."
+3913,"There are a number of basic operations that can be applied on matrices. Some, such as transposition and submatrix do not depend on the nature of the entries. Others, such as matrix addition, scalar multiplication, matrix multiplication, and row operations involve operations on matrix entries and therefore require that matrix entries are numbers or belong to a field or a ring."
+3914,"In this section, it is supposed that matrix entries belong to a fixed ring, that is typically a field of numbers."
+3915,The sum A+B of two m-by-n matrices A and B is calculated entrywise:
+3916,"For example,"
+3917,The product cA of a number  c  and a matrix A is computed by multiplying every entry of A by c:
+3918,"This operation is called scalar multiplication, but its result is not named ""scalar product"" to avoid confusion, since ""scalar product"" is often used as a synonym for ""inner product"". For example:"
+3919,The subtraction of two m×n matrices is defined by composing matrix addition with scalar multiplication by –1:
+3920,The transpose of an m-by-n matrix A is the n-by-m matrix AT  formed by turning rows into columns and vice versa:
+3921,For example:
+3922,"Familiar properties of numbers extend to these operations on matrices: for example, addition is commutative, that is, the matrix sum does not depend on the order of the summands: A + B = B + A.
+The transpose is compatible with addition and scalar multiplication, as expressed by T = c and T = AT + BT. Finally, T = A."
+3923,"Multiplication of two matrices is defined if and only if the number of columns of the left matrix is the same as the number of rows of the right matrix. If A is an m-by-n matrix and B is an n-by-p matrix, then their matrix product AB is the m-by-p matrix whose entries are given by dot product of the corresponding row of A and the corresponding column of B:"
+3924,"where 1 ≤ i ≤ m and 1 ≤ j ≤ p. For example, the underlined entry 2340 in the product is calculated as  +  +  = 2340:"
+3925,"Matrix multiplication satisfies the rules C = A , and C = AC + BC as well as C = CA + CB , whenever the size of the matrices is such that the various products are defined. The product AB may be defined without BA being defined, namely if A and B are m-by-n and n-by-k matrices, respectively, and m ≠ k. Even if both products are defined, they generally need not be equal, that is:"
+3926,"In other words, matrix multiplication is not commutative, in marked contrast to  numbers, whose product is independent of the order of the factors. An example of two matrices not commuting with each other is:"
+3927,"Besides the ordinary matrix multiplication just described, other less frequently used operations on matrices that can be considered forms of multiplication also exist, such as the Hadamard product and the Kronecker product. They arise in solving matrix equations such as the Sylvester equation."
+3928,There are three types of row operations:
+3929,"row addition, that is adding a row to another."
+3930,"row multiplication, that is multiplying all entries of a row by a non-zero constant;"
+3931,"row switching, that is interchanging two rows of a matrix;"
+3932,"These operations are used in several ways, including solving linear equations and finding matrix inverses."
+3933,"A submatrix of a matrix is a matrix obtained by deleting any collection of rows and/or columns. For example, from the following 3-by-4 matrix, we can construct a 2-by-3 submatrix by removing row 3 and column 2:"
+3934,The minors and cofactors of a matrix are found by computing the determinant of certain submatrices.
+3935,"A principal submatrix is a square submatrix obtained by removing certain rows and columns. The definition varies from author to author. According to some authors, a principal submatrix is a submatrix in which the set of row indices that remain is the same as the set of column indices that remain. Other authors define a principal submatrix as one in which the first k rows and columns, for some number k, are the ones that remain; this type of submatrix has also been called a leading principal submatrix."
+3936,"Matrices can be used to compactly write and work with multiple linear equations, that is, systems of linear equations. For example, if A is an m-by-n matrix, x designates a column vector  of n variables x1, x2, ..., xn, and b is an m×1-column vector, then the matrix equation"
+3937,is equivalent to the system of linear equations
+3938,"Using matrices, this can be solved more compactly than would be possible by writing out all the equations separately. If n = m and the equations are independent, then this can be done by writing"
+3939,"where A−1 is the inverse matrix of A. If A has no inverse, solutions—if any—can be found using its generalized inverse."
+3940,"Matrices and matrix multiplication reveal their essential features when related to linear transformations, also known as linear maps. A real m-by-n matrix A gives rise to a linear transformation Rn → Rm mapping each vector x in Rn to the  product Ax, which is a vector in Rm. Conversely, each linear transformation f: Rn → Rm arises from a unique m-by-n matrix A: explicitly, the -entry of A is the ith coordinate of f, where ej =  is the unit vector with 1 in the jth position and 0 elsewhere. The matrix A is said to represent the linear map f, and A is called the transformation matrix of f."
+3941,"For example, the 2×2 matrix"
+3942,The following table shows several 2×2 real matrices with the associated linear maps of R2. The blue original is mapped to the green grid and shapes. The origin  is marked with a black point.
+3943,"Under the 1-to-1 correspondence between matrices and linear maps, matrix multiplication corresponds to composition of maps: if a k-by-m matrix B represents another linear map g: Rm → Rk, then the composition g ∘ f is represented by BA since"
+3944,The last equality follows from the above-mentioned associativity of matrix multiplication.
+3945,"The rank of a matrix A is the maximum number of linearly independent row vectors of the matrix, which is the same as the maximum number of linearly independent column vectors. Equivalently it is the dimension of the image of the linear map represented by A. The rank–nullity theorem states that the dimension of the kernel of a matrix plus the rank equals the number of columns of the matrix."
+3946,"A square matrix is a matrix with the same number of rows and columns. An n-by-n matrix is known as a square matrix of order n. Any two square matrices of the same order can be added and multiplied.
+The entries aii form the main diagonal of a square matrix. They lie on the imaginary line that runs from the top left corner to the bottom right corner of the matrix."
+3947,"If all entries of A below the main diagonal are zero, A is called an upper triangular matrix. Similarly if all entries of A above the main diagonal are zero, A is called a lower triangular matrix. If all entries outside the main diagonal are zero, A is called a diagonal matrix."
+3948,"The identity matrix In of size n is the n-by-n matrix in which all the elements on the main diagonal are equal to 1 and all other elements are equal to 0, for example,"
+3949,"It is a square matrix of order n, and also a special kind of diagonal matrix. It is called an identity matrix because multiplication with it leaves a matrix unchanged:"
+3950,"A nonzero scalar multiple of an identity matrix is called a scalar matrix. If the matrix entries come from a field, the scalar matrices form a group, under matrix multiplication, that is isomorphic to the multiplicative group of nonzero elements of the field."
+3951,"A square matrix A that is equal to its transpose, that is, A = AT, is a symmetric matrix. If instead, A is equal to the negative of its transpose, that is, A = −AT, then A is a skew-symmetric matrix. In complex matrices, symmetry is often replaced by the concept of Hermitian matrices, which satisfy A∗ = A, where the star or asterisk denotes the conjugate transpose of the matrix, that is, the transpose of the complex conjugate of A."
+3952,"By the spectral theorem, real symmetric matrices and complex Hermitian matrices have an eigenbasis; that is, every vector is expressible as a linear combination of eigenvectors. In both cases, all eigenvalues are real. This theorem can be generalized to infinite-dimensional situations related to matrices with infinitely many rows and columns, see below."
+3953,A square matrix A is called invertible or non-singular if there exists a matrix B such that
+3954,"where In is the n×n identity matrix with 1s on the main diagonal and 0s elsewhere. If B exists, it is unique and is called the inverse matrix of A, denoted A−1."
+3955,A symmetric real matrix A is called positive-definite if the associated quadratic form
+3956,"has a positive value for every nonzero vector x in Rn. If f  only yields negative values then A is negative-definite; if f does produce both negative and positive values then A is indefinite. If the quadratic form f yields only non-negative values , the symmetric matrix is called positive-semidefinite ; hence the matrix is indefinite precisely when it is neither positive-semidefinite nor negative-semidefinite."
+3957,"A symmetric matrix is positive-definite if and only if all its eigenvalues are positive, that is, the matrix is positive-semidefinite and it is invertible. The table at the right shows two possibilities for 2-by-2 matrices."
+3958,Allowing as input two different vectors instead yields the bilinear form associated to A:
+3959,"In the case of complex matrices, the same terminology and result apply, with symmetric matrix, quadratic form, bilinear form, and transpose xT replaced respectively by  Hermitian matrix, Hermitian form, sesquilinear form, and conjugate transpose xH."
+3960,"An orthogonal matrix is a square matrix with real entries whose columns and rows are orthogonal unit vectors . Equivalently, a matrix A is orthogonal if its transpose is equal to its inverse:"
+3961,which entails
+3962,where In is the identity matrix of size n.
+3963,"An orthogonal matrix A is necessarily invertible , unitary , and normal . The determinant of any orthogonal matrix is either +1 or −1. A special orthogonal matrix is an orthogonal matrix with determinant +1. As a linear transformation, every orthogonal matrix with determinant +1 is a pure rotation without reflection, i.e., the transformation preserves the orientation of the transformed structure, while every orthogonal matrix with determinant -1 reverses the orientation, i.e., is a composition of a pure reflection and a  rotation. The identity matrices have determinant 1, and are pure rotations by an angle zero."
+3964,The complex analogue of an orthogonal matrix is a unitary matrix.
+3965,"The trace, tr of a square matrix A is the sum of its diagonal entries. While matrix multiplication is not commutative as mentioned above, the trace of the product of two matrices is independent of the order of the factors:"
+3966,This is immediate from the definition of matrix multiplication:
+3967,"It follows that the trace of the product of more than two matrices is independent of cyclic permutations of the matrices, however this does not in general apply for arbitrary permutations  ≠ tr, in general). Also, the trace of a matrix is equal to that of its transpose, that is,"
+3968,"The determinant of a square matrix A  or |A|) is a number encoding certain properties of the matrix. A matrix is invertible if and only if its determinant is nonzero. Its absolute value equals the area  or volume  of the image of the unit square , while its sign corresponds to the orientation of the corresponding linear map: the determinant is positive if and only if the orientation is preserved."
+3969,The determinant of 2-by-2 matrices is given by
+3970,The determinant of 3-by-3 matrices involves 6 terms . The more lengthy Leibniz formula generalises these two formulae to all dimensions.
+3971,The determinant of a product of square matrices equals the product of their determinants:
+3972,"Adding a multiple of any row to another row, or a multiple of any column to another column does not change the determinant. Interchanging two rows or two columns affects the determinant by multiplying it by −1. Using these operations, any matrix can be transformed to a lower  triangular matrix, and for such matrices, the determinant equals the product of the entries on the main diagonal; this provides a method to calculate the determinant of any matrix. Finally, the Laplace expansion expresses the determinant in terms of minors, that is, determinants of smaller matrices. This expansion can be used for a recursive definition of determinants , that can be seen to be equivalent to the Leibniz formula. Determinants can be used to solve linear systems using Cramer's rule, where the division of the determinants of two related square matrices equates to the value of each of the system's variables."
+3973,"are called an eigenvalue and an eigenvector of A, respectively. The number λ is an eigenvalue of an n×n-matrix A if and only if A−λIn is not invertible, which is equivalent to"
+3974,"The polynomial pA in an indeterminate X given by evaluation of the determinant det is called the characteristic polynomial of A. It is a monic polynomial of degree n. Therefore the polynomial equation pA = 0 has at most n different solutions, that is, eigenvalues of the matrix. They may be complex even if the entries of A are real. According to the Cayley–Hamilton theorem, pA = 0, that is, the result of substituting the matrix itself into its own characteristic polynomial yields the zero matrix."
+3975,"Matrix calculations can be often performed with different techniques. Many problems can be solved by both direct algorithms or iterative approaches. For example, the eigenvectors of a square matrix can be obtained by finding a sequence of vectors xn converging to an eigenvector when n tends to infinity."
+3976,"To choose the most appropriate algorithm for each specific problem, it is important to determine both the effectiveness and precision of all the available algorithms. The domain studying these matters is called numerical linear algebra. As with other numerical situations, two main aspects are the complexity of algorithms and their numerical stability."
+3977,"Determining the complexity of an algorithm means finding upper bounds or estimates of how many elementary operations such as additions and multiplications of scalars are necessary to perform some algorithm, for example, multiplication of matrices. Calculating the matrix product of two n-by-n matrices using the definition given above needs n3 multiplications, since for any of the n2 entries of the product, n multiplications are necessary. The Strassen algorithm outperforms this ""naive"" algorithm; it needs only n2.807 multiplications. A refined approach also incorporates specific features of the computing devices."
+3978,"In many practical situations additional information about the matrices involved is known. An important case are sparse matrices, that is, matrices most of whose entries are zero. There are specifically adapted algorithms for, say, solving linear systems Ax = b for sparse matrices A, such as the conjugate gradient method."
+3979,"An algorithm is, roughly speaking, numerically stable, if little deviations in the input values do not lead to big deviations in the result. For example, calculating the inverse of a matrix via Laplace expansion  denotes the adjugate matrix of A)"
+3980,"may lead to significant rounding errors if the determinant of the matrix is very small. The norm of a matrix can be used to capture the conditioning of linear algebraic problems, such as computing a matrix's inverse."
+3981,"Most computer programming languages support arrays but are not designed with built-in commands for matrices. Instead, available external libraries provide matrix operations on arrays, in nearly all currently used programming languages. Matrix manipulation was among the earliest numerical applications of computers. The original Dartmouth BASIC had built-in commands for matrix arithmetic on arrays from its second edition implementation in 1964. As early as the 1970s, some engineering desktop computers such as the HP 9830 had ROM cartridges to add BASIC commands for matrices. Some computer languages such as APL were designed to manipulate matrices, and various mathematical programs can be used to aid computing with matrices. As of 2023, most computers have some form of built-in matrix operations at a low-level implementing the standard BLAS specification, upon which most higher-level matrix and linear algebra libraries  rely. While most of these libraries require a professional level of coding, LAPACK can be accessed by higher-level  bindings such as NumPy/SciPy, R, GNU Octave, MATLAB."
+3982,"There are several methods to render matrices into a more easily accessible form. They are generally referred to as matrix decomposition or matrix factorization techniques. The interest of all these techniques is that they preserve certain properties of the matrices in question, such as determinant, rank, or inverse, so that these quantities can be calculated after applying the transformation, or that certain matrix operations are algorithmically easier to carry out for some types of matrices."
+3983,"The LU decomposition factors matrices as a product of lower  and an upper triangular matrices . Once this decomposition is calculated, linear systems can be solved more efficiently, by a simple technique called forward and back substitution. Likewise, inverses of triangular matrices are algorithmically easier to calculate. The Gaussian elimination is a similar algorithm; it transforms any matrix to row echelon form. Both methods proceed by multiplying the matrix by suitable elementary matrices, which correspond to permuting rows or columns and adding multiples of one row to another row. Singular value decomposition expresses any matrix A as a product UDV∗, where U and V are unitary matrices and D is a diagonal matrix."
+3984,"The eigendecomposition or diagonalization expresses A as a product VDV−1, where D is a diagonal matrix and V is a suitable invertible matrix. If A can be written in this form, it is called diagonalizable. More generally, and applicable to all matrices, the Jordan decomposition transforms a matrix into Jordan normal form, that is to say matrices whose only nonzero entries are the eigenvalues λ1 to λn of A, placed on the main diagonal and possibly entries equal to one directly above the main diagonal, as shown at the right. Given the eigendecomposition, the nth power of A  can be calculated via"
+3985,"and the power of a diagonal matrix can be calculated by taking the corresponding powers of the diagonal entries, which is much easier than doing the exponentiation for A instead. This can be used to compute the matrix exponential eA, a need frequently arising in solving linear differential equations, matrix logarithms and square roots of matrices. To avoid numerically ill-conditioned situations, further algorithms such as the Schur decomposition can be employed."
+3986,"Matrices can be generalized in different ways. Abstract algebra uses matrices with entries in more general fields or even rings, while linear algebra codifies properties of matrices in the notion of linear maps. It is possible to consider matrices with infinitely many columns and rows. Another extension is tensors, which can be seen as higher-dimensional arrays of numbers, as opposed to vectors, which can often be realized as sequences of numbers, while matrices are rectangular or two-dimensional arrays of numbers. Matrices, subject to certain requirements tend to form groups known as matrix groups. Similarly under certain conditions matrices form rings known as matrix rings. Though the product of matrices is not in general commutative yet certain matrices form fields known as matrix fields."
+3987,"This article focuses on matrices whose entries are real or complex numbers. However, matrices can be considered with much more general types of entries than real or complex numbers. As a first step of generalization, any field, that is, a set where addition, subtraction, multiplication, and division operations are defined and well-behaved, may be used instead of R or C, for example rational numbers or finite fields. For example, coding theory makes use of matrices over finite fields. Wherever eigenvalues are considered, as these are roots of a polynomial they may exist only in a larger field than that of the entries of the matrix; for instance, they may be complex in the case of a matrix with real entries. The possibility to reinterpret the entries of a matrix as elements of a larger field  then allows considering each square matrix to possess a full set of eigenvalues. Alternatively one can consider only matrices with entries in an algebraically closed field, such as C, from the outset."
+3988,"More generally, matrices with entries in a ring R are widely used in mathematics. Rings are a more general notion than fields in that a division operation need not exist. The very same addition and multiplication operations of matrices extend to this setting, too. The set M ) of all square n-by-n matrices over R is a ring called matrix ring, isomorphic to the endomorphism ring of the left R-module Rn. If the ring R is commutative, that is, its multiplication is commutative, then the ring M is also an associative algebra over R. The determinant of square matrices over a commutative ring R can still be defined using the Leibniz formula; such a matrix is invertible if and only if its determinant is invertible in R, generalising the situation over a field F, where every nonzero element is invertible. Matrices over superrings are called supermatrices."
+3989,"Matrices do not always have all their entries in the same ring – or even in any ring at all. One special but common case is block matrices, which may be considered as matrices whose entries themselves are matrices. The entries need not be square matrices, and thus need not be members of any ring; but their sizes must fulfill certain compatibility conditions."
+3990,"Linear maps Rn → Rm are equivalent to m-by-n matrices, as described above. More generally, any linear map f: V → W between finite-dimensional vector spaces can be described by a matrix A = , after choosing bases v1, ..., vn of V, and w1, ..., wm of W , which is such that"
+3991,"In other words, column j of A expresses the image of vj in terms of the basis vectors wi of W; thus this relation uniquely determines the entries of the matrix A. The matrix depends on the choice of the bases: different choices of bases give rise to different, but equivalent matrices. Many of the above concrete notions can be reinterpreted in this light, for example, the transpose matrix AT describes the transpose of the linear map given by A, with respect to the dual bases."
+3992,"More generally, the set of m×n matrices can be used to represent the R-linear maps between the free modules Rm and Rn for an arbitrary ring R with unity. When n = m composition of these maps is possible, and this gives rise to the matrix ring of n×n matrices representing the endomorphism ring of Rn."
+3993,"A group is a mathematical structure consisting of a set of objects together with a binary operation, that is, an operation combining any two objects to a third, subject to certain requirements. A group in which the objects are matrices and the group operation is matrix multiplication is called a matrix group. Since a group every element must be invertible, the most general matrix groups are the groups of all invertible matrices of a given size, called the general linear groups."
+3994,"Any property of matrices that is preserved under matrix products and inverses can be used to define further matrix groups. For example, matrices with a given size and with a determinant of 1 form a subgroup of  their general linear group, called a special linear group. Orthogonal matrices, determined by the condition"
+3995,form the orthogonal group. Every orthogonal matrix has determinant 1 or −1. Orthogonal matrices with determinant 1 form a subgroup called special orthogonal group.
+3996,"Every finite group is isomorphic to a matrix group, as one can see by considering the regular representation of the symmetric group. General groups can be studied using matrix groups, which are comparatively well understood, by means of representation theory."
+3997,"It is also possible to consider matrices with infinitely many rows and/or columns even though, being infinite objects, one cannot write down such matrices explicitly. All that matters is that for every element in the set indexing rows, and every element in the set indexing columns, there is a well-defined entry . The basic operations of addition, subtraction, scalar multiplication, and transposition can still be defined without problem; however, matrix multiplication may involve infinite summations to define the resulting entries, and these are not defined in general."
+3998,"If infinite matrices are used to describe linear maps, then only those matrices can be used all of whose columns have but a finite number of nonzero entries, for the following reason. For a matrix A to describe a linear map f: V→W, bases for both spaces must have been chosen; recall that by definition this means that every vector in the space can be written uniquely as a  linear combination of basis vectors, so that written as a  vector v of coefficients, only finitely many entries vi are nonzero. Now the columns of A describe the images by f of individual basis vectors of V in the basis of W, which is only meaningful if these columns have only finitely many nonzero entries. There is no restriction on the rows of A however: in the product A·v there are only finitely many nonzero coefficients of v involved, so every one of its entries, even if it is given as an infinite sum of products, involves only finitely many nonzero terms and is therefore well defined. Moreover, this amounts to forming a linear combination of the columns of A that effectively involves only finitely many of them, whence the result has only finitely many nonzero entries because each of those columns does. Products of two matrices of the given type are well defined , are of the same type, and correspond to the composition of linear maps."
+3999,"If R is a normed ring, then the condition of row or column finiteness can be relaxed. With the norm in place, absolutely convergent series can be used instead of finite sums. For example, the matrices whose column sums are absolutely convergent sequences form a ring. Analogously, the matrices whose row sums are absolutely convergent series also form a ring."
+4000,"Infinite matrices can also be used to describe operators on Hilbert spaces, where convergence and continuity questions arise, which again results in certain constraints that must be imposed. However, the explicit point of view of matrices tends to obfuscate the matter, and the abstract and more powerful tools of functional analysis can be used instead."
+4001,"An empty matrix is a matrix in which the number of rows or columns  is zero. Empty matrices help dealing with maps involving the zero vector space. For example, if A is a 3-by-0 matrix and B is a 0-by-3 matrix, then AB is the 3-by-3 zero matrix corresponding to the null map from a 3-dimensional space V to itself, while BA is a 0-by-0 matrix. There is no common notation for empty matrices, but most computer algebra systems allow creating and computing with them. The determinant of the 0-by-0 matrix is 1 as follows regarding the empty product occurring in the Leibniz formula for the determinant as 1. This value is also consistent with the fact that the identity map from any finite-dimensional space to itself has determinant 1, a fact that is often used as a part of the characterization of determinants."
+4002,"There are numerous applications of matrices, both in mathematics and other sciences. Some of them merely take advantage of the compact representation of a set of numbers in a matrix. For example, in game theory and economics, the payoff matrix encodes the payoff for two players, depending on which out of a given  set of strategies the players choose. Text mining and automated thesaurus compilation makes use of document-term matrices such as tf-idf to track frequencies of certain words in several documents."
+4003,Complex numbers can be represented by particular real 2-by-2 matrices via
+4004,"under which addition and multiplication of complex numbers and matrices correspond to each other. For example, 2-by-2 rotation matrices represent the multiplication with some complex number of absolute value 1, as above. A similar interpretation is possible for quaternions and Clifford algebras in general."
+4005,"Early encryption techniques such as the Hill cipher also used matrices. However, due to the linear nature of matrices, these codes are comparatively easy to break. Computer graphics uses matrices to represent objects; to calculate transformations of objects using affine rotation matrices to accomplish tasks such as projecting a three-dimensional object onto a two-dimensional screen, corresponding to a theoretical camera observation; and to apply image convolutions such as sharpening, blurring, edge detection, and more. Matrices over a polynomial ring are important in the study of control theory."
+4006,"Chemistry makes use of matrices in various ways, particularly since the use of quantum theory to discuss molecular bonding and spectroscopy. Examples are the overlap matrix and the Fock matrix used in solving the Roothaan equations to obtain the molecular orbitals of the Hartree–Fock method."
+4007,"The adjacency matrix of a finite graph is a basic notion of graph theory. It records which vertices of the graph are connected by an edge. Matrices containing just two different values  are called logical matrices. The distance  matrix contains information about distances of the edges. These concepts can be applied to websites connected by hyperlinks or cities connected by roads etc., in which case  the matrices tend to be sparse, that is, contain few nonzero entries. Therefore, specifically tailored matrix algorithms can be used in network theory."
+4008,"The Hessian matrix of a differentiable function ƒ: Rn → R consists of the second derivatives of ƒ with respect to the several coordinate directions, that is,"
+4009,"Another matrix frequently used in geometrical situations is the Jacobi matrix of a differentiable map f: Rn → Rm. If f1, ..., fm denote the components of f, then the Jacobi matrix is defined as"
+4010,"If n > m, and if the rank of the Jacobi matrix attains its maximal value m, f is locally invertible at that point, by the implicit function theorem."
+4011,"Partial differential equations can be classified by considering the matrix of coefficients of the highest-order differential operators of the equation. For elliptic partial differential equations this matrix is positive definite, which has a decisive influence on the set of possible solutions of the equation in question."
+4012,"The finite element method is an important numerical method to solve partial differential equations, widely applied in simulating complex physical systems. It attempts to approximate the solution to some equation by piecewise linear functions, where the pieces are chosen concerning a sufficiently fine grid, which in turn can be recast as a matrix equation."
+4013,"Stochastic matrices are square matrices whose rows are probability vectors, that is, whose entries are non-negative and sum up to one. Stochastic matrices are used to define Markov chains with finitely many states. A row of the stochastic matrix gives the probability distribution for the next position of some particle currently in the state that corresponds to the row. Properties of the Markov chain-like absorbing states, that is, states that any particle attains eventually, can be read off the eigenvectors of the transition matrices."
+4014,"Statistics also makes use of matrices in many different forms. Descriptive statistics is concerned with describing data sets, which can often be represented as data matrices, which may then be subjected to dimensionality reduction techniques. The covariance matrix encodes the mutual variance of several random variables. Another technique using matrices are linear least squares, a method that approximates a finite set of pairs , , ..., , by a linear function"
+4015,"which can be formulated in terms of matrices, related to the singular value decomposition of matrices."
+4016,"Random matrices are matrices whose entries are random numbers, subject to suitable probability distributions, such as matrix normal distribution. Beyond probability theory, they are applied in domains ranging from number theory to physics."
+4017,"Linear transformations and the associated symmetries play a key role in modern physics. For example, elementary particles in quantum field theory are classified as representations of the Lorentz group of special relativity and, more specifically, by their behavior under the spin group. Concrete representations involving the Pauli matrices and more general gamma matrices are an integral part of the physical description of fermions, which behave as spinors. For the three lightest quarks, there is a group-theoretical representation involving the special unitary group SU; for their calculations, physicists use a convenient matrix representation known as the Gell-Mann matrices, which are also used for the SU gauge group that forms the basis of the modern description of strong nuclear interactions, quantum chromodynamics. The Cabibbo–Kobayashi–Maskawa matrix, in turn, expresses the fact that the basic quark states that are important for weak interactions are not the same as, but linearly related to the basic quark states that define particles with specific and distinct masses."
+4018,"The first model of quantum mechanics  represented the theory's operators by infinite-dimensional matrices acting on quantum states. This is also referred to as matrix mechanics. One particular example is the density matrix that characterizes the ""mixed"" state of a quantum system as a linear combination of elementary, ""pure"" eigenstates."
+4019,"Another matrix serves as a key tool for describing the scattering experiments that form the cornerstone of experimental particle physics: Collision reactions such as occur in particle accelerators, where non-interacting particles head towards each other and collide in a small interaction zone, with a new set of non-interacting particles as the result, can be described as the scalar product of outgoing particle states and a linear combination of ingoing particle states. The linear combination is given by a matrix known as the S-matrix, which encodes all information about the possible interactions between particles."
+4020,"A general application of matrices in physics is the description of linearly coupled harmonic systems. The equations of motion of such systems can be described in matrix form, with a mass matrix multiplying a generalized velocity to give the kinetic term, and a force matrix multiplying a displacement vector to characterize the interactions. The best way to obtain solutions is to determine the system's eigenvectors, its normal modes, by diagonalizing the matrix equation. Techniques like this are crucial when it comes to the internal dynamics of molecules: the internal vibrations of systems consisting of mutually bound component atoms. They are also needed for describing mechanical vibrations, and oscillations in electrical circuits."
+4021,"Geometrical optics provides further matrix applications. In this approximative theory, the wave nature of light is neglected. The result is a model in which light rays are indeed geometrical rays. If the deflection of light rays by optical elements is small, the action of a lens or reflective element on a given light ray can be expressed as multiplication of a two-component vector with a two-by-two matrix called ray transfer matrix analysis: the vector's components are the light ray's slope and its distance from the optical axis, while the matrix encodes the properties of the optical element. Actually, there are two kinds of matrices, viz. a refraction matrix describing the refraction at a lens surface, and a translation matrix, describing the translation of the plane of reference to the next refracting surface, where another refraction matrix applies.
+The optical system, consisting of a combination of lenses and/or reflective elements, is simply described by the matrix resulting from the product of the components' matrices."
+4022,Traditional mesh analysis and nodal analysis in electronics lead to a system of linear equations that can be described with a matrix.
+4023,"The behaviour of many electronic components can be described using matrices. Let A be a 2-dimensional vector with the component's input voltage v1 and input current i1 as its elements, and let B be a 2-dimensional vector with the component's output voltage v2 and output current i2 as its elements. Then the behaviour of the electronic component can be described by B = H · A, where H is a 2 x 2 matrix containing one impedance element , one admittance element , and two dimensionless elements . Calculating a circuit now reduces to multiplying matrices."
+4024,"Matrices have a long history of application in solving linear equations but they were known as arrays until the 1800s. The Chinese text The Nine Chapters on the Mathematical Art written in 10th–2nd century BCE is the first example of the use of array methods to solve simultaneous equations, including the concept of determinants. In 1545 Italian mathematician Gerolamo Cardano introduced the method to Europe when he published Ars Magna. The Japanese mathematician Seki used the same array methods to solve simultaneous equations in 1683. The Dutch mathematician Jan de Witt represented transformations using arrays in his 1659 book Elements of Curves . Between 1700 and 1710 Gottfried Wilhelm Leibniz publicized the use of arrays for recording information or solutions and experimented with over 50 different systems of arrays. Cramer presented his rule in 1750."
+4025,"The term ""matrix"" , ""source"", ""origin"", ""list"", ""register"", derived from mater—mother) was coined by James Joseph Sylvester in 1850, who understood a matrix as an object giving rise to several determinants today called minors, that is to say, determinants of smaller matrices that derive from the original one by removing columns and rows. In an 1851 paper, Sylvester explains:"
+4026,"Arthur Cayley published a treatise on geometric transformations using matrices that were not rotated versions of the coefficients being investigated as had previously been done. Instead, he defined operations such as addition, subtraction, multiplication, and division as transformations of those matrices and showed the associative and distributive properties held true. Cayley investigated and demonstrated the non-commutative property of matrix multiplication as well as the commutative property of matrix addition. Early matrix theory had limited the use of arrays almost exclusively to determinants and Arthur Cayley's abstract matrix operations were revolutionary. He was instrumental in proposing a matrix concept independent of equation systems. In 1858 Cayley published his A memoir on the theory of matrices in which he proposed and demonstrated the Cayley–Hamilton theorem."
+4027,"The English mathematician Cuthbert Edmund Cullis was the first to use modern bracket notation for matrices in 1913 and he simultaneously demonstrated the first significant use of the notation A =  to represent a matrix where ai,j refers to the ith row and the jth column."
+4028,"The modern study of determinants sprang from several sources. Number-theoretical problems led Gauss to relate coefficients of quadratic forms, that is, expressions such as x2 + xy − 2y2, and linear maps in three dimensions to matrices. Eisenstein further developed these notions, including the remark that, in modern parlance, matrix products are non-commutative. Cauchy was the first to prove general statements about determinants, using as definition of the determinant of a matrix A =  the following: replace the powers ajk by ajk in the polynomial"
+4029,"Many theorems were first established for small matrices only, for example, the Cayley–Hamilton theorem was proved for 2×2 matrices by Cayley in the aforementioned memoir, and by Hamilton for 4×4 matrices. Frobenius, working on bilinear forms, generalized the theorem to all dimensions . Also at the end of the 19th century, the Gauss–Jordan elimination  was established by Wilhelm Jordan. In the early 20th century, matrices attained a central role in linear algebra, partially due to their use in classification of the hypercomplex number systems of the previous century."
+4030,"The inception of matrix mechanics by Heisenberg, Born and Jordan led to studying matrices with infinitely many rows and columns. Later, von Neumann carried out the mathematical formulation of quantum mechanics, by further developing functional analytic notions such as linear operators on Hilbert spaces, which, very roughly speaking, correspond to Euclidean space, but with an infinity of independent directions."
+4031,The word has been used in unusual ways by at least two authors of historical importance.
+4032,"Bertrand Russell and Alfred North Whitehead in their Principia Mathematica  use the word ""matrix"" in the context of their axiom of reducibility. They proposed this axiom as a means to reduce any function to one of lower type, successively, so that at the ""bottom""  the function is identical to its extension:"
+4033,"For example, a function Φ of two variables x and y can be reduced to a collection of functions of a single variable, for example, y, by ""considering"" the function for all possible values of ""individuals"" ai substituted in place of variable x. And then the resulting collection of functions of the single variable y, that is, ∀ai: Φ, can be reduced to a ""matrix"" of values by ""considering"" the function for all possible values of ""individuals"" bi substituted in place of variable y:"
+4034,"Alfred Tarski in his 1946 Introduction to Logic used the word ""matrix"" synonymously with the notion of truth table as used in mathematical logic."
+4035,"A vector is what is needed to ""carry"" the point A to the point B; the Latin word vector means ""carrier"". It was first used by 18th century astronomers investigating planetary revolution around the Sun. The magnitude of the vector is the distance between the two points, and the direction refers to the direction of displacement from A to B. Many algebraic operations on real numbers such as addition, subtraction, multiplication, and negation have close analogues for vectors, operations which obey the familiar algebraic laws of commutativity, associativity, and distributivity. These operations and associated laws qualify Euclidean vectors as an example of the more generalized concept of vectors defined simply as elements of a vector space."
+4036,"Vectors play an important role in physics: the velocity and acceleration of a moving object and the forces acting on it can all be described with vectors. Many other physical quantities can be usefully thought of as vectors. Although most of them do not represent distances , their magnitude and direction can still be represented by the length and direction of an arrow. The mathematical representation of a physical vector depends on the coordinate system used to describe it. Other vector-like objects that describe physical quantities and transform in a similar way under changes of the coordinate system include pseudovectors and tensors."
+4037,"The vector concept, as it is known today, is the result of a gradual development over a period of more than 200 years. About a dozen people contributed significantly to its development. In 1835, Giusto Bellavitis abstracted the basic idea when he established the concept of equipollence. Working in a Euclidean plane, he made equipollent any pair of parallel line segments of the same length and orientation. Essentially, he realized an equivalence relation on the pairs of points  in the plane, and thus erected the first space of vectors in the plane.: 52–4  The term vector was introduced by William Rowan Hamilton as part of a quaternion, which is a sum q = s + v of a real number s  and a 3-dimensional vector. Like Bellavitis, Hamilton viewed vectors as representative of classes of equipollent directed segments. As complex numbers use an imaginary unit to complement the real line, Hamilton considered the vector v to be the imaginary part of a quaternion:"
+4038,"Several other mathematicians developed vector-like systems in the middle of the nineteenth century,  including Augustin Cauchy, Hermann Grassmann, August Möbius, Comte de Saint-Venant, and Matthew O'Brien. Grassmann's 1840 work Theorie der Ebbe und Flut  was the first system of spatial analysis that is similar to today's system, and had ideas corresponding to the cross product, scalar product and vector differentiation. Grassmann's work was largely neglected until the 1870s. Peter Guthrie Tait carried the quaternion standard after Hamilton. His 1867 Elementary Treatise of Quaternions included extensive treatment of the nabla or del operator ∇. In 1878, Elements of Dynamic was published by William Kingdon Clifford. Clifford simplified the quaternion study by isolating the dot product and cross product of two vectors from the complete quaternion product. This approach made vector calculations available to engineers—and others working in three dimensions and skeptical of the fourth."
+4039,"Josiah Willard Gibbs, who was exposed to quaternions through James Clerk Maxwell's Treatise on Electricity and Magnetism, separated off their vector part for independent treatment. The first half of Gibbs's Elements of Vector Analysis, published in 1881, presents what is essentially the modern system of vector analysis. In 1901, Edwin Bidwell Wilson published Vector Analysis, adapted from Gibbs's lectures, which banished any mention of quaternions in the development of vector calculus."
+4040,"In physics and engineering, a vector is typically regarded as a geometric entity characterized by a magnitude and a direction. It is formally defined as a directed line segment, or arrow, in a Euclidean space. In pure mathematics, a vector is defined more generally as any element of a vector space. In this context, vectors are abstract entities which may or may not be characterized by a magnitude and a direction. This generalized definition implies that the above-mentioned geometric entities are a special kind of vectors, as they are elements of a special kind of vector space called Euclidean space. This particular article is about vectors strictly defined as arrows in Euclidean space. When it becomes necessary to distinguish these special vectors from vectors as defined in pure mathematics, they are sometimes referred to as geometric, spatial, or Euclidean vectors."
+4041,"A Euclidean vector is thus an equivalence class of directed segments with the same magnitude ) and same direction . In physics, Euclidean vectors are used to represent physical quantities that have both magnitude and direction, but are not located at a specific place, in contrast to scalars, which have no direction. For example, velocity, forces and acceleration are represented by vectors."
+4042,"Since the physicist's concept of force has a direction and a magnitude, it may be seen as a vector. As an example, consider a rightward force F of 15 newtons. If the positive axis is also directed rightward, then F is represented by the vector 15 N, and if positive points leftward, then the vector for F is −15 N. In either case, the magnitude of the vector is 15 N. Likewise, the vector representation of a displacement Δs of 4 meters would be 4 m or −4 m, depending on its direction, and its magnitude would be 4 m regardless."
+4043,"Vectors are fundamental in the physical sciences. They can be used to represent any quantity that has magnitude, has direction, and which adheres to the rules of vector addition. An example is velocity, the magnitude of which is speed. For instance, the velocity 5 meters per second upward could be represented by the vector  . Another quantity represented by a vector is force, since it has a magnitude and direction and follows the rules of vector addition. Vectors also describe many other physical quantities, such as linear displacement, displacement, linear acceleration, angular acceleration, linear momentum, and angular momentum. Other physical vectors, such as the electric and magnetic field, are represented as a system of vectors at each point of a physical space; that is, a vector field. Examples of quantities that have magnitude and direction, but fail to follow the rules of vector addition, are angular displacement and electric current. Consequently, these are not vectors."
+4044,"In Cartesian coordinates, a free vector may be thought of in terms of a corresponding bound vector, in this sense, whose initial point has the coordinates of the origin O = . It is then determined by the coordinates of that bound vector's terminal point. Thus the free vector represented by  is a vector of unit length—pointing along the direction of the positive x-axis."
+4045,"This coordinate representation of free vectors allows their algebraic features to be expressed in a convenient numerical fashion. For example, the sum of the two  vectors  and  is the  vector"
+4046,"In the geometrical and physical settings, it is sometimes possible to associate, in a natural way, a length or magnitude and a direction to vectors. In addition, the notion of direction is strictly associated with the notion of an angle between two vectors. If the dot product of two vectors is defined—a scalar-valued product of two vectors—then it is also possible to define a length; the dot product gives a convenient algebraic characterization of both angle  and length . In three dimensions, it is further possible to define the cross product, which supplies an algebraic characterization of the area and orientation in space of the parallelogram defined by two vectors . In any dimension , it is possible to define the exterior product, which  supplies an algebraic characterization of the area and orientation in space of the n-dimensional parallelotope defined by n vectors."
+4047,"In a pseudo-Euclidean space, a vector's squared length can be positive, negative, or zero. An important example is Minkowski space ."
+4048,"However, it is not always possible or desirable to define the length of a vector. This more general type of spatial vector is the subject of vector spaces  and affine spaces . One physical example comes from thermodynamics, where many quantities of interest can be considered vectors in a space with no notion of length or angle."
+4049,"In physics, as well as mathematics, a vector is often identified with a tuple of components, or list of numbers, that act as scalar coefficients for a set of basis vectors. When the basis is transformed, for example by rotation or stretching, then the components of any vector in terms of that basis also transform in an opposite sense. The vector itself has not changed, but the basis has, so the components of the vector must change to compensate. The vector is called covariant or contravariant, depending on how the transformation of the vector's components is related to the transformation of the basis. In general, contravariant vectors are ""regular vectors"" with units of distance , or distance times some other unit ; covariant vectors, on the other hand, have units of one-over-distance such as gradient. If you change units  from meters to millimeters, a scale factor of 1/1000, a displacement of 1 m becomes 1000 mm—a contravariant change in numerical value. In contrast, a gradient of 1 K/m becomes 0.001 K/mm—a covariant change in value . Tensors are another type of quantity that behave in this way; a vector is one type of tensor."
+4050,"In pure mathematics, a vector is any element of a vector space over some field and is often represented as a coordinate vector. The vectors described in this article are a very special case of this general definition, because they are contravariant with respect to the ambient space. Contravariance captures the physical intuition behind the idea that a vector has ""magnitude and direction""."
+4051,"Vectors are usually shown in graphs or other diagrams as arrows , as illustrated in the figure. Here, the point A is called the origin, tail, base, or initial point, and the point B is called the head, tip, endpoint, terminal point or final point. The length of the arrow is proportional to the vector's magnitude, while the direction in which the arrow points indicates the vector's direction."
+4052,"On a two-dimensional diagram, a vector perpendicular to the plane of the diagram is sometimes desired. These vectors are commonly shown as small circles. A circle with a dot at its centre  indicates a vector pointing out of the front of the diagram, toward the viewer. A circle with a cross inscribed in it  indicates a vector pointing into and behind the diagram. These can be thought of as viewing the tip of an arrow head on and viewing the flights of an arrow from the back."
+4053,"In order to calculate with vectors, the graphical representation may be too cumbersome. Vectors in an n-dimensional Euclidean space can be represented as coordinate vectors in a Cartesian coordinate system. The endpoint of a vector can be identified with an ordered list of n real numbers . These numbers are the coordinates of the endpoint of the vector, with respect to a given Cartesian coordinate system, and are typically called the scalar components  of the vector on the axes of the coordinate system."
+4054,"As an example in two dimensions , the vector from the origin O =  to the point A =  is simply written as"
+4055,"In three dimensional Euclidean space , vectors are identified with triples of scalar components:"
+4056,This can be generalised to n-dimensional Euclidean space .
+4057,"These numbers are often arranged into a column vector or row vector, particularly when dealing with matrices, as follows:"
+4058,"Another way to represent a vector in n-dimensions is to introduce the standard basis vectors. For instance, in three dimensions, there are three of them:"
+4059,"The notation ei is compatible with the index notation and the summation convention commonly used in higher level mathematics, physics, and engineering."
+4060,"As explained above, a vector is often described by a set of vector components that add up to form the given vector. Typically, these components are the projections of the vector on a set of mutually perpendicular reference axes . The vector is said to be decomposed or resolved with respect to that set."
+4061,"The decomposition or resolution of a vector into components is not unique, because it depends on the choice of the axes on which the vector is projected."
+4062,The choice of a basis does not affect the properties of a vector or its behaviour under transformations.
+4063,"A vector can also be broken up with respect to ""non-fixed"" basis vectors that change their orientation as a function of time or space. For example, a vector in three-dimensional space can be decomposed with respect to two axes, respectively normal, and tangent to a surface . Moreover, the radial and tangential components of a vector relate to the radius of rotation of an object. The former is parallel to the radius and the latter is orthogonal to it."
+4064,"In these cases, each of the components may be in turn decomposed with respect to a fixed coordinate system or basis set ."
+4065,The following section uses the Cartesian coordinate system with basis vectors
+4066,"Two vectors are parallel if they have the same direction but not necessarily the same magnitude, or antiparallel if they have opposite direction but not necessarily the same magnitude."
+4067,"Two vectors are parallel if they have the same direction but not necessarily the same magnitude, or antiparallel if they have opposite direction but not necessarily the same magnitude."
+4068,The sum of a and b of two vectors may be defined as
+4069,"The addition may be represented graphically by placing the tail of the arrow b at the head of the arrow a, and then drawing an arrow from the tail of a to the head of b. The new arrow drawn represents the vector a + b, as illustrated below:"
+4070,"This addition method is sometimes called the parallelogram rule because a and b form the sides of a parallelogram and a + b is one of the diagonals. If a and b are bound vectors that have the same base point, this point will also be the base point of a + b. One can check geometrically that a + b = b + a and  + c = a + ."
+4071,The difference of a and b is
+4072,"Subtraction of two vectors can be geometrically illustrated as follows: to subtract b from a, place the tails of a and b at the same point, and then draw an arrow from the head of b to the head of a. This new arrow represents the vector  + a, with  being the opposite of  b,  see drawing. And  + a = a − b."
+4073,"A vector may also be multiplied, or re-scaled, by a real number r. In the context of conventional vector algebra, these real numbers are often called scalars  to distinguish them from vectors. The operation of multiplying a vector by a scalar is called scalar multiplication. The resulting vector is"
+4074,"Intuitively, multiplying by a scalar r stretches a vector out by a factor of r. Geometrically, this can be visualized  as placing r copies of the vector in a line where the endpoint of one vector is the initial point of the next vector."
+4075,"If r is negative, then the vector changes direction: it flips around by an angle of 180°. Two examples  are given below:"
+4076,Scalar multiplication is distributive over vector addition in the following sense: r = ra + rb for all vectors a and b and all scalars r. One can also show that a − b = a + b.
+4077,"The length or magnitude or norm of the vector a is denoted by ‖a‖ or, less commonly, |a|, which is not to be confused with the absolute value ."
+4078,"The length of the vector a can be computed with the Euclidean norm,"
+4079,"which is a consequence of the Pythagorean theorem since the basis vectors e1, e2, e3 are orthogonal unit vectors."
+4080,"This happens to be equal to the square root of the dot product, discussed below, of the vector with itself:"
+4081,A unit vector is any vector with a length of one; normally unit vectors are used simply to indicate direction. A vector of arbitrary length can be divided by its length to create a unit vector. This is known as normalizing a vector. A unit vector is often indicated with a hat as in â.
+4082,"To normalize a vector a = , scale the vector by the reciprocal of its length ‖a‖. That is:"
+4083,"where θ is the measure of the angle between a and b . Geometrically, this means that a and b are drawn with a common start point, and then the length of a is multiplied with the length of the component of b that points in the same direction as a."
+4084,The dot product can also be defined as the sum of the products of the components of each vector as
+4085,"The cross product  is only meaningful in three or seven dimensions. The cross product differs from the dot product primarily in that the result of the cross product of two vectors is a vector. The cross product, denoted a × b, is a vector perpendicular to both a and b and is defined as"
+4086,"where θ is the measure of the angle between a and b, and n is a unit vector perpendicular to both a and b which completes a right-handed system. The right-handedness constraint is necessary because there exist two unit vectors that are perpendicular to both a and b, namely, n and ."
+4087,"The cross product a × b is defined so that a, b, and a × b also becomes a right-handed system . This is the right-hand rule."
+4088,The length of a × b can be interpreted as the area of the parallelogram having a and b as sides.
+4089,The cross product can be written as
+4090,For arbitrary choices of spatial orientation  the cross product of two vectors is a pseudovector instead of a vector .
+4091,"The scalar triple product  is not really a new operator, but a way of applying the other two multiplication operators to three vectors. The scalar triple product is sometimes denoted by  and defined as:"
+4092,"It has three primary uses. First, the absolute value of the box product is the volume of the parallelepiped which has edges that are defined by the three vectors. Second, the scalar triple product is zero if and only if the three vectors are linearly dependent, which can be easily proved by considering that in order for the three vectors to not make a volume, they must all lie in the same plane. Third, the box product is positive if and only if the three vectors a, b and c are right-handed."
+4093,"In components , if the three vectors are thought of as rows , the scalar triple product is simply the determinant of the 3-by-3 matrix having the three vectors as rows"
+4094,The scalar triple product is linear in all three entries and anti-symmetric in the following sense:
+4095,"All examples thus far have dealt with vectors expressed in terms of the same basis, namely, the e basis {e1, e2, e3}. However, a vector can be expressed in terms of any number of different bases that are not necessarily aligned with each other, and still remain the same vector.  In the e basis, a vector a is expressed, by definition, as"
+4096,"The scalar components in the e basis are, by definition,"
+4097,"In another orthonormal basis n = {n1, n2, n3} that is not necessarily aligned with e, the vector a is expressed as"
+4098,"and the scalar components in the n basis are, by definition,"
+4099,"The values of p, q, r, and u, v, w relate to the unit vectors in such a way that the resulting vector sum is exactly the same physical vector a in both cases.  It is common to encounter vectors known in terms of different bases .  In such a case it is necessary to develop a method to convert between bases so the basic vector operations such as addition and subtraction can be performed.  One way to express u, v, w in terms of p, q, r is to use column matrices along with a direction cosine matrix containing the information that relates the two bases.  Such an expression can be formed by substitution of the above equations to form"
+4100,Distributing the dot-multiplication gives
+4101,Replacing each dot product with a unique scalar gives
+4102,and these equations can be expressed as the single matrix equation
+4103,"This matrix equation relates the scalar components of a in the n basis  with those in the e basis .  Each matrix element cjk is the direction cosine relating nj to ek. The term direction cosine refers to the cosine of the angle between two unit vectors, which is also equal to their dot product. Therefore,"
+4104,"By referring collectively to e1, e2, e3 as the e basis and to n1, n2, n3 as the n basis, the matrix containing all the cjk is known as the ""transformation matrix from e to n"", or the ""rotation matrix from e to n"" , or the ""direction cosine matrix from e to n"" .  The properties of a rotation matrix are such that its inverse is equal to its transpose. This means that the ""rotation matrix from e to n"" is the transpose of ""rotation matrix from n to e""."
+4105,"The properties of a direction cosine matrix, C are:"
+4106,"The advantage of this method is that a direction cosine matrix can usually be obtained independently by using Euler angles or a quaternion to relate the two vector bases, so the basis conversions can be performed directly, without having to work out all the dot products described above."
+4107,"By applying several matrix multiplications in succession, any vector can be expressed in any basis so long as the set of direction cosines is known relating the successive bases."
+4108,"With the exception of the cross and triple products, the above formulae generalise to two dimensions and higher dimensions. For example, addition generalises to two dimensions as"
+4109,"The cross product does not readily generalise to other dimensions, though the closely related exterior product does, whose result is a bivector. In two dimensions this is simply a pseudoscalar"
+4110,A seven-dimensional cross product is similar to the cross product in that its result is a vector orthogonal to the two arguments; there is however no natural way of selecting one of the possible such products.
+4111,Vectors have many uses in physics and other sciences.
+4112,"In abstract vector spaces, the length of the arrow depends on a dimensionless scale. If it represents, for example, a force, the ""scale"" is of physical dimension length/force. Thus there is typically consistency in scale among quantities of the same dimension, but otherwise scale ratios may vary; for example, if ""1 newton"" and ""5 m"" are both represented with an arrow of 2 cm, the scales are 1 m:50 N and 1:250 respectively. Equal length of vectors of different dimension has no particular significance unless there is some proportionality constant inherent in the system that the diagram represents. Also length of a unit vector  has no coordinate-system-invariant significance."
+4113,"Often in areas of physics and mathematics, a vector evolves in time, meaning that it depends on a time parameter t. For instance, if r represents the position vector of a particle, then r gives a parametric representation of the trajectory of the particle. Vector-valued functions can be differentiated and integrated by differentiating or integrating the components of the vector, and many of the familiar rules from calculus continue to hold for the derivative and integral of vector-valued functions."
+4114,The position of a point x =  in three-dimensional space can be represented as a position vector whose base point is the origin
+4115,"Given two points x = , y =  their displacement is a vector"
+4116,"The velocity v of a point or particle is a vector, its length gives the speed. For constant velocity the position at time t will be"
+4117,Acceleration a of a point is vector which is the time derivative of velocity. Its dimensions are length/time2.
+4118,Force is a vector with dimensions of mass×length/time2 and Newton's second law is the scalar multiplication
+4119,Work is the dot product of force and displacement
+4120,"In the language of differential geometry, the requirement that the components of a vector transform according to the same matrix of the coordinate transition is equivalent to defining a contravariant vector to be a tensor of contravariant rank one. Alternatively, a contravariant vector is defined to be a tangent vector, and the rules for transforming a contravariant vector follow from the chain rule."
+4121,"Some vectors transform like contravariant vectors, except that when they are reflected through a mirror, they flip and gain a minus sign. A transformation that switches right-handedness to left-handedness and vice versa like a mirror does is said to change the orientation of space. A vector which gains a minus sign when the orientation of space changes is called a pseudovector or an axial vector. Ordinary vectors are sometimes called true vectors or polar vectors to distinguish them from pseudovectors. Pseudovectors occur most frequently as the cross product of two ordinary vectors."
+4122,"One example of a pseudovector is angular velocity. Driving in a car, and looking forward, each of the wheels has an angular velocity vector pointing to the left. If the world is reflected in a mirror which switches the left and right side of the car, the reflection of this angular velocity vector points to the right, but the actual angular velocity vector of the wheel still points to the left, corresponding to the minus sign. Other examples of pseudovectors include magnetic field, torque, or more generally any cross product of two  vectors."
+4123,"This distinction between vectors and pseudovectors is often ignored, but it becomes important in studying symmetry properties. See parity ."
+4124,"MIMD architectures may be used in a number of application areas such as computer-aided design/computer-aided manufacturing, simulation, modeling, and as communication switches. MIMD machines can be of either shared memory or distributed memory categories. These classifications are based on how MIMD processors access memory. Shared memory machines may be of the bus-based, extended, or hierarchical type. Distributed memory machines may have hypercube or mesh interconnection schemes."
+4125,"An example of MIMD system is Intel Xeon Phi, descended from Larrabee microarchitecture. These processors have multiple processing cores  that can execute different instructions on different data."
+4126,"Most parallel computers, as of 2013, are MIMD systems."
+4127,"In shared memory model the processors are all connected to a ""globally available"" memory, via either software or hardware means. The operating system usually maintains its memory coherence."
+4128,"From a programmer's point of view, this memory model is better understood than the distributed memory model.  Another advantage is that memory coherence is managed by the operating system and not the written program.  Two known disadvantages are: scalability beyond thirty-two processors is difficult, and the shared memory model is less flexible than the distributed memory model."
+4129,"There are many examples of shared memory : UMA , COMA ."
+4130,"MIMD machines with shared memory have processors which share a common, central memory. In the simplest form, all processors are attached to a bus which connects them to memory. This means that every machine with shared memory shares a specific CM, common bus system for all the clients."
+4131,"For example, if we consider a bus with clients A, B, C connected on one side and P, Q, R connected on the opposite side,
+any one of the clients will communicate with the other by means of the bus interface between them."
+4132,"MIMD machines with hierarchical shared memory use a hierarchy of buses  to give processors access to each other's memory. Processors on different boards may communicate through inter-nodal buses. Buses support communication between boards. With this type of architecture, the machine may support over nine thousand processors."
+4133,"In distributed memory MIMD  machines, each processor has its own individual memory location. Each processor has no direct knowledge about other processor's memory. For data to be shared, it must be passed from one processor to another as a message. Since there is no shared memory, contention is not as great a problem with these machines. It is not economically feasible to connect a large number of processors directly to each other. A way to avoid this multitude of direct connections is to connect each processor to just a few others. This type of design can be inefficient because of the added time required to pass a message from one processor to another along the message path. The amount of time required for processors to perform simple message routing can be substantial. Systems were designed to reduce this time loss and hypercube and mesh are among two of the popular interconnection schemes."
+4134,"Examples of distributed memory  include MPP , COW  and NUMA . The former is complex and expensive: Many super-computers coupled by broad-band networks. Examples include hypercube and mesh interconnections. COW is the ""home-made"" version for a fraction of the price."
+4135,"In an MIMD distributed memory machine with a hypercube system interconnection network containing four processors, a processor and a memory module are placed at each vertex of a square. The diameter of the system is the minimum number of steps it takes for one processor to send a message to the processor that is the farthest away. So, for example, the diameter of a 2-cube is 2. In a hypercube system with eight processors and each processor and memory module being placed in the vertex of a cube, the diameter is 3. In general, a system that contains 2^N processors with each processor directly connected to N other processors, the diameter of the system is N. One disadvantage of a hypercube system is that it must be configured in powers of two, so a machine must be built that could potentially have many more processors than is really needed for the application."
+4136,"In an MIMD distributed memory machine with a mesh interconnection network, processors are placed in a two-dimensional grid. Each processor is connected to its four immediate neighbors. Wrap around connections may be provided at the edges of the mesh. One advantage of the mesh interconnection network over the hypercube is that the mesh system need not be configured in powers of two. A disadvantage is that the diameter of the mesh network is greater than the hypercube for systems with more than four processors."
+4137,"Such machines exploit data level parallelism, but not concurrency: there are simultaneous  computations, but each unit performs the exact same instruction at any given moment . SIMD is particularly applicable to common tasks such as adjusting the contrast in a digital image or adjusting the volume of digital audio. Most modern CPU designs include SIMD instructions to improve the performance of multimedia use. SIMD has three different subcategories in Flynn's 1972 Taxonomy, one of which is SIMT.  SIMT should not be confused with software threads or hardware threads, both of which are task time-sharing .  SIMT is true simultaneous parallel hardware-level execution."
+4138,"Modern graphics processing units  are often wide SIMD implementations, capable of branches, loads, and stores on 128 or 256 bits at a time."
+4139,"The first use of SIMD instructions was in the ILLIAC IV, which was completed in 1966."
+4140,"SIMD was the basis for vector supercomputers of the early 1970s such as the CDC Star-100 and the Texas Instruments ASC, which could operate on a ""vector"" of data with a single instruction. Vector processing was especially popularized by Cray in the 1970s and 1980s. Vector processing architectures are now considered separate from SIMD computers: Duncan's Taxonomy includes them where Flynn's Taxonomy does not, due to Flynn's work  pre-dating the Cray-1 ."
+4141,"The first era of modern SIMD computers was characterized by massively parallel processing-style supercomputers such as the Thinking Machines CM-1 and CM-2. These computers had many limited-functionality processors that would work in parallel. For example, each of 65,536 single-bit processors in a Thinking Machines CM-2 would execute the same instruction at the same time, allowing, for instance, to logically combine 65,536 pairs of bits at a time, using a hypercube-connected network or processor-dedicated RAM to find its operands. Supercomputing moved away from the SIMD approach when inexpensive scalar MIMD approaches based on commodity processors such as the Intel i860 XP became more powerful, and interest in SIMD waned."
+4142,"The current era of SIMD processors grew out of the desktop-computer market rather than the supercomputer market. As desktop processors became powerful enough to support real-time gaming and audio/video processing during the 1990s, demand grew for this particular type of computing power, and microprocessor vendors turned to SIMD to meet the demand. Hewlett-Packard introduced MAX instructions into PA-RISC 1.1 desktops in 1994 to accelerate MPEG decoding. Sun Microsystems introduced SIMD integer instructions in its ""VIS"" instruction set extensions in 1995, in its UltraSPARC I microprocessor. MIPS followed suit with their similar MDMX system."
+4143,"The first widely deployed desktop SIMD was with Intel's MMX extensions to the x86 architecture in 1996. This sparked the introduction of the much more powerful AltiVec system in the Motorola PowerPC and IBM's POWER systems. Intel responded in 1999 by introducing the all-new SSE system. Since then, there have been several extensions to the SIMD instruction sets for both architectures. Advanced vector extensions AVX, AVX2 and AVX-512 are developed by Intel. AMD supports AVX, AVX2, and AVX-512 in their current products."
+4144,"All of these developments have been oriented toward support for real-time graphics, and are therefore oriented toward processing in two, three, or four dimensions, usually with vector lengths of between two and sixteen words, depending on data type and architecture. When new SIMD architectures need to be distinguished from older ones, the newer architectures are then considered ""short-vector"" architectures, as earlier SIMD and vector supercomputers had vector lengths from 64 to 64,000. A modern supercomputer is almost always a cluster of MIMD computers, each of which implements  SIMD instructions."
+4145,"An application that may take advantage of SIMD is one where the same value is being added to  a large number of data points, a common operation in many multimedia applications. One example would be changing the brightness of an image. Each pixel of an image consists of three values for the brightness of the red , green  and blue  portions of the color. To change the brightness, the R, G and B values are read from memory, a value is added to  them, and the resulting values are written back out to memory.  Audio DSPs would likewise, for volume control, multiply both Left and Right channels simultaneously."
+4146,"With a SIMD processor there are two improvements to this process. For one the data is understood to be in blocks, and a number of values can be loaded all at once. Instead of a series of instructions saying ""retrieve this pixel, now retrieve the next pixel"", a SIMD processor will have a single instruction that effectively says ""retrieve n pixels"" . For a variety of reasons, this can take much less time than retrieving each pixel individually, as with a traditional CPU design."
+4147,"Another advantage is that the instruction operates on all loaded data in a single operation. In other words, if the SIMD system works by loading up eight data points at once, the add operation being applied to the data will happen to all eight values at the same time. This parallelism is separate from the parallelism provided by a superscalar processor; the eight values are processed in parallel even on a non-superscalar processor, and a superscalar processor may be able to perform multiple SIMD operations in parallel."
+4148,"To remedy problems 1 and 5, RISC-V's vector extension uses an alternative approach: instead of exposing the sub-register-level details to the programmer, the instruction set abstracts them out as a few ""vector registers"" that use the same interfaces across all CPUs with this instruction set. The hardware handles all alignment issues and ""strip-mining"" of loops. Machines with different vector sizes would be able to run the same code. LLVM calls this vector type ""vscale""."
+4149,"An order of magnitude increase in code size is not uncommon, when compared to equivalent scalar or equivalent vector code, and an order of magnitude or greater effectiveness  is achievable with Vector ISAs."
+4150,"ARM's Scalable Vector Extension takes another approach, known in Flynn's Taxonomy as ""Associative Processing"", more commonly known today as ""Predicated""  SIMD. This approach is not as compact as Vector processing but is still far better than non-predicated SIMD. Detailed comparative examples are given in the Vector processing page."
+4151,"Small-scale  SIMD became popular on general-purpose CPUs in the early 1990s and continued through 1997 and later with Motion Video Instructions  for Alpha. SIMD instructions can be found, to one degree or another, on most CPUs, including IBM's AltiVec and SPE for PowerPC, HP's PA-RISC Multimedia Acceleration eXtensions , Intel's MMX and iwMMXt, SSE, SSE2, SSE3 SSSE3 and SSE4.x, AMD's 3DNow!, ARC's ARC Video subsystem, SPARC's VIS and VIS2, Sun's MAJC, ARM's Neon technology, MIPS' MDMX  and MIPS-3D. The IBM, Sony, Toshiba co-developed Cell Processor's SPU's instruction set is heavily SIMD based. Philips, now NXP, developed several SIMD processors named Xetal. The Xetal has 320 16-bit processor elements especially designed for vision tasks."
+4152,Intel's AVX-512 SIMD instructions process 512 bits of data at once.
+4153,"SIMD instructions are widely used to process 3D graphics, although modern graphics cards with embedded SIMD have largely taken over this task from the CPU. Some systems also include permute functions that re-pack elements inside vectors, making them particularly useful for data processing and compression. They are also used in cryptography. The trend of general-purpose computing on GPUs  may lead to wider use of SIMD in the future."
+4154,"Adoption of SIMD systems in personal computer software was at first slow, due to a number of problems. One was that many of the early SIMD instruction sets tended to slow overall performance of the system due to the re-use of existing floating point registers. Other systems, like MMX and 3DNow!, offered support for data types that were not interesting to a wide audience and had expensive context switching instructions to switch between using the FPU and MMX registers. Compilers also often lacked support, requiring programmers to resort to assembly language coding."
+4155,"SIMD on x86 had a slow start. The introduction of 3DNow! by AMD and SSE by Intel confused matters somewhat, but today the system seems to have settled down  and newer compilers should result in more SIMD-enabled software. Intel and AMD now both provide optimized math libraries that use SIMD instructions, and open source alternatives like libSIMD, SIMDx86 and SLEEF have started to appear ."
+4156,"Apple Computer had somewhat more success, even though they entered the SIMD market later than the rest. AltiVec offered a rich system and can be programmed using increasingly sophisticated compilers from Motorola, IBM and GNU, therefore assembly language programming is rarely needed. Additionally, many of the systems that would benefit from SIMD were supplied by Apple itself, for example iTunes and QuickTime. However, in 2006, Apple computers moved to Intel x86 processors. Apple's APIs and development tools  were modified to support SSE2 and SSE3 as well as AltiVec. Apple was the dominant purchaser of PowerPC chips from IBM and Freescale Semiconductor. Even though Apple has stopped using PowerPC processors in their products, further development of AltiVec is continued in several PowerPC and Power ISA designs from Freescale and IBM."
+4157,"SIMD within a register, or SWAR, is a range of techniques and tricks used for performing SIMD in general-purpose registers on hardware that does not provide any direct support for SIMD instructions. This can be used to exploit parallelism in certain algorithms even on hardware that does not support SIMD directly."
+4158,"It is common for publishers of the SIMD instruction sets to make their own C/C++ language extensions with intrinsic functions or special datatypes  guaranteeing the generation of vector code. Intel, AltiVec, and ARM NEON provide extensions widely adopted by the compilers targeting their CPUs. "
+4159,"The GNU C Compiler takes the extensions a step further by abstracting them into a universal interface that can be used on any platform by providing a way of defining SIMD datatypes. The LLVM Clang compiler also implements the feature, with an analogous interface defined in the IR. Rust's packed_simd crate  uses this interface, and so does Swift 2.0+."
+4160,"C++ has an experimental interface std::experimental::simd that works similarly to the GCC extension. LLVM's libcxx seems to implement it. For GCC and libstdc++, a wrapper library that builds on top of the GCC extension is available."
+4161,"Microsoft added SIMD to .NET in RyuJIT. The System.Numerics.Vector package, available on NuGet, implements SIMD datatypes. Java also has a new proposed API for SIMD instructions  available in OpenJDK 17 in an incubator module. It also has a safe fallback mechanism on unsupported CPUs to simple loops."
+4162,"Instead of providing an SIMD datatype, compilers can also be hinted to auto-vectorize some loops, potentially taking some assertions about the lack of data dependency. This is not as flexible as manipulating SIMD variables directly, but is easier to use. OpenMP 4.0+ has a #pragma omp simd hint. This OpenMP interface has replaced a wide set of nonstandard extensions, including Cilk's #pragma simd, GCC's #pragma GCC ivdep, and many more."
+4163,"Consumer software is typically expected to work on a range of CPUs covering multiple generations, which could limit the programmer's ability to use new SIMD instructions to improve the computational performance of a program. The solution is to include multiple versions of the same code that uses either older or newer SIMD technologies, and pick one that best fits the user's CPU at run-time . There are two main camps of solutions:"
+4164,"FMV, manually coded in assembly language, is quite commonly used in a number of performance-critical libraries such as glibc and libjpeg-turbo. Intel C++ Compiler, GNU Compiler Collection since GCC 6, and Clang since clang 7 allow for a simplified approach, with the compiler taking care of function duplication and selection. GCC and clang requires explicit target_clones labels in the code to ""clone"" functions, while ICC does so automatically . The Rust programming language also supports FMV. The setup is similar to GCC and Clang in that the code defines what instruction sets to compile for, but cloning is manually done via inlining."
+4165,"As using FMV requires code modification on GCC and Clang, vendors more commonly use library multi-versioning: this is easier to achieve as only compiler switches need to be changed. Glibc supports LMV and this functionality is adopted by the Intel-backed Clear Linux project."
+4166,"In 2013 John McCutchan announced that he had created a high-performance interface to SIMD instruction sets for the Dart programming language, bringing the benefits of SIMD to web programs for the first time. The interface consists of two types:"
+4167,"Instances of these types are immutable and in optimized code are mapped directly to SIMD registers. Operations expressed in Dart typically are compiled into a single instruction without any overhead. This is similar to C and C++ intrinsics. Benchmarks for 4×4 matrix multiplication, 3D vertex transformation, and Mandelbrot set visualization show near 400% speedup compared to scalar code written in Dart."
+4168,"McCutchan's work on Dart, now called SIMD.js, has been adopted by ECMAScript and Intel announced at IDF 2013 that they are implementing McCutchan's specification for both V8 and SpiderMonkey. However, by 2017, SIMD.js has been taken out of the ECMAScript standard queue in favor of pursuing a similar interface in WebAssembly. As of August 2020, the WebAssembly interface remains unfinished, but its portable 128-bit SIMD feature has already seen some use in many engines."
+4169,"Emscripten, Mozilla's C/C++-to-JavaScript compiler, with extensions can enable compilation of C++ programs that make use of SIMD intrinsics or GCC-style vector code to the SIMD API of JavaScript, resulting in equivalent speedups compared to scalar code. It also supports  the WebAssembly 128-bit SIMD proposal."
+4170,It has generally proven difficult to find sustainable commercial applications for SIMD-only processors.
+4171,"One that has had some measure of success is the GAPP, which was developed by Lockheed Martin and taken to the commercial sector by their spin-off Teranex. The GAPP's recent incarnations have become a powerful tool in real-time video processing applications like conversion between various video standards and frame rates , deinterlacing, image noise reduction, adaptive video compression, and image enhancement."
+4172,"A more ubiquitous application for SIMD is found in video games: nearly every modern video game console since 1998 has incorporated a SIMD processor somewhere in its architecture. The PlayStation 2 was unusual in that one of its vector-float units could function as an autonomous DSP executing its own instruction stream, or as a coprocessor driven by ordinary CPU instructions. 3D graphics applications tend to lend themselves well to SIMD processing as they rely heavily on operations with 4-dimensional vectors. Microsoft's Direct3D 9.0 now chooses at runtime processor-specific implementations of its own math operations, including the use of SIMD-capable instructions."
+4173,"A later processor that used vector processing is the Cell Processor used in the Playstation 3, which was developed by IBM in cooperation with Toshiba and Sony. It uses a number of SIMD processors  and is geared towards the huge datasets required by 3D and video processing applications. It differs from traditional ISAs by being SIMD from the ground up with no separate scalar registers."
+4174,"Ziilabs produced an SIMD type processor for use on mobile devices, such as media players and mobile phones."
+4175,"Larger scale commercial SIMD processors are available from ClearSpeed Technology, Ltd. and Stream Processors, Inc. ClearSpeed's CSX600  has 96 cores each with two double-precision floating point units while the CSX700  has 192. Stream Processors is headed by computer architect Bill Dally. Their Storm-1 processor  contains 80 SIMD cores controlled by a MIPS CPU."
+4176,"Arcade system boards have used specialized graphics circuits since the 1970s. In early video game hardware, RAM for frame buffers was expensive, so video chips composited data together as the display was being scanned out on the monitor."
+4177,"A specialized barrel shifter circuit helped the CPU animate the framebuffer graphics for various 1970s arcade video games from Midway and Taito, such as Gun Fight , Sea Wolf , and Space Invaders . The Namco Galaxian arcade system in 1979 used specialized graphics hardware that supported RGB color, multi-colored sprites, and tilemap backgrounds. The Galaxian hardware was widely used during the golden age of arcade video games, by game companies such as Namco, Centuri, Gremlin, Irem, Konami, Midway, Nichibutsu, Sega, and Taito."
+4178,"The Atari 2600 in 1977 used a video shifter called the Television Interface Adaptor. Atari 8-bit computers  had ANTIC, a video processor which interpreted instructions describing a ""display list""—the way the scan lines map to specific bitmapped or character modes and where the memory is stored . 6502 machine code subroutines could be triggered on scan lines by setting a bit on a display list instruction. ANTIC also supported smooth vertical and horizontal scrolling independent of the CPU."
+4179,"The NEC µPD7220 was the first implementation of a personal computer graphics display processor as a single large-scale integration  integrated circuit chip. This enabled the design of low-cost, high-performance video graphics cards such as those from Number Nine Visual Technology. It became the best-known GPU until the mid-1980s. It was the first fully integrated VLSI  metal–oxide–semiconductor  graphics display processor for PCs, supported up to 1024×1024 resolution, and laid the foundations for the emerging PC graphics market. It was used in a number of graphics cards and was licensed for clones such as the Intel 82720, the first of Intel's graphics processing units. The Williams Electronics arcade games Robotron 2084, Joust, Sinistar, and Bubbles, all released in 1982, contain custom blitter chips for operating on 16-color bitmaps."
+4180,"In 1984, Hitachi released ARTC HD63484, the first major CMOS graphics processor for personal computers. The ARTC could display up to 4K resolution when in monochrome mode. It was used in a number of graphics cards and terminals during the late 1980s. In 1985, the Amiga was released with a custom graphics chip including a blitter for bitmap manipulation, line drawing, and area fill. It also included a coprocessor with its own simple instruction set, that was capable of manipulating graphics hardware registers in sync with the video beam , or driving the blitter. In 1986, Texas Instruments released the TMS34010, the first fully programmable graphics processor. It could run general-purpose code, but it had a graphics-oriented instruction set. During 1990–1992, this chip became the basis of the Texas Instruments Graphics Architecture  Windows accelerator cards."
+4181,"In 1987, the IBM 8514 graphics system was released. It was one of the first video cards for IBM PC compatibles to implement fixed-function 2D primitives in electronic hardware. Sharp's X68000, released in 1987, used a custom graphics chipset with a 65,536 color palette and hardware support for sprites, scrolling, and multiple playfields. It served as a development machine for Capcom's CP System arcade board. Fujitsu's FM Towns computer, released in 1989, had support for a 16,777,216 color palette. In 1988, the first dedicated polygonal 3D graphics boards were introduced in arcades with the Namco System 21 and Taito Air System."
+4182,"IBM introduced its proprietary Video Graphics Array  display standard in 1987, with a maximum resolution of 640×480 pixels. In November 1988, NEC Home Electronics announced its creation of the Video Electronics Standards Association  to develop and promote a Super VGA  computer display standard as a successor to VGA. Super VGA enabled graphics display resolutions up to 800×600 pixels, a 36% increase."
+4183,"In 1991, S3 Graphics introduced the S3 86C911, which its designers named after the Porsche 911 as an indication of the performance increase it promised. The 86C911 spawned a variety of imitators: by 1995, all major PC graphics chip makers had added 2D acceleration support to their chips. Fixed-function Windows accelerators surpassed expensive general-purpose graphics coprocessors in Windows performance, and such coprocessors faded from the PC market."
+4184,"Throughout the 1990s, 2D GUI acceleration evolved. As manufacturing capabilities improved, so did the level of integration of graphics chips. Additional application programming interfaces  arrived for a variety of tasks, such as Microsoft's WinG graphics library for Windows 3.x, and their later DirectDraw interface for hardware acceleration of 2D games in Windows 95 and later."
+4185,"In the early- and mid-1990s, real-time 3D graphics became increasingly common in arcade, computer, and console games, which led to increasing public demand for hardware-accelerated 3D graphics. Early examples of mass-market 3D graphics hardware can be found in arcade system boards such as the Sega Model 1, Namco System 22, and Sega Model 2, and the fifth-generation video game consoles such as the Saturn, PlayStation, and Nintendo 64. Arcade systems such as the Sega Model 2 and SGI Onyx-based Namco Magic Edge Hornet Simulator in 1993 were capable of hardware T&L  years before appearing in consumer graphics cards. Another early example is the Super FX chip, a RISC-based on-cartridge graphics chip used in some SNES games, notably Doom and Star Fox. Some systems used DSPs to accelerate transformations. Fujitsu, which worked on the Sega Model 2 arcade system, began working on integrating T&L into a single LSI solution for use in home computers in 1995; the Fujitsu Pinolite, the first 3D geometry processor for personal computers, released in 1997. The first hardware T&L GPU on home video game consoles was the Nintendo 64's Reality Coprocessor, released in 1996. In 1997, Mitsubishi released the 3Dpro/2MP, a GPU capable of transformation and lighting, for workstations and Windows NT desktops; ATi used it for its FireGL 4000 graphics card, released in 1997."
+4186,"The term ""GPU"" was coined by Sony in reference to the 32-bit Sony GPU  in the PlayStation video game console, released in 1994."
+4187,"In the PC world, notable failed attempts for low-cost 3D graphics chips included the S3 ViRGE, ATI Rage, and Matrox Mystique. These chips were essentially previous-generation 2D accelerators with 3D features bolted on. Many were pin-compatible with the earlier-generation chips for ease of implementation and minimal cost. Initially, 3D graphics were possible only with discrete boards dedicated to accelerating 3D functions  acceleration entirely) such as the PowerVR and the 3dfx Voodoo. However, as manufacturing technology continued to progress, video, 2D GUI acceleration, and 3D functionality were all integrated into one chip. Rendition's Verite chipsets were among the first to do this well. In 1997, Rendition collaborated with Hercules and Fujitsu on a ""Thriller Conspiracy"" project which combined a Fujitsu FXG-1 Pinolite geometry processor with a Vérité V2200 core to create a graphics card with a full T&L engine years before Nvidia's GeForce 256; This card, designed to reduce the load placed upon the system's CPU, never made it to market. NVIDIA RIVA 128 was one of the first consumer-facing GPU integrated 3D processing unit and 2D processing unit on a chip."
+4188,"OpenGL appeared in the early '90s as a professional graphics API, but originally suffered from performance issues which allowed the Glide API to become a dominant force on the PC in the late '90s. These issues were quickly overcome and the Glide API fell by the wayside. Software implementations of OpenGL were common during this time, although the influence of OpenGL eventually led to widespread hardware support. DirectX became popular among Windows game developers during the late '90s. Unlike OpenGL, Microsoft insisted on providing strict one-to-one support of hardware. The approach made DirectX less popular as a standalone graphics API initially, since many GPUs provided their own specific features, which existing OpenGL applications were already able to benefit from, leaving DirectX often one generation behind. "
+4189,"Microsoft began to work more closely with hardware developers and started to target the releases of DirectX to coincide with those of the supporting graphics hardware. Direct3D 5.0 was the first version of the API to gain widespread adoption in the gaming market, and it competed directly with more-hardware-specific, often proprietary, graphics libraries, while OpenGL maintained a strong following. Direct3D 7.0 introduced support for hardware-accelerated transform and lighting  for Direct3D; OpenGL had this capability from its inception. 3D accelerator cards moved beyond being simple rasterizers to add another significant hardware stage to the 3D rendering pipeline. The Nvidia GeForce 256  was the first consumer-level card with hardware-accelerated T&L; professional 3D cards already had this capability. Hardware transform and lighting—existing features of OpenGL—came to consumer-level hardware in the '90s and set the precedent for later pixel shader and vertex shader units which were far more flexible and programmable."
+4190,"Nvidia was first to produce a chip capable of programmable shading: the GeForce 3. Each pixel could now be processed by a short program that could include additional image textures as inputs, and each geometric vertex could likewise be processed by a short program before it was projected onto the screen. Used in the Xbox console, this chip competed with the one in the PlayStation 2, which used a custom vector unit for hardware accelerated vertex processing . The earliest incarnations of shader execution engines used in Xbox were not general purpose and could not execute arbitrary pixel code. Vertices and pixels were processed by different units which had their own resources, with pixel shaders having tighter constraints . Pixel shading engines were actually more akin to a highly customizable function block and did not really ""run"" a program. Many of these disparities between vertex and pixel shading were not addressed until the Unified Shader Model."
+4191,"In October 2002, with the introduction of the ATI Radeon 9700 , the world's first Direct3D 9.0 accelerator, pixel and vertex shaders could implement looping and lengthy floating point math, and were quickly becoming as flexible as CPUs, yet orders of magnitude faster for image-array operations. Pixel shading is often used for bump mapping, which adds texture to make an object look shiny, dull, rough, or even round or extruded."
+4192,"With the introduction of the Nvidia GeForce 8 series and new generic stream processing units, GPUs became more generalized computing devices. Parallel GPUs are making computational inroads against the CPU, and a subfield of research, dubbed GPU computing or GPGPU for general purpose computing on GPU, has found applications in fields as diverse as machine learning, oil exploration, scientific image processing, linear algebra, statistics, 3D reconstruction, and stock options pricing. GPGPU was the precursor to what is now called a compute shader  and actually abused the hardware to a degree by treating the data passed to algorithms as texture maps and executing algorithms by drawing a triangle or quad with an appropriate pixel shader. This entails some overheads since units like the scan converter are involved where they are not needed ."
+4193,"Nvidia's CUDA platform, first introduced in 2007, was the earliest widely adopted programming model for GPU computing. OpenCL is an open standard defined by the Khronos Group that allows for the development of code for both GPUs and CPUs with an emphasis on portability. OpenCL solutions are supported by Intel, AMD, Nvidia, and ARM, and according to a report in 2011 by Evans Data, OpenCL had become the second most popular HPC tool."
+4194,"In 2010, Nvidia partnered with Audi to power their cars' dashboards, using the Tegra GPU to provide increased functionality to cars' navigation and entertainment systems. Advances in GPU technology in cars helped advance self-driving technology. AMD's Radeon HD 6000 series cards were released in 2010, and in 2011 AMD released its 6000M Series discrete GPUs for mobile devices. The Kepler line of graphics cards by Nvidia were released in 2012 and were used in the Nvidia's 600 and 700 series cards. A feature in this GPU microarchitecture included GPU boost, a technology that adjusts the clock-speed of a video card to increase or decrease it according to its power draw. The Kepler microarchitecture was manufactured on the 28 nm process."
+4195,"The PS4 and Xbox One were released in 2013; they both use GPUs based on AMD's Radeon HD 7850 and 7790. Nvidia's Kepler line of GPUs was followed by the Maxwell line, manufactured on the same process. Nvidia's 28 nm chips were manufactured by TSMC in Taiwan using the 28 nm process. Compared to the 40 nm technology from the past, this manufacturing process allowed a 20 percent boost in performance while drawing less power. Virtual reality headsets have high system requirements; manufacturers recommended the GTX 970 and the R9 290X or better at the time of their release. Cards based on the Pascal microarchitecture were released in 2016. The GeForce 10 series of cards are of this generation of graphics cards. They are made using the 16 nm manufacturing process which improves upon previous microarchitectures. Nvidia released one non-consumer card under the new Volta architecture, the Titan V. Changes from the Titan XP, Pascal's high-end card, include an increase in the number of CUDA cores, the addition of tensor cores, and HBM2. Tensor cores are designed for deep learning, while high-bandwidth memory is on-die, stacked, lower-clocked memory that offers an extremely wide memory bus. To emphasize that the Titan V is not a gaming card, Nvidia removed the ""GeForce GTX"" suffix it adds to consumer gaming cards."
+4196,"In 2018, Nvidia launched the RTX 20 series GPUs that added ray-tracing cores to GPUs, improving their performance on lighting effects. Polaris 11 and Polaris 10 GPUs from AMD are fabricated by a 14 nm process. Their release resulted in a substantial increase in the performance per watt of AMD video cards. AMD also released the Vega GPU series for the high end market as a competitor to Nvidia's high end Pascal cards, also featuring HBM2 like the Titan V."
+4197,"In 2019, AMD released the successor to their Graphics Core Next  microarchitecture/instruction set. Dubbed RDNA, the first product featuring it was the Radeon RX 5000 series of video cards."
+4198,"The company announced that the successor to the RDNA microarchitecture would be incremental . AMD unveiled the Radeon RX 6000 series, its RDNA 2 graphics cards with support for hardware-accelerated ray tracing. The product series, launched in late 2020, consisted of the RX 6800, RX 6800 XT, and RX 6900 XT. The RX 6700 XT, which is based on Navi 22, was launched in early 2021."
+4199,The PlayStation 5 and Xbox Series X and Series S were released in 2020; they both use GPUs based on the RDNA 2 microarchitecture with incremental improvements and different GPU configurations in each system's implementation.
+4200,"Intel first entered the GPU market in the late 1990s, but produced lackluster 3D accelerators compared to the competition at the time. Rather than attempting to compete with the high-end manufacturers Nvidia and ATI/AMD, they began integrating Intel Graphics Technology GPUs into motherboard chipsets, beginning with the Intel 810 for the Pentium III, and later into CPUs. They began with the Intel Atom 'Pineview' laptop processor in 2009, continuing in 2010 with desktop processors in the first generation of the Intel Core line and with contemporary Pentiums and Celerons. This resulted in a large nominal market share, as the majority of computers with an Intel CPU also featured this embedded graphics processor. These generally lagged behind discrete processors in performance. Intel re-entered the discrete GPU market in 2022 with its Arc series, which competed with the then-current GeForce 30 series and Radeon 6000 series cards at competitive prices."
+4201,"In the 2020s, GPUs have been increasingly used for calculations involving embarrassingly parallel problems, such as training of neural networks on enormous datasets that are needed for large language models. Specialized processing cores on some modern workstation's GPUs are dedicated for deep learning since they have significant FLOPS performance increases, using 4×4 matrix multiplication and division, resulting in hardware performance up to 128 TFLOPS in some applications. These tensor cores are expected to appear in consumer cards, as well."
+4202,"Many companies have produced GPUs under a number of brand names. In 2009, Intel, Nvidia, and AMD/ATI were the market share leaders, with 49.4%, 27.8%, and 20.6% market share respectively. In addition, Matrox produces GPUs. Modern smartphones use mostly Adreno GPUs from Qualcomm, PowerVR GPUs from Imagination Technologies, and Mali GPUs from ARM."
+4203,"Modern GPUs have traditionally used most of their transistors to do calculations related to 3D computer graphics. In addition to the 3D hardware, today's GPUs include basic 2D acceleration and framebuffer capabilities . Newer cards such as AMD/ATI HD5000–HD7000 lack dedicated 2D acceleration; it is emulated by 3D hardware. GPUs were initially used to accelerate the memory-intensive work of texture mapping and rendering polygons. Later, units were added to accelerate geometric calculations such as the rotation and translation of vertices into different coordinate systems. Recent developments in GPUs include support for programmable shaders which can manipulate vertices and textures with many of the same operations that are supported by CPUs, oversampling and interpolation techniques to reduce aliasing, and very high-precision color spaces."
+4204,"Several factors of GPU construction affect the performance of the card for real-time rendering, such as the size of the connector pathways in the semiconductor device fabrication, the clock signal frequency, and the number and size of various on-chip memory caches. Performance is also affected by the number of streaming multiprocessors  for NVidia GPUs, or compute units  for AMD GPUs, or Xe cores for Intel discrete GPUs, which describe the number of core on-silicon processor units within the GPU chip that perform the core calculations, typically working in parallel with other SM/CUs on the GPU. GPU performance is typically measured in floating point operations per second ; GPUs in the 2010s and 2020s typically deliver performance measured in teraflops . This is an estimated performance measure, as other factors can affect the actual display rate."
+4205,"Most GPUs made since 1995 support the YUV color space and hardware overlays, important for digital video playback, and many GPUs made since 2000 also support MPEG primitives such as motion compensation and iDCT. This hardware-accelerated video decoding, in which portions of the video decoding process and video post-processing are offloaded to the GPU hardware, is commonly referred to as ""GPU accelerated video decoding"", ""GPU assisted video decoding"", ""GPU hardware accelerated video decoding"", or ""GPU hardware assisted video decoding""."
+4206,"Recent graphics cards decode high-definition video on the card, offloading the central processing unit. The most common APIs for GPU accelerated video decoding are DxVA for Microsoft Windows operating systems and VDPAU, VAAPI, XvMC, and XvBA for Linux-based and UNIX-like operating systems. All except XvMC are capable of decoding videos encoded with MPEG-1, MPEG-2, MPEG-4 ASP , MPEG-4 AVC , VC-1, WMV3/WMV9, Xvid / OpenDivX , and DivX 5 codecs, while XvMC is only capable of decoding MPEG-1 and MPEG-2."
+4207,There are several dedicated hardware video decoding and encoding solutions.
+4208,Video decoding processes that can be accelerated by modern GPU hardware are:
+4209,"These operations also have applications in video editing, encoding, and transcoding."
+4210,"An earlier GPU may support one or more 2D graphics API for 2D acceleration, such as GDI and DirectDraw."
+4211,"A GPU can support one or more 3D graphics API, such as DirectX, Metal, OpenGL, OpenGL ES, Vulkan."
+4212,"In the 1970s, the term ""GPU"" originally stood for graphics processor unit and described a programmable processing unit working independently from the CPU that was responsible for graphics manipulation and output. In 1994, Sony used the term  in reference to the PlayStation console's Toshiba-designed Sony GPU. The term was popularized by Nvidia in 1999, who marketed the GeForce 256 as ""the world's first GPU"". It was presented as a ""single-chip processor with integrated transform, lighting, triangle setup/clipping, and rendering engines"". Rival ATI Technologies coined the term ""visual processing unit"" or VPU with the release of the Radeon 9700 in 2002. The AMD Alveo MA35D features dual VPU’s, each using the 5 nm process in 2023."
+4213,"In personal computers, there are two main forms of GPUs. Each has many synonyms:"
+4214,"Most GPUs are designed for a specific use, real-time 3D graphics, or other mass calculations:"
+4215,"Gaming
+GeForce GTX, RTX
+Nvidia Titan
+Radeon HD, R5, R7, R9, RX, Vega and Navi series
+Radeon VII
+Intel Arc"
+4216,"GeForce GTX, RTX"
+4217,Nvidia Titan
+4218,"Radeon HD, R5, R7, R9, RX, Vega and Navi series"
+4219,Radeon VII
+4220,Intel Arc
+4221,"Cloud Gaming
+Nvidia GRID
+Radeon Sky"
+4222,Nvidia GRID
+4223,Radeon Sky
+4224,"Workstation
+Nvidia Quadro
+Nvidia RTX
+AMD FirePro
+AMD Radeon Pro
+Intel Arc Pro"
+4225,Nvidia Quadro
+4226,Nvidia RTX
+4227,AMD FirePro
+4228,AMD Radeon Pro
+4229,Intel Arc Pro
+4230,"Cloud Workstation
+Nvidia Tesla
+AMD FireStream"
+4231,Nvidia Tesla
+4232,AMD FireStream
+4233,"Artificial Intelligence training and Cloud
+Nvidia Tesla
+AMD Radeon Instinct"
+4234,Nvidia Tesla
+4235,AMD Radeon Instinct
+4236,"Automated/Driverless car
+Nvidia Drive PX"
+4237,Nvidia Drive PX
+4238,"Dedicated graphics processing units are not necessarily removable, nor does it necessarily interface with the motherboard in a standard fashion. The term ""dedicated"" refers to the fact that graphics cards have RAM that is dedicated to the card's use, not to the fact that most dedicated GPUs are removable. This RAM is usually specially selected for the expected serial workload of the graphics card . Sometimes systems with dedicated discrete GPUs were called ""DIS"" systems as opposed to ""UMA"" systems . Dedicated GPUs for portable computers are most commonly interfaced through a non-standard and often proprietary slot due to size and weight constraints. Such ports may still be considered PCIe or AGP in terms of their logical host interface, even if they are not physically interchangeable with their counterparts."
+4239,"Graphics cards with dedicated GPUs typically interface with the motherboard by means of an expansion slot such as PCI Express  or Accelerated Graphics Port . They can usually be replaced or upgraded with relative ease, assuming the motherboard is capable of supporting the upgrade. A few graphics cards still use Peripheral Component Interconnect  slots, but their bandwidth is so limited that they are generally used only when a PCIe or AGP slot is not available."
+4240,"Technologies such as SLI and NVLink by Nvidia and CrossFire by AMD allow multiple GPUs to draw images simultaneously for a single screen, increasing the processing power available for graphics. These technologies, however, are increasingly uncommon; most games do not fully use multiple GPUs, as most users cannot afford them. Multiple GPUs are still used on supercomputers , on workstations to accelerate video  and 3D rendering, for VFX, GPGPU workloads and for simulations, and in AI to expedite training, as is the case with Nvidia's lineup of DGX workstations and servers, Tesla GPUs, and Intel's Ponte Vecchio GPUs."
+4241,"Integrated graphics processing unit , integrated graphics, shared graphics solutions, integrated graphics processors , or unified memory architecture  use a portion of a computer's system RAM rather than dedicated graphics memory. IGPs can be integrated onto a motherboard as part of its northbridge chipset, or on the same die  with the CPU . On certain motherboards, AMD's IGPs can use dedicated sideport memory: a separate fixed block of high performance memory that is dedicated for use by the GPU. As of early 2007 computers with integrated graphics account for about 90% of all PC shipments. They are less costly to implement than dedicated graphics processing, but tend to be less capable. Historically, integrated processing was considered unfit for 3D games or graphically intensive programs but could run less intensive programs such as Adobe Flash. Examples of such IGPs would be offerings from SiS and VIA circa 2004. However, modern integrated graphics processors such as AMD Accelerated Processing Unit and Intel Graphics Technology  can handle 2D graphics or low-stress 3D graphics."
+4242,"Since GPU computations are memory-intensive, integrated processing may compete with the CPU for relatively slow system RAM, as it has minimal or no dedicated video memory. IGPs use system memory with bandwidth up to a current maximum of 128 GB/s, whereas a discrete graphics card may have a bandwidth of more than 1000 GB/s between its VRAM and GPU core. This memory bus bandwidth can limit the performance of the GPU, though multi-channel memory can mitigate this deficiency. Older integrated graphics chipsets lacked hardware transform and lighting, but newer ones include it."
+4243,"On systems with ""Unified Memory Architecture"" , including modern AMD processors with integrated graphics, modern Intel processors with integrated graphics, Apple processors, the PS5 and Xbox Series , the CPU cores and the GPU block share the same pool of RAM and memory address space. This allows the system to dynamically allocate memory between the CPU cores and the GPU block based on memory needs  and thanks to zero copy transfers, removes the need for either copying data over a bus  between physically separate RAM pools or copying between separate address spaces on a single physical pool of RAM, allowing more efficient transfer of data."
+4244,Hybrid GPUs compete with integrated graphics in the low-end desktop and notebook markets. The most common implementations of this are ATI's HyperMemory and Nvidia's TurboCache.
+4245,"Hybrid graphics cards are somewhat more expensive than integrated graphics, but much less expensive than dedicated graphics cards. They share memory with the system and have a small dedicated memory cache, to make up for the high latency of the system RAM. Technologies within PCI Express make this possible. While these solutions are sometimes advertised as having as much as 768 MB of RAM, this refers to how much can be shared with the system memory."
+4246,"It is common to use a general purpose graphics processing unit  as a modified form of stream processor , running compute kernels. This turns the massive computational power of a modern graphics accelerator's shader pipeline into general-purpose computing power. In certain applications requiring massive vector operations, this can yield several orders of magnitude higher performance than a conventional CPU. The two largest discrete  GPU designers, AMD and Nvidia, are pursuing this approach with an array of applications. Both Nvidia and AMD teamed with Stanford University to create a GPU-based client for the Folding@home distributed computing project for protein folding calculations. In certain circumstances, the GPU calculates forty times faster than the CPUs traditionally used by such applications."
+4247,GPGPUs can be used for many types of embarrassingly parallel tasks including ray tracing. They are generally suited to high-throughput computations that exhibit data-parallelism to exploit the wide vector width SIMD architecture of the GPU.
+4248,GPU-based high performance computers play a significant role in large-scale modelling. Three of the ten most powerful supercomputers in the world take advantage of GPU acceleration.
+4249,"GPUs support API extensions to the C programming language such as OpenCL and OpenMP. Furthermore, each GPU vendor introduced its own API which only works with their cards: AMD APP SDK from AMD, and CUDA from Nvidia. These allow functions called compute kernels to run on the GPU's stream processors. This makes it possible for C programs to take advantage of a GPU's ability to operate on large buffers in parallel, while still using the CPU when appropriate. CUDA was the first API to allow CPU-based applications to directly access the resources of a GPU for more general purpose computing without the limitations of using a graphics API."
+4250,"Since 2005 there has been interest in using the performance offered by GPUs for evolutionary computation in general, and for accelerating the fitness evaluation in genetic programming in particular. Most approaches compile linear or tree programs on the host PC and transfer the executable to the GPU to be run. Typically a performance advantage is only obtained by running the single active program simultaneously on many example problems in parallel, using the GPU's SIMD architecture. However, substantial acceleration can also be obtained by not compiling the programs, and instead transferring them to the GPU, to be interpreted there. Acceleration can then be obtained by either interpreting multiple programs simultaneously, simultaneously running multiple example problems, or combinations of both. A modern GPU can simultaneously interpret hundreds of thousands of very small programs."
+4251,"An external GPU is a graphics processor located outside of the housing of the computer, similar to a large external hard drive. External graphics processors are sometimes used with laptop computers. Laptops might have a substantial amount of RAM and a sufficiently powerful central processing unit , but often lack a powerful graphics processor, and instead have a less powerful but more energy-efficient on-board graphics chip. On-board graphics chips are often not powerful enough for playing video games, or for other graphically intensive tasks, such as editing video or 3D animation/rendering."
+4252,"Therefore, it is desirable to attach a GPU to some external bus of a notebook. PCI Express is the only bus used for this purpose. The port may be, for example, an ExpressCard or mPCIe port , a Thunderbolt 1, 2, or 3 port , or an OCuLink port. Those ports are only available on certain notebook systems. eGPU enclosures include their own power supply , because powerful GPUs can consume hundreds of watts."
+4253,Official vendor support for external GPUs has gained traction. A milestone was Apple's decision to support external GPUs with MacOS High Sierra 10.13.4. Several major hardware vendors  released Thunderbolt 3 eGPU enclosures. This support fuels eGPU implementations by enthusiasts.
+4254,"With modern GPUs, energy usage is an important constraint on the maximum computational capabilities that can be achieved. GPU designs are usually highly scalable, allowing the manufacturer to put multiple chips on the same video card, or to use multiple video cards that work in parallel. Peak performance of any system is essentially limited by the amount of power it can draw and the amount of heat it can dissipate. Consequently, performance per watt of a GPU design translates directly into peak performance of a system that uses that design."
+4255,"In 2013, 438.3 million GPUs were shipped globally and the forecast for 2014 was 414.2 million. However, by the third quarter of 2022, shipments of integrated GPUs totaled around 75.5 million units, down 19% year-over-year. "
+4256,"While a superscalar CPU is typically also pipelined, superscalar and pipelining execution are considered different performance enhancement techniques. The former executes multiple instructions in parallel by using multiple execution units, whereas the latter executes multiple instructions in the same execution unit in parallel by dividing the execution unit into different phases."
+4257,The superscalar technique is traditionally associated with several identifying characteristics :
+4258,"Seymour Cray's CDC 6600 from 1964 is often mentioned as the first superscalar design. The 1967 IBM System/360 Model 91 was another superscalar mainframe. The Intel i960CA , the AMD 29000-series 29050 , and the Motorola MC88110 , microprocessors were the first commercial single-chip superscalar microprocessors. RISC microprocessors like these were the first to have superscalar execution, because RISC architectures free transistors and die area which can be used to include multiple execution units ."
+4259,"Except for CPUs used in low-power applications, embedded systems, and battery-powered devices, essentially all general-purpose CPUs developed since about 1998 are superscalar."
+4260,"The P5 Pentium was the first superscalar x86 processor; the Nx586, P6 Pentium Pro and AMD K5 were among the first designs which decode x86-instructions asynchronously into dynamic microcode-like micro-op sequences prior to actual execution on a superscalar microarchitecture; this opened up for dynamic scheduling of buffered partial instructions and enabled more parallelism to be extracted compared to the more rigid methods used in the simpler P5 Pentium; it also simplified speculative execution and allowed higher clock frequencies compared to designs such as the advanced Cyrix 6x86."
+4261,"The simplest processors are scalar processors. Each instruction executed by a scalar processor typically manipulates one or two data items at a time. By contrast, each instruction executed by a vector processor operates simultaneously on many data items. An analogy is the difference between scalar and vector arithmetic. A superscalar processor is a mixture of the two. Each instruction processes one data item, but there are multiple execution units within each CPU thus multiple instructions can be processing separate data items concurrently."
+4262,"Superscalar CPU design emphasizes improving the instruction dispatcher accuracy, and allowing it to keep the multiple execution units in use at all times. This has become increasingly important as the number of units has increased. While early superscalar CPUs would have two ALUs and a single FPU, a later design such as the PowerPC 970 includes four ALUs, two FPUs, and two SIMD units. If the dispatcher is ineffective at keeping all of these units fed with instructions, the performance of the system will be no better than that of a simpler, cheaper design."
+4263,"A superscalar processor usually sustains an execution rate in excess of one instruction per machine cycle. But merely processing multiple instructions concurrently does not make an architecture superscalar, since pipelined, multiprocessor or multi-core architectures also achieve that, but with different methods."
+4264,"In a superscalar CPU the dispatcher reads instructions from memory and decides which ones can be run in parallel, dispatching each to one of the several execution units contained inside a single CPU. Therefore, a superscalar processor can be envisioned having multiple parallel pipelines, each of which is processing instructions simultaneously from a single instruction thread."
+4265,Available performance improvement from superscalar techniques is limited by three key areas:
+4266,"Existing binary executable programs have varying degrees of intrinsic parallelism. In some cases instructions are not dependent on each other and can be executed simultaneously. In other cases they are inter-dependent: one instruction impacts either resources or results of the other. The instructions a = b + c; d = e + f can be run in parallel because none of the results depend on other calculations. However, the instructions a = b + c; b = e + f might not be runnable in parallel, depending on the order in which the instructions complete while they move through the units."
+4267,"Although the instruction stream may contain no inter-instruction dependencies, a superscalar CPU must nonetheless check for that possibility, since there is no assurance otherwise and failure to detect a dependency would produce incorrect results."
+4268,"No matter how advanced the semiconductor process or how fast the switching speed, this places a practical limit on how many instructions can be simultaneously dispatched. While process advances will allow ever greater numbers of execution units , the burden of checking instruction dependencies grows rapidly, as does the complexity of register renaming circuitry to mitigate some dependencies. Collectively the power consumption, complexity and gate delay costs limit the achievable superscalar speedup."
+4269,"However even given infinitely fast dependency checking logic on an otherwise conventional superscalar CPU, if the instruction stream itself has many dependencies, this would also limit the possible speedup. Thus the degree of intrinsic parallelism in the code stream forms a second limitation."
+4270,"Collectively, these limits drive investigation into alternative architectural changes such as very long instruction word , explicitly parallel instruction computing , simultaneous multithreading , and multi-core computing."
+4271,"With VLIW, the burdensome task of dependency checking by hardware logic at run time is removed and delegated to the compiler. Explicitly parallel instruction computing  is like VLIW with extra cache prefetching instructions."
+4272,Simultaneous multithreading  is a technique for improving the overall efficiency of superscalar processors. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures.
+4273,"Superscalar processors differ from multi-core processors in that the several execution units are not entire processors. A single processor is composed of finer-grained execution units such as the ALU, integer multiplier, integer shifter, FPU, etc. There may be multiple versions of each execution unit to enable execution of many instructions in parallel. This differs from a multi-core processor that concurrently processes instructions from multiple threads, one thread per processing unit . It also differs from a pipelined processor, where the multiple instructions can concurrently be in various stages of execution, assembly-line fashion."
+4274,"The various alternative techniques are not mutually exclusive—they can be  combined in a single processor. Thus a multicore CPU is possible where each core is an independent processor containing multiple parallel pipelines, each pipeline being superscalar. Some processors also include vector capability."
+4275,"In imperative programming languages, the term ""conditional statement"" is usually used, whereas in functional programming, the terms ""conditional expression"" or ""conditional construct"" are preferred, because these terms all have distinct meanings."
+4276,"The if–then construct  is common across many programming languages. Although the syntax varies from language to language, the basic structure  looks like this:"
+4277,For example:
+4278,"In the example code above, the part represented by  constitutes a conditional expression, having intrinsic value  but having no intrinsic meaning. In contrast, the combination of this expression, the If and Then surrounding it, and the consequent that follows afterward constitute a conditional statement, having intrinsic meaning  but no intrinsic value."
+4279,"When an interpreter finds an If, it expects a Boolean condition – for example, x > 0, which means ""the variable x contains a number that is greater than zero"" – and evaluates that condition. If the condition is true, the statements following the then are executed. Otherwise, the execution continues in the following branch – either in the else block , or if there is no else branch, then after the end If."
+4280,"After either branch has been executed, control returns to the point after the end If."
+4281,"In early programming languages, especially some dialects of BASIC in the 1980s home computers, an if–then statement could only contain GOTO statements . This led to a hard-to-read style of programming known as spaghetti programming, with programs in this style called spaghetti code.  As a result, structured programming, which allows  arbitrary statements to be put in statement blocks inside an if statement, gained in popularity, until it became the norm even in most BASIC programming circles. Such mechanisms and principles were based on the older but more advanced ALGOL family of languages, and ALGOL-like languages such as Pascal and Modula-2 influenced modern BASIC variants for many years. While it is possible while using only GOTO statements in if–then statements to write programs that are not spaghetti code and are just as well structured and readable as programs written in a structured programming language, structured programming makes this easier and enforces it.  Structured if–then–else statements like the example above are one of the key elements of structured programming, and they are present in most popular high-level programming languages such as C, Java, JavaScript and Visual Basic ."
+4282,"The else keyword is made to target a specific if–then statement preceding it, but for nested if–then statements, classic programming languages such as ALGOL 60 struggled to define which specific statement to target. Without clear boundaries for which statement is which, an else keyword could target any preceding if–then statement in the nest, as parsed."
+4283,can be parsed as
+4284,"depending on whether the else is associated with the first if or second if. This is known as the dangling else problem, and is resolved in various ways, depending on the language ."
+4285,"By using else if, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped."
+4286,"For example, for a shop offering as much as a 30% discount for an item:"
+4287,"In the example above, if the discount is 10%, then the first if statement will be evaluated as true and ""you have to pay $30"" will be printed out. All other statements below that first if statement will be skipped."
+4288,"The elseif statement, in the Ada language for example, is simply syntactic sugar for else followed by if. In Ada, the difference is that only one end if is needed, if one uses elseif instead of else followed by if. PHP uses the elseif keyword both for its curly brackets or colon syntaxes. Perl provides the keyword elsif to avoid the large number of braces that would be required by multiple if and else statements. Python uses the special keyword elif because structure is denoted by indentation rather than braces, so a repeated use of else and if would require increased indentation after every condition. Some implementations of BASIC, such as Visual Basic, use ElseIf too. Similarly, the earlier UNIX shells  use elif too, but giving the choice of delimiting with spaces, line breaks, or both."
+4289,"However, in many languages more directly descended from Algol, such as Simula, Pascal, BCPL and C, this special syntax for the else if construct is not present, nor is it present in the many syntactical derivatives of C, such as Java, ECMAScript, and so on. This works because in these languages, any single statement  can follow a conditional without being enclosed in a block."
+4290,"This design choice has a slight ""cost"". Each else if branch effectively adds an extra nesting level. This complicates the job for the compiler , because the compiler must analyse and implement arbitrarily long else if chains recursively."
+4291,"If all terms in the sequence of conditionals are testing the value of a single expression , an alternative is the switch statement, also called case-statement or select-statement. Conversely, in languages that do not have a switch statement, these can be produced by a sequence of else if statements."
+4292,"Many languages support if expressions, which are similar to if statements, but return a value as a result. Thus, they are true expressions , not statements ."
+4293,ALGOL 60 and some other members of the ALGOL family allow if–then–else as an expression:
+4294,"In dialects of Lisp – Scheme, Racket and Common Lisp – the first of which was inspired to a great extent by ALGOL:"
+4295,"In Haskell 98, there is only an if expression, no if statement, and the else part is compulsory, as every expression must have some value. Logic that would be expressed with conditionals in other languages is usually expressed with pattern matching in recursive functions."
+4296,"Because Haskell is lazy, it is possible to write control structures, such as if, as ordinary expressions; the lazy evaluation means that an if function can evaluate only the condition and proper branch . It can be written like this:"
+4297,C and C-like languages have a special ternary operator  for conditional expressions with a function that may be described by a template like this:
+4298,"This means that it can be inlined into expressions, unlike if-statements, in C-like languages:"
+4299,which can be compared to the Algol-family if–then–else expressions  .
+4300,"To accomplish the same using an if-statement, this would take more than one line of code , and require mentioning ""my_variable"" twice:"
+4301,"Some argue that the explicit if/then statement is easier to read and that it may compile to more efficient code than the ternary operator, while others argue that concise expressions are easier to read than statements spread over several lines containing repetition."
+4302,"First, when the user runs the program, a cursor appears waiting for the reader to type a number. If that number is greater than 10, the text ""My variable is named 'foo'."" is displayed on the screen. If the number is smaller than 10, then the message ""My variable is named 'bar'."" is printed on the screen."
+4303,"In Visual Basic and some other languages, a function called IIf is provided, which can be used as a conditional expression. However, it does not behave like a true conditional expression, because both the true and false branches are always evaluated; it is just that the result of one of them is thrown away, while the result of the other is returned by the IIf function."
+4304,In Tcl if is not a keyword but a function . For example
+4305,invokes a function named if passing 2 arguments: The first one being the condition and the second one being the true branch. Both arguments are passed as strings .
+4306,"In the above example the condition is not evaluated before calling the function. Instead, the implementation of the if function receives the condition as a string value and is responsible to evaluate this string as an expression in the callers scope."
+4307,Such a behavior is possible by using uplevel and expr commands:
+4308,Because if is actually a function it also returns a value:
+4309,"In Rust, if is always an expression. It evaluates to the value of whichever branch is executed, or to the unit type  if no branch is executed. If a branch does not provide a return value, it evaluates to  by default. To ensure the if expression's type is known at compile time, each branch must evaluate to a value of the same type. For this reason, an else branch is effectively compulsory unless the other branches evaluate to , because an if without an else can always evaluate to  by default."
+4310,"Up to Fortran 77, the language Fortran has an ""arithmetic if"" statement which is halfway between a computed IF and a case statement, based on the trichotomy x < 0, x = 0, x > 0. This was the earliest conditional statement in Fortran:"
+4311,Where e is any numeric expression ; this is equivalent to
+4312,"Because this arithmetic IF is equivalent to multiple GOTO statements that could jump to anywhere, it is considered to be an unstructured control statement, and should not be used if more structured statements can be used. In practice it has been observed that most arithmetic IF statements referenced the following statement with one or two of the labels."
+4313,"This was the only conditional control statement in the original implementation of Fortran on the IBM 704 computer. On that computer the test-and-branch op-code had three addresses for those three states. Other computers would have ""flag"" registers such as positive, zero, negative, even, overflow, carry, associated with the last arithmetic operations and would use instructions such as 'Branch if accumulator negative' then 'Branch if accumulator zero' or similar. Note that the expression is evaluated once only, and in cases such as integer arithmetic where overflow may occur, the overflow or carry flags would be considered also."
+4314,"In contrast to other languages, in Smalltalk the conditional statement is not a language construct but defined in the class Boolean as an abstract method that takes two parameters, both closures. Boolean has two subclasses, True and False, which both define the method, True executing the first closure only, False executing the second closure only."
+4315,JavaScript uses if-else statements similar to those in C languages. A Boolean value is accepted within parentheses between the reserved if keyword and a left curly bracket.
+4316,"The above example takes the conditional of Math.random < 0.5 which outputs true if a random float value between 0 and 1 is greater than 0.5. The statement uses it to randomly choose between outputting You got Heads! or You got Tails! to the console. Else and else-if statements can also be chained after the curly bracket of the statement preceding it as many times as necessary, as shown below:"
+4317,"In Lambda calculus, the concept of an if-then-else conditional can be expressed using the following expressions:"
+4318,"true takes up to two arguments and once both are provided , it returns the first argument given."
+4319,"false takes up to two arguments and once both are provided, it returns the second argument given."
+4320,"ifThenElse takes up to three arguments and once all are provided, it passes both second and third argument to the first argument. We expect ifThenElse to only take true or false as an argument, both of which project the given two arguments to their preferred single argument, which is then returned."
+4321,"note: if ifThenElse is passed two functions as the left and right conditionals; it is necessary to also pass an empty tuple  to the result of ifThenElse in order to actually call the chosen function, otherwise ifThenElse will just return the function object without getting called."
+4322,"In a system where numbers can be used without definition , the above can be expressed as a single closure below:"
+4323,"Here, true, false, and ifThenElse are bound to their respective definitions which are passed to their scope at the end of their block."
+4324,A working JavaScript analogy to this is as follows:
+4325,The code above with multivariable functions looks like this:
+4326,Another version of the earlier example without a system where numbers are assumed is below.
+4327,"The first example shows the first branch being taken, while second example shows the second branch being taken."
+4328,"Smalltalk uses a similar idea for its  true and false representations, with True and False being singleton objects that respond to messages ifTrue/ifFalse differently."
+4329,"Haskell used to use this exact model for its Boolean type, but at the time of writing, most Haskell programs use syntactic sugar ""if a then b else c"" construct which unlike ifThenElse does not compose unless
+either wrapped in another function or re-implemented as shown in The Haskell section of this page."
+4330,"Switch statements  compare a given value with specified constants and take action according to the first constant to match. There is usually a provision for a default action  to be taken if no match succeeds. Switch statements can allow compiler optimizations, such as lookup tables. In dynamic languages, the cases may not be limited to constant expressions, and might extend to pattern matching, as in the shell script example on the right, where the '*)' implements the default case as a regular expression matching any string."
+4331,"Pattern matching may be seen as an alternative to both if–then–else, and case statements. It is available in many programming languages with functional programming features, such as Wolfram Language, ML and many others. Here is a simple example written in the OCaml language:"
+4332,The power of pattern matching is the ability to concisely match not only actions but also values to patterns of data. Here is an example written in Haskell which illustrates both of these features:
+4333,"This code defines a function map, which applies the first argument  to each of the elements of the second argument , and returns the resulting list. The two lines are the two definitions of the function for the two kinds of arguments possible in this case – one where the list is empty  and the other case where the list is not empty."
+4334,"Pattern matching is not strictly speaking always a choice construct, because it is possible in Haskell to write only one alternative, which is guaranteed to always be matched – in this situation, it is not being used as a choice construct, but simply as a way to bind names to values. However, it is frequently used as a choice construct in the languages in which it is available."
+4335,"In programming languages that have associative arrays or comparable data structures, such as Python, Perl, PHP or Objective-C, it is idiomatic to use them to implement conditional assignment."
+4336,"In languages that have anonymous functions or that allow a programmer to assign a named function to a variable reference, conditional flow can be implemented by using a hash as a dispatch table."
+4337,An alternative to conditional branch instructions is predication. Predication is an architectural feature that enables instructions to be conditionally executed instead of modifying the control flow.
+4338,"This table refers to the most recent language specification of each language. For languages that do not have a specification, the latest officially released implementation is referred to."
+4339,"An operand of a negation is a negand, or negatum."
+4340,"The truth table of 
+  
+    
+      
+        ¬
+        P
+      
+    
+    {\displaystyle \neg P}
+  
+ is as follows:"
+4341,"Algebraically, classical negation corresponds to complementation in a Boolean algebra, and intuitionistic negation to pseudocomplementation in a Heyting algebra. These algebras provide a semantics for classical and intuitionistic logic."
+4342,"The negation of a proposition p is notated in different ways, in various contexts of discussion and fields of application. The following table documents some of these variants:"
+4343,Here is a table that shows a commonly used precedence of logical operators.
+4344,"As a result, in the propositional case, a sentence is classically provable if its double negation is intuitionistically provable. This result is known as Glivenko's theorem."
+4345,De Morgan's laws provide a way of distributing negation over disjunction and conjunction:
+4346,"Another way to express this is that each variable always makes a difference in the truth-value of the operation, or it never makes a difference. Negation is a linear logical operator."
+4347,"In Boolean algebra, a self dual function is a function such that:"
+4348,"As in mathematics, negation is used in computer science to construct logical statements."
+4349,"The exclamation mark ""!"" signifies logical NOT in B, C, and languages with a C-inspired syntax such as C++, Java, JavaScript, Perl, and PHP. ""NOT"" is the operator used in ALGOL 60, BASIC, and languages with an ALGOL- or BASIC-inspired syntax such as Pascal, Ada, Eiffel and Seed7. Some languages  provide more than one operator for negation. A few languages like PL/I and Ratfor use ¬ for negation. Most modern languages allow the above statement to be shortened from if ) to if , which allows sometimes, when the compiler/interpreter is not able to optimize it, faster programs."
+4350,"In computer science there is also bitwise negation. This takes the value given and switches all the binary 1s to 0s and 0s to 1s.  See bitwise operation.  This is often used to create ones' complement or ""~"" in C or C++ and two's complement  as it basically creates the opposite  or mathematical complement of the value ."
+4351,"To get the absolute  value of a given integer the following would work as the ""-"" changes it from negative to positive "
+4352,To demonstrate logical negation:
+4353,"Inverting the condition and reversing the outcomes produces code that is logically equivalent to the original code, i.e. will have identical results for any input ."
+4354,"This convention occasionally surfaces in ordinary written speech, as computer-related slang for not.  For example, the phrase !voting means ""not voting"".  Another example is the phrase !clue which is used as a synonym for ""no-clue"" or ""clueless""."
+4355,"In Kripke semantics where the semantic values of formulae are sets of possible worlds, negation can be taken to mean set-theoretic complementation ."
+4356,"It gains the name ""exclusive or"" because the meaning of ""or"" is ambiguous when both operands are true. XOR excludes that case. Some informal ways of describing XOR are ""one or the other but not both"", ""either one or the other"", and ""A or B, but not A and B""."
+4357,or:
+4358,This equivalence can be established by applying De Morgan's laws twice to the fourth line of the above proof.
+4359,"The exclusive or is also equivalent to the negation of a logical biconditional, by the rules of material implication  and material equivalence."
+4360,"In summary, we have, in mathematical and in engineering notation:"
+4361,"The spirit of De Morgan's laws can be applied, we have:"
+4362,"Disjunction is often understood exclusively in natural languages. In English, the disjunctive word ""or"" is often understood exclusively, particularly when used with the particle ""either"". The English example below would normally be understood in conversation as implying that Mary is not both a singer and a poet."
+4363,"However, disjunction can also be understood inclusively, even in combination with ""either"". For instance, the first example below shows that ""either"" can be felicitously used in combination with an outright statement that both disjuncts are true. The second example shows that the exclusive inference vanishes away under downward entailing contexts. If disjunction were understood as exclusive in this example, it would leave open the possibility that some people ate both rice and beans."
+4364,"Examples such as the above have motivated analyses of the exclusivity inference as pragmatic conversational implicatures calculated on the basis of an inclusive semantics. Implicatures are typically cancellable and do not arise in downward entailing contexts if their calculation depends on the Maxim of Quantity. However, some researchers have treated exclusivity as a bona fide semantic entailment and proposed nonclassical logics which would validate it."
+4365,"This behavior of English ""or"" is also found in other languages. However, many languages have disjunctive constructions which are robustly exclusive such as French soit... soit."
+4366,"The symbol used for exclusive disjunction varies from one field of application to the next, and even depends on the properties being emphasized in a given context of discussion.  In addition to the abbreviation ""XOR"", any of the following symbols may also be seen:"
+4367,"If using binary values for true  and false , then exclusive or works exactly like addition modulo 2."
+4368,Exclusive disjunction is often used for bitwise operations. Examples:
+4369,"In computer science, exclusive disjunction has several uses:"
+4370,"In logical circuits, a simple adder can be made with an XOR gate to add the numbers, and a series of AND, OR and NOT gates to create the carry output."
+4371,"On some computer architectures, it is more efficient to store a zero in a register by XOR-ing the register with itself  than to load and store the value zero."
+4372,"In cryptography, XOR is sometimes used as a simple, self-inverse mixing function, such as in one-time pad or Feistel network systems. XOR is also heavily used in block ciphers such as AES  or Serpent and in block cipher implementation ."
+4373,"In simple threshold-activated artificial neural networks, modeling the XOR function requires a second layer because XOR is not a linearly separable function."
+4374,"Similarly, XOR can be used in generating entropy pools for hardware random number generators.  The XOR operation preserves randomness, meaning that a random bit XORed with a non-random bit will result in a random bit.  Multiple sources of potentially random data can be combined using XOR, and the unpredictability of the output is guaranteed to be at least as good as the best individual source."
+4375,"XOR is used in RAID 3–6 for creating parity information.  For example, RAID can ""back up"" bytes 100111002 and 011011002 from two  hard drives by XORing the just mentioned bytes, resulting in  and writing it to another drive.  Under this method, if any one of the three hard drives are lost, the lost byte can be re-created by XORing bytes from the remaining drives. For instance,  if the drive containing 011011002 is lost, 100111002 and 111100002 can be XORed to recover the lost byte."
+4376,"XOR is also used to detect an overflow in the result of a signed binary arithmetic operation. If the leftmost retained bit of the result is not the same as the infinite number of digits to the left, then that means overflow occurred. XORing those two bits will give a ""1"" if there is an overflow."
+4377,"XOR can be used to swap two numeric variables in computers, using the XOR swap algorithm; however this is regarded as more of a curiosity and not encouraged in practice."
+4378,XOR linked lists leverage XOR properties in order to save space to represent doubly linked list data structures.
+4379,"In computer graphics, XOR-based drawing methods are often used to manage such items as bounding boxes and cursors on systems without alpha channels or overlay planes."
+4380,An operand of a disjunction is a disjunct.
+4381,"Because the logical ""or"" means a disjunction formula is true when either one or both of its parts are true, it is referred to as an inclusive disjunction.  This is in contrast with an exclusive disjunction, which is true when one or the other of the arguments are true, but not both ."
+4382,"When it is necessary to clarify whether inclusive or exclusive ""or"" is intended, English speakers sometimes uses the phrase ""and/or"". In terms of logic, this phrase is identical to ""or"", but makes the inclusion of both being true explicit."
+4383,"In the semantics of logic, classical disjunction is a truth functional operation which returns the truth value ""true"" unless both of its arguments are ""false"". Its semantic entry is standardly given as follows:"
+4384,This semantics corresponds to the following truth table:
+4385,The latter can be checked by the following truth table:
+4386,It can be checked by the following truth table:
+4387,The following properties apply to disjunction:
+4388,Operators corresponding to logical disjunction exist in most programming languages.
+4389,Disjunction is often used for bitwise operations. Examples:
+4390,"The or operator can be used to set bits in a bit field to 1, by or-ing the field with a constant field with the relevant bits set to 1. For example, x = x | 0b00000001 will force the final bit to 1, while leaving other bits unchanged."
+4391,"Many languages distinguish between bitwise and logical disjunction by providing two distinct operators; in languages following C, bitwise disjunction is performed with the single pipe operator , and logical disjunction with the double pipe  operator."
+4392,"Logical disjunction is usually short-circuited; that is, if the first  operand evaluates to true, then the second  operand is not evaluated. The logical disjunction operator thus usually constitutes a sequence point."
+4393,"In a parallel  language, it is possible to short-circuit both sides: they are evaluated in parallel, and if one terminates with value true, the other is interrupted. This operator is thus called the parallel or."
+4394,"Although the type of a logical disjunction expression is boolean in most languages , in some languages , the logical disjunction operator returns one of its operands: the first operand if it evaluates to a true value, and the second operand otherwise."
+4395,The Curry–Howard correspondence relates a constructivist form of disjunction to tagged union types.
+4396,"This inference has sometimes been understood as an entailment, for instance by Alfred Tarski, who suggested that natural language disjunction is ambiguous between a classical and a nonclassical interpretation. More recent work in pragmatics has shown that this inference can be derived as a conversational implicature on the basis of a semantic denotation which behaves classically. However, disjunctive constructions including Hungarian vagy... vagy and French soit... soit have been argued to be inherently exclusive, rendering ungrammaticality in contexts where an inclusive reading would otherwise be forced."
+4397,"Similar deviations from classical logic have been noted in cases such as free choice disjunction and simplification of disjunctive antecedents, where certain modal operators trigger a conjunction-like interpretation of disjunction. As with exclusivity, these inferences have been analyzed both as implicatures and as entailments arising from a nonclassical interpretation of disjunction."
+4398,"In many languages, disjunctive expressions play a role in question formation. For instance, while the following English example can be interpreted as a polar question asking whether it's true that Mary is either a philosopher or a linguist, it can also be interpreted as an alternative question asking which of the two professions is hers. The role of disjunction in these cases has been analyzed using nonclassical logics such as alternative semantics and inquisitive semantics, which have also been adopted to explain the free choice and simplification inferences."
+4399,"In English, as in many other languages, disjunction is expressed by a coordinating conjunction. Other languages express disjunctive meanings in a variety of ways, though it is unknown whether disjunction itself is a linguistic universal. In many languages such as Dyirbal and Maricopa, disjunction is marked using a verb suffix. For instance, in the Maricopa example below, disjunction is marked by the suffix šaa."
+4400,John-NOM
+4401,Bill-NOM
+4402,3-come-PL-FUT-INFER
+4403,John-NOM Bill-NOM 3-come-PL-FUT-INFER
+4404,'John or Bill will come.'
+4405,An operand of a conjunction is a conjunct.
+4406,"Beyond logic, the term ""conjunction"" also refers to similar concepts in other fields:"
+4407,"Logical conjunction is an operation on two logical values, typically the values of two propositions, that produces a value of true if and only if  both of its operands are true."
+4408,"The conjunctive identity is true, which is to say that AND-ing an expression with true will never change the value of the expression. In keeping with the concept of vacuous truth, when conjunction is defined as an operator or function of arbitrary arity, the empty conjunction  is often defined as having the result true."
+4409,"In systems where logical conjunction is not a primitive, it may be defined as"
+4410,or
+4411,or in logical operator notation:
+4412,Here is an example of an argument that fits the form conjunction introduction:
+4413,"Conjunction elimination is another classically valid, simple argument form. Intuitively, it permits the inference from any conjunction of either element of that conjunction."
+4414,"...or alternatively,"
+4415,In logical operator notation:
+4416,"...or alternatively,"
+4417,This formula can be seen as a special case of
+4418,"In other words, a conjunction can actually be proven false just by knowing about the relation of its conjuncts, and not necessary about their truth values."
+4419,This formula can be seen as a special case of
+4420,Either of the above are constructively valid proofs by contradiction.
+4421,commutativity: yes
+4422,associativity: yes
+4423,"distributivity: with various operations, especially with or"
+4424,with material nonimplication:
+4425,with itself:
+4426,idempotency: yes
+4427,monotonicity: yes
+4428,"truth-preserving: yes
+When all inputs are true, the output is true."
+4429,"falsehood-preserving: yes
+When all inputs are false, the output is false."
+4430,Walsh spectrum: 
+4431,Nonlinearity: 1 
+4432,"If using binary values for true  and false , then logical conjunction works exactly like normal arithmetic multiplication."
+4433,"In high-level computer programming and digital electronics, logical conjunction is commonly represented by an infix operator, usually as a keyword such as ""AND"", an algebraic multiplication, or the ampersand symbol & . Many languages also provide short-circuit control structures corresponding to logical conjunction."
+4434,"Logical conjunction is often used for bitwise operations, where 0 corresponds to false and 1 to true:"
+4435,"The operation can also be applied to two binary words viewed as bitstrings of equal length, by taking the bitwise AND of each pair of bits at corresponding positions. For example:"
+4436,"This can be used to select part of a bitstring using a bit mask.  For example, 10011101 AND 00001000  =  00001000 extracts the fourth bit of an 8-bit bitstring."
+4437,"In computer networking, bit masks are used to derive the network address of a subnet within an existing network from a given IP address, by ANDing the IP address and the subnet mask."
+4438,"Logical conjunction ""AND"" is also used in SQL operations to form database queries."
+4439,The Curry–Howard correspondence relates logical conjunction to product types.
+4440,"As with other notions formalized in mathematical logic, the logical conjunction and is related to, but not the same as, the grammatical conjunction and in natural languages."
+4441,"English ""and"" has properties not captured by logical conjunction.  For example, ""and"" sometimes implies order having the sense of ""then"". For example, ""They got married and had a child"" in common discourse means that the marriage came before the child."
+4442,"The word ""and"" can also imply a partition of a thing into parts, as ""The American flag is red, white, and blue.""  Here, it is not meant that the flag is at once red, white, and blue, but rather that it has a part of each color."
+4443,"In some programming languages, any expression can be evaluated in a context that expects a Boolean data type. Typically  expressions like the number zero, the empty string, empty lists, and null evaluate to false, and strings with content , other numbers, and objects evaluate to true.
+Sometimes these classes of expressions are called ""truthy"" and ""falsy"" / ""false""."
+4444,"In classical logic, with its intended semantics, the truth values are true , and untrue or false ; that is, classical logic is a two-valued logic. This set of two values is also called the Boolean domain. Corresponding semantics of logical connectives are truth functions, whose values are expressed in the form of truth tables. Logical biconditional becomes the equality binary relation, and negation becomes a bijection which permutes true and false. Conjunction and disjunction are dual with respect to negation, which is expressed by De Morgan's laws:"
+4445,Propositional variables become variables in the Boolean domain. Assigning values for propositional variables is referred to as valuation.
+4446,"In intuitionistic logic, and more generally, constructive mathematics, statements are assigned a truth value only if they can be given a constructive proof. It starts with a set of axioms, and a statement is true if one can build a proof of the statement from those axioms. A statement is false if one can deduce a contradiction from it. This leaves open the possibility of statements that have not yet been assigned a truth value.
+Unproven statements in intuitionistic logic are not given an intermediate truth value . Indeed, one can prove that they have no third truth value, a result dating back to Glivenko in 1928."
+4447,"Instead, statements simply remain of unknown truth value, until they are either proven or disproven."
+4448,"There are various ways of interpreting intuitionistic logic, including the Brouwer–Heyting–Kolmogorov interpretation. See also Intuitionistic logic § Semantics."
+4449,"Multi-valued logics  allow for more than two truth values, possibly containing some internal structure. For example, on the unit interval  such structure is a total order; this may be expressed as the existence of various degrees of truth."
+4450,"Not all logical systems are truth-valuational in the sense that logical connectives may be interpreted as truth functions. For example, intuitionistic logic lacks a complete set of truth values because its semantics, the Brouwer–Heyting–Kolmogorov interpretation, is specified in terms of provability conditions, and not directly in terms of the necessary truth of formulae."
+4451,"But even non-truth-valuational logics can associate values with logical formulae, as is done in algebraic semantics. The algebraic semantics of intuitionistic logic is given in terms of Heyting algebras, compared to Boolean algebra semantics of classical propositional calculus."
+4452,Intuitionistic type theory uses types in the place of truth values.
+4453,Topos theory uses truth values in a special sense: the truth values of a topos are the global elements of the subobject classifier.  Having truth values in this sense does not make a logic truth valuational.
+4454,"The real numbers are fundamental in calculus , in particular by their role in the classical definitions of limits, continuity and derivatives."
+4455,"The real numbers include the rational numbers, such as the integer −5 and the fraction 4 / 3. The rest of the real numbers are called irrational numbers.  Some irrational numbers  are the root of a polynomial with integer coefficients, such as the square root √2 = 1.414...; these are called algebraic numbers.  There are also real numbers which are not, such as π = 3.1415...; these are called transcendental numbers."
+4456,"Real numbers can be thought of as all points on a line called the number line or real line, where the points corresponding to integers  are equally spaced."
+4457,"Conversely, analytic geometry is the association of points on lines  to real numbers such that geometric displacements are proportional to differences between corresponding numbers."
+4458,"The informal descriptions above of the real numbers are not sufficient for ensuring the correctness of proofs of theorems involving real numbers. The realization that a better definition was needed, and the elaboration of such a definition was a major development of 19th-century mathematics and is the foundation of real analysis, the study of real functions and real-valued sequences. A current axiomatic definition is that real numbers form the unique  Dedekind-complete ordered field. Other common definitions of real numbers include equivalence classes of Cauchy sequences , Dedekind cuts, and infinite decimal representations. All these definitions satisfy the axiomatic definition and are thus equivalent."
+4459,"Real numbers are completely characterized by their fundamental properties that can be summarized by saying that they form an ordered field that is Dedekind complete. Here, ""completely characterized"" means that there is a unique isomorphism between any two Dedekind complete ordered fields, and thus that their elements have exactly the same properties. This implies that one can manipulate real numbers and compute with them, without knowing how they can be defined; this is what mathematicians and physicists did during several centuries before the first formal definitions were provided in the second half of the 19th century. See Construction of the real numbers for details about these formal definitions and the proof of their equivalence."
+4460,"The real numbers form an ordered field. Intuitively, this means that methods and rules of elementary arithmetic apply to them. More precisely, there are two binary operations, addition and multiplication, and a total order that have the following properties."
+4461,Many other properties can be deduced from the above ones. In particular:
+4462,"Several other operations are commonly used, which can be deduced from the above ones."
+4463,The real numbers 0 and 1 are commonly identified with the natural numbers 0 and 1. This allows identifying any natural number n with the sum of n real numbers equal to 1.
+4464,"The above identifications make sense, since natural numbers, integers and real numbers are generally not defined by their individual nature, but by defining properties . So, the identification of natural numbers with some real numbers is justified by the fact that Peano axioms are satisfied by these real numbers, with the addition with 1 taken as the successor function."
+4465,"These identifications are formally abuses of notation, and are generally harmless. It is only in very specific situations, that one must avoid them and replace them by using explicitly the above homomorphisms. This is the case in constructive mathematics and computer programming. In the latter case, these homomorphisms are interpreted as type conversions that can often be done  automatically by the compiler."
+4466,"Dedekind completeness implies other sorts of completeness , but also has some important consequences."
+4467,"The last two properties are summarized by saying that the real numbers form a real closed field. This implies the real version of the fundamental theorem of algebra, namely that every polynomial with real coefficients can be factored into polynomials with real coefficients of degree at most two."
+4468,A key property of real numbers is their decimal representation. A decimal representation consists of a nonnegative integer k and an infinite sequence of decimal digits 
+4469,that is written
+4470,"The real number defined by the sequence is the least upper bound of the 
+  
+    
+      
+        
+          D
+          
+            n
+          
+        
+        ,
+      
+    
+    {\displaystyle D_{n},}
+  
+ which exists by Dedekind completeness."
+4471,"In summary, there is a bijection between the real numbers and the decimal representations that do not end with infinitely many trailing 9."
+4472,"A main reason for using real numbers is so that many sequences have limits. More formally, the reals are complete :"
+4473,"A sequence  of real numbers is called a Cauchy sequence if for any ε > 0 there exists an integer N  such that the distance |xn − xm| is less than ε for all n and m that are both greater than N. This definition, originally provided by Cauchy, formalizes the fact that the xn eventually come and remain arbitrarily close to each other."
+4474,"A sequence  converges to the limit x if its elements eventually come and remain arbitrarily close to x, that is, if for any ε > 0 there exists an integer N  such that the distance |xn − x| is less than ε for n greater than N."
+4475,"Every convergent sequence is a Cauchy sequence, and the converse is true for real numbers, and this means that the topological space of the real numbers is complete."
+4476,"The set of rational numbers is not complete. For example, the sequence , where each term adds a digit of the decimal expansion of the positive square root of 2, is Cauchy but it does not converge to a rational number ."
+4477,"The completeness property of the reals is the basis on which calculus, and more generally mathematical analysis, are built. In particular, the test that a sequence is a Cauchy sequence allows proving that a sequence has a limit, without computing it, and even without knowing it."
+4478,"For example, the standard series of the exponential function"
+4479,"converges to a real number for every x, because the sums"
+4480,"The real numbers are often described as ""the complete ordered field"", a phrase that can be interpreted in several ways."
+4481,"First, an order can be lattice-complete. It is easy to see that no ordered field can be lattice-complete, because it can have no largest element ."
+4482,"Additionally, an order can be Dedekind-complete, see § Axiomatic approach. The uniqueness result at the end of that section justifies using the word ""the"" in the phrase ""complete ordered field"" when this is the sense of ""complete"" that is meant. This sense of completeness is most closely related to the construction of the reals from Dedekind cuts, since that construction starts from an ordered field  and then forms the Dedekind-completion of it in a standard way."
+4483,"As a topological space, the real numbers are separable. This is because the set of rationals, which is countable, is dense in the real numbers. The irrational numbers are also dense in the real numbers, however they are uncountable and have the same cardinality as the reals."
+4484,"The real numbers form a metric space: the distance between x and y is defined as the absolute value |x − y|. By virtue of being a totally ordered set, they also carry an order topology; the topology arising from the metric and the one arising from the order are identical, but yield different presentations for the topology—in the order topology as ordered intervals, in the metric topology as epsilon-balls. The Dedekind cuts construction uses the order topology presentation, while the Cauchy sequences construction uses the metric topology presentation. The reals form a contractible , separable and complete metric space of Hausdorff dimension 1. The real numbers are locally compact but not compact. There are various properties that uniquely specify them; for instance, all unbounded, connected, and separable order topologies are necessarily homeomorphic to the reals."
+4485,"The reals carry a canonical measure, the Lebesgue measure, which is the Haar measure on their structure as a topological group normalized such that the unit interval  has measure 1. There exist sets of real numbers that are not Lebesgue measurable, e.g. Vitali sets."
+4486,A real number may be either computable or uncomputable; either algorithmically random or not; and either arithmetically random or not.
+4487,"Simple fractions were used by the Egyptians around 1000 BC; the Vedic ""Shulba Sutras""  in c. 600 BC include what may be the first ""use"" of irrational numbers. The concept of irrationality was implicitly accepted by early Indian mathematicians such as Manava , who was aware that the square roots of certain numbers, such as 2 and 61, could not be exactly determined. Around 500 BC, the Greek mathematicians led by Pythagoras also realized that the square root of 2 is irrational."
+4488,"The Middle Ages brought about the acceptance of zero, negative numbers, integers, and fractional numbers, first by Indian and Chinese mathematicians, and then by Arabic mathematicians, who were also the first to treat irrational numbers as algebraic objects . Arabic mathematicians merged the concepts of ""number"" and ""magnitude"" into a more general idea of real numbers. The Egyptian mathematician Abū Kāmil Shujā ibn Aslam  was the first to accept irrational numbers as solutions to quadratic equations, or as coefficients in an equation . In Europe, such numbers, not commensurable with the numerical unit, were called irrational or surd ."
+4489,"In the 16th century, Simon Stevin created the basis for modern decimal notation, and insisted that there is no difference between rational and irrational numbers in this regard."
+4490,"In the 17th century, Descartes introduced the term ""real"" to describe roots of a polynomial, distinguishing them from ""imaginary"" ones."
+4491,"In the 18th and 19th centuries, there was much work on irrational and transcendental numbers. Lambert  gave a flawed proof that π cannot be rational; Legendre  completed the proof and showed that π is not the square root of a rational number. Liouville  showed that neither e nor e2 can be a root of an integer quadratic equation, and then established the existence of transcendental numbers; Cantor  extended and greatly simplified this proof. Hermite  proved that e is transcendental, and Lindemann , showed that π is transcendental. Lindemann's proof was much simplified by Weierstrass , Hilbert , Hurwitz, and Gordan."
+4492,"The developers of calculus used real numbers without having defined them rigorously. The first rigorous definition was published by Cantor in 1871. In 1874, he showed that the set of all real numbers is uncountably infinite, but the set of all algebraic numbers is countably infinite. Cantor's first uncountability proof was different from his famous diagonal argument published in 1891."
+4493,"These properties imply the Archimedean property , which states that the set of integers has no upper bound in the reals. In fact, if this were false, then the integers would have a least upper bound N; then, N – 1 would not be an upper bound, and there would be an integer n such that n > N – 1, and thus n + 1 > N, which is a contradiction with the upper-bound property of N."
+4494,"The real numbers can be constructed as a completion of the rational numbers, in such a way that a sequence defined by a decimal or binary expansion like  converges to a unique real number—in this case π. For details and other constructions of real numbers, see construction of the real numbers."
+4495,"In the physical sciences, most physical constants such as the universal gravitational constant, and physical variables, such as position, mass, speed, and electric charge, are modeled using real numbers. In fact, the fundamental physical theories such as classical mechanics, electromagnetism, quantum mechanics, general relativity and the standard model are described using mathematical structures, typically smooth manifolds or Hilbert spaces, that are based on the real numbers, although actual measurements of physical quantities are of finite accuracy and precision."
+4496,"Physicists have occasionally suggested that a more fundamental theory would replace the real numbers with quantities that do not form a continuum, but such proposals remain speculative."
+4497,"The real numbers are most often formalized using the Zermelo–Fraenkel axiomatization of set theory, but some mathematicians study the real numbers with other logical foundations of mathematics. In particular, the real numbers are also studied in reverse mathematics and in constructive mathematics."
+4498,"The hyperreal numbers as developed by Edwin Hewitt, Abraham Robinson and others extend the set of the real numbers by introducing infinitesimal and infinite numbers, allowing for building infinitesimal calculus in a way closer to the original intuitions of Leibniz, Euler, Cauchy and others."
+4499,"Edward Nelson's internal set theory enriches the Zermelo–Fraenkel set theory syntactically by introducing a unary predicate ""standard"". In this approach, infinitesimals are  elements of the set of the real numbers ."
+4500,"Electronic calculators and computers cannot operate on arbitrary real numbers, because finite computers cannot directly store infinitely many digits or other infinite representations. Nor do they usually even operate on arbitrary definable real numbers, which are inconvenient to manipulate."
+4501,"Instead, computers typically work with finite-precision approximations called floating-point numbers, a representation similar to scientific notation. The achievable precision is limited by the data storage space allocated for each number, whether as fixed-point, floating-point, or arbitrary-precision numbers, or some other representation. Most scientific computation uses binary floating-point arithmetic, often a 64-bit representation with around 16 decimal digits of precision. Real numbers satisfy the usual rules of arithmetic, but floating-point numbers do not. The field of numerical analysis studies the stability and accuracy of numerical algorithms implemented with approximate arithmetic."
+4502,"A real number is called computable if there exists an algorithm that yields its digits. Because there are only countably many algorithms, but an uncountable number of reals, almost all real numbers fail to be computable. Moreover, the equality of two computable numbers is an undecidable problem. Some constructivists accept the existence of only those reals that are computable. The set of definable numbers is broader, but still only countable."
+4503,"In set theory, specifically descriptive set theory, the Baire space is used as a surrogate for the real numbers since the latter have some topological properties  that are a technical inconvenience. Elements of Baire space are referred to as ""reals""."
+4504,"In mathematics, real is used as an adjective, meaning that the underlying field is the field of the real numbers . For example, real matrix, real polynomial and real Lie algebra. The word is also used as a noun, meaning a real number ."
+4505,The real numbers can be generalized and extended in several different directions:
+4506,"In computing, floating-point arithmetic  is arithmetic that represents subsets of real numbers using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base.
+Numbers of this form are called floating-point numbers.: 3 : 10 
+For example, 12.345 is a floating-point number in base ten with five digits of precision:"
+4507,"However, unlike 12.345, 12.3456 is not a floating-point number in base ten with five digits of precision—it needs six digits of precision; the nearest floating-point number with only five digits is 12.346.
+In practice, most floating-point systems use base two, though base ten  is also common."
+4508,"Floating-point arithmetic operations, such as addition and division, approximate the corresponding real number arithmetic operations by rounding any result that is not a floating-point number itself to a nearby floating-point number.: 22 : 10 
+For example, in a floating-point arithmetic with five base-ten digits of precision, the sum 12.345 + 1.0001 = 13.3451 might be rounded to 13.345."
+4509,"The term floating point refers to the fact that the number's radix point can ""float"" anywhere to the left, right, or between the significant digits of the number. This position is indicated by the exponent, so floating point can be considered a form of scientific notation."
+4510,"A floating-point system can be used to represent, with a fixed number of digits, numbers of very different orders of magnitude — such as the number of meters between galaxies or between protons in an atom.  For this reason, floating-point arithmetic is often used to allow very small and very large real numbers that require fast processing times.  The result of this dynamic range is that the numbers that can be represented are not uniformly spaced; the difference between two consecutive representable numbers varies with their exponent."
+4511,"Over the years, a variety of floating-point representations have been used in computers.  In 1985, the IEEE 754 Standard for Floating-Point Arithmetic was established, and since the 1990s, the most commonly encountered representations are those defined by the IEEE."
+4512,"The speed of floating-point operations, commonly measured in terms of FLOPS, is an important characteristic of a computer system, especially for applications that involve intensive mathematical calculations."
+4513,A floating-point unit  is a part of a computer system specially designed to carry out operations on floating-point numbers.
+4514,"A number representation specifies some way of encoding a number, usually as a string of digits."
+4515,"There are several mechanisms by which strings of digits can represent numbers. In standard mathematical notation, the digit string can be of any length, and the location of the radix point is indicated by placing an explicit ""point"" character  there. If the radix point is not specified, then the string implicitly represents an integer and the unstated radix point would be off the right-hand end of the string, next to the least significant digit. In fixed-point systems, a position in the string is specified for the radix point. So a fixed-point scheme might use a string of 8 decimal digits with the decimal point in the middle, whereby ""00012345"" would represent 0001.2345."
+4516,"In scientific notation, the given number is scaled by a power of 10, so that it lies within a specific range—typically between 1 and 10, with the radix point appearing immediately after the first digit.  As a power of ten, the scaling factor is then indicated separately at the end of the number.  For example, the orbital period of Jupiter's moon Io is 152,853.5047 seconds, a value that would be represented in standard-form scientific notation as 1.528535047×105 seconds."
+4517,"Floating-point representation is similar in concept to scientific notation. Logically, a floating-point number consists of:"
+4518,"To derive the value of the floating-point number, the significand is multiplied by the base raised to the power of the exponent, equivalent to shifting the radix point from its implied position by a number of places equal to the value of the exponent—to the right if the exponent is positive or to the left if the exponent is negative."
+4519,"Using base-10  as an example, the number 152,853.5047, which has ten decimal digits of precision, is represented as the significand 1,528,535,047 together with 5 as the exponent.  To determine the actual value, a decimal point is placed after the first digit of the significand and the result is multiplied by 105 to give 1.528535047×105, or 152,853.5047. In storing such a number, the base  need not be stored, since it will be the same for the entire range of supported numbers, and can thus be inferred."
+4520,"where s is the significand , p is the precision , b is the base , and e is the exponent."
+4521,"Historically, several number bases have been used for representing floating-point numbers, with base two  being the most common, followed by base ten , and other less common varieties, such as base sixteen , base eight , base four , base three  and even base 256 and base 65,536."
+4522,"A floating-point number is a rational number, because it can be represented as one integer divided by another; for example 1.45×103 is ×1000 or 145,000/100. The base determines the fractions that can be represented; for instance, 1/5 cannot be represented exactly as a floating-point number using a binary base, but 1/5 can be represented exactly using a decimal base . However, 1/3 cannot be represented exactly by either binary  or decimal , but in base 3, it is trivial  . The occasions on which infinite expansions occur depend on the base and its prime factors."
+4523,"In this binary expansion, let us denote the positions from 0  to 32 . The 24-bit significand will stop at position 23, shown as the underlined bit 0 above. The next bit, at position 24, is called the round bit or rounding bit. It is used to round the 33-bit approximation to the nearest 24-bit number . This bit, which is 1 in this example, is added to the integer formed by the leftmost 24 bits, yielding:"
+4524,"When this is stored in memory using the IEEE 754 encoding, this becomes the significand s. The significand is assumed to have a binary point to the right of the leftmost bit. So, the binary representation of π is calculated from left-to-right as follows:"
+4525,"where p is the precision , n is the position of the bit of the significand from the left  and e is the exponent ."
+4526,"It can be required that the most significant digit of the significand of a non-zero number be non-zero . This process is called normalization. For binary formats , this non-zero digit is necessarily 1. Therefore, it does not need to be represented in memory, allowing the format to have one more bit of precision. This rule is variously called the leading bit convention, the implicit bit convention, the hidden bit convention, or the assumed bit convention."
+4527,"The floating-point representation is by far the most common way of representing in computers an approximation to real numbers. However, there are alternatives:"
+4528,"The first commercial computer with floating-point hardware was Zuse's Z4 computer, designed in 1942–1945. In 1946, Bell Laboratories introduced the Model V, which implemented decimal floating-point numbers."
+4529,"The Pilot ACE has binary floating-point arithmetic, and it became operational in 1950 at National Physical Laboratory, UK. Thirty-three were later sold commercially as the English Electric DEUCE. The arithmetic is actually implemented in software, but with a one megahertz clock rate, the speed of floating-point and fixed-point operations in this machine were initially faster than those of many competing computers."
+4530,"The mass-produced IBM 704 followed in 1954; it introduced the use of a biased exponent. For many decades after that, floating-point hardware was typically an optional feature, and computers that had it were said to be ""scientific computers"", or to have ""scientific computation""  capability ). It was not until the launch of the Intel i486 in 1989 that general-purpose personal computers had floating-point capability in hardware as a standard feature."
+4531,"The UNIVAC 1100/2200 series, introduced in 1962, supported two floating-point representations:"
+4532,"The IBM 7094, also introduced in 1962, supported single-precision and double-precision representations, but with no relation to the UNIVAC's representations. Indeed, in 1964, IBM introduced hexadecimal floating-point representations in its System/360 mainframes; these same representations are still available for use in modern z/Architecture systems.  In 1998, IBM implemented IEEE-compatible binary floating-point arithmetic in its mainframes; in 2005, IBM also added IEEE-compatible decimal floating-point arithmetic."
+4533,"Initially, computers used many different representations for floating-point numbers. The lack of standardization at the mainframe level was an ongoing problem by the early 1970s for those writing and maintaining higher-level source code; these manufacturer floating-point standards differed in the word sizes, the representations, and the rounding behavior and general accuracy of operations. Floating-point compatibility across multiple computing systems was in desperate need of standardization by the early 1980s, leading to the creation of the IEEE 754 standard once the 32-bit  word had become commonplace. This standard was significantly based on a proposal from Intel, which was designing the i8087 numerical coprocessor; Motorola, which was designing the 68000 around the same time, gave significant input as well."
+4534,"In 1989, mathematician and computer scientist William Kahan was honored with the Turing Award for being the primary architect behind this proposal; he was aided by his student Jerome Coonen and a visiting professor, Harold Stone."
+4535,Among the x86 innovations are these:
+4536,"A floating-point number consists of two fixed-point components, whose range depends exclusively on the number of bits or digits in their representation. Whereas components linearly depend on their range, the floating-point range linearly depends on the significand range and exponentially on the range of exponent component, which attaches outstandingly wider range to the number."
+4537,"On a typical computer system, a double-precision  binary floating-point number has a coefficient of 53 bits , an exponent of 11 bits, and 1 sign bit. Since 210 = 1024, the complete range of the positive normal floating-point numbers in this format is from 2−1022 ≈ 2 × 10−308 to approximately 21024 ≈ 2 × 10308."
+4538,The number of normal floating-point numbers in a system  where
+4539,"There is a smallest positive normal floating-point number,"
+4540,"which has a 1 as the leading digit and 0 for the remaining digits of the significand, and the smallest possible value for the exponent."
+4541,"There is a largest floating-point number,"
+4542,which has B − 1 as the value for each digit of the significand and the largest possible value for the exponent.
+4543,"In addition, there are representable values strictly between −UFL and UFL. Namely, positive and negative zeros, as well as subnormal numbers."
+4544,"The IEEE standardized the computer representation for binary floating-point numbers in IEEE 754  in 1985. This first standard is followed by almost all modern machines. It was revised in 2008. IBM mainframes support IBM's own hexadecimal floating point format and IEEE 754-2008 decimal floating point in addition to the IEEE 754 binary format. The Cray T90 series had an IEEE version, but the SV1 still uses Cray floating-point format."
+4545,"The standard provides for many closely related formats, differing in only a few details. Five of these formats are called basic formats, and others are termed extended precision formats and extendable precision format. Three formats are especially widely used in computer hardware and languages:"
+4546,"Increasing the precision of the floating-point representation generally reduces the amount of accumulated round-off error caused by intermediate calculations.
+Other IEEE formats include:"
+4547,"Any integer with absolute value less than 224 can be exactly represented in the single-precision format, and any integer with absolute value less than 253 can be exactly represented in the double-precision format. Furthermore, a wide range of powers of 2 times such a number can be represented. These properties are sometimes used for purely integer data, to get 53-bit integers on platforms that have double-precision floats but only 32-bit integers."
+4548,"The standard specifies some special values, and their representation: positive infinity , negative infinity , a negative zero  distinct from ordinary  zero, and ""not a number"" values ."
+4549,"Comparison of floating-point numbers, as defined by the IEEE standard, is a bit different from usual integer comparison. Negative and positive zero compare equal, and every NaN compares unequal to every value, including itself. All finite floating-point numbers are strictly smaller than +∞ and strictly greater than −∞, and they are ordered in the same way as their values ."
+4550,"Floating-point numbers are typically packed into a computer datum as the sign bit, the exponent field, and the significand or mantissa, from left to right.  For the IEEE 754 binary formats  which have extant hardware implementations, they are apportioned as follows:"
+4551,"While the exponent can be positive or negative, in binary formats it is stored as an unsigned number that has a fixed ""bias"" added to it. Values of all 0s in this field are reserved for the zeros and subnormal numbers; values of all 1s are reserved for the infinities and NaNs. The exponent range for normal numbers is  for single precision,  for double, or  for quad. Normal numbers exclude subnormal values, zeros, infinities, and NaNs."
+4552,"In the IEEE binary interchange formats the leading 1 bit of a normalized significand is not actually stored in the computer datum. It is called the ""hidden"" or ""implicit"" bit. Because of this, the single-precision format actually has a significand with 24 bits of precision, the double-precision format has 53, and quad has 113."
+4553,"For example, it was shown above that π, rounded to 24 bits of precision, has:"
+4554,"The sum of the exponent bias  and the exponent  is 128, so this is represented in the single-precision format as"
+4555,An example of a layout for 32-bit floating point is
+4556,and the 64-bit  layout is similar.
+4557,"In addition to the widely used IEEE 754 standard formats, other floating-point formats are used, or have been used, in certain domain-specific areas."
+4558,"By their nature, all numbers expressed in floating-point format are rational numbers with a terminating expansion in the relevant base . Irrational numbers, such as π or √2, or non-terminating rational numbers, must be approximated. The number of digits  of precision also limits the set of rational numbers that can be represented exactly. For example, the decimal number 123456789 cannot be exactly represented if only eight decimal digits of precision are available , the same applies to non-terminating digits ."
+4559,"When a number is represented in some format  which is not a native floating-point representation supported in a computer implementation, then it will require a conversion before it can be used in that implementation. If the number can be represented exactly in the floating-point format then the conversion is exact. If there is not an exact representation then the conversion requires a choice of which floating-point number to use to represent the original value. The representation chosen will have a different value from the original, and the value thus adjusted is called the rounded value."
+4560,"Whether or not a rational number has a terminating expansion depends on the base. For example, in base-10 the number 1/2 has a terminating expansion  while the number 1/3 does not . In base-2 only rationals with denominators that are powers of 2  are terminating. Any rational with a denominator that has a prime factor other than 2 will have an infinite binary expansion. This means that numbers that appear to be short and exact when written in decimal format may need to be approximated when converted to binary floating-point. For example, the decimal number 0.1 is not representable in binary floating-point of any finite precision;  the exact binary representation would have a ""1100"" sequence continuing endlessly:"
+4561,"where, as previously, s is the significand and e is the exponent."
+4562,When rounded to 24 bits this becomes
+4563,which is actually 0.100000001490116119384765625 in decimal.
+4564,"As a further example, the real number π, represented in binary as an infinite sequence of bits is"
+4565,when approximated by rounding to a precision of 24 bits.
+4566,"In binary single-precision floating-point, this is represented as s = 1.10010010000111111011011 with e = 1.
+This has a decimal value of"
+4567,whereas a more accurate approximation of the true value of π is
+4568,"The result of rounding differs from the true value by about 0.03 parts per million, and matches the decimal representation of π in the first 7 digits. The difference is the discretization error and is limited by the machine epsilon."
+4569,"The arithmetical difference between two consecutive representable floating-point numbers which have the same exponent is called a unit in the last place . For example, if there is no representable number lying between the representable numbers 1.45a70c22hex and 1.45a70c24hex, the ULP is 2×16−8, or 2−31. For numbers with a base-2 exponent part of 0, i.e. numbers with an absolute value higher than or equal to 1 but lower than 2, an ULP is exactly 2−23 or about 10−7 in single precision, and exactly 2−53 or about 10−16 in double precision. The mandated behavior of IEEE-compliant hardware is that the result be within one-half of a ULP."
+4570,"Rounding is used when the exact result of a floating-point operation  would need more digits than there are digits in the significand. IEEE 754 requires correct rounding: that is, the rounded result is as if infinitely precise arithmetic was used to compute the value and then rounded . There are several different rounding schemes . Historically, truncation was the typical approach. Since the introduction of IEEE 754, the default method  is more commonly used. This method rounds the ideal  result of an arithmetic operation to the nearest representable value, and gives that representation as the result. In the case of a tie, the value that would make the significand end in an even digit is chosen. The IEEE 754 standard requires the same rounding to be applied to all fundamental algebraic operations, including square root and conversions, when there is a numeric  result. It means that the results of IEEE 754 operations are completely determined in all bits of the result, except for the representation of NaNs. "
+4571,Alternative rounding options are also available. IEEE 754 specifies the following rounding modes:
+4572,"Alternative modes are useful when the amount of error being introduced must be bounded. Applications that require a bounded error are multi-precision floating-point, and interval arithmetic.
+The alternative rounding modes are also useful in diagnosing numerical instability: if the results of a subroutine vary substantially between rounding to + and − infinity then it is likely numerically unstable and affected by round-off error."
+4573,"Converting a double-precision binary floating-point number to a decimal string is a common operation, but an algorithm producing results that are both accurate and minimal did not appear in print until 1990, with Steele and White's Dragon4. Some of the improvements since then include:"
+4574,Many modern language runtimes use Grisu3 with a Dragon4 fallback.
+4575,"The problem of parsing a decimal string into a binary FP representation is complex, with an accurate parser not appearing until Clinger's 1990 work . Further work has likewise progressed in the direction of faster parsing."
+4576,"For ease of presentation and understanding, decimal radix with 7 digit precision will be used in the examples, as in the IEEE 754 decimal32 format. The fundamental principles are the same in any radix or precision, except that normalization is optional . Here, s denotes the significand and e denotes the exponent."
+4577,"A simple method to add floating-point numbers is to first represent them with the same exponent. In the example below, the second number is shifted right by three digits, and one then proceeds with the usual addition method:"
+4578,In detail:
+4579,"This is the true result, the exact sum of the operands. It will be rounded to seven digits and then normalized if necessary. The final result is"
+4580,"The lowest three digits of the second operand  are essentially lost. This is round-off error. In extreme cases, the sum of two non-zero numbers may be equal to one of them:"
+4581,"In the above conceptual examples it would appear that a large number of extra digits would need to be provided by the adder to ensure correct rounding; however, for binary addition or subtraction using careful implementation techniques only a guard bit, a rounding bit and one extra sticky bit need to be carried beyond the precision of the operands.: 218–220"
+4582,Another problem of loss of significance occurs when approximations to two nearly equal numbers are subtracted. In the following example e = 5; s = 1.234571 and e = 5; s = 1.234567 are approximations to the rationals 123457.1467 and 123456.659.
+4583,"The floating-point difference is computed exactly because the numbers are close—the Sterbenz lemma guarantees this, even in case of underflow when gradual underflow is supported. Despite this, the difference of the original numbers is e = −1; s = 4.877000, which differs more than 20% from the difference e = −1; s = 4.000000 of the approximations. In extreme cases, all significant digits of precision can be lost. This cancellation illustrates the danger in assuming that all of the digits of a computed result are meaningful. Dealing with the consequences of these errors is a topic in numerical analysis; see also Accuracy problems."
+4584,"To multiply, the significands are multiplied while the exponents are added, and the result is rounded and normalized."
+4585,"Similarly, division is accomplished by subtracting the divisor's exponent from the dividend's exponent, and dividing the dividend's significand by the divisor's significand."
+4586,"There are no cancellation or absorption problems with multiplication or division, though small errors may accumulate as operations are performed in succession. In practice, the way these operations are carried out in digital logic can be quite complex .
+For a fast, simple method, see the Horner method."
+4587,"Literals for floating-point numbers depend on languages. They typically use e or E to denote scientific notation. The C programming language and the IEEE 754 standard also define a hexadecimal literal syntax with a base-2 exponent instead of 10. In languages like C, when the decimal exponent is omitted, a decimal point is needed to differentiate them from integers. Other languages do not have an integer type , or allow overloading of numeric types . In these cases, digit strings such as 123 may also be floating-point literals."
+4588,Examples of floating-point literals are:
+4589,Floating-point computation in a computer can run into three kinds of problems:
+4590,"Prior to the IEEE standard, such conditions usually caused the program to terminate, or triggered some kind of trap that the programmer might be able to catch. How this worked was system-dependent, meaning that floating-point programs were not portable. "
+4591,"Here, the required default method of handling exceptions according to IEEE 754 is discussed . Arithmetic exceptions are  required to be recorded in ""sticky"" status flag bits. That they are ""sticky"" means that they are not reset by the next  operation, but stay set until explicitly reset. The use of ""sticky"" flags thus allows for testing of exceptional conditions to be delayed until after a full floating-point expression or subroutine: without them exceptional conditions that could not be otherwise ignored would require explicit testing immediately after every floating-point operation. By default, an operation always returns a result according to specification without interrupting computation. For instance, 1/0 returns +∞, while also setting the divide-by-zero flag bit ."
+4592,"The original IEEE 754 standard, however, failed to recommend operations to handle such sets of arithmetic exception flag bits. So while these were implemented in hardware, initially programming language implementations typically did not provide a means to access them . Over time some programming language standards  have been updated to specify methods to access and change status flag bits. The 2008 version of the IEEE 754 standard now specifies a few operations for accessing and handling the arithmetic flag bits. The programming model is based on a single thread of execution and use of them by multiple threads has to be handled by a means outside of the standard ."
+4593,IEEE 754 specifies five arithmetic exceptions that are to be recorded in the status flags :
+4594,"Overflow and invalid exceptions can typically not be ignored, but do not necessarily represent errors: for example, a root-finding routine, as part of its normal operation, may evaluate a passed-in function at values outside of its domain, returning NaN and an invalid exception flag to be ignored until finding a useful start point."
+4595,"The fact that floating-point numbers cannot accurately represent all real numbers, and that floating-point operations cannot accurately represent true arithmetic operations, leads to many surprising situations. This is related to the finite precision with which computers generally represent numbers."
+4596,"For example, the decimal numbers 0.1 and 0.01 cannot be represented exactly as binary floating-point numbers. In the IEEE 754 binary32 format with its 24-bit significand, the result of attempting to square the approximation to 0.1 is neither 0.01 nor the representable number closest to it. The decimal number 0.1 is represented in binary as e = −4; s = 110011001100110011001101, which is"
+4597,Squaring this number gives
+4598,Squaring it with rounding to the 24-bit precision gives
+4599,But the representable number closest to 0.01 is
+4600,"Also, the non-representability of π  means that an attempted computation of tan will not yield a result of infinity, nor will it even overflow in the usual floating-point formats . It is simply not possible for standard floating-point hardware to attempt to compute tan, because π/2 cannot be represented exactly. This computation in C:"
+4601,"will give a result of 16331239353195370.0. In single precision , the result will be −22877332.0."
+4602,"By the same token, an attempted computation of sin will not yield zero. The result will be  0.1225×10−15 in double precision, or −0.8742×10−7 in single precision."
+4603,"While floating-point addition and multiplication are both commutative , they are not necessarily associative. That is,  + c is not necessarily equal to a + . Using 7-digit significand decimal arithmetic:"
+4604,"They are also not necessarily distributive. That is,  × c may not be the same as a × c + b × c:"
+4605,"In addition to loss of significance, inability to represent numbers such as π and 0.1 exactly, and other slight inaccuracies, the following phenomena may occur:"
+4606,"Machine precision is a quantity that characterizes the accuracy of a floating-point system, and is used in backward error analysis of floating-point algorithms. It is also known as unit roundoff or machine epsilon. Usually denoted Εmach, its value depends on the particular rounding being used."
+4607,"With rounding to zero,"
+4608,This is important since it bounds the relative error in representing any non-zero real number x within the normalized range of a floating-point system:
+4609,"Backward error analysis, the theory of which was developed and popularized by James H. Wilkinson, can be used to establish that an algorithm implementing a numerical function is numerically stable. The basic approach is to show that although the calculated result, due to roundoff errors, will not be exactly correct, it is the exact solution to a nearby problem with slightly perturbed input data. If the perturbation required is small, on the order of the uncertainty in the input data, then the results are in some sense as accurate as the data ""deserves"". The algorithm is then defined as backward stable. Stability is a measure of the sensitivity to rounding errors of a given numerical procedure; by contrast, the condition number of a function for a given problem indicates the inherent sensitivity of the function to small perturbations in its input and is independent of the implementation used to solve the problem."
+4610,"by definition, which is the sum of two slightly perturbed  input data, and so is backward stable. For more realistic examples in numerical linear algebra, see Higham 2002 and other references below."
+4611,"Although individual arithmetic operations of IEEE 754 are guaranteed accurate to within half a ULP, more complicated formulae can suffer from larger errors for a variety of reasons. The loss of accuracy can be substantial if a problem or its data are ill-conditioned, meaning that the correct result is hypersensitive to tiny perturbations in its data. However, even functions that are well-conditioned can suffer from large loss of accuracy if an algorithm numerically unstable for that data is used: apparently equivalent formulations of expressions in a programming language can differ markedly in their numerical stability. One approach to remove the risk of such loss of accuracy is the design and analysis of numerically stable algorithms, which is an aim of the branch of mathematics known as numerical analysis. Another approach that can protect against the risk of numerical instabilities is the computation of intermediate  values in an algorithm at a higher precision than the final result requires, which can remove, or reduce by orders of magnitude, such risk: IEEE 754 quadruple precision and extended precision are designed for this purpose when computing at double precision."
+4612,"For example, the following algorithm is a direct implementation to compute the function A =  /  − 1) which is well-conditioned at 1.0, however it can be shown to be numerically unstable and lose up to half the significant digits carried by the arithmetic when computed near 1.0."
+4613,"If, however, intermediate computations are all performed in extended precision , then up to full precision in the final double result can be maintained. Alternatively, a numerical analysis of the algorithm reveals that if the following non-obvious change to line  is made:"
+4614,then the algorithm becomes numerically stable and can compute to full double precision.
+4615,"To maintain the properties of such carefully constructed numerically stable programs, careful handling by the compiler is required. Certain ""optimizations"" that compilers might make  can work against the goals of well-behaved software. There is some controversy about the failings of compilers and language designs in this area: C99 is an example of a language where such optimizations are carefully specified to maintain numerical precision. See the external references at the bottom of this article."
+4616,"A detailed treatment of the techniques for writing high-quality floating-point software is beyond the scope of this article, and the reader is referred to, and the other references at the bottom of this article. Kahan suggests several rules of thumb that can substantially decrease by orders of magnitude the risk of numerical anomalies, in addition to, or in lieu of, a more careful numerical analysis. These include: as noted above, computing all expressions and intermediate results in the highest precision supported in hardware ; and rounding input data and results to only the precision required and supported by the input data . Brief descriptions of several additional issues and techniques follow."
+4617,"As decimal fractions can often not be exactly represented in binary floating-point, such arithmetic is at its best when it is simply being used to measure real-world quantities over a wide range of scales , and at its worst when it is expected to model the interactions of quantities expressed as decimal strings that are expected to be exact. An example of the latter case is financial calculations. For this reason, financial software tends not to use a binary floating-point number representation. The ""decimal"" data type of the C# and Python programming languages, and the decimal formats of the IEEE 754-2008 standard, are designed to avoid the problems of binary floating-point representations when applied to human-entered exact decimal values, and make the arithmetic always behave as expected when numbers are printed in decimal."
+4618,"The use of the equality test  ...) requires care when dealing with floating-point numbers. Even simple expressions like 0.6/0.2-3==0 will, on most computers, fail to be true . Consequently, such tests are sometimes replaced with ""fuzzy"" comparisons  < epsilon) ..., where epsilon is sufficiently small and tailored to the application, such as 1.0E−13). The wisdom of doing this varies greatly, and can require numerical analysis to bound epsilon. Values derived from the primary data representation and their comparisons should be performed in a wider, extended, precision to minimize the risk of such inconsistencies due to round-off errors. It is often better to organize the code in such a way that such tests are unnecessary. For example, in computational geometry, exact tests of whether a point lies off or on a line or plane defined by other points can be performed using adaptive precision or exact arithmetic methods."
+4619,"Small errors in floating-point arithmetic can grow when mathematical algorithms perform operations an enormous number of times. A few examples are matrix inversion, eigenvector computation, and differential equation solving. These algorithms must be very carefully designed, using numerical approaches such as iterative refinement, if they are to work well."
+4620,"Summation of a vector of floating-point values is a basic algorithm in scientific computing, and so an awareness of when loss of significance can occur is essential. For example, if one is adding a very large number of numbers, the individual addends are very small compared with the sum. This can lead to loss of significance. A typical addition would then be something like"
+4621,"The low 3 digits of the addends are effectively lost. Suppose, for example, that one needs to add many numbers, all approximately equal to 3. After 1000 of them have been added, the running sum is about 3000; the lost digits are not regained. The Kahan summation algorithm may be used to reduce the errors."
+4622,"Round-off error can affect the convergence and accuracy of iterative numerical procedures. As an example, Archimedes approximated π by calculating the perimeters of polygons inscribing and circumscribing a circle, starting with hexagons, and successively doubling the number of sides. As noted above, computations may be rearranged in a way that is mathematically equivalent but less prone to error . Two forms of the recurrence formula for the circumscribed polygon are:"
+4623,"Here is a computation using IEEE ""double""  arithmetic:"
+4624,"While the two forms of the recurrence formula are clearly mathematically equivalent, the first subtracts 1 from a number extremely close to 1, leading to an increasingly problematic loss of significant digits. As the recurrence is applied repeatedly, the accuracy improves at first, but then it deteriorates. It never gets better than about 8 digits, even though 53-bit arithmetic should be capable of about 16 digits of precision. When the second form of the recurrence is used, the value converges to 15 digits of precision."
+4625,"The aforementioned lack of associativity of floating-point operations in general means that compilers cannot as effectively reorder arithmetic expressions as they could with integer and fixed-point arithmetic, presenting a roadblock in optimizations such as common subexpression elimination and auto-vectorization. The ""fast math"" option on many compilers  turns on reassociation along with unsafe assumptions such as a lack of NaN and infinite numbers in IEEE 754. Some compilers also offer more granular options to only turn on reassociation. In either case, the programmer is exposed to many of the precision pitfalls mentioned above for the portion of the program using ""fast"" math."
+4626,"In some compilers , turning on ""fast"" math may cause the program to disable subnormal floats at startup, affecting the floating-point behavior of not only the generated code, but also any program using such code as a library."
+4627,"In most Fortran compilers, as allowed by the ISO/IEC 1539-1:2004 Fortran standard, reassociation is the default, with breakage largely prevented by the ""protect parens"" setting . This setting stops the compiler from reassociating beyond the boundaries of parentheses. Intel Fortran Compiler is a notable outlier."
+4628,"A common problem in ""fast"" math is that subexpressions may not be optimized identically from place to place, leading to unexpected differences. One interpretation of the issue is that ""fast"" math as implemented currently has a poorly defined semantics. One attempt at formalizing ""fast"" math optimizations is seen in Icing, a verified compiler."
+4629,"The integers form the smallest group and the smallest ring containing the natural numbers. In algebraic number theory, the integers are sometimes qualified as rational integers to distinguish them from the more general algebraic integers. In fact,  integers are algebraic integers that are also rational numbers."
+4630,"The word integer comes from the Latin integer meaning ""whole"" or  ""untouched"", from in  plus tangere . ""Entire"" derives from the same origin via the French word entier, which means both entire and integer. Historically the term was used for a number that was a multiple of 1, or to the whole part of a mixed number. Only positive integers were considered, making the term synonymous with the natural numbers. The definition of integer expanded over time to include negative numbers as their usefulness was recognized. For example Leonhard Euler in his 1765 Elements of Algebra defined integers to include both positive and negative numbers. However, European mathematicians, for the most part, resisted the concept of negative numbers until the middle of the 19th century."
+4631,"The use of the letter Z to denote the set of integers comes from the German word Zahlen  and has been attributed to David Hilbert. The earliest known use of the notation in a textbook occurs in Algébre written by the collective Nicolas Bourbaki, dating to 1947. The notation was not adopted immediately, for example another textbook used the letter J and a 1960 paper used Z to denote the non-negative integers. But by 1961, Z was generally used by modern algebra texts to denote the positive and negative integers."
+4632,"The whole numbers were synonymous with the integers up until the early 1950s. In the late 1950s, as part of the New Math movement, American elementary school teachers began teaching that ""whole numbers"" referred to the natural numbers, excluding negative numbers, while ""integer"" included the negative numbers. ""Whole number"" remains ambiguous to the present day."
+4633,Algebraic structures
+4634,Related structures
+4635,p-adic number theory and decimals
+4636,Algebraic geometry
+4637,Free algebra
+4638,Clifford algebra
+4639,"The traditional arithmetic operations can then be defined on the integers in a piecewise fashion, for each of positive numbers, negative numbers, and zero. For example negation is defined as follows:"
+4640,The traditional style of definition leads to many different cases  and makes it tedious to prove that integers obey the various laws of arithmetic.
+4641,"In modern set-theoretic mathematics, a more abstract construction allowing one to define arithmetical operations without any case distinction is often used instead. The integers can thus be formally constructed as the equivalence classes of ordered pairs of natural numbers ."
+4642,"The intuition is that  stands for the result of subtracting b from a. To confirm our expectation that 1 − 2 and 4 − 5 denote the same number, we define an equivalence relation ~ on these pairs with the following rule:"
+4643,precisely when
+4644,"Addition and multiplication of integers can be defined in terms of the equivalent operations on the natural numbers; by using  to denote the equivalence class having  as a member, one has:"
+4645,The negation  of an integer is obtained by reversing the order of the pair:
+4646,Hence subtraction can be defined as the addition of the additive inverse:
+4647,The standard ordering on the integers is given by:
+4648,It is easily verified that these definitions are independent of the choice of representatives of the equivalence classes.
+4649,"Every equivalence class has a unique member that is of the form  or  . The natural number n is identified with the class  , and the class  is denoted −n (this covers all remaining classes, and gives the class  a second time since −0 = 0."
+4650,"Thus,  is denoted by"
+4651,"If the natural numbers are identified with the corresponding integers , this convention creates no ambiguity."
+4652,"This notation recovers the familiar representation of the integers as {..., −2, −1, 0, 1, 2, ...} ."
+4653,Some examples are:
+4654,"In theoretical computer science, other approaches for the construction of integers are used by automated theorem provers and term rewrite engines.
+Integers are represented as algebraic terms built using a few basic operations  and, possibly, using natural numbers, which are assumed to be already constructed ."
+4655,"There exist at least ten such constructions of signed integers. These constructions differ in several ways: the number of basic operations used for the construction, the number  and the types of arguments accepted by these operations; the presence or absence of natural numbers as arguments of some of these operations, and the fact that these operations are free constructors or not, i.e., that the same integer can be represented using only one or many algebraic terms."
+4656,"An integer is often a primitive data type in computer languages. However, integer data types can only represent a subset of all integers, since practical computers are of finite capacity. Also, in the common two's complement representation, the inherent definition of sign distinguishes between ""negative"" and ""non-negative"" rather than ""negative, positive, and 0"".  Fixed length integer approximation data types  are denoted int or Integer in several programming languages ."
+4657,"Variable-length representations of integers, such as bignums, can store any integer that fits in the computer's memory. Other integer data types are implemented with a fixed size, usually a number of bits which is a power of 2  or a memorable number of decimal digits ."
+4658,"The set of integers is countably infinite, meaning it is possible to pair each integer with a unique natural number. An example of such a pairing is"
+4659,"This article incorporates material from Integer on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License."
+4660,"Square roots of negative numbers can be discussed within the framework of complex numbers. More generally, square roots can be considered in any context in which a notion of the ""square"" of a mathematical object is defined. These include function spaces and square matrices, among other mathematical structures."
+4661,The Rhind Mathematical Papyrus is a copy from 1650 BC of an earlier Berlin Papyrus and other texts – possibly the Kahun Papyrus – that shows how the Egyptians extracted square roots by an inverse proportion method.
+4662,"In the Chinese mathematical work Writings on Reckoning, written between 202 BC and 186 BC during the early Han Dynasty, the square root is approximated by using an ""excess and deficiency"" method, which says to ""...combine the excess and deficiency as the divisor;  the deficiency numerator multiplied by the excess denominator and the excess numerator times the deficiency denominator, combine them as the dividend."""
+4663,"A symbol for square roots, written as an elaborate R, was invented by Regiomontanus . An R was also used for radix to indicate square roots in Gerolamo Cardano's Ars Magna."
+4664,"According to historian of mathematics D.E. Smith, Aryabhata's method for finding the square root was first introduced in Europe by Cataneo—in 1546."
+4665,"According to Jeffrey A. Oaks, Arabs used the letter jīm/ĝīm , the first letter of the word ""جذر"" , placed in its initial form  over a number to indicate its square root. The letter jīm resembles the present square root shape. Its usage goes as far as the end of the twelfth century in the works of the Moroccan mathematician Ibn al-Yasamin."
+4666,"The symbol ""√"" for the square root was first used in print in 1525, in Christoph Rudolff's Coss."
+4667,"The square root of x is rational if and only if x is a rational number that can be represented as a ratio of two perfect squares.  The square root function maps rational numbers into algebraic numbers, the latter being a superset of the rational numbers)."
+4668,"For all real numbers x,"
+4669,"For all nonnegative real numbers x and y,"
+4670,"The square root function is continuous for all nonnegative x, and differentiable for all positive x. If f denotes the square root function, whose derivative is given by:"
+4671,"The square root of a nonnegative number is used in the definition of Euclidean norm , as well as in generalizations such as Hilbert spaces. It defines an important concept of standard deviation used in probability theory and statistics. It has a major use in the formula for roots of a quadratic equation; quadratic fields and rings of quadratic integers, which are based on square roots, are important in algebra and have uses in geometry. Square roots frequently appear in mathematical formulas elsewhere, as well as in many physical laws."
+4672,"A positive number has two square roots, one positive, and one negative, which are opposite to each other. When talking of the square root of a positive integer, it is usually the positive square root that is meant."
+4673,The square roots of an integer are algebraic integers—more specifically quadratic integers.
+4674,"The square roots of the perfect squares  are integers. In all other cases, the square roots of positive integers are irrational numbers, and hence have non-repeating decimals in their decimal representations. Decimal approximations of the square roots of the first few natural numbers are given in the following table."
+4675,"As with before, the square roots of the perfect squares  are integers. In all other cases, the square roots of positive integers are irrational numbers, and therefore have non-repeating digits in any standard positional notation system."
+4676,The square roots of small integers are used in both the SHA-1 and SHA-2 hash function designs to provide nothing up my sleeve numbers.
+4677,"One of the most intriguing results from the study of irrational numbers as continued fractions was obtained by Joseph Louis Lagrange c. 1780. Lagrange found that the representation of the square root of any non-square positive integer as a continued fraction is periodic. That is, a certain pattern of partial denominators repeats indefinitely in the continued fraction. In a sense these square roots are the very simplest irrational numbers, because they can be represented with a simple repeating pattern of integers."
+4678,"The square bracket notation used above is a short form for a continued fraction. Written in the more suggestive algebraic form, the simple continued fraction for the square root of 11, , looks like this:"
+4679,"where the two-digit pattern {3, 6} repeats over and over again in the partial denominators. Since 11 = 32 + 2, the above is also identical to the following generalized continued fractions:"
+4680,"Square roots of positive numbers are not in general rational numbers, and so cannot be written as a terminating or recurring decimal expression. Therefore in general any attempt to compute a square root expressed in decimal form can only yield an approximation, though a sequence of increasingly accurate approximations can be obtained."
+4681,"Most pocket calculators have a square root key. Computer spreadsheets and other software are also frequently used to calculate square roots. Pocket calculators typically implement efficient routines, such as the Newton's method , to compute the square root of a positive real number.  When computing square roots with logarithm tables or slide rules, one can exploit the identities"
+4682,"The most common iterative method of square root calculation by hand is known as the ""Babylonian method"" or ""Heron's method"" after the first-century Greek philosopher Heron of Alexandria, who first described it.
+The method uses the same iterative scheme as the Newton–Raphson method yields when applied to the function y = f = x2 − a, using the fact that its slope at any point is dy/dx = f′ = 2x, but predates it by many centuries.
+The algorithm is to repeat a simple calculation that results in a number closer to the actual square root each time it is repeated with its result as the new input. The motivation is that if x is an overestimate to the square root of a nonnegative real number a then a/x will be an underestimate and so the average of these two numbers is a better approximation than either of them. However, the inequality of arithmetic and geometric means shows this average is always an overestimate of the square root , and so it can serve as a new overestimate with which to repeat the process, which converges as a consequence of the successive overestimates and underestimates being closer to each other after each iteration. To find x:"
+4683,"Start with an arbitrary positive start value x. The closer to the square root of a, the fewer the iterations that will be needed to achieve the desired precision."
+4684,Replace x by the average  / 2 between x and a/x.
+4685,"Repeat from step 2, using this average as the new value of x."
+4686,Using the identity
+4687,The time complexity for computing a square root with n digits of precision is equivalent to that of multiplying two n-digit numbers.
+4688,"Another useful method for calculating the square root is the shifting nth root algorithm, applied for n = 2."
+4689,"The name of the square root function varies from programming language to programming language, with sqrt  being common, used in C and derived languages like C++, JavaScript, PHP, and Python."
+4690,"The square of any positive or negative number is positive, and the square of 0 is 0. Therefore, no negative number can have a real square root.  However, it is possible to work with a more inclusive set of numbers, called the complex numbers, that does contain solutions to the square root of a negative number.  This is done by introducing a new number, denoted by i  and called the imaginary unit, which is defined such that i2 = −1. Using this notation, we can think of i as the square root of −1, but we also have 2 = i2 = −1 and so −i is also a square root of −1. By convention, the principal square root of −1 is i, or more generally, if x is any nonnegative number, then the principal square root of −x is"
+4691,"The right side  is indeed a square root of −x, since"
+4692,"For every non-zero complex number z there exist precisely two numbers w such that w2 = z: the principal square root of z , and its negative."
+4693,The above can also be expressed in terms of trigonometric functions:
+4694,"When the number is expressed using its real and imaginary parts, the following formula can be used for the principal square root:"
+4695,"where sgn = 1 if y ≥ 0 and sgn =  −1 otherwise. In particular, the imaginary parts of the original number and the principal value of its square root have the same sign. The real part of the principal value of the square root is always nonnegative."
+4696,"For example, the principal square roots of ±i are given by:"
+4697,"In the following, the complex z and w may be expressed as:"
+4698,"Because of the discontinuous nature of the square root function in the complex plane, the following laws are not true in general."
+4699,"A similar problem appears with other complex functions with branch cuts, e.g., the complex logarithm and the relations logz + logw = log or log = log* which are not true in general."
+4700,"Wrongly assuming one of these laws underlies several faulty ""proofs"", for instance the following one showing that −1 = 1:"
+4701,"Abel–Ruffini theorem states that, in general, the roots of a polynomial of degree five or higher cannot be expressed in terms of n-th roots."
+4702,"If A is a positive-definite matrix or operator, then there exists precisely one positive definite matrix or operator B with B2 = A; we then define A1/2 = B. In general matrices may have multiple square roots or even an infinitude of them.  For example, the 2 × 2 identity matrix has an infinity of square roots, though only one of them is positive definite."
+4703,"Each element of an integral domain has no more than 2 square roots. The difference of two squares identity u2 − v2 =  is proved using the commutativity of multiplication. If u and v are square roots of the same element, then u2 − v2 = 0. Because there are no zero divisors this implies u = v or u + v = 0, where the latter means that two roots are additive inverses of each other. In other words if an element a square root u of an element a exists, then the only square roots of a are u and −u. The only square root of 0 in an integral domain is 0 itself."
+4704,"In a field of characteristic 2, an element either has one square root or does not have any at all, because each element is its own additive inverse, so that −u = u. If the field is finite of characteristic 2 then every element has a unique square root. In a field of any other characteristic, any non-zero element either has two square roots, as explained above, or does not have any."
+4705,"Given an odd prime number p, let q = pe for some positive integer e. A non-zero element of the field Fq with q elements is a quadratic residue if it has a square root in Fq. Otherwise, it is a quadratic non-residue. There are /2 quadratic residues and /2 quadratic non-residues; zero is not counted in either class. The quadratic residues form a group under multiplication. The properties of quadratic residues are widely used in number theory."
+4706,"The construction is also given by Descartes in his La Géométrie, see figure 2 on page 2. However, Descartes made no claim to originality and his audience would have been quite familiar with Euclid."
+4707,"The MOSFET  is a type of insulated-gate field-effect transistor  that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals."
+4708,"The MOSFET is the basic building block of most modern electronics, and the most frequently manufactured device in history, with an estimated total of 13 sextillion  MOSFETs manufactured between 1960 and 2018.  It is the most common semiconductor device in digital and analog circuits, and the most common power device. It was the first truly compact transistor that could be miniaturized and mass-produced for a wide range of uses. MOSFET scaling and miniaturization has been driving the rapid exponential growth of electronic semiconductor technology since the 1960s, and enable high-density integrated circuits  such as memory chips and microprocessors."
+4709,"MOSFETs in integrated circuits are the primary elements of computer processors, semiconductor memory, image sensors, and most other types of integrated circuits. Discrete MOSFET devices are widely used in applications such as switch mode power supplies, variable-frequency drives, and other power electronics applications where each device may be switching thousands of watts. Radio-frequency amplifiers up to the UHF spectrum use MOSFET transistors as analog signal and power amplifiers. Radio systems also use MOSFETs as oscillators, or mixers to convert frequencies. MOSFET devices are also applied in audio-frequency power amplifiers for public address systems, sound reinforcement, and home and automobile sound systems."
+4710,"The MOSFET is the most widely used type of transistor and the most critical device component in integrated circuit  chips.  Planar process, developed by Jean Hoerni at Fairchild Semiconductor in early 1959, was critical to the invention of the monolithic integrated circuit chip by Robert Noyce later in 1959. The same year, Mohamed M. Atalla used his surface passivation process to make the first working MOSFET with Dawon Kahng at Bell Labs. This was followed by the development of clean rooms to reduce contamination to levels never before thought necessary, and coincided with the development of photolithography which, along with surface passivation and the planar process, allowed circuits to be made in few steps."
+4711,"Atalla realised that the main advantage of a MOS transistor was its ease of fabrication, particularly suiting it for use in the recently invented integrated circuits. In contrast to bipolar transistors which required a number of steps for the p–n junction isolation of transistors on a chip, MOSFETs required no such steps but could be easily isolated from each other. Its advantage for integrated circuits was re-iterated by Dawon Kahng in 1961. The Si–SiO2 system possessed the technical attractions of low cost of production  and ease of integration. These two factors, along with its rapidly scaling miniaturization and low energy consumption, led to the MOSFET becoming the most widely used type of transistor in IC chips."
+4712,"The earliest experimental MOS IC to be demonstrated was a 16-transistor chip built by Fred Heiman and Steven Hofstein at RCA in 1962. General Microelectronics later introduced the first commercial MOS integrated circuits in 1964, consisting of 120 p-channel transistors. It was a 20-bit shift register, developed by Robert Norman and Frank Wanlass. In 1967, Bell Labs researchers Robert Kerwin, Donald Klein and John Sarace developed the self-aligned gate  MOS transistor, which Fairchild Semiconductor researchers Federico Faggin and Tom Klein used to develop the first silicon-gate MOS IC."
+4713,"There are various different types of MOS IC chips, which include the following."
+4714,"With its high scalability, and much lower power consumption and higher density than bipolar junction transistors, the MOSFET made it possible to build high-density IC chips. By 1964, MOS chips had reached higher transistor density and lower manufacturing costs than bipolar chips. MOS chips further increased in complexity at a rate predicted by Moore's law, leading to large-scale integration  with hundreds of MOSFETs on a chip by the late 1960s. MOS technology enabled the integration of more than 10,000 transistors on a single LSI chip by the early 1970s, before later enabling very large-scale integration ."
+4715,"The MOSFET is the basis of every microprocessor, and was responsible for the invention of the microprocessor. The origins of both the microprocessor and the microcontroller can be traced back to the invention and development of MOS technology. The application of MOS LSI chips to computing was the basis for the first microprocessors, as engineers began recognizing that a complete computer processor could be contained on a single MOS LSI chip."
+4716,"The earliest microprocessors were all MOS chips, built with MOS LSI circuits. The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. The first commercial single-chip microprocessor, the Intel 4004, was developed by Federico Faggin, using his silicon-gate MOS IC technology, with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima. With the arrival of CMOS microprocessors in 1975, the term ""MOS microprocessors"" began to refer to chips fabricated entirely from PMOS logic or fabricated entirely from NMOS logic, contrasted with ""CMOS microprocessors"" and ""bipolar bit-slice processors""."
+4717,"Complementary metal–oxide–semiconductor  logic was developed by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963. CMOS had lower power consumption, but was initially slower than NMOS, which was more widely used for computers in the 1970s. In 1978, Hitachi introduced the twin-well CMOS process, which allowed CMOS to match the performance of NMOS with less power consumption. The twin-well CMOS process eventually overtook NMOS as the most common semiconductor manufacturing process for computers in the 1980s. By the 1980s CMOS logic consumed over 7 times less power than NMOS logic, and about 100,000 times less power than bipolar transistor-transistor logic ."
+4718,"The growth of digital technologies like the microprocessor has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor. A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs. Bipolar transistor-based logic  does not have such a high fanout capacity. This isolation also makes it easier for the designers to ignore to some extent loading effects between logic stages independently. That extent is defined by the operating frequency: as frequencies increase, the input impedance of the MOSFETs decreases."
+4719,"The MOSFET's advantages in digital circuits do not translate into supremacy in all analog circuits. The two types of circuit draw upon different features of transistor behavior. Digital circuits switch, spending most of their time either fully on or fully off. The transition from one to the other is only of concern with regards to speed and charge required. Analog circuits depend on operation in the transition region where small changes to Vgs can modulate the output  current. The JFET and bipolar junction transistor  are preferred for accurate matching , higher transconductance and certain temperature characteristics which simplify keeping performance predictable as circuit temperature varies."
+4720,"Nevertheless, MOSFETs are widely used in many types of analog circuits because of their own advantages . The characteristics and performance of many analog circuits can be scaled up or down by changing the sizes  of the MOSFETs used. By comparison, in bipolar transistors the size of the device does not significantly affect its performance. MOSFETs' ideal characteristics regarding gate current  and drain-source offset voltage  also make them nearly ideal switch elements, and also make switched capacitor analog circuits practical. In their linear region, MOSFETs can be used as precision resistors, which can have a much higher controlled resistance than BJTs. In high power circuits, MOSFETs sometimes have the advantage of not suffering from thermal runaway as BJTs do. Also, MOSFETs can be configured to perform as capacitors and gyrator circuits which allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices on a chip  to be built entirely out of MOSFETs. This means that complete analog circuits can be made on a silicon chip in a much smaller space and with simpler fabrication techniques. MOSFETS are ideally suited to switch inductive loads because of tolerance to inductive kickback."
+4721,"Some ICs combine analog and digital MOSFET circuitry on a single mixed-signal integrated circuit, making the needed board space even smaller. This creates a need to isolate the analog circuits from the digital circuits on a chip level, leading to the use of isolation rings and silicon on insulator . Since MOSFETs require more space to handle a given amount of power than a BJT, fabrication processes can incorporate BJTs and MOSFETs into a single device. Mixed-transistor devices are called bi-FETs  if they contain just one BJT-FET and BiCMOS  if they contain complementary BJT-FETs. Such devices have the advantages of both insulated gates and higher current density."
+4722,"In the late 1980s, Asad Abidi pioneered RF CMOS technology, which uses MOS VLSI circuits, while working at UCLA. This changed the way in which RF circuits were designed, away from discrete bipolar transistors and towards CMOS integrated circuits. As of 2008, the radio transceivers in all wireless networking devices and modern mobile phones are mass-produced as RF CMOS devices. RF CMOS is also used in nearly all modern Bluetooth and wireless LAN  devices."
+4723,"MOSFET analog switches use the MOSFET to pass analog signals when on, and as a high impedance when off. Signals flow in both directions across a MOSFET switch. In this application, the drain and source of a MOSFET exchange places depending on the relative voltages of the source/drain electrodes. The source is the more negative side for an N-MOS or the more positive side for a P-MOS. All of these switches are limited on what signals they can pass or stop by their gate–source, gate–drain, and source–drain voltages; exceeding the voltage, current, or power limits will potentially damage the switch."
+4724,This analog switch uses a four-terminal simple MOSFET of either P or N type.
+4725,"In the case of an n-type switch, the body is connected to the most negative supply  and the gate is used as the switch control. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS switch passes all voltages less than Vgate − Vtn. When the switch is conducting, it typically operates in the linear  mode of operation, since the source and drain voltages will typically be nearly equal."
+4726,"In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than Vgate − Vtp ."
+4727,"This ""complementary"" or CMOS type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the single-type switch. The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential  and the body of the N-MOS is connected to the low potential . To turn the switch on, the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential. For voltages between VDD − Vtn and gnd − Vtp, both FETs conduct the signal; for voltages less than gnd − Vtp, the N-MOS conducts alone; and for voltages greater than VDD − Vtn, the P-MOS conducts alone."
+4728,"The voltage limits for this switch are the gate–source, gate–drain and source–drain voltage limits for both FETs. Also, the P-MOS is typically two to three times wider than the N-MOS, so the switch will be balanced for speed in the two directions."
+4729,"Tri-state circuitry sometimes incorporates a CMOS MOSFET switch on its output to provide for a low-ohmic, full-range output when on, and a high-ohmic, mid-level signal when off."
+4730,"The advent of the MOSFET enabled the practical use of MOS transistors as memory cell storage elements, a function previously served by magnetic cores in computer memory. The first modern computer memory was introduced in 1965, when John Schmidt at Fairchild Semiconductor designed the first MOS semiconductor memory, a 64-bit MOS SRAM . SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each bit of data."
+4731,"MOS technology is the basis for DRAM . In 1966, Dr. Robert H. Dennard at the IBM Thomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of building capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. In 1967, Dennard filed a patent under IBM for a single-transistor DRAM  memory cell, based on MOS technology. MOS memory enabled higher performance, was cheaper, and consumed less power, than magnetic-core memory, leading to MOS memory overtaking magnetic core memory as the dominant computer memory technology by the early 1970s."
+4732,"Frank Wanlass, while studying MOSFET structures in 1963, noted the movement of charge through oxide onto a gate. While he did not pursue it, this idea would later become the basis for EPROM  technology. In 1967, Dawon Kahng and Simon Sze proposed that floating-gate memory cells, consisting of floating-gate MOSFETs , could be used to produce reprogrammable ROM . Floating-gate memory cells later became the basis for non-volatile memory  technologies including EPROM, EEPROM  and flash memory."
+4733,There are various different types of MOS memory. The following list includes various different MOS memory types.
+4734,"A number of MOSFET sensors have been developed, for measuring physical, chemical, biological and environmental parameters. The earliest MOSFET sensors include the open-gate FET  introduced by Johannessen in 1970, the ion-sensitive field-effect transistor  invented by Piet Bergveld in 1970, the adsorption FET  patented by P.F. Cox in 1974, and a hydrogen-sensitive MOSFET demonstrated by I. Lundstrom, M.S. Shivaraman, C.S. Svenson and L. Lundkvist in 1975. The ISFET is a special type of MOSFET with a gate at a certain distance, and where the metal gate is replaced by an ion-sensitive membrane, electrolyte solution and reference electrode."
+4735,"By the mid-1980s, numerous other MOSFET sensors had been developed, including the gas sensor FET , surface accessible FET , charge flow transistor , pressure sensor FET , chemical field-effect transistor , reference ISFET , biosensor FET , enzyme-modified FET  and immunologically modified FET . By the early 2000s, BioFET types such as the DNA field-effect transistor , gene-modified FET  and cell-potential BioFET  had been developed."
+4736,"The two main types of image sensors used in digital imaging technology are the charge-coupled device  and the active-pixel sensor . Both CCD and CMOS sensors are based on MOS technology, with the CCD based on MOS capacitors and the CMOS sensor based on MOS transistors."
+4737,"MOS technology is the basis for modern image sensors, including the charge-coupled device  and the CMOS active-pixel sensor , used in digital imaging and digital cameras. Willard Boyle and George E. Smith developed the CCD in 1969. While researching the MOS process, they realized that an electric charge was the analogy of the magnetic bubble and that it could be stored on a tiny MOS capacitor. As it was fairly straightforward to fabricate a series of MOS capacitors in a row, they connected a suitable voltage to them so that the charge could be stepped along from one to the next. The CCD is a semiconductor circuit that was later used in the first digital video cameras for television broadcasting."
+4738,The MOS active-pixel sensor  was developed by Tsutomu Nakamura at Olympus in 1985. The CMOS active-pixel sensor was later developed by Eric Fossum and his team at NASA's Jet Propulsion Laboratory in the early 1990s.
+4739,"MOS image sensors are widely used in optical mouse technology. The first optical mouse, invented by Richard F. Lyon at Xerox in 1980, used a 5 µm NMOS sensor chip. Since the first commercial optical mouse, the IntelliMouse introduced in 1999, most optical mouse devices use CMOS sensors."
+4740,"MOS sensors, also known as MOSFET sensors, are widely used to measure physical, chemical, biological and environmental parameters. The ion-sensitive field-effect transistor , for example, is widely used in biomedical applications."
+4741,"MOSFETs are also widely used in microelectromechanical systems , as silicon MOSFETs could interact and communicate with the surroundings and process things such as chemicals, motions and light. An early example of a MEMS device is the resonant-gate transistor, an adaptation of the MOSFET, developed by Harvey C. Nathanson in 1965."
+4742,Common applications of other MOS sensors include the following.
+4743,"The power MOSFET, which is commonly used in power electronics, was developed in the early 1970s. The power MOSFET enables low gate drive power, fast switching speed, and advanced paralleling capability."
+4744,"The power MOSFET is the most widely used power device in the world. Advantages over bipolar junction transistors in power electronics include MOSFETs not requiring a continuous flow of drive current to remain in the ON state, offering higher switching speeds, lower switching power losses, lower on-resistances, and reduced susceptibility to thermal runaway. The power MOSFET had an impact on power supplies, enabling higher operating frequencies, size and weight reduction, and increased volume production."
+4745,"Switching power supplies are the most common applications for power MOSFETs. They are also widely used for MOS RF power amplifiers, which enabled the transition of mobile networks from analog to digital in the 1990s. This led to the wide proliferation of wireless mobile networks, which revolutionised telecommunication systems. The LDMOS in particular is the most widely used power amplifier in mobile networks such as 2G, 3G, 4G and 5G, as well as broadcasting and amateur radio. Over 50 billion discrete power MOSFETs are shipped annually, as of 2018. They are widely used for automotive, industrial and communications systems in particular. Power MOSFETs are commonly used in automotive electronics, particularly as switching devices in electronic control units, and as power converters in modern electric vehicles. The insulated-gate bipolar transistor , a hybrid MOS-bipolar transistor, is also used for a wide variety of applications."
+4746,"LDMOS, a power MOSFET with lateral structure, is commonly used in high-end audio amplifiers and high-power PA systems. Their advantage is a better behaviour in the saturated region  than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications."
+4747,"Power MOSFETs, including DMOS, LDMOS and VMOS devices, are commonly used for a wide range of other applications, which include the following."
+4748,"RF DMOS, also known as RF power MOSFET, is a type of DMOS power transistor designed for radio-frequency  applications. It is used in various radio and RF applications, which include the following."
+4749,"MOSFETs are fundamental to the consumer electronics industry. According to Colinge, numerous consumer electronics would not exist without the MOSFET, such as digital wristwatches, pocket calculators, and video games, for example."
+4750,"MOSFETs are commonly used for a wide range of consumer electronics, which include the following devices listed. Computers or telecommunication devices  are not included here, but are listed separately in the Information and communications technology  section below."
+4751,"One of the earliest influential consumer electronic products enabled by MOS LSI circuits was the electronic pocket calculator, as MOS LSI technology enabled large amounts of computational capability in small packages. In 1965, the Victor 3900 desktop calculator was the first MOS LSI calculator, with 29 MOS LSI chips. In 1967 the Texas Instruments Cal-Tech was the first prototype electronic handheld calculator, with three MOS LSI chips, and it was later released as the Canon Pocketronic in 1970. The Sharp QT-8D desktop calculator was the first mass-produced LSI MOS calculator in 1969, and the Sharp EL-8 which used four MOS LSI chips was the first commercial electronic handheld calculator in 1970. The first true electronic pocket calculator was the Busicom LE-120A HANDY LE, which used a single MOS LSI calculator-on-a-chip from Mostek, and was released in 1971. By 1972, MOS LSI circuits were commercialized for numerous other applications."
+4752,"MOSFETs are commonly used for a wide range of audio-visual  media technologies, which include the following list of applications."
+4753,Power MOSFETs are commonly used for a wide range of consumer electronics. Power MOSFETs are widely used in the following consumer applications.
+4754,"MOSFETs are fundamental to information and communications technology , including modern computers, modern computing, telecommunications, the communications infrastructure, the Internet, digital telephony, wireless telecommunications, and mobile networks. According to Colinge, the modern computer industry and digital telecommunication systems would not exist without the MOSFET. Advances in MOS technology has been the most important contributing factor in the rapid rise of network bandwidth in telecommunication networks, with bandwidth doubling every 18 months, from bits per second to terabits per second ."
+4755,"MOSFETs are commonly used in a wide range of computers and computing applications, which include the following."
+4756,"MOSFETs are commonly used in a wide range of telecommunications, which include the following applications."
+4757,"The insulated-gate bipolar transistor  is a power transistor with characteristics of both a MOSFET and bipolar junction transistor . As of 2010, the IGBT is the second most widely used power transistor, after the power MOSFET. The IGBT accounts for 27% of the power transistor market, second only to the power MOSFET , and ahead of the RF amplifier  and bipolar junction transistor . The IGBT is widely used in consumer electronics, industrial technology, the energy sector, aerospace electronic devices, and transportation."
+4758,The IGBT is widely used in the following applications.
+4759,"In quantum physics and quantum mechanics, the MOSFET is the basis for two-dimensional electron gas  and the quantum Hall effect. The MOSFET enables physicists to study electron behavior in a two-dimensional gas, called a two-dimensional electron gas. In a MOSFET, conduction electrons travel in a thin surface layer, and a ""gate"" voltage controls the number of charge carriers in this layer. This allows researchers to explore quantum effects by operating high-purity MOSFETs at liquid helium temperatures."
+4760,"In 1978, the Gakushuin University researchers Jun-ichi Wakabayashi and Shinji Kawaji observed the Hall effect in experiments carried out on the inversion layer of MOSFETs. In 1980, Klaus von Klitzing, working at the high magnetic field laboratory in Grenoble with silicon-based MOSFET samples developed by Michael Pepper and Gerhard Dorda, made the unexpected discovery of the quantum Hall effect."
+4761,The MOSFET is used in quantum technology. A quantum field-effect transistor  or quantum well field-effect transistor  is a type of MOSFET that takes advantage of quantum tunneling to greatly increase the speed of transistor operation.
+4762,"MOSFETs are widely used in transportation. For example, they are commonly used for automotive electronics in the automotive industry. MOS technology is commonly used for a wide range of vehicles and transportation, which include the following applications."
+4763,"MOSFETs are widely used in the automotive industry, particularly for automotive electronics in motor vehicles. Automotive applications include the following."
+4764,"Power MOSFETs are widely used in transportation technology, which includes the following vehicles."
+4765,"In the automotive industry, power MOSFETs are widely used in automotive electronics, which include the following."
+4766,The insulated-gate bipolar transistor  is a power transistor with characteristics of both a MOSFET and bipolar junction transistor . IGBTs are widely used in the following transportation applications.
+4767,"In the space industry, MOSFET devices were adopted by NASA for space research in 1964, for its Interplanetary Monitoring Platform  program and Explorers space exploration program. The use of MOSFETs was a major step forward in the electronics design of spacecraft and satellites. The IMP D , launched in 1966, was the first spacecraft to use the MOSFET. Data gathered by IMP spacecraft and satellites were used to support the Apollo program, enabling the first crewed Moon landing with the Apollo 11 mission in 1969."
+4768,"The Cassini–Huygens to Saturn in 1997 had spacecraft power distribution accomplished 192 solid-state power switch  devices, which also functioned as circuit breakers in the event of an overload condition. The switches were developed from a combination of two semiconductor devices with switching capabilities: the MOSFET and the ASIC . This combination resulted in advanced power switches that had better performance characteristics than traditional mechanical switches."
+4769,"MOSFETs are commonly used for a wide range of other applications, which include the following."
+4770,"Housed in special high-speed memory, microcode translates machine instructions, state machine data, or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying electronics, thereby enabling greater flexibility in designing and altering instructions. Moreover, it facilitates the construction of complex multi-step instructions, while simultaneously reducing the complexity of computer circuits. The act of writing microcode is often referred to as microprogramming, and the microcode in a specific processor implementation is sometimes termed a microprogram."
+4771,"Through extensive microprogramming, microarchitectures of smaller scale and simplicity can emulate more robust architectures with wider word lengths, additional execution units, and so forth. This approach provides a relatively straightforward method of ensuring software compatibility between different products within a processor family."
+4772,"Some hardware vendors, notably IBM/Lenovo, use the term microcode interchangeably with firmware. In this context, all code within a device is termed microcode, whether it is microcode or machine code. For instance, updates to a hard disk drive's microcode often encompass updates to both its microcode and firmware."
+4773,"At the hardware level, processors contain a number of separate areas of circuity, or ""units"", that perform different tasks. Commonly found units include the arithmetic logic unit  which performs instructions such as addition or comparing two numbers, circuits for reading and writing data to external memory, and small areas of onboard memory to store these values while they are being processed. In most designs, additional high-performance memory, the register file, is used to store temporary values, not just those needed by the current instruction."
+4774,"To properly perform an instruction, the various circuits have to be activated in order. For instance, it is not possible to add two numbers if they have not yet been loaded from memory. In RISC designs, the proper ordering of these instructions is largely up to the programmer, or at least to the compiler of the programming language they are using. So to add two numbers, for instance, the compiler may output instructions to load one of the values into one register, the second into another, call the addition function in the ALU, and then write the result back out to memory."
+4775,"As the sequence of instructions needed to complete this higher-level concept, ""add these two numbers in memory"", may require multiple instructions, this can represent a performance bottleneck if those instructions are stored in main memory. Reading those instructions one by one is taking up time that could be used to read and write the actual data. For this reason, it is common for non-RISC designs to have many different instructions that differ largely on where they store data. For instance, the MOS 6502 has eight variations of the addition instruction, ADC, which differ only in where they look to find the two operands."
+4776,"Using the variation of the instruction, or ""opcode"", that most closely matches the ultimate operation can reduce the number of instructions to one, saving memory used by the program code and improving performance by leaving the data bus open for other operations. Internally, however, these instructions are not separate operations, but sequences of the operations the units actually perform. Converting a single instruction read from memory into the sequence of internal actions is the duty of the control unit, another unit within the processor."
+4777,"The basic idea behind microcode is to replace the custom logic implementing the instruction sequencing with a series of simple instructions run in a ""microcode engine"" in the processor. Whereas a custom logic system might have a series of diodes and gates that output a series of voltages on various control lines, the microcode engine is connected to these lines instead, and these are turned on and off as the engine reads the microcode instructions in sequence. The microcode instructions are often bit encoded to those lines, for instance, if bit 8 is true, that might mean that the ALU should be paused awaiting data. In this respect microcode is somewhat similar to the paper rolls in a player piano, where the holes represent which key should be pressed."
+4778,"The distinction between custom logic and microcode may seem small, one uses a pattern of diodes and gates to decode the instruction and produce a sequence of signals, whereas the other encodes the signals as microinstructions that are read in sequence to produce the same results. The critical difference is that in a custom logic design, changes to the individual steps require the logic to be redesigned. Using microcode, all that changes is the code stored in the associated read only memory . This makes it much easier to fix problems in a microcode system. It also means that there is no effective limit to the complexity of the instructions, it is only limited by the amount of ROM one is willing to use."
+4779,"The lowest layer in a computer's software stack is traditionally raw machine code instructions for the processor. In microcoded processors, fetching and decoding those instructions, and executing them, may be done by microcode. To avoid confusion, each microprogram-related element is differentiated by the micro prefix: microinstruction, microassembler, microprogrammer, etc."
+4780,"Complex digital processors may also employ more than one  control unit in order to delegate sub-tasks that must be performed essentially asynchronously in parallel.  For example, the VAX 9000 has an IBox unit to fetch and decode instructions, which it hands to a microcoded EBox unit to be executed, and the VAX 8800 has both a microcoded IBox and a microcoded EBox."
+4781,"A high-level programmer, or even an assembly language programmer, does not normally see or change microcode. Unlike machine code, which often retains some backward compatibility among different processors in a family, microcode only runs on the exact electronic circuitry for which it is designed, as it constitutes an inherent part of the particular processor design itself."
+4782,"Engineers normally write the microcode during the design phase of a processor, storing it in a read-only memory  or programmable logic array  structure, or in a combination of both. However, machines also exist that have some or all microcode stored in static random-access memory  or flash memory. This is traditionally denoted as writeable control store in the context of computers, which can be either read-only or read–write memory. In the latter case, the CPU initialization process loads microcode into the control store from another storage medium, with the possibility of altering the microcode to correct bugs in the instruction set, or to implement new machine instructions."
+4783,"Microprograms consist of series of microinstructions, which control the CPU at a very fundamental level of hardware circuitry. For example, a single typical horizontal microinstruction might specify the following operations:"
+4784,"To simultaneously control all processor's features in one cycle, the microinstruction is often wider than 50 bits; e.g., 128 bits on a 360/85 with an emulator feature. Microprograms are carefully designed and optimized for the fastest possible execution, as a slow microprogram would result in a slow machine instruction and degraded performance for related application programs that use such instructions."
+4785,"Microcode was originally developed as a simpler method of developing the control logic for a computer. Initially, CPU instruction sets were hardwired. Each step needed to fetch, decode, and execute the machine instructions  was controlled directly by combinational logic and rather minimal sequential state machine circuitry. While such hard-wired processors were very efficient, the need for powerful instruction sets with multi-step addressing and complex operations  made them difficult to design and debug; highly encoded and varied-length instructions can contribute to this as well, especially when very irregular encodings are used."
+4786,"Microcode simplified the job by allowing much of the processor's behaviour and programming model to be defined via microprogram routines rather than by dedicated circuitry. Even late in the design process, microcode could easily be changed, whereas hard-wired CPU designs were very cumbersome to change. Thus, this greatly facilitated CPU design."
+4787,"From the 1940s to the late 1970s, a large portion of programming was done in assembly language; higher-level instructions mean greater programmer productivity, so an important advantage of microcode was the relative ease by which powerful machine instructions can be defined. The ultimate extension of this are ""Directly Executable High Level Language"" designs, in which each statement of a high-level language such as PL/I is entirely and directly executed by microcode, without compilation. The IBM Future Systems project and Data General Fountainhead Processor are examples of this. During the 1970s, CPU speeds grew more quickly than memory speeds and numerous techniques such as memory block transfer, memory pre-fetch and multi-level caches were used to alleviate this. High-level machine instructions, made possible by microcode, helped further, as fewer more complex machine instructions require less memory bandwidth. For example, an operation on a character string can be done as a single machine instruction, thus avoiding multiple instruction fetches."
+4788,"Architectures with instruction sets implemented by complex microprograms included the IBM System/360 and Digital Equipment Corporation VAX. The approach of increasingly complex microcode-implemented instruction sets was later called complex instruction set computer . An alternate approach, used in many microprocessors, is to use one or more programmable logic array  or read-only memory   mainly for instruction decoding, and let a simple state machine  do most of the sequencing. The MOS Technology 6502 is an example of a microprocessor using a PLA for instruction decode and sequencing. The PLA is visible in photomicrographs of the chip, and its operation can be seen in the transistor-level simulation."
+4789,"Microprogramming is still used in modern CPU designs. In some cases, after the microcode is debugged in simulation, logic functions are substituted for the control store. Logic functions are often faster and less expensive than the equivalent microprogram memory."
+4790,"A processor's microprograms operate on a more primitive, totally different, and much more hardware-oriented architecture than the assembly instructions visible to normal programmers. In coordination with the hardware, the microcode implements the programmer-visible architecture. The underlying hardware need not have a fixed relationship to the visible architecture. This makes it easier to implement a given instruction set architecture on a wide variety of underlying hardware micro-architectures."
+4791,"The IBM System/360 has a 32-bit architecture with 16 general-purpose registers, but most of the System/360 implementations use hardware that implements a much simpler underlying microarchitecture; for example, the System/360 Model 30 has 8-bit data paths to the arithmetic logic unit  and main memory and implemented the general-purpose registers in a special unit of higher-speed core memory, and the System/360 Model 40 has 8-bit data paths to the ALU and 16-bit data paths to main memory and also implemented the general-purpose registers in a special unit of higher-speed core memory. The Model 50 has full 32-bit data paths and implements the general-purpose registers in a special unit of higher-speed core memory. The Model 65 through the Model 195 have larger data paths and implement the general-purpose registers in faster transistor circuits. In this way, microprogramming enabled IBM to design many System/360 models with substantially different hardware and spanning a wide range of cost and performance, while making them all architecturally compatible. This dramatically reduces the number of unique system software programs that must be written for each model."
+4792,"A similar approach was used by Digital Equipment Corporation  in their VAX family of computers. As a result, different VAX processors use different microarchitectures, yet the programmer-visible architecture does not change."
+4793,Microprogramming also reduces the cost of field changes to correct defects  in the processor; a bug can often be fixed by replacing a portion of the microprogram rather than by changes being made to hardware logic and wiring.
+4794,"In 1947, the design of the MIT Whirlwind introduced the concept of a control store as a way to simplify computer design and move beyond ad hoc methods. The control store is a diode matrix: a two-dimensional lattice, where one dimension accepts ""control time pulses"" from the CPU's internal clock, and the other connects to control signals on gates and other circuits. A ""pulse distributor"" takes the pulses generated by the CPU clock and breaks them up into eight separate time pulses, each of which activates a different row of the lattice. When the row is activated, it activates the control signals connected to it."
+4795,"In 1951, Maurice Wilkes enhanced this concept by adding conditional execution, a concept akin to a conditional in computer software. His initial implementation consisted of a pair of matrices: the first one generated signals in the manner of the Whirlwind control store, while the second matrix selected which row of signals  to invoke on the next cycle. Conditionals were implemented by providing a way that a single line in the control store could choose from alternatives in the second matrix. This made the control signals conditional on the detected internal signal. Wilkes coined the term microprogramming to describe this feature and distinguish it from a simple control store."
+4796,"Microcode remained relatively rare in computer design as the cost of the ROM needed to store the code was not significantly different than using a custom control store. This changed through the early 1960s with the introduction of mass-produced core memory and core rope, which was far less expensive that dedicated logic based on diode arrays or similar solutions. The first to take real advantage of this was IBM in their 1964 System/360 series. This allowed the machines to have a very complex instruction set, including operations that matched high-level language constructs like formatting binary values as decimal strings, storing the complex series of instructions needed for this task in low cost memory."
+4797,"But the real value in the 360 line was that one could build a series of machines that were completely different internally, yet run the same ISA. For a low-end machine, one might use an 8-bit ALU that requires multiple cycles to complete a single 32-bit addition, while a higher end machine might have a full 32-bit ALU that performs the same addition in a single cycle. These differences could be implemented in control logic, but the cost of implementing a completely different decoder for each machine would be prohibitive. Using microcode meant all that changed was the code in the ROM. For instance, one machine might include a floating point unit and thus its microcode for multiplying two numbers might be only a few lines line, whereas on the same machine without the FPU this would be a program that did the same using multiple additions, and all that changed was the ROM."
+4798,"The outcome of this design was that customers could use a low-end model of the family to develop their software, knowing that if more performance was ever needed, they could move to a faster version and nothing else would change. This lowered the barrier to entry and the 360 was a runaway success. By the end of the decade, the use of microcode was de rigueur across the mainframe industry."
+4799,"Early minicomputers were far too simple to require microcode, and were more similar to earlier mainframes in terms of their instruction sets and the way they were decoded. But it was not long before their designers began using more powerful integrated circuits that allowed for more complex ISAs. By the mid-1970s, most new minicomputers and superminicomputers were using microcode as well, such as most models of the PDP-11 and, most notably, most models of the VAX, which included high-level instruction not unlike those found in the 360."
+4800,"The same basic evolution occurred with microprocessors as well. Early designs were extremely simple, and even the more powerful 8-bit designs of the mid-1970s like the Zilog Z80 had instruction sets that were simple enough to be implemented in dedicated logic. By this time, the control logic could be patterned into the same die as the CPU, making the difference in cost between ROM and logic less of an issue. However, it was not long before these companies were also facing the problem of introducing higher-performance designs but still wanting to offer backward compatibility. Among early examples of microcode in micros was the Intel 8086."
+4801,"Among the ultimate implementations of microcode in microprocessors is the Motorola 68000. This offered a highly orthogonal instruction set with a wide variety of addressing modes, all implemented in microcode. This did not come without cost, according to early articles, about 20% of the chip's surface area  is the microcode system. and later estimates suggest approximately 23,000 of the systems 68,000 gates were part of the microcode system."
+4802,"While companies continued to compete on the complexity of their instruction sets, and the use of microcode to implement these was unquestioned, in the mid-1970s an internal project in IBM was raising serious questions about the entire concept. As part of a project to develop a high-performance all-digital telephone switch, a team led by John Cocke began examining huge volumes of performance data from their customer's 360  programs. This led them to notice a curious pattern: when the ISA presented multiple versions of an instruction, the compiler almost always used the simplest one, instead of the one most directly representing the code. They learned that this was because those instructions were always implemented in hardware, and thus run the fastest. Using the other instruction might offer higher performance on some machines, but there was no way to know what machine they were running on, that was the whole reason for using microcode in the first place, to hide these distinctions."
+4803,"The team came to a radical conclusion: ""Imposing microcode between a computer and its users imposes an expensive overhead in performing the most frequently executed instructions."""
+4804,"The result of this discovery was what is today known as the RISC concept. The complex microcode engine and its associated ROM is reduced or eliminated completely, and those circuits instead dedicated to things like additional registers or a wider ALU, which increases the performance of every program. When complex sequences of instructions are needed, this is left to the compiler, which is the entire purpose of using a compiler in the first place. The basic concept was soon picked up by university researchers in California, where simulations suggested such designs would trivially outperform even the fastest conventional designs. It was one such project, at the University of California, Berkeley, that introduced the term RISC."
+4805,"The industry responded to the concept of RISC with both confusion and hostility, including a famous dismissive article by the VAX team at Digital. A major point of contention was that implementing the instructions outside of the processor meant it would spend much more time reading those instructions from memory, thereby slowing overall performance no matter how fast the CPU itself ran. Proponents pointed out that simulations clearly showed the number of instructions was not much greater, especially when considering compiled code."
+4806,"The debate raged until the first commercial RISC designs emerged in the second half of the 1980s, which easily outperformed the most complex designs from other companies. By the late 1980s it was over; even DEC was abandoning microcode for their DEC Alpha designs, and CISC processors switched to using hardwired circuitry, rather than microcode, to perform many functions.  For example, the Intel 80486 uses hardwired circuitry to fetch and decode instructions, using microcode only to execute instructions; register-register move and arithmetic instructions required only one microinstruction, allowing them to be completed in one clock cycle.  The Pentium Pro's fetch and decode hardware fetches instructions and decodes them into series of micro-operations that are passed on to the execution unit, which schedules and executes the micro-operations, possibly doing so out-of-order.  Complex instructions are implemented by microcode that consists of pre-defined sequences of micro-operations."
+4807,"Some processor designs use machine code that runs in a special mode, with special instructions, available only in that mode, that have access to processor-dependent hardware, to implement some low-level features of the instruction set. The DEC Alpha, a pure RISC design, used PALcode to implement features such as translation lookaside buffer  miss handling and interrupt handling, as well as providing, for Alpha-based systems running OpenVMS, instructions requiring interlocked memory acces that are similar to instructions provided by the VAX architecture. CMOS IBM System/390 CPUs, starting with the G4 processor, and z/Architecture CPUs use millicode to implement some instructions."
+4808,"Each microinstruction in a microprogram provides the bits that control the functional elements that internally compose a CPU. The advantage over a hard-wired CPU is that internal CPU control becomes a specialized form of a computer program. Microcode thus transforms a complex electronic design challenge  into a less complex programming challenge. To take advantage of this, a CPU is divided into several parts:"
+4809,"There may also be a memory address register and a memory data register, used to access the main computer storage. Together, these elements form an ""execution unit"". Most modern CPUs have several execution units. Even simple computers usually have one unit to read and write memory, and another to execute user code. These elements could often be brought together as a single chip. This chip comes in a fixed width that would form a ""slice"" through the execution unit. These are known as ""bit slice"" chips. The AMD Am2900 family is one of the best known examples of bit slice elements. The parts of the execution units and the whole execution units are interconnected by a bundle of wires called a bus."
+4810,"Programmers develop microprograms, using basic software tools. A microassembler allows a programmer to define the table of bits symbolically. Because of its close relationship to the underlying architecture, ""microcode has several properties that make it difficult to generate using a compiler."" A simulator program is intended to execute the bits in the same way as the electronics, and allows much more freedom to debug the microprogram. After the microprogram is finalized, and extensively tested, it is sometimes used as the input to a computer program that constructs logic to produce the same data. This program is similar to those used to optimize a programmable logic array. Even without fully optimal logic, heuristically optimized logic can vastly reduce the number of transistors from the number needed for a read-only memory  control store. This reduces the cost to produce, and the electricity used by, a CPU."
+4811,"Microcode can be characterized as horizontal or vertical, referring primarily to whether each microinstruction controls CPU elements with little or no decoding  or requires extensive decoding by combinatorial logic before doing so . Consequently, each horizontal microinstruction is wider  and occupies more storage space than a vertical microinstruction."
+4812,"""Horizontal microcode has several discrete micro-operations that are combined in a single microinstruction for simultaneous operation."" Horizontal microcode is typically contained in a fairly wide control store; it is not uncommon for each word to be 108 bits or more. On each tick of a sequencer clock a microcode word is read, decoded, and used to control the functional elements that make up the CPU."
+4813,"In a typical implementation a horizontal microprogram word comprises fairly tightly defined groups of bits. For example, one simple arrangement might be:"
+4814,"For this type of micromachine to implement a JUMP instruction with the address following the opcode, the microcode might require two clock ticks. The engineer designing it would write microassembler source code looking something like this:"
+4815,"For each tick it is common to find that only some portions of the CPU are used, with the remaining groups of bits in the microinstruction being no-ops. With careful design of hardware and microcode, this property can be exploited to parallelise operations that use different areas of the CPU; for example, in the case above, the ALU is not required during the first tick, so it could potentially be used to complete an earlier arithmetic instruction."
+4816,"In vertical microcode, each microinstruction is significantly encoded, that is, the bit fields generally pass through intermediate combinatory logic that, in turn, generates the control and sequencing signals for internal CPU elements . This is in contrast with horizontal microcode, in which the bit fields either directly produce the control and sequencing signals or are only minimally encoded. Consequently, vertical microcode requires smaller instruction lengths and less storage, but requires more time to decode, resulting in a slower CPU clock."
+4817,"Some vertical microcode is just the assembly language of a simple conventional computer that is emulating a more complex computer. Some processors, such as DEC Alpha processors and the CMOS microprocessors on later IBM mainframes System/390 and z/Architecture, use machine code, running in a special mode that gives it access to special instructions, special registers, and other hardware resources unavailable to regular machine code, to implement some instructions and other functions, such as page table walks on Alpha processors.  This is called PALcode on Alpha processors and millicode on IBM mainframe processors."
+4818,Another form of vertical microcode has two fields:
+4819,"The field select selects which part of the CPU will be controlled by this word of the control store. The field value controls that part of the CPU. With this type of microcode, a designer explicitly chooses to make a slower CPU to save money by reducing the unused bits in the control store; however, the reduced complexity may increase the CPU's clock frequency, which lessens the effect of an increased number of cycles per instruction."
+4820,"As transistors grew cheaper, horizontal microcode came to dominate the design of CPUs using microcode, with vertical microcode being used less often."
+4821,"When both vertical and horizontal microcode are used, the horizontal microcode may be referred to as nanocode or picocode."
+4822,"A few computers were built using writable microcode. In this design, rather than storing the microcode in ROM or hard-wired logic, the microcode is stored in a RAM called a writable control store or WCS. Such a computer is sometimes called a writable instruction set computer ."
+4823,"Many experimental prototype computers use writable control stores; there are also commercial machines that use writable microcode, such as the Burroughs Small Systems, early Xerox workstations, the DEC VAX 8800  family, the Symbolics L- and G-machines, a number of IBM System/360 and System/370 implementations, some DEC PDP-10 machines, and the Data General Eclipse MV/8000."
+4824,"The IBM System/370 includes a facility called Initial-Microprogram Load  that can be invoked from the console, as part of power-on reset  or from another processor in a tightly coupled multiprocessor complex."
+4825,"Some commercial machines, for example IBM 360/85, have both a read-only storage and a writable control store for microcode."
+4826,"WCS offers several advantages including the ease of patching the microprogram and, for certain hardware generations, faster access than ROMs can provide. User-programmable WCS allows the user to optimize the machine for specific purposes."
+4827,"Starting with the Pentium Pro in 1995, several x86 CPUs have writable Intel Microcode. This, for example, has allowed bugs in the Intel Core 2 and Intel Xeon microcodes to be fixed by patching their microprograms, rather than requiring the entire chips to be replaced. A second prominent example is the set of microcode patches that Intel offered for some of their processor architectures of up to 10 years in age, in a bid to counter the security vulnerabilities discovered in their designs – Spectre and Meltdown – which went public at the start of 2018. A microcode update can be installed by Linux, FreeBSD, Microsoft Windows, or the motherboard BIOS."
+4828,"Some machines offer user-programmable writable control stores as an option, including the HP 2100, DEC PDP-11/60, TI-990/12, and Varian Data Machines V-70 series minicomputers."
+4829,The design trend toward heavily microcoded processors with complex instructions began in the early 1960s and continued until roughly the mid-1980s. At that point the RISC design philosophy started becoming more prominent.
+4830,"A CPU that uses microcode generally takes several clock cycles to execute a single instruction, one clock cycle for each step in the microprogram for that instruction. Some CISC processors include instructions that can take a very long time to execute. Such variations interfere with both interrupt latency and, what is far more important in modern systems, pipelining."
+4831,"When designing a new processor, a hardwired control RISC has the following advantages over microcoded CISC:"
+4832,There are counterpoints as well:
+4833,"Many RISC and VLIW processors are designed to execute every instruction  in a single cycle. This is very similar to the way CPUs with microcode execute one microinstruction per cycle. VLIW processors have instructions that behave similarly to very wide horizontal microcode, although typically without such fine-grained control over the hardware as provided by microcode. RISC instructions are sometimes similar to the narrow vertical microcode."
+4834,"Microcode has been popular in application-specific processors such as network processors, digital signal processors, channel controllers, disk controllers, network interface controllers, flash memory controllers, graphics processing units, and in other hardware."
+4835,"Modern CISC implementations, such as the x86 family, decode instructions into dynamically buffered micro-operations with an instruction encoding similar to RISC or traditional microcode. A hardwired instruction decode unit directly emits microoperations for common x86 instructions, but falls back to a more traditional microcode ROM containing microoperations for more complex or rarely used instructions."
+4836,"For example, an x86 might look up microoperations from microcode to handle complex multistep operations such as loop or string instructions, floating-point unit transcendental functions or unusual values such as denormal numbers, and special-purpose instructions such as CPUID."
+4837,"Usually the addresses are generated by some combination of a counter, a field from a microinstruction, and some subset of the instruction register.  A counter is used for the typical case, that the next microinstruction is the one to execute.  A field from the microinstruction is used for jumps, or other logic."
+4838,"Since CPUs implement an instruction set, it's very useful to be able to decode the instruction's bits directly into the sequencer, to select a set of microinstructions to perform a CPU's instructions."
+4839,"Most modern CISC processors use a combination of pipelined logic to process lower complexity opcodes which can be completed in one clock cycle, and microcode to implement ones that take multiple clock cycles to complete."
+4840,"One of the first integrated microcoded processors was the IBM PALM Processor, which emulated all of the processor's instruction in microcode and was used on the IBM 5100, one of the first personal computers."
+4841,"Recent examples of similar open-sourced microsequencer-based processors are the MicroCore Labs MCL86,  MCL51 , and MCL65 cores which emulate the Intel 8086/8088, 8051 and MOS 6502 instruction sets entirely in microcode."
+4842,"The Digital Scientific Corp. Meta 4 Series 16 computer system was a user-microprogrammable system first available in 1970. Branches in the microcode sequence occur in
+one of three ways."
+4843,"One more sequencing option allowed on a branch instruction is the execute  option. When specified, the single instruction at the branch address is executed, but then execution continues after the original branch instruction. The IX option can be used with the XQ option."
+4844,"The IBM System/360 was a series of compatible computers introduced in 1964, many of which were microprogrammed. The System/360 Model 40 is a good example of a microprogrammed machine with complex microsequencing."
+4845,"The microstore consists of 4,096 56-bit microinstructions that operate in
+a horizontal microprogramming style. The store is addressed by the 12-bit
+read-only address register . Unlike most registers in the
+S/360 architecture, bits in the ROAR are numbered from bit 0 on the right
+to bit 11 on the left."
+4846,"The model 40 performs no sequential execution of microinstructions and
+therefore the microsequencer doesn't really branch in the conventional
+sense. Instead, each microinstruction specifies the address of the next one
+to be executed. Four fields in the microinstruction contribute to the new
+address."
+4847,There are essentially three combinations or formats of these fields.
+4848,"When the CB field contains 15, a functional branch occurs. The bits of
+the new microstore address in the ROAR are determined as follows."
+4849,"The CC field can specify various tests of the state of the machine. It can
+also specify a constant 0 or 1 for an unconditional bit."
+4850,"This format alters the flow of control to 1 of 16 instruction pairs
+within the low 32 words of a 64-word block of microstore .  The CC field then determines which instruction of the pair
+receives control."
+4851,"When the CD field is 0, 1, or 3, flow of control is directed to an
+instruction within the current 64-word block. The bits of the new microstore
+address are determined as follows."
+4852,"The CA field selects 1 of 16 4-word groups within the current 64-word block.
+The CB and CC fields then determine which instruction of the 4 receives
+control."
+4853,"When the CD field is 2, flow of control is directed in a nonobvious manner. The
+bits of the new microstore address are determined as follows:"
+4854,"The next instruction is in the same 1K-word region as the current
+instruction, because bits 11–10 remain the same. The CA field determines
+the 64-word block within the region. The instruction is in the same 4-word
+group within the new block as the current instruction is within the current
+block, because bits 5–2 remain the same.
+The CB and CC fields then determine which instruction of the 4 receives
+control."
+4855,This description has been simplified. It ignores the following features.
+4856,"Almost all computers, whether load/store architecture or not, load items of data from a larger memory into registers where they are used for arithmetic operations, bitwise operations, and other operations, and are manipulated or tested by machine instructions. Manipulated items are then often stored back to main memory, either by the same instruction or by a subsequent one. Modern processors use either static or dynamic RAM as main memory, with the latter usually accessed via one or more cache levels."
+4857,"Processor registers are normally at the top of the memory hierarchy, and provide the fastest way to access data. The term normally refers only to the group of registers that are directly encoded as part of an instruction, as defined by the instruction set. However, modern high-performance CPUs often have duplicates of these ""architectural registers"" in order to improve performance via register renaming, allowing parallel and speculative execution. Modern x86 design acquired these techniques around 1995 with the releases of Pentium Pro, Cyrix 6x86, Nx586, and AMD K5."
+4858,"When a computer program accesses the same data repeatedly, this is called locality of reference.  Holding frequently used values in registers can be critical to a program's performance. Register allocation is performed either by a compiler in the code generation phase, or manually by an assembly language programmer."
+4859,"Registers are normally measured by the number of bits they can hold, for example, an ""8-bit register"", ""32-bit register"", ""64-bit register"", or even more. In some instruction sets, the registers can operate in various modes, breaking down their storage memory into smaller parts  to which multiple data  can be loaded and operated upon at the same time. Typically it is implemented by adding extra registers that map their memory into a larger register. Processors that have the ability to execute single instructions on multiple data are called vector processors."
+4860,"A processor often contains several kinds of registers, which can be classified according to the types of values they can store or the instructions that operate on them:"
+4861,"Hardware registers are similar, but occur outside CPUs."
+4862,"In some architectures , the first or last register in the integer register file is a pseudo-register in that it is hardwired to always return zero when read , and it cannot be overwritten. In Alpha, this is also done for the floating-point register file. As a result of this, register files are commonly quoted as having one register more than how many of them are actually usable; for example, 32 registers are quoted when only 31 of them fit within the above definition of a register."
+4863,"The following table shows the number of registers in several mainstream CPU architectures. Note that in x86-compatible processors, the stack pointer  is counted as an integer register, even though there are a limited number of instructions that may be used to operate on its contents. Similar caveats apply to most architectures."
+4864,"Although all of the below-listed architectures are different, almost all are in a basic arrangement known as the von Neumann architecture, first proposed by the Hungarian-American mathematician John von Neumann. It is also noteworthy that the number of registers on GPUs is much higher than that on CPUs."
+4865,The number of registers available on a processor and the operations that can be performed using those registers has a significant impact on the efficiency of code generated by optimizing compilers. The Strahler number of an expression tree gives the minimum number of registers required to evaluate that expression tree.
+4866,"The program counter , commonly called the instruction pointer  in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register , the instruction counter, or just part of the instruction sequencer, is a processor register that indicates where a computer is in its program sequence."
+4867,"Usually, the PC is incremented after fetching an instruction, and holds the memory address of  the next instruction that would be executed."
+4868,"Processors usually fetch instructions sequentially from memory, but control transfer instructions change the sequence by placing a new value in the PC. These include branches , subroutine calls, and returns. A transfer that is conditional on the truth of some assertion lets the computer follow a different sequence under different conditions."
+4869,"A branch provides that the next instruction is fetched from elsewhere in memory. A subroutine call not only branches but saves the preceding contents of the PC somewhere. A return retrieves the saved contents of the PC and places it back in the PC, resuming sequential execution with the instruction following the subroutine call."
+4870,"In a simple central processing unit , the PC is a digital counter  that may be one of several hardware registers. The instruction cycle begins with a fetch, in which the CPU places the value of the PC on the address bus to send it to the memory. The memory responds by sending the contents of that memory location on the data bus.  Following the fetch, the CPU proceeds to execution, taking some action based on the memory contents that it obtained. At some point in this cycle, the PC will be modified so that the next instruction executed is a different one ."
+4871,"Like other processor registers, the PC may be a bank of binary latches, each one representing one bit of the value of the PC. The number of bits  relates to the processor architecture. For instance, a “32-bit” CPU may use 32 bits to be able to address 232 units of memory. On some processors, the width of program counter instead depends on the addressable memory; for example, some AVR microcontrollers have a PC which wraps around after 12 bits."
+4872,"If the PC is a binary counter, it may increment when a pulse is applied to its COUNT UP input, or the CPU may compute some other value and load it into the PC by a pulse to its LOAD input."
+4873,"To identify the current instruction, the PC may be combined with other registers that identify a segment or page. This approach permits a PC with fewer bits by assuming that most memory units of interest are within the current vicinity."
+4874,"Use of a PC that normally increments assumes that what a computer does is execute a usually linear sequence of instructions.  Such a PC is central to the von Neumann architecture. Thus programmers write a sequential control flow even for algorithms that do not have to be sequential.  The resulting “von Neumann bottleneck” led to research into parallel computing, including non-von Neumann or dataflow models that did not use a PC; for example, rather than specifying sequential steps, the high-level programmer might specify desired function and the low-level programmer might specify this using combinatory logic."
+4875,"This research also led to ways to making conventional, PC-based, CPUs run faster, including:"
+4876,"Modern high-level programming languages still follow the sequential-execution model and, indeed, a common way of identifying programming errors is with a “procedure execution” in which the programmer's finger identifies the point of execution as a PC would.  The high-level language is essentially the machine language of a virtual machine, too complex to be built as hardware but instead emulated or interpreted by software."
+4877,"However, new programming models transcend sequential-execution programming:"
+4878,"The design process involves choosing an instruction set and a certain execution paradigm  and results in a microarchitecture, which might be described in e.g. VHDL or Verilog. For microprocessor design, this description is then manufactured employing some of the various semiconductor device fabrication processes, resulting in a die which is bonded onto a chip carrier. This chip carrier is then soldered onto, or inserted into a socket on, a printed circuit board ."
+4879,"The mode of operation of any processor is the execution of lists of instructions. Instructions typically include those to compute or manipulate data values using registers, change or retrieve values in read/write memory, perform relational tests between data values and to control program flow."
+4880,Processor designs are often tested and validated on one or several FPGAs before sending the design of the processor to a foundry for semiconductor fabrication.
+4881,"CPU design is divided into multiple components. Information is transferred through datapaths . These datapaths are controlled through logic by control units. Memory components include register files and caches to retain information, or certain actions. Clock circuitry maintains internal rhythms and timing through clock drivers, PLLs, and clock distribution networks. Pad transceiver circuitry with allows signals to be received and sent and a logic gate cell library which is used to implement the logic."
+4882,"CPUs designed for high-performance markets might require custom ) designs for each of these items to achieve frequency, power-dissipation, and chip-area goals whereas CPUs designed for lower performance markets might lessen the implementation burden by acquiring some of these items by purchasing them as intellectual property. Control logic implementation techniques  can be used to implement datapaths, register files, and clocks. Common logic styles used in CPU design include unstructured random logic, finite-state machines, microprogramming , and Programmable logic arrays ."
+4883,Device types used to implement the logic include:
+4884,A CPU design project generally has these major tasks:
+4885,"Re-designing a CPU core to a smaller die area helps to shrink everything , resulting in the same number of transistors on a smaller die. It improves performance , reduces power  and reduces cost . Releasing a CPU on the same size die, but with a smaller CPU core, keeps the cost about the same but allows higher levels of integration within one very-large-scale integration chip , improving performance and reducing overall system cost."
+4886,"As with most complex electronic designs, the logic verification effort  now dominates the project schedule of a CPU."
+4887,"Key CPU architectural innovations include index register, cache, virtual memory, instruction pipelining, superscalar, CISC, RISC, virtual machine, emulators, microprogram, and stack."
+4888,"A variety of new CPU design ideas have been proposed,
+including reconfigurable logic, clockless CPUs, computational RAM, and optical computing."
+4889,"Benchmarking is a way of testing CPU speed. Examples include SPECint and SPECfp, developed by Standard Performance Evaluation Corporation, and ConsumerMark developed by the Embedded Microprocessor Benchmark Consortium EEMBC."
+4890,Some of the commonly used metrics include:
+4891,"There may be tradeoffs in optimizing some of these metrics.  In particular, many design techniques that make a CPU run faster make the ""performance per watt"", ""performance per dollar"", and ""deterministic response"" much worse, and vice versa."
+4892,"There are several different markets in which CPUs are used. Since each of these markets differ in their requirements for CPUs, the devices designed for one market are in most cases inappropriate for the other markets."
+4893,"As of 2010, in the general-purpose computing market, that is, desktop, laptop, and server computers commonly used in businesses and homes, the Intel IA-32 and the 64-bit version x86-64 architecture dominate the market, with its rivals PowerPC and SPARC maintaining much smaller customer bases. Yearly, hundreds of millions of IA-32 architecture CPUs are used by this market.  A growing percentage of these processors are for mobile implementations such as netbooks and laptops."
+4894,"Since these devices are used to run countless different types of programs, these CPU designs are not specifically targeted at one type of application or one function. The demands of being able to run a wide range of programs efficiently has made these CPU designs among the more advanced technically, along with some disadvantages of being relatively costly, and having high power consumption."
+4895,"In 1984, most high-performance CPUs required four to five years to develop."
+4896,"Scientific computing is a much smaller niche market .  It is used in government research labs and universities. Before 1990, CPU design was often done for this market, but mass market CPUs organized into large clusters have proven to be more affordable. The main remaining area of active hardware design and research for scientific computing is for high-speed data transmission systems to connect mass market CPUs."
+4897,"As measured by units shipped, most CPUs are embedded in other machinery, such as telephones, clocks, appliances, vehicles, and infrastructure. Embedded processors sell in the volume of many billions of units per year, however, mostly at much lower price points than that of the general purpose processors."
+4898,These single-function devices differ from the more familiar general-purpose CPUs in several ways:
+4899,"The embedded CPU family with the largest number of total units shipped is the 8051, averaging nearly a billion units per year.  The 8051 is widely used because it is very inexpensive.  The design time is now roughly zero, because it is widely available as commercial intellectual property.  It is now often embedded as a small part of a larger system on a chip.  The silicon cost of an 8051 is now as low as US$0.001, because some implementations use as few as 2,200 logic gates and take 0.4730 square millimeters of silicon."
+4900,"As of 2009, more CPUs are produced using the ARM architecture family instruction sets than any other 32-bit instruction set.
+The ARM architecture and the first ARM chip were designed in about one and a half years and 5 human years of work time."
+4901,The 32-bit Parallax Propeller microcontroller architecture and the first chip were designed by two people in about 10 human years of work time.
+4902,The 8-bit AVR architecture and first AVR microcontroller was conceived and designed by two students at the Norwegian Institute of Technology.
+4903,The 8-bit 6502 architecture and the first MOS Technology 6502 chip were designed in 13 months by a group of about 9 people.
+4904,"The 32-bit Berkeley RISC I and RISC II processors were mostly designed by a series of students as part of a four quarter sequence of graduate courses.
+This design became the basis of the commercial SPARC processor design."
+4905,"For about a decade, every student taking the 6.004 class at MIT was part of a team—each team had one semester to design and build a simple 8 bit CPU out of 7400 series integrated circuits.
+One team of 4 students designed and built a simple 32 bit CPU during that semester."
+4906,"Some undergraduate courses require a team of 2 to 5 students to design, implement, and test a simple CPU in a FPGA in a single 15-week semester."
+4907,"The MultiTitan CPU was designed with 2.5 man years of effort, which was considered ""relatively little design effort"" at the time.
+24 people contributed to the 3.5 year MultiTitan research project, which included designing and building a prototype CPU."
+4908,"For embedded systems, the highest performance levels are often not needed or desired due to the power consumption requirements. This allows for the use of processors which can be totally implemented by logic synthesis techniques. These synthesized processors can be implemented in a much shorter amount of time, giving quicker time-to-market."
+4909,"A graphics card  is a computer expansion card that generates a feed of graphics output to a display device such as a monitor. Graphics cards are sometimes called discrete or dedicated graphics cards to emphasize their distinction to an integrated graphics processor on the motherboard or the central processing unit . A graphics processing unit  that performs the necessary computations is the main component in a graphics card, but the acronym ""GPU"" is sometimes also used to erroneously refer to the graphics card as a whole."
+4910,"Most graphics cards are not limited to simple display output. The graphics processing unit can be used for additional processing, which reduces the load from the central processing unit. Additionally, computing platforms such as OpenCL and CUDA allow using graphics cards for general-purpose computing. Applications of general-purpose computing on graphics cards include AI training, cryptocurrency mining, and molecular simulation."
+4911,"Usually, a graphics card comes in the form of a printed circuit board  which is to be inserted into an expansion slot. Others may have dedicated enclosures, and they are connected to the computer via a docking station or a cable. These are known as external GPUs ."
+4912,Graphics cards are often preferred over integrated graphics for increased performance.
+4913,"Graphics cards historically supported different computer display standards as they evolved. For the IBM PC compatibles, common early standards were MDA, CGA, Hercules, EGA and VGA."
+4914,In the late 1980s the like of Radius produced graphics cards for the Apple Macintosh II with discrete 2D QuickDraw capabilities.
+4915,"3dfx Interactive was one of the first companies to develop a consumer-facing GPU with 3D acceleration  and the first to develop a graphical chipset dedicated to 3D, but without 2D support ."
+4916,NVIDIA RIVA 128 was one of the first consumer-facing GPU integrated 3D processing unit and 2D processing unit on a chip.
+4917,"The majority of modern graphics cards are built with either AMD-sourced or Nvidia-sourced graphics chips. Most graphics cards offer various functions such as 3D rendering, 2D graphics, video decoding, TV output, and the ability to connect multiple monitors . Graphics cards also have sound card capabilities to output sound along with video output for connected TVs or monitors with integrated speakers."
+4918,"Within the industry, graphics cards are sometimes called graphics add-in-boards, abbreviated as AIBs, with the word ""graphics"" usually omitted."
+4919,"As an alternative to the use of a graphics card, video hardware can be integrated into the motherboard, CPU, or a system-on-chip as integrated graphics. Motherboard-based implementations are sometimes called ""on-board video"". Some motherboards support using both integrated graphics and the graphics card simultaneously to feed separate displays. The main advantages of integrated graphics are: a low cost, compactness, simplicity, and low energy consumption. Integrated graphics often has less performance than a graphics card because the graphics processing unit inside integrated graphics needs to share system resources with the CPU. On the other hand, a graphics card has a separate random access memory , cooling system, and dedicated power regulators. A graphics card can offload work and reduce memory-bus-contention from the CPU and system RAM, therefore the overall performance for a computer could improve in addition to increased performance in graphics processing. Such improvements to performance can be seen in video gaming, 3D animation, and video editing."
+4920,"Both AMD and Intel have introduced CPUs and motherboard chipsets which support the integration of a GPU into the same die as the CPU. AMD advertises CPUs with integrated graphics under the trademark Accelerated Processing Unit , while Intel brands similar technology under ""Intel Graphics Technology""."
+4921,"As the processing power of graphics cards increased, so did their demand for electrical power. Current high-performance graphics cards tend to consume large amounts of power. For example, the thermal design power  for the GeForce Titan RTX is 280 watts. When tested with video games, the GeForce RTX 2080 Ti Founder's Edition averaged 300 watts of power consumption. While CPU and power supply manufacturers have recently aimed toward higher efficiency, power demands of graphics cards continued to rise, with the largest power consumption of any individual part in a computer. Although power supplies have also increased their power output, the bottleneck occurs in the PCI-Express connection, which is limited to supplying 75 watts."
+4922,Modern graphics cards with a power consumption of over 75 watts usually include a combination of six-pin  or eight-pin  sockets that connect directly to the power supply. Providing adequate cooling becomes a challenge in such computers. Computers with multiple graphics cards may require power supplies over 750 watts. Heat extraction becomes a major design consideration for computers with two or more high-end graphics cards.
+4923,"As of the Nvidia GeForce RTX 30 series, Ampere architecture, a custom flashed RTX 3090 named ""Hall of Fame"" has been recorded to reach a peak power draw as high as 630 watts. A standard RTX 3090 can peak at up to 450 watts. The RTX 3080 can reach up to 350 watts, while a 3070 can reach a similar, if not slightly lower peak power draw. Ampere cards of the Founders Edition variant feature a ""dual axial flow through"" cooler design, which includes fans above and below the card to dissipate as much heat as possible towards the rear of the computer case. A similar design was used by the Sapphire Radeon RX Vega 56 Pulse graphics card."
+4924,"Graphics cards for desktop computers have different size profiles, which allows graphics cards to be added to smaller-sized computers. Some graphics cards are not of the usual size, and are named as ""low profile"". Graphics card profiles are based on height only, with low-profile cards taking up less than the height of a PCIe slot, some can be as low as ""half-height"". Length and thickness can vary greatly, with high-end cards usually occupying two or three expansion slots, and with modern high-end graphics cards such as the RTX 4090 exceeding 300mm in length. A lower profile card is preferred when trying to fit multiple cards or if graphics cards run into clearance issues with other motherboard components like the DIMM or PCIE slots. This can be fixed with a larger computer case such as mid-tower or full tower. Full towers are usually able to fit larger motherboards in sizes like ATX and micro ATX."
+4925,"Some graphics cards can be linked together to allow scaling graphics processing across multiple cards. This is done using either the PCIe bus on the motherboard or, more commonly, a data bridge. Usually, the cards must be of the same model to be linked, and most low end cards are not able to be linked in this way. AMD and Nvidia both have proprietary scaling methods, CrossFireX for AMD, and SLI  for Nvidia. Cards from different chip-set manufacturers or architectures cannot be used together for multi-card scaling. If graphics cards have different sizes of memory, the lowest value will be used, with the higher values disregarded. Currently, scaling on consumer-grade cards can be done using up to four cards. The use of four cards requires a large motherboard with a proper configuration. Nvidia's GeForce GTX 590 graphics card can be configured in a four-card configuration. As stated above, users will want to stick to cards with the same performances for optimal use. Motherboards including ASUS Maximus 3 Extreme and Gigabyte GA EX58 Extreme are certified to work with this configuration. A large power supply is necessary to run the cards in SLI or CrossFireX. Power demands must be known before a proper supply is installed. For the four card configuration, a 1000+ watt supply is needed. With any relatively powerful graphics card, thermal management cannot be ignored. Graphics cards require well-vented chassis and good thermal solutions. Air or water cooling are usually required, though low end GPUs can use passive cooling. Larger configurations use water solutions or immersion cooling to achieve proper performance without thermal throttling."
+4926,"SLI and Crossfire have become increasingly uncommon as most games do not fully utilize multiple GPUs, due to the fact that most users cannot afford them. Multiple GPUs are still used on supercomputers , on workstations to accelerate video and 3D rendering, visual effects, for simulations, and for training artificial intelligence."
+4927,"A graphics driver usually supports one or multiple cards by the same vendor and has to be written for a specific operating system. Additionally, the operating system or an extra software package may provide certain programming APIs for applications to perform 3D rendering."
+4928,Some GPUs are designed with specific usage in mind:
+4929,"Gaming
+GeForce GTX
+GeForce RTX
+Nvidia Titan
+Radeon HD
+Radeon RX
+Intel Arc"
+4930,GeForce GTX
+4931,GeForce RTX
+4932,Nvidia Titan
+4933,Radeon HD
+4934,Radeon RX
+4935,Intel Arc
+4936,"Cloud gaming
+Nvidia Grid
+Radeon Sky"
+4937,Nvidia Grid
+4938,Radeon Sky
+4939,"Workstation
+Nvidia Quadro
+AMD FirePro
+Radeon Pro
+Intel Arc Pro"
+4940,Nvidia Quadro
+4941,AMD FirePro
+4942,Radeon Pro
+4943,Intel Arc Pro
+4944,"Cloud Workstation
+Nvidia Tesla
+AMD FireStream"
+4945,Nvidia Tesla
+4946,AMD FireStream
+4947,"Artificial Intelligence Cloud
+Nvidia Tesla
+Radeon Instinct"
+4948,Nvidia Tesla
+4949,Radeon Instinct
+4950,"Automated/Driverless car
+Nvidia Drive PX"
+4951,Nvidia Drive PX
+4952,"As of 2016, the primary suppliers of the GPUs  used in graphics cards are AMD and Nvidia. In the third quarter of 2013, AMD had a 35.5% market share while Nvidia had 64.5%, according to Jon Peddie Research. In economics, this industry structure is termed a duopoly. AMD and Nvidia also build and sell graphics cards, which are termed graphics add-in-boards  in the industry.  In addition to marketing their own graphics cards, AMD and Nvidia sell their GPUs to authorized AIB suppliers, which AMD and Nvidia refer to as ""partners"". The fact that Nvidia and AMD compete directly with their customer/partners complicates relationships in the industry. AMD and Intel being direct competitors in the CPU industry is also noteworthy, since AMD-based graphics cards may be used in computers with Intel CPUs. Intel's integrated graphics may weaken AMD, in which the latter derives a significant portion of its revenue from its APUs. As of the second quarter of 2013, there were 52 AIB suppliers. These AIB suppliers may market graphics cards under their own brands, produce graphics cards for private label brands, or produce graphics cards for computer manufacturers. Some AIB suppliers such as MSI build both AMD-based and Nvidia-based graphics cards. Others, such as EVGA, build only Nvidia-based graphics cards, while XFX, now builds only AMD-based graphics cards. Several AIB suppliers are also motherboard suppliers. Most of the largest AIB suppliers are based in Taiwan and they include ASUS, MSI, GIGABYTE, and Palit. Hong Kong–based AIB manufacturers include Sapphire and Zotac. Sapphire and Zotac also sell graphics cards exclusively for AMD and Nvidia GPUs respectively."
+4953,"Graphics card shipments peaked at a total of 114 million in 1999. By contrast, they totaled 14.5 million units in the third quarter of 2013, a 17% fall from Q3 2012 levels. Shipments reached an annual total of 44 million in 2015. The sales of graphics cards have trended downward due to improvements in integrated graphics technologies; high-end, CPU-integrated graphics can provide competitive performance with low-end graphics cards. At the same time, graphics card sales have grown within the high-end segment, as manufacturers have shifted their focus to prioritize the gaming and enthusiast market."
+4954,"Beyond the gaming and multimedia segments, graphics cards have been increasingly used for general-purpose computing, such as big data processing. The growth of cryptocurrency has placed a severely high demand on high-end graphics cards, especially in large quantities, due to their advantages in the process of cryptocurrency mining. In January 2018, mid- to high-end graphics cards experienced a major surge in price, with many retailers having stock shortages due to the significant demand among this market. Graphics card companies released mining-specific cards designed to run 24 hours a day, seven days a week, and without video output ports. The graphics card industry took a setback due to the 2020–21 chip shortage."
+4955,A modern graphics card consists of a printed circuit board on which the components are mounted. These include:
+4956,"A graphics processing unit , also occasionally called visual processing unit , is a specialized electronic circuit designed to rapidly manipulate and alter memory to accelerate the building of images in a frame buffer intended for output to a display. Because of the large degree of programmable computational complexity for such a task, a modern graphics card is also a computer unto itself."
+4957,"A heat sink is mounted on most modern graphics cards. A heat sink spreads out the heat produced by the graphics processing unit evenly throughout the heat sink and unit itself. The heat sink commonly has a fan mounted to cool the heat sink and the graphics processing unit. Not all cards have heat sinks, for example, some cards are liquid-cooled and instead have a water block; additionally, cards from the 1980s and early 1990s did not produce much heat, and did not require heat sinks. Most modern graphics cards need proper thermal solutions. They can be water-cooled or through heat sinks with additional connected heat pipes usually made of copper for the best thermal transfer."
+4958,"The video BIOS or firmware contains a minimal program for the initial set up and control of the graphics card. It may contain information on the memory and memory timing, operating speeds and voltages of the graphics processor, and other details which can sometimes be changed."
+4959,"Modern Video BIOSes do not support full functionalities of graphics cards; they are only sufficient to identify and initialize the card to display one of a few frame buffer or text display modes. It does not support YUV to RGB translation, video scaling, pixel copying, compositing or any of the multitude of other 2D and 3D features of the graphics card, which must be accessed by software drivers."
+4960,"The memory capacity of most modern graphics cards ranges from 2 to 24 GB. But with up to 32 GB as of the last 2010s, the applications for graphics use are becoming more powerful and widespread. Since video memory needs to be accessed by the GPU and the display circuitry, it often uses special high-speed or multi-port memory, such as VRAM, WRAM, SGRAM, etc. Around 2003, the video memory was typically based on DDR technology. During and after that year, manufacturers moved towards DDR2, GDDR3, GDDR4, GDDR5, GDDR5X, and GDDR6. The effective memory clock rate in modern cards is generally between 2 and 15 GHz."
+4961,"Video memory may be used for storing other data as well as the screen image, such as the Z-buffer, which manages the depth coordinates in 3D graphics, as well as textures, vertex buffers, and compiled shader programs."
+4962,"The RAMDAC, or random-access-memory digital-to-analog converter, converts digital signals to analog signals for use by a computer display that uses analog inputs such as cathode-ray tube  displays. The RAMDAC is a kind of RAM chip that regulates the functioning of the graphics card. Depending on the number of bits used and the RAMDAC-data-transfer rate, the converter will be able to support different computer-display refresh rates. With CRT displays, it is best to work over 75 Hz and never under 60 Hz, to minimize flicker.  Due to the growing popularity of digital computer displays and the integration of the RAMDAC onto the GPU die, it has mostly disappeared as a discrete component. All current LCD/plasma monitors and TVs and projectors with only digital connections work in the digital domain and do not require a RAMDAC for those connections. There are displays that feature analog inputs  only. These require a RAMDAC, but they reconvert the analog signal back to digital before they can display it, with the unavoidable loss of quality stemming from this digital-to-analog-to-digital conversion. With the VGA standard being phased out in favor of digital formats, RAMDACs have started to disappear from graphics cards."
+4963,The most common connection systems between the graphics card and the computer display are:
+4964,"Also known as D-sub, VGA is an analog-based standard adopted in the late 1980s designed for CRT displays, also called VGA connector. Today, the VGA analog interface is used for high definition video resolutions including 1080p and higher. Some problems of this standard are electrical noise, image distortion and sampling error in evaluating pixels. While the VGA transmission bandwidth is high enough to support even higher resolution playback, the picture quality can degrade depending on cable quality and length. The extent of quality difference depends on the individual's eyesight and the display; when using a DVI or HDMI connection, especially on larger sized LCD/LED monitors or TVs, quality degradation, if present, is prominently visible. Blu-ray playback at 1080p is possible via the VGA analog interface, if Image Constraint Token  is not enabled on the Blu-ray disc."
+4965,"Digital Visual Interface is a digital-based standard designed for displays such as flat-panel displays  and video projectors. There were also some rare high-end CRT monitors that use DVI. It avoids image distortion and electrical noise, corresponding each pixel from the computer to a display pixel, using its native resolution. It is worth noting that most manufacturers include a DVI-I connector, allowing  standard RGB signal output to an old CRT or LCD monitor with VGA input."
+4966,"These connectors are included to allow connection with televisions, DVD players, video recorders and video game consoles. They often come in two 10-pin mini-DIN connector variations, and the VIVO splitter cable generally comes with either 4 connectors , or 6 connectors ."
+4967,"HDMI is a compact audio/video interface for transferring uncompressed video data and compressed/uncompressed digital audio data from an HDMI-compliant device  to a compatible digital audio device, computer monitor, video projector, or digital television. HDMI is a digital replacement for existing analog video standards. HDMI supports copy protection through HDCP."
+4968,"DisplayPort is a digital display interface developed by the Video Electronics Standards Association . The interface is primarily used to connect a video source to a display device such as a computer monitor, though it can also be used to transmit audio, USB, and other forms of data.
+The VESA specification is royalty-free. VESA designed it to replace VGA, DVI, and LVDS. Backward compatibility to VGA and DVI by using adapter dongles enables consumers to use DisplayPort fitted video sources without replacing existing display devices. Although DisplayPort has a greater throughput of the same functionality as HDMI, it is expected to complement the interface, not replace it."
+4969,"Chronologically, connection systems between graphics card and motherboard were, mainly:"
+4970,The following table is a comparison between features of some interfaces listed above.
+4971,Microphone via one of:
+4972,A sound card  is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term sound card is also applied to external audio interfaces used for professional audio applications.
+4973,"Sound functionality can also be integrated onto the motherboard, using components similar to those found on plug-in cards. The integrated sound system is often still referred to as a sound card. Sound processing hardware is also present on modern video cards with HDMI to output sound along with the video using that connector; previously they used a S/PDIF connection to the motherboard or sound card."
+4974,"Typical uses of sound cards or sound card functionality include providing the audio component for multimedia applications such as music composition, editing video or audio, presentation, education and entertainment  and video projection. Sound cards are also used for computer-based communication such as voice over IP and teleconferencing."
+4975,"Sound cards use a digital-to-analog converter , which converts recorded or generated digital signal data into an analog format. The output signal is connected to an amplifier, headphones, or external device using standard interconnects, such as a TRS phone connector."
+4976,"A common external connector is the microphone connector. Input through a microphone connector can be used, for example, by speech recognition or voice over IP applications. Most sound cards have a line in connector for an analog input from a sound source that has higher voltage levels than a microphone. In either case, the sound card uses an analog-to-digital converter  to digitize this signal."
+4977,"Some cards include a sound chip to support the production of synthesized sounds, usually for real-time generation of music and sound effects using minimal data and CPU time."
+4978,"The card may use direct memory access to transfer the samples to and from main memory, from where a recording and playback software may read and write it to the hard disk for storage, editing, or further processing."
+4979,"An important sound card characteristic is polyphony, which refers to its ability to process and output multiple independent voices or sounds simultaneously. These distinct channels are seen as the number of audio outputs, which may correspond to a speaker configuration such as 2.0 , 2.1 , 5.1 , or other configurations. Sometimes, the terms voice and channel are used interchangeably to indicate the degree of polyphony, not the output speaker configuration. For example, much older sound chips could accommodate three voices, but only one output audio channel , requiring all voices to be mixed together. Later cards, such as the AdLib sound card, had a 9-voice polyphony combined in 1 mono output channel."
+4980,"Early PC sound cards had multiple FM synthesis voices  which were used for MIDI music. The full capabilities of advanced cards are often not fully used; only one  or two  voice and channel are usually dedicated to playback of digital sound samples, and playing back more than one digital sound sample usually requires a software downmix at a fixed sampling rate. Modern low-cost integrated sound cards  such as audio codecs like those meeting the AC'97 standard and even some lower-cost expansion sound cards still work this way. These devices may provide more than two sound output channels , but they usually have no actual hardware polyphony for either sound effects or MIDI reproduction –  these tasks are performed entirely in software. This is similar to the way inexpensive softmodems perform modem tasks in software rather than in hardware."
+4981,"In the early days of wavetable synthesis, some sound card manufacturers advertised polyphony solely on the MIDI capabilities alone. In this case, typically, the card is only capable of two channels of digital sound and the polyphony specification solely applies to the number of MIDI instruments the sound card is capable of producing at once."
+4982,"Modern sound cards may provide more flexible audio accelerator capabilities which can be used in support of higher levels of polyphony or other purposes such as hardware acceleration of 3D sound, positional audio and real-time DSP effects."
+4983,"Connectors on the sound cards are color-coded as per the PC System Design Guide. They may also have symbols of arrows, holes and soundwaves that are associated with each jack position."
+4984,"Sound cards for IBM PC compatible computers were very uncommon until 1988. For the majority IBM PC users, the internal PC speaker was the only way for early PC software to produce sound and music. The speaker hardware was typically limited to square waves. The resulting sound was generally described as ""beeps and boops"" which resulted in the common nickname beeper. Several companies, most notably Access Software, developed techniques for digital sound reproduction over the PC speaker like RealSound. The resulting audio, while functional, suffered from the heavily distorted output and low volume, and usually required all other processing to be stopped while sounds were played. Other home computers of the 1980s like the Commodore 64 included hardware support for digital sound playback or music synthesis, leaving the IBM PC at a disadvantage when it came to multimedia applications. Early sound cards for the IBM PC platform were not designed for gaming or multimedia applications, but rather on specific audio applications, such as music composition with the AdLib Personal Music System, IBM Music Feature Card, and Creative Music System, or on speech synthesis like Digispeech DS201, Covox Speech Thing, and Street Electronics Echo."
+4985,"In 1988, a panel of computer-game CEOs stated at the Consumer Electronics Show that the PC's limited sound capability prevented it from becoming the leading home computer, that it needed a $49–79 sound card with better capability than current products, and that once such hardware was widely installed, their companies would support it. Sierra On-Line, which had pioneered supporting EGA and VGA video, and 3-1/2"" disks, promised that year to support the AdLib, IBM Music Feature, and Roland MT-32 sound cards in its games. A 1989 Computer Gaming World survey found that 18 of 25 game companies planned to support AdLib, six Roland and Covox, and seven Creative Music System/Game Blaster."
+4986,"One of the first manufacturers of sound cards for the IBM PC was AdLib, which produced a card based on the Yamaha YM3812 sound chip, also known as the OPL2. The AdLib had two modes: A 9-voice mode where each voice could be fully programmed, and a less frequently used percussion mode with 3 regular voices producing 5 independent percussion-only voices for a total of 11."
+4987,"Creative Labs also marketed a sound card about the same time called the Creative Music System . Although the C/MS had twelve voices to AdLib's nine, and was a stereo card while the AdLib was mono, the basic technology behind it was based on the Philips SAA1099 chip which was essentially a square-wave generator. It sounded much like twelve simultaneous PC speakers would have except for each channel having amplitude control, and failed to sell well, even after Creative renamed it the Game Blaster a year later, and marketed it through RadioShack in the US. The Game Blaster retailed for under $100 and was compatible with many popular games, such as Silpheed."
+4988,"A large change in the IBM PC-compatible sound card market happened when Creative Labs introduced the Sound Blaster card. Recommended by Microsoft to developers creating software based on the Multimedia PC standard, the Sound Blaster cloned the AdLib and added a sound coprocessor for recording and playback of digital audio. The card also included a game port for adding a joystick, and the capability to interface to MIDI equipment using the game port and a special cable. With AdLib compatibility and more features at nearly the same price, most buyers chose the Sound Blaster. It eventually outsold the AdLib and dominated the market."
+4989,"Roland also made sound cards in the late 1980s such as the MT-32 and LAPC-I. Roland cards sold for hundreds of dollars. Many games had music written for their cards, such as Silpheed and Police Quest II. The cards were often poor at sound effects such as laughs, but for music was by far the best sound cards available until the mid-nineties. Some Roland cards, such as the SCC, and later versions of the MT-32 were made to be less expensive."
+4990,"By 1992, one sound card vendor advertised that its product was ""Sound Blaster, AdLib, Disney Sound Source and Covox Speech Thing Compatible!"" Responding to readers complaining about an article on sound cards that unfavorably mentioned the Gravis Ultrasound, Computer Gaming World stated in January 1994 that, ""The de facto standard in the gaming world is Sound Blaster compatibility ... It would have been unfair to have recommended anything else."" The magazine that year stated that Wing Commander II was ""Probably the game responsible"" for making it the standard card. The Sound Blaster line of cards, together with the first inexpensive CD-ROM drives and evolving video technology, ushered in a new era of multimedia computer applications that could play back CD audio, add recorded dialogue to video games, or even reproduce full motion video . The widespread decision to support the Sound Blaster design in multimedia and entertainment titles meant that future sound cards such as Media Vision's Pro Audio Spectrum and the Gravis Ultrasound had to be Sound Blaster compatible if they were to sell well. Until the early 2000s, when the AC'97 audio standard became more widespread and eventually usurped the SoundBlaster as a standard due to its low cost and integration into many motherboards, Sound Blaster compatibility was a standard that many other sound cards supported to maintain compatibility with many games and applications released."
+4991,"When game company Sierra On-Line opted to support add-on music hardware in addition to built-in hardware such as the PC speaker and built-in sound capabilities of the IBM PCjr and Tandy 1000, what could be done with sound and music on the IBM PC changed dramatically. Two of the companies Sierra partnered with were Roland and AdLib, opting to produce in-game music for King's Quest 4 that supported the MT-32 and AdLib Music Synthesizer. The MT-32 had superior output quality, due in part to its method of sound synthesis as well as built-in reverb. Since it was the most sophisticated synthesizer they supported, Sierra chose to use most of the MT-32's custom features and unconventional instrument patches, producing background sound effects  before the Sound Blaster brought digital audio playback to the PC. Many game companies also supported the MT-32, but supported the Adlib card as an alternative because of the latter's higher market base. The adoption of the MT-32 led the way for the creation of the MPU-401, Roland Sound Canvas and General MIDI standards as the most common means of playing in-game music until the mid-1990s."
+4992,"Early ISA bus sound cards were half-duplex, meaning they couldn't record and play digitized sound simultaneously. Later, ISA cards like the SoundBlaster AWE series and Plug-and-play Soundblaster clones supported simultaneous recording and playback, but at the expense of using up two IRQ and DMA channels instead of one. Conventional PCI bus cards generally do not have these limitations and are mostly full-duplex."
+4993,"Sound cards have evolved in terms of digital audio sampling rate . Along the way, some cards started offering wavetable synthesis, which provides superior MIDI synthesis quality relative to the earlier Yamaha OPL based solutions, which uses FM-synthesis. Some higher-end cards introduced their own RAM and processor for user-definable sound samples and MIDI instruments as well as to offload audio processing from the CPU."
+4994,"With some exceptions, for years, sound cards, most notably the Sound Blaster series and their compatibles, had only one or two channels of digital sound. Early games and MOD-players needing more channels than a card could support had to resort to mixing multiple channels in software. Even today, the tendency is still to mix multiple sound streams in software, except in products specifically intended for gamers or professional musicians."
+4995,"The number of physical sound channels has also increased. The first sound card solutions were mono. Stereo sound was introduced in the early 1980s, and quadraphonic sound came in 1989. This was shortly followed by 5.1 channel audio. The latest sound cards support up to 8 audio channels for the 7.1 speaker setup."
+4996,"A few early sound cards had sufficient power to drive unpowered speakers directly – for example, two watts per channel. With the popularity of amplified speakers, sound cards no longer have a power stage, though in many cases they can adequately drive headphones."
+4997,"Professional sound cards are sound cards optimized for high-fidelity, low-latency multichannel sound recording and playback. Their drivers usually follow the Audio Stream Input/Output protocol for use with professional sound engineering and music software."
+4998,"Professional sound cards are usually described as audio interfaces, and sometimes have the form of external rack-mountable units using USB, FireWire, or an optical interface, to offer sufficient data rates. The emphasis in these products is, in general, on multiple input and output connectors, direct hardware support for multiple input and output sound channels, as well as higher sampling rates and fidelity as compared to the usual consumer sound card."
+4999,"On the other hand, certain features of consumer sound cards such as support for 3D audio, hardware acceleration in video games, or real-time ambiance effects are secondary, nonexistent or even undesirable in professional audio interfaces, and as such audio interfaces are not recommended for the typical home user."
+5000,"The typical consumer-grade sound card is intended for generic home, office, and entertainment purposes with an emphasis on playback and casual use, rather than catering to the needs of audio professionals. In general, consumer-grade sound cards impose several restrictions and inconveniences that would be unacceptable to an audio professional. Consumer sound cards are also limited in the effective sampling rates and bit depths they can actually manage and have lower numbers of less flexible input channels. Professional studio recording use typically requires more than the two channels that consumer sound cards provide, and more accessible connectors, unlike the variable mixture of internal—and sometimes virtual—and external connectors found in consumer-grade sound cards."
+5001,"In 1984, the first IBM PCjr had a rudimentary 3-voice sound synthesis chip  which was capable of generating three square-wave tones with variable amplitude, and a pseudo-white noise channel that could generate primitive percussion sounds. The Tandy 1000, initially a clone of the PCjr, duplicated this functionality, with the Tandy 1000 TL/SL/RL models adding digital sound recording and playback capabilities. Many games during the 1980s that supported the PCjr's video standard  also supported PCjr/Tandy 1000 audio."
+5002,"In the late 1990s, many computer manufacturers began to replace plug-in sound cards with an audio codec chip  integrated into the motherboard. Many of these used Intel's AC'97 specification. Others used inexpensive ACR slot accessory cards."
+5003,"From around 2001, many motherboards incorporated full-featured sound cards, usually in the form of a custom chipset, providing something akin to full Sound Blaster compatibility and relatively high-quality sound. However, these features were dropped when AC'97 was superseded by Intel's HD Audio standard, which was released in 2004, again specified the use of a codec chip, and slowly gained acceptance. As of 2011, most motherboards have returned to using a codec chip, albeit an HD Audio compatible one, and the requirement for Sound Blaster compatibility relegated to history."
+5004,"Various non-IBM PC compatible computers, such as early home computers like the Commodore 64   and Amiga , NEC's PC-88, Fujitsu's FM-7 and FM Towns, Sharp's X1 and X68000, Acorn's BBC Micro, Electron and Archimedes, Atari's 8-bit home computers, ST and Falcon, Amstrad's CPC, later revisions of Sinclair's ZX Spectrum, the MSX, Apple's Macintosh and IIGS, and workstations from manufacturers like Sun, Silicon Graphics and NeXT have had their own motherboard-integrated sound devices. In some cases, most notably in those of the Macintosh, IIGS, Amiga, C64, SGI Indigo, X68000, MSX, Falcon, Archimedes, FM-7 and FM Towns, they provide very advanced capabilities , in others they are only minimal capabilities. Some of these platforms have also had sound cards designed for their bus architectures that cannot be used in a standard PC."
+5005,"Several Japanese computer platforms, including the MSX, X1, X68000, FM Towns and FM-7, featured built-in FM synthesis sound from Yamaha by the mid-1980s. By 1989, the FM Towns computer platform featured built-in PCM sample-based sound and supported the CD-ROM format."
+5006,"The custom sound chip on Amiga, named Paula, had four digital sound channels  with 8-bit resolution for each channel and a 6-bit volume control per channel. Sound playback on Amiga was done by reading directly from the chip RAM without using the main CPU."
+5007,"Most arcade games have integrated sound chips, the most popular being the Yamaha OPL chip for music coupled with a variety of DACs for sampled audio and sound effects."
+5008,"The earliest known sound card used by computers was the Gooch Synthetic Woodwind, a music device for PLATO terminals, and is widely hailed as the precursor to sound cards and MIDI. It was invented in 1972."
+5009,"Certain early arcade machines made use of sound cards to achieve playback of complex audio waveforms and digital music, despite being already equipped with onboard audio. An example of a sound card used in arcade machines is the Digital Compression System card, used in games from Midway. For example, Mortal Kombat II on the Midway T-Unit hardware. The T-Unit hardware already has an onboard YM2151 OPL chip coupled with an OKI 6295 DAC, but said game uses an added-on DCS card instead. The card is also used in the arcade version of Midway and Aerosmith's Revolution X for complex looping music and speech playback."
+5010,"MSX computers, while equipped with built-in sound capabilities, also relied on sound cards to produce better quality audio. The card, known as Moonsound, uses a Yamaha OPL4 sound chip. Prior to the Moonsound, there were also sound cards called MSX Music and MSX Audio for the system, which uses OPL2 and OPL3 chipsets."
+5011,"The Apple II computers, which did not have sound capabilities beyond rapidly clicking a speaker until the IIGS, could use plug-in sound cards from a variety of manufacturers. The first, in 1978, was ALF's Apple Music Synthesizer, with 3 voices; two or three cards could be used to create 6 or 9 voices in stereo. Later ALF created the Apple Music II, a 9-voice model. The most widely supported card, however, was the Mockingboard. Sweet Micro Systems sold the Mockingboard in various models. Early Mockingboard models ranged from 3 voices in mono, while some later designs had 6 voices in stereo. Some software supported use of two Mockingboard cards, which allowed 12-voice music and sound. A 12-voice, single-card clone of the Mockingboard called the Phasor was made by Applied Engineering."
+5012,"The ZX Spectrum that initially only had a beeper had some sound cards made for it. Examples include TurboSound Other examples are the Fuller Box, and Zon X-81."
+5013,"The Commodore 64, while having an integrated SID  chip, also had sound cards made for it. For example, the Sound Expander, which added on an OPL FM synthesizer."
+5014,"The PC-98 series of computers, like their IBM PC cousins, also do not have integrated sound contrary to popular belief, and their default configuration is a PC speaker driven by a timer. Sound cards were made for the C-Bus expansion slots that these computers had, most of which used Yamaha's FM and PSG chips and made by NEC themselves, although aftermarket clones can also be purchased, and Creative did release a C-Bus version of the SoundBlaster line of sound cards for the platform."
+5015,"Devices such as the Covox Speech Thing could be attached to the parallel port of an IBM PC and fed 6- or 8-bit PCM sample data to produce audio. Also, many types of professional sound cards take the form of an external FireWire or USB unit, usually for convenience and improved fidelity."
+5016,"Sound cards using the PC Card interface were available before laptop and notebook computers routinely had onboard sound. Most of these units were designed for mobile DJs, providing separate outputs to allow both playback and monitoring from one system, however, some also target mobile gamers."
+5017,USB sound cards are external devices that plug into the computer via USB. They are often used in studios and on stage by electronic musicians including live PA performers and DJs. DJs who use DJ software typically use sound cards integrated into DJ controllers or specialized DJ sound cards. DJ sound cards sometimes have inputs with phono preamplifiers to allow turntables to be connected to the computer to control the software's playback of music files with vinyl emulation.
+5018,"The USB specification defines a standard interface, the USB audio device class, allowing a single driver to work with the various USB sound devices and interfaces on the market. Mac OS X, Windows, and Linux support this standard. However, some USB sound cards do not conform to the standard and require proprietary drivers from the manufacturer."
+5019,"Cards meeting the older USB 1.1 specification are capable of high-quality sound with a limited number of channels, but USB 2.0 or later is more capable with their higher bandwidths."
+5020,"The main function of a sound card is to play audio, usually music, with varying formats  and degrees of control. The source may be a CD or DVD, a file, streamed audio, or any external source connected to a sound card input. Audio may be recorded. Sometimes sound card hardware and drivers do not support recording a source that is being played."
+5021,"Sound cards can be used to generate  arbitrary electrical waveforms, as any digital waveform played by the soundcard is converted to the desired output within the bounds of its capabilities. In other words, sound cards are consumer-grade arbitrary waveform generators. A number of free and commercial software allow sound cards to act like function generators by generating desired waveforms from functions; there are also online services that generate audio files for any desired waveforms, playable through a sound card."
+5022,"Sound cards can also be used to record electrical waveforms, in the same way it records an analog audio input. The recording can be displayed by special or general-purpose audio-editing software  or further transformed and analyzed. A protection circuit should be used to keep the input voltage within acceptable bounds."
+5023,"As general-purpose waveform generators and analyzers, sound cards are bound by several design and physical limitations."
+5024,Sound cards have been used to analyze and generate the following types of signals:
+5025,"To use a sound card, the operating system  typically requires a specific device driver, a low-level program that handles the data connections between the physical hardware and the operating system. Some operating systems include the drivers for many cards; for cards not so supported, drivers are supplied with the card, or available for download."
+5026,"The most common type of projector used today is called a video projector. Video projectors are digital replacements for earlier types of projectors such as slide projectors and overhead projectors. These earlier types of projectors were mostly replaced with digital video projectors throughout the 1990s and early 2000s, but old analog projectors are still used at some places. The newest types of projectors are handheld projectors that use lasers or LEDs to project images."
+5027,"Movie theaters used a type of projector called a movie projector, nowadays mostly replaced with digital cinema video projectors."
+5028,"Projectors can be roughly divided into three categories, based on the type of input. Some of the listed projectors were capable of projecting several types of input. For instance: video projectors were basically developed for the projection of prerecorded moving images, but are regularly used for still images in PowerPoint presentations and can easily be connected to a video camera for real-time input. The magic lantern is best known for the projection of still images, but was capable of projecting moving images from mechanical slides since its invention and was probably at its peak of popularity when used in phantasmagoria shows to project moving images of ghosts."
+5029,"There probably existed quite a few other types of projectors than the examples described below, but evidence is scarce and reports are often unclear about their nature. Spectators did not always provide the details needed to differentiate between for instance a shadow play and a lantern projection. Many did not understand the nature of what they had seen and few had ever seen other comparable media. Projections were often presented or perceived as magic or even as religious experiences, with most projectionists unwilling to share their secrets. Joseph Needham sums up some possible projection examples from China in his 1962 book series Science and Civilization in China"
+5030,"The earliest projection of images was most likely done in primitive shadowgraphy dating back to prehistory. Shadow play usually does not involve a projection device, but can be seen as a first step in the development of projectors. It evolved into more refined forms of shadow puppetry in Asia, where it has a long history in Indonesia , Malaysia, Thailand, Cambodia, China , India and Nepal."
+5031,Projectors share a common history with cameras in the camera obscura. Camera obscura  is the natural optical phenomenon that occurs when an image of a scene at the other side of a screen  is projected through a small hole in that screen to form an inverted image  on a surface opposite to the opening. The oldest known record of this principle is a description by Han Chinese philosopher Mozi . Mozi correctly asserted that the camera obscura image is inverted because light travels in straight lines.
+5032,"In the early 11th century, Arab physicist Ibn al-Haytham  described experiments with light through a small opening in a darkened room and realized that a smaller hole provided a sharper image."
+5033,"The oldest known objects that can project images are Chinese magic mirrors. The origins of these mirrors have been traced back to the Chinese Han dynasty  and are also found in Japan. The mirrors were cast in bronze with a pattern embossed at the back and a mercury amalgam laid over the polished front. The pattern on the back of the mirror is seen in a projection when light is reflected from the polished front onto a wall or other surface. No trace of the pattern can be discerned on the reflecting surface with the naked eye, but minute undulations on the surface are introduced during the manufacturing process and cause the reflected rays of light to form the pattern. It is very likely that the practice of image projection via drawings or text on the surface of mirrors predates the very refined ancient art of the magic mirrors, but no evidence seems to be available."
+5034,"Revolving lanterns have been known in China as ""trotting horse lamps""  since before 1000 CE. A trotting horse lamp is a hexagonal, cubical or round lantern which on the inside has cut-out silhouettes attached to a shaft with a paper vane impeller on top, rotated by heated air rising from a lamp. The silhouettes are projected on the thin paper sides of the lantern and appear to chase each other. Some versions showed some extra motion in the heads, feet and/or hands of figures by connecting them with a fine iron wire to an extra inner layer that would be triggered by a transversely connected iron wire. The lamp would typically show images of horses and horse-riders."
+5035,"In France, similar lanterns were known as ""lanterne vive""  in Medieval times. and as ""lanterne tournante"" since the 18th century. An early variation was described in 1584 by Jean Prevost in his small octavo book La Premiere partie des subtiles et plaisantes inventions. In his ""lanterne"", cut-out figures of a small army were placed on a wooden platform rotated by a cardboard propeller above a candle. The figures cast their shadows on translucent, oiled paper on the outside of the lantern. He suggested to take special care that the figures look lively: with horses raising their front legs as if they were jumping and soldiers with drawn swords, a dog chasing a hare, etcetera. According to Prevost barbers were skilled in this art and it was common to see these night lanterns in their shop windows."
+5036,"A more common version had the figures, usually representing grotesque or devilish creatures, painted on a transparent strip. The strip was rotated inside a cylinder by a tin impeller above a candle. The cylinder could be made of paper or of sheet metal perforated with decorative patterns. Around 1608 Mathurin Régnier mentioned the device in his Satire XI as something used by a patissier to amuse children. Régnier compared the mind of an old nagger with the lantern's effect of birds, monkeys, elephants, dogs, cats, hares, foxes and many strange beasts chasing each other."
+5037,"John Locke  referred to a similar device when wondering if ideas are formed in the human mind at regular intervals,""not much unlike the images in the inside of a lantern, turned round by the heat of a candle."" Related constructions were commonly used as Christmas decorations in England  and parts of Europe. A still relatively common type of rotating device that is closely related does not really involve light and shadows, but it simply uses candles and an impeller to rotate a ring with tiny figurines standing on top."
+5038,"Many modern electric versions of this type of lantern use all kinds of colorful transparent cellophane figures which are projected across the walls, especially popular for nurseries."
+5039,"The inverted real image of an object reflected by a concave mirror can appear at the focal point in front of the mirror. In a construction with an object at the bottom of two opposing concave mirrors  on top of each other, the top one with an opening in its center, the reflected image can appear at the opening as a very convincing 3D optical illusion."
+5040,The earliest description of projection with concave mirrors has been traced back to a text by French author Jean de Meun in his part of Roman de la Rose . A theory known as the Hockney-Falco thesis claims that artists used either concave mirrors or refractive lenses to project images onto their canvas/board as a drawing/painting aid as early as circa 1430.
+5041,It has also been thought that some encounters with spirits or gods since antiquity may have been conjured up with  mirrors.
+5042,"Around 1420 the Venetian scholar and engineer Giovanni Fontana included a drawing of a person with a lantern projecting an image of a demon in his book about mechanical instruments ""Bellicorum Instrumentorum Liber"". The Latin text ""Apparentia nocturna ad terrorem videntium"" "" clarifies its purpose, but the meaning of the undecipherable other lines is unclear. The lantern seems to simply have the light of an oil lamp or candle go through a transparent cylindrical case on which the figure is drawn to project the larger image, so it probably couldn't project an image as clearly defined as Fontana's drawing suggests."
+5043,"In 1437 Italian humanist author, artist, architect, poet, priest, linguist, philosopher and cryptographer Leon Battista Alberti is thought to have possibly projected painted pictures from a small closed box with a small hole, but it is unclear whether this actually was a projector or rather a type of show box with transparent pictures illuminated from behind and viewed through the hole."
+5044,"Leonardo da Vinci is thought to have had a projecting lantern - with a condensing lens, candle and chimney - based on a small sketch from around 1515."
+5045,"In his Three Books of Occult Philosophy  Heinrich Cornelius Agrippa claimed that it was possible to project ""images artificially painted, or written letters"" onto the surface of the Moon with the means of moonbeams and their ""resemblances being multiplied in the air"". Pythagoras would have often performed this trick."
+5046,In 1589 Giambattista della Porta published about the ancient art of projecting mirror writing in his book Magia Naturalis.
+5047,"Dutch inventor Cornelis Drebbel, who is a likely inventor of the microscope, is thought to have had some kind of projector that he used in magical performances. In a 1608 letter he described the many marvelous transformations he performed and the apparitions that he summoned by the means of his new invention based on optics. It included giants that rose from the earth and moved all their limbs very lifelike. The letter was found in the papers of his friend Constantijn Huygens, father of the likely inventor of the magic lantern Christiaan Huygens."
+5048,"In 1612 Italian mathematician Benedetto Castelli wrote to his mentor, the Italian astronomer, physicist, engineer, philosopher and mathematician Galileo Galilei about projecting images of the sun through a telescope  to study the recently discovered sunspots. Galilei wrote about Castelli's technique to the German Jesuit priest, physicist and astronomer Christoph Scheiner."
+5049,"From 1612 to at least 1630 Christoph Scheiner would keep on studying sunspots and constructing new telescopic solar projection systems. He called these ""Heliotropii Telioscopici"", later contracted to helioscope."
+5050,"The 1645 first edition of German Jesuit scholar Athanasius Kircher's book Ars Magna Lucis et Umbrae included a description of his invention, the steganographic mirror: a primitive projection system with a focusing lens and text or pictures painted on a concave mirror reflecting sunlight, mostly intended for long distance communication. He saw limitations in the increase of size and diminished clarity over a long distance and expressed his hope that someone would find a method to improve on this. Kircher also suggested projecting live flies and shadow puppets from the surface of the mirror. The book was quite influential and inspired many scholars, probably including Christiaan Huygens who would invent the magic lantern. Kircher was often credited as the inventor of the magic lantern, although in his 1671 edition of Ars Magna Lucis et Umbrae Kircher himself credited Danish mathematician Thomas Rasmussen Walgensten for the magic lantern, which Kircher saw as a further development of his own projection system."
+5051,"Although Athanasius Kircher claimed the Steganographic mirror as his own invention and wrote not to have read about anything like it, it has been suggested that Rembrandt's 1635 painting of ""Belshazzar's Feast"" depicts a steganographic mirror projection with God's hand writing Hebrew letters on a dusty mirror's surface."
+5052,"In 1654 Belgian Jesuit mathematician André Tacquet used Kircher's technique to show the journey from China to Belgium of Italian Jesuit missionary Martino Martini. It is sometimes reported that Martini lectured throughout Europe with a magic lantern which he might have imported from China, but there's no evidence that anything other than Kircher's technique was used."
+5053,"By 1659 Dutch scientist Christiaan Huygens had developed the magic lantern, which used a concave mirror to reflect and direct as much of the light of a lamp as possible through a small sheet of glass on which was the image to be projected, and onward into a focusing lens at the front of the apparatus to project the image onto a wall or screen . He did not publish nor publicly demonstrate his invention as he thought it was too frivolous."
+5054,The magic lantern became a very popular medium for entertainment and educational purposes in the 18th and 19th century. This popularity waned after the introduction of cinema in the 1890s. The magic lantern remained a common medium until slide projectors came into widespread use during the 1950s.
+5055,"A few years before his death in 1736 Polish-German-Dutch physicist Daniel Gabriel Fahrenheit reportedly constructed a solar microscope, which basically was a combination of the compound microscope with camera obscura projection. It needed bright sunlight as a light source to project a clear magnified image of transparent objects. Fahrenheit's instrument may have been seen by German physician Johann Nathanael Lieberkühn who introduced the instrument in England, where optician John Cuff improved it with a stationary optical tube and an adjustable mirror. In 1774 English instrument maker Benjamin Martin introduced his ""Opake Solar Microscope"" for the enlarged projection of opaque objects. He claimed:"
+5056,"The solar microscope, was employed in experiments with photosensitive silver nitrate by Thomas Wedgwood in collaboration with Humphry Davy in making the first, but impermanent, photographic enlargements. Their discoveries, regarded as the earliest deliberate and successful form of photography, were published in June 1802 by Davy in his An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver. Invented by T. Wedgwood, Esq. With Observations by H. Davy in the first issue of the Journals of the Royal Institution of Great Britain."
+5057,"Swiss mathematician, physicist, astronomer, logician and engineer Leonhard Euler demonstrated an opaque projector, now commonly known as an episcope, around 1756. It could project a clear image of opaque images and  objects."
+5058,"French scientist Jacques Charles is thought to have invented the similar ""megascope"" in 1780. He used it for his lectures. Around 1872 Henry Morton used an opaque projector in demonstrations for huge audiences, for example in the Philadelphia Opera House which could seat 3500 people. His machine did not use a condenser or reflector, but used an oxyhydrogen lamp close to the object in order to project huge clear images."
+5059,See main article: Solar camera
+5060,"Known equally, though later, as a solar enlarger, the solar camera is a photographic application of the solar microscope and an ancestor of the darkroom enlarger, and was used, mostly by portrait photographers and as an aid to portrait artists, in the mid-to-late 19th century to make photographic enlargements from negatives using the Sun as a light source powerful enough to expose the then available low-sensitivity photographic materials. It was superseded in the 1880s when other light sources, including the incandescent bulb, were developed for the darkroom enlarger and materials became ever more photo-sensitive."
+5061,"In the early and middle parts of the 20th century, low-cost opaque projectors were produced and marketed as a toy for children. The light source in early opaque projectors was often limelight, with incandescent light bulbs and halogen lamps taking over later. Episcopes are still marketed as artists’ enlargement tools to allow images to be traced on surfaces such as prepared canvas."
+5062,"In the late 1950s and early 1960s, overhead projectors began to be widely used in schools and businesses. The first overhead projector was used for police identification work. It used a celluloid roll over a 9-inch stage allowing facial characteristics to be rolled across the stage. The United States military in 1940 was the first to use it in quantity for training."
+5063,"From the 1950s to the 1990s slide projectors for 35 mm photographic positive film slides were common for presentations and as a form of entertainment; family members and friends would occasionally gather to view slideshows, typically of vacation travels."
+5064,"Complex Multi-image shows of the 1970s to 1990s, purposed usually for marketing, promotion or community service or artistic displays, used 35mm and 46mm transparency slides  projected by single or multiple slide projectors onto one or more screens in synchronization with an audio voice-over and/or music track controlled by a pulsed-signal tape or cassette. Multi-image productions are also known as multi-image slide presentations, slide shows and diaporamas and are a specific form of multimedia or audio-visual production."
+5065,"Digital cameras had become commercialised by 1990, and in 1997 Microsoft PowerPoint was updated to include image files,  accelerating the transition from 35 mm slides to digital images, and thus digital projectors, in pedagogy and training. Production of all Kodak Carousel slide projectors ceased in 2004, and in 2009 manufacture and processing of Kodachrome film was discontinued."
+5066,In Mad Men's first series the final episode presents the protagonist Don Draper's presentation  of a plan to market the Kodak slide carrier a 'carousel'.
+5067,"A PC speaker is a loudspeaker built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven  speaker. More recent computers use a tiny moving-iron or piezo speaker instead. The speaker allows software and firmware to provide auditory feedback to a user, such as to report a hardware fault. A PC speaker generates waveforms using the programmable interval timer, an Intel 8253 or 8254 chip."
+5068,"The PC speaker is used during the power-on self-test  sequence to indicate errors during the boot process. Since it is active before the graphics card, it can be used to communicate ""beep codes"" related to problems that prevent the much more complex initialization of the graphics card to take place. For example, the Video BIOS usually cannot activate a graphics card unless working RAM is present in the system while beeping the speaker is possible with just ROM and the CPU registers. Usually, different error codes will be signalled by specific beeping patterns, such as e.g. ""one beep; pause; three beeps; pause; repeat"". These patterns are specific to the BIOS/UEFI manufacturer and are usually documented in the technical manual of the motherboard."
+5069,"Several programs, including music software, operating systems or games, could play pulse-code modulation  sound through the PC speaker using special Pulse-width Modulation techniques explained later in this article."
+5070,"The PC speaker was often used in very innovative ways to create the impression of polyphonic music or sound effects within computer games of its era, such as the LucasArts series of adventure games from the mid-1980s, using swift arpeggios.  Several games such as Space Hulk and Pinball Fantasies were noted for their elaborate sound effects; Space Hulk, in particular, even had full speech."
+5071,"However, because the method used to reproduce PCM was very sensitive to timing issues, these effects either caused noticeable sluggishness on slower PCs or sometimes failed on faster PCs . Also, it was difficult for programs to do much else, even update the display, during the playing of such sounds. Thus, when sound cards  became mainstream in the PC market after 1990, they quickly replaced the PC speaker as the preferred output device for sound effects. Most newly-released PC games stopped supporting the speaker during the second half of the 1990s."
+5072,"Several programs, including MP , Scream Tracker, Fast Tracker, Impulse Tracker, and even device drivers for Linux and Microsoft Windows, could play PCM sound through the PC speaker."
+5073,"Modern Microsoft Windows systems have PC speaker support as a separate device with special capabilities – that is, it cannot be configured as a normal audio output device. Some software uses this special sound channel to produce sounds. For example, Skype can use it as a reserve calling signal device for the case where the primary audio output device cannot be heard ."
+5074,"In the 1990s, a computer virus for Microsoft DOS named ""Techno"" appeared, playing a melody through the PC speaker while printing the word ""TECHNO"" on the screen until filled."
+5075,"In some applications, the PC speaker is affixed directly to the computer's motherboard; in others, including the first IBM Personal Computer, the speaker is attached by wire to a connector on the motherboard. Some PC cases come with a PC speaker preinstalled. A wired PC speaker connector may have a two-, three-, or four-pin configuration, and either two or three wires. The female connector of the speaker connects to pin headers on the motherboard, which are sometimes labeled SPEAKER or SPKR."
+5076,"The PC speaker is normally meant to reproduce a square wave via only 2 levels of output , driven by channel 2 of the Intel 8253  or 8254  Programmable Interval Timer operating in mode three . The speaker hardware itself is directly accessible via PC I/O port 61H  via bit 1 and can be physically manipulated for 2 levels of output .  However, by carefully timing a short pulse , and by relying on the speaker's physical filtering properties , it is possible to drive the speaker to various intermediate output levels, functioning as a crude digital-to-analog converter.  This technique is called pulse-width modulation  and allows approximate playback of PCM audio.  "
+5077,"With the PC speaker this method achieves limited quality playback, but a commercial solution named RealSound used it to provide improved sound on several games."
+5078,"Obtaining a high fidelity sound output using this technique requires a switching frequency much higher than the audio frequencies meant to be reproduced , and the output voltage to be bipolar, in order to make better use of the output devices' dynamic range and power. On the PC speaker, however, the output voltage is either zero or at a Transistor-Transistor Logic  level ."
+5079,"The quality depends on a trade-off between the PWM carrier frequency  and the number of output levels .  The clock rate of the PC's programmable interval timer which drives the speaker is fixed at 1,193,180 Hz, and the product of the audio sample rate times the maximum DAC value must equal this.  
+Typically, a 6-bit DAC with a maximum value of 63 is used at a sample rate of 18,939.4 Hz, producing poor but recognizable audio."
+5080,"The audio fidelity of this technique is further decreased by the lack of a properly sized dynamic loudspeaker, specially in modern machines and particularly laptops that use a tiny moving-iron speaker . The reason for this is that PWM-produced audio requires a low-pass filter before the final output in order to suppress switching noise and high harmonics. A normal dynamic loudspeaker does this naturally, but the tiny metal diaphragm of the moving-iron speaker will let much switching noise pass, as will many direct couplings ."
+5081,This use of the PC speaker for complex audio output became less common with the introduction of Sound Blaster and other sound cards.
+5082,"The display in modern monitors is typically an LCD with LED backlight, having by the 2010s replaced CCFL backlit LCDs. Before the mid-2000s, most monitors used a cathode-ray tube  as the image output technology. A monitor is typically connected to its host computer via DisplayPort, HDMI, USB-C, DVI, or VGA. Monitors sometimes use other proprietary connectors and signals to connect to a computer, which is less common."
+5083,"Originally, computer monitors were used for data processing while television sets were used for video. From the 1980s onward, computers  have been used for both data processing and video, while televisions have implemented some computer functionality. In the 2000s, the typical display aspect ratio of both televisions and computer monitors changed from 4:3 to 16:9."
+5084,"Modern computer monitors are often functionally interchangeable with television sets and vice versa. As most computer monitors do not include integrated speakers, TV tuners, or remote controls, external components such as a DTA box may be needed to use a computer monitor as a TV set."
+5085,"Early electronic computer front panels were fitted with an array of light bulbs where the state of each particular bulb would indicate the on/off state of a particular register bit inside the computer. This allowed the engineers operating the computer to monitor the internal state of the machine, so this panel of lights came to be known as the 'monitor'. As early monitors were only capable of displaying a very limited amount of information and were very transient, they were rarely considered for program output. Instead, a line printer was the primary output device, while the monitor was limited to keeping track of the program's operation."
+5086,"One of the first uses of a standalone computer monitor with a personal computer was with the Apple 1, which connected directly to a consumer television as a monitor instead of using a glass terminal as its output."
+5087,"Computer monitors were formerly known as visual display units , particularly in British English. This term mostly fell out of use by the 1990s."
+5088,Multiple technologies have been used for computer monitors. Until the 21st century most used cathode-ray tubes but they have largely been superseded by LCD monitors.
+5089,"The first computer monitors used cathode-ray tubes . Prior to the advent of home computers in the late 1970s, it was common for a video display terminal  using a CRT to be physically integrated with a keyboard and other components of the workstation in a single large chassis, typically limiting them to emulation of a paper teletypewriter, thus the early epithet of 'glass TTY'. The display was monochromatic and far less sharp and detailed than on a modern monitor, necessitating the use of relatively large text and severely limiting the amount of information that could be displayed at one time. High-resolution CRT displays were developed for specialized military, industrial and scientific applications but they were far too costly for general use; wider commercial use became possible after the release of a slow, but affordable Tektronix 4010 terminal in 1972."
+5090,"Some of the earliest home computers  were limited to monochrome CRT displays, but color display capability was already a possible feature for a few MOS 6500 series-based machines , and the color output was a specialty of the more graphically sophisticated Atari 800 computer, introduced in 1979. Either computer could be connected to the antenna terminals of an ordinary color TV set or used with a purpose-made CRT color monitor for optimum resolution and color quality. Lagging several years behind, in 1981 IBM introduced the Color Graphics Adapter, which could display four colors with a resolution of 320 × 200 pixels, or it could produce 640 × 200 pixels with two colors. In 1984 IBM introduced the Enhanced Graphics Adapter which was capable of producing 16 colors and had a resolution of 640 × 350."
+5091,"By the end of the 1980s color progressive scan CRT monitors were widely available and increasingly affordable, while the sharpest prosumer monitors could clearly display high-definition video, against the backdrop of efforts at HDTV standardization from the 1970s to the 1980s failing continuously, leaving consumer SDTVs to stagnate increasingly far behind the capabilities of computer CRT monitors well into the 2000s. During the following decade, maximum display resolutions gradually increased and prices continued to fall as CRT technology remained dominant in the PC monitor market into the new millennium, partly because it remained cheaper to produce. CRTs still offer color, grayscale, motion, and latency advantages over today's LCDs, but improvements to the latter have made them much less obvious. The dynamic range of early LCD panels was very poor, and although text and other motionless graphics were sharper than on a CRT, an LCD characteristic known as pixel lag caused moving graphics to appear noticeably smeared and blurry."
+5092,"There are multiple technologies that have been used to implement liquid-crystal displays . Throughout the 1990s, the primary use of LCD technology as computer monitors was in laptops where the lower power consumption, lighter weight, and smaller physical size of LCDs justified the higher price versus a CRT. Commonly, the same laptop would be offered with an assortment of display options at increasing price points:  monochrome, passive color, or active matrix color . As volume and manufacturing capability have improved, the monochrome and passive color technologies were dropped from most product lines."
+5093,TFT-LCD is a variant of LCD which is now the dominant technology used for computer monitors.
+5094,"The first standalone LCDs appeared in the mid-1990s selling for high prices. As prices declined they became more popular, and by 1997 were competing with CRT monitors. Among the first desktop LCD computer monitors were the Eizo FlexScan L66 in the mid-1990s, the SGI 1600SW, Apple Studio Display and the ViewSonic VP140 in 1998. In 2003, LCDs outsold CRTs for the first time, becoming the primary technology used for computer monitors. The physical advantages of LCD over CRT monitors are that LCDs are lighter, smaller, and consume less power. In terms of performance, LCDs produce less or no flicker, reducing eyestrain, sharper image at native resolution, and better checkerboard contrast. On the other hand, CRT monitors have superior blacks, viewing angles, and response time, can use arbitrary lower resolutions without aliasing, and flicker can be reduced with higher refresh rates, though this flicker can also be used to reduce motion blur compared to less flickery displays such as most LCDs. Many specialized fields such as vision science remain dependent on CRTs, the best LCD monitors having achieved moderate temporal accuracy, and so can be used only if their poor spatial accuracy is unimportant."
+5095,"High dynamic range  has been implemented into high-end LCD monitors to improve grayscale accuracy. Since around the late 2000s, widescreen LCD monitors have become popular, in part due to television series, motion pictures and video games transitioning to widescreen, which makes squarer monitors unsuited to display them correctly."
+5096,"Organic light-emitting diode  monitors provide most of the benefits of both LCD and CRT monitors with few of their drawbacks, though much like plasma panels or very early CRTs they suffer from burn-in, and remain very expensive."
+5097,The performance of a monitor is measured by the following parameters:
+5098,"On two-dimensional display devices such as computer monitors the display size or viewable image size is the actual amount of screen space that is available to display a picture, video or working space, without obstruction from the bezel or other aspects of the unit's design. The main measurements for display devices are width, height, total area and the diagonal."
+5099,"The size of a display is usually given by manufacturers diagonally, i.e. as the distance between two opposite screen corners. This method of measurement is inherited from the method used for the first generation of CRT television when picture tubes with circular faces were in common use. Being circular, it was the external diameter of the glass envelope that described their size. Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangular image was smaller than the diameter of the tube's face . This method continued even when cathode-ray tubes were manufactured as rounded rectangles; it had the advantage of being a single number specifying the size and was not confusing when the aspect ratio was universally 4:3."
+5100,"With the introduction of flat panel technology, the diagonal measurement became the actual diagonal of the visible display. This meant that an eighteen-inch LCD had a larger viewable area than an eighteen-inch cathode-ray tube."
+5101,"Estimation of monitor size by the distance between opposite corners does not take into account the display aspect ratio, so that for example a 16:9 21-inch  widescreen display has less area, than a 21-inch  4:3 screen. The 4:3 screen has dimensions of 16.8 in × 12.6 in  and an area 211 sq in , while the widescreen is 18.3 in × 10.3 in , 188 sq in ."
+5102,"Until about 2003, most computer monitors had a 4:3 aspect ratio and some had 5:4. Between 2003 and 2006, monitors with 16:9 and mostly 16:10  aspect ratios became commonly available, first in laptops and later also in standalone monitors. Reasons for this transition included productive uses  such as the word processor display of two standard letter pages side by side, as well as CAD displays of large-size drawings and application menus at the same time. In 2008 16:10 became the most common sold aspect ratio for LCD monitors and the same year 16:10 was the mainstream standard for laptops and notebook computers."
+5103,"In 2010, the computer industry started to move over from 16:10 to 16:9 because 16:9 was chosen to be the standard high-definition television display size, and because they were cheaper to manufacture."
+5104,"In 2011, non-widescreen displays with 4:3 aspect ratios were only being manufactured in small quantities. According to Samsung, this was because the ""Demand for the old 'Square monitors' has decreased rapidly over the last couple of years,"" and ""I predict that by the end of 2011, production on all 4:3 or similar panels will be halted due to a lack of demand."""
+5105,"The resolution for computer monitors has increased over time. From 280 × 192 during the late 1970s, to 1024 × 768 during the late 1990s. Since 2009, the most commonly sold resolution for computer monitors is 1920 × 1080, shared with the 1080p of HDTV. Before 2013 mass market LCD monitors were limited to 2560 × 1600 at 30 in , excluding niche professional monitors. By 2015 most major display manufacturers had released 3840 × 2160  displays, and the first 7680 × 4320  monitors had begun shipping."
+5106,"Every RGB monitor has its own color gamut, bounded in chromaticity by a color triangle. Some of these triangles are smaller than the sRGB triangle, some are larger. Colors are typically encoded by 8 bits per primary color. The RGB value  represents red, but slightly different colors in different color spaces such as Adobe RGB and sRGB. Displaying sRGB-encoded data on wide-gamut devices can give an unrealistic result. The gamut is a property of the monitor; the image color space can be forwarded as Exif metadata in the picture. As long as the monitor gamut is wider than the color space gamut, correct display is possible, if the monitor is calibrated. A picture that uses colors that are outside the sRGB color space will display on an sRGB color space monitor with limitations. Still today, many monitors that can display the sRGB color space are not factory nor user-calibrated to display it correctly. Color management is needed both in electronic publishing  and in desktop publishing targeted to print."
+5107,Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to turn off a monitor after a specified period of inactivity. This also extends the monitor's service life. Some monitors will also switch themselves off after a time period on standby.
+5108,Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.
+5109,"Most modern monitors have two different indicator light colors wherein if video-input signal was detected, the indicator light is green and when the monitor is in power-saving mode, the screen is black and the indicator light is orange. Some monitors have different indicator light colors and some monitors have blinking indicator light when in power-saving mode."
+5110,"Many monitors have other accessories  integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers. These monitors have advanced microprocessors which contain codec information, Windows interface drivers and other small software which help in proper functioning of these functions."
+5111,Monitors that feature an aspect ratio greater than 2:1 .Monitors with an aspect ratio greater than 3:1 are marketed as super ultrawide monitors. These are typically massive curved screens intended to replace a multi-monitor deployment.
+5112,"These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. The screen will need frequent cleaning due to image degradation from fingerprints."
+5113,"Some displays, especially newer flat panel monitors, replace the traditional anti-glare matte finish with a glossy one. This increases color saturation and sharpness but reflections from lights and windows are more visible. Anti-reflective coatings are sometimes applied to help reduce reflections, although this only partly mitigates the problem."
+5114,"Most often using nominally flat-panel display technology such as LCD or OLED, a concave rather than convex curve is imparted, reducing geometric distortion, especially in extremely large and wide seamless desktop monitors intended for close viewing range."
+5115,"Newer monitors are able to display a different image for each eye, often with the help of special glasses and polarizers, giving the perception of depth. An autostereoscopic screen can generate 3D images without headgear."
+5116,Features for medical using or for outdoor placement.
+5117,Narrow viewing angle screens are used in some security-conscious applications.
+5118,"Integrated screen calibration tools, screen hoods, signal transmitters; Protective screens."
+5119,A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch without the use of one or more special tools' pressure. Newer models however are now able to detect touch from any pressure and often have the ability to detect tool tilt and rotation as well.
+5120,"Touch and tablet sensors are often used on sample and hold displays such as LCDs to substitute for the light pen, which can only work on CRTs."
+5121,The option for using the display as a reference monitor; these calibration features can give an advanced color management control for take a near-perfect image.
+5122,"Option for professional LCD monitors, inherent to OLED & CRT; professional feature with mainstream tendency."
+5123,Near to mainstream professional feature; advanced hardware driver for backlit modules with local zones of uniformity correction.
+5124,Computer monitors are provided with a variety of methods for mounting them depending on the application and environment.
+5125,"Raw monitors are raw framed LCD monitors, to install a monitor on a not so common place, ie, on the car door or you need it in the trunk. It is usually paired with a power adapter to have a versatile monitor for home or commercial use."
+5126,"A desktop monitor is typically provided with a stand from the manufacturer which lifts the monitor up to a more ergonomic viewing height. The stand may be attached to the monitor using a proprietary method or may use, or be adaptable to, a VESA mount. A VESA standard mount allows the monitor to be used with more after-market stands if the original stand is removed. Stands may be fixed or offer a variety of features such as height adjustment, horizontal swivel, and landscape or portrait screen orientation."
+5127,"The Flat Display Mounting Interface , also known as VESA Mounting Interface Standard  or colloquially as a VESA mount, is a family of standards defined by the Video Electronics Standards Association for mounting flat panel displays to stands or wall mounts. It is implemented on most modern flat-panel monitors and TVs."
+5128,"For computer monitors, the VESA Mount typically consists of four threaded holes on the rear of the display that will mate with an adapter bracket."
+5129,Rack mount computer monitors are available in two styles and are intended to be mounted into a 19-inch rack:
+5130,"A fixed rack mount monitor is mounted directly to the rack with the flat-panel or CRT visible at all times. The height of the unit is measured in rack units  and 8U or 9U are most common to fit 17-inch or 19-inch screens. The front sides of the unit are provided with flanges to mount to the rack, providing appropriately spaced holes or slots for the rack mounting screws. A 19-inch diagonal screen is the largest size that will fit within the rails of a 19-inch rack. Larger flat-panels may be accommodated but are 'mount-on-rack' and extend forward of the rack. There are smaller display units, typically used in broadcast environments, which fit multiple smaller screens side by side into one rack mount."
+5131,"A stowable rack mount monitor is 1U, 2U or 3U high and is mounted on rack slides allowing the display to be folded down and the unit slid into the rack for storage as a drawer. The flat display is visible only when pulled out of the rack and deployed. These units may include only a display or may be equipped with a keyboard creating a KVM . Most common are systems with a single LCD but there are systems providing two or three displays in a single rack mount system."
+5132,"A panel mount computer monitor is intended for mounting into a flat surface with the front of the display unit protruding just slightly. They may also be mounted to the rear of the panel. A flange is provided around the screen, sides, top and bottom, to allow mounting. This contrasts with a rack mount display where the flanges are only on the sides. The flanges will be provided with holes for thru-bolts or may have studs welded to the rear surface to secure the unit in the hole in the panel. Often a gasket is provided to provide a water-tight seal to the panel and the front of the screen will be sealed to the back of the front panel to prevent water and dirt contamination."
+5133,"An open frame monitor provides the display and enough supporting structure to hold associated electronics and to minimally support the display. Provision will be made for attaching the unit to some external structure for support and protection. Open frame monitors are intended to be built into some other piece of equipment providing its own case. An arcade video game would be a good example with the display mounted inside the cabinet. There is usually an open frame display inside all end-use displays with the end-use display simply providing an attractive protective enclosure. Some rack mount monitor manufacturers will purchase desktop displays, take them apart, and discard the outer plastic parts, keeping the inner open-frame display for inclusion into their product."
+5134,"According to an NSA document leaked to Der Spiegel, the NSA sometimes swaps the monitor cables on targeted computers with a bugged monitor cable in order to allow the NSA to remotely see what is being displayed on the targeted computer monitor."
+5135,"Van Eck phreaking is the process of remotely displaying the contents of a CRT or LCD by detecting its electromagnetic emissions. It is named after Dutch computer researcher Wim van Eck, who in 1985 published the first paper on it, including proof of concept. Phreaking more generally is the process of exploiting telephone networks."
+5136,A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's cathode-ray tube  display.
+5137,"It allows the user to point to displayed objects or draw on the screen in a similar way to a touchscreen but with greater positional accuracy.  A light pen can work with any CRT-based display, but its ability to be used with LCDs was unclear ."
+5138,"A light pen detects changes in brightness of nearby screen pixels when scanned by cathode-ray tube electron beam and communicates the timing of this event to the computer. Since a CRT scans the entire screen one pixel at a time, the computer can keep track of the expected time of scanning various locations on screen by the beam and infer the pen's position from the latest time stamps."
+5139,"The first light pen, at this time still called ""light gun"", was created around 1951–1955 as part of the Whirlwind I project at MIT, where it was used to select discrete symbols on the screen, and later at the SAGE project, where it was used for tactical real-time-control of a radar-networked airspace."
+5140,"One of the first more widely deployed uses was in the Situation Display consoles of the AN/FSQ-7 for military airspace surveillance. This is not very surprising, given its relationship with the Whirlwind projects. See Semi-Automatic Ground Environment for more details."
+5141,"During the 1960s, light pens were common on graphics terminals such as the IBM 2250 and were also available for the IBM 3270 text-only terminal."
+5142,"Light pen usage was expanded in the early 1980s to music workstations such as the Fairlight CMI and personal computers such as the BBC Micro. IBM PC-compatible MDA , CGA, HGC  and some EGA graphics cards also featured a connector compatible with a light pen, as did early Tandy 1000 computers, the Thomson MO5 computer family, the Amiga, Atari 8-bit, Commodore 8-bit, some MSX computers and Amstrad PCW home computers. For the MSX computers, Sanyo produced a light pen interface cartridge."
+5143,"Because the user was required to hold their arm in front of the screen for long periods of time  or to use a desk that tilts the monitor, the light pen fell out of use as a general-purpose input device. Light pen was also perceived as working well only on displays with low persistance, which tend to flicker."
+5144,"With most trackballs, operators have to lift their finger, thumb or hand and reposition in on the ball to continue rolling, whereas a mouse would have to be lifted itself and re-positioned. Some trackballs have notably low friction, as well as being made of a dense material such as phenolic resin, so they can be spun to make them coast. The trackball's buttons may be in similar positions to those of a mouse, or configured to suit the user."
+5145,"Large trackballs are common on CAD workstations for easy precision. Before the advent of the touchpad, small trackballs were common on portable computers  where there may be no desk space on which to run a mouse. Some small ""thumballs"" are designed to clip onto the side of the keyboard and have integral buttons with the same function as mouse buttons."
+5146,"The trackball was invented as part of a post-World War II-era radar plotting system named Comprehensive Display System  by Ralph Benjamin when working for the British Royal Navy Scientific Service. Benjamin's project used analog computers to calculate the future position of target aircraft based on several initial input points provided by a user with a joystick. Benjamin felt that a more elegant input device was needed and invented a ball tracker system called the roller ball for this purpose in 1946. The device was patented in 1947, but only a prototype using a metal ball rolling on two rubber-coated wheels was ever built and the device was kept as a military secret. Production versions of the CDS used joysticks."
+5147,"The CDS system had also been viewed by a number of engineers from Ferranti Canada, who returned to Canada and began development of the Royal Canadian Navy's DATAR system in 1952. Principal designers Tom Cranston, Fred Longstaff and Kenyon Taylor chose the trackball as the primary input, using a standard five-pin bowling ball as the roller. DATAR was similar in concept to Benjamin's display, but used a digital computer to calculate tracks, and sent the resulting data to other ships in a task force using pulse-code modulation radio signals."
+5148,"DATAR's trackball used four disks to pick up motion, two each for the X and Y directions. Several additional rollers provided mechanical support. When the ball was rolled, the pickup discs spun and contacts on their outer rim made periodic contact with wires, producing pulses of output with each movement of the ball. By counting the pulses, the physical movement of the ball could be determined."
+5149,"Since 1966, the American company Orbit Instrument Corporation produced a device named X-Y Ball Tracker, a trackball, which was embedded into radar flight control desks."
+5150,"A similar trackball device at the German Bundesanstalt für Flugsicherung  was constructed by a team around Rainer Mallebrein  of Telefunken Konstanz as part of the development for the Telefunken computer infrastructure around the main frame TR 440 , process computer TR 86 and video terminal SIG 100-86, which began in 1965. This trackball was called Rollkugel . Somewhat later, the idea of ""reversing"" this device led to the introduction of the first computer ball mouse , which was offered as an alternative input device to light pens and trackballs for Telefunken's computer systems since 1968."
+5151,"In later trackball models the electrical contacts were replaced by an optical chopper wheel, which had small slots cut into it in rather than electrical contacts. With an LED for illumination from one side and an optical sensor on the other, rotation of the wheel periodically blocks and unblocks the light, so the sensor produces electrical pulses to indicate that rotation is occurring."
+5152,"Mice used the same basic system for determining motion, but had the problem that the ball was in contact with the desk or mousepad. In order to provide smooth motion the balls were often covered with an anti-slip surface treatment, which was, by design, sticky. Rolling the mouse tended to pick up any dirt and drag it into the system where it would clog the chopper wheels, demanding cleanup. In contrast the trackball is in contact only with the user's hand, which tends to be cleaner. In the late 1990s both mice and trackballs began using direct optical tracking which follows dots on the ball, avoiding the need for anti-slip surface treatment."
+5153,"As with modern mice, most trackballs now have an auxiliary device primarily intended for scrolling. Some have a scroll wheel like most mice, but the most common type is a “scroll ring” which is spun around the ball. Kensington's SlimBlade Trackball similarly tracks the ball itself in three dimensions for scrolling."
+5154,"As of 1989 and into the 2020s, two major companies developed and produce consumer trackballs, Logitech and Kensington, although Logitech has narrowed its product line to two models. Other smaller companies occasionally offer a trackball in their product line. Microsoft produced popular models including The Microsoft Trackball Explorer, but has since discontinued all of its products."
+5155,"In September 2017 Logitech announced release of MX-Ergo Mouse, which was released after 6 years of its last trackball mouse."
+5156,"Large trackballs are sometimes seen on computerized special-purpose workstations, such as the radar consoles in an air-traffic control room or sonar equipment on a ship or submarine. Modern installations of such equipment may use mice instead, since most people now already know how to use one. However, military mobile anti-aircraft radars, commercial airliners  and submarine sonars tend to continue using trackballs, since they can be made more durable and more fit for fast emergency use.  Large and well made ones allow easier high precision work, for which reason they may still be used in these applications  and in computer-aided design."
+5157,"Trackballs have appeared in video games, particularly early arcade games . In March 1978, Sega released World Cup, an association football game with trackball controls. In October 1978, Atari released Atari Football, which popularized the use of a trackball, with the game's developers mentioning it was inspired by an earlier Japanese association football game. Other notable trackball games include Atari's Centipede and Missile Command – Atari trademarked it ""TRAK-BALL""."
+5158,"Console trackballs, now fairly rare, were common in the early 1980s: the Atari 2600 and 5200 consoles, as well as the competing ColecoVision console, though using a joystick as their standard controller, each had one as an optional peripheral.  The Apple Pippin, a console introduced in 1996, had a trackball built into its gamepad as standard. Trackballs were occasionally used in e-sports prior to the mainstreaming of optical mice in the early 2000s because they were more reliable than ball mice, but now they are extremely rare because optical mice offer superior speed and precision. Trackballs remain in use in pub golf machines  to simulate swinging the club."
+5159,"Trackballs have also been regarded as excellent complements to analog joysticks, as pioneered by the Assassin 3D, a trackball released in 1996 with joystick pass-through capability. Later in 1996, Mad Catz released the Panther XL, which was based on the Assassin 3D. This combination provides for two-hand aiming and a high accuracy and consistency replacement for the traditional mouse and keyboard combo generally used on first-person shooter games. Many such games natively support joysticks and analog player movement, like Valve's Half-Life and id Software's Quake series. As of 2020, one professional eSport player was known for using a trackball."
+5160,"Trackballs are provided as the pointing device in some public internet access terminals. Unlike a mouse, a trackball can easily be built into a console, and cannot be ripped away or easily vandalized. Two examples are the Internet browsing consoles provided in some UK McDonald's outlets, and the BT Broadband Internet public phone boxes. This simplicity and ruggedness also makes them ideal for use in industrial computers."
+5161,"Because trackballs for personal computers are stationary, they may require less space for operation than a mouse, simplifying use in confined or cluttered areas such as a small desk or a rack-mounted terminal. They are generally preferred in laboratory setting for the same reason."
+5162,"Trackballs were often included in laptop computers, but since the late 1990s these have been replaced by touchpads and pointing sticks. Trackballs are still used as separate input devices with standard desktop computers, but this application is also moving to touchpads due to the prevalence of multi touch gesture control in new desktop operating systems."
+5163,People with a mobility impairment use trackballs as an assistive technology input device. Access to an alternative pointing device has become even more important for them with the dominance of graphically-oriented operating systems. There are many alternative systems to be considered. The control surface of a trackball is easier to manipulate and the buttons can be activated without affecting the pointer position.
+5164,"Trackball users also often state that they are not limited to using the device on a flat desk surface. Trackballs can be used whilst browsing a laptop in bed, or wirelessly from an armchair to a PC playing a movie. They are also useful for computing on boats or other unstable platforms where a rolling deck could produce undesirable input."
+5165,"Trackballs are generally either thumb-operated, with a ball about an inch in diameter or smaller moved by one digit  and the buttons clicked by others, or finger-operated, with a ball over two inches in diameter operated by the middle fingers and the buttons by the thumb and little finger. Users favor one format or another for reasons of comfort, mobility, precision, or because it reduces strain on one part of the hand/wrist. Most, but not all, finger-operated designs are symmetrical in design, making them usable by both hands, while thumb-operated designs are by their nature asymmetric or “handed,” allowing the smallest examples to be held in the air.  Thumb-operated trackballs are not generally available in left-handed configurations, due to small demand."
+5166,"Some computer users prefer a trackball over the more common mouse for ergonomic reasons.  There seems to be no conclusive evidence from studies performed to determine which type of pointing device works best for most applications.  Application users are encouraged to test different devices, and to maintain proper posture and scheduled breaks for comfort. Some disabled users find trackballs easier since they only have to move their thumb relative to their hand, instead of moving the whole hand, while others incur unacceptable fatigue of the thumb.  Elderly people sometimes have difficulty holding a mouse still while double-clicking; the trackball allows them to let go of the ball while using the button."
+5167,"At times when a user is browsing menus or websites rather than typing, it is also possible to hold a trackball in the right hand like a television remote control, operating the ball with the right thumb and pressing the buttons with the left thumb, thus giving the fingers a rest."
+5168,"Some mobile devices have trackballs, including those in the BlackBerry range, the T-Mobile Sidekick 3, and many early HTC smartphones.  These miniature trackballs are made to fit within the thickness of a mobile device, and are controlled by the tip of a finger or thumb. These have mostly been replaced on smartphones by touch screens, although on the BlackBerry range they were replaced by an ""optical trackball"" or ""optical trackpad"" before later being replaced with touch screens."
+5169,"In lieu of a scroll wheel, some mice include a tiny trackball sometimes called a scroll ball. A popular example is Apple's Mighty Mouse. Mice with a larger trackball on a side may be designed to stay stationary, using the trackball to move the mouse cursor instead of moving the mouse."
+5170,"Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time of day."
+5171,"The term real-time clock is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units. RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day."
+5172,"Although keeping time can be done without an RTC, using one has benefits:"
+5173,"A GPS receiver can shorten its startup time by comparing the current time, according to its RTC, with the time at which it last had a valid signal. If it has been less than a few hours, then the previous ephemeris is still usable."
+5174,Some motherboards are made without RTCs. The RTC may be omitted out of desire to save money or reduce possible sources of hardware failure.
+5175,"RTCs often have an alternate source of power, so they can continue to keep time while the primary source of power is off or unavailable. This alternate source of power is normally a lithium battery in older systems, but some newer systems use a supercapacitor, because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM."
+5176,"Most RTCs use a crystal oscillator, but some have the option of using the power line frequency. The crystal frequency is usually 32.768 kHz, the same frequency used in quartz clocks and watches. Being exactly 215 cycles per second, it is a convenient rate to use with simple binary counter circuits. The low frequency saves power, while remaining above human hearing range.  The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much."
+5177,"Some RTCs use a micromechanical resonator on the silicon chip of the RTC. This reduces the size and cost of an RTC by reducing its parts count. Micromechanical resonators are much more sensitive to temperature than quartz resonators. So, these compensate for temperature changes using an electronic thermometer and electronic logic."
+5178,"Typical crystal RTC accuracy specifications are from ±100 to ±20 parts per million , but temperature-compensated RTC ICs are available accurate to less than 5 parts per million. In practical terms, this is good enough to perform celestial navigation, the classic task of a chronometer. In 2011, chip-scale atomic clocks became available. Although vastly more expensive and power-hungry , they keep time within 50 parts per trillion ."
+5179,"Many integrated circuit manufacturers make RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductors, Texas Instruments, STMicroelectronics and Ricoh. A common RTC used in single-board computers is the Maxim Integrated DS1307."
+5180,"The RTC was introduced to PC compatibles by the IBM PC/AT in 1984, which used a Motorola MC146818 RTC. Later, Dallas Semiconductor made compatible RTCs, which were often used in older personal computers, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. A standard CMOS interface is available for the PC RTC."
+5181,"In newer computer systems, the RTC is integrated into the southbridge chip."
+5182,"Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals."
+5183,"Some modern computers receive clock information by digital radio and use it to promote time-standards. There are two common methods: Most cell phone protocols  directly provide the current local time.  If an internet radio is available, a computer may use the network time protocol. Computers used as local time servers occasionally use GPS or ultra-low frequency radio transmissions broadcast by a national standards organization ."
+5184,"The following system is well-known to embedded systems programmers, who sometimes must construct RTCs in systems that lack them. Most computers have one or more hardware timers that use timing signals from quartz crystals or ceramic resonators. These have inaccurate absolute timing  that is yet very repeatable . Software can do the math to make these into accurate RTCs. The hardware timer can produce a periodic interrupt, e.g. 50 Hz, to mimic a historic RTC .  However, it uses math to adjust the timing chain for accuracy:"
+5185,time = time + rate.
+5186,"When the ""time"" variable exceeds a constant, usually a power of two, the nominal, calculated clock time  is subtracted from ""time"", and the clock's timing-chain software is invoked to count fractions of seconds, seconds, etc. With 32-bit variables for time and rate, the mathematical resolution of ""rate"" can exceed one part per billion. The clock remains accurate because it will occasionally skip a fraction of a second, or increment by two fractions. The tiny skip  is imperceptible for almost all real uses of an RTC."
+5187,"The complexity with this system is determining the instantaneous corrected value for the variable ""rate"".  The simplest system tracks RTC time and reference time between two settings of the clock, and divides reference time by RTC time to find ""rate"". Internet time is often accurate to less than 20 milliseconds, so 8000 or more seconds  of separation between settings can usually divide the forty milliseconds  of error to less than 5 parts per million to get chronometer-like accuracy. The main complexity with this system is converting dates and times to counts of seconds, but methods are well known."
+5188,"If the RTC runs when a unit is off, usually the RTC will run at two rates, one when the unit is on and another when off. This is because the temperature and power-supply voltage in each state is consistent. To adjust for these states, the software calculates two rates. First, software records the RTC time, reference time, on seconds and off seconds for the two intervals between the last three times that the clock is set. Using this, it can measure the accuracy of the two intervals, with each interval having a different distribution of on and off seconds. The rate math solves two linear equations to calculate two rates, one for on and the other for off."
+5189,"Another approach measures the temperature of the oscillator with an electronic thermometer,  and uses a polynomial to calculate ""rate"" about once per minute. These require a calibration that measures the frequency at several temperatures, and then a linear regression to find the equation of temperature. The most common quartz crystals in a system are SC-cut crystals, and their rates over temperature can be characterized with a 3rd-degree polynomial. So, to calibrate these, the frequency is measured at four temperatures. The common tuning-fork-style crystals used in watches and many RTC components have parabolic  equations of temperature, and can be calibrated with only 3 measurements. MEMS oscillators vary, from 3rd degree to fifth degree polynomials, depending on their mechanical design, and so need from four to six calibration measurements. Something like this approach might be used in commercial RTC ICs, but the actual methods of efficient high-speed manufacturing are proprietary."
+5190,"Some computer designs such as smaller IBM System/360s,
+PDP-8s and Novas used a real-time clock that was accurate, simple and low cost. In Europe, North America and some other grids, the frequency of the AC mains is adjusted to the long-term frequency accuracy of the national standards. In those grids, clocks using AC mains can keep perfect time without adjustment. Such clocks are not practical in portable computers or grids  that do not regulate the frequency of AC mains."
+5191,"These computers' power supplies use a transformer or resistor divider to produce a sine wave at logic voltages. This signal is conditioned by a zero crossing detector, either using a linear amplifier, or a schmitt trigger. The result is a square wave with single, fast edges at the mains frequency. This logic signal triggers an interrupt. The interrupt handler software usually counts cycles, seconds, etc. In this way, it can provide an entire clock and calendar. In the IBM 360, the interrupt updates a 64-bit count of microseconds utilized by standardized systems software. The clock's jitter error is half if the clock interrupts for each zero crossing, instead of each cycle."
+5192,"The clock also usually formed the basis of computers' software timing chains; e.g. it was usually the timer used to switch tasks in an operating system. Counting timers used in modern computers provide similar features at lower precision, and may trace their requirements to this type of clock. "
+5193,"A software-based clock must be set each time its computer is turned on. Originally this was done by computer operators. When the Internet became commonplace, network time protocols were used to automatically set clocks of this type."
+5194,"Overlay keyboards generally consist of a flat grid of unmarked buttons. A sheet called an overlay is placed on the keyboard to identify each key, after the keyboard is programmed. The overlay can consist of any combination of words, symbols, or pictures."
+5195,"Overlay keyboards have several advantages over conventional keyboards or mice. They do not require memorization of shortcut keys  nor do they require a great deal of fine motor control, making them ideal for people who have difficulty using a conventional keyboard. Overlay keyboards are easy to clean, and resistant to spills or dust. The ability to change overlay sheets also makes it easy for a single overlay keyboard to have several different uses."
+5196,"Overlay keyboards are probably most often found in fast food restaurants, where they reduce the amount of time required to enter items. Overlay keyboards are also used in education, especially at the primary level. They can also be used by disabled people who have sensory or motor control difficulties."
+5197,"A computer mouse  is a hand-held pointing device that detects two-dimensional motion relative to a surface. This motion is typically translated into the motion of the pointer  on a display, which allows a smooth control of the graphical user interface of a computer."
+5198,"The first public demonstration of a mouse controlling a computer system was done by Doug Engelbart in 1968 as part of the Mother of All Demos. Mice originally used two separate wheels to directly track movement across a surface: one in the x-dimension and one in the Y. Later, the standard design shifted to use a ball rolling on a surface to detect motion, in turn connected to internal rollers. Most modern mice use optical movement detection with no moving parts. Though originally all mice were connected to a computer by a cable, many modern mice are cordless, relying on short-range radio communication with the connected system."
+5199,"In addition to moving a cursor, computer mice have one or more buttons to allow operations such as the selection of a menu item on a display. Mice often also feature other elements, such as touch surfaces and scroll wheels, which enable additional control and dimensional input."
+5200,"The earliest known written use of the term mouse or mice in reference to a computer pointing device is in Bill English's July 1965 publication, ""Computer-Aided Display Control"". This likely originated from its resemblance to the shape and size of a mouse, with the cord resembling its tail. The popularity of wireless mice without cords makes the resemblance less obvious."
+5201,"According to Roger Bates, a hardware designer under English, the term also came about because the cursor on the screen was for some unknown reason referred to as ""CAT"" and was seen by the team as if it would be chasing the new desktop device."
+5202,"The plural for the small rodent is always ""mice"" in modern usage. The plural for a computer mouse is either ""mice"" or ""mouses"" according to most dictionaries, with ""mice"" being more common. The first recorded plural usage is ""mice""; the online Oxford Dictionaries cites a 1984 use, and earlier uses include J. C. R. Licklider's ""The Computer as a Communication Device"" of 1968."
+5203,"The trackball, a related pointing device, was invented in 1946 by Ralph Benjamin as part of a post-World War II-era fire-control radar plotting system called the Comprehensive Display System . Benjamin was then working for the British Royal Navy Scientific Service. Benjamin's project used analog computers to calculate the future position of target aircraft based on several initial input points provided by a user with a joystick. Benjamin felt that a more elegant input device was needed and invented what they called a ""roller ball"" for this purpose."
+5204,"The device was patented in 1947, but only a prototype using a metal ball rolling on two rubber-coated wheels was ever built, and the device was kept as a military secret."
+5205,"Another early trackball was built by Kenyon Taylor, a British electrical engineer working in collaboration with Tom Cranston and Fred Longstaff. Taylor was part of the original Ferranti Canada, working on the Royal Canadian Navy's DATAR  system in 1952."
+5206,"DATAR was similar in concept to Benjamin's display. The trackball used four disks to pick up motion, two each for the X and Y directions. Several rollers provided mechanical support. When the ball was rolled, the pickup discs spun and contacts on their outer rim made periodic contact with wires, producing pulses of output with each movement of the ball. By counting the pulses, the physical movement of the ball could be determined. A digital computer calculated the tracks and sent the resulting data to other ships in a task force using pulse-code modulation radio signals. This trackball used a standard Canadian five-pin bowling ball. It was not patented, since it was a secret military project."
+5207,"Douglas Engelbart of the Stanford Research Institute  has been credited in published books by Thierry Bardini, Paul Ceruzzi, Howard Rheingold, and several others as the inventor of the computer mouse. Engelbart was also recognized as such in various obituary titles after his death in July 2013."
+5208,"By 1963, Engelbart had already established a research lab at SRI, the Augmentation Research Center , to pursue his objective of developing both hardware and software computer technology to ""augment"" human intelligence. That November, while attending a conference on computer graphics in Reno, Nevada, Engelbart began to ponder how to adapt the underlying principles of the planimeter to inputting X- and Y-coordinate data. On 14 November 1963, he first recorded his thoughts in his personal notebook about something he initially called a ""bug"", which is a ""3-point"" form could have a ""drop point and 2 orthogonal wheels"". He wrote that the ""bug"" would be ""easier"" and ""more natural"" to use, and unlike a stylus, it would stay still when let go, which meant it would be ""much better for coordination with the keyboard""."
+5209,"In 1964, Bill English joined ARC, where he helped Engelbart build the first mouse prototype. They christened the device the mouse as early models had a cord attached to the rear part of the device which looked like a tail, and in turn, resembled the common mouse. According to Roger Bates, a hardware designer in English, another reason for choosing this name was because the cursor on the screen was also referred to as ""CAT"" at this time."
+5210,"As noted above, this ""mouse"" was first mentioned in print in a July 1965 report, on which English was the lead author. On 9 December 1968, Engelbart publicly demonstrated the mouse at what would come to be known as The Mother of All Demos. Engelbart never received any royalties for it, as his employer SRI held the patent, which expired before the mouse became widely used in personal computers. In any event, the invention of the mouse was just a small part of Engelbart's much larger project of augmenting human intellect."
+5211,"Several other experimental pointing-devices developed for Engelbart's oN-Line System  exploited different body movements – for example, head-mounted devices attached to the chin or nose – but ultimately the mouse won out because of its speed and convenience. The first mouse, a bulky device  used two potentiometers perpendicular to each other and connected to wheels: the rotation of each wheel translated into motion along one axis. At the time of the ""Mother of All Demos"", Engelbart's group had been using their second-generation, 3-button mouse for about a year."
+5212,"On 2 October 1968, three years after Engelbart's prototype but more than two months before his public demo, a mouse device named Rollkugelsteuerung  was shown in a sales brochure by the German company AEG-Telefunken as an optional input device for the SIG 100 vector graphics terminal, part of the system around their process computer TR 86 and the TR 440  main frame. Based on an even earlier trackball device, the mouse device had been developed by the company in 1966 in what had been a parallel and independent discovery. As the name suggests and unlike Engelbart's mouse, the Telefunken model already had a ball  and two mechanical 4-bit rotational position transducers with Gray code-like states, allowing easy movement in any direction. The bits remained stable for at least two successive states to relax debouncing requirements. This arrangement was chosen so that the data could also be transmitted to the TR 86 front-end process computer and over longer distance telex lines with c. 50 baud. Weighing 465 grams , the device with a total height of about 7 cm  came in a c. 12 cm  diameter hemispherical injection-molded thermoplastic casing featuring one central push button."
+5213,"As noted above, the device was based on an earlier trackball-like device  that was embedded into radar flight control desks. This trackball had been originally developed by a team led by Rainer Mallebrein  at Telefunken Konstanz for the German Bundesanstalt für Flugsicherung  . It was part of the corresponding workstation system SAP 300 and the terminal SIG 3001, which had been designed and developed since 1963. Development for the TR 440 main frame began in 1965. This led to the development of the TR 86 process computer system with its SIG 100-86 terminal. Inspired by a discussion with a university customer, Mallebrein came up with the idea of ""reversing"" the existing Rollkugel trackball into a moveable mouse-like device in 1966, so that customers did not have to be bothered with mounting holes for the earlier trackball device. The device was finished in early 1968, and together with light pens and trackballs, it was commercially offered as an optional input device for their system starting later that year. Not all customers opted to buy the device, which added costs of DM 1,500 per piece to the already up to 20-million DM deal for the main frame, of which only a total of 46 systems were sold or leased. They were installed at more than 20 German universities including RWTH Aachen, Technical University Berlin, University of Stuttgart and Konstanz. Several Rollkugel mice installed at the Leibniz Supercomputing Centre in Munich in 1972 are well preserved in a museum, two others survived in a museum at Stuttgart University, two in Hamburg, the one from Aachen at the Computer History Museum in the US, and yet another sample was recently donated to the Heinz Nixdorf MuseumsForum  in Paderborn. Anecdotal reports claim that Telefunken's attempt to patent the device was rejected by the German Patent Office due to lack of inventiveness. For the air traffic control system, the Mallebrein team had already developed a precursor to touch screens in form of an ultrasonic-curtain-based pointing device in front of the display. In 1970, they developed a device named ""Touchinput-Einrichtung""  based on a conductively coated glass screen."
+5214,"The Xerox Alto was one of the first computers designed for individual use in 1973 and is regarded as the first modern computer to use a mouse. Alan Kay designed the 16-by-16 mouse cursor icon with its left edge vertical and right edge 45-degrees so it displays well on the bitmap.Inspired by PARC's Alto, the Lilith, a computer which had been developed by a team around Niklaus Wirth at ETH Zürich between 1978 and 1980, provided a mouse as well. The third marketed version of an integrated mouse shipped as a part of a computer and intended for personal computer navigation came with the Xerox 8010 Star in 1981."
+5215,"By 1982, the Xerox 8010 was probably the best-known computer with a mouse. The Sun-1 also came with a mouse, and the forthcoming Apple Lisa was rumored to use one, but the peripheral remained obscure; Jack Hawley of The Mouse House reported that one buyer for a large organization believed at first that his company sold lab mice. Hawley, who manufactured mice for Xerox, stated that ""Practically, I have the market all to myself right now""; a Hawley mouse cost $415. In 1982, Logitech introduced the P4 Mouse at the Comdex trade show in Las Vegas, its first hardware mouse. That same year Microsoft made the decision to make the MS-DOS program Microsoft Word mouse-compatible, and developed the first PC-compatible mouse. The Microsoft Mouse shipped in 1983, thus beginning the Microsoft Hardware division of the company. However, the mouse remained relatively obscure until the appearance of the Macintosh 128K  in 1984, and of the Amiga 1000 and the Atari ST in 1985."
+5216,"A mouse typically controls the motion of a pointer in two dimensions in a graphical user interface . The mouse turns movements of the hand backward and forward, left and right into equivalent electronic signals that in turn are used to move the pointer."
+5217,"The relative movements of the mouse on the surface are applied to the position of the pointer on the screen, which signals the point where actions of the user take place, so hand movements are replicated by the pointer. Clicking or pointing  can select files, programs or actions from a list of names, or  through small images called ""icons"" and other elements. For example, a text file might be represented by a picture of a paper notebook and clicking while the cursor points at this icon might cause a text editing program to open the file in a window."
+5218,Different ways of operating the mouse cause specific things to happen in the GUI:
+5219,"The Concept of Gestural Interfaces Gestural interfaces have become an integral part of modern computing, allowing users to interact with their devices in a more intuitive and natural way. In addition to traditional pointing-and-clicking actions, users can now employ gestural inputs to issue commands or perform specific actions. These stylized motions of the mouse cursor, known as ""gestures"", have the potential to enhance user experience and streamline workflow."
+5220,"Mouse Gestures in Action To illustrate the concept of gestural interfaces, let's consider a drawing program as an example. In this scenario, a user can employ a gesture to delete a shape on the canvas. By rapidly moving the mouse cursor in an ""x"" motion over the shape, the user can trigger the command to delete the selected shape. This gesture-based interaction enables users to perform actions quickly and efficiently without relying solely on traditional input methods."
+5221,"Challenges and Benefits of Gestural Interfaces While gestural interfaces offer a more immersive and interactive user experience, they also present challenges. One of the primary difficulties lies in the requirement of finer motor control from users. Gestures demand precise movements, which can be more challenging for individuals with limited dexterity or those who are new to this mode of interaction."
+5222,"However, despite these challenges, gestural interfaces have gained popularity due to their ability to simplify complex tasks and improve efficiency. Several gestural conventions have become widely adopted, making them more accessible to users. One such convention is the drag and drop gesture, which has become pervasive across various applications and platforms."
+5223,The Drag and Drop Gesture The drag and drop gesture is a fundamental gestural convention that enables users to manipulate objects on the screen seamlessly. It involves a series of actions performed by the user:
+5224,Pressing the mouse button while the cursor hovers over an interface object.
+5225,Moving the cursor to a different location while holding the button down.
+5226,Releasing the mouse button to complete the action.
+5227,"This gesture allows users to transfer or rearrange objects effortlessly. For instance, a user can drag and drop a picture representing a file onto an image of a trash can, indicating the intention to delete the file. This intuitive and visual approach to interaction has become synonymous with organizing digital content and simplifying file management tasks."
+5228,"Standard Semantic Gestures In addition to the drag and drop gesture, several other semantic gestures have emerged as standard conventions within the gestural interface paradigm. These gestures serve specific purposes and contribute to a more intuitive user experience. Some of the notable semantic gestures include:"
+5229,"Crossing-based goal: This gesture involves crossing a specific boundary or threshold on the screen to trigger an action or complete a task. For example, swiping across the screen to unlock a device or confirm a selection."
+5230,Menu traversal: Menu traversal gestures facilitate navigation through hierarchical menus or options. Users can perform gestures such as swiping or scrolling to explore different menu levels or activate specific commands.
+5231,"Pointing: Pointing gestures involve positioning the mouse cursor over an object or element to interact with it. This fundamental gesture enables users to select, click, or access contextual menus."
+5232,"Mouseover : Mouseover gestures occur when the cursor is positioned over an object without clicking. This action often triggers a visual change or displays additional information about the object, providing users with real-time feedback."
+5233,"These standard semantic gestures, along with the drag and drop convention, form the building blocks of gestural interfaces, allowing users to interact with digital content using intuitive and natural movements."
+5234,"At the end of 20th century, digitizer mice  with magnifying glass was used with AutoCAD for the digitizations of blueprints."
+5235,"Other uses of the mouse's input occur commonly in special application domains. In interactive three-dimensional graphics, the mouse's motion often translates directly into changes in the virtual objects' or camera's orientation. For example, in the first-person shooter genre of games , players usually employ the mouse to control the direction in which the virtual player's ""head"" faces: moving the mouse up will cause the player to look up, revealing the view above the player's head. A related function makes an image of an object rotate so that all sides can be examined. 3D design and animation software often modally chord many different combinations to allow objects and cameras to be rotated and moved through space with the few axes of movement mice can detect."
+5236,"When mice have more than one button, the software may assign different functions to each button. Often, the primary  button on the mouse will select items, and the secondary  button will bring up a menu of alternative actions applicable to that item. For example, on platforms with more than one button, the Mozilla web browser will follow a link in response to a primary button click, will bring up a contextual menu of alternative actions for that link in response to a secondary-button click, and will often open the link in a new tab or window in response to a click with the tertiary  mouse button."
+5237,"The German company Telefunken published on their early ball mouse on 2 October 1968. Telefunken's mouse was sold as optional equipment for their computer systems. Bill English, builder of Engelbart's original mouse, created a ball mouse in 1972 while working for Xerox PARC."
+5238,"The ball mouse replaced the external wheels with a single ball that could rotate in any direction. It came as part of the hardware package of the Xerox Alto computer. Perpendicular chopper wheels housed inside the mouse's body chopped beams of light on the way to light sensors, thus detecting in their turn the motion of the ball. This variant of the mouse resembled an inverted trackball and became the predominant form used with personal computers throughout the 1980s and 1990s. The Xerox PARC group also settled on the modern technique of using both hands to type on a full-size keyboard and grabbing the mouse when required."
+5239,"The ball mouse has two freely rotating rollers. These are located 90 degrees apart. One roller detects the forward-backward motion of the mouse and the other the left-right motion. Opposite the two rollers is a third one  that is spring-loaded to push the ball against the other two rollers. Each roller is on the same shaft as an encoder wheel that has slotted edges; the slots interrupt infrared light beams to generate electrical pulses that represent wheel movement. Each wheel's disc has a pair of light beams, located so that a given beam becomes interrupted or again starts to pass light freely when the other beam of the pair is about halfway between changes."
+5240,"Simple logic circuits interpret the relative timing to indicate which direction the wheel is rotating. This incremental rotary encoder scheme is sometimes called quadrature encoding of the wheel rotation, as the two optical sensors produce signals that are in approximately quadrature phase. The mouse sends these signals to the computer system via the mouse cable, directly as logic signals in very old mice such as the Xerox mice, and via a data-formatting IC in modern mice. The driver software in the system converts the signals into motion of the mouse cursor along X and Y axes on the computer screen."
+5241,"The ball is mostly steel, with a precision spherical rubber surface. The weight of the ball, given an appropriate working surface under the mouse, provides a reliable grip so the mouse's movement is transmitted accurately. Ball mice and wheel mice were manufactured for Xerox by Jack Hawley, doing business as The Mouse House in Berkeley, California, starting in 1975. Based on another invention by Jack Hawley, proprietor of the Mouse House, Honeywell produced another type of mechanical mouse. Instead of a ball, it had two wheels rotating at off axes. Key Tronic later produced a similar product."
+5242,"Modern computer mice took form at the École Polytechnique Fédérale de Lausanne  under the inspiration of Professor Jean-Daniel Nicoud and at the hands of engineer and watchmaker André Guignard. This new design incorporated a single hard rubber mouseball and three buttons, and remained a common design until the mainstream adoption of the scroll-wheel mouse during the 1990s. In 1985, René Sommer added a microprocessor to Nicoud's and Guignard's design. Through this innovation, Sommer is credited with inventing a significant component of the mouse, which made it more ""intelligent""; though optical mice from Mouse Systems had incorporated microprocessors by 1984."
+5243,"Another type of mechanical mouse, the ""analog mouse"" , uses potentiometers rather than encoder wheels, and is typically designed to be plug compatible with an analog joystick. The ""Color Mouse"", originally marketed by RadioShack for their Color Computer  was the best-known example."
+5244,"Early optical mice relied entirely on one or more light-emitting diodes  and an imaging array of photodiodes to detect movement relative to the underlying surface, eschewing the internal moving parts a mechanical mouse uses in addition to its optics. A laser mouse is an optical mouse that uses coherent  light."
+5245,"The earliest optical mice detected movement on pre-printed mousepad surfaces, whereas the modern LED optical mouse works on most opaque diffuse surfaces; it is usually unable to detect movement on specular surfaces like polished stone. Laser diodes provide good resolution and precision, improving performance on opaque specular surfaces. Later, more surface-independent optical mice use an optoelectronic sensor  to take successive images of the surface on which the mouse operates. Battery powered, wireless optical mice flash the LED intermittently to save power, and only glow steadily when movement is detected."
+5246,"Often called ""air mice"" since they do not require a surface to operate, inertial mice use a tuning fork or other accelerometer  to detect rotary movement for every axis supported. The most common models  work using 2 degrees of rotational freedom and are insensitive to spatial translation. The user requires only small wrist rotations to move the cursor, reducing user fatigue or ""gorilla arm""."
+5247,"Usually cordless, they often have a switch to deactivate the movement circuitry between use, allowing the user freedom of movement without affecting the cursor position. A patent for an inertial mouse claims that such mice consume less power than optically based mice, and offer increased sensitivity, reduced weight and increased ease-of-use. In combination with a wireless keyboard an inertial mouse can offer alternative ergonomic arrangements which do not require a flat work surface, potentially alleviating some types of repetitive motion injuries related to workstation posture."
+5248,"A 3D mouse is a computer input device for viewport interaction with at least three degrees of freedom , e.g. in 3D computer graphics software for manipulating virtual objects, navigating in the viewport, defining camera paths, posing, and desktop motion capture. 3D mice can also be used as spatial controllers for video game interaction, e.g. SpaceOrb 360. To perform such different tasks the used transfer function and the device stiffness are essential for efficient interaction."
+5249,The virtual motion is connected to the 3D mouse control handle via a transfer function. Position control means that the virtual position and orientation is proportional to the mouse handle's deflection whereas velocity control means that translation and rotation velocity of the controlled object is proportional to the handle deflection. A further essential property of a transfer function is its interaction metaphor:
+5250,"Ware and Osborne performed an experiment investigating these metaphors whereby it was shown that there is no single best metaphor. For manipulation tasks, the object-in-hand metaphor was superior, whereas for navigation tasks the camera-in-hand metaphor was superior."
+5251,Zhai used and the following three categories for device stiffness:
+5252,"Logitech 3D Mouse  was the first ultrasonic mouse and is an example of an isotonic 3D mouse having six degrees of freedom . Isotonic devices have also been developed with less than 6DoF, e.g. the Inspector at Technical University of Denmark ."
+5253,"Other examples of isotonic 3D mice are motion controllers, i.e. is a type of game controller that typically uses accelerometers to track motion. Motion tracking systems are also used for motion capture e.g. in the film industry, although that these tracking systems are not 3D mice in a strict sense, because motion capture only means recording 3D motion and not 3D interaction."
+5254,"Early 3D mice for velocity control were almost ideally isometric, e.g. SpaceBall 1003, 2003, 3003, and a device developed at Deutsches Zentrum für Luft und Raumfahrt , cf. US patent US4589810A."
+5255,"At DLR an elastic 6DoF sensor was developed that was used in Logitech's SpaceMouse and in the products of 3DConnexion. SpaceBall 4000 FLX has a maximum deflection of approximately 3 mm  at a maximum force of approximately 10N, that is, a stiffness of approximately 33 N/cm . SpaceMouse has a maximum deflection of 1.5 mm  at a maximum force of 4.4 N , that is, a stiffness of approximately 30 N/cm . Taking this development further, the softly elastic Sundinlabs SpaceCat was developed. SpaceCat has a maximum translational deflection of approximately 15 mm  and maximum rotational deflection of approximately 30° at a maximum force less than 2N, that is, a stiffness of approximately 1.3 N/cm . With SpaceCat Sundin and Fjeld reviewed five comparative experiments performed with different device stiffness and transfer functions and performed a further study comparing 6DoF softly elastic position control with 6DoF stiffly elastic velocity control in a positioning task. They concluded that for positioning tasks position control is to be preferred over velocity control. They could further conjecture the following two types of preferred 3D mouse usage:"
+5256,"3DConnexion's 3D mice have been commercially successful over decades. They are used in combination with the conventional mouse for CAD. The Space Mouse is used to orient the target object or change the viewpoint with the non-dominant hand, whereas the dominant hand operates the computer mouse for conventional CAD GUI operation. This is a kind of space-multiplexed input where the 6 DoF input device acts as a graspable user interface that is always connected to the view port."
+5257,"With force feedback the device stiffness can dynamically be adapted to the task just performed by the user, e.g. performing positioning tasks with less stiffness than navigation tasks."
+5258,"In 2000, Logitech introduced a ""tactile mouse"" known as the ""iFeel Mouse"" developed by Immersion Corporation that contained a small actuator to enable the mouse to generate simulated physical sensations. Such a mouse can augment user-interfaces with haptic feedback, such as giving feedback when crossing a window boundary. To surf the internet by touch-enabled mouse was first developed in 1996 and first implemented commercially by the Wingman Force Feedback Mouse. It requires the user to be able to feel depth or hardness; this ability was realized with the first electrorheological tactile mice but never marketed."
+5259,"Tablet digitizers are sometimes used with accessories called pucks, devices which rely on absolute positioning, but can be configured for sufficiently mouse-like relative tracking that they are sometimes marketed as mice."
+5260,"As the name suggests, this type of mouse is intended to provide optimum comfort and avoid injuries such as carpal tunnel syndrome, arthritis, and other repetitive strain injuries. It is designed to fit natural hand position and movements, to reduce discomfort."
+5261,"When holding a typical mouse, the ulna and radius bones on the arm are crossed. Some designs attempt to place the palm more vertically, so the bones take more natural parallel position."
+5262,"Increasing mouse height and angling the mouse topcase can improve wrist posture without negatively affecting performance. Some limit wrist movement, encouraging arm movement instead, that may be less precise but more optimal from the health point of view. A mouse may be angled from the thumb downward to the opposite side – this is known to reduce wrist pronation. However such optimizations make the mouse right or left hand specific, making more problematic to change the tired hand. Time has criticized manufacturers for offering few or no left-handed ergonomic mice: ""Oftentimes I felt like I was dealing with someone who'd never actually met a left-handed person before."""
+5263,"Another solution is a pointing bar device. The so-called roller bar mouse is positioned snugly in front of the keyboard, thus allowing bi-manual accessibility."
+5264,"These mice are specifically designed for use in computer games. They typically employ a wider array of controls and buttons and have designs that differ radically from traditional mice. They may also have decorative monochrome or programmable RGB LED lighting. The additional buttons can often be used for changing the sensitivity of the mouse or they can be assigned  to macros . It is also common for game mice, especially those designed for use in real-time strategy games such as StarCraft, or in multiplayer online battle arena games such as League of Legends to have a relatively high sensitivity, measured in dots per inch , which can be as high as 25,600. DPI and CPI are the same values that refer to the mouse's sensitivity. DPI is a misnomer used in the gaming world, and many manufacturers use it to refer to CPI, counts per inch. Some advanced mice from gaming manufacturers also allow users to adjust the weight of the mouse by adding or subtracting weights to allow for easier control. Ergonomic quality is also an important factor in gaming mouse, as extended gameplay times may render further use of the mouse to be uncomfortable. Some mice have been designed to have adjustable features such as removable and/or elongated palm rests, horizontally adjustable thumb rests and pinky rests. Some mice may include several different rests with their products to ensure comfort for a wider range of target consumers. Gaming mice are held by gamers in three styles of grip:"
+5265,"Palm Grip: the hand rests on the mouse, with extended fingers."
+5266,"Claw Grip: palm rests on the mouse, bent fingers."
+5267,"Finger-Tip Grip: bent fingers, palm does not touch the mouse."
+5268,"To transmit their input, typical cabled mice use a thin electrical cord terminating in a standard connector, such as RS-232C, PS/2, ADB, or USB. Cordless mice instead transmit data via infrared radiation  or radio , although many such cordless interfaces are themselves connected through the aforementioned wired serial buses."
+5269,"While the electrical interface and the format of the data transmitted by commonly available mice is currently standardized on USB, in the past it varied between different manufacturers. A bus mouse used a dedicated interface card for connection to an IBM PC or compatible computer."
+5270,"Mouse use in DOS applications became more common after the introduction of the Microsoft Mouse, largely because Microsoft provided an open standard for communication between applications and mouse driver software. Thus, any application written to use the Microsoft standard could use a mouse with a driver that implements the same API, even if the mouse hardware itself was incompatible with Microsoft's. This driver provides the state of the buttons and the distance the mouse has moved in units that its documentation calls ""mickeys""."
+5271,"In the 1970s, the Xerox Alto mouse, and in the 1980s the Xerox optical mouse, used a quadrature-encoded X and Y interface. This two-bit encoding per dimension had the property that only one bit of the two would change at a time, like a Gray code or Johnson counter, so that the transitions would not be misinterpreted when asynchronously sampled."
+5272,"The earliest mass-market mice, such as on the original Macintosh, Amiga, and Atari ST mice used a D-subminiature 9-pin connector to send the quadrature-encoded X and Y axis signals directly, plus one pin per mouse button. The mouse was a simple optomechanical device, and the decoding circuitry was all in the main computer."
+5273,"The DE-9 connectors were designed to be electrically compatible with the joysticks popular on numerous 8-bit systems, such as the Commodore 64 and the Atari 2600. Although the ports could be used for both purposes, the signals must be interpreted differently. As a result, plugging a mouse into a joystick port causes the ""joystick"" to continuously move in some direction, even if the mouse stays still, whereas plugging a joystick into a mouse port causes the ""mouse"" to only be able to move a single pixel in each direction."
+5274,"Because the IBM PC did not have a quadrature decoder built in, early PC mice used the RS-232C serial port to communicate encoded mouse movements, as well as provide power to the mouse's circuits. The Mouse Systems Corporation  version used a five-byte protocol and supported three buttons. The Microsoft version used a three-byte protocol and supported two buttons. Due to the incompatibility between the two protocols, some manufacturers sold serial mice with a mode switch: ""PC"" for MSC mode, ""MS"" for Microsoft mode."
+5275,"In 1986 Apple first implemented the Apple Desktop Bus allowing the daisy chaining of up to 16 devices, including mice and other devices on the same bus with no configuration whatsoever. Featuring only a single data pin, the bus used a purely polled approach to device communications and survived as the standard on mainstream models  until 1998 when Apple's iMac line of computers joined the industry-wide switch to using USB. Beginning with the Bronze Keyboard PowerBook G3 in May 1999, Apple dropped the external ADB port in favor of USB, but retained an internal ADB connection in the PowerBook G4 for communication with its built-in keyboard and trackpad until early 2005."
+5276,"With the arrival of the IBM PS/2 personal-computer series in 1987, IBM introduced the eponymous PS/2 port for mice and keyboards, which other manufacturers rapidly adopted. The most visible change was the use of a round 6-pin mini-DIN, in lieu of the former 5-pin MIDI style full sized DIN 41524 connector. In default mode  a PS/2 mouse communicates motion, and the state of each button, by means of 3-byte packets. For any motion, button press or button release event, a PS/2 mouse sends, over a bi-directional serial port, a sequence of three bytes, with the following format:"
+5277,"Here, XS and YS represent the sign bits of the movement vectors, XV and YV indicate an overflow in the respective vector component, and LB, MB and RB indicate the status of the left, middle and right mouse buttons . PS/2 mice also understand several commands for reset and self-test, switching between different operating modes, and changing the resolution of the reported motion vectors."
+5278,"A Microsoft IntelliMouse relies on an extension of the PS/2 protocol: the ImPS/2 or IMPS/2 protocol . It initially operates in standard PS/2 format, for backward compatibility. After the host sends a special command sequence, it switches to an extended format in which a fourth byte carries information about wheel movements. The IntelliMouse Explorer works analogously, with the difference that its 4-byte packets also allow for two additional buttons ."
+5279,"Mouse vendors also use other extended formats, often without providing public documentation. The Typhoon mouse uses 6-byte packets which can appear as a sequence of two standard 3-byte packets, such that an ordinary PS/2 driver can handle them. For 3D  input, vendors have made many extensions both to the hardware and to software. In the late 1990s, Logitech created ultrasound based tracking which gave 3D input to a few millimeters accuracy, which worked well as an input device but failed as a profitable product. In 2008, Motion4U introduced its ""OptiBurst"" system using IR tracking for use as a Maya  plugin."
+5280,Almost all wired mice today use USB and the USB human interface device class for communication.
+5281,"Cordless or wireless mice transmit data via radio. Some mice connect to the computer through Bluetooth or Wi-Fi, while others use a receiver that plugs into the computer, for example through a USB port."
+5282,"Many mice that use a USB receiver have a storage compartment for it inside the mouse. Some ""nano receivers"" are designed to be small enough to remain plugged into a laptop during transport, while still being large enough to easily remove."
+5283,"MS-DOS and Windows 1.0 support connecting a mouse such as a Microsoft Mouse via multiple interfaces: BallPoint, Bus , Serial port or PS/2."
+5284,"Windows 98 added built-in support for USB Human Interface Device class , with native vertical scrolling support. Windows 2000 and Windows Me expanded this built-in support to 5-button mice."
+5285,"Windows XP Service Pack 2 introduced a Bluetooth stack, allowing Bluetooth mice to be used without any USB receivers. Windows Vista added native support for horizontal scrolling and standardized wheel movement granularity for finer scrolling."
+5286,Windows 8 introduced BLE  mouse/HID support.
+5287,"Some systems allow two or more mice to be used at once as input devices. Late-1980s era home computers such as the Amiga used this to allow computer games with two players interacting on the same computer . The same idea is sometimes used in collaborative software, e.g. to simulate a whiteboard that multiple users can draw on without passing a single mouse around."
+5288,"Microsoft Windows, since Windows 98, has supported multiple simultaneous pointing devices. Because Windows only provides a single screen cursor, using more than one device at the same time requires cooperation of users or applications designed for multiple input devices."
+5289,Multiple mice are often used in multi-user gaming in addition to specially designed devices that provide several input interfaces.
+5290,Windows also has full support for multiple input/mouse configurations for multi-user environments.
+5291,"Starting with Windows XP, Microsoft introduced an SDK for developing applications that allow multiple input devices to be used at the same time with independent cursors and independent input points. However, it no longer appears to be available."
+5292,"The introduction of Windows Vista and Microsoft Surface  introduced a new set of input APIs that were adopted into Windows 7, allowing for 50 points/cursors, all controlled by independent users. The new input points provide traditional mouse input; however, they were designed with other input technologies like touch and image in mind. They inherently offer 3D coordinates along with pressure, size, tilt, angle, mask, and even an image bitmap to see and recognize the input point/object on the screen."
+5293,"As of 2009, Linux distributions and other operating systems that use X.Org, such as OpenSolaris and FreeBSD, support 255 cursors/input points through Multi-Pointer X. However, currently no window managers support Multi-Pointer X leaving it relegated to custom software usage."
+5294,There have also been propositions of having a single operator use two mice simultaneously as a more sophisticated means of controlling various graphics and multimedia applications.
+5295,"Mouse buttons are microswitches which can be pressed to select or interact with an element of a graphical user interface, producing a distinctive clicking sound."
+5296,"Since around the late 1990s, the three-button scrollmouse has become the de facto standard. Users most commonly employ the second button to invoke a contextual menu in the computer's software user interface, which contains options specifically tailored to the interface element over which the mouse cursor currently sits. By default, the primary mouse button sits located on the left-hand side of the mouse, for the benefit of right-handed users; left-handed users can usually reverse this configuration via software."
+5297,"Nearly all mice now have an integrated input primarily intended for scrolling on top, usually a single-axis digital wheel or rocker switch which can also be depressed to act as a third button. Though less common, many mice instead have two-axis inputs such as a tiltable wheel, trackball, or touchpad. Those with a trackball may be designed to stay stationary, using the trackball instead of moving the mouse."
+5298,"Mickeys per second is a unit of measurement for the speed and movement direction of a computer mouse, where direction is often expressed as ""horizontal"" versus ""vertical"" mickey count. However, speed can also refer to the ratio between how many pixels the cursor moves on the screen and how far the mouse moves on the mouse pad, which may be expressed as pixels per mickey, pixels per inch, or pixels per centimeter."
+5299,"The computer industry often measures mouse sensitivity in terms of counts per inch , commonly expressed as dots per inch  – the number of steps the mouse will report when it moves one inch. In early mice, this specification was called pulses per inch . The mickey originally referred to one of these counts, or one resolvable step of motion. If the default mouse-tracking condition involves moving the cursor by one screen-pixel or dot on-screen per reported step, then the CPI does equate to DPI: dots of cursor motion per inch of mouse motion. The CPI or DPI as reported by manufacturers depends on how they make the mouse; the higher the CPI, the faster the cursor moves with mouse movement. However, software can adjust the mouse sensitivity, making the cursor move faster or slower than its CPI. As of 2007, software can change the speed of the cursor dynamically, taking into account the mouse's absolute speed and the movement from the last stop-point. In most software, an example being the Windows platforms, this setting is named ""speed"", referring to ""cursor precision"". However, some operating systems name this setting ""acceleration"", the typical Apple OS designation. This term is incorrect. Mouse acceleration in most mouse software refers to the change in speed of the cursor over time while the mouse movement is constant."
+5300,"For simple software, when the mouse starts to move, the software will count the number of ""counts"" or ""mickeys"" received from the mouse and will move the cursor across the screen by that number of pixels . The cursor will move slowly on the screen, with good precision. When the movement of the mouse passes the value set for some threshold, the software will start to move the cursor faster, with a greater rate factor. Usually, the user can set the value of the second rate factor by changing the ""acceleration"" setting."
+5301,"Operating systems sometimes apply acceleration, referred to as ""ballistics"", to the motion reported by the mouse. For example, versions of Windows prior to Windows XP doubled reported values above a configurable threshold, and then optionally doubled them again above a second configurable threshold. These doublings applied separately in the X and Y directions, resulting in very nonlinear response."
+5302,"Engelbart's original mouse did not require a mousepad; the mouse had two large wheels which could roll on virtually any surface. However, most subsequent mechanical mice starting with the steel roller ball mouse have required a mousepad for optimal performance."
+5303,"The mousepad, the most common mouse accessory, appears most commonly in conjunction with mechanical mice, because to roll smoothly the ball requires more friction than common desk surfaces usually provide. So-called ""hard mousepads"" for gamers or optical/laser mice also exist."
+5304,"Most optical and laser mice do not require a pad, the notable exception being early optical mice which relied on a grid on the pad to detect movement . Whether to use a hard or soft mousepad with an optical mouse is largely a matter of personal preference. One exception occurs when the desk surface creates problems for the optical or laser tracking, for example, a transparent or reflective surface, such as glass."
+5305,"Some mice also come with small ""pads"" attached to the bottom surface, also called mouse feet or mouse skates, that help the user slide the mouse smoothly across surfaces."
+5306,"Around 1981, Xerox included mice with its Xerox Star, based on the mouse used in the 1970s on the Alto computer at Xerox PARC. Sun Microsystems, Symbolics, Lisp Machines Inc., and Tektronix also shipped workstations with mice, starting in about 1981. Later, inspired by the Star, Apple Computer released the Apple Lisa, which also used a mouse. However, none of these products achieved large-scale success. Only with the release of the Apple Macintosh in 1984 did the mouse see widespread use."
+5307,"The Macintosh design, commercially successful and technically influential, led many other vendors to begin producing mice or including them with their other computer products ."
+5308,"The widespread adoption of graphical user interfaces in the software of the 1980s and 1990s made mice all but indispensable for controlling computers. In November 2008, Logitech built their billionth mouse."
+5309,The device often functions as an interface for PC-based computer games and sometimes for video game consoles. The Classic Mac OS Desk Accessory Puzzle in 1984 was the first game designed specifically for a mouse.
+5310,"FPSs naturally lend themselves to separate and simultaneous control of the player's movement and aim, and on computers this has traditionally been achieved with a combination of keyboard and mouse. Players use the X-axis of the mouse for looking  left and right, and the Y-axis for looking up and down; the keyboard is used for movement and supplemental inputs."
+5311,"Many shooting genre players prefer a mouse over a gamepad analog stick because the wide range of motion offered by a mouse allows for faster and more varied control. Although an analog stick allows the player more granular control, it is poor for certain movements, as the player's input is relayed based on a vector of both the stick's direction and magnitude. Thus, a small but fast movement  using a gamepad requires the player to quickly move the stick from its rest position to the edge and back again in quick succession, a difficult maneuver. In addition the stick also has a finite magnitude; if the player is currently using the stick to move at a non-zero velocity their ability to increase the rate of movement of the camera is further limited based on the position their displaced stick was already at before executing the maneuver. The effect of this is that a mouse is well suited not only to small, precise movements but also to large, quick movements and immediate, responsive movements; all of which are important in shooter gaming. This advantage also extends in varying degrees to similar game styles such as third-person shooters."
+5312,"Some incorrectly ported games or game engines have acceleration and interpolation curves which unintentionally produce excessive, irregular, or even negative acceleration when used with a mouse instead of their native platform's non-mouse default input device. Depending on how deeply hardcoded this misbehavior is, internal user patches or external 3rd-party software may be able to fix it. Individual game engines will also have their own sensitivities. This often restricts one from taking a game's existing sensitivity, transferring it to another, and acquiring the same 360 rotational measurements. A sensitivity converter is required in order to translate rotational movements properly."
+5313,"Due to their similarity to the WIMP desktop metaphor interface for which mice were originally designed, and to their own tabletop game origins, computer strategy games are most commonly played with mice. In particular, real-time strategy and MOBA games usually require the use of a mouse."
+5314,"The left button usually controls primary fire. If the game supports multiple fire modes, the right button often provides secondary fire from the selected weapon. Games with only a single fire mode will generally map secondary fire to aim down the weapon sights. In some games, the right button may also invoke accessories for a particular weapon, such as allowing access to the scope of a sniper rifle or allowing the mounting of a bayonet or silencer."
+5315,"Players can use a scroll wheel for changing weapons . On most first person shooter games, programming may also assign more functions to additional buttons on mice with more than three controls. A keyboard usually controls movement  and other functions such as changing posture. Since the mouse serves for aiming, a mouse that tracks movement accurately and with less lag  will give a player an advantage over players with less accurate or slower mice. In some cases the right mouse button may be used to move the player forward, either in lieu of, or in conjunction with the typical WASD configuration."
+5316,"Many games provide players with the option of mapping their own choice of a key or button to a certain control. An early technique of players, circle strafing, saw a player continuously strafing while aiming and shooting at an opponent by walking in circle around the opponent with the opponent at the center of the circle. Players could achieve this by holding down a key for strafing while continuously aiming the mouse toward the opponent."
+5317,"Games using mice for input are so popular that many manufacturers make mice specifically for gaming. Such mice may feature adjustable weights, high-resolution optical or laser components, additional buttons, ergonomic shape, and other features such as adjustable CPI. Mouse Bungees are typically used with gaming mice because it eliminates the annoyance of the cable."
+5318,"Many games, such as first- or third-person shooters, have a setting named ""invert mouse"" or similar  which allows the user to look downward by moving the mouse forward and upward by moving the mouse backward . This control system resembles that of aircraft control sticks, where pulling back causes pitch up and pushing forward causes pitch down; computer joysticks also typically emulate this control-configuration."
+5319,"After id Software's commercial hit of Doom, which did not support vertical aiming, competitor Bungie's Marathon became the first first-person shooter to support using the mouse to aim up and down. Games using the Build engine had an option to invert the Y-axis. The ""invert"" feature actually made the mouse behave in a manner that users now regard as non-inverted . Soon after, id Software released Quake, which introduced the invert feature as users now know it."
+5320,"In 1988, the VTech Socrates educational video game console featured a wireless mouse with an attached mouse pad as an optional controller used for some games. In the early 1990s, the Super Nintendo Entertainment System video game system featured a mouse in addition to its controllers. A mouse was also released for the Nintendo 64, although it was only released in Japan. The 1992 game Mario Paint in particular used the mouse's capabilities, as did its Japanese-only successor Mario Artist on the N64 for its 64DD disk drive peripheral in 1999. Sega released official mice for their Genesis/Mega Drive, Saturn and Dreamcast consoles. NEC sold official mice for its PC Engine and PC-FX consoles. Sony released an official mouse product for the PlayStation console, included one along with the Linux for PlayStation 2 kit, as well as allowing owners to use virtually any USB mouse with the PS2, PS3, and PS4. Nintendo's Wii also had this feature implemented in a later software update, and this support was retained on its successor, the Wii U. Microsoft's Xbox line of game consoles  also had universal-wide mouse support using USB."
+5321,"A microphone, colloquially called a mic , or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live and recorded audio engineering, sound recording, two-way radios, megaphones, and radio and television broadcasting. They are also used in computers and other electronic devices, such as mobile phones, for recording sounds, speech recognition, VoIP, and other purposes, such as ultrasonic sensors or knock sensors."
+5322,"Several types of microphone are used today, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal.  The most common are the dynamic microphone, which uses a coil of wire suspended in a magnetic field; the condenser microphone, which uses the vibrating diaphragm as a capacitor plate; and the contact microphone, which uses a crystal of piezoelectric material.  Microphones typically need to be connected to a preamplifier before the signal can be recorded or reproduced."
+5323,"In order to speak to larger groups of people, a need arose to increase the volume of the human voice. The earliest devices used to achieve this were acoustic megaphones.  Some of the first examples, from fifth-century-BC Greece, were theater masks with horn-shaped mouth openings that acoustically amplified the voice of actors in amphitheaters. In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the ""lovers' telephone"" made of stretched wire with a cup attached at each end."
+5324,"In 1856, Italian inventor Antonio Meucci developed a dynamic microphone based on the generation of electric current by moving a coil of wire to various depths in a magnetic field. This method of modulation was also the most enduring method for the technology of the telephone as well. Speaking of his device, Meucci wrote in 1857, ""It consists of a vibrating diaphragm and an electrified magnet with a spiral wire that wraps around it. The vibrating diaphragm alters the current of the magnet. These alterations of current, transmitted to the other end of the wire, create analogous vibrations of the receiving diaphragm and reproduce the word."""
+5325,"In 1861, German inventor Johann Philipp Reis built an early sound transmitter  that used a metallic strip attached to a vibrating membrane that would produce intermittent current. Better results were achieved in 1876 with the ""liquid transmitter"" design in early telephones from Alexander Graham Bell and Elisha Gray – the diaphragm was attached to a conductive rod in an acid solution. These systems, however, gave a very poor sound quality."
+5326,"The first microphone that enabled proper voice telephony was the  carbon microphone. This was independently developed by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent  in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention. The Berliner microphone found commercial success through the use by Alexander Graham Bell for his telephone and Berliner became employed by Bell. The carbon microphone was critical in the development of telephony, broadcasting and the recording industries. Thomas Edison refined the carbon microphone into his carbon-button transmitter of 1886. This microphone was employed at the first radio broadcast ever, a performance at the New York Metropolitan Opera House in 1910."
+5327,"In 1916, E.C. Wente of Western Electric developed the next breakthrough with the first condenser microphone. In 1923, the first practical moving coil microphone was built. The Marconi-Sykes magnetophone, developed by Captain H. J. Round, became the standard for BBC studios in London. This was improved in 1930 by Alan Blumlein and Herbert Holman who released the HB1A and was the best standard of the day."
+5328,"Also in 1923, the ribbon microphone was introduced, another electromagnetic type, believed to have been developed by Harry F. Olson, who applied the concept used in a ribbon speaker to making a microphone. Over the years these microphones were developed by several companies, most notably RCA that made large advancements in pattern control, to give the microphone directionality. With television and film technology booming there was a demand for high-fidelity microphones and greater directionality. Electro-Voice responded with their Academy Award-winning shotgun microphone in 1963."
+5329,"During the second half of the 20th century, development advanced quickly with the Shure Brothers bringing out the SM58 and SM57."
+5330,"Microphones are categorized by their transducer principle, such as condenser, dynamic, etc., and by their directional characteristics. Sometimes other characteristics such as diaphragm size, intended use or orientation of the principal sound input to the principal axis  of the microphone are used to describe the microphone."
+5331,"The condenser microphone, invented at Western Electric in 1916 by E. C. Wente, is also called a capacitor microphone or electrostatic microphone—capacitors were historically called condensers. The diaphragm acts as one plate of a capacitor, and audio vibrations produce changes in the distance between the plates. Because the capacitance of the plates is inversely proportional to the distance between them, the vibrations produce changes in capacitance. These changes in capacitance are used to measure the audio signal. The assembly of fixed and movable plates is called an ""element"" or ""capsule""."
+5332,"Condenser microphones span the range from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones. They generally produce a high-quality audio signal and are now the popular choice in laboratory and recording studio applications. The inherent suitability of this technology is due to the very small mass that must be moved by the incident sound wave, unlike other microphone types that require the sound wave to do more work."
+5333,"Condenser microphones require a power source, provided either via microphone inputs on equipment as phantom power or from a small battery. Power is necessary for establishing the capacitor plate voltage and is also needed to power the microphone electronics . Condenser microphones are also available with two diaphragms that can be electrically connected to provide a range of polar patterns , such as cardioid, omnidirectional, and figure-eight. It is also possible to vary the pattern continuously with some microphones, for example, the Røde NT2000 or CAD M179."
+5334,"There are two main categories of condenser microphones, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, and radio frequency  or high frequency  condenser microphones."
+5335,"With a DC-biased condenser microphone, the plates are biased with a fixed charge . The voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation , where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor does change very slightly, but at audible frequencies it is sensibly constant. The capacitance of the capsule  and the value of the bias resistor  form a filter that is high-pass for the audio signal, and low-pass for the bias voltage. Note that the time constant of an RC circuit equals the product of the resistance and capacitance."
+5336,"Within the time frame of the capacitance change , the charge is practically constant and the voltage across the capacitor changes instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified for performance or recording. In most cases, the electronics in the microphone itself contribute no voltage gain as the voltage differential is quite significant, up to several volts for high sound levels. Since this is a very high impedance circuit, only current gain is usually needed, with the voltage remaining constant."
+5337,"RF condenser microphones use a comparatively low RF voltage, generated by a low-noise oscillator. The signal from the oscillator may either be amplitude modulated by the capacitance changes produced by the sound waves moving the capsule diaphragm, or the capsule may be part of a resonant circuit that modulates the frequency of the oscillator signal. Demodulation yields a low-noise audio frequency signal with a very low source impedance. The absence of a high bias voltage permits the use of a diaphragm with looser tension, which may be used to achieve wider frequency response due to higher compliance. The RF biasing process results in a lower electrical impedance capsule, a useful by-product of which is that RF condenser microphones can be operated in damp weather conditions that could create problems in DC-biased microphones with contaminated insulating surfaces. The Sennheiser ""MKH"" series of microphones use the RF biasing technique. A covert, remotely energised application of the same physical principle was devised by Soviet Russian inventor Leon Theremin and used to bug the US Ambassador's Residence in Moscow between 1945 and 1952."
+5338,"An electret microphone is a type of condenser microphone invented by Gerhard Sessler and Jim West at Bell laboratories in 1962. The externally applied charge used for a conventional condenser microphone is replaced by a permanent charge in an electret material. An electret is a ferroelectric material that has been permanently electrically charged or polarized. The name comes from electrostatic and magnet; a static charge is embedded in an electret by the alignment of the static charges in the material, much the way a permanent magnet is made by aligning the magnetic domains in a piece of iron."
+5339,"Due to their good performance and ease of manufacture, hence low cost, the vast majority of microphones made today are electret microphones; a semiconductor manufacturer estimates annual production at over one billion units. They are used in many applications, from high-quality recording and lavalier  use to built-in microphones in small sound recording devices and telephones. Prior to the proliferation of MEMS microphones, nearly all cell-phone, computer, PDA and headset microphones were electret types."
+5340,"Unlike other capacitor microphones, they require no polarizing voltage, but often contain an integrated preamplifier that does require power . This preamplifier is frequently phantom powered in sound reinforcement and studio applications. Monophonic microphones designed for personal computers , sometimes called multimedia microphones, use a 3.5 mm plug as usually used, without power, for stereo; the ring, instead of carrying the signal for a second channel, carries power via a resistor from  a 5 V supply in the computer. Stereophonic microphones use the same connector; there is no obvious way to determine which standard is used by equipment and microphones."
+5341,A valve microphone is a condenser microphone that uses a vacuum tube  amplifier. They remain popular with enthusiasts of tube sound.
+5342,"The dynamic microphone  works via electromagnetic induction. They are robust, relatively inexpensive and resistant to moisture. This, coupled with their potentially high gain before feedback, makes them ideal for on-stage use."
+5343,"Dynamic microphones use the same dynamic principle as in a loudspeaker, only reversed. A small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. When sound enters through the windscreen of the microphone, the sound wave moves the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying voltage across the coil through electromagnetic induction. A single dynamic membrane does not respond linearly to all audio frequencies. For this reason, some microphones utilize multiple membranes for the different parts of the audio spectrum and then combine the resulting signals. Combining the multiple signals correctly is difficult; designs that do this are rare and tend to be expensive. On the other hand, there are several designs that are more specifically aimed towards isolated parts of the audio spectrum. The AKG D112, for example, is designed for bass response rather than treble."
+5344,"Ribbon microphones use a thin, usually corrugated metal ribbon suspended in a magnetic field. The ribbon is electrically connected to the microphone's output, and its vibration within the magnetic field generates the electrical signal. Ribbon microphones are similar to moving coil microphones in the sense that both produce sound by means of magnetic induction. Basic ribbon microphones detect sound in a bi-directional  pattern because the ribbon is open on both sides.  Also, because the ribbon has much less mass it responds to the air velocity rather than the sound pressure. Though the symmetrical front and rear pickup can be a nuisance in normal stereo recording, the high side rejection can be used to advantage by positioning a ribbon microphone horizontally, for example above cymbals, so that the rear lobe picks up sound only from the cymbals. Crossed figure 8, or Blumlein pair, stereo recording is gaining in popularity, and the figure-eight response of a ribbon microphone is ideal for that application."
+5345,"Other directional patterns are produced by enclosing one side of the ribbon in an acoustic trap or baffle, allowing sound to reach only one side. The classic RCA Type 77-DX microphone has several externally adjustable positions of the internal baffle, allowing the selection of several response patterns ranging from ""figure-eight"" to ""unidirectional"". Such older ribbon microphones, some of which still provide high-quality sound reproduction, were once valued for this reason, but a good low-frequency response could be obtained only when the ribbon was suspended very loosely, which made them relatively fragile. Modern ribbon materials, including new nanomaterials, have now been introduced that eliminate those concerns and even improve the effective dynamic range of ribbon microphones at low frequencies. Protective wind screens can reduce the danger of damaging a vintage ribbon, and also reduce plosive artifacts in the recording. Properly designed wind screens produce negligible treble attenuation. In common with other classes of dynamic microphone, ribbon microphones do not require phantom power; in fact, this voltage can damage some older ribbon microphones. Some new modern ribbon microphone designs incorporate a preamplifier and, therefore, do require phantom power, and circuits of modern passive ribbon microphones  are specifically designed to resist damage to the ribbon and transformer by phantom power. Also there are new ribbon materials available that are immune to wind blasts and phantom power."
+5346,"The carbon microphone was the earliest type of microphone. The carbon button microphone , uses a capsule or button containing carbon granules pressed between two metal plates like the Berliner and Edison microphones. A voltage is applied across the metal plates, causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. The changes in resistance cause a corresponding change in the current flowing through the microphone, producing the electrical signal. Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range but are very robust devices. The Boudet microphone, which used relatively large carbon balls, was similar to the granule carbon button microphones."
+5347,"Unlike other microphone types, the carbon microphone can also be used as a type of amplifier, using a small amount of sound energy to control a larger amount of electrical energy. Carbon microphones found use as early telephone repeaters, making long-distance phone calls possible in the era before vacuum tubes. Called a Brown's relay, these repeaters worked by mechanically coupling a magnetic telephone receiver to a carbon microphone: the faint signal from the receiver was transferred to the microphone, where it modulated a stronger electric current, producing a stronger electrical signal to send down the line. One illustration of this amplifier effect was the oscillation caused by feedback, resulting in an audible squeal from the old ""candlestick"" telephone if its earphone was placed near the carbon microphone."
+5348,"A crystal microphone or piezo microphone uses the phenomenon of piezoelectricity—the ability of some materials to produce a voltage when subjected to pressure—to convert vibrations into an electrical signal. An example of this is potassium sodium tartrate, which is a piezoelectric crystal that works as a transducer, both as a microphone and as a slimline loudspeaker component. Crystal microphones were once commonly supplied with vacuum tube  equipment, such as domestic tape recorders.  Their high output impedance matched the high input impedance  of the vacuum tube input stage well. They were difficult to match to early transistor equipment and were quickly supplanted by dynamic microphones for a time, and later small electret condenser devices. The high impedance of the crystal microphone made it very susceptible to handling noise, both from the microphone itself and from the connecting cable."
+5349,"Piezoelectric transducers are often used as contact microphones to amplify sound from acoustic musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in challenging environments, such as underwater under high pressure. Saddle-mounted pickups on acoustic guitars are generally piezoelectric devices that contact the strings passing over the saddle. This type of microphone is different from magnetic coil pickups commonly visible on typical electric guitars, which use magnetic induction, rather than mechanical coupling, to pick up vibration."
+5350,"A fiber-optic microphone converts acoustic waves into electrical signals by sensing changes in light intensity, instead of sensing changes in capacitance or magnetic fields as with conventional microphones."
+5351,"During operation, light from a laser source travels through an optical fiber to illuminate the surface of a reflective diaphragm. Sound vibrations of the diaphragm modulate the intensity of light reflecting off the diaphragm in a specific direction. The modulated light is then transmitted over a second optical fiber to a photodetector, which transforms the intensity-modulated light into analog or digital audio for transmission or recording. Fiber-optic microphones possess high dynamic and frequency range, similar to the best high fidelity conventional microphones."
+5352,"Fiber-optic microphones do not react to or influence any electrical, magnetic, electrostatic or radioactive fields . The fiber-optic microphone design is therefore ideal for use in areas where conventional microphones are ineffective or dangerous, such as inside industrial turbines or in magnetic resonance imaging  equipment environments."
+5353,"Fiber-optic microphones are robust, resistant to environmental changes in heat and moisture, and can be produced for any directionality or impedance matching. The distance between the microphone's light source and its photodetector may be up to several kilometers without need for any preamplifier or another electrical device, making fiber-optic microphones suitable for industrial and surveillance acoustic monitoring."
+5354,"Fiber-optic microphones are used in very specific application areas such as for infrasound monitoring and noise cancellation. They have proven especially useful in medical applications, such as allowing radiologists, staff and patients within the powerful and noisy magnetic field to converse normally, inside the MRI suites as well as in remote control rooms. Other uses include industrial equipment monitoring and  audio calibration and measurement, high-fidelity recording and law enforcement."
+5355,"Laser microphones are often portrayed in movies as spy gadgets because they can be used to pick up sound at a distance from the microphone equipment. A laser beam is aimed at the surface of a window or other plane surface that is affected by sound. The vibrations of this surface change the angle at which the beam is reflected, and the motion of the laser spot from the returning beam is detected and converted to an audio signal."
+5356,"In a more robust and expensive implementation, the returned light is split and fed to an interferometer, which detects movement of the surface by changes in the optical path length of the reflected beam. The former implementation is a tabletop experiment; the latter requires an extremely stable laser and precise optics."
+5357,"A new type of laser microphone is a device that uses a laser beam and smoke or vapor to detect sound vibrations in free air. On August 25, 2009, U.S. patent 7,580,533 issued for a Particulate Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or vapor in the laser beam's path. Sound pressure waves cause disturbances in the smoke that in turn cause variations in the amount of laser light reaching the photodetector. A prototype of the device was demonstrated at the 127th Audio Engineering Society convention in New York City from 9 through October 12, 2009."
+5358,"Early microphones did not produce intelligible speech, until Alexander Graham Bell made improvements including a variable-resistance microphone/transmitter.  Bell's liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added. A sound wave caused the diaphragm to move, forcing a needle to move up and down in the water. The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around the submerged needle.  Elisha Gray filed a caveat for a version using a brass rod instead of the needle.  Other minor variations and improvements were made to the liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version was patented by Reginald Fessenden in 1903. These were the first working microphones, but they were not practical for commercial application.  The famous first phone conversation between Bell and Watson took place using a liquid microphone."
+5359,"The MEMS  microphone is also called a microphone chip or silicon microphone. A pressure-sensitive diaphragm is etched directly into a silicon wafer by MEMS processing techniques and is usually accompanied with an integrated preamplifier. Most MEMS microphones are variants of the condenser microphone design. Digital MEMS microphones have built-in analog-to-digital converter  circuits on the same CMOS chip making the chip a digital microphone and so more readily integrated with modern digital products. Major manufacturers producing MEMS silicon microphones are Wolfson Microelectronics  now Cirrus Logic, InvenSense , Akustica , Infineon , Knowles Electronics, Memstech , NXP Semiconductors , Sonion MEMS, Vesper, AAC Acoustic Technologies, and Omron."
+5360,"More recently, since the 2010s, there has been increased interest and research into making piezoelectric MEMS microphones which are a significant architectural and material change from existing condenser style MEMS designs."
+5361,In a plasma microphone a plasma arc of ionized gas is used. The sound waves cause variations in the pressure around the plasma in turn causing variations in temperature which alter the conductance of the plasma. These variations in conductance can be picked up as variations superimposed on the electrical supply to the plasma. This is a very rare form of microphone.
+5362,"A loudspeaker, a transducer that turns an electrical signal into sound waves, is the functional opposite of a microphone. Since a conventional speaker is similar in construction to a dynamic microphone , speakers can actually work ""in reverse"" as microphones. Reciprocity applies, so the resulting microphone has the same impairments as a single-driver loudspeaker: limited low- and high-end frequency response, poorly controlled directivity, and low sensitivity. In practical use, speakers are sometimes used as microphones in applications where high bandwidth and sensitivity are not needed such as intercoms, walkie-talkies or video game voice chat peripherals, or when conventional microphones are in short supply."
+5363,"However, there is at least one practical application that exploits those weaknesses: the use of a medium-size woofer placed closely in front of a ""kick drum""  in a drum set to act as a microphone.  A commercial product example is the Yamaha Subkick, a 6.5-inch  woofer shock-mounted into a 10"" drum shell used in front of kick drums. Since a relatively massive membrane is unable to transduce high frequencies while being capable of tolerating strong low-frequency transients, the speaker is often ideal for picking up the kick drum while reducing bleed from the nearby cymbals and snare drums."
+5364,"The inner elements of a microphone are the primary source of differences in directivity. A pressure microphone uses a diaphragm between a fixed internal volume of air and the environment and responds uniformly to pressure from all directions, so it is said to be omnidirectional. A pressure-gradient microphone uses a diaphragm that is at least partially open on both sides. The pressure difference between the two sides produces its directional characteristics. Other elements such as the external shape of the microphone and external devices such as interference tubes can also alter a microphone's directional response. A pure pressure-gradient microphone is equally sensitive to sounds arriving from front or back but insensitive to sounds arriving from the side because sound arriving at the front and back at the same time creates no gradient between the two. The characteristic directional pattern of a pure pressure-gradient microphone is like a figure-8. Other polar patterns are derived by creating a capsule that combines these two effects in different ways. The cardioid, for instance, features a partially closed backside, so its response is a combination of pressure and pressure-gradient characteristics."
+5365,"A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis.  The polar patterns illustrated above represent the locus of points in polar coordinates that produce the same signal level output in the microphone if a given sound pressure level  is generated from that point. How the physical body of the microphone is oriented relative to the diagrams depends on the microphone design. For large-membrane microphones such as in the Oktava , the upward direction in the polar diagram is usually perpendicular to the microphone body, commonly known as ""side fire"" or ""side address"". For small diaphragm microphones such as the Shure , it usually extends from the axis of the microphone commonly known as ""end fire"" or ""top/end address""."
+5366,Some microphone designs combine several principles in creating the desired polar pattern. This ranges from shielding  by the housing itself to electronically combining dual membranes.
+5367,"An omnidirectional  microphone's response is generally considered to be a perfect sphere in three dimensions. In the real world, this is not the case. As with directional microphones, the polar pattern for an ""omnidirectional"" microphone is a function of frequency. The body of the microphone is not infinitely small and, as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response. This flattening increases as the diameter of the microphone  reaches the wavelength of the frequency in question. Therefore, the smallest diameter microphone gives the best omnidirectional characteristics at high frequencies."
+5368,"The wavelength of sound at 10 kHz is 1.4"" . The smallest measuring microphones are often 1/4""  in diameter, which practically eliminates directionality even up to the highest frequencies. Omnidirectional microphones, unlike cardioids, do not employ resonant cavities as delays, and so can be considered the ""purest"" microphones in terms of low coloration; they add very little to the original sound. Being pressure-sensitive they can also have a very flat low-frequency response down to 20 Hz or below. Pressure-sensitive microphones also respond much less to wind noise and plosives than directional  microphones."
+5369,"Areas of application: studios, old churches, theaters, on-site TV interviews, etc."
+5370,An example of a nondirectional microphone is the round black eight ball.
+5371,A unidirectional microphone is primarily sensitive to sounds from only one direction. The diagram above  illustrates a number of these patterns. The microphone faces upwards in each diagram. The sound intensity for a particular frequency is plotted for angles radially from 0 to 360°. 
+5372,"The most common unidirectional microphone is a cardioid microphone, so named because the sensitivity pattern is ""heart-shaped"" . The cardioid family of microphones are commonly used as vocal or speech microphones since they are good at rejecting sounds from other directions. In three dimensions, the cardioid is shaped like an apple centered around the microphone, which is the ""stem"" of the apple. The cardioid response reduces pickup from the side and rear, helping to avoid feedback from the monitors. Since these directional transducer microphones achieve their patterns by sensing pressure gradient, putting them very close to the sound source  results in a bass boost due to the increased gradient. This is known as the proximity effect. The SM58 has been the most commonly used microphone for live vocals for more than 50 years demonstrating the importance and popularity of cardioid mics."
+5373,"The cardioid is effectively a superposition of an omnidirectional  and a figure-8  microphone; for sound waves coming from the back, the negative signal from the figure-8 cancels the positive signal from the omnidirectional element, whereas, for sound waves coming from the front, the two add to each other. However, in low frequencies a cardioid microphone behaves as an omnidirectional microphone."
+5374,"By combining the two components in different ratios, any pattern between omni and figure-8 can be achieved, which comprise the first-order cardioid family.  Common shapes include:"
+5375,Three such cardioid microphones/hydrophones could be orthogonally oriented as a collocated triad to improve the gain and also create a steerable beam pattern.
+5376,"""Figure-8"" or bi-directional microphones receive sound equally from both the front and back of the element. Most ribbon microphones are of this pattern.  In principle they do not respond to sound pressure at all, only to the change in pressure between front and back; since sound arriving from the side reaches front and back equally there is no difference in pressure and therefore no sensitivity to sound from that direction. In more mathematical terms, while omnidirectional microphones are scalar transducers responding to pressure from any direction, bi-directional microphones are vector transducers responding to the gradient along an axis normal to the plane of the diaphragm. This also has the effect of inverting the output polarity for sounds arriving from the back side."
+5377,"Shotgun microphones are the most highly directional of simple first-order unidirectional types.  At low frequencies, they have the classic polar response of a hypercardioid, while at medium and higher frequencies an interference tube gives them an increased forward response. This is achieved by a process of cancellation of off-axis waves entering the longitudinal array of slots. A consequence of this technique is the presence of some rear lobes that vary in level and angle with frequency and can cause some coloration effects."
+5378,"Several approaches have been developed for effectively using a microphone in less-than-ideal acoustic spaces, which often suffer from excessive reflections from one or more of the surfaces  that make up the space. If the microphone is placed in, or very close to, one of these boundaries, the reflections from that surface have the same timing as the direct sound, thus giving the microphone a hemispherical polar pattern and improved intelligibility. Initially, this was done by placing an ordinary microphone adjacent to the surface, sometimes in a block of acoustically transparent foam. Sound engineers Ed Long and Ron Wickersham developed the concept of placing the diaphragm parallel to and facing the boundary. While the patent has expired, Pressure Zone Microphone and PZM are still active trademarks of Crown International."
+5379,"While a boundary microphone was initially implemented using an omnidirectional element, it is also possible to mount a directional microphone close enough to the surface to gain some of the benefits of this technique while retaining the directional properties of the element. Crown's trademark on this approach is Phase Coherent Cardioid or PCC, but there are other makers who employ this technique as well."
+5380,"A lavalier microphone is made for hands-free operation. These small microphones are worn on the body. Originally, they were held in place with a lanyard worn around the neck, but more often they are fastened to clothing with a clip, pin, tape or magnet. The lavalier cord may be hidden by clothes and either run to an RF transmitter in a pocket or clipped to a belt , or run directly to the mixer ."
+5381,"A wireless microphone transmits the audio as a radio or optical signal rather than via a cable. It usually sends its signal using a small radio transmitter to a nearby receiver connected to the sound system, but it can also use infrared waves if the transmitter and receiver are within sight of each other."
+5382,"A contact microphone picks up vibrations directly from a solid surface or object, as opposed to sound vibrations carried through air. One use for this is to detect sounds of a very low level, such as those from small objects or insects. The microphone commonly consists of a magnetic  transducer, contact plate and contact pin. The contact plate is placed directly on the vibrating part of a musical instrument or other surface, and the contact pin transfers vibrations to the coil. Contact microphones have been used to pick up the sound of a snail's heartbeat and the footsteps of ants. A portable version of this microphone has recently been developed. A throat microphone is a variant of the contact microphone that picks up speech directly from a person's throat, which it is strapped to. This lets the device be used in areas with ambient sounds that would otherwise make the speaker inaudible."
+5383,"A parabolic microphone uses a parabolic reflector to collect and focus sound waves onto a microphone receiver, in much the same way that a parabolic antenna  does with radio waves. Typical uses of this microphone, which has unusually focused front sensitivity and can pick up sounds from many meters away, include nature recording, outdoor sporting events, eavesdropping, law enforcement, and even espionage. Parabolic microphones are not typically used for standard recording applications, because they tend to have a poor low-frequency response as a side effect of their design."
+5384,A stereo microphone integrates two microphones in one unit to produce a stereophonic signal. A stereo microphone is often used for broadcast applications or field recording where it would be impractical to configure two separate condenser microphones in a classic X-Y configuration  for stereophonic recording. Some such microphones have an adjustable angle of coverage between the two channels.
+5385,"A noise-canceling microphone is a highly directional design intended for noisy environments.  One such use is in aircraft cockpits where they are normally installed as boom microphones on headsets. Another use is in live event support on loud concert stages for vocalists involved with live performances. Many noise-canceling microphones combine signals received from two diaphragms that are in opposite electrical polarity or are processed electronically. In dual diaphragm designs, the main diaphragm is mounted closest to the intended source and the second is positioned farther away from the source so that it can pick up environmental sounds to be subtracted from the main diaphragm's signal. After the two signals have been combined, sounds other than the intended source are greatly reduced, substantially increasing intelligibility. Other noise-canceling designs use one diaphragm that is affected by ports open to the sides and rear of the microphone, with the sum being a 16 dB rejection of sounds that are farther away. One noise-canceling headset design using a single diaphragm has been used prominently by vocal artists such as Garth Brooks and Janet Jackson. A few noise-canceling microphones are throat microphones."
+5386,"Various standard techniques are used with microphones used in sound reinforcement at live performances, or for recording in a studio or on a motion picture set. By suitable arrangement of one or more microphones, desirable features of the sound to be collected can be kept, while rejecting unwanted sounds."
+5387,"Microphones containing active circuitry, such as most condenser microphones, require power to operate the active components. The first of these used vacuum-tube circuits with a separate power supply unit, using a multi-pin cable and connector. With the advent of solid-state amplification, the power requirements were greatly reduced and it became practical to use the same cable conductors and connector for audio and power. During the 1960s several powering methods were developed, mainly in Europe. The two dominant methods were initially defined in German DIN 45595 as de:Tonaderspeisung or T-power and DIN 45596 for phantom power. Since the 1980s, phantom power has become much more common, because the same input may be used for both powered and unpowered microphones. In consumer electronics such as DSLRs and camcorders, ""plug-in power"" is more common, for microphones using a 3.5 mm phone plug connector. Phantom, T-power and plug-in power are described in international standard IEC 61938."
+5388,The most common connectors used by microphones are:
+5389,"Some microphones use other connectors, such as a 5-pin XLR, or mini XLR for connection to portable equipment. Some lavalier  microphones use a proprietary connector for connection to a wireless transmitter, such as a radio pack. Since 2005, professional-quality microphones with USB connections have begun to appear, designed for direct recording into computer-based software."
+5390,"When choosing a pre-amplifier for a certain microphone, the microphone's impedance must be known. Impedance is a frequency-dependent electrical characteristic, measured in ohms , that relates voltage to current. When not concerned with power transfer, signals are generally transferred as varying voltages and this is also the case for microphones. To obtain the highest signal amplitude one uses a method called impedance bridging. In this configuration the output impedance of the microphone should be negligible in comparison with the input impedance of the pre-amplifier . By doing so, the signal is attenuated minimally and almost no power is used in the process."
+5391,"The main alternative to impedance bridging is impedance matching which maximizes power transfer for a given source impedance. However, this has not been relevant since the early 20th century when amplifiers were very expensive and produced a lot of heat. To reduce the number of amplifiers in telephone lines, power loss needed to be minimal so source and load impedances were matched. A downside to impedance matching is the 6 dB loss in signal that occurs as only half the voltage level appears at the pre-amplifier's input. Certain ribbon and dynamic microphones however are exceptions, due to the designers' assumption of a certain load impedance being part of the internal electro-acoustical damping circuit of the microphone."
+5392,"Different microphones can have vastly different impedances and this depends on the design. In passive microphones, this value relates closely to the impedance of the coil . In active microphones, this value describes the output impedance of its internal amplifier circuitry."
+5393,"Low impedance is considered under 600 Ω. Medium impedance is considered between 600 Ω and 10 kΩ. High impedance is above 10 kΩ. Owing to their built-in amplifier, condenser microphones typically have an output impedance between 50 and 200 Ω."
+5394,"The AES42 standard, published by the Audio Engineering Society, defines a digital interface for microphones. Microphones conforming to this standard directly output a digital audio stream through an XLR or XLD male connector, rather than producing an analog output. Digital microphones may be used either with new equipment with appropriate input connections that conform to the AES42 standard, or else via a suitable interface box. Studio-quality microphones that operate in accordance with the AES42 standard are now available from a number of microphone manufacturers."
+5395,"Because of differences in their construction, microphones have their own characteristic responses to sound. This difference in response produces non-uniform phase and frequency responses. Additionally, microphones are not uniformly sensitive to sound pressure and can accept differing levels without distorting. Although for scientific applications microphones with a more uniform response are desirable, this is often not the case for music recording, as the non-uniform response of a microphone can produce a desirable coloration of the sound. There is an international standard for microphone specifications, but few manufacturers adhere to it. As a result, comparison of published data from different manufacturers is difficult because different measurement techniques are used. Caution should be used in drawing any solid conclusions from this or any other published data, however, unless it is known that the manufacturer has supplied specifications in accordance with IEC 60268-4."
+5396,"A frequency response diagram plots the microphone sensitivity in decibels over a range of frequencies , generally for perfectly on-axis sound . Frequency response may be less informatively stated textually like so: ""30 Hz–16 kHz ±3 dB"". This is interpreted as meaning a nearly flat, linear, plot between the stated frequencies, with variations in amplitude of no more than plus or minus 3 dB. However, one cannot determine from this information how smooth the variations are, nor in what parts of the spectrum they occur. Note that commonly made statements such as ""20 Hz–20 kHz"" are meaningless without a decibel measure of tolerance. Directional microphones' frequency response varies greatly with distance from the sound source, and with the geometry of the sound source. IEC 60268-4 specifies that frequency response should be measured in plane progressive wave conditions  but this is seldom practical. Close talking microphones may be measured with different sound sources and distances, but there is no standard and therefore no way to compare data from different models unless the measurement technique is described."
+5397,"The self-noise or equivalent input noise level is the sound level that creates the same output voltage as the microphone does in the absence of sound. This represents the lowest point of the microphone's dynamic range, and is particularly important should you wish to record sounds that are quiet. The measure is often stated in dB, which is the equivalent loudness of the noise on a decibel scale frequency-weighted for how the ear hears, for example: ""15 dBA SPL"" . The lower the number the better. Some microphone manufacturers state the noise level using ITU-R 468 noise weighting, which more accurately represents the way we hear noise, but gives a figure some 11–14 dB higher. A quiet microphone typically measures 20 dBA SPL or 32 dB SPL 468-weighted. Very quiet microphones have existed for years for special applications, such the Brüel & Kjaer 4179, with a noise level around 0 dB SPL. Recently some microphones with low noise specifications have been introduced in the studio/entertainment market, such as models from Neumann and Røde that advertise noise levels between 5–7 dBA. Typically this is achieved by altering the frequency response of the capsule and electronics to result in lower noise within the A-weighting curve while broadband noise may be increased."
+5398,"The clipping level is an important indicator of maximum usable level, as the 1% total harmonic distortion  figure usually quoted under max SPL is really a very mild level of distortion, quite inaudible especially on brief high peaks. Clipping is much more audible. For some microphones, the clipping level may be much higher than the max SPL."
+5399,"The dynamic range of a microphone is the difference in SPL between the noise floor and the maximum SPL. If stated on its own, for example, ""120 dB"", it conveys significantly less information than having the self-noise and maximum SPL figures individually."
+5400,"Sensitivity indicates how well the microphone converts acoustic pressure to an output voltage. A high sensitivity microphone creates more voltage and so needs less amplification at the mixer or recording device.  This is a practical concern but is not directly an indication of the microphone's quality, and in fact the term sensitivity is something of a misnomer, ""transduction gain"" being perhaps more meaningful,  because true sensitivity is generally set by the noise floor, and too much ""sensitivity"" in terms of output level compromises the clipping level. There are two common measures. The  international standard is made in millivolts per pascal at 1 kHz. A higher value indicates greater sensitivity. The older American method is referred to a 1 V/Pa standard and measured in plain decibels, resulting in a negative value. Again, a higher value indicates greater sensitivity, so −60  dB is more sensitive than −70 dB."
+5401,"Some microphones are intended for testing speakers, measuring noise levels and otherwise quantifying an acoustic experience. These are calibrated transducers and are usually supplied with a calibration certificate that states absolute sensitivity against frequency. The quality of measurement microphones is often referred to using the designations ""Class 1,"" ""Type 2,"" etc., which are references not to microphone specifications but to sound level meters. A more comprehensive standard for the description of measurement microphone performance was recently adopted."
+5402,"Measurement microphones are generally scalar sensors of pressure; they exhibit an omnidirectional response, limited only by the scattering profile of their physical dimensions. Sound intensity or sound power measurements require pressure-gradient measurements, which are typically made using arrays of at least two microphones, or with hot-wire anemometers."
+5403,"To take a scientific measurement with a microphone, its precise sensitivity must be known . Since this may change over the lifetime of the device, it is necessary to regularly calibrate measurement microphones. This service is offered by some microphone manufacturers and by independent certified testing labs. All microphone calibration is ultimately traceable to primary standards at a national measurement institute such as NPL in the UK, PTB in Germany and NIST in the United States, which most commonly calibrate using the reciprocity primary standard. Measurement microphones calibrated using this method can then be used to calibrate other microphones using comparison calibration techniques."
+5404,"Depending on the application, measurement microphones must be tested periodically  and after any potentially damaging event, such as being dropped  or exposed to sounds beyond the acceptable level."
+5405,A microphone array is any number of microphones operating in tandem. There are many applications:
+5406,"Typically, an array is made up of omnidirectional microphones distributed about the perimeter of a space, linked to a computer that records and interprets the results into a coherent form."
+5407,"Windscreens  provide a method of reducing the effect of wind on microphones. While pop-screens give protection from unidirectional blasts, foam ""hats"" shield wind into the grille from all directions, and blimps, zeppelins, and baskets entirely enclose the microphone and protect its body as well. The latter is important because, given the extreme low-frequency content of wind noise, vibration induced in the housing of the microphone can contribute substantially to the noise output."
+5408,"The shielding material used – wire gauze, fabric or foam – is designed to have a significant acoustic impedance. The relatively low particle-velocity air pressure changes that constitute sound waves can pass through with minimal attenuation, but higher particle-velocity wind is impeded to a far greater extent. Increasing the thickness of the material improves wind attenuation but also begins to compromise high-frequency audio content. This limits the practical size of simple foam screens. While foams and wire meshes can be partly or wholly self-supporting, soft fabrics and gauzes require stretching on frames or laminating with coarser structural elements."
+5409,"Since all wind noise is generated at the first surface the air hits, the greater the spacing between the shield periphery and microphone capsule, the greater the noise attenuation. For an approximately spherical shield, attenuation increases by  the cube of that distance. With full basket windshields there is an additional pressure chamber effect, first explained by Joerg Wuttke, which, for two-port  microphones, allows the shield and microphone combination to act as a high-pass acoustic filter."
+5410,"Since turbulence at a surface is the source of wind noise, reducing gross turbulence can add to noise reduction. Both aerodynamically smooth surfaces, and ones that prevent powerful vortices being generated, have been used successfully. Historically, artificial fur has proved very useful for this purpose since the fibers produce micro-turbulence and absorb energy silently. If not matted by wind and rain, the fur fibers are very transparent acoustically, but the woven or knitted backing can give significant attenuation. As a material, it suffers from being difficult to manufacture with consistency and is hard to keep in pristine condition on location. Thus there is an interest in moving away from its use."
+5411,"Modern scanners typically use a charge-coupled device  or a contact image sensor  as the image sensor, whereas drum scanners, developed earlier and still used for the highest possible image quality, use a photomultiplier tube  as the image sensor. A rotary scanner, used for high-speed document scanning, is a type of drum scanner that uses a CCD array instead of a photomultiplier. Non-contact planetary scanners essentially photograph delicate books and documents. All these scanners produce two-dimensional images of subjects that are usually flat, but sometimes solid; 3D scanners produce information on the three-dimensional structure of solid objects."
+5412,"Digital cameras can be used for the same purposes as dedicated scanners. When compared to a true scanner, a camera image is subject to a degree of distortion, reflections, shadows, low contrast, and blur due to camera shake . Resolution is sufficient for less demanding applications. Digital cameras offer the advantages of speed, portability, and non-contact digitizing of thick documents without damaging the book spine. In 2010 scanning technologies were combining 3D scanners with digital cameras to create full-color, photo-realistic 3D models of objects."
+5413,Scans are usually downloaded by a computer the unit is attached to. Some scanners are able to store scans on standalone flash media .
+5414,"In the biomedical research area, detection devices for DNA microarrays are called scanners as well. These scanners are high-resolution systems , similar to microscopes. The detection is done via CCD or photomultiplier tubes."
+5415,Modern scanners are considered the successors of early telephotography and fax input devices.
+5416,"The pantelegraph  was an early form of facsimile machine transmitting over normal telegraph lines developed by Giovanni Caselli, used commercially in the 1860s, that was the first such device to enter practical service. It used electromagnets to drive and synchronize the movement of pendulums at the source and the distant location, to scan and reproduce images. It could transmit handwriting, signatures, or drawings within an area of up to 150 × 100 mm."
+5417,"Édouard Belin's Belinograph of 1913, scanned using a photocell and transmitted over ordinary phone lines, formed the basis for the AT&T Wirephoto service. In Europe, services similar to a wirephoto were called a Belino. It was used by news agencies from the 1920s to the mid-1990s and consisted of a rotating drum with a single photodetector at a standard speed of 60 or 120 rpm . They sent a linear analog AM signal through standard telephone voice lines to receptors, which synchronously print the proportional intensity on special paper. Color photos were sent as three separated RGB filtered images consecutively, but only for special events due to transmission costs."
+5418,"Drum scanners capture image information with photomultiplier tubes , rather than the charge-coupled device  arrays found in flatbed scanners and inexpensive film scanners. ""Reflective and transmissive originals are mounted on an acrylic cylinder, the scanner drum, which rotates at high speed while it passes the object being scanned in front of precision optics that deliver image information to the PMTs. Modern color drum scanners use three matched PMTs, which read red, blue, and green light, respectively. Light from the original artwork is split into separate red, blue, and green beams in the optical bench of the scanner with dichroic filters."" Photomultipliers offer superior dynamic range and for this reason, drum scanners can extract more detail from very dark shadow areas of a transparency than flatbed scanners using CCD sensors. The smaller dynamic range of the CCD sensors, versus photomultiplier tubes, can lead to loss of shadow detail, especially when scanning very dense transparency film. While mechanics vary by manufacturer, most drum scanners pass light from halogen lamps though a focusing system to illuminate both reflective and transmissive originals."
+5419,"The drum scanner gets its name from the clear acrylic cylinder, the drum, on which the original artwork is mounted for scanning. Depending on size, it is possible to mount originals up to 20 by 28 inches , but the maximum size varies by manufacturer. ""One of the unique features of drum scanners is the ability to control sample area and aperture size independently. The sample size is the area that the scanner encoder reads to create an individual pixel. The aperture is the actual opening that allows light into the optical bench of the scanner. The ability to control aperture and sample size separately are particularly useful for smoothing film grain when scanning black-and-white and color negative originals."""
+5420,"While drum scanners are capable of scanning both reflective and transmissive artwork, a good-quality flatbed scanner can produce good scans from reflective artwork. As a result, drum scanners are rarely used to scan prints now that high-quality, inexpensive flatbed scanners are readily available. Film, however, is where drum scanners continue to be the tool of choice for high-end applications. Because film can be wet-mounted to the scanner drum, which enhances sharpness and masks dust and scratches, and because of the exceptional sensitivity of the PMTs, drum scanners are capable of capturing very subtle details in film originals."
+5421,"The situation as of 2014 was that only a few companies continued to manufacture and service drum scanners. While prices of both new and used units dropped from the start of the 21st century, they were still much more costly than CCD flatbed and film scanners. Image quality produced by flatbed scanners had improved to the degree that the best ones were suitable for many graphic-arts operations, and they replaced drum scanners in many cases as they were less expensive and faster. However, drum scanners with their superior resolution , color gradation, and value structure continued to be used for scanning images to be enlarged, and for museum-quality archiving of photographs and print production of high-quality books and magazine advertisements. As second-hand drum scanners became more plentiful and less costly, many fine-art photographers acquired them."
+5422,"This type of scanner is sometimes called a reflective scanner because it works by shining white light onto the object to be scanned and reading the intensity and color of light that is reflected from it, usually a line at a time. They are designed for scanning prints or other flat, opaque materials but some have available transparency adapters, which for a number of reasons, in most cases, are not very well suited to scanning film. Some flatbed scanners incorporate sheet feeding mechanisms called ADFs ."
+5423,"""A flatbed scanner is usually composed of a glass pane , under which there is a bright light  which illuminates the pane, and a moving optical array in CCD scanning. CCD-type scanners typically contain three rows  of sensors with red, green, and blue filters."""
+5424,"Contact image sensor  scanning consists of a moving set of red, green, and blue LEDs strobed for illumination and a connected monochromatic photodiode array under a rod lens array for light collection. ""Images to be scanned are placed face down on the glass, an opaque cover is lowered over it to exclude ambient light, and the sensor array and light source move across the pane, reading the entire area. An image is therefore visible to the detector only because of the light it reflects. Transparent images do not work in this way and require special accessories that illuminate them from the upper side. Many scanners offer this as an option."""
+5425,"Sheetfed scanners do not have a scanning bed, have a mechanism to feed paper through the scanner, and some are capable of scanning several sheets at once using an ADF. A printer cartridge, the Canon IS-22, was released that could be used to convert an inkjet printer into a sheetfed scanner."
+5426,"These scanners have an overhead scanning mechanism that moves a beam of light, or have a fixed camera, and a scanning area defined by a mat to easily scan books."
+5427,"This type of scanner is sometimes called a slide or transparency scanner and it works by passing a narrowly focused beam of light through the film and reading the intensity and color of the light that emerges. ""Usually, uncut film strips of up to six frames, or four mounted slides, are inserted in a carrier, which is moved by a stepper motor across a lens and CCD sensor inside the scanner. Some models are mainly used for same-size scans. Film scanners vary a great deal in price and quality."" The lowest-cost dedicated film scanners can be had for less than $50 and they might be sufficient for modest needs. From there they inch up in staggered levels of quality and advanced features upward of five figures. ""The specifics vary by brand and model and the end results are greatly determined by the level of sophistication of the scanner's optical system and, equally important, the sophistication of the scanning software."""
+5428,"Scanners are available that pull a flat sheet over the scanning element between rotating rollers. They can only handle single sheets up to a specified width, typically 8.5 inches  to accommodate both US letter and standard A4 sizes, but can be very compact, just requiring a pair of narrow rollers between which the document is passed."
+5429,"A roller scanner may be embedded inside a computer keyboard, with a footprint no larger than a computer keyboard."
+5430,"Some roller scanners are portable, powered by batteries and with their own storage, eventually transferring stored scans to a computer over a USB or other interface."
+5431,3D scanners collect data on the three-dimensional shape and appearance of an object.
+5432,Planetary scanners scan a delicate object without physical contact.
+5433,Hand scanners are moved over the subject to be imaged by hand. There are two different types: document and 3D scanners.
+5434,"Hand-held document scanners are manual devices that are dragged across the surface of the image to be scanned by hand. Scanning documents in this manner requires a steady hand, as an uneven scanning rate produces distorted images; an indicator light on the scanner indicates if motion is too fast. They typically have a ""start"" button, which is held by the user for the duration of the scan; some switches to set the optical resolution; and a roller, which generates a clock pulse for synchronization with the computer. Older hand scanners were monochrome, and produced light from an array of green LEDs to illuminate the image""; later ones scan in monochrome or color, as desired. A hand scanner may have a small window through which the document being scanned could be viewed. In the early 1990s, many hand scanners had a proprietary interface module specific to a particular type of computer, such as an Atari ST or Commodore Amiga. Since the introduction of the USB standard, it is the interface most commonly used. As hand scanners are much narrower than most normal document or book sizes, software  needed to combine several narrow ""strips"" of scanned documents to produce the finished article."
+5435,"Inexpensive portable battery-powered or USB-powered ""glide-over"" hand or pen scanners, typically capable of scanning an area as wide as a normal letter and much longer remain available as of 2014. Some computer mice can also scan documents."
+5436,"Handheld 3D scanners are used in industrial design, reverse engineering, inspection and analysis, digital manufacturing, and medical applications. ""To compensate for the uneven motion of the human hand, most 3D scanning systems rely on the placement of reference markers, typically adhesive reflective tabs that the scanner uses to align elements and mark positions in space."""
+5437,"Image scanners are usually used in conjunction with a computer which controls the scanner and stores scans. Small portable scanners, either roller-fed or ""glide-over"" hand-operated, operated by batteries and with storage capability, are available for use away from a computer; stored scans can be transferred later. Many can scan both small documents such as business cards and till receipts, and letter-sized documents."
+5438,"The higher-resolution cameras fitted to some smartphones can produce reasonable quality document scans by taking a photograph with the phone's camera and post-processing it with a scanning app, a range of which are available for most phone operating systems, to whiten the background of a page, correct perspective distortion so that the shape of a rectangular document is corrected, convert to black-and-white, etc. Many such apps can scan multiple-page documents with successive camera exposures and output them either as a single file or multiple-page files. Some smartphone scanning apps can save documents directly to online storage locations, such as Dropbox and Evernote, send via email or fax documents via email-to-fax gateways."
+5439,Smartphone scanner apps can be broadly divided into three categories:
+5440,"Document scanning apps primarily designed to handle documents and output PDF, and sometimes JPEG, files"
+5441,"Photo scanning apps that output JPEG files, and have editing functions useful for photo rather than document editing;"
+5442,Barcode-like QR code scanning apps that then search the internet for information associated with the code.
+5443,"Color scanners typically read RGB  data from the array. This data is then processed with some proprietary algorithm to correct for different exposure conditions, and sent to the computer via the device's input/output interface ."
+5444,"Color depth varies depending on the scanning array characteristics, but is usually at least 24 bits. High-quality models have 36-48 bits of color depth."
+5445,"Another qualifying parameter for a scanner is its resolution, measured in pixels per inch , sometimes more accurately referred to as Samples per inch . Instead of using the scanner's true optical resolution, the only meaningful parameter, manufacturers like to refer to the interpolated resolution, which is much higher thanks to software interpolation. As of 2009, a high-end flatbed scanner can scan up to 5400 ppi and drum scanners have an optical resolution of between 3,000 and 24,000 ppi."
+5446,"""Effective resolution"" is the true resolution of a scanner, and is determined by using a resolution test chart.  The effective resolution of most all consumer flatbed scanners is considerably lower than the manufactures' given optical resolution.  Example is the Epson V750 Pro with an optical resolution given by manufacturer as being 4800dpi and 6400dpi , but tested ""According to this we get a resolution of only about 2300 dpi - that's just 40% of the claimed resolution!""  Dynamic range is claimed to be 4.0 Dmax, but ""Regarding the density range of the Epson Perfection V750 Pro, which is indicated as 4.0, one must say that here it doesn't reach the high-quality  film scanners either."""
+5447,"Manufacturers often claim interpolated resolutions as high as 19,200 ppi; but such numbers carry little meaningful value because the number of possible interpolated pixels is unlimited, and doing so does not increase the level of captured detail."
+5448,"The size of the file created increases with the square of the resolution; doubling the resolution quadruples the file size. A resolution must be chosen that is within the capabilities of the equipment, preserves sufficient detail, and does not produce a file of excessive size. The file size can be reduced for a given resolution by using ""lossy"" compression methods such as JPEG, at some cost in quality. If the best possible quality is required lossless compression should be used; reduced-quality files of smaller size can be produced from such an image when required ."
+5449,"Purity can be diminished by scanner noise, optical flare, poor analog to digital conversion, scratches, dust, Newton's rings, out-of-focus sensors, improper scanner operation, and poor software. Drum scanners are said to produce the purest digital representations of the film, followed by high-end film scanners that use the larger Kodak Tri-Linear sensors."
+5450,"The third important parameter for a scanner is its density range  or Drange . A high-density range means that the scanner is able to record shadow details and brightness details in one scan. Density of film is measured on a base 10 log scale and varies between 0.0  and 5.0, about 16 stops.  Density range is the space taken up in the 0 to 5 scale, and Dmin and Dmax denote where the least dense and most dense measurements on a negative or positive film. The density range of negative film is up to 3.6d, while slide film dynamic range is 2.4d.  Color negative density range after processing is 2.0d thanks to the compression of the 12 stops into a small density range.  Dmax will be the densest on slide film for shadows, and densest on negative film for highlights.  Some slide films can have a Dmax close to 4.0d with proper exposure, and so can black-and-white negative film."
+5451,"Consumer-level flatbed photo scanners have a dynamic range in the 2.0–3.0 range, which can be inadequate for scanning all types of photographic film, as Dmax can be and often is between 3.0d and 4.0d with traditional black-and-white film. Color film compresses its 12 stops of a possible 16 stops  into just 2.0d of space via the process of dye coupling and removal of all silver from the emulsion.  Kodak Vision 3 has 18 stops.  So, color-negative film scans the easiest of all film types on the widest range of scanners. Because traditional black-and-white film retains the image creating silver after processing, density range can be almost twice that of color film.  This makes scanning traditional black-and-white film more difficult and requires a scanner with at least a 3.6d dynamic range, but also a Dmax between 4.0d to 5.0d. High-end  flatbed scanners can reach a dynamic range of 3.7, and Dmax around 4.0d. Dedicated film scanners  have a dynamic range between 3.0d–4.0d.  Office document scanners can have a dynamic range of less than 2.0d. Drum scanners have a dynamic range of 3.6–4.5."
+5452,"By combining full-color imagery with 3D models, modern hand-held scanners are able to completely reproduce objects electronically. The addition of 3D color printers enables accurate miniaturization of these objects, with applications across many industries and professions."
+5453,"For scanner apps, the scan quality is highly dependent on the quality of the phone camera and on the framing chosen by the user of the app."
+5454,Scans must virtually always be transferred from the scanner to a computer or information storage system for further processing or storage. There are two basic issues:  how the scanner is physically connected to the computer and  how the application retrieves the information from the scanner.
+5455,"The file size of a scan can be up to about 100 megabytes for a 600 DPI 23 x 28 cm   uncompressed 24-bit image. Scanned files must be transferred and stored. Scanners can generate this volume of data in a matter of seconds, making a fast connection desirable."
+5456,"Scanners communicate to their host computer using one of the following physical interfaces, listing roughly from slow to fast:"
+5457,"During the early 1990s professional flatbed scanners were available over a local computer network. This proved useful to publishers, print shops, etc. This functionality largely fell out of use as the cost of flatbed scanners reduced enough to make sharing unnecessary."
+5458,"From 2000 all-in-one multi-purpose devices became available which were suitable for both small offices and consumers, with printing, scanning, copying, and fax capability in a single apparatus that can be made available to all members of a workgroup."
+5459,"Battery-powered portable scanners store scans on internal memory; they can later be transferred to a computer either by direct connection, typically USB, or in some cases a memory card may be removed from the scanner and plugged into the computer."
+5460,"A paint application such as GIMP or Adobe Photoshop must communicate with the scanner. There are many different scanners, and many of those scanners use different protocols. In order to simplify applications programming, some Applications programming interfaces  were developed. The API presents a uniform interface to the scanner. This means that the application does not need to know the specific details of the scanner in order to access it directly. For example, Adobe Photoshop supports the TWAIN standard; therefore in theory Photoshop can acquire an image from any scanner that has a TWAIN driver."
+5461,"In practice, there are often problems with an application communicating with a scanner. Either the application or the scanner manufacturer  may have faults in their implementation of the API."
+5462,"Typically, the API is implemented as a dynamically linked library. Each scanner manufacturer provides software that translates the API procedure calls into primitive commands that are issued to a hardware controller . The manufacturer's part of the API is commonly called a device driver, but that designation is not strictly accurate: the API does not run in kernel mode and does not directly access the device. Rather the scanner API library translates application requests into hardware requests."
+5463,Common scanner software API:
+5464,"SANE  is a free/open-source API for accessing scanners. Originally developed for Unix and Linux operating systems, it has been ported to OS/2, Mac OS X, and Microsoft Windows. Unlike TWAIN, SANE does not handle the user interface. This allows batch scans and transparent network access without any special support from the device driver."
+5465,"TWAIN is used by most scanners. Originally used for low-end and home-use equipment, it is now widely used for large-volume scanning."
+5466,"ISIS  created by Pixel Translations, which still uses SCSI-II for performance reasons, is used by large, departmental-scale, machines."
+5467,WIA  is an API provided by Microsoft for use on Microsoft Windows.
+5468,"Although no software beyond a scanning utility is a feature of any scanner, many scanners come bundled with software. Typically, in addition to the scanning utility, some type of image-editing application , and optical character recognition  software are supplied. OCR software converts graphical images of text into standard text that can be edited using common word-processing and text-editing software; accuracy is rarely perfect."
+5469,"Some scanners, especially those designed for scanning printed documents, only work in black-and-white but most modern scanners work in color. For the latter, the scanned result is a non-compressed RGB image, which can be transferred to a computer's memory. The color output of different scanners is not the same due to the spectral response of their sensing elements, the nature of their light source and the correction applied by the scanning software. While most image sensors have a linear response, the output values are usually gamma compressed. Some scanners compress and clean up the image using embedded firmware. Once on the computer, the image can be processed with a raster graphics program  and saved on a storage device ."
+5470,"Images are usually stored on a hard disk. Pictures are normally stored in image formats such as uncompressed Bitmap, ""non-lossy""  compressed TIFF and PNG, and ""lossy"" compressed JPEG. Documents are best stored in TIFF or PDF format; JPEG is particularly unsuitable for text. Optical character recognition  software allows a scanned image of text to be converted into editable text with reasonable accuracy, so long as the text is cleanly printed and in a typeface and size that can be read by the software. OCR capability may be integrated into the scanning software, or the scanned image file can be processed with a separate OCR program."
+5471,"Document imaging requirements differ from those of image scanning. These requirements include scanning speed, automated paper feed, and the ability to automatically scan both the front and the back of a document. On the other hand, image scanning typically requires the ability to handle fragile and or three-dimensional objects as well as scan at a much higher resolution."
+5472,"Document scanners have document feeders, usually larger than those sometimes found on copiers or all-purpose scanners. Scans are made at high speed, from 20 up to 280 or 420 pages per minute, often in grayscale, although many scanners support color. Many scanners can scan both sides of double-sided originals . Sophisticated document scanners have firmware or software that cleans up scans of text as they are produced, eliminating accidental marks and sharpening type; this would be unacceptable for photographic work, where marks cannot reliably be distinguished from desired fine detail. Files created are compressed as they are made."
+5473,"The resolution used is usually from 150 to 300 dpi, although the hardware may be capable of 600 or higher resolution; this produces images of text good enough to read and for optical character recognition , without the higher demands on storage space required by higher-resolution images."
+5474,"Document scans are often processed using OCR technology to create editable and searchable files. Most scanners use ISIS or TWAIN device drivers to scan documents into TIFF format so that the scanned pages can be fed into a document management system that will handle the archiving and retrieval of the scanned pages. Lossy JPEG compression, which is very efficient for pictures, is undesirable for text documents, as slanted straight edges take on a jagged appearance, and solid black  text on a light background compresses well with lossless compression formats."
+5475,"While paper feeding and scanning can be done automatically and quickly, preparation and indexing are necessary and require much work by humans. Preparation involves manually inspecting the papers to be scanned and making sure that they are in order, unfolded, without staples or anything else that might jam the scanner. Additionally, some industries such as legal and medical may require documents to have Bates Numbering or some other mark giving a document identification number and date/time of the document scan."
+5476,"Indexing involves associating relevant keywords to files so that they can be retrieved by content. This process can sometimes be automated to some extent, but it often requires manual labour performed by data-entry clerks. One common practice is the use of barcode-recognition technology: during preparation, barcode sheets with folder names or index information are inserted into the document files, folders, and document groups. Using automatic batch scanning, the documents are saved into appropriate folders, and an index is created for integration into document-management systems."
+5477,"A specialized form of document scanning is book scanning. Technical difficulties arise from the books usually being bound and sometimes fragile and irreplaceable, but some manufacturers have developed specialized machinery to deal with this. Often special robotic mechanisms are used to automate the page-turning and scanning process."
+5478,"Another category of document scanner is the document camera. Capturing images on document cameras differs from that of flatbed and Automatic document feeder  scanners in that there are no moving parts required to scan the object. Conventionally either the illumination/reflector rod inside the scanner must be moved over the document , or the document must be passed over the rod  in order to produce a scan of a whole image. Document cameras capture the whole document or object in one step, usually instantly. Typically, documents are placed on a flat surface, usually the office desk, underneath the capture area of the document camera. The process of whole-surface-at-once capturing has the benefit of increasing reaction time for the workflow of scanning. After being captured, the images are usually processed through software that may enhance the image and perform such tasks like automatically rotating, cropping, and straightening them."
+5479,"It is not required that the documents or objects being scanned make contact with the document camera, therefore increasing flexibility of the types of documents which are able to be scanned. Objects that have previously been difficult to scan on conventional scanners are now able to be done so with one device. This includes in particular documents that are of varying sizes and shapes, stapled, in folders, or bent/crumpled which may get jammed in a feed scanner. Other objects include books, magazines, receipts, letters, tickets etc. No moving parts can also remove the need for maintenance, a consideration in the Total cost of ownership, which includes the continuing operational costs of scanners."
+5480,"Increased reaction time whilst scanning also has benefits in the realm of context-scanning. ADF scanners, whilst very fast and very good at batch scanning, also require pre- and post-processing of the documents. Document cameras can be integrated directly into a Workflow or process, for example, a teller at a bank. The document is scanned directly in the context of the customer, in which it is to be placed or used. Reaction time is an advantage in these situations. Document cameras usually also require a small amount of space and are often portable."
+5481,"Whilst scanning with document cameras may have a quick reaction time, large amounts of batch scanning of even, unstapled documents is more efficient with an ADF scanner. There are challenges that face this kind of technology regarding external factors  which may have influence on the scan results. The way in which these issues are resolved strongly depends on the sophistication of the product and how it deals with these issues."
+5482,"Infrared cleaning is a technique used to remove the effects of dust and scratches on images scanned from film; many modern scanners incorporate this feature. It works by scanning the film with infrared light; the dyes in typical color film emulsions are transparent to infrared light, but dust and scratches are not, and block infrared; scanner software can use the visible and infrared information to detect scratches and process the image to greatly reduce their visibility, considering their position, size, shape, and surroundings."
+5483,"Scanner manufacturers usually have their own names attached to this technique. For example, Epson, Minolta, Nikon, Konica Minolta, Microtek, and others use Digital ICE, while Canon uses its own system FARE . Plustek uses LaserSoft Imaging iSRD.  Some independent software developers design infrared cleaning tools."
+5484,"Flatbed scanners have been used as digital backs for large-format cameras to create high-resolution digital images of static subjects. A modified flatbed scanner has been used for documentation and quantification of thin layer chromatograms detected by fluorescence quenching on silica gel layers containing an ultraviolet  indicator. 'ChromImage' is allegedly the first commercial flatbed scanner densitometer. It enables acquisition of TLC plate images and quantification of chromatograms by use of Galaxie-TLC software. Other than being turned into densitometers, flatbed scanners were also turned into colorimeters using different methods. Trichromatic Color Analyser is allegedly the first distributable system using a flatbed scanner as a tristimulus colorimetric device."
+5485,"The device consists of a rough surface upon which the user may ""draw"" or trace an image using the attached stylus, a pen-like drawing apparatus. The image is shown on the computer monitor, though some graphic tablets now also incorporate an LCD screen for  more realistic or natural experience and usability."
+5486,Some tablets are intended as a replacement for the computer mouse as the primary pointing and navigation device for desktop computers.
+5487,"The first electronic handwriting device was the Telautograph, patented by Elisha Gray in 1888."
+5488,"The first graphic tablet resembling contemporary tablets and used for handwriting recognition by a computer was the Stylator in 1957. Better known  is the RAND Tablet also known as the Grafacon , introduced in 1964. The RAND Tablet employed a grid of wires under the surface of the pad that encoded horizontal and vertical coordinates in a small electrostatic signal. The stylus received the signal by capacitive coupling, which could then be decoded back as coordinate information."
+5489,"The acoustic tablet, or spark tablet, used a stylus that generated clicks with a spark plug. The clicks were then triangulated by a series of microphones to locate the pen in space. The system was fairly complex and expensive, and the sensors were susceptible to interference by external noise."
+5490,"Digitizers were popularized in the mid-1970s and early 1980s by the commercial success of the ID  and BitPad manufactured by the Summagraphics Corp. The Summagraphics digitizers were sold under the company's name but were also private labeled for HP, Tektronix, Apple, Evans and Sutherland and several other graphic system manufacturers. The ID model was the first graphics tablet to make use of what was at the time, the new Intel microprocessor technology. This embedded processing power allowed the ID models to have twice the accuracy of previous models while still making use of the same foundation technology. Key to this accuracy improvement were two US Patents issued to Stephen Domyan, Robert Davis, and Edward Snyder. The Bit Pad model was the first attempt at a low cost graphics tablet with an initial selling price of $555 when other graphics tablets were selling in the $2,000 to $3,000 price range. This lower cost opened up the opportunities for would be entrepreneurs to be able to write graphics software for a multitude of new applications. These digitizers were used as the input device for many high-end CAD  systems as well as bundled with PCs and PC-based CAD software like AutoCAD. These tablets used a magnetostriction technology which used wires made of a special alloy stretched over a solid substrate to accurately locate the tip of a stylus or the center of a digitizer cursor on the surface of the tablet. This technology also allowed Proximity or ""Z"" axis measurement."
+5491,"In 1981, musician Todd Rundgren created the first color graphic tablet software for personal computers, which was licensed to Apple as the Utopia Graphic Tablet System."
+5492,"In 1981, the Quantel Paintbox color graphic workstation was released; This model was equipped with the first pressure sensitive tablet."
+5493,"The first home computer graphic tablet was the KoalaPad, released in 1983. Though originally designed for the Apple II, the Koala eventually broadened its applicability to practically all home computers with graphic support, examples of which include the TRS-80 Color Computer, Commodore 64, and Atari 8-bit family. Competing tablets were eventually produced; the tablets produced by Atari were generally considered to be of high quality."
+5494,"In the 1980s, several vendors of graphic tablets began to include additional functions, such as handwriting recognition and on-tablet menus."
+5495,"Typically tablets are characterized by size of the device, drawing area, its resolution size , pressure sensitivity , number of buttons and types and number of interfaces: Bluetooth, USB; etc. The actual drawing accuracy is restricted to pen's nib size."
+5496,There have been many attempts to categorize the technologies that have been used for graphic tablets:
+5497,"For all these technologies, the tablet can use the received signal to also determine the distance of the stylus from the surface of the tablet, the tilt  of the stylus, and other information in addition to the horizontal and vertical positions, such as clicking buttons of the stylus or the rotation of the stylus."
+5498,"Compared to touchscreens, a graphic tablet generally offers much higher precision, the ability to track an object which is not touching the tablet, and can gather much more information about the stylus, but is typically more expensive, and can only be used with the special stylus or other accessories."
+5499,"Some tablets, especially inexpensive ones aimed at young children, come with a corded stylus, using technology similar to older RAND tablets."
+5500,"After styluses, pucks are the most commonly used tablet accessory. A puck is a mouse-like device that can detect its absolute position and rotation. This is opposed to a mouse, which can only sense its relative velocity on a surface . Pucks range in size and shape; some are externally indistinguishable from a mouse, while others are a fairly large device with dozens of buttons and controls. Professional pucks often have a reticle or loupe which allows the user to see the exact point on the tablet's surface targeted by the puck, for detailed tracing and computer aided design  work."
+5501,"Pucks are still used on the Microsoft Surface range and were recently used on the Dell Canvas. However, they have been largely discontinued by most manufactures in favour of physical hotkeys and dials."
+5502,"Some graphics tablets incorporate an LCD into the tablet itself, allowing the user to draw directly on the display surface."
+5503,"Graphics tablet/screen hybrids offer advantages over both standard PC touchscreens and ordinary graphics tablets. Unlike touchscreens, they offer pressure sensitivity, and their input resolution is generally higher. While their pressure sensitivity and resolution are typically no better than those of ordinary tablets, they offer the additional advantage of directly seeing the location of the physical pen device relatively to the image on the screen. This often allows for increased accuracy and a more tactile, ""real"" feeling to the use of the device."
+5504,"The graphics tablet manufacturer Wacom holds many patents on key technologies for graphics tablets, which forces competitors to use other technologies or license Wacom's patents. The displays are often sold for thousands of dollars. For instance, the Wacom Cintiq series ranges from just below US$1,000 to over US$2,000."
+5505,Some commercially available graphics tablet/screen hybrids include:
+5506,There have also been do-it-yourself projects where conventional used LCD monitors and graphics tablets have been converted to a graphics tablet-screen hybrid.
+5507,"Graphic tablets, because of their stylus-based interface and ability to detect some or all of pressure, tilt, and other attributes of the stylus and its interaction with the tablet, are widely considered to offer a very natural way to create computer graphics, especially two-dimensional computer graphics. Indeed, many graphic packages can make use of the pressure  information generated by a tablet, by modifying the brush size, shape, opacity, color, or other attributes based on data received from the graphic tablet."
+5508,"In East Asia, graphic tablets, known as ""pen tablets"", are widely used in conjunction with input-method editor software  to write Chinese, Japanese, and Korean characters . The technology is popular and inexpensive and offers a method for interacting with the computer in a more natural way than typing on the keyboard, with the pen tablet supplanting the role of the computer mouse. Uptake of handwriting recognition among users who use alphabetic scripts has been slower."
+5509,"Graphic tablets are commonly used in the artistic world. Using a pen-like stylus on a graphic tablet combined with a graphics-editing program, such as Illustrator, Photoshop by Adobe Systems, Corelpainter, or Krita gives artists a lot of precision when creating digital drawings or artwork. Photographers can also find working with a graphic tablet during their post processing can really speed up tasks like creating a detailed layer mask or dodging and burning."
+5510,"Educators make use of tablets in classrooms to project handwritten notes or lessons and to allow students to do the same, as well as providing feedback on student work submitted electronically. Online teachers may also use a tablet for marking student work, or for live tutorials or lessons, especially where complex visual information or mathematical equations are required. Students are also increasingly using them as note-taking devices, especially during university lectures while following along with the lecturer. They facilitate smooth online teaching process and are popularly used along with face-cam to mimic classroom experience."
+5511,"Tablets are also popular for technical drawings and CAD, as one can typically put a piece of paper on them without interfering with their function."
+5512,"Finally, tablets are gaining popularity as a replacement for the computer mouse as a pointing device. They can feel more intuitive to some users than a mouse, as the position of a pen on a tablet typically corresponds to the location of the pointer on the GUI shown on the computer screen. Those artists using a pen for graphic work may, as a matter of convenience, use a tablet and pen for standard computer operations rather than put down the pen and find a mouse. Popular rhythm game osu! allows utilizing a tablet as a way of playing."
+5513,Graphic tablets are available in various sizes and price ranges; A6-sized tablets being relatively inexpensive and A3-sized tablets far more expensive. Modern tablets usually connect to the computer via a USB or HDMI interface.
+5514,"Interactive whiteboards offer high-resolution wall size graphic tablets up to 95""  along with options for pressure and multiple input. These are becoming commonplace in schools and meeting rooms around the world."
+5515,"Earlier resistive touch screen devices  were typically equipped with styluses, but accuracy of stylus input was very limited."
+5516,"The more modern capacitive touch screens such as those found on some table computers, tablet computers and laptops operate in similar ways, but they usually use either optical grids or a pressure-sensitive film instead so do not need a special pointing device. Some of the latest models with capacitive input can be equipped with specialized styluses, and then these input devices can be used similar to full-function graphics tablet."
+5517,A graphic tablet is also used for Audio-Haptic products where blind or visually impaired people touch swelled graphics on a graphic tablet and get audio feedback from that. The product that is using this technology is called Tactile Talking Tablet or T3.
+5518,"Digital and digital movie cameras share an optical system, typically using a lens with a variable diaphragm to focus light onto an image pickup device. The diaphragm and shutter admit a controlled amount of light to the image, just as with film, but the image pickup device is electronic rather than chemical. However, unlike film cameras, digital cameras can display images on a screen immediately after being recorded, and store and delete images from memory. Many digital cameras can also record moving videos with sound. Some digital cameras can crop and stitch pictures and perform other kinds of image editing."
+5519,"The first semiconductor image sensor was the charge-coupled device , invented by Willard S. Boyle and George E. Smith at Bell Labs in 1969, based on MOS capacitor technology. The NMOS active-pixel sensor was later invented by Tsutomu Nakamura's team at Olympus in 1985, which led to the development of the CMOS active-pixel sensor  at the NASA Jet Propulsion Laboratory in 1993."
+5520,"In the 1960s, Eugene F. Lally of the Jet Propulsion Laboratory was thinking about how to use a mosaic photosensor to capture digital images. His idea was to take pictures of the planets and stars while travelling through space to give information about the astronauts' position. As with Texas Instruments employee Willis Adcock's film-less camera  in 1972, the technology had yet to catch up with the concept."
+5521,"In 1972, the Landsat 1 satellite's multispectral scanner  started taking digital images of Earth. The MSS, designed by Virginia Norwood at Hughes Aircraft Company starting in 1969, captured and transmitted image data from green, red, and two infrared bands with 6 bits per channel, using a mechanical rocking mirror and an array of 24 detectors. Operating for six years, it transmitted more than 300,000 digital photographs of Earth, while orbiting the planet about 14 times per day."
+5522,"Also in 1972, Thomas McCord from MIT and James Westphal from Cal Tech together developed a digital camera for use with telescopes. Their 1972 ""photometer-digitizer system"" used an analog-to-digital converter and a digital frame memory to store  256 x 256-pixel images of planets and stars, which were then recorded on digital magnetic tape.  CCD sensors were not yet commercially available, and the camera used a silicon diode vidicon tube detector which was cooled using dry ice to reduce dark current, allowing exposure times of up to one hour."
+5523,"The Cromemco Cyclops was an all-digital camera introduced as a commercial product in 1975. Its design was published as a hobbyist construction project in the February 1975 issue of Popular Electronics magazine. It used a 32×32 metal–oxide–semiconductor  image sensor, which was a modified MOS dynamic RAM  memory chip."
+5524,"Steven Sasson, an engineer at Eastman Kodak, built a self-contained electronic camera that used a monochrome Fairchild CCD image sensor in 1975. Around the same time, Fujifilm began developing CCD technology in the 1970s. Early uses were mainly military and scientific; followed by medical and news applications."
+5525,"The first filmless SLR  camera was publicly demonstrated by Sony in August 1981. The Sony ""Mavica""  used a color-striped 2/3"" format CCD sensor with 280K pixels, along with analogue video signal processing and recording. The Mavica electronic still camera recorded FM modulated analog video signals on a newly developed 2"" magnetic floppy disk, dubbed the ""Mavipak"".  The disk format was later standardized as the ""Still Video Floppy"", or ""SVF""."
+5526,"The Canon RC-701, introduced in May 1986, was the first SVF camera  sold in the US. It employed an SLR viewfinder, included a 2/3"" format color CCD sensor with 380K pixels, and was sold along with a removable 11-66mm and 50-150mm zoom lens."
+5527,"Over the next few years, many other companies began selling SVF cameras. These analog electronic cameras included the Nikon QV-1000C, which had an SLR viewfinder and a 2/3"" format monochrome CCD sensor with 380K pixels, and recorded analog black and white images on a Still Video Floppy."
+5528,"At Photokina 1988, Fujifilm introduced the FUJIX DS-1P, the first fully digital camera, which recorded digital images using a semiconductor memory card. The camera's memory card had a capacity of 2 MB of SRAM , and could hold up to ten photographs. In 1989, Fujifilm released the FUJIX DS-X, the first fully digital camera to be commercially released. In 1996, Toshiba's 40 MB flash memory card was adopted for several digital cameras."
+5529,"The first commercial camera phone was the Kyocera Visual Phone VP-210, released in Japan in May 1999. It was called a ""mobile videophone"" at the time, and had a 110,000-pixel front-facing camera. It stored up to 20 JPEG digital images, which could be sent over e-mail, or the phone could send up to two images per second over Japan's Personal Handy-phone System  cellular network. The Samsung SCH-V200, released in South Korea in June 2000, was also one of the first phones with a built-in camera. It had a TFT liquid-crystal display  and stored up to 20 digital photos at 350,000-pixel resolution. However, it could not send the resulting image over the telephone function but required a computer connection to access photos. The first mass-market camera phone was the J-SH04, a Sharp J-Phone model sold in Japan in November 2000. It could instantly transmit pictures via cell phone telecommunication. By the mid-2000s, higher-end cell phones had an integrated digital camera and by the early 2010s, almost all smartphones had an integrated digital camera."
+5530,"The two major types of digital image sensors are CCD and CMOS. A CCD sensor has one amplifier for all the pixels, while each pixel in a CMOS active-pixel sensor has its own amplifier. Compared to CCDs, CMOS sensors use less power. Cameras with a small sensor use a back-side-illuminated CMOS  sensor. The image processing capabilities of the camera determine the outcome of the final image quality much more than the sensor type."
+5531,"The resolution of a digital camera is often limited by the image sensor that turns light into discrete signals. The brighter the image at a given point on the sensor, the larger the value that is read for that pixel.
+Depending on the physical structure of the sensor, a color filter array may be used, which requires demosaicing to recreate a full-color image. The number of pixels in the sensor determines the camera's ""pixel count"".
+In a typical sensor, the pixel count is the product of the number of rows and the number of columns. For example, a 1,000 by 1,000-pixel sensor would have 1,000,000 pixels, or 1 megapixel."
+5532,"Firmwares' resolution selector allows the user to optionally lower the resolution, to reduce the file size per picture, and extend lossless digital zooming. The bottom resolution option is typically 640×480 pixels ."
+5533,"A lower resolution extends the number of remaining photos in free space, postponing the exhaustion of space storage, which is of use where no further data storage device is available, and for captures of lower significance, where the benefit from less space storage consumption outweighs the disadvantage from reduced detail."
+5534,"An image's sharpness is presented through the crisp detail, defined lines, and its depicted contrast. Sharpness is a factor of multiple systems throughout the DSLR camera by its ISO, resolution, lens and the lens settings, the environment of the image, and its post-processing. Images have a possibility of being too sharp but they can never be too in focus."
+5535,"A digital camera resolution is determined by a digital sensor. The digital sensor indicates a high level of sharpness can be produced through the amount of noise and grain that is tolerated through the lens of the camera. Resolution within the field of digital still and digital movies is indicated through the camera's ability to determine detail based on the distance which is then measured by frame size, pixel type, number, and organization although some DSLR cameras have resolutions limited, it is almost impossible to not have the proper sharpness for an image. The ISO choice when taking a photo affects the quality of the image as high ISO settings equate to an image that is less sharp due to the increased amount of noise allowed into the image along with too little noise can also produce an image that is not sharp."
+5536,"Since the first digital backs were introduced, there have been three main methods of capturing the image, each based on the hardware configuration of the sensor and color filters."
+5537,"Single-shot capture systems use either one sensor chip with a Bayer filter mosaic, or three separate image sensors  which are exposed to the same image via a beam splitter ."
+5538,Multi-shot exposes the sensor to the image in a sequence of three or more openings of the lens aperture. There are several methods of application of the multi-shot technique. The most common was originally to use a single image sensor with three filters passed in front of the sensor in sequence to obtain the additive color information. Another multiple-shot method is called microscanning. This method uses a single sensor chip with a Bayer filter and physically moves the sensor on the focus plane of the lens to construct a higher resolution image than the native resolution of the chip. A third version combines these two methods without a Bayer filter on the chip.
+5539,"The third method is called scanning because the sensor moves across the focal plane much like the sensor of an image scanner. The linear or tri-linear sensors in scanning cameras utilize only a single line of photosensors, or three lines for the three colors. Scanning may be accomplished by moving the sensor  or by rotating the whole camera. A digital rotating line camera offers images consisting of a total resolution that is very high."
+5540,"The choice of method for a given capture is determined largely by the subject matter. It is usually inappropriate to attempt to capture a subject that moves with anything but a single-shot system. However, the higher color fidelity and larger file sizes and resolutions that are available with multi-shot and scanning backs make them more attractive for commercial photographers who are working with stationary subjects and large-format photographs."
+5541,"Improvements in single-shot cameras and image file processing at the beginning of the 21st century made single-shot cameras almost completely dominant, even in high-end commercial photography."
+5542,"Most current  consumer digital cameras use a Bayer filter mosaic in combination with an optical anti-aliasing filter to reduce the aliasing due to the reduced sampling of the different primary-color images.
+A demosaicing algorithm is used to interpolate color information to create a full array of RGB image data."
+5543,"Cameras that use a beam-splitter single-shot 3CCD approach, three-filter multi-shot approach, color co-site sampling or Foveon X3 sensor do not use anti-aliasing filters, nor demosaicing."
+5544,"Firmware in the camera, or a software in a raw converter program such as Adobe Camera Raw, interprets the raw data from the sensor to obtain a full-color image, because the RGB color model requires three intensity values for each pixel: one each for the red, green, and blue .
+A single sensor element cannot simultaneously record these three intensities, so a color filter array  must be used to selectively filter a particular color for each pixel."
+5545,"The Bayer filter pattern is a repeating 2x2 mosaic pattern of light filters, with green ones at opposite corners and red and blue in the other two positions. The high proportion of green takes advantage of the properties of the human visual system, which determines brightness mostly from green and is far more sensitive to brightness than to hue or saturation. Sometimes a 4-color filter pattern is used, often involving two different hues of green. This provides potentially more accurate color, but requires a slightly more complicated interpolation process."
+5546,The color intensity values not captured for each pixel can be interpolated from the values of adjacent pixels which represent the color being calculated.
+5547,Cameras with digital image sensors that are smaller than the typical 35 mm film size have a smaller field or angle of view when used with a lens of the same focal length. This is because the angle of view is a function of both focal length and the sensor or film size used.
+5548,"The crop factor is relative to the 35mm film format. If a smaller sensor is used, as in most digicams, the field of view is cropped by the sensor to smaller than the 35 mm full-frame format's field of view. This narrowing of the field of view may be described as crop factor, a factor by which a longer focal length lens would be needed to get the same field of view on a 35 mm film camera. Full-frame digital SLRs utilize a sensor of the same size as a frame of 35 mm film."
+5549,"Common values for field of view crop in DSLRs using active pixel sensors include 1.3x for some Canon  sensors, 1.5x for Sony APS-C sensors used by Nikon, Pentax and Konica Minolta and for Fujifilm sensors, 1.6  for most Canon sensors, ~1.7x for Sigma's Foveon sensors and 2x for Kodak and Panasonic 4/3-inch sensors currently used by Olympus and Panasonic. Crop factors for non-SLR consumer compact and bridge cameras are larger, frequently 4x or more."
+5550,"The resolution of a digital camera is often limited by the image sensor that turns light into discrete signals. The brighter the image at a given point on the sensor, the larger the value that is read for that pixel. Depending on the physical structure of the sensor, a color filter array may be used, which requires demosaicing to recreate a full-color image. The number of pixels in the sensor determines the camera's ""pixel count"". In a typical sensor, the pixel count is the product of the number of rows and the number of columns. Pixels are square and is often equal to 1, for example, a 1,000 by 1,000-pixel sensor would have 1,000,000 pixels, or 1 megapixel. On full-frame sensors , some cameras propose images with 20–25 million pixels that were captured by 7.5–m photosites, or a surface that is 50 times larger.  "
+5551,"Digital cameras come in a wide range of sizes, prices, and capabilities. In addition to general-purpose digital cameras, specialized cameras including multispectral imaging equipment and astrographs are used for scientific, military, medical, and other special purposes."
+5552,"Compact cameras are intended to be portable  and are particularly suitable for casual ""snapshots"". Point-and-shoot cameras usually fall under this category."
+5553,"Many incorporate a retractable lens assembly that provides optical zoom. In most models, an auto-actuating lens cover protects the lens from elements. Most ruggedized or water-resistant models do not retract, and most with superzoom capability do not retract fully."
+5554,"Compact cameras are usually designed to be easy to use. Almost all include an automatic mode, or ""auto mode"", which automatically makes all camera settings for the user. Some also have manual controls. Compact digital cameras typically contain a small sensor that trades-off picture quality for compactness and simplicity; images can usually only be stored using lossy compression . Most have a built-in flash usually of low power, sufficient for nearby subjects. A few high-end compact digital cameras have a hotshoe for connecting to an external flash. Live preview is almost always used to frame the photo on an integrated LCD. In addition to being able to take still photographs almost all compact cameras have the ability to record video."
+5555,"Compacts often have macro capability and zoom lenses, but the zoom range  is generally enough for candid photography but less than is available on bridge cameras , or the interchangeable lenses of DSLR cameras available at a much higher cost. Autofocus systems in compact digital cameras generally are based on a contrast-detection methodology using the image data from the live preview feed of the main imager. Some compact digital cameras use a hybrid autofocus system similar to what is commonly available on DSLRs."
+5556,"Typically, compact digital cameras incorporate a nearly silent leaf shutter into the lens but play a simulated camera sound for skeuomorphic purposes."
+5557,"For low cost and small size, these cameras typically use image sensor formats with a diagonal between 6 and 11 mm, corresponding to a crop factor between 7 and 4. This gives them weaker low-light performance, greater depth of field, generally closer focusing ability, and smaller components than cameras using larger sensors. Some cameras use a larger sensor including, at the high end, a pricey full-frame sensor compact camera, such as Sony Cyber-shot DSC-RX1, but have capability near that of a DSLR."
+5558,"A variety of additional features are available depending on the model of the camera. Such features include GPS, compass, barometers and altimeters."
+5559,"Starting in 2010, some compact digital cameras can take 3D still photos. These 3D compact stereo cameras can capture 3D panoramic photos with dual lens or even a single lens for playback on a 3D TV."
+5560,"In 2013, Sony released two add-on camera models without display, to be used with a smartphone or tablet, controlled by a mobile application via WiFi."
+5561,"Rugged compact cameras typically include protection against submersion, hot and cold conditions, shock, and pressure. Terms used to describe such properties include waterproof, freeze-proof, heatproof, shockproof, and crushproof, respectively. Nearly all major camera manufacturers have at least one product in this category. Some are waterproof to a considerable depth up to 100 feet ; others only 10 feet , but only a few will float. Ruggeds often lack some of the features of ordinary compact camera, but they have video capability and the majority can record sound. Most have image stabilization and built-in flash. Touchscreen LCD and GPS do not work underwater."
+5562,"GoPro and other brands offer action cameras that are rugged, small, and can be easily attached to helmets, arms, bicycles, etc. Most have a wide angle and fixed focus and can take still pictures and video, typically with sound."
+5563,"The 360-degree camera can take picture or video 360 degrees using two lenses back-to-back and shooting at the same time. Some of the cameras are Ricoh Theta S, Nikon Keymission 360 and Samsung Gear 360. Nico360 was launched in 2016 and claimed as the world's smallest 360-degree camera with size 46 x 46 x 28 mm  and price less than $200. With virtual reality mode built-in stitching, Wifi, and Bluetooth, live streaming can be done. Due to it also being water resistant, the Nico360 can be used as action camera."
+5564,There are tend that action cameras have capabilities to shoot 360 degrees with at least 4K resolution.
+5565,"Bridge cameras physically resemble DSLRs, and are sometimes called DSLR-shape or DSLR-like. They provide some similar features but, like compacts, they use a fixed lens and a small sensor. Some compact cameras have also PSAM mode. Most use live preview to frame the image. Their usual autofocus is by the same contrast-detect mechanism as compacts, but many bridge cameras have a manual focus mode and some have a separate focus ring for greater control."
+5566,"The big physical size and small sensor allow superzoom and wide aperture. Bridge cameras generally include an image stabilization system to enable longer handheld exposures, sometimes better than DSLR for low light conditions."
+5567,"As of 2014, bridge cameras come in two principal classes in terms of sensor size, firstly the more traditional 1/2.3"" sensor  which gives more flexibility in lens design and allows for handholdable zoom from 20 to 24 mm  wide angle all the way up to over 1000 mm supertele, and secondly a 1"" sensor that allows better image quality particularly in low light  but puts greater constraints on lens design, resulting in zoom lenses that stop at 200 mm  or 400 mm  equivalent, corresponding to an optical zoom factor of roughly 10 to 15."
+5568,"Some bridge cameras have a lens thread to attach accessories such as wide-angle or telephoto converters as well as filters such as UV or Circular Polarizing filter and lens hoods. The scene is composed by viewing the display or the electronic viewfinder . Most have a slightly longer shutter lag than a DSLR. Many of these cameras can store images in a raw format in addition to supporting JPEG. The majority have a built-in flash, but only a few have a hotshoe."
+5569,"In bright sun, the quality difference between a good compact camera and a digital SLR is minimal but bridge cameras are more portable, cost less and have a greater zoom ability. Thus a bridge camera may better suit outdoor daytime activities, except when seeking professional-quality photos."
+5570,"In late 2008, a new type of camera emerged, called a mirrorless interchangeable-lens camera. It is technically a DSLR camera that does not require a reflex mirror, a key component of the former. While a typical DSLR has a mirror that reflects light from the lens up to the optical viewfinder, in a mirrorless camera, there is no optical viewfinder. The image sensor is exposed to light at all times, giving the user a digital preview of the image either on the built-in rear LCD screen or an electronic viewfinder ."
+5571,"These are simpler and more compact than DSLRs due to not having a lens reflex system. MILCs, or mirrorless cameras for short, come with various sensor sizes depending on the brand and manufacturer, these include: a small 1/2.3 inch sensor, as is commonly used in bridge cameras such as the original Pentax Q ; a 1-inch sensor; a Micro Four Thirds sensor; an APS-C sensor found in Sony NEX series and α ""DSLR-likes"", Fujifilm X series, Pentax K-01, and Canon EOS M; and some, such as the Sony α7, use a full frame  sensor, with the Hasselblad X1D being the first medium format mirrorless camera. Some MILCs have a separate electronic viewfinder to compensate the lack of an optical one. In other cameras, the back display is used as the primary viewfinder in the same way as in compact cameras. One disadvantage of mirrorless cameras compared to a typical DSLR is its battery life due to the energy consumption of the electronic viewfinder, but this can be mitigated by a setting inside the camera in some models. Many mirrorless cameras have a hotshoe."
+5572,"Olympus and Panasonic released many Micro Four Thirds cameras with interchangeable lenses that are fully compatible with each other without any adapter, while others have proprietary mounts. In 2014, Kodak released its first Micro Four Third system camera."
+5573,"As of March 2014, mirrorless cameras are fast becoming appealing to both amateurs and professionals alike due to their simplicity, compatibility with some DSLR lenses, and features that match most DSLRs today."
+5574,"While most digital cameras with interchangeable lenses feature a lens-mount of some kind, there are also a number of modular cameras, where the shutter and sensor are incorporated into the lens module."
+5575,"The first such modular camera was the Minolta Dimâge V in 1996, followed by the Minolta Dimâge EX 1500 in 1998 and the Minolta MetaFlash 3D 1500 in 1999. In 2009, Ricoh released the Ricoh GXR modular camera."
+5576,"At CES 2013, Sakar International announced the Polaroid iM1836, an 18MP camera with 1""-sensor with interchangeable sensor-lens. An adapter for Micro Four Thirds, Nikon and K-mount lenses was planned to ship with the camera."
+5577,"There are also a number of add-on camera modules for smartphones, they are called lens-style cameras . They contain all the essential components of a digital camera inside a DSLR lens-shaped module, hence the name, but lack any sort of viewfinder and most controls of a regular camera. Instead, they are connected wirelessly and/or mounted to a smartphone to be used as its display output and operate the camera's various controls."
+5578,Lens-style cameras include:
+5579,"Digital single-lens reflex cameras  is a camera with a digital sensor that utilizes a reflex mirror to split or direct light into the viewfinder to produce an image. The reflex mirror finds the image by blocking light to the camera's sensor and then reflecting it into the camera's pentaprism which allows it to be seen through the viewfinder. When the shutter release is fully pressed the reflex mirror pulls out horizontally below the pentaprism briefly darkening the viewfinder and then opening up the sensor for exposure which creates the photo. The digital image is produced by the sensor which is an array of photoreceptors on a microchip capable of recording light values. Many modern DSLRs offer the ability for ""live view"" or the framing of the subject emitted from the sensor onto a digital screen, and many have a hotshoe."
+5580,"The sensor also known as a full-frame sensor is much larger than the other types, typically 18mm to 36mm on the diagonal . The larger sensor permits more light to be received by each pixel; this, combined with the relatively large lenses provides superior low-light performance. For the same field of view and the same aperture, a larger sensor gives shallower focus. DSLRs can equip interchangeable lenses for versatility by removing it from the lens mount of the camera, typically a silver ring on the front side of DSLRs. These lenses work in tandem with the mechanics of the DSLR to adjust aperture and focus. Autofocus is accomplished using sensors in the mirror box and on most modern lenses can be activated from the lens itself which will trigger upon shutter release."
+5581,"Digital Still Camera , such as the Sony DSC cameras, is a type of camera that does not use a reflex mirror. DSCs are like point-and-shoot cameras and are the most common type of cameras, due to their comfortable price and its quality."
+5582,Here are a list of DSCs: List of Sony Cyber-shot cameras
+5583,"Cameras with fixed semi-transparent mirrors, also known as DSLT cameras, such as the Sony SLT cameras, are single-lens without a moving reflex mirror as in a conventional DSLR. A semi-transparent mirror transmits some of the light to the image sensor and reflects some of the light along the path to a pentaprism/pentamirror which then goes to an optical view finder  as is done with a reflex mirror in DSLR cameras. The total amount of light is not changed, just some of the light travels one path and some of it travels the other. The consequences are that DSLT cameras should shoot a half stop differently from DSLR. One advantage of using a DSLT camera is the blind moments a DSLR user experiences while the reflecting mirror is moved to send the light to the sensor instead of the viewfinder do not exist for DSLT cameras. Because there is no time at which light is not traveling along both paths, DSLT cameras get the benefit of continuous auto-focus tracking. This is especially beneficial for burst-mode shooting in low-light conditions and also for tracking when taking video."
+5584,"A rangefinder is a device to measure subject distance, with the intent to adjust the focus of a camera's objective lens accordingly . The rangefinder and lens focusing mechanism may or may not be coupled. In common parlance, the term ""rangefinder camera"" is interpreted very narrowly to denote manual-focus cameras with a visually-read out optical rangefinder based on parallax. Most digital cameras achieve focus through analysis of the image captured by the objective lens and distance estimation, if it is provided at all, is only a byproduct of the focusing process ."
+5585,"A line-scan camera traditionally has a single row of pixel sensors, instead of a matrix of them. The lines are continuously fed to a computer that joins them to each other and makes an image. This is most commonly done by connecting the camera output to a frame grabber which resides in a PCI slot of an industrial computer. The frame grabber acts to buffer the image and sometimes provide some processing before delivering to the computer software for processing. Industrial processes often require height and width measurements performed by digital line-scan systems."
+5586,"Multiple rows of sensors may be used to make colored images, or to increase sensitivity by TDI ."
+5587,"Many industrial applications require a wide field of view. Traditionally maintaining consistent light over large 2D areas is quite difficult. With a line scan camera all that is necessary is to provide even illumination across the ""line"" currently being viewed by the camera. This makes sharp pictures of objects that pass the camera at high speed."
+5588,"Such cameras are also commonly used to make photo finishes, to determine the winner when multiple competitors cross the finishing line at nearly the same time. They can also be used as industrial instruments for analyzing fast processes."
+5589,"Line-scan cameras are also extensively used in imaging from satellites . In this case the row of sensors is perpendicular to the direction of satellite motion. Line-scan cameras are widely used in scanners. In this case, the camera moves horizontally."
+5590,Digital superzoom cameras are digital cameras that can zoom in very far. These superzoom cameras are suitable for people who have nearsightedness.
+5591,"The HX series is a series containing Sony's superzoom cameras like HX20V, HX90V and the newest HX99. HX stands for HyperXoom."
+5592,"This type of digital camera captures information about the light field emanating from a scene; that is, the intensity of light in a scene, and also the direction that the light rays are traveling in space. This contrasts with a conventional digital camera, which records only light intensity."
+5593,"Instead of measuring the intensity of light over some predetermined time interval , event cameras detect when the intensity of light changes by some threshold for each pixel independently, usually with microsecond precision."
+5594,"Many devices have a built-in digital camera, including, for example, smartphones, mobile phones, PDAs and laptop computers. Built-in cameras generally store the images in the JPEG file format."
+5595,"Mobile phones incorporating digital cameras were introduced in Japan in 2001 by J-Phone. In 2003 camera phones outsold stand-alone digital cameras, and in 2006 they outsold film and digital stand-alone cameras. Five billion camera phones were sold in five years, and by 2007 more than half of the installed base of all mobile phones were camera phones. Sales of separate cameras peaked in 2008."
+5596,"There are many manufacturers that lead in the production of digital cameras . Each brand embodies different mission statements that differ them from each other outside of the physical technology that they produce. While the majority of manufacturers share modern features amongst their production of cameras, some specialize in specific details either physically on camera or within the system and image quality."
+5597,"Sales of traditional digital cameras have declined due to the increasing use of smartphones for casual photography, which also enable easier manipulation and sharing of photos through the use of apps and web-based services. ""Bridge cameras"", in contrast, have held their ground with functionality that most smartphone cameras lack, such as optical zoom and other advanced features. DSLRs have also lost ground to Mirrorless interchangeable-lens camera s offering the same sensor size in a smaller camera. A few expensive ones use a full-frame sensor, just like DSLR professional cameras."
+5598,"In response to the convenience and flexibility of smartphone cameras, some manufacturers produced ""smart"" digital cameras that combine features of traditional cameras with those of a smartphone. In 2012, Nikon and Samsung released the Coolpix S800c and Galaxy Camera, the first two digital cameras to run the Android operating system. Since this software platform is used in many smartphones, they can integrate with some of the same services  that smartphones do and use other Android-compatible software."
+5599,"In an inversion, some phone makers have introduced smartphones with cameras designed to resemble traditional digital cameras. Nokia released the 808 PureView and Lumia 1020 in 2012 and 2013; the two devices respectively run the Symbian and Windows Phone operating systems, and both include a 41-megapixel camera . Similarly, Samsung introduced the Galaxy S4 Zoom, having a 16-megapixel camera and 10x optical zoom, combining traits from the Galaxy S4 Mini with the Galaxy Camera. Panasonic Lumix DMC-CM1 is an Android KitKat 4.4 smartphone with 20MP, 1"" sensor, the largest sensor for a smartphone ever, with Leica fixed lens equivalent of 28 mm at F2.8, can take RAW image and 4K video, has 21 mm thickness. Furthermore, in 2018 Huawei P20 Pro is an android Oreo 8.1 has triple Leica lenses in the back of the smartphone with 40MP 1/1.7"" RGB sensor as first lens, 20MP 1/2.7"" monochrome sensor as second lens and 8MP 1/4"" RGB sensor with 3x optical zoom as third lens. Combination of first lens and second lens will produce bokeh image with larger high dynamic range, whereas combination of mega pixel first lens and optical zoom will produce maximum 5x digital zoom without loss of quality by reducing the image size to 8MP."
+5600,Light-field cameras were introduced in 2013 with one consumer product and several professional ones.
+5601,"After a big dip of sales in 2012, consumer digital camera sales declined again in 2013 by 36 percent. In 2011, compact digital cameras sold 10 million per month. In 2013, sales fell to about 4 million per month. DSLR and MILC sales also declined in 2013 by 10–15% after almost ten years of double digit growth.
+Worldwide unit sales of digital cameras is continuously declining from 148 million in 2011 to 58 million in 2015 and tends to decrease more in the following years."
+5602,"Film camera sales hit their peak at about 37 million units in 1997, while digital camera sales began in 1989. By 2008, the film camera market had died and digital camera sales hit their peak at 121 million units in 2010. In 2002, cell phones with an integrated camera had been introduced and in 2003 the cell phone with an integrated camera had sold 80 million units per year. By 2011, cell phones with an integrated camera were selling hundreds of millions per year, which were causing a decline in digital cameras. In 2015, digital camera sales were 35 million units or only less than a third of digital camera sales numbers at their peak and also slightly less than film camera sold number at their peak."
+5603,Many digital cameras can connect directly to a computer to transfer data:-
+5604,"A common alternative is the use of a card reader which may be capable of reading several types of storage media, as well as high speed transfer of data to the computer. Use of a card reader also avoids draining the camera battery during the download process. An external card reader allows convenient direct access to the images on a collection of storage media. But if only one storage card is in use, moving it back and forth between the camera and the reader can be inconvenient. Many computers have a card reader built in, at least for SD cards."
+5605,"Many modern cameras support the PictBridge standard, which allows them to send data directly to a PictBridge-capable printer without the need for a computer.  PictBridge uses PTP to transfer images and control information."
+5606,Wireless connectivity can also provide for printing photos without a cable connection.
+5607,"An instant-print camera, is a digital camera with a built-in printer. This confers a similar functionality as an instant camera which uses instant film to quickly generate a physical photograph. Such non-digital cameras were popularized by Polaroid with the SX-70 in 1972."
+5608,"Many digital cameras include a video output port. Usually sVideo, it sends a standard-definition video signal to a television, allowing the user to show one picture at a time. Buttons or menus on the camera allow the user to select the photo, advance from one to another, or automatically send a ""slide show"" to the TV."
+5609,"HDMI has been adopted by many high-end digital camera makers, to show photos in their high-resolution quality on an HDTV."
+5610,"In January 2008, Silicon Image announced a new technology for sending video from mobile devices to a television in digital form. MHL sends pictures as a video stream, up to 1080p resolution, and is compatible with HDMI."
+5611,Some DVD recorders and television sets can read memory cards used in cameras; alternatively several types of flash card readers have TV output capability.
+5612,"Cameras can be equipped with a varying amount of environmental sealing to provide protection against splashing water, moisture , dust and sand, or complete waterproofness to a certain depth and for a certain duration. The latter is one of the approaches to allow underwater photography, the other approach being the use of waterproof housings. Many waterproof digital cameras are also shockproof and resistant to low temperatures."
+5613,Some waterproof cameras can be fitted with a waterproof housing to increase the operational depth range. The Olympus 'Tough' range of compact cameras is an example.
+5614,"Many digital cameras have preset modes for different applications. Within the constraints of correct exposure various parameters can be changed, including exposure, aperture, focusing, light metering, white balance, and equivalent sensitivity. For example, a portrait might use a wider aperture to render the background out of focus, and would seek out and focus on a human face rather than other image content."
+5615,Few cameras are equipped with a voice note  recording feature.
+5616,"Vendors implement a variety scene modes in cameras' firmwares for various purposes, such as a ""landscape mode"" which prevents focusing on rainy and/or stained window glass such as a windshield, and a ""sports mode"" which reduces motion blur of moving subjects by reducing exposure time with the help of increased light sensitivity. Firmwares may be equipped with the ability to select a suitable scene mode automatically through artificial intelligence."
+5617,"Many camera phones and most stand alone digital cameras store image data in flash memory cards or other removable media. Most stand-alone cameras use SD format, while a few use CompactFlash or other types. In January 2012, a faster XQD card format was announced. In early 2014, some high end cameras have two hot-swappable memory slots. Photographers can swap one of the memory card with camera-on. Each memory slot can accept either Compact Flash or SD Card. All new Sony cameras also have two memory slots, one for its Memory Stick and one for SD Card, but not hot-swapable."
+5618,"The approximate count of remaining photos until space exhaustion is calculated by the firmware throughout use and indicated in the viewfinder, to prepare the user for an impending necessary hot swap of the memory card, and/or file offload."
+5619,"A few cameras used other removable storage such as Microdrives , CD single , and 3.5"" floppy disks . Other unusual formats include:"
+5620,"Most manufacturers of digital cameras do not provide drivers and software to allow their cameras to work with Linux or other free software. Still, many cameras use the standard USB mass storage and/or Media Transfer Protocol, and are thus widely supported. Other cameras are supported by the gPhoto project, and many computers are equipped with a memory card reader."
+5621,The Joint Photography Experts Group standard  is the most common file format for storing image data. Other file types include Tagged Image File Format  and various Raw image formats.
+5622,"Many cameras, especially high-end ones, support a raw image format. A raw image is the unprocessed set of pixel data directly from the camera's sensor, often saved in a proprietary format. Adobe Systems has released the DNG format, a royalty-free raw image format used by at least 10 camera manufacturers."
+5623,"Raw files initially had to be processed in specialized image editing programs, but over time many mainstream editing programs, such as Google's Picasa, have added support for raw images. Rendering to standard images from raw sensor data allows more flexibility in making major adjustments without losing image quality or retaking the picture."
+5624,"Formats for movies are AVI, DV, MPEG, MOV , WMV, and ASF . Recent formats include MP4, which is based on the QuickTime format and uses newer compression algorithms to allow longer recording times in the same space."
+5625,"Other formats that are used in cameras  are the Design Rule for Camera Format , an ISO specification, used in almost all camera since 1998, which defines an internal file structure and naming. Also used is the Digital Print Order Format , which dictates what order images are to be printed in and how many copies. The DCF 1998 defines a logical file system with 8.3 filenames and makes the usage of either FAT12, FAT16, FAT32 or exFAT mandatory for its physical layer in order to maximize platform interoperability."
+5626,"Most cameras include Exif data that provides metadata about the picture. Exif data may include aperture, exposure time, focal length, date and time taken. Some are able to tag the location."
+5627,"In order to guarantee interoperability, DCF specifies the file system for image and sound files to be used on formatted DCF media  as FAT12, FAT16, FAT32, or exFAT. Media with a capacity of more than 2 GB must be formatted using FAT32 or exFAT."
+5628,"The filesystem in a digital camera contains a DCIM  directory, which can contain multiple subdirectories with names such as ""123ABCDE"" that consist of a unique directory number  and five alphanumeric characters, which may be freely chosen and often refer to a camera maker. These directories contain files with names such as ""ABCD1234.JPG"" that consist of four alphanumeric characters , followed by a number. Handling of directories with possibly user-created duplicate numbers may vary among camera firmwares."
+5629,"DCF 2.0 adds support for DCF optional files recorded in an optional color space . Such files must be indicated by a leading ""_"" ."
+5630,"To enable loading many images in miniature view quickly and efficiently, and to retain meta data, some vendors' firmwares generate accompanying low-resolution thumbnail files for videos and raw photos. For example, those of Canon cameras end with .THM. JPEG can already store a thumbnail image standalone."
+5631,"Digital cameras have become smaller over time, resulting in an ongoing need to develop a battery small enough to fit in the camera and yet able to power it for a reasonable length of time."
+5632,"Digital cameras utilize either proprietary or standard consumer batteries. As of March 2014, most cameras use proprietary lithium-ion batteries while some use standard AA batteries or primarily use a proprietary Lithium-ion rechargeable battery pack but have an optional AA battery holder available."
+5633,"The most common class of battery used in digital cameras is proprietary battery formats. These are built to a manufacturer's custom specifications. Almost all proprietary batteries are lithium-ion. In addition to being available from the OEM, aftermarket replacement batteries are commonly available for most camera models."
+5634,"Digital cameras that utilize off-the-shelf batteries are typically designed to be able to use both single-use disposable and rechargeable batteries, but not with both types in use at the same time. The most common off-the-shelf battery size used is AA. CR2, CR-V3 batteries, and AAA batteries are also used in some cameras. The CR2 and CR-V3 batteries are lithium based, intended for a single use. Rechargeable RCR-V3 lithium-ion batteries are also available as an alternative to non-rechargeable CR-V3 batteries."
+5635,Some battery grips for DSLRs come with a separate holder to accommodate AA cells as an external power source.
+5636,"When digital cameras became common, many photographers asked whether their film cameras could be converted to digital. The answer was not immediately clear, as it differed among models. For the majority of 35 mm film cameras the answer is no, the reworking and cost would be too great, especially as lenses have been evolving as well as cameras. For most a conversion to digital, to give enough space for the electronics and allow a liquid crystal display to preview, would require removing the back of the camera and replacing it with a custom built digital unit."
+5637,"Many early professional SLR cameras, such as the Kodak DCS series, were developed from 35 mm film cameras. The technology of the time, however, meant that rather than being digital ""backs"" the bodies of these cameras were mounted on large, bulky digital units, often bigger than the camera portion itself. These were factory built cameras, however, not aftermarket conversions."
+5638,"A notable exception is the Nikon E2 and Nikon E3, using additional optics to convert the 35 mm format to a 2/3 CCD-sensor."
+5639,"A few 35 mm cameras have had digital camera backs made by their manufacturer, Leica being a notable example. Medium format and large format cameras , have a low unit production, and typical digital backs for them cost over $10,000. These cameras also tend to be highly modular, with handgrips, film backs, winders, and lenses available separately to fit various needs."
+5640,"The very large sensor these backs use leads to enormous image sizes. For example, Phase One's P45 39 MP image back creates a single TIFF image of size up to 224.6 MB, and even greater pixel counts are available. Medium format digitals such as this are geared more towards studio and portrait photography than their smaller DSLR counterparts; the ISO speed in particular tends to have a maximum of 400, versus 6400 for some DSLR cameras. ."
+5641,"In the industrial and high-end professional photography market, some camera systems use modular  image sensors. For example, some medium format SLR cameras, such as the Mamiya 645D series, allow installation of either a digital camera back or a traditional photographic film back."
+5642,Linear array cameras are also called scan backs.
+5643,"Most earlier digital camera backs used linear array sensors, moving vertically to digitize the image. Many of them only capture grayscale images. The relatively long exposure times, in the range of seconds or even minutes generally limit scan backs to studio applications, where all aspects of the photographic scene are under the photographer's control."
+5644,Some other camera backs use CCD arrays similar to typical cameras. These are called single-shot backs.
+5645,"Since it is much easier to manufacture a high-quality linear CCD array with only thousands of pixels than a CCD matrix with millions, very high resolution linear CCD camera backs were available much earlier than their CCD matrix counterparts. For example, you could buy an  camera back with over 7,000 pixel horizontal resolution in the mid-1990s. However, as of 2004, it is still difficult to buy a comparable CCD matrix camera of the same resolution. Rotating line cameras, with about 10,000 color pixels in its sensor line, are able, as of 2005, to capture about 120,000 lines during one full 360 degree rotation, thereby creating a single digital image of 1,200 Megapixels."
+5646,"Most modern digital camera backs use CCD or CMOS matrix sensors. The matrix sensor captures the entire image frame at once, instead of incrementing scanning the frame area through the prolonged exposure. For example, Phase One produces a 39 million pixel digital camera back with a 49.1 x 36.8 mm CCD in 2008. This CCD array is a little smaller than a frame of 120 film and much larger than a 35 mm frame . In comparison, consumer digital cameras use arrays ranging from 36 x 24 mm  to 1.28 x 0.96 mm  CMOS sensor."
+5647,"A computer keyboard is a peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or keys to act as mechanical levers or electronic switches. Replacing early punched cards and paper tape technology, interaction via teleprinter-style keyboards have been the main input method for computers since the 1970s, supplemented by the computer mouse since the 1980s."
+5648,"Keyboard keys  typically have a set of characters engraved or printed on them, and each press of a key typically corresponds to a single written symbol. However, producing some symbols may require pressing and holding several keys simultaneously or in sequence. 
+While most keys produce characters , other keys  can prompt the computer to execute system commands. In a modern computer, the interpretation of key presses is generally left to the software: the information sent to the computer, the scan code, tells it only which physical key  was pressed or released."
+5649,"In normal usage, the keyboard is used as a text entry interface for typing text, numbers, and symbols into application software such as a word processor, web browser or social media app."
+5650,Touchscreens use virtual keyboards.
+5651,"Typewriters are the definitive ancestor of all key-based text entry devices, but the computer keyboard as a device for electromechanical data entry and communication largely comes from the utility of two devices: teleprinters  and keypunches. It was through such devices that modern computer keyboards inherited their layouts."
+5652,"As early as the 1870s, teleprinter-like devices were used to simultaneously type and transmit stock market text data from the keyboard across telegraph lines to stock ticker machines to be immediately copied and displayed onto ticker tape. The teleprinter, in its more contemporary form, was developed from 1907 to 1910 by American mechanical engineer Charles Krum and his son Howard, with early contributions by electrical engineer Frank Pearne. Earlier models were developed separately by individuals such as Royal Earl House and Frederick G. Creed."
+5653,"Earlier, Herman Hollerith developed the first keypunch devices, which soon evolved to include keys for text and number entry akin to normal typewriters by the 1930s."
+5654,"The keyboard on the teleprinter played a strong role in point-to-point and point-to-multipoint communication for most of the 20th century, while the keyboard on the keypunch device played a strong role in data entry and storage for just as long. The development of some of the earliest computers incorporated electric typewriter keyboards: the development of the ENIAC computer incorporated a keypunch device as both the input and paper-based output device, and the BINAC computer made use of an electromechanically controlled typewriter for both data entry onto magnetic tape  and data output."
+5655,"The keyboard remained the primary, most integrated computer peripheral well into the era of personal computing until the introduction of the mouse as a consumer device in 1984. By this time, text-only user interfaces with sparse graphics gave way to comparatively graphics-rich icons on screen. However, keyboards remain central to human-computer interaction to the present though mobile personal computing devices such as smartphones and tablets use a virtual keyboard."
+5656,"Different types of keyboards are available and each is designed with a focus on specific features that suit particular needs. Today, most full-size keyboards use one of three different mechanical layouts, usually referred to as simply ISO , ANSI , and JIS , referring roughly to the organizations issuing the relevant worldwide, United States, and Japanese standards, respectively.  ANSI standard alphanumeric keyboards have keys that are on three-quarter inch centers ), and have a key travel of at least 0.15 inches ."
+5657,"Modern keyboard models contain a set number of total keys according to their given standard, described as 101, 104, 105, etc. and sold as ""Full-size"" keyboards. Modern keyboards matching US conventions typically have 104 keys while the 105 key layout is the norm in the rest of the world. This number is not always followed, and individual keys or whole sections are commonly skipped for the sake of compactness or user preference. The most common choice is to not include the numpad, which can usually be fully replaced by the alphanumeric section; such designs are referred to as ""tenkeyless"". Laptops and wireless peripherals often lack duplicate keys and ones seldom used. Function- and arrow keys are nearly always present."
+5658,"Another factor determining the size of a keyboard is the size and spacing of the keys. The reduction is limited by the practical consideration that the keys must be large enough to be easily pressed by fingers. Alternatively, a tool is used for pressing small keys."
+5659,"Desktop computer keyboards include alphabetic characters and numerals , typographical symbols and punctuation marks, one or more currency symbols and other special characters, diacritics and a variety of function keys. The repertoire of glyphs engraved on the keys of a keyboard accords with national conventions and language needs. Computer keyboards are similar to electric-typewriter keyboards but contain additional keys, such as the command key or Windows keys."
+5660,"Keyboards on laptops and notebook computers usually have a shorter travel distance for the keystroke, shorter over travel distance, and a reduced set of keys. They may not have a numeric keypad, and the function keys may be placed in locations that differ from their placement on a standard, full-sized keyboard. The switch mechanism for a laptop keyboard is more likely to be a scissor switch than a rubber dome; this is opposite the trend for full-size keyboards."
+5661,"Flexible keyboards are a junction between normal type and laptop type keyboards: normal from the full arrangement of keys, and laptop from the short key distance. Additionally, the flexibility allows the user to fold/roll the keyboard for better storage and transfer. However, for typing the keyboard must be resting on a hard surface. The vast majority of flexible keyboards in the market are made from silicone; this material makes them water- and dust-proof. This is useful in hospitals, where keyboards are subjected to frequent washing,
+and other dirty or must-be-clean environments."
+5662,"Handheld ergonomic keyboards are designed to be held like a game controller, and can be used as such, instead of laid out flat on top of a table surface."
+5663,"Typically handheld keyboards hold all the alphanumeric keys and symbols that a standard keyboard would have, yet only be accessed by pressing two sets of keys at once; one acting as a function key similar to a 'Shift' key that would allow for capital letters on a standard keyboard. Handheld keyboards allow the user the ability to move around a room or to lean back on a chair while also being able to type in front or away from the computer. Some variations of handheld ergonomic keyboards also include a trackball mouse that allow mouse movement and typing included in one handheld device."
+5664,"Smaller external keyboards have been introduced for devices without a built-in keyboard, such as PDAs, and smartphones. Small keyboards are also useful where there is a limited workspace."
+5665,A thumb keyboard  is used in some personal digital assistants such as the Palm Treo and BlackBerry and some Ultra-Mobile PCs such as the OQO.
+5666,"Numeric keyboards contain only numbers, mathematical symbols for addition, subtraction, multiplication, and division, a decimal point, and several function keys. They are often used to facilitate data entry with smaller keyboards that do not have a numeric keypad, commonly those of laptop computers. These keys are collectively known as a numeric pad, numeric keys, or a numeric keypad, and it can consist of the following types of keys: Arithmetic operators, numbers, arrow keys, Navigation keys, Num Lock and Enter key."
+5667,"Multifunctional keyboards provide additional function beyond the standard keyboard. Many are programmable, configurable computer keyboards and some control multiple PCs, workstations and other information sources, usually in multi-screen work environments. Users have additional key functions as well as the standard functions and can typically use a single keyboard and mouse to access multiple sources."
+5668,"Multifunctional keyboards may feature customised keypads, fully programmable function or soft keys for macros/pre-sets, biometric or smart card readers, trackballs, etc. New generation multifunctional keyboards feature a touchscreen display to stream video, control audio visual media and alarms, execute application inputs, configure individual desktop environments, etc. Multifunctional keyboards may also permit users to share access to PCs and other information sources. Multiple interfaces  are used to integrate external devices. Some multifunctional keyboards are also used to directly and intuitively control video walls."
+5669,"Common environments for multifunctional keyboards are complex, high-performance workplaces for financial traders and control room operators ."
+5670,"Many keyboards have been designed for one-handed operation.
+The first one, a chorded keyboard, was invented by Douglas Engelbart.
+Other types of one-handed keyboards include the FrogPad, the Half-keyboard,
+and  one-handed Dvorak keyboard layouts designed for one hand typing."
+5671,"While other keyboards generally associate one action with each key, chorded keyboards associate actions with combinations of key presses. Since there are many combinations available, chorded keyboards can effectively produce more actions on a board with fewer keys. Court reporters' stenotype machines use chorded keyboards to enable them to enter text much faster by typing a syllable with each stroke instead of one letter at a time. The fastest typists  use a stenograph, a kind of chorded keyboard used by most court reporters and closed-caption reporters. Some chorded keyboards are also made for use in situations where fewer keys are preferable, such as on devices that can be used with only one hand, and on small mobile devices that don't have room for larger keyboards. Chorded keyboards are less desirable in many cases because it usually takes practice and memorization of the combinations to become proficient."
+5672,"Virtual keyboards, sometimes called on-screen keyboards , consist of computer programs that display an image of a keyboard on the screen. Another input device such as a mouse or a touchscreen can be used to operate each virtual key to enter text. Virtual keyboards have become very popular in touchscreen enabled cell phones due to the additional cost and space requirements of other types of hardware keyboards. Microsoft Windows, Mac OS X, and some varieties of Linux include on-screen keyboards that can be controlled with the mouse. In these, the mouse has to be maneuvered onto the on-screen letters given by the software. On the click of a letter, the software writes the respective letter in the respective spot."
+5673,"Projection keyboards project an image of keys, usually with a laser, onto a flat surface. The device then uses a camera or infrared sensor to ""watch"" where the user's fingers move, and will count a key as being pressed when it ""sees"" the user's finger touch the projected image. Projection keyboards can simulate a full size keyboard from a very small projector. Because the ""keys"" are simply projected images, they cannot be felt when pressed. Users of projected keyboards often experience increased discomfort in their fingertips because of the lack of ""give"" when typing. A flat, non-reflective surface is also required for the keys to be projected. Most projection keyboards are made for use with PDAs and smartphones due to their small form factor."
+5674,"Also known as photo-optical keyboard, light responsive keyboard, photo-electric keyboard and optical key actuation detection technology."
+5675,"An optical keyboard technology utilizes LEDs and photo sensors to optically detect actuated keys. Most commonly the emitters and sensors are located in the perimeter, mounted on a small PCB. The light is directed from side to side of the keyboard interior and it can only be blocked by the actuated keys. Most optical keyboards require at least 2 beams  to determine the actuated key. Some optical keyboards use a special key structure that blocks the light in a certain pattern, allowing only one beam per row of keys ."
+5676,"Alphabetical, numeric, and punctuation keys are used in the same fashion as a typewriter keyboard to enter their respective symbol into a word processing program, text editor, data spreadsheet, or other program. Many of these keys will produce different symbols when modifier keys or shift keys are pressed. The alphabetic characters become uppercase when the shift key or Caps Lock key is depressed. The numeric characters become symbols or punctuation marks when the shift key is depressed. The alphabetical, numeric, and punctuation keys can also have other functions when they are pressed at the same time as some modifier keys."
+5677,"The Space bar is a horizontal bar in the lowermost row, which is significantly wider than other keys. Like the alphanumeric characters, it is also descended from the mechanical typewriter. Its main purpose is to enter the space between words during typing. It is large enough so that a thumb from either hand can use it easily. Depending on the operating system, when the space bar is used with a modifier key such as the control key, it may have functions such as resizing or closing the current window, half-spacing, or backspacing. In computer games and other applications the key has myriad uses in addition to its normal purpose in typing, such as jumping and adding marks to check boxes. In certain programs for playback of digital video, the space bar is used for pausing and resuming the playback."
+5678,"Modifier keys are special keys that modify the normal action of another key, when the two are pressed in combination. For example, Alt+F4 in Microsoft Windows will close the program in an active window. In contrast, pressing just F4 will probably do nothing, unless assigned a specific function in a particular program. By themselves, modifier keys usually do nothing."
+5679,"The most widely used modifier keys include the Control key, Shift key and the Alt key. The AltGr key is used to access additional symbols for keys that have three symbols printed on them. On the Macintosh and Apple keyboards, the modifier keys are the Option key and Command key, respectively. On Sun Microsystems and Lisp machine keyboards, the Meta key is used as a modifier and for Windows keyboards, there is a Windows key. Compact keyboard layouts often use a Fn key. ""Dead keys"" allow placement of a diacritic mark, such as an accent, on the following letter ."
+5680,"The enter/return key typically causes a command line, window form or dialog box to operate its default function, which is typically to finish an ""entry"" and begin the desired process. In word processing applications, pressing the enter key ends a paragraph and starts a new one."
+5681,"Navigation keys or cursor keys include a variety of keys which move the cursor to different positions on the screen. Arrow keys are programmed to move the cursor in a specified direction; page scroll keys, such as the Page Up and Page Down keys, scroll the page up and down. The Home key is used to return the cursor to the beginning of the line where the cursor is located; the End key puts the cursor at the end of the line. The Tab key advances the cursor to the next tab stop.
+The Insert key is mainly used to switch between overtype mode, in which the cursor overwrites any text that is present on and after its current location, and insert mode, where the cursor inserts a character at its current position, forcing all characters past it one position further. The Delete key discards the character ahead of the cursor's position, moving all following characters one position ""back"" towards the freed place. On many notebook computer keyboards the key labeled Delete  serves the same purpose as a Backspace key. The Backspace key deletes the preceding character.
+Lock keys lock part of a keyboard, depending on the settings selected. The lock keys are scattered around the keyboard. Most styles of keyboards have three LEDs indicating which locks are enabled, in the upper right corner above the numeric pad. The lock keys include Scroll lock, Num lock , and Caps lock."
+5682,"The SysRq and Print screen commands often share the same key. SysRq was used in earlier computers as a ""panic"" button to recover from crashes . The Print screen command used to capture the entire screen and send it to the printer, but in the present it usually puts a screenshot in the clipboard."
+5683,"The Break key/Pause key no longer has a well-defined purpose. Its origins go back to teleprinter users, who wanted a key that would temporarily interrupt the communications line. The Break key can be used by software in several different ways, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection.
+In programming, especially old DOS-style BASIC, Pascal and C, Break is used  to stop program execution. In addition to this, Linux and variants, as well as many DOS programs, treat this combination the same as Ctrl+C. On modern keyboards, the break key is usually labeled Pause/Break. In most Windows environments, the key combination Windows key+Pause brings up the system properties."
+5684,"The escape key  has a variety of meanings according to Operating System, application or both.  ""Nearly all of the time"", it signals Stop, QUIT, or ""let me get out of a dialog"" . It triggers the Stop function in many web browsers."
+5685,"The escape key was part of the standard keyboard of the Teletype Model 33 . The DEC VT50, introduced July 1974, also had an Esc key. The TECO text editor  and its descendant Emacs  use the Esc key extensively."
+5686,"Historically it also served as a type of shift key, such that one or more following characters were interpreted differently, hence the term escape sequence, which refers to a series of characters, usually preceded by the escape character."
+5687,"On machines running Microsoft Windows, prior to the implementation of the Windows key on keyboards, the typical practice for invoking the ""start"" button was to hold down the control key and press escape. This process still works in Windows 95, 98, Me, NT 4, 2000, XP, Vista, 7, 8, and 10."
+5688,"The 'enter key' ⌅ Enter and 'return key' ↵ Return are two closely related keys with overlapping and distinct functions dependent on operating system and application. On full-size keyboards, there are two such keys, one in the alphanumeric keys and the other one is in the numeric keys. The purpose of the enter key is to confirm what has been typed. The return key is based on the original line feed/carriage return function of typewriters: in many word processors, for example, the return key ends a paragraph; in a spreadsheet, it completes the current cell and move to the next cell."
+5689,"The shape of the Enter key differs between ISO and ANSI keyboards: in the latter, the Enter key is in a single row  while in the former it spans over two rows and has an inverse L shape."
+5690,"The purpose of the ⇧ Shift key is to invoke the first alternative function of the key with which it is pressed concurrently. For alphabetic keys, shift+letter gives the upper case version of that letter. For other keys, the key is engraved with symbols for both the unshifted and shifted result. When used in combination with other control keys , the effect is system and application dependent."
+5691,"The Menu key or Application key is a key found on Windows-oriented computer keyboards. It is used to launch a context menu with the keyboard rather than with the usual right mouse button. The key's symbol is usually a small icon depicting a cursor hovering above a menu. On some Samsung keyboards the cursor in the icon is not present, showing the menu only.  This key was created at the same time as the Windows key. This key is normally used when the right mouse button is not present on the mouse. Some Windows public terminals do not have a Menu key on their keyboard to prevent users from right-clicking ."
+5692,"Many, but not all, computer keyboards have a numeric keypad to the right of the alphabetic keyboard, often separated from the other groups of keys such as the function keys and system command keys, which contains numbers, basic mathematical symbols , and a few function keys.  In addition to the row of number keys above the top alphabetic row, most desktop keyboards have a number pad or accounting pad, on the right hand side of the keyboard. While num lock is set, the numbers on these keys duplicate the number row; if not, they have alternative functions as engraved. In addition to numbers, this pad has command symbols concerned with calculations such as addition, subtraction, multiplication and division symbols. The enter key in this keys indicate the equal sign."
+5693,"On Japanese/Korean keyboards, there may be Language input keys for changing the language to use."
+5694,"Some keyboards have power management keys ; Internet keys to access a web browser or E-mail; and/or multimedia keys, such as volume controls; or keys that can be programmed by the user to launch a specified application or a command like minimizing all windows."
+5695,"It is possible to install multiple keyboard layouts within an operating system and switch between them, either through features implemented within the OS, or through an external application. Microsoft Windows, Linux, and Mac provide support to add keyboard layouts and choose from them."
+5696,"Keyboards and keypads may be illuminated from inside, especially on equipment for mobile use. Both keyboards built into computers and external ones may support backlighting; external backlit keyboards may have a wired USB connection, or be connected wirelessly and powered by batteries. Illumination facilitates the use of the keyboard or keypad in dark environments."
+5697,"For general productivity, only the keys may be uniformly backlit, without distracting light around the keys."
+5698,"Many gaming keyboards are designed to have an aesthetic as well as functional appeal, with multiple colours, and colour-coded keys to make it easier for gamers to find command keys while playing in a dark room. Many keyboards not otherwise illuminated may have small LED indicator lights in a few important function keys, or elsewhere on the housing, if their function is activated ."
+5699,"In the first electronic keyboards in the early 1970s, the key switches were individual switches inserted into holes in metal frames. These keyboards cost from 80 to 120 USD and were used in mainframe data terminals. The most popular switch types were reed switches ."
+5700,"In the mid-1970s, lower-cost direct-contact key switches were introduced, but their life in switch cycles was much shorter  because they were open to the environment. This became more acceptable, however, for use in computer terminals at the time, which began to see increasingly shorter model lifespans as they advanced."
+5701,"In 1978, Key Tronic Corporation introduced keyboards with capacitive-based switches, one of the first keyboard technologies not to use self-contained switches. There was simply a sponge pad with a conductive-coated Mylar plastic sheet on the switch plunger, and two half-moon trace patterns on the printed circuit board below. As the key was depressed, the capacitance between the plunger pad and the patterns on the PCB below changed, which was detected by integrated circuits . These keyboards were claimed to have the same reliability as the other ""solid-state switch"" keyboards such as inductive and Hall-effect, but competitive with direct-contact keyboards. Prices of $60 for keyboards were achieved, and Key Tronic rapidly became the largest independent keyboard manufacturer."
+5702,"Meanwhile, IBM made their own keyboards, using their own patented technology: Keys on older IBM keyboards were made with a ""buckling spring"" mechanism, in which a coil spring under the key buckles under pressure from the user's finger, triggering a hammer that presses two plastic sheets  with conductive traces together, completing a circuit. This produces a clicking sound and gives physical feedback for the typist, indicating that the key has been depressed."
+5703,"The first electronic keyboards had a typewriter key travel distance of 0.187 inches , keytops were a half-inch  high, and keyboards were about two inches  thick. Over time, less key travel was accepted in the market, finally landing on 0.110 inches . Coincident with this, Key Tronic was the first company to introduce a keyboard that was only about one inch thick. And now keyboards measure only about a half-inch thick."
+5704,"Keytops are an important element of keyboards. In the beginning, keyboard keytops had a ""dish shape"" on top, like typewriters before them. Keyboard key legends must be extremely durable over tens of millions of depressions, since they are subjected to extreme mechanical wear from fingers and fingernails, and subject to hand oils and creams, so engraving and filling key legends with paint, as was done previously for individual switches, was never acceptable. So, for the first electronic keyboards, the key legends were produced by two-shot  molding, where either the key shell or the inside of the key with the key legend was molded first, and then the other color molded second. But, to save cost, other methods were explored, such as sublimation printing and laser engraving, both methods which could be used to print a whole keyboard at the same time."
+5705,"Initially, sublimation printing, where a special ink is printed onto the keycap surface and the application of heat causes the ink molecules to penetrate and commingle with the plastic modules, had a problem because finger oils caused the molecules to disperse, but then a necessarily very hard clear coating was applied to prevent this. Coincident with sublimation printing, which was first used in high volume by IBM on their keyboards, was the introduction by IBM of single-curved-dish keycaps to facilitate quality printing of key legends by having a consistently curved surface instead of a dish. But one problem with sublimation or laser printing was that the processes took too long and only dark legends could be printed on light-colored keys. On another note, IBM was unique in using separate shells, or ""keycaps"", on keytop bases. This might have made their manufacturing of different keyboard layouts more flexible, but the reason for doing this was that the plastic material that needed to be used for sublimation printing was different from standard ABS keytop plastic material."
+5706,"Three final mechanical technologies brought keyboards to where they are today, driving the cost well under $10:"
+5707,"""Monoblock"" keyboard designs were developed where individual switch housings were eliminated and a one-piece ""monoblock"" housing used instead. This was possible because of molding techniques that could provide very tight tolerances for the switch-plunger holes and guides across the width of the keyboard so that the key plunger-to-housing clearances were not too tight or too loose, either of which could cause the keys to bind."
+5708,"The use of contact-switch membrane sheets under the monoblock. This technology came from flat-panel switch membranes, where the switch contacts are printed inside of a top and bottom layer, with a spacer layer in between, so that when pressure is applied to the area above, a direct electrical contact is made.  The membrane layers can be printed by very-high volume, low-cost ""reel-to-reel"" printing machines, with each keyboard membrane cut and punched out afterwards."
+5709,"Plastic materials played a very important part in the development and progress of electronic keyboards. Until ""monoblocks"" came along, GE's ""self-lubricating"" Delrin was the only plastic material for keyboard switch plungers that could withstand the beating over tens of millions of cycles of lifetime use. Greasing or oiling switch plungers was undesirable because it would attract dirt over time which would eventually affect the feel and even bind the key switches . But Delrin was only available in black and white, and was not suitable for keytops , so keytops use ABS plastic.  However, as plastic molding advanced in maintaining tight tolerances, and as key travel length reduced from 0.187-inch to 0.110-inch , single-part keytop/plungers could be made of ABS, with the keyboard monoblocks also made of ABS."
+5710,"In common use, the term ""mechanical keyboard"" refers to a keyboard with individual mechanical key switches, each of which contains a fully encased plunger with a spring below it and metallic electrical contacts on a side. The plunger sits on the spring and the key will often close the contacts when the plunger is pressed half-way. Other switches require the plunger to be fully pressed down. The depth at which the plunger must be pressed for the contacts to close is known as the activation distance. Analog keyboards with key switches whose activation distance can be reconfigured through software, optical switches that work by blocking laser beams, and Hall Effect keyboards that use key switches that use a magnet to activate a hall sensor, are also available."
+5711,"Computer keyboards include control circuitry to convert key presses into key codes  that the computer's electronics can understand. The key switches are connected via the printed circuit board in an electrical X-Y matrix where a voltage is provided sequentially to the Y lines and, when a key is depressed, detected sequentially by scanning the X lines."
+5712,"The first computer keyboards were for mainframe computer data terminals and used discrete electronic parts. The first keyboard microprocessor was introduced in 1972 by General Instruments, but keyboards have been using the single-chip 8048 microcontroller variant since it became available in 1978. The keyboard switch matrix is wired to its inputs, it converts the keystrokes to key codes, and, for a detached keyboard, sends the codes down a serial cable  to the main processor on the computer motherboard. This serial keyboard cable communication is only bi-directional to the extent that the computer's electronics controls the illumination of the caps lock, num lock and scroll lock lights."
+5713,"One test for whether the computer has crashed is pressing the caps lock key. The keyboard sends the key code to the keyboard driver running in the main computer; if the main computer is operating, it commands the light to turn on. All the other indicator lights work in a similar way. The keyboard driver also tracks the Shift, alt and control state of the keyboard."
+5714,"Some lower-quality keyboards have multiple or false key entries due to inadequate electrical designs.  These are caused by inadequate keyswitch ""debouncing"" or inadequate keyswitch matrix layout that don't allow multiple keys to be depressed at the same time, both circumstances which are explained below:"
+5715,"When pressing a keyboard key, the key contacts may ""bounce"" against each other for several milliseconds before they settle into firm contact. When released, they bounce some more until they revert to the uncontacted state. If the computer were watching for each pulse, it would see many keystrokes for what the user thought was just one. To resolve this problem, the processor in a keyboard  ""debounces"" the keystrokes, by aggregating them across time to produce one ""confirmed"" keystroke."
+5716,"Some low-quality keyboards also suffer problems with rollover . Early ""solid-state"" keyswitch keyboards did not have this problem because the keyswitches are electrically isolated from each other, and early ""direct-contact"" keyswitch keyboards avoided this problem by having isolation diodes for every keyswitch. These early keyboards had ""n-key"" rollover, which means any number of keys can be depressed and the keyboard will still recognize the next key depressed. But when three keys are pressed  at the same time in a ""direct contact"" keyswitch matrix that doesn't have isolation diodes, the keyboard electronics can see a fourth ""phantom"" key which is the intersection of the X and Y lines of the three keys. Some types of keyboard circuitry will register a maximum number of keys at one time. ""Three-key"" rollover, also called ""phantom key blocking"" or ""phantom key lockout"", will only register three keys and ignore all others until one of the three keys is lifted. This is undesirable, especially for fast typing , and games ."
+5717,"As direct-contact membrane keyboards became popular, the available rollover of keys was optimized by analyzing the most common key sequences and placing these keys so that they do not potentially produce phantom keys in the electrical key matrix , so that blocking a third key usually isn't a problem. But lower-quality keyboard designs and unknowledgeable engineers may not know these tricks, and it can still be a problem in games due to wildly different or configurable layouts in different games."
+5718,"There are several ways of connecting a keyboard to a system unit  using cables, including the standard AT connector commonly found on motherboards, which was eventually replaced by the PS/2 and the USB connection. Prior to the iMac line of systems, Apple used the proprietary Apple Desktop Bus for its keyboard connector."
+5719,"Wireless keyboards have become popular. A wireless keyboard must have a transmitter built in, and a receiver connected to the computer's keyboard port; it communicates either by radio frequency  or infrared  signals. A wireless keyboard may use industry standard Bluetooth radio communication,  in which case the receiver may be built into the computer. Wireless keyboards need batteries for power, and may be at risk of data eavesdropping. Wireless solar keyboards charge their batteries from small solar panels using natural or artificial light. The 1984 Apricot Portable is an early example of an IR keyboard."
+5720,"Optical character recognition  is preferable to rekeying for converting existing text that is already written down but not in machine-readable format . In other words, to convert the text from an image to editable text , a person could re-type it, or a computer could look at the image and deduce what each character is. OCR technology has already reached an impressive state  and promises more for the future."
+5721,"Speech recognition converts speech into machine-readable text . This technology has also reached an advanced state and is implemented in various software products. For certain uses  speech recognition is starting to replace the keyboard.  However, the lack of privacy when issuing voice commands and dictation makes this kind of input unsuitable for many environments."
+5722,"Pointing devices can be used to enter text or characters in contexts where using a physical keyboard would be inappropriate or impossible. These accessories typically present characters on a display, in a layout that provides fast access to the more frequently used characters or character combinations. Popular examples of this kind of input are Graffiti, Dasher and on-screen virtual keyboards."
+5723,Unencrypted wireless Bluetooth keyboards are known to be vulnerable to signal theft by placing a covert listening device in the same room as the keyboard to sniff and record Bluetooth packets for the purpose of logging keys typed by the user. Microsoft wireless keyboards 2011 and earlier are documented to have this vulnerability.
+5724,"Keystroke logging  is a method of capturing and recording user keystrokes. While it is used legally to measure employee productivity on certain clerical tasks, or by law enforcement agencies to find out about illegal activities, it is also used by hackers for various illegal or malicious acts. Hackers use keyloggers as a means to obtain passwords or encryption keys and thus bypass other security measures."
+5725,"Keystroke logging can be achieved by both hardware and software means. Hardware key loggers are attached to the keyboard cable or installed inside standard keyboards. Software keyloggers work on the target computer's operating system and gain unauthorized access to the hardware, hook into the keyboard with functions provided by the OS, or use remote access software to transmit recorded data out of the target computer to a remote location. Some hackers also use wireless keylogger sniffers to collect packets of data being transferred from a wireless keyboard and its receiver, and then they crack the encryption key being used to secure wireless communications between the two devices."
+5726,"Anti-spyware applications are able to detect many keyloggers and cleanse them. Responsible vendors of monitoring software support detection by anti-spyware programs, thus preventing abuse of the software. Enabling a firewall does not stop keyloggers per se, but can possibly prevent transmission of the logged material over the net if properly configured. Network monitors  can be used to alert the user whenever an application attempts to make a network connection. This gives the user the chance to prevent the keylogger from ""phoning home"" with his or her typed information. Automatic form-filling programs can prevent keylogging entirely by not using the keyboard at all. Historically, most keyloggers could be fooled by alternating between typing the login credentials and typing characters somewhere else in the focus window."
+5727,"Keyboards are also known to emit electromagnetic signatures that can be detected using special spying equipment to reconstruct the keys pressed on the keyboard. Neal O'Farrell, executive director of the Identity Theft Council, revealed to InformationWeek that ""More than 25 years ago, a couple of former spooks showed me how they could capture a user's ATM PIN, from a van parked across the street, simply by capturing and decoding the electromagnetic signals generated by every keystroke,"" O'Farrell said. ""They could even capture keystrokes from computers in nearby offices, but the technology wasn't sophisticated enough to focus in on any specific computer."""
+5728,"The use of any keyboard may cause serious injury  to hands, wrists, arms, neck or back. The risks of injuries can be reduced by taking frequent short breaks to get up and walk around a couple of times every hour. As well, users should vary tasks throughout the day, to avoid overuse of the hands and wrists. When inputting at the keyboard, a person should keep the shoulders relaxed with the elbows at the side, with the keyboard and mouse positioned so that reaching is not necessary. The chair height and keyboard tray should be adjusted so that the wrists are straight, and the wrists should not be rested on sharp table edges. Wrist or palm rests should not be used while typing."
+5729,"Some adaptive technology ranging from special keyboards, mouse replacements and pen tablet interfaces to speech recognition software can reduce the risk of injury. Pause software reminds the user to pause frequently. Switching to a much more ergonomic mouse, such as a vertical mouse or joystick mouse may provide relief."
+5730,"By using a touchpad or a stylus pen with a graphic tablet, in place of a mouse, one can lessen the repetitive strain on the arms and hands."
+5731,"A logic gate is a device that performs a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has, for instance, zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device ."
+5732,"The primary way of building logic gates uses diodes or transistors acting as electronic switches. Today, most logic gates are made from MOSFETs . They can also be constructed using vacuum tubes, electromagnetic relays with relay logic, fluidic logic, pneumatic logic, optics, molecules, acoustics, or even mechanical or thermal elements."
+5733,"Logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic. Logic circuits include such devices as multiplexers, registers, arithmetic logic units , and computer memory, all the way up through complete microprocessors, which may contain more than 100 million logic gates."
+5734,Compound logic gates AND-OR-Invert  and OR-AND-Invert  are often employed in circuit design because their construction using MOSFETs is simpler and more efficient than the sum of the individual gates.
+5735,"The binary number system was refined by Gottfried Wilhelm Leibniz , influenced by the ancient I Ching's binary system. Leibniz established that using the binary system combined the principles of arithmetic and logic."
+5736,"In an 1886 letter, Charles Sanders Peirce described how logical operations could be carried out by electrical switching circuits. Early electro-mechanical computers were constructed from switches and relay logic rather than the later innovations of vacuum tubes  or transistors . Ludwig Wittgenstein introduced a version of the 16-row truth table as proposition 5.101 of Tractatus Logico-Philosophicus . Walther Bothe, inventor of the coincidence circuit, got part of the 1954 Nobel Prize in physics, for the first modern electronic AND gate in 1924. Konrad Zuse designed and built electromechanical logic gates for his computer Z1 ."
+5737,"From 1934 to 1936, NEC engineer Akira Nakashima, Claude Shannon and Victor Shestakov introduced switching circuit theory in a series of papers showing that two-valued Boolean algebra, which they discovered independently, can describe the operation of switching circuits. Using this property of electrical switches to implement logic is the fundamental concept that underlies all electronic digital computers. Switching circuit theory became the foundation of digital circuit design, as it became widely known in the electrical engineering community during and after World War II, with theoretical rigor superseding the ad hoc methods that had prevailed previously."
+5738,Metal–oxide–semiconductor  devices in the forms of PMOS and NMOS were demonstrated by Bell Labs engineers Mohamed M. Atalla and Dawon Kahng in 1960. Both types were later combined and adapted into complementary MOS  logic by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963.
+5739,"There are two sets of symbols for elementary logic gates in common use, both defined in ANSI/IEEE Std 91-1984 and its supplement ANSI/IEEE Std 91a-1991. The ""distinctive shape"" set, based on traditional schematics, is used for simple drawings and derives from United States Military Standard MIL-STD-806 of the 1950s and 1960s. It is sometimes unofficially described as ""military"", reflecting its origin. The ""rectangular shape"" set, based on ANSI Y32.14 and other early industry standards as later refined by IEEE and IEC, has rectangular outlines for all types of gate and allows representation of a much wider range of devices than is possible with the traditional symbols. The IEC standard, IEC 60617-12, has been adopted by other standards, such as EN 60617-12:1999 in Europe, BS EN 60617-12:1999 in the United Kingdom, and DIN EN 60617-12:1998 in Germany."
+5740,The mutual goal of IEEE Std 91-1984 and IEC 617-12 was to provide a uniform method of describing the complex logic functions of digital circuits with schematic symbols. These functions were more complex than simple AND and OR gates. They could be medium-scale circuits such as a 4-bit counter to a large-scale circuit such as a microprocessor.
+5741,"IEC 617-12 and its renumbered successor IEC 60617-12 do not explicitly show the ""distinctive shape"" symbols, but do not prohibit them. These are, however, shown in ANSI/IEEE Std 91  with this note: ""The distinctive-shape symbol is, according to IEC Publication 617, Part 12, not preferred, but is not considered to be in contradiction to that standard."" IEC 60617-12 correspondingly contains the note  ""Although non-preferred, the use of other symbols recognized by official national standards, that is distinctive shapes in place of symbols , shall not be considered to be in contradiction with this standard. Usage of these other symbols in combination to form complex symbols  is discouraged."" This compromise was reached between the respective IEEE and IEC working groups to permit the IEEE and IEC standards to be in mutual compliance with one another."
+5742,"In the 1980s, schematics were the predominant method to design both circuit boards and custom ICs known as gate arrays. Today custom ICs and the field-programmable gate array are typically designed with Hardware Description Languages  such as Verilog or VHDL."
+5743,"By use of De Morgan's laws, an AND function is identical to an OR function with negated inputs and outputs. Likewise, an OR function is identical to an AND function with negated inputs and outputs. A NAND gate is equivalent to an OR gate with negated inputs, and a NOR gate is equivalent to an AND gate with negated inputs."
+5744,"This leads to an alternative set of symbols for basic gates that use the opposite core symbol  but with the inputs and outputs negated. Use of these alternative symbols can make logic circuit diagrams much clearer and help to show accidental connection of an active high output to an active low input or vice versa. Any connection that has logic negations at both ends can be replaced by a negationless connection and a suitable change of gate or vice versa. Any connection that has a negation at one end and no negation at the other can be made easier to interpret by instead using the De Morgan equivalent symbol at either of the two ends. When negation or polarity indicators on both ends of a connection match, there is no logic negation in that path , making it easier to follow logic states from one symbol to the next. This is commonly seen in real logic diagrams – thus the reader must not get into the habit of associating the shapes exclusively as OR or AND shapes, but also take into account the bubbles at both inputs and outputs in order to determine the ""true"" logic function indicated."
+5745,"A De Morgan symbol can show more clearly a gate's primary logical purpose and the polarity of its nodes that are considered in the ""signaled""  state. Consider the simplified case where a two-input NAND gate is used to drive a motor when either of its inputs are brought low by a switch. The ""signaled"" state  occurs when either one OR the other switch is on. Unlike a regular NAND symbol, which suggests AND logic, the De Morgan version, a two negative-input OR gate, correctly shows that OR is of interest. The regular NAND symbol has a bubble at the output and none at the inputs , but the De Morgan symbol shows both inputs and output in the polarity that will drive the motor."
+5746,"De Morgan's theorem is most commonly used to implement logic gates as combinations of only NAND gates, or as combinations of only NOR gates, for economic reasons."
+5747,Output comparison of various logic gates:
+5748,"Charles Sanders Peirce  showed that NOR gates alone  can be used to reproduce the functions of all the other logic gates, but his work on it was unpublished until 1933. The first published proof was by Henry M. Sheffer in 1913, so the NAND logical operation is sometimes called Sheffer stroke; the logical NOR is sometimes called Peirce's arrow. Consequently, these gates are sometimes called universal logic gates."
+5749,"Logic gates can also be used to hold a state, allowing data storage. A storage element can be constructed by connecting several gates in a ""latch"" circuit. Latching circuitry is used in static random-access memory. More complicated designs that use clock signals and that change only on a rising or falling edge of the clock are called edge-triggered ""flip-flops"". Formally, a flip-flop is called a bistable circuit, because it has two stable states which it can maintain indefinitely. The combination of multiple flip-flops in parallel, to store a multiple-bit value, is known as a register. When using any of these gate setups the overall system has memory; it is then called a sequential logic system since its output can be influenced by its previous state, i.e. by the sequence of input states.  In contrast, the output from combinational logic is purely a combination of its present inputs, unaffected by the previous input and output states."
+5750,"These logic circuits are used in computer memory. They vary in performance, based on factors of speed, complexity, and reliability of storage, and many different types of designs are used based on the application."
+5751,"A functionally complete logic system may be composed of relays, valves , or transistors. The simplest family of logic gates uses bipolar transistors, and is called resistor–transistor logic . Unlike simple diode logic gates , RTL gates can be cascaded indefinitely to produce more complex logic functions. RTL gates were used in early integrated circuits. For higher speed and better density, the resistors used in RTL were replaced by diodes resulting in diode–transistor logic . Transistor–transistor logic  then supplanted DTL."
+5752,"As integrated circuits became more complex, bipolar transistors were replaced with smaller field-effect transistors ; see PMOS and NMOS. To reduce power consumption still further, most contemporary chip implementations of digital systems now use CMOS logic.  CMOS uses complementary  MOSFET devices to achieve a high speed with low power dissipation."
+5753,"For small-scale logic, designers now use prefabricated logic gates from families of devices such as the TTL 7400 series by Texas Instruments, the CMOS 4000 series by RCA, and their more recent descendants. Increasingly, these fixed-function logic gates are being replaced by programmable logic devices, which allow designers to pack many mixed logic gates into a single integrated circuit. The field-programmable nature of programmable logic devices such as FPGAs has reduced the 'hard' property of hardware; it is now possible to change the logic design of a hardware system by reprogramming some of its components, thus allowing the features or function of a hardware implementation of a logic system to be changed. Other types of logic gates include, but are not limited to:"
+5754,"There are several logic families with different characteristics  such as: RDL , RTL , DTL , TTL  and CMOS. There are also sub-variants, e.g. standard CMOS logic vs. advanced types using still CMOS technology, but with some optimizations for avoiding loss of speed due to slower PMOS transistors."
+5755,"Electronic logic gates differ significantly from their relay-and-switch equivalents. They are much faster, consume much less power, and are much smaller . Also, there is a fundamental structural difference. The switch circuit creates a continuous metallic path for current to flow  between its input and its output. The semiconductor logic gate, on the other hand, acts as a high-gain voltage amplifier, which sinks a tiny current at its input and produces a low-impedance voltage at its output. It is not possible for current to flow between the output and the input of a semiconductor logic gate."
+5756,"Another important advantage of standardized integrated circuit logic families, such as the 7400 and 4000 families, is that they can be cascaded. This means that the output of one gate can be wired to the inputs of one or several other gates, and so on. Systems with varying degrees of complexity can be built without great concern of the designer for the internal workings of the gates, provided the limitations of each integrated circuit are considered."
+5757,"The output of one gate can only drive a finite number of inputs to other gates, a number called the 'fan-out limit'. Also, there is always a delay, called the 'propagation delay', from a change in input of a gate to the corresponding change in its output. When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed synchronous circuits. Additional delay can be caused when many inputs are connected to an output, due to the distributed capacitance of all the inputs and wiring and the finite amount of current that each output can provide."
+5758,Logic built with FeFET  transistors can retain their state to speed recovery in case of a power loss.
+5759,"A three-state logic gate is a type of logic gate that can have three different outputs: high , low  and high-impedance . The high-impedance state plays no role in the logic, which is strictly binary. These devices are used on buses of the CPU to allow multiple chips to send data. A group of three-states driving a line with a suitable control circuit is basically equivalent to a multiplexer, which may be physically distributed over separate devices or plug-in cards."
+5760,"In electronics, a high output would mean the output is sourcing current from the positive power terminal . A low output would mean the output is sinking current to the negative power terminal . High impedance would mean that the output is effectively disconnected from the circuit."
+5761,"Non-electronic implementations are varied, though few of them are used in practical applications. Many early electromechanical digital computers, such as the Harvard Mark I, were built from relay logic gates, using electro-mechanical relays. Logic gates can be made using pneumatic devices, such as the Sorteberg relay or mechanical logic gates, including on a molecular scale. Various types of fundamental logic gates have been constructed using molecules , which are based on chemical inputs and spectroscopic outputs. Logic gates have been made out of DNA  and used to create a computer called MAYA . Logic gates can be made from quantum mechanical effects, see quantum logic gate. Photonic logic gates use nonlinear optical effects."
+5762,"In principle any method that leads to a gate that is functionally complete  can be used to make any kind of digital logic circuit. Note that the use of 3-state logic for bus systems is not needed, and can be replaced by digital multiplexers, which can be built using only simple logic gates ."
+5763,"In computer architecture, a bus  is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components  and software, including communication protocols."
+5764,"Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical busbar. Modern computer buses can use both parallel and bit serial connections, and can be wired in either a multidrop  or daisy chain topology, or connected by switched hubs, as in the case of Universal Serial Bus ."
+5765,Computer systems generally consist of three main parts:
+5766,"An early computer might contain a hand-wired CPU of vacuum tubes, a magnetic drum for main memory, and a punch tape and printer for reading and writing data respectively. A modern system might have a multi-core CPU, DDR4 SDRAM for memory, a solid-state drive for secondary storage, a graphics card and LCD as a display system, a mouse and keyboard for interaction, and a Wi-Fi connection for networking. In both examples, computer buses of one form or another move data between all of these devices."
+5767,"In most traditional computer architectures, the CPU and main memory tend to be tightly coupled. A microprocessor conventionally is a single chip which has a number of electrical connections on its pins that can be used to select an ""address"" in the main memory and another set of pins to read and write the data stored at that location. In most cases, the CPU and memory share signalling characteristics and operate in synchrony. The bus connecting the CPU and memory is one of the defining characteristics of the system, and often referred to simply as the system bus."
+5768,"It is possible to allow peripherals to communicate with memory in the same fashion, attaching adapters, either on the motherboard or in the form of expansion cards, directly to the system bus. This is commonly accomplished through some sort of standardized electrical connector, several of these forming the expansion bus or local bus. However, as the performance differences between the CPU and peripherals varies widely, some solution is generally needed to ensure that peripherals do not slow overall system performance. Many CPUs feature a second set of pins similar to those for communicating with memory, but able to operate at very different speeds and using different protocols . Others use smart controllers to place the data directly in memory, a concept known as direct memory access. Most modern systems combine both solutions, where appropriate."
+5769,"As the number of potential peripherals grew, using an expansion card for every peripheral became increasingly untenable. This has led to the introduction of bus systems designed specifically to support multiple peripherals. Common examples are the SATA ports in modern computers, which allow a number of hard drives to be connected without the need for a card. However, these high-performance systems are generally too expensive to implement in low-end devices, like a mouse. This has led to the parallel development of a number of low-performance bus systems for these solutions, the most common example being the standardized Universal Serial Bus . All such examples may be referred to as peripheral buses, although this terminology is not universal."
+5770,"In modern systems the performance difference between the CPU and main memory has grown so great that increasing amounts of high-speed memory is built directly into the CPU, known as a cache. In such systems, CPUs communicate using high-performance buses that operate at speeds much greater than memory, and communicate with memory using protocols similar to those used solely for peripherals in the past. These system buses are also used to communicate with most  other peripherals, through adaptors, which in turn talk to other peripherals and controllers. Such systems are architecturally more similar to multicomputers, communicating over a bus rather than a network. In these cases, expansion buses are entirely separate and no longer share any architecture with their host CPU . What would have formerly been a system bus is now often known as a front-side bus."
+5771,"Given these changes, the classical terms ""system"", ""expansion"" and ""peripheral"" no longer have the same connotations. Other common categorization systems are based on the bus's primary role, connecting devices internally or externally, PCI vs. SCSI for instance. However, many common modern bus systems can be used for both; SATA and the associated eSATA are one example of a system that would formerly be described as internal, while certain automotive applications use the primarily external IEEE 1394 in a fashion more similar to a system bus. Other examples, like InfiniBand and I²C were designed from the start to be used both internally and externally."
+5772,"The internal bus, also known as internal data bus, memory bus, system bus or front-side bus, connects all the internal components of a computer, such as CPU and memory, to the motherboard. Internal data buses are also referred to as local buses, because they are intended to connect to local devices. This bus is typically rather quick and is independent of the rest of the computer operations."
+5773,"The external bus, or expansion bus, is made up of the electronic pathways that connect the different external devices, such as printer etc., to the computer."
+5774,"An address bus is a bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus . The width of the address bus determines the amount of memory a system can address. For example, a system with a 32-bit address bus can address 232  memory locations. If each memory location holds one byte, the addressable memory space is 4 GB."
+5775,"Early processors used a wire for each bit of the address width. For example, a 16-bit address bus had 16 physical wires making up the bus. As the buses became wider and lengthier, this approach became expensive in terms of the number of chip pins and board traces. Beginning with the Mostek 4096 DRAM, address multiplexing implemented with multiplexers became common. In a multiplexed address scheme, the address is sent in two equal parts on alternate bus cycles. This halves the number of address bus signals required to connect to the memory. For example, a 32-bit address bus can be implemented by using 16 lines and sending the first half of the memory address, immediately followed by the second half memory address."
+5776,Typically two additional pins in the control bus—a row-address strobe  and the column-address strobe  -- are used to tell the DRAM whether the address bus is currently sending the first half of the memory address or the second half.
+5777,"Accessing an individual byte frequently requires reading or writing the full bus width  at once. In these instances the least significant bits of the address bus may not even be implemented - it is instead the responsibility of the controlling device to isolate the individual byte required from the complete word transmitted. This is the case, for instance, with the VESA Local Bus which lacks the two least significant bits, limiting this bus to aligned 32-bit transfers."
+5778,"Historically, there were also some examples of computers which were only able to address words -- word machines."
+5779,"The memory bus is the bus which connects the main memory to the memory controller in computer systems.  Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory buses are designed to connect directly to DRAM chips, and thus are designed by chip standards bodies such as JEDEC.   Examples are the various generations of SDRAM, and serial point-to-point buses like SLDRAM and RDRAM.  An exception is the Fully Buffered DIMM which, despite being carefully designed to minimize the effect, has been criticized for its higher latency."
+5780,"Buses can be parallel buses, which carry data words in parallel on multiple wires, or serial buses, which carry data in bit-serial form. The addition of extra power and control connections, differential drivers, and data connections in each direction usually means that most serial buses have more conductors than the minimum of one used in 1-Wire and UNI/O. As data rates increase, the problems of timing skew, power consumption, electromagnetic interference and crosstalk across parallel buses become more and more difficult to circumvent. One partial solution to this problem has been to double pump the bus. Often, a serial bus can be operated at higher overall data rates than a parallel bus, despite having fewer electrical connections, because a serial bus inherently has no timing skew or crosstalk. USB, FireWire, and Serial ATA are examples of this. Multidrop connections do not work well for fast serial buses, so most modern serial buses use daisy-chain or hub designs."
+5781,"Network connections such as Ethernet are not generally regarded as buses, although the difference is largely conceptual rather than practical. An attribute generally used to characterize a bus is that power is provided by the bus for the connected hardware. This emphasizes the busbar origins of bus architecture as supplying switched or distributed power. This excludes, as buses, schemes such as serial RS-232, parallel Centronics, IEEE 1284 interfaces and Ethernet, since these devices also needed separate power supplies. Universal Serial Bus devices may use the bus supplied power, but often use a separate power source. This distinction is exemplified by a telephone system with a connected modem, where the RJ11 connection and associated modulated signalling scheme is not considered a bus, and is analogous to an Ethernet connection. A phone line connection scheme is not considered to be a bus with respect to signals, but the Central Office uses buses with cross-bar switches for connections between phones."
+5782,"However, this distinction‍—‌that power is provided by the bus‍—‌is not the case in many avionic systems, where data connections such as ARINC 429, ARINC 629, MIL-STD-1553B , and EFABus  are commonly referred to as “data buses” or, sometimes, ""databuses"". Such avionic data buses are usually characterized by having several equipments or Line Replaceable Items/Units  connected to a common, shared media. They may, as with ARINC 429, be simplex, i.e. have a single source LRI/LRU or, as with ARINC 629, MIL-STD-1553B, and STANAG 3910, be duplex, allow all the connected LRI/LRUs to act, at different times , as transmitters and receivers of data."
+5783,"The simplest system bus has completely separate input data lines, output data lines, and address lines.
+To reduce cost, most microcomputers have a bidirectional data bus, re-using the same wires for input and output at different times."
+5784,"Some processors use a dedicated wire for each bit of the address bus, data bus, and the control bus.
+For example, the 64-pin STEbus is composed of 8 physical wires dedicated to the 8-bit data bus, 20 physical wires dedicated to the 20-bit address bus, 21 physical wires dedicated to the control bus, and 15 physical wires dedicated to various power buses."
+5785,"Bus multiplexing requires fewer wires, which reduces costs in many early microprocessors and DRAM chips.
+One common multiplexing scheme, address multiplexing, has already been mentioned.
+Another multiplexing scheme re-uses the address bus pins as the data bus pins, an approach used by conventional PCI and the 8086.
+The various ""serial buses"" can be seen as the ultimate limit of multiplexing, sending each of the address bits and each of the data bits, one at a time, through a single pin ."
+5786,"Over time, several groups of people worked on various computer bus standards, including the IEEE Bus Architecture Standards Committee , the IEEE ""Superbus"" study group, the open microprocessor initiative , the open microsystems initiative , the ""Gang of Nine"" that developed EISA, etc."
+5787,"Early computer buses were bundles of wire that attached computer memory and peripherals. Anecdotally termed the ""digit trunk"" in the early Australian CSIRAC computer, they were named after electrical power buses, or busbars. Almost always, there was one bus for memory, and one or more separate buses for peripherals. These were accessed by separate instructions, with completely different timings and protocols."
+5788,"One of the first complications was the use of interrupts. Early computer programs performed I/O by waiting in a loop for the peripheral to become ready. This was a waste of time for programs that had other tasks to do. Also, if the program attempted to perform those other tasks, it might take too long for the program to check again, resulting in loss of data. Engineers thus arranged for the peripherals to interrupt the CPU. The interrupts had to be prioritized, because the CPU can only execute code for one peripheral at a time, and some devices are more time-critical than others."
+5789,"High-end systems introduced the idea of channel controllers, which were essentially small computers dedicated to handling the input and output of a given bus. IBM introduced these on the IBM 709 in 1958, and they became a common feature of their platforms. Other high-performance vendors like Control Data Corporation implemented similar designs. Generally, the channel controllers would do their best to run all of the bus operations internally, moving data when the CPU was known to be busy elsewhere if possible, and only using interrupts when necessary. This greatly reduced CPU load, and provided better overall system performance."
+5790,"To provide modularity, memory and I/O buses can be combined into a unified system bus. In this case, a single mechanical and electrical system can be used to connect together many of the system components, or in some cases, all of them."
+5791,"Later computer programs began to share memory common to several CPUs. Access to this memory bus had to be prioritized, as well. The simple way to prioritize interrupts or bus access was with a daisy chain. In this case signals will naturally flow through the bus in physical or logical order, eliminating the need for complex scheduling."
+5792,"Digital Equipment Corporation  further reduced cost for mass-produced minicomputers, and mapped peripherals into the memory bus, so that the input and output devices appeared to be memory locations.  This was implemented in the Unibus of the PDP-11 around 1969."
+5793,"Early microcomputer bus systems were essentially a passive backplane connected directly or through buffer amplifiers to the pins of the CPU. Memory and other devices would be added to the bus using the same address and data pins as the CPU itself used, connected in parallel.  Communication was controlled by the CPU, which read and wrote data from the devices as if they are blocks of memory, using the same instructions, all timed by a central clock controlling the speed of the CPU. Still, devices interrupted the CPU by signaling on separate CPU pins."
+5794,"For instance, a disk drive controller would signal the CPU that new data was ready to be read, at which point the CPU would move the data by reading the ""memory location"" that corresponded to the disk drive. Almost all early microcomputers were built in this fashion, starting with the S-100 bus in the Altair 8800 computer system."
+5795,"In some instances, most notably in the IBM PC, although similar physical architecture can be employed, instructions to access peripherals  and memory  have not been made uniform at all, and still generate distinct CPU signals, that could be used to implement a separate I/O bus."
+5796,"These simple bus systems had a serious drawback when used for general-purpose computers.  All the equipment on the bus had to talk at the same speed, as it shared a single clock."
+5797,"Increasing the speed of the CPU becomes harder, because the speed of all the devices must increase as well. When it is not practical or economical to have all devices as fast as the CPU, the CPU must either enter a wait state, or work at a slower clock frequency temporarily, to talk to other devices in the computer. While acceptable in embedded systems, this problem was not tolerated for long in general-purpose, user-expandable computers."
+5798,"Such bus systems are also difficult to configure when constructed from common off-the-shelf equipment.  Typically each added expansion card requires many jumpers in order to set memory addresses, I/O addresses, interrupt priorities, and interrupt numbers."
+5799,"""Second generation"" bus systems like NuBus addressed some of these problems. They typically separated the computer into two ""worlds"", the CPU and memory on one side, and the various devices on the other. A bus controller accepted data from the CPU side to be moved to the peripherals side, thus shifting the communications protocol burden from the CPU itself. This allowed the CPU and memory side to evolve separately from the device bus, or just ""bus"". Devices on the bus could talk to each other with no CPU intervention. This led to much better ""real world"" performance, but also required the cards to be much more complex. These buses also often addressed speed issues by being ""bigger"" in terms of the size of the data path, moving from 8-bit parallel buses in the first generation, to 16 or 32-bit in the second, as well as adding software setup  to supplant or replace the jumpers."
+5800,"However, these newer systems shared one quality with their earlier cousins, in that everyone on the bus had to talk at the same speed. While the CPU was now isolated and could increase speed, CPUs and memory continued to increase in speed much faster than the buses they talked to. The result was that the bus speeds were now very much slower than what a modern system needed, and the machines were left starved for data. A particularly common example of this problem was that video cards quickly outran even the newer bus systems like PCI, and computers began to include AGP just to drive the video card. By 2004 AGP was outgrown again by high-end video cards and other peripherals and has been replaced by the new PCI Express bus."
+5801,"An increasing number of external devices started employing their own bus systems as well. When disk drives were first introduced, they would be added to the machine with a card plugged into the bus, which is why computers have so many slots on the bus. But through the 1980s and 1990s, new systems like SCSI and IDE were introduced to serve this need, leaving most slots in modern systems empty. Today there are likely to be about five different buses in the typical machine, supporting various devices."
+5802,"""Third generation"" buses have been emerging into the market since about 2001, including HyperTransport and InfiniBand. They also tend to be very flexible in terms of their physical connections, allowing them to be used both as internal buses, as well as connecting different machines together. This can lead to complex problems when trying to service different requests, so much of the work on these systems concerns software design, as opposed to the hardware itself. In general, these third generation buses tend to look more like a network than the original concept of a bus, with a higher protocol overhead needed than early systems, while also allowing multiple devices to use the bus at once."
+5803,Buses such as Wishbone have been developed by the open source hardware movement in an attempt to further remove legal and patent constraints from computer design.
+5804,"The Compute Express Link  is an open standard interconnect for high-speed CPU-to-device and CPU-to-memory, designed to accelerate next-generation data center performance."
+5805,"Many field buses are serial data buses , several of which use the RS-485 electrical characteristics and then specify their own protocol and connector:"
+5806,Other serial buses include:
+5807,"I/O devices are the pieces of hardware used by a human  to communicate with a computer. For instance, a keyboard or computer mouse is an input device for a computer, while monitors and printers are output devices. Devices for communication between computers, such as modems and network cards, typically perform both input and output operations. Any interaction with the system by an interactor is an input and the reaction the system responds is called the output."
+5808,"The designation of a device as either input or output depends on perspective. Mice and keyboards take physical movements that the human user outputs and convert them into input signals that a computer can understand; the output from these devices is the computer's input. Similarly, printers and monitors take signals that computers output as input, and they convert these signals into a representation that human users can understand. From the human user's perspective, the process of reading or seeing these representations is receiving output; this type of interaction between computers and humans is studied in the field of human–computer interaction. A further complication is that a device traditionally considered an input device, e.g., card reader, keyboard, may accept control commands to, e.g., select stacker, display keyboard lights, while a device traditionally considered as an output device may provide status data ."
+5809,"In computer architecture, the combination of the CPU and main memory, to which the CPU can read or write directly using individual instructions, is considered the brain of a computer. Any transfer of information to or from the CPU/memory combo, for example by reading data from a disk drive, is considered I/O. The CPU and its supporting circuitry may provide memory-mapped I/O that is used in low-level computer programming, such as in the implementation of device drivers, or may provide access to I/O channels. An I/O algorithm is one designed to exploit locality and perform efficiently when exchanging data with a secondary storage device, such as a disk drive."
+5810,"An I/O interface is required whenever the I/O device is driven by a processor. Typically a CPU communicates with devices via a bus. The interface must have the necessary logic to interpret the device address generated by the processor. Handshaking should be implemented by the interface using appropriate commands , and the processor can communicate with an I/O device through the interface. If different data formats are being exchanged, the interface must be able to convert serial data to parallel form and vice versa. Because it would be a waste for a processor to be idle while it waits for data from an input device there must be provision for generating interrupts and the corresponding type numbers for further processing by the processor if required."
+5811,"A computer that uses memory-mapped I/O accesses hardware by reading and writing to specific memory locations, using the same assembly language instructions that computer would normally use to access memory. An alternative method is via instruction-based I/O which requires that a CPU have specialized instructions for I/O. Both input and output devices have a data processing rate that can vary greatly. With some devices able to exchange data at very high speeds direct access to memory  without the continuous aid of a CPU is required."
+5812,"Higher-level operating system and programming facilities employ separate, more abstract I/O concepts and primitives. For example, most operating systems provide application programs with the concept of files. The C and C++ programming languages, and operating systems in the Unix family, traditionally abstract files and devices as streams, which can be read or written, or sometimes both. The C standard library provides functions for manipulating streams for input and output."
+5813,"In the context of the ALGOL 68 programming language, the input and output facilities are collectively referred to as transput. The ALGOL 68 transput library recognizes the following standard files/devices: stand in, stand out, stand errors and stand back."
+5814,"An alternative to special primitive functions is the I/O monad, which permits programs to just describe I/O, and the actions are carried out outside the program. This is notable because the I/O functions would introduce side-effects to any programming language, but this allows purely functional programming to be practical."
+5815,"Channel I/O requires the use of instructions that are specifically designed to perform I/O operations. The I/O instructions address the channel or the channel and device; the channel asynchronously accesses all other required addressing and control information. This is similar to DMA, but more flexible."
+5816,"Port-mapped I/O also requires the use of special I/O instructions. Typically one or more ports are assigned to the device, each with a special purpose. The port numbers are in a separate address space from that used by normal instructions."
+5817,Direct memory access  is a means for devices to transfer large chunks of data to and from memory independently of the CPU.
+5818,Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.: 15–16
+5819,"The central processing unit  of a computer is what manipulates data by performing computations. In practice, almost all computers use a storage hierarchy,: 468–473  which puts fast but expensive and small storage options close to the CPU and slower but less expensive and larger options further away. Generally, the fast technologies are referred to as ""memory"", while slower persistent technologies are referred to as ""storage""."
+5820,"Even the first computer designs, Charles Babbage's Analytical Engine and Percy Ludgate's Analytical Machine, clearly distinguished between processing and memory . This distinction was extended in the Von Neumann architecture, where the CPU consists of two main parts: The control unit and the arithmetic logic unit . The former controls the flow of data between the CPU and memory, while the latter performs arithmetic and logical operations on data."
+5821,"Without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behavior. This is acceptable for devices such as desk calculators, digital signal processors, and other specialized devices. Von Neumann machines differ in having a memory in which they store their operating instructions and data.: 20  Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in-memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep state between successive computations to build up complex procedural results. Most modern computers are von Neumann machines."
+5822,"A modern digital computer represents data using the binary numeral system. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of bits, or binary digits, each of which has a value of 0 or 1. The most common unit of storage is the byte, equal to 8 bits. A piece of information can be handled by any computer or device whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. For example, the complete works of Shakespeare, about 1250 pages in print, can be stored in about five megabytes  with one byte per character."
+5823,"Data are encoded by assigning a bit pattern to each character, digit, or multimedia object. Many standards exist for encoding ."
+5824,"By adding bits to each encoded unit, redundancy allows the computer to detect errors in coded data and correct them based on mathematical algorithms. Errors generally occur in low probabilities due to random bit value flipping, or ""physical bit fatigue"", loss of the physical bit in the storage of its ability to maintain a distinguishable value , or due to errors in inter or intra-computer communication. A random bit flip  is typically corrected upon detection. A bit or a group of malfunctioning physical bits  is typically automatically fenced out, taken out of use by the device, and replaced with another functioning equivalent group in the device, where the corrected bit values are restored . The cyclic redundancy check  method is typically used in communications and storage for error detection. A detected error is then retried."
+5825,Data compression methods allow in many cases  to represent a string of bits by a shorter bit string  and reconstruct the original string  when needed. This utilizes substantially less storage  for many types of data at the cost of more computation . Analysis of the trade-off between storage cost saving and costs of related computations and possible delays in data availability is done before deciding whether to keep certain data compressed or not.
+5826,"For security reasons, certain types of data  may be kept encrypted in storage to prevent the possibility of unauthorized information reconstruction from chunks of storage snapshots."
+5827,"Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. This traditional division of storage to primary, secondary, tertiary, and off-line storage is also guided by cost per bit."
+5828,"In contemporary usage, memory is usually fast but temporary semiconductor read-write memory, typically DRAM  or other such devices. Storage consists of storage devices and their media not directly accessible by the CPU , typically hard disk drives, optical disc drives, and other devices slower than RAM but non-volatile ."
+5829,"Historically, memory has, depending on technology, been called central memory, core memory, core storage, drum, main memory, real storage, or internal memory. Meanwhile, slower persistent storage devices have been referred to as secondary storage, external memory, or auxiliary/peripheral storage."
+5830,"Primary storage , often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in a uniform manner."
+5831,"Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by magnetic-core memory. Core memory remained dominant until the 1970s, when advances in integrated circuit technology allowed semiconductor memory to become economically competitive."
+5832,"This led to modern random-access memory . It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are volatile, meaning that they lose the information when not powered. Besides storing opened programs, it serves as disk cache and write buffer to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it's not needed by running software. Spare memory can be utilized as RAM drive for temporary high-speed data storage."
+5833,"As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:"
+5834,"Main memory is directly or indirectly connected to the central processing unit via a memory bus. It is actually two buses : an address bus and a data bus. The CPU firstly sends a number through an address bus, a number called memory address, that indicates the desired location of data. Then it reads or writes the data in the memory cells using the data bus. Additionally, a memory management unit  is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks."
+5835,"As the RAM types used for primary storage are volatile , a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program  is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for read-only memory ."
+5836,"Many types of ""ROM"" are not literally read only, as updates to them are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some embedded systems run programs directly from ROM , because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, and rather, use large capacities of secondary storage, which is non-volatile as well, and not as costly."
+5837,"Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage."
+5838,Secondary storage  differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfer the desired data to primary storage. Secondary storage is non-volatile . Modern computer systems typically have two orders of magnitude more secondary storage than primary storage because secondary storage is less expensive.
+5839,"In modern computers, hard disk drives  or solid-state drives  are usually used as secondary storage. The access time per byte for HDDs or SSDs is typically measured in milliseconds , while the access time per byte for primary storage is measured in nanoseconds . Thus, secondary storage is significantly slower than primary storage. Rotating optical storage devices, such as CD and DVD drives, have even longer access times. Other examples of secondary storage technologies include USB flash drives, floppy disks, magnetic tape, paper tape, punched cards, and RAM disks."
+5840,"Once the disk read/write head on HDDs reaches the proper placement and the data, subsequent data on the track are very fast to access. To reduce the seek time and rotational latency, data are transferred to and from disks in large contiguous blocks. Sequential or block access on disks is orders of magnitude faster than random access, and many sophisticated paradigms have been developed to design efficient algorithms based on sequential and block access. Another way to reduce the I/O bottleneck is to use multiple disks in parallel to increase the bandwidth between primary and secondary memory."
+5841,"Secondary storage is often formatted according to a file system format, which provides the abstraction necessary to organize data into files and directories, while also providing metadata describing the owner of a certain file, the access time, the access permissions, and other information."
+5842,"Most computer operating systems use the concept of virtual memory, allowing the utilization of more primary storage capacity than is physically available in the system. As the primary memory fills up, the system moves the least-used chunks  to a swap file or page file on secondary storage, retrieving them later when needed. If a lot of pages are moved to slower secondary storage, the system performance is degraded."
+5843,"Tertiary storage or tertiary memory is a level below secondary storage. Typically, it involves a robotic mechanism which will mount  and dismount removable mass storage media into a storage device according to the system's demands; such data are often copied to secondary storage before use. It is primarily used for archiving rarely accessed information since it is much slower than secondary storage . This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes."
+5844,"When a computer needs to read information from the tertiary storage, it will first consult a catalog database to determine which tape or disc contains the information. Next, the computer will instruct a robotic arm to fetch the medium and place it in a drive. When the computer has finished reading the information, the robotic arm will return the medium to its place in the library."
+5845,"Tertiary storage is also known as nearline storage because it is ""near to online"". The formal distinction between online, nearline, and offline storage is:"
+5846,"For example, always-on spinning hard disk drives are online storage, while spinning drives that spin down automatically, such as in massive arrays of idle disks , are nearline storage. Removable media such as tape cartridges that can be automatically loaded, as in tape libraries, are nearline storage, while tape cartridges that must be manually loaded are offline storage."
+5847,"Off-line storage is computer data storage on a medium or a device that is not under the control of a processing unit. The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction."
+5848,"Off-line storage is used to transfer information since the detached medium can easily be physically transported. Additionally, it is useful for cases of disaster, where, for example, a fire destroys the original data, a medium in a remote location will be unaffected, enabling disaster recovery. Off-line storage increases general information security since it is physically inaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques. Also, if the information stored for archival purposes is rarely accessed, off-line storage is less expensive than tertiary storage."
+5849,"In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are the most popular, and to a much lesser extent removable hard disk drives; older examples include floppy disks and Zip disks. In enterprise uses, magnetic tape cartridges are predominant; older examples include open-reel magnetic tape and punched cards."
+5850,"Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressability. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance."
+5851,"Non-volatile memory retains the stored information even if not constantly supplied with electric power. It is suitable for long-term storage of information. Volatile memory requires constant power to maintain the stored information. The fastest memory technologies are volatile ones, although that is not a universal rule. Since the primary storage is required to be very fast, it predominantly uses volatile memory."
+5852,"Dynamic random-access memory is a form of volatile memory that also requires the stored information to be periodically reread and rewritten, or refreshed, otherwise it would vanish. Static random-access memory is a form of volatile memory similar to DRAM with the exception that it never needs to be refreshed as long as power is applied; it loses its content when the power supply is lost."
+5853,"An uninterruptible power supply  can be used to give a computer a brief window of time to move information from primary volatile storage into non-volatile storage before the batteries are exhausted. Some systems, for example EMC Symmetrix, have integrated batteries that maintain volatile storage for several minutes."
+5854,Utilities such as hdparm and sar can be used to measure IO performance in Linux.
+5855,"Full disk encryption, volume and virtual disk encryption, andor file/folder encryption is readily available for most storage devices."
+5856,"Hardware memory encryption is available in Intel Architecture, supporting Total Memory Encryption  and page granular memory encryption with multiple keys . and in SPARC M7 generation since October 2015."
+5857,Distinct types of data storage have different points of failure and various methods of predictive failure analysis.
+5858,Vulnerabilities that can instantly lead to total loss are head crashing on mechanical hard drives and failure of electronic components on flash storage.
+5859,"Impending failure on hard disk drives is estimable using S.M.A.R.T. diagnostic data that includes the hours of operation and the count of spin-ups, though its reliability is disputed."
+5860,"Flash storage may experience downspiking transfer rates as a result of accumulating errors, which the flash memory controller attempts to correct."
+5861,"The health of optical media can be determined by measuring correctable minor errors, of which high counts signify deteriorating and/or low-quality media. Too many consecutive minor errors can lead to data corruption. Not all vendors and models of optical drives support error scanning."
+5862,"As of 2011, the most commonly used data storage media are semiconductor, magnetic, and optical, while paper still sees some limited usage. Some other fundamental storage technologies, such as all-flash arrays  are proposed for development."
+5863,"Semiconductor memory uses semiconductor-based integrated circuit  chips to store information. Data are typically stored in metal–oxide–semiconductor  memory cells. A semiconductor memory chip may contain millions of memory cells, consisting of tiny MOS field-effect transistors  and/or MOS capacitors. Both volatile and non-volatile forms of semiconductor memory exist, the former using standard MOSFETs and the latter using floating-gate MOSFETs."
+5864,"In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor random-access memory , particularly dynamic random-access memory . Since the turn of the century, a type of non-volatile floating-gate semiconductor memory known as flash memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers that are designed for them."
+5865,"As early as 2006, notebook and desktop computer manufacturers started using flash-based solid-state drives  as default configuration options for the secondary storage either in addition to or instead of the more traditional HDD."
+5866,"Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers. A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms:"
+5867,"In early computers, magnetic storage was also used as:"
+5868,"Magnetic storage does not have a definite limit of rewriting cycles like flash storage and re-writeable optical media, as altering magnetic fields causes no physical wear. Rather, their life span is limited by mechanical parts."
+5869,"Optical storage, the typical optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile. The deformities may be permanent , formed once  or reversible . The following forms are in common use as of 2009:"
+5870,"Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage."
+5871,3D optical data storage has also been proposed.
+5872,Light induced magnetization melting in magnetic photoconductors has also been proposed for high-speed low-energy consumption magneto-optical storage.
+5873,"Paper data storage, typically in the form of paper tape or punched cards, has long been used to store information for automatic processing, particularly before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium and was read mechanically  to determine whether a particular location on the medium was solid or contained a hole. Barcodes make it possible for objects that are sold or transported to have some computer-readable information securely attached."
+5874,"Relatively small amounts of digital data  may be backed up on paper as a matrix barcode for very long-term storage, as the longevity of paper typically exceeds even magnetic data storage."
+5875,"While a group of bits malfunction may be resolved by error detection and correction mechanisms , storage device malfunction requires different solutions. The following solutions are commonly used and valid for most storage devices:"
+5876,"Device mirroring and typical RAID are designed to handle a single device failure in the RAID group of devices. However, if a second failure occurs before the RAID group is completely repaired from the first failure, then data can be lost. The probability of a single failure is typically small. Thus the probability of two failures in the same RAID group in time proximity is much smaller . If a database cannot tolerate even such a smaller probability of data loss, then the RAID group itself is replicated . In many cases such mirroring is done geographically remotely, in a different storage array, to handle recovery from disasters ."
+5877,"A secondary or tertiary storage may connect to a computer utilizing computer networks. This concept does not pertain to the primary storage, which is shared between multiple processors to a lesser degree."
+5878,"Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as tape libraries, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. The smallest forms of either technology containing just one drive device are referred to as autoloaders or autochangers."
+5879,"Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots."
+5880,"Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is a most known archiving strategy of automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are retrieved back to disk."
+5881,This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 22 January 2022.
+5882,"Most computer resources are managed by the CU. It directs the flow of data between the CPU and the other devices. John von Neumann included the control unit as part of the Von Neumann architecture. In modern computer designs, the control unit is typically an internal part of the CPU with its overall role and operation unchanged since its introduction."
+5883,"The simplest computers use a multicycle microarchitecture. These were the earliest designs. They are still popular in the very smallest computers, such as the embedded systems that operate machinery."
+5884,"In a  computer, the control unit often steps through the instruction cycle successively. This consists of fetching the instruction, fetching the operands, decoding the instruction, executing the instruction, and then writing the results back to memory. When the next instruction is placed in the control unit, it changes the behavior of the control unit to complete the instruction correctly. So, the bits of the instruction directly control the control unit, which in turn controls the computer."
+5885,The control unit may include a binary counter to tell the control unit's logic what step it should do.
+5886,"Multicycle control units typically use both the rising and falling edges of their square-wave timing clock. They operate a step of their operation on each edge of the timing clock, so that a four-step operation completes in two clock cycles. This doubles the speed of the computer, given the same logic family."
+5887,Many computers have two different types of unexpected events. An interrupt occurs because some type of input or output needs software attention in order to operate correctly. An exception is caused by the computer's operation. One crucial difference is that the timing of an interrupt cannot be predicted. Another is that some exceptions  can be caused by an instruction that needs to be restarted.
+5888,"Control units can be designed to handle interrupts in one of two typical ways. If a quick response is most important, a control unit is designed to abandon work to handle the interrupt. In this case, the work in process will be restarted after the last completed instruction. If the computer is to be very inexpensive, very simple, very reliable, or to get more work done, the control unit will finish the work in process before handling the interrupt. Finishing the work is inexpensive, because it needs no register to record the last finished instruction. It is simple and reliable because it has the fewest states. It also wastes the least amount of work."
+5889,"Exceptions can be made to operate like interrupts in very simple computers. If virtual memory is required, then a memory-not-available exception must retry the failing instruction."
+5890,"It is common for multicycle computers to use more cycles. Sometimes it takes longer to take a conditional jump, because the program counter has to be reloaded. Sometimes they do multiplication or division instructions by a process, something like binary long multiplication and division. Very small computers might do arithmetic, one or a few bits at a time. Some other computers have very complex instructions that take many steps."
+5891,Many medium-complexity computers pipeline instructions. This design is popular because of its economy and speed.
+5892,"In a pipelined computer, instructions flow through the computer. This design has several stages. For example, it might have one stage for each step of the Von Neumann cycle. A pipelined computer usually has ""pipeline registers"" after each stage. These store the bits calculated by a stage so that the logic gates of the next stage can use the bits to do the next step."
+5893,"It is common for even numbered stages to operate on one edge of the square-wave clock, while odd-numbered stages operate on the other edge. This speeds the computer by a factor of two compared to single-edge designs."
+5894,"In a pipelined computer, the control unit arranges for the flow to start, continue, and stop as a program commands. The instruction data is usually passed in pipeline registers from one stage to the next, with a somewhat separated piece of control logic for each stage. The control unit also assures that the instruction in each stage does not harm the operation of instructions in other stages.  For example, if two stages must use the same piece of data, the control logic assures that the uses are done in the correct sequence."
+5895,"When operating efficiently, a pipelined computer will have an instruction in each stage. It is then working on all of those instructions at the same time. It can finish about one instruction for each cycle of its clock. When a program makes a decision, and switches to a different sequence of instructions, the pipeline sometimes must discard the data in process and restart. This is called a ""stall."" When two instructions could interfere, sometimes the control unit must stop processing a later instruction until an earlier instruction completes. This is called a ""pipeline bubble"" because a part of the pipeline is not processing instructions. Pipeline bubbles can occur when two instructions operate on the same register."
+5896,"Interrupts and unexpected exceptions also stall the pipeline. If a pipelined computer abandons work for an interrupt, more work is lost than in a multicycle computer.  Predictable exceptions do not need to stall. For example, if an exception instruction is used to enter the operating system, it does not cause a stall."
+5897,"For the same speed of electronic logic, a pipelined computer can execute more instructions per second than a multicycle computer. Also, even though the electronic logic has a fixed maximum speed, a pipelined computer can be made faster or slower by varying the number of stages in the pipeline. With more stages, each stage does less work, and so the stage has fewer delays from the logic gates."
+5898,A pipelined model of a computer often has less logic gates per instruction per second than multicycle and out-of-order computers. This is because the average stage is less complex than a multicycle computer. An out-of-order computer usually has large amounts of idle logic at any given instant. Similar calculations usually show that a pipelined computer uses less energy per instruction.
+5899,"However, a pipelined computer is usually more complex and more costly than a comparable multicycle computer. It typically has more logic gates, registers and a more complex control unit. In a like way, it might use more total energy, while using less energy per instruction. Out-of-order CPUs can usually do more instructions per second because they can do several instructions at once."
+5900,"Control units use many methods to keep a pipeline full and avoid stalls. For example, even simple control units can assume that a backwards branch, to a lower-numbered, earlier instruction, is a loop, and will be repeated. So, a control unit with this design will always fill the pipeline with the backwards branch path. If a compiler can detect the most frequently-taken direction of a branch, the compiler can just produce instructions so that the most frequently taken branch is the preferred direction of branch. In a like way, a control unit might get hints from the compiler: Some computers have instructions that can encode hints from the compiler about the direction of branch."
+5901,"Some control units do branch prediction: A control unit keeps an electronic list of the recent branches, encoded by the address of the branch instruction. This list has a few bits for each branch to remember the direction that was taken most recently."
+5902,"Some control units can do speculative execution, in which a computer might have two or more pipelines, calculate both directions of a branch, and then discard the calculations of the unused direction."
+5903,"Results from memory can become available at unpredictable times because very fast computers cache memory. That is, they copy limited amounts of memory data into very fast memory. The CPU must be designed to process at the very fast speed of the cache memory. Therefore, the CPU might stall when it must access main memory directly. In modern PCs, main memory is as much as three hundred times slower than cache."
+5904,"To help this, out-of-order CPUs and control units were developed to process data as it becomes available. "
+5905,"But what if all the calculations are complete, but the CPU is still stalled, waiting for main memory? Then, a control unit can switch to an alternative thread of execution whose data has been fetched while the thread was idle. A thread has its own program counter, a stream of instructions and a separate set of registers. Designers vary the number of threads depending on current memory technologies and the type of computer. Typical computers such as PCs and smart phones usually have control units with a few threads, just enough to keep busy with affordable memory systems. Database computers often have about twice as many threads, to keep their much larger memories busy. Graphic processing units  usually have hundreds or thousands of threads, because they have hundreds or thousands of execution units doing repetitive graphic calculations."
+5906,"When a control unit permits threads, the software also has to be designed to handle them. In general-purpose CPUs like PCs and smartphones, the threads are usually made to look very like normal time-sliced processes. At most, the operating system might need some awareness of them. In GPUs, the thread scheduling usually cannot be hidden from the application software, and is often controlled with a specialized subroutine library."
+5907,"A control unit can be designed to finish what it can. If several instructions can be completed at the same time, the control unit will arrange it. So, the fastest computers can process instructions in a sequence that can vary somewhat, depending on when the operands or instruction destinations become available. Most supercomputers and many PC CPUs use this method. The exact organization of this type of control unit depends on the slowest part of the computer."
+5908,"When the execution of calculations is the slowest, instructions flow from memory into pieces of electronics called ""issue units."" An issue unit holds an instruction until both its operands and an execution unit are available. Then, the instruction and its operands are ""issued"" to an execution unit. The execution unit does the instruction. Then the resulting data is moved into a queue of data to be written back to memory or registers. If the computer has multiple execution units, it can usually do several instructions per clock cycle."
+5909,"It is common to have specialized execution units. For example, a modestly priced computer might have only one floating-point execution unit, because floating point units are expensive. The same computer might have several integer units, because these are relatively inexpensive, and can do the bulk of instructions."
+5910,"One kind of control unit for issuing uses an array of electronic logic, a ""scoreboard"""" that detects when an instruction can be issued. The ""height"" of the array is the number of execution units, and the ""length"" and ""width"" are each the number of sources of operands. When all the items come together, the signals from the operands and execution unit will cross. The logic at this intersection detects that the instruction can work, so the instruction is ""issued"" to the free execution unit. An alternative style of issuing control unit implements the Tomasulo algorithm, which reorders a hardware queue of instructions. In some sense, both styles utilize a queue. The scoreboard is an alternative way to encode and reorder a queue of instructions, and some designers call it a queue table."
+5911,"With some additional logic, a scoreboard can compactly combine execution reordering, register renaming and precise exceptions and interrupts. Further it can do this without the power-hungry, complex content-addressable memory used by the Tomasulo algorithm."
+5912,"If the execution is slower than writing the results, the memory write-back queue always has free entries. But what if the memory writes slowly? Or what if the destination register will be used by an ""earlier"" instruction that has not yet issued? Then the write-back step of the instruction might need to be scheduled. This is sometimes called ""retiring"" an instruction. In this case, there must be scheduling logic on the back end of execution units. It schedules access to the registers or memory that will get the results."
+5913,"Retiring logic can also be designed into an issuing scoreboard or a Tomasulo queue, by including memory or register access in the issuing logic."
+5914,"Out of order controllers require special design features to handle interrupts. When there are several instructions in progress, it is not clear where in the instruction stream an interrupt occurs. For input and output interrupts, almost any solution works. However, when a computer has virtual memory, an interrupt occurs to indicate that a memory access failed. This memory access must be associated with an exact instruction and an exact processor state, so that the processor's state can be saved and restored by the interrupt. A usual solution preserves copies of registers until a memory access completes."
+5915,"Also, out of order CPUs have even more problems with stalls from branching, because they can complete several instructions per clock cycle, and usually have many instructions in various stages of progress. So, these control units might use all of the solutions used by pipelined processors."
+5916,"Some computers translate each single instruction into a sequence of simpler instructions. The advantage is that an out of order computer can be simpler in the bulk of its logic, while handling complex multi-step instructions. x86 Intel CPUs since the Pentium Pro translate complex CISC x86 instructions to more RISC-like internal micro-operations."
+5917,"In these, the ""front"" of the control unit manages the translation of instructions. Operands are not translated. The ""back"" of the CU is an out-of-order CPU that issues the micro-operations and operands to the execution units and data paths."
+5918,"Many modern computers have controls that minimize power usage. In battery-powered computers, such as those in cell-phones, the advantage is longer battery life. In computers with utility power, the justification is to reduce the cost of power, cooling or noise."
+5919,"Most modern computers use CMOS logic. CMOS wastes power in two common ways: By changing state, i.e. ""active power,"" and by unintended leakage.  The active power of a computer can be reduced by turning off control signals. Leakage current can be reduced by reducing the electrical pressure, the voltage, making the transistors with larger depletion regions or turning off the logic completely."
+5920,Active power is easier to reduce because data stored in the logic is not affected. The usual method reduces the CPU's clock rate. Most computer systems use this method. It is common for a CPU to idle during the transition to avoid side-effects from the changing clock.
+5921,"Most computers also have a ""halt"" instruction. This was invented to stop non-interrupt code so that interrupt code has reliable timing. However, designers soon noticed that a halt instruction was also a good time to turn off a CPU's clock completely, reducing the CPU's active power to zero. The interrupt controller might continue to need a clock, but that usually uses much less power than the CPU."
+5922,"These methods are relatively easy to design, and became so common that others were invented for commercial advantage. Many modern low-power CMOS CPUs stop and start specialized execution units and bus interfaces depending on the needed instruction. Some computers even arrange the CPU's microarchitecture to use transfer-triggered multiplexers so that each instruction only utilises the exact pieces of logic needed."
+5923,"One common method is to spread the load to many CPUs, and turn off unused CPUs as the load reduces. The operating system's task switching logic saves the CPUs' data to memory. In some cases, one of the CPUs can be simpler and smaller, literally with fewer logic gates. So, it has low leakage, and it is the last to be turned off, and the first to be turned on. Also it then is the only CPU that requires special low-power features. A similar method is used in most PCs, which usually have an auxiliary embedded CPU that manages the power system. However, in PCs, the software is usually in the BIOS, not the operating system."
+5924,"Theoretically, computers at lower clock speeds could also reduce leakage by reducing the voltage of the power supply. This affects the reliability of the computer in many ways, so the engineering is expensive, and it is uncommon except in relatively expensive computers such as PCs or cellphones."
+5925,"Some designs can use very low leakage transistors, but these usually add cost. The depletion barriers of the transistors can be made larger to have less leakage, but this makes the transistor larger and thus both slower and more expensive. Some vendors use this technique in selected portions of an IC by constructing low leakage logic from large transistors that some processes provide for analog circuits. Some processes place the transistors above the surface of the silicon, in ""fin fets"", but these processes have more steps, so are more expensive. Special transistor doping materials  can also reduce leakage, but this adds steps to the processing, making it more expensive. Some semiconductors have a larger band-gap than silicon. However, these materials and processes are currently  more expensive than silicon."
+5926,"Managing leakage is more difficult, because before the logic can be turned-off, the data in it must be moved to some type of low-leakage storage."
+5927,"Some CPUs make use of a special type of flip-flop  that couples a fast, high-leakage storage cell to a slow, large  low-leakage cell. These two cells have separated power supplies. When the CPU enters a power saving mode , data is transferred to the low-leakage cells, and the others are turned off.  When the CPU leaves a low-leakage mode , the process is reversed."
+5928,"Older designs would copy the CPU state to memory, or even disk, sometimes with specialized software. Very simple embedded systems sometimes just restart."
+5929,"All modern CPUs have control logic to attach the CPU to the rest of the computer. In modern computers, this is usually a bus controller. When an instruction reads or writes memory, the control unit either controls the bus directly, or controls a bus controller. Many modern computers use the same bus interface for memory, input and output. This is called ""memory-mapped I/O"". To a programmer, the registers of the I/O devices appear as numbers at specific memory addresses. x86 PCs use an older method, a separate I/O bus accessed by I/O instructions."
+5930,A modern CPU also tends to include an interrupt controller. It handles interrupt signals from the system bus. The control unit is the part of the computer that responds to the interrupts.
+5931,"There is often a cache controller to cache memory. The cache controller and the associated cache memory is often the largest physical part of a modern, higher-performance CPU. When the memory, bus or cache is shared with other CPUs, the control logic must communicate with them to assure that no computer ever gets out-of-date old data."
+5932,"Many historic computers built some type of input and output directly into the control unit. For example, many historic computers had a front panel with switches and lights directly controlled by the control unit. These let a programmer directly enter a program and debug it. In later production computers, the most common use of a front panel was to enter a small bootstrap program to read the operating system from disk. This was annoying. So, front panels were replaced by bootstrap programs in read-only memory."
+5933,"Most PDP-8 models had a data bus designed to let I/O devices borrow the control unit's memory read and write logic. This reduced the complexity and expense of high speed I/O controllers, e.g. for disk."
+5934,"The Xerox Alto had a multitasking microprogrammable control unit that performed almost all I/O. This design provided most of the features of a modern PC with only a tiny fraction of the electronic logic. The dual-thread computer was run by the two lowest-priority microthreads. These performed calculations whenever I/O was not required. High priority microthreads provided  video, network, disk, a periodic timer, mouse, and keyboard. The microprogram did the complex logic of the I/O device, as well as the logic to integrate the device with the computer. For the actual hardware I/O, the microprogram read and wrote shift registers for most I/O, sometimes with resistor networks and transistors to shift output voltage levels . To handle outside events, the microcontroller had microinterrupts to switch threads at the end of a thread's cycle, e.g. at the end of an instruction, or after a shift-register was accessed. The microprogram could be rewritten and reinstalled, which was very useful for a research computer."
+5935,Thus a program of instructions in memory will cause the CU to configure a CPU's data flows to manipulate the data correctly between instructions. This results in a computer that could run a complete program and require no human intervention to make hardware changes between instructions .
+5936,"Hardwired control units are implemented through use of combinational logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses. Hardwired control units are generally faster than the microprogrammed designs."
+5937,"This design uses a fixed architecture—it requires changes in the wiring if the instruction set is modified or changed. It can be convenient for simple, fast computers."
+5938,"A controller that uses this approach can operate at high speed; however, it has little flexibility. A complex instruction set can overwhelm a designer who uses ad hoc logic design."
+5939,"The hardwired approach has become less popular as computers have evolved. Previously, control units for CPUs used ad hoc logic, and they were difficult to design."
+5940,"The idea of microprogramming was introduced by Maurice Wilkes in 1951 as an intermediate level to execute computer program instructions. Microprograms were organized as a sequence of microinstructions and stored in special control memory. The algorithm for the microprogram control unit, unlike the hardwired control unit, is usually specified by flowchart description. The main advantage of a microprogrammed control unit is the simplicity of its structure. Outputs from the controller are by microinstructions. The microprogram can be debugged and replaced similarly to software."
+5941,"A popular variation on microcode is to debug the microcode using a software simulator. Then, the microcode is a table of bits. This is a logical truth table, that translates a microcode address into the control unit outputs.  This truth table can be fed to a computer program that produces optimized electronic logic. The resulting control unit is almost as easy to design as microprogramming, but it has the fast speed and low number of logic elements of a hard wired control unit. The practical result resembles a Mealy machine or Richards controller."
+5942,"A resistive network is a network containing only resistors and ideal current and voltage sources. Analysis of resistive networks is less complicated than analysis of networks containing capacitors and inductors. If the sources are constant  sources, the result is a DC network. The effective resistance and current distribution properties of arbitrary resistor networks can be modeled in terms of their graph measures and geometrical properties."
+5943,A network that contains active electronic components is known as an electronic circuit. Such networks are generally nonlinear and require more complex design and analysis tools.
+5944,An active network contains at least one voltage source or current source that can supply energy to the network indefinitely. A passive network  does not contain an active source.
+5945,"An active network contains one or more sources of electromotive force. Practical examples of such sources include a battery or a generator. Active elements can inject power to the circuit, provide power gain, and control the current flow within the circuit."
+5946,Passive networks do not contain any sources of electromotive force. They consist of passive elements like resistors and capacitors.
+5947,"A network is linear if its signals obey the principle of superposition; otherwise it is non-linear.  Passive networks are generally taken to be linear, but there are exceptions.  For instance, an inductor with an iron core can be driven into saturation if driven with a large enough current.  In this region, the behaviour of the inductor is very non-linear."
+5948,"Discrete passive components  are called lumped elements because all of their, respectively, resistance, capacitance and inductance is assumed to be located  at one place.  This design philosophy is called the lumped-element model and networks so designed are called lumped-element circuits.  This is the conventional approach to circuit design.  At high enough frequencies, or for long enough circuits , the lumped assumption no longer holds because there is a significant fraction of a wavelength across the component dimensions.  A new design model is needed for such cases called the distributed-element model.  Networks designed to this model are called distributed-element circuits."
+5949,A distributed-element circuit that includes some lumped components is called a semi-lumped design.  An example of a semi-lumped circuit is the combline filter.
+5950,Sources can be classified as independent sources and dependent sources.
+5951,An ideal independent source maintains the same voltage or current regardless of the other elements present in the circuit. Its value is either constant  or sinusoidal . The strength of voltage or current is not changed by any variation in the connected network.
+5952,Dependent sources depend upon a particular element of the circuit for delivering the power or voltage or current depending upon the type of source it is.
+5953,A number of electrical laws apply to all linear resistive networks. These include:
+5954,Applying these laws results in a set of simultaneous equations that can be solved either algebraically or numerically.  The laws can generally be extended to networks containing reactances.  They cannot be used in networks that contain nonlinear or time-varying components.
+5955,"To design any electrical circuit, either analog or digital, electrical engineers need to be able to predict the voltages and currents at all places within the circuit.  Simple linear circuits can be analyzed by hand using complex number theory. In more complex cases the circuit may be analyzed with specialized computer programs or estimation techniques such as the piecewise-linear model."
+5956,"Circuit simulation software, such as HSPICE , and languages such as VHDL-AMS and verilog-AMS allow engineers to design circuits without the time, cost and risk of error involved in building circuit prototypes."
+5957,"More complex circuits can be analyzed numerically with software such as SPICE or GNUCAP, or symbolically using software such as SapWin."
+5958,"When faced with a new circuit, the software first tries to find a steady state solution, that is, one where all nodes conform to Kirchhoff's current law and the voltages across and through each element of the circuit conform to the voltage/current equations governing that element."
+5959,"Once the steady state solution is found, the operating points of each element in the circuit are known.  For a small signal analysis, every non-linear element can be linearized around its operation point to obtain the small-signal estimate of the voltages and currents.  This is an application of Ohm's Law.  The resulting linear circuit matrix can be solved with Gaussian elimination."
+5960,"Software such as the PLECS interface to Simulink uses piecewise-linear approximation of the equations governing the elements of a circuit.  The circuit is treated as a completely linear network of ideal diodes.  Every time a diode switches from on to off or vice versa, the configuration of the linear network changes.  Adding more detail to the approximation of equations increases the accuracy of the simulation, but also increases its running time."
+5961,"Computer hardware includes the physical parts of a computer, such as the central processing unit , random access memory , motherboard, computer data storage, graphics card, sound card, and computer case. It includes external devices such as a monitor, mouse, keyboard, and speakers."
+5962,"By contrast, software is the set of instructions that can be stored and run by hardware. Hardware is so-termed because it is hard or rigid with respect to changes, whereas software is soft because it is easy to change."
+5963,"Hardware is typically directed by the software to execute any command or instruction. A combination of hardware and software forms a usable computing system, although other systems exist with only hardware."
+5964,"The template for all modern computers is the Von Neumann architecture, detailed in a 1945 paper by Hungarian mathematician John von Neumann. The paper describes a design architecture for an electronic digital computer with subdivisions of a processing unit consisting of an arithmetic logic unit and processor registers, a control unit containing an instruction register and program counter, a memory to store both data and instructions, external mass storage, and input and output mechanisms. The meaning of the term has evolved to mean a stored-program computer in which an instruction fetch and a data operation cannot occur at the same time because they share a common bus. This is referred to as the Von Neumann bottleneck and often limits the performance of the system."
+5965,"The personal computer is one of the most common types of computer due to its versatility and relatively low price. Desktop personal computers have  a monitor, a keyboard, a mouse, and a computer case. The computer case holds the motherboard, fixed or removable disk drives for data storage, the power supply, and may contain other peripheral devices such as modems or network interfaces.  Some models of desktop computers integrated the monitor and keyboard into the same case as the processor and power supply. Separating the elements allows the user to arrange the components in a pleasing, comfortable array, at the cost of managing power and data cables between them."
+5966,"Laptops are designed for portability but operate similarly to desktop PCs. They may use lower-power or reduced size components, with lower performance than a similarly priced desktop computer. Laptops contain the keyboard, display, and processor in one case. The monitor in the folding upper cover of the case can be closed for transportation, to protect the screen and keyboard.  Instead of a mouse, laptops may have a touchpad or pointing stick."
+5967,Tablets are portable computers that use a touch screen as the primary input device. Tablets generally weigh less and are smaller than laptops.
+5968,"Some tablets include fold-out keyboards or offer connections to separate external keyboards. Some models of laptop computers have a detachable keyboard, which allows the system to be configured as a touch-screen tablet. They are sometimes called ""2-in-1 detachable laptops"" or ""tablet-laptop hybrids""."
+5969,"A computer case encloses most of the components of a desktop computer system. It provides mechanical support and protection for internal elements such as the motherboard, disk drives, and power supply, and controls and directs the flow of cooling air over internal components. The case is also part of the system to control electromagnetic interference radiated by the computer and protects internal parts from electrostatic discharge. Large tower cases provide space for multiple disk drives or other peripherals and usually stand on the floor, while desktop cases provide less expansion room. All-in-one style designs include a video display built into the same case. Portable and laptop computers require cases that provide impact protection for the unit. Hobbyists may decorate the cases with colored lights, paint, or other features, in an activity called case modding."
+5970,"A power supply unit  converts alternating current  electric power to low-voltage direct current  power for the computer. The PSU typically uses a switched-mode power supply , with power MOSFETs  used in the converters and regulator circuits of the SMPS."
+5971,"Laptops can run on a built-in rechargeable battery.
+Laptops can run on a built-in rechargeable battery."
+5972,"The motherboard is the main component of a computer. It is a board with integrated circuitry that connects the other parts of the computer including the CPU, the RAM, the disk drives  as well as any peripherals connected via the ports or the expansion slots. The integrated circuit  chips in a computer typically contain billions of tiny metal–oxide–semiconductor field-effect transistors ."
+5973,Components directly attached to or to part of the motherboard include:
+5974,An expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus. Expansion cards can be used to obtain or expand on features not offered by the motherboard.
+5975,"A storage device is computer hardware or digital media that is used for storing, porting, and extracting data files and objects. It can hold and store information either temporarily or permanently and can be internal or external to a computer. Data storage is a core function and fundamental component of computers. Dedicated storage devices include RAIDs and tape libraries."
+5976,"Data is stored by a computer using a variety of media. Hard disk drives  are found in virtually all older computers, due to their high capacity and low cost, but solid-state drives  are faster and more power efficient, although currently more expensive than hard drives in terms of dollar per gigabyte, so are often found in personal computers built post-2007. SSDs use flash memory, which stores data on MOS memory chips consisting of floating-gate MOSFET memory cells. Some systems may use a disk array controller for greater performance or reliability."
+5977,"To transfer data between computers, an external flash memory device  or optical disc  may be used. Their usefulness depends on being readable by other systems; the majority of machines have an optical disk drive , and virtually all have at least one Universal Serial Bus  port. USB sticks are typically pre-formatted with the FAT32 file system, which is widely supported across operating systems."
+5978,Input and output devices are typically housed externally to the main computer chassis. The following are either standard or very common to many computer systems.
+5979,"Input devices allow the user to enter information into the system, or control its operation. Most personal computers have a mouse and keyboard, but laptop systems typically use a touchpad instead of a mouse. Other input devices include webcams, microphones, joysticks, and image scanners."
+5980,"Output devices are designed around the senses of human beings. For example, monitors display text that can be read, speakers produce sound that can be heard. Such devices also could include printers or a Braille embosser."
+5981,A mainframe computer is a much larger computer that typically fills a room and may cost many hundreds or thousands of times as much as a personal computer. They are designed to perform large numbers of calculations for governments and large enterprises.
+5982,"In the 1960s and 1970s, more and more departments started to use cheaper and dedicated systems for specific purposes like process control and laboratory automation. A minicomputer, or colloquially mini, is a class of smaller computers that was developed in the mid-1960s and sold for much less than mainframe and mid-size computers from IBM and its direct competitors."
+5983,"A supercomputer is superficially similar to a mainframe but is instead intended for extremely demanding computational tasks. As of November 2021, the fastest supercomputer on the TOP500 supercomputer list is Fugaku, in Japan, with a LINPACK benchmark score of 415 PFLOPS, superseding the second fastest, Summit, in the United States, by around 294 PFLOPS."
+5984,"The term supercomputer does not refer to a specific technology. Rather it indicates the fastest computations available at any given time. In mid-2011, the fastest supercomputers boasted speeds exceeding one petaflop, or 1 quadrillion  floating-point operations per second.
+Supercomputers are fast but extremely costly, so they are generally used by large organizations to execute computationally demanding tasks involving large data sets. Supercomputers typically run military and scientific applications. Although costly, they are also being used for commercial applications where huge amounts of data must be analyzed. For example, large banks employ supercomputers to calculate the risks and returns of various investment strategies, and healthcare organizations use them to analyze giant databases of patient data to determine optimal treatments for various diseases and problems incurring to the country."
+5985,"When using computer hardware, an upgrade means adding new or additional hardware to a computer that improves its performance, increases its capacity, or adds new features. For example, a user could perform a hardware upgrade to replace the hard drive with a faster one or a solid state drive  to get a boost in performance. The user may also install more Random Access Memory  so the computer can store additional temporary data, or retrieve such data at a faster rate. The user may add a USB 3.0 expansion card to fully use USB 3.0 devices, or could upgrade the Graphics Processing Unit  for cleaner, more advanced graphics, or more monitors. Performing such hardware upgrades may be necessary for aged computers to meet a new, or updated program's system requirements."
+5986,"In large organizations, hardware upgrades are handled by administrators who are also in charge of keeping networks running smoothly. They replace network devices like servers, routers and storage devices based on new demands and capacities."
+5987,Global revenue from computer hardware in 2023 reached $705.17 billion.
+5988,"Because computer parts contain hazardous materials, there is a growing movement to recycle old and outdated parts. Computer hardware contain dangerous chemicals such as lead, mercury, nickel, and cadmium. According to the EPA these e-wastes have a harmful effect on the environment unless they are disposed of properly. Making hardware requires energy, and recycling parts will reduce air pollution, water pollution, as well as greenhouse gas emissions. Disposing unauthorized computer equipment is in fact illegal. Legislation makes it mandatory to recycle computers through the government approved facilities. Recycling a computer can be made easier by taking out certain reusable parts. For example, the RAM, DVD drive, the graphics card, hard drive or SSD, and other similar removable parts can be reused."
+5989,"Many materials used in computer hardware can be recovered by recycling for use in future production. Reuse of tin, silicon, iron, aluminum, and a variety of plastics that are present in bulk in computers or other electronics can reduce the costs of constructing new systems. Components frequently contain copper, gold, tantalum, silver, platinum, palladium, and lead as well as other valuable materials suitable for reclamation."
+5990,"The central processing unit contains many toxic materials. It contains lead and chromium in the metal plates. Resistors, semiconductors, infrared detectors, stabilizers, cables, and wires contain cadmium. The circuit boards in a computer contain mercury, and chromium. When these types of materials, and chemicals are disposed improperly will become hazardous for the environment."
+5991,"According to the United States Environmental Protection Agency only around 15% of the e-waste actually is recycled. When e-waste byproducts leach into groundwater, are burned, or get mishandled during recycling, it causes harm. Health problems associated with such toxins include impaired mental development, cancer, and damage to the lungs, liver, and kidneys. That is why even wires have to be recycled. Different companies have different techniques to recycle a wire. The most popular one is the grinder that separates the copper wires from the plastic/rubber casing. When the processes are done there are two different piles left; one containing the copper powder, and the other containing plastic/rubber pieces. Computer monitors, mice, and keyboards all have a similar way of being recycled. For example, first, each of the parts are taken apart then all of the inner parts get separated and placed into its own bin."
+5992,"Computer components contain many toxic substances, like dioxins, polychlorinated biphenyls , cadmium, chromium, radioactive isotopes and mercury. Circuit boards contain considerable quantities of lead-tin solders that are more likely to leach into groundwater or create air pollution due to incineration. In US landfills, about 40% of the lead content levels are from e-waste. The processing  required to reclaim these precious substances may release, generate, or synthesize toxic byproducts."
+5993,"Recycling of computer hardware is considered environmentally friendly because it prevents hazardous waste, including heavy metals and carcinogens, from entering the atmosphere, landfill or waterways. While electronics consist a small fraction of total waste generated, they are far more dangerous. There is stringent legislation designed to enforce and encourage the sustainable disposal of appliances, the most notable being the Waste Electrical and Electronic Equipment Directive of the European Union and the United States National Computer Recycling Act."
+5994,"As computer hardware contain a wide number of metals inside, the United States Environmental Protection Agency  encourages the collection and recycling of computer hardware. ""E-cycling"", the recycling of computer hardware, refers to the donation, reuse, shredding and general collection of used electronics. Generically, the term refers to the process of collecting, brokering, disassembling, repairing and recycling the components or metals contained in used or discarded electronic equipment, otherwise known as electronic waste . ""E-cyclable"" items include, but are not limited to: televisions, computers, microwave ovens, vacuum cleaners, telephones and cellular phones, stereos, and VCRs and DVDs just about anything that has a cord, light or takes some kind of battery."
+5995,"Some companies, such as Dell and Apple, will recycle computers of their make or any other make. Otherwise, a computer can be donated to Computer Aid International which is an organization that recycles and refurbishes old computers for hospitals, schools, universities, etc."
+5996,"In modern terminology, a microcontroller is similar to, but less sophisticated than, a system on a chip . A SoC may include a microcontroller as one of its components, but usually integrates it with advanced peripherals like a graphics processing unit , a Wi-Fi module, or one or more coprocessors."
+5997,"Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, toys and other embedded systems. By reducing the size and cost compared to a design that uses a separate microprocessor, memory, and input/output devices, microcontrollers make digital control of more devices and processes practical.  Mixed-signal microcontrollers are common, integrating analog components needed to control non-digital electronic systems. In the context of the internet of things, microcontrollers are an economical and popular means of data collection, sensing and actuating the physical world as edge devices."
+5998,"Some microcontrollers may use four-bit words and operate at frequencies as low as 4 kHz for low power consumption . They generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping  may be just nanowatts, making many of them well suited for long lasting battery applications. Other microcontrollers may serve performance-critical roles, where they may need to act more like a digital signal processor , with higher clock speeds and power consumption."
+5999,"The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. The first single-chip microprocessor was the Intel 4004, released on a single MOS LSI chip in 1971. It was developed by Federico Faggin, using his silicon-gate MOS technology, along with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima. It was followed by the 4-bit Intel 4040, the 8-bit Intel 8008, and the 8-bit Intel 8080. All of these processors required several external chips to implement a working system, including memory and peripheral interface chips.  As a result, the total system cost was several hundred  dollars, making it impossible to economically computerize small appliances."
+6000,"MOS Technology introduced its sub-$100 microprocessors in 1975, the 6501 and 6502. Their chief aim was to reduce this cost barrier but these microprocessors still required external support, memory, and peripheral chips which kept the total system cost in the hundreds of dollars."
+6001,"One book credits TI engineers Gary Boone and Michael Cochran with the successful creation of the first microcontroller in 1971. The result of their work was the TMS 1000, which became commercially available in 1974. It combined read-only memory, read/write memory, processor and clock on one chip and was targeted at embedded systems."
+6002,"During the early-to-mid-1970s, Japanese electronics manufacturers began producing microcontrollers for automobiles, including 4-bit MCUs for in-car entertainment, automatic wipers, electronic locks, and dashboard, and 8-bit MCUs for engine control."
+6003,"Partly in response to the existence of the single-chip TMS 1000, Intel developed a computer system on a chip optimized for control applications, the Intel 8048, with commercial parts first shipping in 1977. It combined RAM and ROM on the same chip with a microprocessor. Among numerous applications, this chip would eventually find its way into over one billion PC keyboards. At that time Intel's President, Luke J. Valenter, stated that the microcontroller was one of the most successful products in the company's history, and he expanded the microcontroller division's budget by over 25%."
+6004,"Most microcontrollers at this time had concurrent variants. One had EPROM program memory, with a transparent quartz window in the lid of the package to allow it to be erased by exposure to ultraviolet light. These erasable chips were often used for prototyping. The other variant was either a mask-programmed ROM or a PROM variant which was only programmable once. For the latter, sometimes the designation OTP was used, standing for ""one-time programmable"". In an OTP microcontroller, the PROM was usually of identical type as the EPROM, but the chip package had no quartz window; because there was no way to expose the EPROM to ultraviolet light, it could not be erased. Because the erasable versions required ceramic packages with quartz windows, they were significantly more expensive than the OTP versions, which could be made in lower-cost opaque plastic packages. For the erasable variants, quartz was required, instead of less expensive glass, for its transparency to ultraviolet light—to which glass is largely opaque—but the main cost differentiator was the ceramic package itself."
+6005,"In 1993, the introduction of EEPROM memory allowed microcontrollers  to be electrically erased quickly without an expensive package as required for EPROM, allowing both rapid prototyping, and in-system programming.  The same year, Atmel introduced the first microcontroller using Flash memory, a special type of EEPROM. Other companies rapidly followed suit, with both memory types."
+6006,"Nowadays microcontrollers are cheap and readily available for hobbyists, with large online communities around certain processors."
+6007,"In 2002, about 55% of all CPUs sold in the world were 8-bit microcontrollers and microprocessors."
+6008,"Over two billion 8-bit microcontrollers were sold in 1997, and according to Semico, over four billion 8-bit microcontrollers were sold in 2006. More recently, Semico has claimed the MCU market grew 36.5% in 2010 and 12% in 2011."
+6009,"A typical home in a developed country is likely to have only four general-purpose microprocessors but around three dozen microcontrollers. A typical mid-range automobile has about 30 microcontrollers. They can also be found in many electrical devices such as washing machines, microwave ovens, and telephones."
+6010,Cost to manufacture can be under US$0.10 per unit.
+6011,"Cost has plummeted over time, with the cheapest 8-bit microcontrollers being available for under US$0.03 in 2018, and some 32-bit microcontrollers around US$1 for similar quantities."
+6012,"In 2012, following a global crisis—a worst ever annual sales decline and recovery and average sales price year-over-year plunging 17%—the biggest reduction since the 1980s—the average price for a microcontroller was US$0.88 ."
+6013,"In 2012, worldwide sales of 8-bit microcontrollers were around US$4 billion, while 4-bit microcontrollers also saw significant sales."
+6014,"In 2015, 8-bit microcontrollers could be bought for US$0.311 , 16-bit for US$0.385 , and 32-bit for US$0.378 ."
+6015,"In 2018, 8-bit microcontrollers could be bought for US$0.03, 16-bit for US$0.393 , and 32-bit for US$0.503 ."
+6016,"In 2018, the low-priced microcontrollers above from 2015 were all more expensive  at: the 8-bit microcontroller could be bought for US$0.319  or 2.6% higher, the 16-bit one for US$0.464  or 21% higher, and the 32-bit one for US$0.503  or 33% higher."
+6017,"On 21 June 2018, the ""world's smallest computer"" was announced by the University of Michigan. The device is a ""0.04 mm3 16 nW wireless and batteryless sensor system with integrated Cortex-M0+ processor and optical communication for cellular temperature measurement."" It ""measures just 0.3 mm to a side—dwarfed by a grain of rice.  In addition to the RAM and photovoltaics, the new computing devices have processors and wireless transmitters and receivers. Because they are too small to have conventional radio antennae, they receive and transmit data with visible light. A base station provides light for power and programming, and it receives the data."" The device is 1⁄10th the size of IBM's previously claimed world-record-sized computer from months back in March 2018, which is ""smaller than a grain of salt"", has a million transistors, costs less than $0.10 to manufacture, and, combined with blockchain technology, is intended for logistics and ""crypto-anchors""—digital fingerprint applications."
+6018,"A microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used as an embedded system. The majority of microcontrollers in use today are embedded in other machinery, such as automobiles, telephones, appliances, and peripherals for computer systems."
+6019,"While some embedded systems are very sophisticated, many have minimal requirements for memory and program length, with no operating system, and low software complexity. Typical input and output devices include switches, relays, solenoids, LED's, small or custom liquid-crystal displays, radio frequency devices, and sensors for data such as temperature, humidity, light level etc. Embedded systems usually have no keyboard, screen, disks, printers, or other recognizable I/O devices of a personal computer, and may lack human interaction devices of any kind."
+6020,"Microcontrollers must provide real-time  response to events in the embedded system they are controlling. When certain events occur, an interrupt system can signal the processor to suspend processing the current instruction sequence and to begin an interrupt service routine  which will perform any processing required based on the source of the interrupt, before returning to the original instruction sequence. Possible interrupt sources are device-dependent and often include events such as an internal timer overflow, completing an analog-to-digital conversion, a logic-level change on an input such as from a button being pressed, and data received on a communication link. Where power consumption is important as in battery devices, interrupts may also wake a microcontroller from a low-power sleep state where the processor is halted until required to do something by a peripheral event."
+6021,"Typically microcontroller programs must fit in the available on-chip memory, since it would be costly to provide a system with external, expandable memory. Compilers and assemblers are used to convert both high-level and assembly language code into a compact machine code for storage in the microcontroller's memory. Depending on the device, the program memory may be permanent, read-only memory that can only be programmed at the factory, or it may be field-alterable flash or erasable read-only memory."
+6022,"Manufacturers have often produced special versions of their microcontrollers in order to help the hardware and software development of the target system. Originally these included EPROM versions that have a ""window"" on the top of the device through which program memory can be erased by ultraviolet light, ready for reprogramming after a programming  and test cycle. Since 1998, EPROM versions are rare and have been replaced by EEPROM and flash, which are easier to use  and cheaper to manufacture."
+6023,"Other versions may be available where the ROM is accessed as an external device rather than as internal memory, however these are becoming rare due to the widespread availability of cheap microcontroller programmers."
+6024,The use of field-programmable devices on a micro controller may allow field update of the firmware or permit late factory revisions to products that have been assembled but not yet shipped. Programmable memory also reduces the lead time required for deployment of a new product.
+6025,"Where hundreds of thousands of identical devices are required, using parts programmed at the time of manufacture can be economical. These ""mask-programmed"" parts have the program laid down in the same way as the logic of the chip, at the same time."
+6026,"A customized microcontroller incorporates a block of digital logic that can be personalized for additional processing capability, peripherals and interfaces that are adapted to the requirements of the application. One example is the AT91CAP from Atmel."
+6027,"Microcontrollers usually contain from several to dozens of general purpose input/output pins . GPIO pins are software configurable to either an input or an output state. When GPIO pins are configured to an input state, they are often used to read sensors or external signals. Configured to the output state, GPIO pins can drive external devices such as LEDs or motors, often indirectly, through external power electronics."
+6028,"Many embedded systems need to read sensors that produce analog signals. This is the purpose of the analog-to-digital converter . Since processors are built to interpret and process digital data, i.e. 1s and 0s, they are not able to do anything with the analog signals that may be sent to it by a device. So the analog-to-digital converter is used to convert the incoming data into a form that the processor can recognize. A less common feature on some microcontrollers is a digital-to-analog converter  that allows the processor to output analog signals or voltage levels."
+6029,"In addition to the converters, many embedded microprocessors include a variety of timers as well. One of the most common types of timers is the programmable interval timer . A PIT may either count down from some value to zero, or up to the capacity of the count register, overflowing to zero. Once it reaches zero, it sends an interrupt to the processor indicating that it has finished counting. This is useful for devices such as thermostats, which periodically test the temperature around them to see if they need to turn the air conditioner on/off, the heater on/off, etc."
+6030,"A dedicated pulse-width modulation  block makes it possible for the CPU to control power converters, resistive loads, motors, etc., without using many CPU resources in tight timer loops."
+6031,"A universal asynchronous receiver/transmitter  block makes it possible to receive and transmit data over a serial line with very little load on the CPU. Dedicated on-chip hardware also often includes capabilities to communicate with other devices  in digital formats such as Inter-Integrated Circuit , Serial Peripheral Interface , Universal Serial Bus , and Ethernet."
+6032,"Microcontrollers may not implement an external address or data bus as they integrate RAM and non-volatile memory on the same chip as the CPU. Using fewer pins, the chip can be placed in a much smaller, cheaper package."
+6033,"Integrating the memory and other peripherals on a single chip and testing them as a unit increases the cost of that chip, but often results in decreased net cost of the embedded system as a whole. Even if the cost of a CPU that has integrated peripherals is slightly more than the cost of a CPU and external peripherals, having fewer chips typically allows a smaller and cheaper circuit board, and reduces the labor required to assemble and test the circuit board, in addition to tending to decrease the defect rate for the finished assembly."
+6034,"A microcontroller is a single integrated circuit, commonly with the following features:"
+6035,"This integration drastically reduces the number of chips and the amount of wiring and circuit board space that would be needed to produce equivalent systems using separate chips. Furthermore, on low pin count devices in particular, each pin may interface to several internal peripherals, with the pin function selected by software. This allows a part to be used in a wider variety of applications than if pins had dedicated functions."
+6036,Microcontrollers have proved to be highly popular in embedded systems since their introduction in the 1970s.
+6037,"Some microcontrollers use a Harvard architecture: separate memory buses for instructions and data, allowing accesses to take place concurrently. Where a Harvard architecture is used, instruction words for the processor may be a different bit size than the length of internal memory and registers; for example: 12-bit instructions used with 8-bit data registers."
+6038,The decision of which peripheral to integrate is often difficult. The microcontroller vendors often trade operating frequencies and system design flexibility against time-to-market requirements from their customers and overall lower system cost. Manufacturers have to balance the need to minimize the chip size against additional functionality.
+6039,"Microcontroller architectures vary widely. Some designs include general-purpose microprocessor cores, with one or more ROM, RAM, or I/O functions integrated onto the package. Other designs are purpose-built for control applications. A microcontroller instruction set usually has many instructions intended for bit manipulation  to make control programs more compact. For example, a general-purpose processor might require several instructions to test a bit in a register and branch if the bit is set, where a microcontroller could have a single instruction to provide that commonly required function."
+6040,"Microcontrollers historically have not had math coprocessors, so floating-point arithmetic has been performed by software. However, some recent designs do include FPUs and DSP-optimized features. An example would be Microchip's PIC32 MIPS-based line."
+6041,"Microcontrollers were originally programmed only in assembly language, but various high-level programming languages, such as C, Python and JavaScript, are now also in common use to target microcontrollers and embedded systems. Compilers for general purpose languages will typically have some restrictions as well as enhancements to better support the unique characteristics of microcontrollers. Some microcontrollers have environments to aid developing certain types of applications. Microcontroller vendors often make tools freely available to make it easier to adopt their hardware."
+6042,"Microcontrollers with specialty hardware may require their own non-standard dialects of C, such as SDCC for the 8051, which prevent using standard tools  even for code unrelated to hardware features. Interpreters may also contain nonstandard features, such as MicroPython, although a fork, CircuitPython, has looked to move hardware dependencies to libraries and have the language adhere to a more CPython standard."
+6043,"Interpreter firmware is also available for some microcontrollers. For example, BASIC on the early microcontroller Intel 8052; BASIC and FORTH on the Zilog Z8 as well as some modern devices. Typically these interpreters support interactive programming."
+6044,"Simulators are available for some microcontrollers. These allow a developer to analyze what the behavior of the microcontroller and their program should be if they were using the actual part. A simulator will show the internal processor state and also that of the outputs, as well as allowing input signals to be generated. While on the one hand most simulators will be limited from being unable to simulate much other hardware in a system, they can exercise conditions that may otherwise be hard to reproduce at will in the physical implementation, and can be the quickest way to debug and analyze problems."
+6045,"Recent microcontrollers are often integrated with on-chip debug circuitry that when accessed by an in-circuit emulator  via JTAG, allow debugging of the firmware with a debugger. A real-time ICE may allow viewing and/or manipulating of internal states while running. A tracing ICE can record executed program and MCU states before/after a trigger point."
+6046,"As of 2008, there are several dozen microcontroller architectures and vendors including:"
+6047,"Many others exist, some of which are used in very narrow range of applications or are more like applications processors than microcontrollers. The microcontroller market is extremely fragmented, with numerous vendors, technologies, and markets. Note that many vendors sell or have sold multiple architectures."
+6048,"In contrast to general-purpose computers, microcontrollers used in embedded systems often seek to optimize interrupt latency over instruction throughput. Issues include both reducing the latency, and making it be more predictable ."
+6049,"When an electronic device causes an interrupt, during the context switch the intermediate results  have to be saved before the software responsible for handling the interrupt can run. They must also be restored after that interrupt handler is finished. If there are more processor registers, this saving and restoring process may take more time, increasing the latency.  Ways to reduce such context/restore latency include having relatively few registers in their central processing units , or at least having the hardware not save them all . Another technique involves spending silicon gates on ""shadow registers"": One or more duplicate registers used only by the interrupt software, perhaps supporting a dedicated stack."
+6050,Other factors affecting interrupt latency include:
+6051,Lower end microcontrollers tend to support fewer interrupt latency controls than higher end ones.
+6052,"Two different kinds of memory are commonly used with microcontrollers, a non-volatile memory for storing firmware and a read-write memory for temporary data."
+6053,"From the earliest microcontrollers to today, six-transistor SRAM is almost always used as the read/write working memory, with a few more transistors per bit used in the register file."
+6054,"In addition to the SRAM, some microcontrollers also have internal EEPROM and/or NVRAM for data storage; and ones that do not have any , or where the internal memory is insufficient, are often connected to an external EEPROM or flash memory chip."
+6055,"A few microcontrollers beginning in 2003 have ""self-programmable"" flash memory."
+6056,"The earliest microcontrollers used mask ROM to store firmware. Later microcontrollers  had EPROM memory, which used a translucent window to allow erasure via UV light, while production versions had no such window, being OTP . Firmware updates were equivalent to replacing the microcontroller itself, thus many products were not upgradeable."
+6057,"Motorola MC68HC805 was the first microcontroller to use EEPROM to store the firmware. EEPROM microcontrollers became more popular in 1993 when Microchip introduced PIC16C84 and Atmel introduced an 8051-core microcontroller that was first one to use NOR Flash memory to store the firmware. Today's microcontrollers almost all use flash memory, with a few models using FRAM and some ultra-low-cost parts still using OTP or Mask ROM."
+6058,"A system on a chip or system-on-chip  is an integrated circuit that integrates most or all components of a computer or other electronic system. These components almost always include on-chip central processing unit , memory interfaces, input/output devices and interfaces, and secondary storage interfaces, often alongside other components such as radio modems and a graphics processing unit  – all on a single substrate or microchip. SoCs may contain digital and also analog, mixed-signal and often radio frequency signal processing functions ."
+6059,"Higher-performance SoCs are often paired with dedicated and physically separate memory and secondary storage  chips, that may be layered on top of the SoC in what's known as a package on package  configuration, or be placed close to the SoC. Additionally, SoCs may use separate wireless modems."
+6060,"An SoC integrates a microcontroller, microprocessor or perhaps several processor cores with peripherals like a GPU, Wi-Fi and cellular network radio modems, and/or one or more coprocessors. Similar to how a microcontroller integrates a microprocessor with peripheral circuits and memory, an SoC can be seen as integrating a microcontroller with even more advanced peripherals. For an overview of integrating system components, see system integration."
+6061,"Compared to a multi-chip architecture, an SoC with equivalent functionality will have reduced power consumption as well as a smaller semiconductor die area. This comes at the cost of reduced replaceability of components. By definition, SoC designs are fully or nearly fully integrated across different component modules. For these reasons, there has been a general trend towards tighter integration of components in the computer hardware industry, in part due to the influence of SoCs and lessons learned from the mobile and embedded computing markets."
+6062,SoCs are very common in the mobile computing  and edge computing markets.
+6063,"In general, there are three distinguishable types of SoCs:"
+6064,"SoCs can be applied to any computing task. However, they are typically used in mobile computing such as tablets, smartphones, smartwatches and netbooks as well as embedded systems and in applications where previously microcontrollers would be used."
+6065,"Where previously only microcontrollers could be used, SoCs are rising to prominence in the embedded systems market. Tighter system integration offers better reliability and mean time between failure, and SoCs offer more advanced functionality and computing power than microcontrollers. Applications include AI acceleration, embedded machine vision, data collection, telemetry, vector processing and ambient intelligence. Often embedded SoCs target the internet of things, multimedia, networking, telecommunications and edge computing markets."
+6066,"Mobile computing based SoCs always bundle processors, memories, on-chip caches, wireless networking capabilities and often digital camera hardware and firmware. With increasing memory sizes, high end SoCs will often have no memory and flash storage and instead, the memory and flash memory will be placed right next to, or above , the SoC. Some examples of mobile computing SoCs include:"
+6067,"In 1992, Acorn Computers produced the A3010, A3020 and A4000 range of personal computers with the ARM250 SoC. It combined the original Acorn ARM2 processor with a memory controller , video controller , and I/O controller . In previous Acorn ARM-powered computers, these were four discrete chips. The ARM7500 chip was their second-generation SoC, based on the ARM700, VIDC20 and IOMD controllers, and was widely licensed in embedded devices such as set-top-boxes, as well as later Acorn personal computers."
+6068,"Tablet and laptop manufacturers have learned lessons from embedded systems and smartphone markets about reduced power consumption, better performance and reliability from tighter integration of hardware and firmware modules, and LTE and other wireless network communications integrated on chip ."
+6069,"An SoC consists of hardware functional units, including microprocessors that run software code, as well as a communications subsystem to connect, control, direct and interface between these functional modules."
+6070,"An SoC must have at least one processor core, but typically an SoC has more than one core. Processor cores can be a microcontroller, microprocessor , digital signal processor  or application-specific instruction set processor  core. ASIPs have instruction sets that are customized for an application domain and designed to be more efficient than general-purpose instructions for a specific type of workload. Multiprocessor SoCs have more than one processor core by definition. The ARM architecture is a common choice for SoC processor cores because some ARM-architecture cores are soft processors specified as IP cores."
+6071,"SoCs must have semiconductor memory blocks to perform their computation, as do microcontrollers and other embedded systems. Depending on the application, SoC memory may form a memory hierarchy and cache hierarchy. In the mobile computing market, this is common, but in many low-power embedded microcontrollers, this is not necessary. Memory technologies for SoCs include read-only memory , random-access memory , Electrically Erasable Programmable ROM  and flash memory. As in other computer systems, RAM can be subdivided into relatively faster but more expensive static RAM  and the slower but cheaper dynamic RAM . When an SoC has a cache hierarchy, SRAM will usually be used to implement processor registers and cores' built-in caches whereas DRAM will be used for main memory. ""Main memory"" may be specific to a single processor  when the SoC has multiple processors, in this case it is distributed memory and must be sent via § Intermodule communication on-chip to be accessed by a different processor. For further discussion of multi-processing memory issues, see cache coherence and memory latency."
+6072,"SoCs include external interfaces, typically for communication protocols. These are often based upon industry standards such as USB, FireWire, Ethernet, USART, SPI, HDMI, I²C, CSI, etc. These interfaces will differ according to the intended application. Wireless networking protocols such as Wi-Fi, Bluetooth, 6LoWPAN and near-field communication may also be supported."
+6073,"When needed, SoCs include analog interfaces including analog-to-digital and digital-to-analog converters, often for signal processing. These may be able to interface with different types of sensors or actuators, including smart transducers. They may interface with application-specific modules or shields. Or they may be internal to the SoC, such as if an analog sensor is built in to the SoC and its readings must be converted to digital signals for mathematical processing."
+6074,"Digital signal processor  cores are often included on SoCs. They perform signal processing operations in SoCs for sensors, actuators, data collection, data analysis and multimedia processing. DSP cores typically feature very long instruction word  and single instruction, multiple data  instruction set architectures, and are therefore highly amenable to exploiting instruction-level parallelism through parallel processing and superscalar execution.: 4   SP cores most often feature application-specific instructions, and as such are typically application-specific instruction set processors . Such application-specific instructions correspond to dedicated hardware functional units that compute those instructions."
+6075,"Typical DSP instructions include multiply-accumulate, Fast Fourier transform, fused multiply-add, and convolutions."
+6076,"As with other computer systems, SoCs require timing sources to generate clock signals, control execution of SoC functions and provide time context to signal processing applications of the SoC, if needed. Popular time sources are crystal oscillators and phase-locked loops."
+6077,"SoC peripherals including counter-timers, real-time timers and power-on reset generators. SoCs also include voltage regulators and power management circuits."
+6078,"SoCs comprise many execution units. These units must often send data and instructions back and forth. Because of this, all but the most trivial SoCs require communications subsystems. Originally, as with other microcomputer technologies, data bus architectures were used, but recently designs based on sparse intercommunication networks known as networks-on-chip  have risen to prominence and are forecast to overtake bus architectures for SoC design in the near future."
+6079,"Historically, a shared global computer bus typically connected the different components, also called ""blocks"" of the SoC. A very common bus for SoC communications is ARM's royalty-free Advanced Microcontroller Bus Architecture  standard."
+6080,"Direct memory access controllers route data directly between external interfaces and SoC memory, bypassing the CPU or control unit, thereby increasing the data throughput of the SoC. This is similar to some device drivers of peripherals on component-based multi-chip module PC architectures."
+6081,"Wire delay is not scalable due to continued miniaturization, system performance does not scale with the number of cores attached, the SoC's operating frequency must decrease with each additional core attached for power to be sustainable, and long wires consume large amounts of electrical power. These challenges are prohibitive to supporting manycore systems on chip.: xiii"
+6082,"In the late 2010s, a trend of SoCs implementing communications subsystems in terms of a network-like topology instead of bus-based protocols has emerged. A trend towards more processor cores on SoCs has caused on-chip communication efficiency to become one of the key factors in determining the overall system performance and cost.: xiii  This has led to the emergence of interconnection networks with router-based packet switching known as ""networks on chip""  to overcome the bottlenecks of bus-based networks.: xiii"
+6083,"Networks-on-chip have advantages including destination- and application-specific routing, greater power efficiency and reduced possibility of bus contention. Network-on-chip architectures take inspiration from communication protocols like TCP and the Internet protocol suite for on-chip communication, although they typically have fewer network layers. Optimal network-on-chip network architectures are an ongoing area of much research interest. NoC architectures range from traditional distributed computing network topologies such as torus, hypercube, meshes and tree networks to genetic algorithm scheduling to randomized algorithms such as random walks with branching and randomized time to live ."
+6084,"Many SoC researchers consider NoC architectures to be the future of SoC design because they have been shown to efficiently meet power and throughput needs of SoC designs. Current NoC architectures are two-dimensional. 2D IC design has limited floorplanning choices as the number of cores in SoCs increase, so as three-dimensional integrated circuits  emerge, SoC designers are looking towards building three-dimensional on-chip networks known as 3DNoCs."
+6085,"A system on a chip consists of both the hardware, described in § Structure, and the software controlling the microcontroller, microprocessor or digital signal processor cores, peripherals and interfaces. The design flow for an SoC aims to develop this hardware and software at the same time, also known as architectural co-design. The design flow must also take into account optimizations  and constraints."
+6086,"Most SoCs are developed from pre-qualified hardware component IP core specifications for the hardware elements and execution units, collectively ""blocks"", described above, together with software device drivers that may control their operation. Of particular importance are the protocol stacks that drive industry-standard interfaces like USB.  The hardware blocks are put together using computer-aided design tools, specifically electronic design automation tools; the software modules are integrated using a software integrated development environment."
+6087,"SoCs components are also often designed in high-level programming languages such as C++, MATLAB or SystemC and converted to RTL designs through high-level synthesis  tools such as C to HDL or flow to HDL. HLS products called ""algorithmic synthesis"" allow designers to use C++ to model and synthesize system, circuit, software and verification levels all in one high level language commonly known to computer engineers in a manner independent of time scales, which are typically specified in HDL. Other components can remain software and be compiled and embedded onto soft-core processors included in the SoC as modules in HDL as IP cores."
+6088,"Once the architecture of the SoC has been defined, any new hardware elements are written in an abstract hardware description language termed register transfer level  which defines the circuit behavior, or synthesized into RTL from a high level language through high-level synthesis. These elements are connected together in a hardware description language to create the full SoC design. The logic specified to connect these components and convert between possibly different interfaces provided by different vendors is called glue logic."
+6089,"Chips are verified for validation correctness before being sent to a semiconductor foundry. This process is called functional verification and it accounts for a significant portion of the time and energy expended in the chip design life cycle, often quoted as 70%. With the growing complexity of chips, hardware verification languages like SystemVerilog, SystemC, e, and OpenVera are being used. Bugs found in the verification stage are reported to the designer."
+6090,"Traditionally, engineers have employed simulation acceleration, emulation or prototyping on reprogrammable hardware to verify and debug hardware and software for SoC designs prior to the finalization of the design, known as tape-out. Field-programmable gate arrays  are favored for prototyping SoCs because FPGA prototypes are reprogrammable, allow debugging and are more flexible than application-specific integrated circuits ."
+6091,"With high capacity and fast compilation time, simulation acceleration and emulation are powerful technologies that provide wide visibility into systems. Both technologies, however, operate slowly, on the order of MHz, which may be significantly slower – up to 100 times slower – than the SoC's operating frequency. Acceleration and emulation boxes are also very large and expensive at over US$1 million."
+6092,"FPGA prototypes, in contrast, use FPGAs directly to enable engineers to validate and test at, or close to, a system's full operating frequency with real-world stimuli. Tools such as Certus are used to insert probes in the FPGA RTL that make signals available for observation. This is used to debug hardware, firmware and software interactions across multiple FPGAs with capabilities similar to a logic analyzer."
+6093,"In parallel, the hardware elements are grouped and passed through a process of logic synthesis, during which performance constraints, such as operational frequency and expected signal delays, are applied. This generates an output known as a netlist describing the design as a physical circuit and its interconnections. These netlists are combined with the glue logic connecting the components to produce the schematic description of the SoC as a circuit which can be printed onto a chip. This process is known as place and route and precedes tape-out in the event that the SoCs are produced as application-specific integrated circuits ."
+6094,"SoCs must optimize power use, area on die, communication, positioning for locality between modular units and other factors. Optimization is necessarily a design goal of SoCs. If optimization was not necessary, the engineers would use a multi-chip module architecture without accounting for the area use, power consumption or performance of the system to the same extent."
+6095,"Common optimization targets for SoC designs follow, with explanations of each. In general, optimizing any of these quantities may be a hard combinatorial optimization problem, and can indeed be NP-hard fairly easily. Therefore, sophisticated optimization algorithms are often required and it may be practical to use approximation algorithms or heuristics in some cases. Additionally, most SoC designs contain multiple variables to optimize simultaneously, so Pareto efficient solutions are sought after in SoC design. Oftentimes the goals of optimizing some of these quantities are directly at odds, further adding complexity to design optimization of SoCs and introducing trade-offs in system design."
+6096,"For broader coverage of trade-offs and requirements analysis, see requirements engineering."
+6097,"SoCs are optimized to minimize the electrical power used to perform the SoC's functions. Most SoCs must use low power. SoC systems often require long battery life , can potentially spend months or years without a power source while needing to maintain autonomous function, and often are limited in power use by a high number of embedded SoCs being networked together in an area. Additionally, energy costs can be high and conserving energy will reduce the total cost of ownership of the SoC. Finally, waste heat from high energy consumption can damage other circuit components if too much heat is dissipated, giving another pragmatic reason to conserve energy. The amount of energy used in a circuit is the integral of power consumed with respect to time, and the average rate of power consumption is the product of current by voltage. Equivalently, by Ohm's law, power is current squared times resistance or voltage squared divided by resistance:"
+6098,"SoCs are optimized to maximize power efficiency in performance per watt: maximize the performance of the SoC given a budget of power usage. Many applications such as edge computing, distributed processing and ambient intelligence require a certain level of computational performance, but power is limited in most SoC environments."
+6099,"SoC designs are optimized to minimize waste heat output on the chip. As with other integrated circuits, heat generated due to high power density are the bottleneck to further miniaturization of components.: 1  The power densities of high speed integrated circuits, particularly microprocessors and including SoCs, have become highly uneven. Too much waste heat can damage circuits and erode reliability of the circuit over time. High temperatures and thermal stress negatively impact reliability, stress migration, decreased mean time between failures, electromigration, wire bonding, metastability and other performance degradation of the SoC over time.: 2–9"
+6100,"In particular, most SoCs are in a small physical area or volume and therefore the effects of waste heat are compounded because there is little room for it to diffuse out of the system. Because of high transistor counts on modern devices, oftentimes a layout of sufficient throughput and high transistor density is physically realizable from fabrication processes but would result in unacceptably high amounts of heat in the circuit's volume.: 1"
+6101,"These thermal effects force SoC and other chip designers to apply conservative design margins, creating less performant devices to mitigate the risk of catastrophic failure. Due to increased transistor densities as length scales get smaller, each process generation produces more heat output than the last. Compounding this problem, SoC architectures are usually heterogeneous, creating spatially inhomogeneous heat fluxes, which cannot be effectively mitigated by uniform passive cooling.: 1"
+6102,SoCs are optimized to maximize computational and communications throughput.
+6103,"SoCs are optimized to minimize latency for some or all of their functions. This can be accomplished by laying out elements with proper proximity and locality to each-other to minimize the interconnection delays and maximize the speed at which data is communicated between modules, functional units and memories. In general, optimizing to minimize latency is an NP-complete problem equivalent to the boolean satisfiability problem."
+6104,"For tasks running on processor cores, latency and throughput can be improved with task scheduling. Some tasks run in application-specific hardware units, however, and even task scheduling may not be sufficient to optimize all software-based tasks to meet timing and throughput constraints."
+6105,"Systems on chip are modeled with standard hardware verification and validation techniques, but additional techniques are used to model and optimize SoC design alternatives to make the system optimal with respect to multiple-criteria decision analysis on the above optimization targets."
+6106,"Task scheduling is an important activity in any computer system with multiple processes or threads sharing a single processor core. It is important to reduce § Latency and increase § Throughput for embedded software running on an SoC's § Processor cores.  Not every important computing activity in a SoC is performed in software running on on-chip processors, but scheduling can drastically improve performance of software-based tasks and other tasks involving shared resources."
+6107,Software running on SoCs often schedules tasks according to network scheduling and randomized scheduling algorithms.
+6108,"Hardware and software tasks are often pipelined in processor design. Pipelining is an important principle for speedup in computer architecture. They are frequently used in GPUs  and RISC processors , but are also applied to application-specific tasks such as digital signal processing and multimedia manipulations in the context of SoCs."
+6109,"SoCs are often analyzed though probabilistic models, queueing networks, and Markov chains. For instance, Little's law allows SoC states and NoC buffers to be modeled as arrival processes and analyzed through Poisson random variables and Poisson processes."
+6110,"SoCs are often modeled with Markov chains, both discrete time and continuous time variants. Markov chain modeling allows asymptotic analysis of the SoC's steady state distribution of power, heat, latency and other factors to allow design decisions to be optimized for the common case."
+6111,"SoC chips are typically fabricated using metal–oxide–semiconductor  technology. The netlists described above are used as the basis for the physical design  flow to convert the designers' intent into the design of the SoC. Throughout this conversion process, the design is analyzed with static timing modeling, simulation and other tools to ensure that it meets the specified operational parameters such as frequency, power consumption and dissipation, functional integrity  and electrical integrity."
+6112,"When all known bugs have been rectified and these have been re-verified and all physical design checks are done, the physical design files describing each layer of the chip are sent to the foundry's mask shop where a full set of glass lithographic masks will be etched. These are sent to a wafer fabrication plant to create the SoC dice before packaging and testing."
+6113,"SoCs can be fabricated by several technologies, including:"
+6114,"ASICs consume less power and are faster than FPGAs but cannot be reprogrammed and are expensive to manufacture. FPGA designs are more suitable for lower volume designs, but after enough units of production ASICs reduce the total cost of ownership."
+6115,"SoC designs consume less power and have a lower cost and higher reliability than the multi-chip systems that they replace. With fewer packages in the system, assembly costs are reduced as well."
+6116,"However, like most very-large-scale integration  designs, the total cost is higher for one large chip than for the same functionality distributed over several smaller chips, because of lower yields and higher non-recurring engineering costs."
+6117,"When it is not feasible to construct an SoC for a particular application, an alternative is a system in package  comprising a number of chips in a single package. When produced in large volumes, SoC is more cost-effective than SiP because its packaging is simpler. Another reason SiP may be preferred is waste heat may be too high in a SoC for a given purpose because functional components are too close together, and in an SiP heat will dissipate better from different functional modules since they are physically further apart."
+6118,Some examples of systems on a chip are:
+6119,"SoC research and development often compares many options. Benchmarks, such as COSMIC, are developed to help such evaluations."
+6120,"Computer science is the study of computation, information, and automation. Computer science spans theoretical disciplines  to applied disciplines . Though more often considered an academic discipline, computer science is closely related to computer programming."
+6121,"Algorithms and data structures are central to computer science.
+The theory of computation concerns abstract models of computation and general classes of problems that can be solved using them. The fields of cryptography and computer security involve studying the means for secure communication and for preventing security vulnerabilities. Computer graphics and computational geometry address the generation of images. Programming language theory considers different ways to describe computational processes, and database theory concerns the management of repositories of data. Human–computer interaction investigates the interfaces through which humans and computers interact, and software engineering focuses on the design and principles behind developing software. Areas such as operating systems, networks and embedded systems investigate the principles and design behind complex systems. Computer architecture describes the construction of computer components and computer-operated equipment. Artificial intelligence and machine learning aim to synthesize goal-orientated processes such as problem-solving, decision-making, environmental adaptation, planning and learning found in humans and animals. Within artificial intelligence, computer vision aims to understand and process image and video data, while natural language processing aims to understand and process textual and linguistic data."
+6122,The fundamental concern of computer science is determining what can and cannot be automated. The Turing Award is generally recognized as the highest distinction in computer science.
+6123,"The earliest foundations of what would become computer science predate the invention of the modern digital computer. Machines for calculating fixed numerical tasks such as the abacus have existed since antiquity, aiding in computations such as multiplication and division. Algorithms for performing computations have existed since antiquity, even before the development of sophisticated computing equipment."
+6124,"Wilhelm Schickard designed and constructed the first working mechanical calculator in 1623. In 1673, Gottfried Leibniz demonstrated a digital mechanical calculator, called the Stepped Reckoner. Leibniz may be considered the first computer scientist and information theorist, because of various reasons, including the fact that he documented the binary number system. In 1820, Thomas de Colmar launched the mechanical calculator industry when he invented his simplified arithmometer, the first calculating machine strong enough and reliable enough to be used daily in an office environment. Charles Babbage started the design of the first automatic mechanical calculator, his Difference Engine, in 1822, which eventually gave him the idea of the first programmable mechanical calculator, his Analytical Engine. He started developing this machine in 1834, and ""in less than two years, he had sketched out many of the salient features of the modern computer"". ""A crucial step was the adoption of a punched card system derived from the Jacquard loom"" making it infinitely programmable. In 1843, during the translation of a French article on the Analytical Engine, Ada Lovelace wrote, in one of the many notes she included, an algorithm to compute the Bernoulli numbers, which is considered to be the first published algorithm ever specifically tailored for implementation on a computer. Around 1885, Herman Hollerith invented the tabulator, which used punched cards to process statistical information; eventually his company became part of IBM. Following Babbage, although unaware of his earlier work, Percy Ludgate in 1909 published the 2nd of the only two designs for mechanical analytical engines in history. In 1914, the Spanish engineer Leonardo Torres Quevedo published his Essays on Automatics, and designed, inspired by Babbage, a theoretical electromechanical calculating machine which was to be controlled by a read-only program. The paper also introduced the idea of floating-point arithmetic. In 1920, to celebrate the 100th anniversary of the invention of the arithmometer, Torres presented in Paris the Electromechanical Arithmometer, a prototype that demonstrated the feasibility of an electromechanical analytical engine, on which commands could be typed and the results printed automatically. In 1937, one hundred years after Babbage's impossible dream, Howard Aiken convinced IBM, which was making all kinds of punched card equipment and was also in the calculator business to develop his giant programmable calculator, the ASCC/Harvard Mark I, based on Babbage's Analytical Engine, which itself used cards and a central computing unit. When the machine was finished, some hailed it as ""Babbage's dream come true""."
+6125,"During the 1940s, with the development of new and more powerful computing machines such as the Atanasoff–Berry computer and ENIAC, the term computer came to refer to the machines rather than their human predecessors. As it became clear that computers could be used for more than just mathematical calculations, the field of computer science broadened to study computation in general. In 1945, IBM founded the Watson Scientific Computing Laboratory at Columbia University in New York City. The renovated fraternity house on Manhattan's West Side was IBM's first laboratory devoted to pure science. The lab is the forerunner of IBM's Research Division, which today operates research facilities around the world. Ultimately, the close relationship between IBM and Columbia University was instrumental in the emergence of a new scientific discipline, with Columbia offering one of the first academic-credit courses in computer science in 1946. Computer science began to be established as a distinct academic discipline in the 1950s and early 1960s. The world's first computer science degree program, the Cambridge Diploma in Computer Science, began at the University of Cambridge Computer Laboratory in 1953. The first computer science department in the United States was formed at Purdue University in 1962. Since practical computers became available, many applications of computing have become distinct areas of study in their own rights."
+6126,"Although first proposed in 1956, the term ""computer science"" appears in a 1959 article in Communications of the ACM,
+in which Louis Fein argues for the creation of a Graduate School in Computer Sciences analogous to the creation of Harvard Business School in 1921. Louis justifies the name by arguing that, like management science, the subject is applied and interdisciplinary in nature, while having the characteristics typical of an academic discipline.
+His efforts, and those of others such as numerical analyst George Forsythe, were rewarded: universities went on to create such departments, starting with Purdue in 1962. Despite its name, a significant amount of computer science does not involve the study of computers themselves. Because of this, several alternative names have been proposed. Certain departments of major universities prefer the term computing science, to emphasize precisely that difference. Danish scientist Peter Naur suggested the term datalogy, to reflect the fact that the scientific discipline revolves around data and data treatment, while not necessarily involving computers. The first scientific institution to use the term was the Department of Datalogy at the University of Copenhagen, founded in 1969, with Peter Naur being the first professor in datalogy. The term is used mainly in the Scandinavian countries. An alternative term, also proposed by Naur, is data science; this is now used for a multi-disciplinary field of data analysis, including statistics and databases."
+6127,"In the early days of computing, a number of terms for the practitioners of the field of computing were suggested in the Communications of the ACM—turingineer, turologist, flow-charts-man, applied meta-mathematician, and applied epistemologist. Three months later in the same journal, comptologist was suggested, followed next year by hypologist. The term computics has also been suggested. In Europe, terms derived from contracted translations of the expression ""automatic information""  or ""information and mathematics"" are often used, e.g. informatique , Informatik , informatica , informática , informatika  or pliroforiki  in Greek. Similar words have also been adopted in the UK . ""In the U.S., however, informatics is linked with applied computing, or computing in the context of another domain."""
+6128,"A folkloric quotation, often attributed to—but almost certainly not first formulated by—Edsger Dijkstra, states that ""computer science is no more about computers than astronomy is about telescopes."" The design and deployment of computers and computer systems is generally considered the province of disciplines other than computer science. For example, the study of computer hardware is usually considered part of computer engineering, while the study of commercial computer systems and their deployment is often called information technology or information systems. However, there has been exchange of ideas between the various computer-related disciplines. Computer science research also often intersects other disciplines, such as cognitive science, linguistics, mathematics, physics, biology, Earth science, statistics, philosophy, and logic."
+6129,"Computer science is considered by some to have a much closer relationship with mathematics than many scientific disciplines, with some observers saying that computing is a mathematical science. Early computer science was strongly influenced by the work of mathematicians such as Kurt Gödel, Alan Turing, John von Neumann, Rózsa Péter and Alonzo Church and there continues to be a useful interchange of ideas between the two fields in areas such as mathematical logic, category theory, domain theory, and algebra."
+6130,"The relationship between computer science and software engineering is a contentious issue, which is further muddied by disputes over what the term ""software engineering"" means, and how computer science is defined. David Parnas, taking a cue from the relationship between other engineering and science disciplines, has claimed that the principal focus of computer science is studying the properties of computation in general, while the principal focus of software engineering is the design of specific computations to achieve practical goals, making the two separate but complementary disciplines."
+6131,"The academic, political, and funding aspects of computer science tend to depend on whether a department is formed with a mathematical emphasis or with an engineering emphasis. Computer science departments with a mathematics emphasis and with a numerical orientation consider alignment with computational science. Both types of departments tend to make efforts to bridge the field educationally if not across all research."
+6132,"Despite the word science in its name, there is debate over whether or not computer science is a discipline of science, mathematics, or engineering. Allen Newell and Herbert A. Simon argued in 1975,"
+6133,"It has since been argued that computer science can be classified as an empirical science since it makes use of empirical testing to evaluate the correctness of programs, but a problem remains in defining the laws and theorems of computer science  and defining the nature of experiments in computer science. Proponents of classifying computer science as an engineering discipline argue that the reliability of computational systems is investigated in the same way as bridges in civil engineering and airplanes in aerospace engineering. They also argue that while empirical sciences observe what presently exists, computer science observes what is possible to exist and while scientists discover laws from observation, no proper laws have been found in computer science and it is instead concerned with creating phenomena."
+6134,Proponents of classifying computer science as a mathematical discipline argue that computer programs are physical realizations of mathematical entities and programs can be deductively reasoned through mathematical formal methods. Computer scientists Edsger W. Dijkstra and Tony Hoare regard instructions for computer programs as mathematical sentences and interpret formal semantics for programming languages as mathematical axiomatic systems.
+6135,"A number of computer scientists have argued for the distinction of three separate paradigms in computer science. Peter Wegner argued that those paradigms are science, technology, and mathematics. Peter Denning's working group argued that they are theory, abstraction , and design. Amnon H. Eden described them as the ""rationalist paradigm"" , the ""technocratic paradigm"" , and the ""scientific paradigm"" .
+Computer science focuses on methods involved in design, specification, programming, verification, implementation and testing of human-made computing systems."
+6136,"As a discipline, computer science spans a range of topics from theoretical studies of algorithms and the limits of computation to the practical issues of implementing computing systems in hardware and software.
+CSAB, formerly called Computing Sciences Accreditation Board—which is made up of representatives of the Association for Computing Machinery , and the IEEE Computer Society —identifies four areas that it considers crucial to the discipline of computer science: theory of computation, algorithms and data structures, programming methodology and languages, and computer elements and architecture. In addition to these four areas, CSAB also identifies fields such as software engineering, artificial intelligence, computer networking and communication, database systems, parallel computation, distributed computation, human–computer interaction, computer graphics, operating systems, and numerical and symbolic computation as being important areas of computer science."
+6137,"Theoretical Computer Science is mathematical and abstract in spirit, but it derives its motivation from the practical and everyday computation. Its aim is to understand the nature of computation and, as a consequence of this understanding, provide more efficient methodologies."
+6138,"According to Peter Denning, the fundamental question underlying computer science is, ""What can be automated?"" Theory of computation is focused on answering fundamental questions about what can be computed and what amount of resources are required to perform those computations. In an effort to answer the first question, computability theory examines which computational problems are solvable on various theoretical models of computation. The second question is addressed by computational complexity theory, which studies the time and space costs associated with different approaches to solving a multitude of computational problems."
+6139,"The famous P = NP? problem, one of the Millennium Prize Problems, is an open problem in the theory of computation."
+6140,"Information theory, closely related to probability and statistics, is related to the quantification of information. This was developed by Claude Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and communicating data.
+Coding theory is the study of the properties of codes  and their fitness for a specific application. Codes are used for data compression, cryptography, error detection and correction, and more recently also for network coding. Codes are studied for the purpose of designing efficient and reliable data transmission methods.
+"
+6141,Data structures and algorithms are the studies of commonly used computational methods and their computational efficiency.
+6142,"Programming language theory is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of programming languages and their individual features. It falls within the discipline of computer science, both depending on and affecting mathematics, software engineering, and linguistics. It is an active research area, with numerous dedicated academic journals."
+6143,"Formal methods are a particular kind of mathematically based technique for the specification, development and verification of software and hardware systems. The use of formal methods for software and hardware design is motivated by the expectation that, as in other engineering disciplines, performing appropriate mathematical analysis can contribute to the reliability and robustness of a design. They form an important theoretical underpinning for software engineering, especially where safety or security is involved. Formal methods are a useful adjunct to software testing since they help avoid errors and can also give a framework for testing. For industrial use, tool support is required. However, the high cost of using formal methods means that they are usually only used in the development of high-integrity and life-critical systems, where safety or security is of utmost importance. Formal methods are best described as the application of a fairly broad variety of theoretical computer science fundamentals, in particular logic calculi, formal languages, automata theory, and program semantics, but also type systems and algebraic data types to problems in software and hardware specification and verification."
+6144,"Computer graphics is the study of digital visual contents and involves the synthesis and manipulation of image data. The study is connected to many other fields in computer science, including computer vision, image processing, and computational geometry, and is heavily applied in the fields of special effects and video games."
+6145,"Information can take the form of images, sound, video or other multimedia. Bits of information can be streamed via signals. Its processing is the central notion of informatics, the European view on computing, which studies information processing algorithms independently of the type of information carrier – whether it is electrical, mechanical or biological. This field plays important role in information theory, telecommunications, information engineering and has applications in medical image computing and speech synthesis, among others. What is the lower bound on the complexity of fast Fourier transform algorithms? is one of unsolved problems in theoretical computer science."
+6146,"Scientific computing  is the field of study concerned with constructing mathematical models and quantitative analysis techniques and using computers to analyze and solve scientific problems. A major usage of scientific computing is simulation of various processes, including computational fluid dynamics, physical, electrical, and electronic systems and circuits, as well as societies and social situations  along with their habitats, among many others. Modern computers enable optimization of such designs as complete aircraft. Notable in electrical and electronic circuit design are SPICE, as well as software for physical realization of new  designs. The latter includes essential design software for integrated circuits."
+6147,"Social computing is an area that is concerned with the intersection of social behavior and computational systems. Human–computer interaction research develops theories, principles, and guidelines for user interface designers."
+6148,"Software engineering is the study of designing, implementing, and modifying the software in order to ensure it is of high quality, affordable, maintainable, and fast to build. It is a systematic approach to software design, involving the application of engineering practices to software. Software engineering deals with the organizing and analyzing of software—it does not just deal with the creation or manufacture of new software, but its internal arrangement and maintenance. For example software testing, systems engineering, technical debt and software development processes."
+6149,"Artificial intelligence  aims to or is required to synthesize goal-orientated processes such as problem-solving, decision-making, environmental adaptation, learning, and communication found in humans and animals. From its origins in cybernetics and in the Dartmouth Conference , artificial intelligence research has been necessarily cross-disciplinary, drawing on areas of expertise such as applied mathematics, symbolic logic, semiotics, electrical engineering, philosophy of mind, neurophysiology, and social intelligence. AI is associated in the popular mind with robotic development, but the main field of practical application has been as an embedded component in areas of software development, which require computational understanding. The starting point in the late 1940s was Alan Turing's question ""Can computers think?"", and the question remains effectively unanswered, although the Turing test is still used to assess computer output on the scale of human intelligence. But the automation of evaluative and predictive tasks has been increasingly successful as a substitute for human monitoring and intervention in domains of computer application involving complex real-world data."
+6150,"Computer architecture, or digital computer organization, is the conceptual design and fundamental operational structure of a computer system. It focuses largely on the way by which the central processing unit performs internally and accesses addresses in memory. Computer engineers study computational logic and design of computer hardware, from individual processor components, microcontrollers, personal computers to supercomputers and embedded systems. The term ""architecture"" in computer literature can be traced to the work of Lyle R. Johnson and Frederick P. Brooks Jr., members of the Machine Organization department in IBM's main research center in 1959."
+6151,"Concurrency is a property of systems in which several computations are executing simultaneously, and potentially interacting with each other. A number of mathematical models have been developed for general concurrent computation including Petri nets, process calculi and the Parallel Random Access Machine model. When multiple computers are connected in a network while using concurrency, this is known as a distributed system. Computers within that distributed system have their own private memory, and information can be exchanged to achieve common goals."
+6152,This branch of computer science aims to manage networks between computers worldwide.
+6153,"Computer security is a branch of computer technology with the objective of protecting information from unauthorized access, disruption, or modification while maintaining the accessibility and usability of the system for its intended users."
+6154,"Historical cryptography is the art of writing and deciphering secret messages. Modern cryptography is the scientific study of problems relating to distributed computations that can be attacked. Technologies studied in modern cryptography include symmetric and asymmetric encryption, digital signatures, cryptographic hash functions, key-agreement protocols, blockchain, zero-knowledge proofs, and garbled circuits."
+6155,"A database is intended to organize, store, and retrieve large amounts of data easily. Digital databases are managed using database management systems to store, create, maintain, and search data, through database models and query languages. Data mining is a process of discovering patterns in large data sets."
+6156,The philosopher of computing Bill Rapaport noted three Great Insights of Computer Science:
+6157,"Gottfried Wilhelm Leibniz's, George Boole's, Alan Turing's, Claude Shannon's, and Samuel Morse's insight: there are only two objects that a computer has to deal with in order to represent ""anything""."
+6158,All the information about any computable problem can be represented using only 0 and 1 .
+6159,"Alan Turing's insight: there are only five actions that a computer has to perform in order to do ""anything""."
+6160,five actions
+6161,"Every algorithm can be expressed in a language for a computer consisting of only five basic instructions:
+move left one location;
+move right one location;
+read symbol at current location;
+print 0 at current location;
+print 1 at current location."
+6162,"Corrado Böhm and Giuseppe Jacopini's insight: there are only three ways of combining these actions  that are needed in order for a computer to do ""anything""."
+6163,"Only three rules are needed to combine any set of basic instructions into more complex ones:
+sequence: first do this, then do that;
+ selection: IF such-and-such is the case, THEN do this, ELSE do that;
+repetition: WHILE such-and-such is the case, DO this."
+6164,Programming languages can be used to accomplish different tasks in different ways. Common programming paradigms include:
+6165,"Functional programming, a style of building the structure and elements of computer programs that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions or declarations instead of statements."
+6166,"Imperative programming, a programming paradigm that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program consists of commands for the computer to perform. Imperative programming focuses on describing how a program operates."
+6167,"Object-oriented programming, a programming paradigm based on the concept of ""objects"", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods. A feature of objects is that an object's procedures can access and often modify the data fields of the object with which they are associated. Thus object-oriented computer programs are made out of objects that interact with one another."
+6168,"Service-oriented programming, a programming paradigm that uses ""services"" as the unit of computer work, to design and implement integrated business applications and mission critical software programs"
+6169,"Many languages offer support for multiple paradigms, making the distinction more a matter of style than of technical capabilities."
+6170,"Conferences are important events for computer science research. During these conferences, researchers from the public and private sectors present their recent work and meet. Unlike in most other academic fields, in computer science, the prestige of conference papers is greater than that of journal publications. One proposed explanation for this is the quick development of this relatively new field requires rapid review and distribution of results, a task better handled by conferences than by journals."
+6171,"Computer Science, known by its near synonyms, Computing, Computer Studies, has been taught in UK schools since the days of batch processing, mark sensitive cards and paper tape but usually to a select few students. In 1981, the BBC produced a micro-computer and classroom network and Computer Studies became common for GCE O level students , and Computer Science to A level students. Its importance was recognised, and it became a compulsory part of the National Curriculum, for Key Stage 3 & 4. In September 2014 it became an entitlement for all pupils over the age of 4."
+6172,"In the US, with 14,000 school districts deciding the curriculum, provision was fractured. According to a 2010 report by the Association for Computing Machinery  and Computer Science Teachers Association , only 14 out of 50 states have adopted significant education standards for high school computer science. According to a 2021 report, only 51% of high schools in the US offer computer science."
+6173,"Israel, New Zealand, and South Korea have included computer science in their national secondary education curricula, and several others are following."
+6174,DBLP Computer Science Bibliography
+6175,Association for Computing Machinery
+6176,Institute of Electrical and Electronics Engineers
+6177,"CSTA has more than 50 chapters in the United States and international affiliates in Israel, New Zealand, and the United Kingdom."
+6178,"CSTA publishes a monthly newsletter, the CSTA Voice, that highlights issues related to computer science education."
+6179,The 2022-23 CSTA Board of Directors consists of:
+6180,Dan Blier
+6181,"Board Chair, School District Representative "
+6182,Nimmi Arunachalam
+6183,"At Large Representative, "
+6184,Greg Bianchi
+6185,"Partner Representative, Microsoft "
+6186,Cindi Chang
+6187,State Department Representative 
+6188,Charity Freeman
+6189,"Board Chair-Elect, Teacher Education Representative "
+6190,Michelle Friend
+6191,At-Large Representative 
+6192,Abigail Joseph
+6193,K-8 Teacher Representative 
+6194,Audra Kaplan
+6195,K-8 Representative 
+6196,Richard Ladner
+6197,University Representative 
+6198,Art Lopez
+6199,9–12 Representative 
+6200,Yolanda Lozano
+6201,"9-12 Teacher Representative, "
+6202,Andrew McGettrick
+6203,"ACM Representative, "
+6204,Michelle Page
+6205,CSTA Committees include:
+6206,"The United States of America , commonly known as the United States  or America, is a country primarily located in North America. It is a federation of 50 states, a federal capital district , and 326 Indian reservations. Outside the union of states, it asserts sovereignty over five major unincorporated island territories and various uninhabited islands. The country has the world's third-largest land area, second-largest exclusive economic zone, and third-largest population, exceeding 334 million."
+6207,"Paleo-Indians migrated across the Bering land bridge more than 12,000 years ago. British colonization led to the first settlement of the Thirteen Colonies in Virginia in 1607. Clashes with the British Crown over taxation and political representation sparked the American Revolution, with the Second Continental Congress formally declaring independence on July 4, 1776. Following its victory in the Revolutionary War , the country continued to expand across North America. As more states were admitted, sectional division over slavery led to the secession of the Confederate States of America, which fought the remaining states of the Union during the 1861–1865 American Civil War. With the Union's victory and preservation, slavery was abolished nationally. By 1890, the United States had established itself as a great power. After Japan's attack on Pearl Harbor in December 1941, the U.S. entered World War II. The aftermath of the war left the U.S. and the Soviet Union as the world's two superpowers and led to the Cold War, during which both countries engaged in a struggle for ideological dominance and international influence. Following the Soviet Union's collapse and the end of the Cold War in 1991, the U.S. emerged as the world's sole superpower."
+6208,"The U.S. national government is a presidential constitutional republic and liberal democracy with three separate branches: legislative, executive, and judicial. It has a bicameral national legislature composed of the House of Representatives, a lower house based on population; and the Senate, an upper house based on equal representation for each state. Substantial autonomy is given to states and several territories, with a political culture that emphasizes liberty, equality under the law, individualism, and limited government."
+6209,"One of the world's most developed countries, the United States has had the largest nominal GDP since 1890 and accounted for 15% of the global economy in 2023. It possesses by far the largest amount of wealth of any country and the highest median income per capita of any non-microstate. The U.S. ranks among the world's highest in economic competitiveness, productivity, innovation, human rights, and higher education. Its hard power and cultural influence have a global reach. The U.S. is a founding member of the World Bank, IMF, Organization of American States, NATO, World Health Organization, and a permanent member of the UN Security Council."
+6210,"The first documentary evidence of the phrase ""United States of America"" dates back to a letter from January 2, 1776, written by Stephen Moylan, a Continental Army aide to General George Washington, to Joseph Reed, Washington's aide-de-camp. Moylan expressed his desire to go ""with full and ample powers from the United States of America to Spain"" to seek assistance in the Revolutionary War effort. The first known publication of the phrase ""United States of America"" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, on April 6, 1776."
+6211,"By June 1776, the name ""United States of America"" appeared in drafts of the Articles of Confederation and Perpetual Union, authored by John Dickinson, a Founding Father from the Province of Pennsylvania, and in the Declaration of Independence, written primarily by Thomas Jefferson and adopted by the Second Continental Congress in Philadelphia, on July 4, 1776."
+6212,"The first inhabitants of North America migrated from Siberia across the Bering land bridge at least 12,000 years ago; the Clovis culture, which appeared around 11,000 BC, is believed to be the first widespread culture in the Americas. Over time, indigenous North American cultures grew increasingly sophisticated, and some, such as the Mississippian culture, developed agriculture, architecture, and complex societies. Indigenous peoples and cultures such as the Algonquian peoples, Ancestral Puebloans, and the Iroquois developed across the present-day United States. Native population estimates of what is now the United States before the arrival of European immigrants range from around 500,000 to nearly 10 million."
+6213,"Christopher Columbus began exploring the Caribbean in 1492, leading to Spanish settlements in present-day Puerto Rico, Florida, and New Mexico. France established its own settlements along the Mississippi River and Gulf of Mexico. British colonization of the East Coast began with the Virginia Colony  and Plymouth Colony . The Mayflower Compact and the Fundamental Orders of Connecticut established precedents for representative self-governance and constitutionalism that would develop throughout the American colonies. While European settlers in what is now the United States experienced conflicts with Native Americans, they also engaged in trade, exchanging European tools for food and animal pelts. Relations ranged from close cooperation to warfare and massacres. The colonial authorities often pursued policies that forced Native Americans to adopt European lifestyles, including conversion to Christianity. Along the eastern seaboard, settlers trafficked African slaves through the Atlantic slave trade."
+6214,"The original Thirteen Colonies that would later found the United States were administered by Great Britain, and had local governments with elections open to most white male property owners. The colonial population grew rapidly, eclipsing Native American populations; by the 1770s, the natural increase of the population was such that only a small minority of Americans had been born overseas. The colonies' distance from Britain allowed for the development of self-governance, and the First Great Awakening—a series of Christian revivals—fueled colonial interest in religious liberty."
+6215,"After winning the French and Indian War, Britain began to assert greater control over local colonial affairs, creating colonial political resistance; one of the primary colonial grievances was a denial of their rights as Englishmen, particularly the right to representation in the British government that taxed them. In 1774, the First Continental Congress met in Philadelphia, and passed a colonial boycott of British goods that proved effective. The British attempt to then disarm the colonists resulted in the 1775 Battles of Lexington and Concord, igniting the American Revolutionary War. At the Second Continental Congress, the colonies appointed George Washington commander-in-chief of the Continental Army and created a committee led by Thomas Jefferson to write the Declaration of Independence, adopted on July 4, 1776. The political values of the American Revolution included liberty, inalienable individual rights; and the sovereignty of the people; supporting republicanism and rejecting monarchy, aristocracy, and hereditary political power; virtue and faithfulness in the performance of civic duties; and vilification of corruption. The Founding Fathers of the United States, which included George Washington, Benjamin Franklin, Alexander Hamilton, Thomas Jefferson, John Jay, James Madison, Thomas Paine, and John Adams, took inspiration from Ancient Greco-Roman, Renaissance, and Age of Enlightenment philosophies and ideas."
+6216,"After the British surrender at the siege of Yorktown in 1781, American sovereignty was internationally recognized by the Treaty of Paris , through which the U.S. gained territory stretching west to the Mississippi River, north to present-day Canada, and south to Spanish Florida. Ratified in 1781, the Articles of Confederation established a decentralized government that operated until 1789. The Northwest Ordinance  established the precedent by which the country's territory would expand with the admission of new states, rather than the expansion of existing states. The U.S. Constitution was drafted at the 1787 Constitutional Convention to overcome the limitations of the Articles; it went into effect in 1789, creating a federation administered by three branches on the principle of checks and balances. Washington was elected the country's first president under the Constitution, and the Bill of Rights was adopted in 1791 to allay concerns by skeptics of the more centralized government; his resignations first as commander-in-chief after the Revolution and later as president set a precedent followed by John Adams, establishing the peaceful transfer of power between rival parties."
+6217,"In the late 18th century, American settlers began to expand westward, some with a sense of manifest destiny. The Louisiana Purchase  from France nearly doubled the territory of the United States. Lingering issues with Britain remained, leading to the War of 1812, which was fought to a draw. Spain ceded Florida and its Gulf Coast territory in 1819. The Missouri Compromise attempted to balance desires of northern states to prevent expansion of slavery in the country with those of southern states to expand it, admitting Missouri as a slave state and Maine as a free state and declared a policy of prohibiting slavery in the remaining Louisiana Purchase lands north of the 36°30′ parallel. As Americans expanded further into land inhabited by Native Americans, the federal government often applied policies of Indian removal or assimilation. The infamous Trail of Tears  was a U.S.
+government policy that forcibly removed and displaced most Native Americans living east of the Mississippi River to lands far to the west. These and earlier organized displacements prompted a long series of American Indian Wars west of the Mississippi. The Republic of Texas was annexed in 1845, and the 1846 Oregon Treaty led to U.S. control of the present-day American Northwest. Victory in the Mexican–American War resulted in the 1848 Mexican Cession of California and much of the present-day American Southwest. The California Gold Rush of 1848–1849 spurred a huge migration of white settlers to the Pacific coast, leading to even more confrontations with Native populations. One of the most violent, the California genocide of thousands of Native inhabitants, lasted into the early 1870s, just as additional western territories and states were created."
+6218,"During the colonial period, slavery was legal in the American colonies, though the practice began to be significantly questioned during the American Revolution. States in The North enacted abolition laws, though support for slavery strengthened in Southern states, as inventions such as the cotton gin made the institution increasingly profitable for Southern elites. This sectional conflict regarding slavery culminated in the American Civil War ."
+6219,"Eleven slave states seceded and formed the Confederate States of America, while the other states remained in the Union. War broke out in April 1861 after the Confederacy bombarded Fort Sumter. After the January 1863 Emancipation Proclamation, many freed slaves joined the Union Army. The war began to turn in the Union's favor following the 1863 Siege of Vicksburg and Battle of Gettysburg, and the Confederacy surrendered in 1865 after the Union's victory in the Battle of Appomattox Court House."
+6220,"The Reconstruction era followed the war. After the assassination of President Abraham Lincoln, Reconstruction Amendments were passed to protect the rights of African Americans. National infrastructure, including transcontinental telegraph and railroads, spurred growth in the American frontier."
+6221,"From 1865 through 1917 an unprecedented stream of immigrants arrived in the United States, including 24.4 million from Europe. Most came through the port of New York City, and New York City and other large cities on the East Coast became home to large Jewish, Irish, and Italian populations, while many Germans and Central Europeans moved to the Midwest. At the same time, about one million French Canadians migrated from Quebec to New England. During the Great Migration, millions of African Americans left the rural South for urban areas in the North. Alaska was purchased from Russia in 1867."
+6222,"The Compromise of 1877 effectively ended Reconstruction and white supremacists took local control of Southern politics. African Americans endured a period of heightened, overt racism following Reconstruction, a time often called the nadir of American race relations. A series of Supreme Court decisions, including Plessy v. Ferguson, emptied the Fourteenth and Fifteenth Amendments of their force, allowing Jim Crow laws in the South to remain unchecked, sundown towns in the Midwest, and segregation in cities across the country, which would be reinforced by the policy of redlining later adopted by the federal Home Owners' Loan Corporation."
+6223,"An explosion of technological advancement accompanied by the exploitation of cheap immigrant labor led to rapid economic development during the late 19th and early 20th centuries, allowing the United States to outpace England, France, and Germany combined. This fostered the amassing of power by a few prominent industrialists, largely by their formation of trusts and monopolies to prevent competition. Tycoons led the nation's expansion in the railroad, petroleum, and steel industries. The United States emerged as a pioneer of the automotive industry. These changes were accompanied by significant increases in economic inequality, slum conditions, and social unrest, creating the environment for labor unions to begin to flourish. This period eventually ended with the advent of the Progressive Era, which was characterized by significant reforms."
+6224,"Pro-American elements in Hawaii overthrew the Hawaiian monarchy; the islands were annexed in 1898. Puerto Rico, Guam, and the Philippines were ceded by Spain following the Spanish–American War. American Samoa was acquired by the United States in 1900 after the Second Samoan Civil War. The U.S. Virgin Islands were purchased from Denmark in 1917. The United States entered World War I alongside the Allies of World War I, helping to turn the tide against the Central Powers. In 1920, a constitutional amendment granted nationwide women's suffrage. During the 1920s and 30s, radio for mass communication and the invention of early television transformed communications nationwide. The Wall Street Crash of 1929 triggered the Great Depression, which President Franklin D. Roosevelt responded to with New Deal social and economic policies."
+6225,"At first neutral during World War II, the U.S. began supplying war materiel to the Allies of World War II in March 1941 and entered the war in December after the Empire of Japan's attack on Pearl Harbor. The U.S. developed the first nuclear weapons and used them against the Japanese cities of Hiroshima and Nagasaki in August 1945, ending the war. The United States was one of the ""Four Policemen"" who met to plan the postwar world, alongside the United Kingdom, Soviet Union, and China. The U.S. emerged relatively unscathed from the war, with even greater economic and international political influence."
+6226,"After World War II, the United States entered the Cold War, where geopolitical tensions between the U.S. and the Soviet Union led the two countries to dominate world affairs. The U.S. engaged in regime change against governments perceived to be aligned with the Soviet Union, and competed in the Space Race, culminating in the first crewed Moon landing in 1969."
+6227,"Domestically, the U.S. experienced economic growth, urbanization, and population growth following World War II. The civil rights movement emerged, with Martin Luther King Jr. becoming a prominent leader in the early 1960s. The Great Society plan of President Lyndon Johnson's administration resulted in groundbreaking and broad-reaching laws, policies and a constitutional amendment to counteract some of the worst effects of lingering institutional racism. The counterculture movement in the U.S. brought significant social changes, including the liberalization of attitudes toward recreational drug use and sexuality. It also encouraged open defiance of the military draft  and wide opposition to U.S. intervention in Vietnam . The societal shift in the roles of women partly resulted in large increases in female labor participation in the 1970s, and by 1985 the majority of women aged 16 and older were employed. The late 1980s and early 1990s saw the collapse of the Warsaw Pact and the dissolution of the Soviet Union, which marked the end of the Cold War and solidified the U.S. as the world's sole superpower."
+6228,"The 1990s saw the longest recorded economic expansion in American history, a dramatic decline in crime, and advances in technology, with the World Wide Web, the evolution of the Pentium microprocessor in accordance with Moore's law, rechargeable lithium-ion batteries, the first gene therapy trial, and cloning all emerging and being improved upon throughout the decade. The Human Genome Project was formally launched in 1990, while Nasdaq became the first stock market in the United States to trade online in 1998. In 1991, an American-led international coalition of states expelled an Iraqi invasion force from Kuwait in the Gulf War."
+6229,"The September 11, 2001 attacks by the pan-Islamist militant organization Al-Qaeda led to the war on terror and subsequent military interventions in Afghanistan and Iraq. The cultural impact of the attacks was profound and long-lasting."
+6230,"The U.S. housing bubble culminated in 2007 with the Great Recession, the largest economic contraction since the Great Depression. Coming to a head in the 2010s, political polarization increased as sociopolitical debates on cultural issues dominated politics. This polarization was capitalized upon in the January 2021 Capitol attack, when a mob of protesters entered the U.S. Capitol building and attempted to prevent the peaceful transfer of power."
+6231,"The United States is the world's third-largest country by total area behind Russia and Canada. The 48 contiguous states and the District of Columbia occupy a combined area of 3,119,885 square miles . The coastal plain of the Atlantic seaboard gives way to inland forests and rolling hills in the Piedmont plateau region."
+6232,"The Appalachian Mountains and the Adirondack massif separate the East Coast from the Great Lakes and the grasslands of the Midwest. The Mississippi River System—the world's fourth longest river system—runs mainly north–south through the heart of the country. The flat, fertile prairie of the Great Plains stretches to the west, interrupted by a highland region in the southeast."
+6233,"The Rocky Mountains, west of the Great Plains, extend north to south across the country, peaking at over 14,000 feet  in Colorado. Farther west are the rocky Great Basin and Chihuahua, Sonoran, and Mojave deserts. In the northwest corner of Arizona, carved by the Colorado River over millions of years, is the Grand Canyon, a steep-sided canyon and popular tourist destination known for its overwhelming visual size and intricate, colorful landscape."
+6234,"The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast. The lowest and highest points in the contiguous United States are in the state of California, about 84 miles  apart. At an elevation of 20,310 feet , Alaska's Denali is the highest peak in the country and continent. Active volcanoes are common throughout Alaska's Alexander and Aleutian Islands, and Hawaii consists of volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature. In 2021, the United States had 8% of global permanent meadows and pastures and 10% of cropland."
+6235,"With its large size and geographic variety, the United States includes most climate types. East of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The western Great Plains are semi-arid. Many mountainous areas of the American West have an alpine climate. The climate is arid in the Southwest, Mediterranean in coastal California, and oceanic in coastal Oregon, Washington, and southern Alaska. Most of Alaska is subarctic or polar. Hawaii and the southern tip of Florida are tropical, as well as its territories in the Caribbean and the Pacific."
+6236,"States bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes occur in the country, mainly in Tornado Alley. Overall, the United States receives more high-impact extreme weather incidents than any other country. Extreme weather became more frequent in the U.S. in the 21st century, with three times the number of reported heat waves as in the 1960s. In the American Southwest, droughts became more persistent and more severe."
+6237,"The U.S. is one of 17 megadiverse countries containing large numbers of endemic species: about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and over 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland. The United States is home to 428 mammal species, 784 birds, 311 reptiles, 295 amphibians, and 91,000 insect species."
+6238,"There are 63 national parks, and hundreds of other federally managed parks, forests, and wilderness areas, managed by the National Park Service and other agencies. About 28% of the country's land is publicly owned and federally managed, primarily in the western states. Most of this land is protected, though some is leased for commercial use, and less than one percent is used for military purposes."
+6239,"Environmental issues in the United States include debates on non-renewable resources and nuclear energy, air and water pollution, biodiversity, logging and deforestation, and climate change. The U.S. Environmental Protection Agency  is the federal agency charged with addressing most environmental-related issues. The idea of wilderness has shaped the management of public lands since 1964, with the Wilderness Act. The Endangered Species Act of 1973 provides a way to protect threatened and endangered species and their habitats. The United States Fish and Wildlife Service implements and enforces the Act. As of 2022, the U.S. ranked 43rd among 180 countries in the Environmental Performance Index. The country joined the Paris Agreement on climate change in 2016 and has many other environmental commitments."
+6240,"The United States is a federal republic of 50 states, with its capital in a federal district, asserting sovereignty over five unincorporated territories and several uninhabited island possessions . It is the world's oldest surviving federation, and, according to the World Economic Forum, the oldest democracy as well. It is a liberal representative democracy ""in which majority rule is tempered by minority rights protected by law."" The Constitution of the United States serves as the country's supreme legal document, also establishing the structure and responsibilities of the national federal government and its relationship with the individual states."
+6241,"Composed of three branches, all headquartered in Washington, D.C., the federal government is the national government of the United States. It is regulated by a strong system of checks and balances."
+6242,"The three-branch system is known as the presidential system, in contrast to the parliamentary system, where the executive is part of the legislative body. Many countries around the world copied this aspect of the 1789 Constitution of the United States, especially in the Americas."
+6243,"The Constitution is silent on political parties. However, they developed independently in the 18th century with the Federalist and Anti-Federalist parties. Since then, the United States has operated as a de facto two-party system, though the parties in that system have been different at different times."
+6244,"The two main national parties are presently the Democratic and the Republican. The former is perceived as relatively liberal in its political platform while the latter is perceived as relatively conservative. Each has a primary system to nominate a presidential ticket, and each runs candidates for other offices in every state in the Union. Other smaller and less influential parties exist but do not have the national scope and breadth of the two main parties."
+6245,"In the American federal system, sovereign powers are shared between two levels of elected government: national and state. People in the states are also represented by local elected governments, which are administrative divisions of the states. States are subdivided into counties or county equivalents, and further divided into municipalities. The District of Columbia is a federal district that contains the capital of the United States, the city of Washington. The territories and the District of Columbia are administrative divisions of the federal government. Federally recognized tribes govern 326 Indian reservations."
+6246,"The United States has an established structure of foreign relations, and it has the world's second-largest diplomatic corps as of 2024. It is a permanent member of the United Nations Security Council, and home to the United Nations headquarters. The United States is a member of the G7, G20, and OECD intergovernmental organizations. Almost all countries have embassies and many have consulates  in the country. Likewise, nearly all countries host formal diplomatic missions with the United States, except Iran, North Korea, and Bhutan. Though Taiwan does not have formal diplomatic relations with the U.S., it maintains close unofficial relations. The United States regularly supplies Taiwan with military equipment to deter potential Chinese aggression. Its geopolitical attention also turned to the Indo-Pacific when the United States joined the Quadrilateral Security Dialogue with Australia, India, and Japan."
+6247,"The United States has a ""Special Relationship"" with the United Kingdom and strong ties with Canada, Australia, New Zealand, the Philippines, Japan, South Korea, Israel, and several European Union countries . The U.S. works closely with its NATO allies on military and national security issues, and with countries in the Americas through the Organization of American States and the United States–Mexico–Canada Free Trade Agreement. In South America, Colombia is traditionally considered to be the closest ally of the United States. The U.S. exercises full international defense authority and responsibility for Micronesia, the Marshall Islands, and Palau through the Compact of Free Association. It has increasingly conducted strategic cooperation with India, but its ties with China have steadily deteriorated. Since 2014, the U.S. has become a key ally of Ukraine; it has also provided the country with significant military equipment and other support in response to Russia's 2022 invasion."
+6248,"The President is the commander-in-chief of the United States Armed Forces and appoints its leaders, the secretary of defense and the Joint Chiefs of Staff. The Department of Defense, which is headquartered at the Pentagon near Washington, D.C., administers five of the six service branches, which are made up of the Army, Marine Corps, Navy, Air Force, and Space Force. The Coast Guard is administered by the Department of Homeland Security in peacetime and can be transferred to the Department of the Navy in wartime."
+6249,"The United States spent $877 billion on its military in 2022, which is by far the largest amount of any country, making up 39% of global military spending and accounting for 3.5% of the country's GDP. The U.S. has 45% of the world's nuclear weapons, the second-largest amount after Russia."
+6250,"The United States has the third-largest combined armed forces in the world, behind the Chinese People's Liberation Army and Indian Armed Forces. The military operates about 800 bases and facilities abroad, and maintains deployments greater than 100 active duty personnel in 25 foreign countries."
+6251,"There are about 18,000 U.S. police agencies from local to national level in the United States. Law in the United States is mainly enforced by local police departments and sheriff departments in their municipal or county jurisdictions. The state police departments have authority in their respective state, and federal agencies such as the Federal Bureau of Investigation  and the U.S. Marshals Service have national jurisdiction and specialized duties, such as protecting civil rights, national security and enforcing U.S. federal courts' rulings and federal laws. State courts conduct most civil and criminal trials, and federal courts handle designated crimes and appeals of state court decisions."
+6252,"As of January 2023, the United States has the sixth highest per-capita incarceration rate in the world, at 531 people per 100,000; and the largest prison and jail population in the world with almost 2 million people incarcerated. An analysis of the World Health Organization Mortality Database from 2010 showed U.S. homicide rates ""were 7 times higher than in other high-income countries, driven by a gun homicide rate that was 25 times higher."""
+6253,"The U.S. has been the world's largest economy nominally since about 1890. The 2023 nominal U.S. gross domestic product  of $27 trillion was the largest in the world, constituting over 25% of the global economy or 15% at purchasing power parity . From 1983 to 2008, U.S. real compounded annual GDP growth was 3.3%, compared to a 2.3% weighted average for the rest of the Group of Seven. The country ranks first in the world by disposable income per capita and nominal GDP; second by GDP , after China; and ninth by GDP  per capita."
+6254,"Of the world's 500 largest companies, 136 are headquartered in the U.S. The U.S. dollar is the currency most used in international transactions and is the world's foremost reserve currency, backed by the country's dominant economy, its military, the petrodollar system, and its linked eurodollar and large U.S. treasuries market. Several countries use it as their official currency and in others it is the de facto currency. It has free trade agreements with several countries, including the USMCA. The U.S. ranked second in the Global Competitiveness Report in 2019, after Singapore. While its economy has reached a post-industrial level of development, the United States remains an industrial power. As of 2021, the U.S. is the second-largest manufacturing country after China."
+6255,"New York City is the world's principal financial center and the epicenter of the world's largest metropolitan economy. The New York Stock Exchange and Nasdaq, both located in New York City, are the world's two largest stock exchanges by market capitalization and trade volume. The United States is at or near the forefront of technological advancement and innovation in many economic fields, especially in artificial intelligence; computers; pharmaceuticals; and medical, aerospace and military equipment. The country's economy is fueled by abundant natural resources, a well-developed infrastructure, and high productivity. The largest U.S. trading partners are the European Union, Mexico, Canada, China, Japan, South Korea, the United Kingdom, Vietnam, India, and Taiwan. The United States is the world's largest importer and the second-largest exporter after China. It is by far the world's largest exporter of services."
+6256,"Americans have the highest average household and employee income among OECD member states, and the fourth-highest median household income, up from sixth-highest in 2013. Wealth in the United States is highly concentrated; the richest 10% of the adult population own 72% of the country's household wealth, while the bottom 50% own just 2%. Income inequality in the U.S. remains at record highs, with the top fifth of earners taking home more than half of all income and giving the U.S. one of the widest income distributions among OECD members. The U.S. ranks first in the number of dollar billionaires and millionaires, with 735 billionaires and nearly 22 million millionaires . There were about 582,500 sheltered and unsheltered homeless persons in the U.S. in 2022, with 60% staying in an emergency shelter or transitional housing program. In 2018, six million children experienced food insecurity. Feeding America estimates that around one in seven, or approximately 11 million, children experience hunger and do not know where they will get their next meal or when. As of 2021, 38 million people, about 12% of the U.S. population, were living in poverty."
+6257,"The United States has a smaller welfare state and redistributes less income through government action than most other high-income countries. It is the only advanced economy that does not guarantee its workers paid vacation nationally and is one of a few countries in the world without federal paid family leave as a legal right. The United States has a higher percentage of low-income workers than almost any other developed country, largely because of a weak collective bargaining system and lack of government support for at-risk workers."
+6258,"The United States has been a leader in technological innovation since the late 19th century and scientific research since the mid-20th century. Methods for producing interchangeable parts and the establishment of a machine tool industry enabled the large-scale manufacturing of U.S. consumer products in the late 19th century. By the early 20th century, factory electrification, the introduction of the assembly line, and other labor-saving techniques created the system of mass production. The United States is a leader in the development of artificial intelligence technology and has maintained a space program since the late 1950s, with plans for long-term habitation of the Moon."
+6259,"In 2022, the United States was the country with the second-highest number of published scientific papers. As of 2021, the U.S. ranked second by the number of patent applications, and third by trademark and industrial design applications. In 2023, the United States ranked 3rd in the Global Innovation Index."
+6260,"As of 2022, the United States receives approximately 81% of its energy from fossil fuel and the largest source of the country's energy came from petroleum , followed by natural gas , renewable sources , coal , and nuclear power . The United States constitutes less than 5% of the world's population, but consumes 17% of the world's energy. The U.S. ranks as the second-highest emitter of greenhouse gases."
+6261,"Personal transportation in the United States is dominated by automobiles, which operate on a network of 4 million miles  of public roads, making it the longest network in the world. The Oldsmobile Curved Dash and the Ford Model T, both American cars, are considered the first mass-produced and mass-affordable cars, respectively. As of 2022, the United States is the second-largest manufacturer of motor vehicles and is home to Tesla, the world's most valuable car company. American automotive company General Motors held the title of the world's best-selling automaker from 1931 to 2008. Currently, the American automotive industry is the world's second-largest automobile market by sales, and the U.S. has the highest vehicle ownership per capita in the world, with 910 vehicles per 1000 people. The United States's rail transport network, the longest network in the world, handles mostly freight."
+6262,"The American civil airline industry is entirely privately owned and has been largely deregulated since 1978, while most major airports are publicly owned. The three largest airlines in the world by passengers carried are U.S.-based; American Airlines is number one after its 2013 acquisition by US Airways. Of the world's 50 busiest passenger airports, 16 are in the United States, including the top five and the busiest, Hartsfield–Jackson Atlanta International Airport. As of 2022, there are 19,969 airports in the U.S., of which 5,193 are designated as ""public use"", including for general aviation and other activities."
+6263,"Of the fifty busiest container ports, four are located in the United States, of which the busiest is the Port of Los Angeles. The country's inland waterways are the world's fifth-longest, and total 41,009 km ."
+6264,"The U.S. Census Bureau reported 331,449,281 residents as of April 1, 2020, making the United States the third-most-populous country in the world, after China and India. According to the Bureau's U.S. Population Clock, on January 28, 2021, the U.S. population had a net gain of one person every 100 seconds, or about 864 people per day. In 2018, 52% of Americans age 15 and over were married, 6% were widowed, 10% were divorced, and 32% had never been married. In 2021, the total fertility rate for the U.S. stood at 1.7 children per woman, and it had the world's highest rate of children  living in single-parent households in 2019."
+6265,"The United States has a diverse population; 37 ancestry groups have more than one million members. White Americans with ancestry from Europe, the Middle East or North Africa, form the largest racial and ethnic group at 57.8% of the United States population. Hispanic and Latino Americans form the second-largest group and are 18.7% of the United States population. African Americans constitute the country's third-largest ancestry group and are 12.1% of the total U.S. population. Asian Americans are the country's fourth-largest group, composing 5.9% of the United States population. The country's 3.7 million Native Americans account for about 1%, and some 574 native tribes are  recognized by the federal government. In 2020, the median age of the United States population was 38.5 years."
+6266,"While many languages are spoken in the United States, English is by far the most commonly spoken and written. Although there is no official language at the federal level, some laws, such as U.S. naturalization requirements, standardize English, and most states have declared it the official language. Three states and four U.S. territories have recognized local or indigenous languages in addition to English, including Hawaii , Alaska , South Dakota , American Samoa , Puerto Rico , Guam , and the Northern Mariana Islands . In total, 169 Native American languages are spoken in the United States. In Puerto Rico, Spanish is more widely spoken than English."
+6267,"According to the American Community Survey in 2010, some 229 million people out of the total U.S. population of 308 million spoke only English at home. About 37 million spoke Spanish at home, making it the second most commonly used language. Other languages spoken at home by one million people or more include Chinese , Tagalog , Vietnamese , French , Korean , and German ."
+6268,"America's immigrant population, 51 million, is by far the world's largest in absolute terms. In 2022, there were 87.7 million immigrants and U.S.-born children of immigrants in the United States, accounting for nearly 27% of the overall U.S. population. In 2017, out of the U.S. foreign-born population, some 45%  were naturalized citizens, 27%  were lawful permanent residents, 6%  were temporary lawful residents, and 23%  were unauthorized immigrants. In 2019, the top countries of origin for immigrants were Mexico , India , China , the Philippines , and El Salvador . The United States has led the world in refugee resettlement for decades, admitting more refugees than the rest of the world combined."
+6269,"The First Amendment guarantees the free exercise of religion and forbids Congress from passing laws respecting its establishment. Religious practice is widespread, among the most diverse in the world, and profoundly vibrant. The country has the world's largest Christian population. A majority of the global Jewish population lives in the United States, as measured by the Law of Return. Other notable faiths include Buddhism, Hinduism, Islam, many New Age movements, and Native American religions. Religious practice varies significantly by region. ""Ceremonial deism"" is common in American culture."
+6270,"The overwhelming majority of Americans believe in a higher power or spiritual force, engage in spiritual practices such as prayer, and consider themselves religious or spiritual. In the ""Bible Belt"", located within the Southern United States, evangelical Protestantism plays a significant role culturally, whereas New England and the Western United States tend to be more secular. Mormonism—a Restorationist movement, whose members migrated westward from Missouri and Illinois under the leadership of Brigham Young in 1847 after the assassination of Joseph Smith—remains the predominant religion in Utah to this day."
+6271,"About 82% of Americans live in urban areas, including suburbs; about half of those reside in cities with populations over 50,000. In 2022, 333 incorporated municipalities had populations over 100,000, nine cities had more than one million residents, and four cities  had populations exceeding two million. Many U.S. metropolitan populations are growing rapidly, particularly in the South and West."
+6272,"According to the Centers for Disease Control , average American life expectancy at birth was 77.5 years in 2022 . This was a gain of 1.1 years from 76.4 years in 2021, but the CDC noted that the new average ""didn't fully offset the loss of 2.4 years between 2019 and 2021"". The COVID pandemic and higher overall mortality due to opioid overdoses and suicides were held mostly responsible for the previous drop in life expectancy. The same report stated that the 2022 gains in average U.S. life expectancy were especially significant for men, Hispanics, and American Indian–Alaskan Native people . Starting in 1998, the life expectancy in the U.S. fell behind that of other wealthy industrialized countries, and Americans' ""health disadvantage"" gap has been increasing ever since. The U.S. has one of the highest suicide rates among high-income countries. Approximately one-third of the U.S. adult population is obese and another third is overweight. The U.S. healthcare system far outspends that of any other country, measured both in per capita spending and as a percentage of GDP, but attains worse healthcare outcomes when compared to peer countries for reasons that are debated. The United States is the only developed country without a system of universal healthcare, and a significant proportion of the population that does not carry health insurance. Government-funded healthcare coverage for the poor  and for those age 65 and older  is available to Americans who meet the programs' income or age qualifications. In 2010, former President Obama passed the Patient Protection and Affordable Care Act."
+6273,"American primary and secondary education  is decentralized. It is operated by state, territorial, and sometimes municipal governments and regulated by the U.S. Department of Education. In general, children are required to attend school or an approved homeschool from the age of five or six  until they are 18 years old. This often brings students through the 12th grade, the final year of a U.S. high school, but some states and territories allow them to leave school earlier, at age 16 or 17. The U.S. spends more on education per student than any country in the world, an average of $12,794 per year per public elementary and secondary school student in 2016–2017. Among Americans age 25 and older, 84.6% graduated from high school, 52.6% attended some college, 27.2% earned a bachelor's degree, and 9.6% earned a graduate degree. The U.S. literacy rate is near-universal. The country has the most Nobel Prize winners in history, with 411 ."
+6274,"U.S. tertiary or higher education has earned a global reputation. Many of the world's top universities, as listed by various ranking organizations, are in the United States, including 19 of the top 25. American higher education is dominated by state university systems, although the country's many private universities and colleges enroll about 20% of all American students. Large amounts of federal financial aid are provided to students in the form of grants and loans."
+6275,"Colleges and universities directly funded by the federal government are limited to military personnel and government employees and include the U.S. service academies, the Naval Postgraduate School, and military staff colleges. Local community colleges generally offer coursework and degree programs covering the first two years of college study. They often have more open admission policies, shorter academic programs, and lower tuition."
+6276,"As for public expenditures on higher education, the U.S. spends more per student than the OECD average, and more than all nations in combined public and private spending. Despite some student loan forgiveness programs in place, student loan debt has increased by 102% in the last decade, and exceeded 1.7 trillion dollars as of 2022."
+6277,"Americans have traditionally been characterized by a unifying political belief in an ""American creed"" emphasizing liberty, equality under the law, democracy, social equality, property rights, and a preference for limited government. Culturally, the country has been described as having the values of individualism and personal autonomy, having a strong work ethic, competitiveness, and voluntary altruism towards others. According to a 2016 study by the Charities Aid Foundation, Americans donated 1.44% of total GDP to charity, the highest rate in the world by a large margin. The United States is home to a wide variety of ethnic groups, traditions, and values. It has acquired significant cultural and economic soft power."
+6278,"Nearly all present Americans or their ancestors came from Europe, Africa, and Asia  within the past five centuries. Mainstream American culture is a Western culture largely derived from the traditions of European immigrants with influences from many other sources, such as traditions brought by slaves from Africa. More recent immigration from Asia and especially Latin America has added to a cultural mix that has been described as a homogenizing melting pot, and a heterogeneous salad bowl, with immigrants contributing to, and often assimilating into, mainstream American culture. The American Dream, or the perception that Americans enjoy high social mobility, plays a key role in attracting immigrants. Whether this perception is accurate has been a topic of debate. While mainstream culture holds that the United States is a classless society, scholars identify significant differences between the country's social classes, affecting socialization, language, and values. Americans tend to greatly value socioeconomic achievement, but being ordinary or average is promoted by some as a noble condition as well."
+6279,"The United States is considered to have the strongest protections of free speech of any country under the First Amendment, which protects flag desecration, hate speech, blasphemy, and lese-majesty as forms of protected expression. A 2016 Pew Research Center poll found that Americans were the most supportive of free expression of any polity measured. They are the ""most supportive of freedom of the press and the right to use the Internet without government censorship."" It is a socially progressive country with permissive attitudes surrounding human sexuality. LGBT rights in the United States are advanced by global standards."
+6280,"Colonial American authors were influenced by John Locke and various other Enlightenment philosophers. Before and shortly after the Revolutionary War, the newspaper rose to prominence, filling a demand for anti-British national literature. Led by Ralph Waldo Emerson and Margaret Fuller in New England, transcendentalism branched from Unitarianism as the first major American philosophical movement. During the nineteenth-century American Renaissance, writers like Walt Whitman and Harriet Beecher Stowe established a distinctive American literary tradition. As literacy rates rose, periodicals published more stories centered around industrial workers, women, and the rural poor. Naturalism, regionalism, and realism—the latter associated with Mark Twain—were the major literary movements of the period."
+6281,"While modernism generally took on an international character, modernist authors working within the United States more often rooted their work in specific regions, peoples, and cultures. Following the Great Migration to northern cities, African-American and black West Indian authors of the Harlem Renaissance developed an independent tradition of literature that rebuked a history of inequality and celebrated black culture. An important cultural export during the Jazz Age, these writings were a key influence on the négritude philosophy. In the 1950s, an ideal of homogeneity led many authors to attempt to write the Great American Novel, while the Beat Generation rejected this conformity, using styles that elevated the impact of the spoken word over mechanics to describe drug use, sexuality, and the failings of society. Contemporary literature is more pluralistic than in previous eras, with the closest thing to a unifying feature being a trend toward self-conscious experiments with language."
+6282,"Media is broadly uncensored, with the First Amendment providing significant protections, as reiterated in New York Times Co. v. United States. The four major broadcasters in the U.S. are the National Broadcasting Company , Columbia Broadcasting System , American Broadcasting Company , and Fox Broadcasting Company . The four major broadcast television networks are all commercial entities. Cable television offers hundreds of channels catering to a variety of niches. As of 2021, about 83% of Americans over age 12 listen to broadcast radio, while about 40% listen to podcasts. As of 2020, there were 15,460 licensed full-power radio stations in the U.S. according to the Federal Communications Commission . Much of the public radio broadcasting is supplied by NPR, incorporated in February 1970 under the Public Broadcasting Act of 1967."
+6283,"U.S. newspapers with a global reach and reputation include The Wall Street Journal, The New York Times, The Washington Post, and USA Today. About 800 publications are produced in Spanish. With few exceptions, newspapers are privately owned, either by large chains such as Gannett or McClatchy, which own dozens or even hundreds of newspapers; by small chains that own a handful of papers; or, in a situation that is increasingly rare, by individuals or families. Major cities often have alternative newspapers to complement the mainstream daily papers, such as The Village Voice in New York City and LA Weekly in Los Angeles. The five most popular websites used in the U.S. are Google, YouTube, Amazon, Yahoo, and Facebook, with all of them being American companies."
+6284,"As of 2022, the video game market of the United States is the world's largest by revenue. There are 444 publishers, developers, and hardware companies in California alone."
+6285,"The United States is well known for its cinema and theater. Mainstream theater in the United States derives from the old European theatrical tradition and has been heavily influenced by the British theater. By the middle of the 19th century America had created new distinct dramatic forms in the Tom Shows, the showboat theater and the minstrel show. The central hub of the American theater scene is Manhattan, with its divisions of Broadway, off-Broadway, and off-off-Broadway."
+6286,"Many movie and television stars have gotten their big break working in New York productions. Outside New York City, many cities have professional regional or resident theater companies that produce their own seasons. The biggest-budget theatrical productions are musicals. U.S. theater has an active community theater culture."
+6287,"The Tony Awards recognizes excellence in live Broadway theatre and are presented at an annual ceremony in Manhattan. The awards are given for Broadway productions and performances. One is also given for regional theatre. Several discretionary non-competitive awards are given as well, including a Special Tony Award, the Tony Honors for Excellence in Theatre, and the Isabelle Stevenson Award."
+6288,"In the visual arts, the Hudson River School was a mid-19th-century movement in the tradition of European naturalism. The 1913 Armory Show in New York City, an exhibition of European modernist art, shocked the public and transformed the U.S. art scene."
+6289,"Georgia O'Keeffe, Marsden Hartley, and others experimented with new and individualistic styles, which would become known as American modernism. Major artistic movements such as the abstract expressionism of Jackson Pollock and Willem de Kooning and the pop art of Andy Warhol and Roy Lichtenstein developed largely in the United States. Major photographers include Alfred Stieglitz, Edward Steichen, Dorothea Lange, Edward Weston, James Van Der Zee, Ansel Adams, and Gordon Parks."
+6290,"The tide of modernism and then postmodernism has brought global fame to American architects, including Frank Lloyd Wright, Philip Johnson, and Frank Gehry. The Metropolitan Museum of Art in Manhattan is the largest art museum in the United States."
+6291,"American folk music encompasses numerous music genres, variously known as traditional music, traditional folk music, contemporary folk music, or roots music. Many traditional songs have been sung within the same family or folk group for generations, and sometimes trace back to such origins as the British Isles, Mainland Europe, or Africa. The rhythmic and lyrical styles of African-American music in particular have influenced American music. Banjos were brought to America through the slave trade. Minstrel shows incorporating the instrument into their acts led to its increased popularity and widespread production in the 19th century. The electric guitar, first invented in the 1930s, and mass-produced by the 1940s, had an enormous influence on popular music, in particular due to the development of rock and roll."
+6292,"Elements from folk idioms such as the blues and old-time music were adopted and transformed into popular genres with global audiences. Jazz grew from blues and ragtime in the early 20th century, developing from the innovations and recordings of composers such as W.C. Handy and Jelly Roll Morton. Louis Armstrong and Duke Ellington increased its popularity early in the 20th century. Country music developed in the 1920s, rock and roll in the 1930s, and bluegrass and rhythm and blues in the 1940s. In the 1960s, Bob Dylan emerged from the folk revival to become one of the country's most celebrated songwriters. The musical forms of punk and hip hop both originated in the United States in the 1970s."
+6293,"The United States has the world's largest music market with a total retail value of $15.9 billion in 2022. Most of the world's major record companies are based in the U.S.; they are represented by the Recording Industry Association of America . Mid-20th-century American pop stars, such as Frank Sinatra and Elvis Presley, became global celebrities and best-selling music artists, as have artists of the late 20th century, such as Michael Jackson, Madonna, Whitney Houston, and Prince, and of early 21st century such as Taylor Swift and Beyoncé."
+6294,"The United States and China collectively account for the majority of global apparel demand. Apart from professional business attire, American fashion is eclectic and predominantly informal. While Americans' diverse cultural roots are reflected in their clothing, sneakers, jeans, T-shirts, and baseball caps are emblematic of American styles. New York is considered to be one of the ""big four"" global fashion capitals, along with Paris, Milan, and London. A study demonstrated that general proximity to Manhattan's Garment District has been synonymous with American fashion since its inception in the early 20th century."
+6295,"The headquarters of many designer labels reside in Manhattan. Labels cater to niche markets, such as pre teens. There has been a trend in the United States fashion towards sustainable clothing. New York Fashion Week is one of the most influential fashion weeks in the world, and occurs twice a year."
+6296,"The U.S. film industry has a worldwide influence and following. Hollywood, a district in northern Los Angeles, the nation's second-most populous city, is also metonymous for the American filmmaking industry, the third-largest in the world, following India and Nigeria. The major film studios of the United States are the primary source of the most commercially successful and most ticket-selling movies in the world. Since the early 20th century, the U.S. film industry has largely been based in and around Hollywood, although in the 21st century an increasing number of films are not made there, and film companies have been subject to the forces of globalization. The Academy Awards, popularly known as the Oscars, have been held annually by the Academy of Motion Picture Arts and Sciences since 1929, and the Golden Globe Awards have been held annually since January 1944."
+6297,"The industry enjoyed its golden years, in what is commonly referred to as the ""Golden Age of Hollywood"", from the early sound period until the early 1960s, with screen actors such as John Wayne and Marilyn Monroe becoming iconic figures. In the 1970s, ""New Hollywood"" or the ""Hollywood Renaissance"" was defined by grittier films influenced by French and Italian realist pictures of the post-war period. The 21st century was marked by the rise of American streaming platforms, which came to rival traditional cinema."
+6298,"Early settlers were introduced by Native Americans to foods such as turkey, sweet potatoes, corn, squash, and maple syrup. Of the most enduring and pervasive examples are variations of the native dish called succotash. Early settlers and later immigrants combined these with foods they were familiar with, such as wheat flour, beef, and milk to create a distinctive American cuisine. New World crops, especially pumpkin, corn, potatoes, and turkey as the main course are part of a shared national menu on Thanksgiving, when many Americans prepare or purchase traditional dishes to celebrate the occasion."
+6299,"Characteristic American dishes such as apple pie, fried chicken, doughnuts, french fries, macaroni and cheese, ice cream, pizza, hamburgers, and hot dogs derive from the recipes of various immigrant groups. Mexican dishes such as burritos and tacos preexisted the United States in areas later annexed from Mexico, and adaptations of Chinese cuisine as well as pasta dishes freely adapted from Italian sources are all widely consumed. American chefs have had a significant impact on society both domestically and internationally. In 1946, the Culinary Institute of America was founded by Katharine Angell and Frances Roth. This would become the United States' most prestigious culinary school, where many of the most talented American chefs would study prior to successful careers."
+6300,"The United States restaurant industry was projected at $899 billion in sales for 2020, and employed more than 15 million people, representing 10% of the nation's workforce directly. It is the country's second-largest private employer and the third-largest employer overall. The United States is home to over 220 Michelin Star rated restaurants, 70 of which are in New York City alone. Wine has been produced in what is now the United States since the 1500s, with the first widespread production beginning in what is now New Mexico in 1628. Today, wine production is undertaken in all fifty states, with California producing 84 percent of all US wine. With more than 1,100,000 acres  under vine, the United States is the fourth-largest wine producing country in the world, after Italy, Spain, and France."
+6301,"The American fast-food industry, the world's first and largest, pioneered the drive-through format in the 1940s and is often viewed as being a symbol of U.S. marketing dominance. American companies such as McDonald's, Burger King, Pizza Hut, Kentucky Fried Chicken, and Domino's Pizza, among many others, have numerous outlets around the world."
+6302,"The most popular spectator sports in the U.S. are American football, basketball, baseball, soccer, and ice hockey. While most major U.S. sports such as baseball and American football have evolved out of European practices, basketball, volleyball, skateboarding, and snowboarding are American inventions, many of which have become popular worldwide. Lacrosse and surfing arose from Native American and Native Hawaiian activities that predate European contact. The market for professional sports in the United States was approximately $69 billion in July 2013, roughly 50% larger than that of all of Europe, the Middle East, and Africa combined."
+6303,"American football is by several measures the most popular spectator sport in the United States; the National Football League has the highest average attendance of any sports league in the world, and the Super Bowl is watched by tens of millions globally. However, baseball has been regarded as the U.S. ""national sport"" since the late 19th century. After American football, the next four most popular professional team sports are basketball, baseball, soccer, and ice hockey. Their premier leagues are, respectively, the National Basketball Association, Major League Baseball, Major League Soccer, and the National Hockey League. The most-watched individual sports in the U.S. are golf and auto racing, particularly NASCAR and IndyCar."
+6304,"On the collegiate level, earnings for the member institutions exceed $1 billion annually, and college football and basketball attract large audiences, as the NCAA March Madness tournament and the College Football Playoff are some of the most watched national sporting events. In the U.S., the intercollegiate sports level serves as a feeder system for professional sports. This differs greatly from practices in nearly all other countries, where publicly and privately funded sports organizations serve this function."
+6305,"Eight Olympic Games have taken place in the United States. The 1904 Summer Olympics in St. Louis, Missouri, were the first-ever Olympic Games held outside of Europe. The Olympic Games will be held in the U.S. for a ninth time when Los Angeles hosts the 2028 Summer Olympics. U.S. athletes have won a total of 2,959 medals  at the Olympic Games, the most of any country."
+6306,"In international competition, the U.S. men's national soccer team has qualified for eleven World Cups, while the women's national team has won the FIFA Women's World Cup and Olympic soccer tournament four times each. The United States hosted the 1994 FIFA World Cup and will co-host, along with Canada and Mexico, the 2026 FIFA World Cup. The 1999 FIFA Women's World Cup was also hosted by the United States. Its final match was watched by 90,185, setting the world record for most-attended women's sporting event."
+6307,Key Development Forecasts for the United States from International Futures
+6308,WikiMiniAtlas40°N 100°W / 40°N 100°W / 40; -100 
+6309,National curriculum assessment generally means testing of students as to whether they meet the national standards.
+6310,Notable national curricula are:
+6311,"The United States notably does not have one. The establishment of a national curriculum was explicitly banned in 1965, in Section 604 of the Elementary and Secondary Education Act .  This act provided federal funding for primary and secondary education  as part of President Lyndon B. Johnson's War on Poverty. However, most states in the United States voluntarily abide by the Common Core State Standards Initiative, which provides certain uniform standards. See Education in the United States."
+6312,"The A-level  is a subject-based qualification conferred as part of the General Certificate of Education, as well as a school leaving qualification offered by the educational bodies in the United Kingdom and the educational authorities of British Crown dependencies to students completing secondary or pre-university education. They were introduced in England and Wales in 1951 to replace the Higher School Certificate. The A-level permits students to have potential access to a chosen university they applied to with UCAS points. They could be accepted into it should they meet the requirements of the university."
+6313,"A number of Commonwealth countries have developed qualifications with the same name as and a similar format to the British A-levels. Obtaining an A-level, or equivalent qualifications, is generally required across the board for university entrance, with universities granting offers based on grades achieved. Particularly in Singapore, its A-level examinations have been regarded as being much more challenging than those in the United Kingdom and Hong Kong."
+6314,"A-levels are typically worked towards over two years. Normally, students take three or four A-level courses in their first year of sixth form, and most taking four cut back to three in their second year. This is because university offers are normally based on three A-level grades, and taking a fourth can have an impact on grades. Unlike other level-3 qualifications, such as the International Baccalaureate, A-levels have no specific subject requirements, so students have the opportunity to combine any subjects they wish to take. However, students normally pick their courses based on the degree they wish to pursue at university: most degrees require specific A-levels for entry."
+6315,"In legacy modular courses , A-levels are split into two parts, with students within their first year of study pursuing an Advanced Subsidiary qualification, commonly referred to as an AS or AS-level, which can either serve as an independent qualification or contribute 40% of the marks towards a full A-level award. The second part is known as an A2 or A2-level, which is generally more in-depth and academically rigorous than the AS. The AS and A2 marks are combined for a full A-level award. The A2-level is not a qualification on its own and must be accompanied by an AS-level in the same subject for certification."
+6316,"In September 2023, it was announced by Prime Minister Rishi Sunak that the A-level would be scrapped in England and merged with T-levels to form the Advanced British Standard. This would incorporate compulsory English and mathematics studies. There are no plans to scrap A-levels in Wales."
+6317,Several countries use A-levels as a school-leaving qualification. The A-levels taken by students in other countries may differ from the A-levels taken in the United Kingdom.
+6318,"In Bangladesh, the GCE AS and A-level are offered by Cambridge Assessment International Education  and Pearson Edexcel after completion of GCE O-level or IGCSE , and is conducted by the British Council. GCE Advanced Level qualifications are offered by some of the private, public, and international schools as an alternative to HSC  offered by the Government Board of Education. It has become a popular choice among students, but due to financial implications, its reach is limited to the middle and upper classes in major cities such as Dhaka and Chittagong."
+6319,"In Brunei, the A-level qualification is offered, with examinations conducted by Cambridge International Examinations . Some subjects are unique to Brunei or have a format, curriculum, or syllabus that is unique to Brunei."
+6320,"The Advanced Level of Cameroon is based on the Cambridge International Examinations and similarly, conducted by the government of Cameroon in collaboration with Cambridge University. All the courses taken are related to what the candidate is willing to pursue a career in university and these courses are on a recognizable international standard for university entrance; as they are major relevant courses. You can select between 3 and 5 courses during your advanced level studies, prior to taking your advanced level examination on which."
+6321,"The British A-level qualifications such as GCE A-level and International A-level are widely accepted in Hong Kong as an alternative to the Hong Kong Diploma of Secondary Education for both admission and employment purposes. It is one of the most popular qualifications used for university admission via the non-JUPAS channel. For example, average non-JUPAS offers from the Hong Kong University of Science and Technology require one to three A*s . Since the introduction of the high distinction grade  in 2010, the British A-level examination has regained its power to differentiate the very top levels of ability. According to the British Department for Education, in the academic year 2014/15, approximately 7.3%, 2.7%, 1.0%, and 0.3% of all the candidates from the GCSE cohort  achieved one to four A*s or a better result in the GCE A-level examination. This percentile rank is one important input for equating the levels in both examinations. On the sole basis of percentile ranks and the grade statistics from HKEAA in 2017, a score of 29/35 from the best five subjects in Hong Kong Diploma of Secondary Education is comparable to 1A*2A in the best three British A-levels, 32/35 to 2A*1A, 33/35 to 3A* and 34/35 to 4A*. In deriving inferences from these statistics, it is important to note that slightly more than a third of the GCSE candidates can go on to study in the sixth form before applying to universities via the Universities and Colleges Admissions Service  while almost all Hong Kong students can study Form 6 before sitting the HKDSE examination."
+6322,"Relevant authorities such as the Hong Kong Examination and Assessment Authority  and UCAS also sought to connect the results from both exam systems to open doors for the holders of these qualifications who wish to study locally or overseas. Officially, the top distinction levels, A* in A-level and 5** in HKDSE, are currently recognized as broadly equivalent. In particular, in the light of statistical evidence, the 1% cut-off line is often set by admission offices at 2A*1A as compared to a score of 43/45 in the International Baccalaureate Diploma but the Uniform Mark Scale  instead of the letter grades would be used in many cases to offer a much more subtle view of the candidate's academic potential. Typical offers from Cambridge Medical School require 2A*1A and Oxford University Medical School require 1A*2A whereas those from the medical and dentistry schools of the University of Hong Kong and the Chinese University of Hong Kong require 4A* and 3A* respectively in A-level due to fierce competition."
+6323,"In India, Cambridge International Examinations , and Learning Resource Network  GCE Advanced Level qualifications are offered at private and international schools as an alternative to the conventional Higher Secondary Certificate ."
+6324,"In Malaysia, the A-level qualification is offered, with examinations conducted by Cambridge International Examinations . Some subjects are unique to Malaysia or have a format, curriculum, or syllabus that is unique to Malaysia. For instance, there are two types of A-level in Malaysia, Cambridge A-level  and Edexcel International Advanced Level ."
+6325,"In Mauritius, A/AS-level qualifications are taken as part of the Higher School Certificate, awarded upon successful completion of secondary school after passing of examinations jointly administered by the Mauritius Examinations Syndicate and the University of Cambridge Local Examinations Syndicate . Some exam papers offered, such as French, are customized to support the national educational standards. Additionally, International A-level qualifications from Edexcel are available, for which exams may be registered through the Mauritius Examinations Syndicate."
+6326,"In Nepal, A-level offered from Cambridge International Examinations  is the most popular and is presented by several public, private, and international institutions as an alternative to the National Examination Board 's 10+2 curriculum. A-level has become a popular choice for many students in Nepal. Besides A-level, International Baccalaureate and Advanced Placement are also offered at this level. In Nepali, students generally appear for 4 subjects at A-level and 5 at AS-level."
+6327,"A-levels are offered in Pakistan by non-governmental, private institutions, along with International Baccalaureate and other international examinations like the Advanced Placement. Examinations are handled by international British boards and the program is equivalent to Higher Secondary School Certificate. Cram Schools are established all around the country which prepares the students to take the examinations as private candidates."
+6328,"In Seychelles, the A-level qualification is offered, with examinations conducted by Cambridge International Examinations . Some subjects are unique to Seychelles or have a format, curriculum, or syllabus that is unique to Seychelles."
+6329,"In Singapore, H1/H2/H3 level qualifications are awarded upon successful completion of examinations jointly administered by Singapore Ministry of Education , Singapore Examinations and Assessment Board  and the University of Cambridge Local Examinations Syndicate . Singaporean A-levels are notably much more difficult than the British A-levels but a little bit easier than the Hong Kong A-level Examination in terms of the proportion of students achieving the highest grades, as part of the country's wider robust educational policy."
+6330,"In Sri Lanka, A-level qualifications are offered by governmental and non-governmental schools . The qualifications are awarded upon successful completion of examinations called Local A-levels while most of the private schools award them upon London A-levels. Local GCE Advanced Level qualifications are offered by the Department of Examinations. Private Schools  in Sri Lanka provide A-level qualifications which are offered by:"
+6331,"Passing A-levels is a major requirement for applying to local universities and for private universities in Sri Lanka. In the recent past, Universities in Sri Lanka have given the opportunity to apply for their foundation courses which only require O-level  results."
+6332,"A-level is an education structure after the completion of 11 years of Universal Primary Education . Universal Primary Education consists of two levels, the primary school level of seven years and ordinarily secondary level, of four years."
+6333,"Students are then required to sit for the National Examination which is done on a specific month in a certain year. The exams are done throughout the country on the same date provided by the examination board. Examinations will depend on the cluster and core subjects in which a student shall take. There are three major clusters, namely science, business, and liberal arts plus core subjects of which are seven in total or more. Students who will then pass according to the National Examination Standards shall be selected to join Advanced Secondary Education and are selected according to the subjects they passed the most and by their will to pursue certain subjects."
+6334,There are only three core subjects to study in Advanced Level and one or more optional subjects to sit for the Advanced Certificate of Secondary Education Examination . Then students are awarded Advanced Certificate of Secondary Education only if they have passed. The pass marks are arranged according to grades and the grades will determine whether a student will qualify to join tertiary education.
+6335,A-level education is a two years education which is offered by the government and non-government schools. It is regulated by the Ministry of Education which assures both qualitative and quantitative subject matters and there is a special independent council that is responsible in composing the Advanced Certificate of Secondary Education Examination  namely The National Examination Council of Tanzania .
+6336,"In Uganda, a Uganda Advanced Certificate of Education is awarded to students who have passed their national examinations that are set and marked by the Uganda National Examination Board . It is among the requirements requirements for one to join a university in Uganda. The advanced level takes two years for one to complete."
+6337,"A-levels are a college or sixth form leaving qualification offered in England, Wales, and Northern Ireland. These are not compulsory, unlike GCSEs. In Scotland, A-levels are also offered by some schools as an alternative school-leaving qualification in place of the Scottish Advanced Higher. The main examination boards which administer British A-levels in the UK are:"
+6338,"Oxford AQA, Edexcel, Cambridge International Examinations , and Learning Resource Network  also offer international versions of the British A-levels in the United Kingdom and worldwide."
+6339,"The British variant of A/AS-levels is also taken in many Commonwealth and former Commonwealth countries, as well as in examination centers worldwide. British international schools in foreign countries generally offer the British A-levels as offered through Edexcel or Cambridge International Examinations. At select examination centers, the British A-level exams may also be available to private candidates."
+6340,"A wide variety of subjects are offered at A-level by the five exam boards. Although exam boards often alter their curricula, this table shows the majority of subjects that are consistently available for study."
+6341,"Between 2015 and 2018 , A-levels in England were reformed, transitioning from a modular to linear structure . This means all A-level exams are taken in one sitting as a set of terminal exams , and there is no coursework set for many subjects. For A-levels that retain a coursework element, the percentage of the final grade determined by coursework has been reduced. An example of this can be seen in Edexcel's new English Literature A-level, reformed in 2015, which reduces the amount of coursework to 20% . A-levels are no longer separated into units, and students must resit all of their exams if they wish to resit the qualification. While these reforms were expected to be complete for first teaching in 2017, this was extended to 2018 to include the reforms of less common languages such as Modern Hebrew and Bengali."
+6342,"The AS-level is now a separate qualification and is not required for an A-level award, although it still encompasses the first year of the full A-level content. However, unlike AS-levels in the old modular courses, they are now worth only 40% as many UCAS points as a full A-level , as content from the second year of A-level is considered more academically challenging than that of the first year."
+6343,"As these reforms took place in stages, many students took a combination of modular and linear courses before all reforms were complete, with AS-levels still being part of an A-level in older modular courses."
+6344,"These reforms look to combat grade inflation, where the proportion of students achieving the highest grades increases year upon year, causing the value of those grades to be eroded. The modular system has also been criticised for nurturing a 'resit culture', while new linear courses give no opportunity to resit individual units."
+6345,"Controversially, some A-level course subjects have been abolished since 2017 as part of these reforms. These include archaeology, anthropology, creative writing, critical thinking, general studies, and home economics. Many universities criticized the scrapping of exams taken at the end of AS-level, which used to be worth 50% of the overall A-level grade. This is because the universities used the grades achieved at AS-level  as an indication of a student's ability and thus whether to give said student an offer."
+6346,Opposition to these reforms in Wales and Northern Ireland has resulted in maintaining the modular structure of their qualification.
+6347,"The GCE Advanced Level qualification is offered by the Zimbabwe School Examinations Council . Before, this qualification was jointly offered by Cambridge International Examinations and the Council in Zimbabwe."
+6348,"In 2002, there has been a move away from the GCE Advanced Level to the CXC CAPE examinations, making them a de facto university entrance examination. Some universities also require applicants to take separate entrance examinations. The International Baccalaureate and European Baccalaureate are also accepted."
+6349,Qualifications and Curriculum Development Agency: A Level resources
+6350,University of Cambridge: International A & AS levels
+6351,The Guardian 2005 A Level results
+6352,2005 A Level results
+6353,CIE O level and A level Timetable 2018
+6354,"The O-Level  is a subject-based qualification conferred as part of the General Certificate of Education. It began in the United Kingdom and has been adopted, often with modifications, in several other countries."
+6355,"In the United Kingdom, it was introduced in place of the School Certificate in 1951 as part of an educational reform alongside the more in-depth and academically rigorous A-Level  in England, Wales and Northern Ireland. Those three jurisdictions replaced O-Level gradually with General Certificate of Secondary Education  completely by 1988 and, the International General Certificate of Secondary Education  over time. The Scottish equivalent was the O-grade ."
+6356,"The AO-Level  was formerly available in most subject areas. Sometimes incorrectly known as the Advanced Ordinary Level, the AO-Level syllabus and examination both assumed a higher degree of maturity on the part of candidates, and employed teaching methods more commonly associated with A-Level study. The AO Level was discontinued, with final qualifications awarded in 1988."
+6357,"The O-Level qualification is still awarded by CIE Cambridge International Examinations, the international counterpart of the British examination Board OCR , in select locations, instead of or alongside the International General Certificate of Secondary Education qualifications. Both CIE and OCR have Cambridge Assessment as their parent organisation. The Cambridge O-Level has already been phased out and is no longer available in certain administrative regions."
+6358,"In Bangladesh, the International GCSE  qualification is offered, which is colloquially known as O-Levels, with examinations conducted by Cambridge Assessment International Examinations  and  Pearson Edexcel both under the board of British Council. Both Pearson Edexcel and CAIE offer International GCSE qualifications. O-Level qualification has become a replacement for the matriculation qualification  offered by the Government boards of education. However, due to the high costs associated with IGCSE qualifications, their reach is limited to middle to elite class families."
+6359,"In Brunei, the O-Level qualification is offered, with examinations conducted by Cambridge Assessment International Examinations ."
+6360,"A number of subjects: English Language, English Literature, Bahasa Melayu , Malay Literature, Islamic Religious Knowledge, Ulum al-Quran, Hafaz al-Quran, Tafsir al-Quran , History, Geography, Pure sciences , Arabic, Art and Design offer exam papers and syllabuses unique to Brunei. Additionally, the Mathematics  subject previously offered an exam paper and a syllabus unique to Brunei, but this has since been retired and the regular syllabus used worldwide is offered. This is equivalent to secondary education now."
+6361,"In India, Cambridge International Examinations  GCE Ordinary-Level qualifications are offered at private and international schools as an alternative to the conventional Indian School Certificate ."
+6362,"In Malaysia, the O-Level qualification is offered as the Sijil Pelajaran Malaysia , with examinations conducted by the Malaysian Examination Syndicate . The examinations were formerly conducted by the  University of Cambridge Local Examinations Syndicate , which still advises the national examination board on standards."
+6363,"The English Language subject was previously offered with an exam paper and a syllabus unique to Malaysia, but this Malaysia-specific qualification has since expired, and the regular English Language exam paper and syllabus used worldwide is now used within the country. The English paper is separately graded by the national examination board and UCLES, and both grades are displayed on the result slip."
+6364,"In Mauritius, the O-Level qualification is awarded as part of the School Certificate, which is awarded upon successful completion of Form V in secondary school. The O-Level examinations are jointly conducted by the Mauritius Examinations Syndicate and the University of Cambridge Local Examinations Syndicate . The International General Certificate of Secondary Education from Edexcel is also offered as an equivalent alternative qualification, for which exam registration may be done through the Mauritius Examinations Syndicate."
+6365,"A number of subjects, including English Language, English Language , History, Mathematics , Mathematics , offer exam papers and syllabuses unique to Mauritius. Additionally, the subject of Art and Design, the offering of which is restricted to a limited geographic region, is available in Mauritius."
+6366,"The GCE O-Level qualification is offered in Pakistan by the CIE and conducted by the British Council. Due to the high costs associated with O Level Schooling, it is mostly opted by privileged citizens."
+6367,"In Seychelles, the O-Level qualification is offered, with examinations conducted by Cambridge International Examinations . Some subjects are unique to Seychelles or have a format, curriculum, or syllabus that is unique to Seychelles."
+6368,"In Singapore, the O-Level qualification is offered jointly by Cambridge International Examinations  and the Singapore Ministry of Education. The examinations are mainly conducted by CIE, with select subject examinations conducted by the Singapore Ministry of Education, including select mother tongue subjects, such as Chinese, Malay, and Tamil, and humanities subject of Social Studies."
+6369,"A number of subjects previously offered exam papers and syllabuses unique to Singapore, but these have since been retired or planned to be phased out. After taking O-levels, some Singapore students go on to GCE Advanced Level exams, which are also marked by Cambridge International Examinations. In 2027, the O levels as well as the N levels will be phased out for a new local examination known as the Singapore-Cambridge Secondary Education Certificate ."
+6370,"The GCE Ordinary-Level qualification is offered by the British council of Sri Lanka School . Before, this qualification was jointly offered by Cambridge International Examinations and the Ministry of Education in Sri Lanka.
+the examinations in Sri Lanka are taken very seriously and the exam is considered a starting point to ones higher education
+this is the reason multiple ""top in the world"" prize students are from Sri Lanka however the amount is still less than India's 200"
+6371,"The GCE Ordinary-Level qualification is offered by the Zimbabwe School Examinations Council . Before, this qualification was jointly offered by Cambridge International Examinations and the Ministry of Education in Zimbabwe."
+6372,"The O-Level qualification was previously awarded in the Caribbean. However, many Caribbean countries have now switched to awarding Caribbean Secondary Education Certification  qualifications based on successful completion of examinations administered by the Caribbean Examinations Council ."
+6373,"The O-Level qualification was previously awarded in Hong Kong, along with the Hong Kong version of the A-Level qualification. However, this was replaced with the Hong Kong Certificate of Education Examination  which was then replaced with the Hong Kong Diploma of Secondary Education . The HKCEE was previously benchmarked against the O Levels for comparable subjects, but with the introduction of the International General Certificate of Secondary Education  qualification the Hong Kong equivalent qualification was then benchmarked against the IGCSE."
+6374,"The GCE O-Level qualification originated in the United Kingdom, where it was awarded as the secondary school-leaving qualification. It was primarily an examinations-based qualification, with a grading system that changed over the years. In the United Kingdom, the O-Level qualifications were replaced in 1988 with the General Certificate of Secondary Education  and the International General Certificate of Secondary Education ."
+6375,"Econet software was later mostly superseded by Acorn Universal Networking , though some suppliers were still offering bridging kits to interconnect old and new networks. AUN was in turn superseded by the Acorn Access+ software."
+6376,"Econet was specified in 1980, and first developed for the Acorn Atom and Acorn System 2/3/4 computers in 1981. Also in that year the BBC Micro was released, initially with provision for floppy disc and Econet interface ports, but without the necessary supporting ICs fitted, optionally to be added in a post sale upgrade."
+6377,"In 1982, the Tasmania Department of Education requested a tender for the supply of personal computers to their schools. Earlier that year Barson Computers, Acorn's Australian computer distributor, had released the BBC Microcomputer with floppy disc storage as part of a bundle. Acorn's Hermann Hauser and Chris Curry agreed to allow it to be also offered with Econet fitted, as they had previously done with the disc interface. As previously with the Disc Filing System, they stipulated that Barson would need to adapt the network filing system from the System 2 without assistance from Acorn. Barson's engineers applied a few modifications to fix bugs on the early BBC Micro motherboards, which were adopted by Acorn in later releases. With both floppy disc and networking available, the BBC Micro was approved for use in schools by all state and territory education authorities in Australia and New Zealand, and quickly overtook the Apple II as the computer of choice in private schools."
+6378,"With no other supporting documentation available, the head of Barson's Acorn division, Rob Napier, published Networking with the BBC Microcomputer, the first reference documentation for Econet."
+6379,"Econet was officially released for the BBC Micro in the UK in 1984, and it later became popular as a networking system for the Acorn Archimedes.  Econet was eventually officially supported on all post-Atom Acorn machines, apart from the Electron , along with 3rd party ISA cards for the IBM PC. The ""Ecolink"" ISA interface card for IBM-compatible PCs was available. It used Microsoft's MS-NET Redirector for MS-DOS to provide file and printer sharing via the NET USE command."
+6380,"File, Print and Tape servers, for the architecture were also supplied by 3rd party vendors such as S J Research."
+6381,"Econet was supported by Acorn MOS, RISC OS, RISC iX, FreeBSD and Linux operating systems."
+6382,"Acorn once received an offer from Commodore International to license the technology, which it refused."
+6383,"With the falling prices and widespread adoption of IP networking in the early 1990s, Acorn Universal Networking , an implementation of Econet protocols and addressing over TCP/IP , was developed to provide legacy support for Econet on Ethernet-connected machines."
+6384,"Support for the Econet protocol and AUN was removed from the Linux kernel in 2012 from version 3.5, due to lack of use and privilege escalation vulnerabilities."
+6385,"Econet was supported by a large number of different computer and server systems, produced both by Acorn and by other companies. As well as Acorn's MOS and RISC OS these also used other operating systems such as CP/M, DR-DOS, Unix, Linux or Microsoft Windows."
+6386,"The Econet API includes an Econet_MachinePeek command, which can be used by software to determine if a machine is present on the network and its hardware platform. The machine-type codes which can be returned by that command
+are a useful indication of the range of hardware that offered Econet as their primary networking function or as an option:"
+6387,"The manual includes an assembly language program to report a machine type, software version and release numbers."
+6388,"An update to the list in volume 5A of the PRM
+lists the following additions to the table above:"
+6389,"Econet is a five-wire bus network composed of one or more segments , where each segment may be up to 500 m long. One pair of wires is used for the clock signal to synchronise devices on the network, one pair for data, and one wire as a common ground. In many but not all implementations, a dedicated external board is required to supply the clock signal known as the ""clock box"", usually positioned in the middle of the network. Some Econet devices have an internal capability to generate the clock signal. Signalling used the RS-422 5-volt differential standard, with one bit transferred per clock cycle. Unshielded cable was used for short lengths, and shielded cable for longer networks. The cable was terminated at each end to prevent reflections and to guarantee high logic levels when the bus was undriven."
+6390,"The original connectors were five-pin circular 180° DIN types. On later 32-bit machines , the Econet connection was made via five of the pins on their 15-pin D-type Network port, which could also accept MAUs  to allow other types of network to be connected via the same socket. This port looks similar to an AUI port, but is not compatible. The Acorn A4 laptop used another implementation, in the form of a 5 pin mini-DIN."
+6391,"Each Econet interface was controlled by a Motorola MC68B54 Advanced Data Link Controller  chip, which handled electrical transmission/reception, frame checksumming and collision detection."
+6392,"Econet used a connectionless transmission model, similar to UDP, with no checksumming or error correction at this layer. Each packet had a four byte header consisting of:"
+6393,"A single data transmission consisted of four frames, each with a header as above:"
+6394,"Each device on a network segment is identified by a ""station ID"" number which serves a similar purpose to a MAC address. The address is not set at time of manufacture but is set manually and a network administrator must ensure addresses do not collide when new machines are installed. In the BBC Micro the station ID is set using jumper pins on the motherboard. Conventionally a file server would be assigned station ID 254 but there is no specific requirement for this to be the case."
+6395,"There was provision for broadcast transmissions, a single frame sent with its destination station and network numbers set to 255.  There was also provision for promiscuous mode reception, termed wild receive
+in the PRM, requested by listening for station and network numbers both being zero."
+6396,"Technical details of packets and frames, the Econet API, and worked examples in ARM assembler and BBC BASIC
+are given in the RISC OS Programmer's Reference Manual."
+6397,"At the time and in the markets for which Econet was developed, the main purpose of computer networking was to provide local area shared access to expensive hardware such as disc storage and printers.  Acorn provided software for the BBC Micro to implement a file server, and optionally a printer server also.  The original file server was very basic, essentially allowing limited access to a floppy disc over the network. The server software was further developed over many years, and Acorn and other manufacturers also produced dedicated Econet servers based on various technologies.  So the servers available fell into roughly three categories:"
+6398,"The machine type numbers listed in the ""Supported systems"" section above are an indication of the range of hardware that was available or planned."
+6399,"Additional services could be implemented, using the network API provided. Short utilities such as network chat programs were often published in magazines or distributed by sharing among users; these made use of the Econet protocols to work alongside the basic file and print services. Larger software packages  were available that provided services such as Teletext and modem drivers."
+6400,"Acorn emphasised the Filestore in the late 1980s as a solution for small workgroups, offering a base unit with optional hard disk storage modules. The device was similar in concept to what is now termed Network Attached Storage, being a standalone unit dedicated to serving files over a network."
+6401,"The Filestore was a 65C102-based machine with 64 KB of RAM,  64 KB of ROM, Econet connectivity, two 3.5"" floppy drives, a parallel printer interface, expansion bus, Econet clock and termination circuits, a real-time clock, and a quantity of battery-backed RAM. The battery-backed RAM was used to hold configuration and authentication details."
+6402,"Initially, hard disk expansion was offered in the form of the E20 module providing a 3.5"" 20 MB Winchester disk drive  for the E01 base unit; later expansions in the form of the E40S and E60S provided 40 MB and 60 MB storage respectively for the E01S base unit. The ""S"" suffix reportedly signifies that the units are ""stacking""."
+6403,"Acorn also offered the Level 1, Level 2 and Level 3 Fileserver solutions running on sufficiently upgraded BBC Micro or BBC Master computers."
+6404,"The Level 1 product offered access to existing Acorn DFS discs via a BBC Model B with Econet, a disc interface and single or dual drives. Printer sharing was also possible. This was intended for small networks, typically in an educational setting, to solve a narrowly defined problem of sharing what were at the time expensive peripherals. An Econet upgrade originally cost significantly less than a disk drive. The only available access control mechanism was that files for each station  on the network could be isolated from each other but anyone with physical access to the relevant station could access the files. Typically each station would only be able to access a small number of files, with only 31 files being stored in total on a side of one floppy disk and split between all of the stations."
+6405,"Level 2 delivered significant enhancements over level 1 but elevated the system requirements for the file server host machine to necessitate a 6502 second processor unit with 64K RAM. Hierarchical directories were possible with the number of files limited only by the amount of storage available, enhanced access controls, random access to data files, and authentication support."
+6406,Level 3 introduced Winchester hard drive support.
+6407,"With the release of the Level 4 Fileserver software providing a means to ""extend the life of existing Acorn computers, such as the A310"", allowing ""any Archimedes computer to act as a fileserver"", the emphasis had evidently shifted away from the Filestore and towards the Level 4 product at the start of the 1990s. A base Filestore E01S unit had a price inclusive of VAT of £1148.85 in February 1989, whereas an Archimedes 310 with 1 MB of RAM cost only £958.00 and an Econet module £56.35, illustrating the pricing considerations for potential buyers. By 1991, the Filestore was apparently no longer offered in Acorn's pricing , but the Level 4 software was priced at £233.83 and an Archimedes 410/1 with 1 MB of RAM at £1049.33."
+6408,"With the introduction of Acorn's Unix workstations running RISC iX, an envisaged application for Econet was the use of Master 128 computers acting as terminals to these Unix systems.: 4  Such systems also offered the capability to act as bridges between Econet and Ethernet networks, offering routing facilities to any Unix machines attached to the Econet,: 5  this being enabled by the IP-over-Econet support in RISC iX."
+6409,"An Econet X.25 gateway product was offered by Acorn, providing access to X.25 networks for computers on an Econet, with the X25 Terminal ROM and the existing Acorn DNFS ROM needing to be fitted to computers to enable access to X.25 services, with the Terminal ROM providing terminal emulation and file transfer functionality."
+6410,"The gateway hardware consisted of the core functionality of a BBC Micro, this being the network service module connected to the Econet, combined with a Z80 second processor connected via the Tube interface, this acting as the gateway module and having 16 KB ROM and 32 KB of private RAM, augmented by another board with a Z80 processor with 32 KB of private RAM, this being the X25 module accessing the X.25 line. The gateway and X25 modules communicated via 16 KB of dual-ported shared RAM. The X25 module was designed by Symicron and ran the ""proven"" Symicron Telematics Software ."
+6411,"Econet users would send network service requests to the gateway that would be forwarded by the STS functionality of the gateway to the X.25 network. Incoming X.25 calls would be forwarded by the STS functionality to the network service functionality and on to the Econet. Network service requests could employ X.25, Yellow Book Transport Service, and X.29 protocols."
+6412,"While Econet was essentially specific to the Acorn range of computers, it does share common concepts with modern network file systems and protocols:"
+6413,The Econet Enthusiasts Area
+6414,Chris' Acorns
+6415,Econet documentation at 8-bit software
+6416,RISC OS Programmer's Reference Manuals –  the latest versions as of May 2014
+6417,"The British Broadcasting Corporation Microcomputer System, or BBC Micro, is a series of microcomputer designed and built by Acorn Computers Limited in the 1980s for the Computer Literacy Project of the BBC. Designed with an emphasis on education, it was notable for its ruggedness, expandability, and the quality of its operating system.  The machine was the focus of a number of educational BBC TV programmes on computer literacy, starting with The Computer Programme in 1982, followed by Making the Most of the Micro,  Computers in Control in 1983, and finally Micro Live in 1985."
+6418,"After the Literacy Project's call for bids for a computer to accompany the television programmes and literature, Acorn won the contract with the Proton, a successor of its Atom computer prototyped at short notice. Renamed the BBC Micro, the system was adopted by most schools in the United Kingdom, changing Acorn's fortunes. It was also successful as a home computer in the UK, despite its high price compared to some other home computers sold in the UK at the time. Acorn later employed the machine to simulate and develop the ARM architecture."
+6419,"While nine models were eventually produced with the BBC brand, the phrase ""BBC Micro"" is usually used colloquially to refer to the first six ; subsequent BBC models are considered part of Acorn's Archimedes series."
+6420,"During the early 1980s, the BBC started what became known as the BBC Computer Literacy Project. The project was initiated partly in response to an ITV documentary series The Mighty Micro, in which Christopher Evans of the UK's National Physical Laboratory predicted the coming microcomputer revolution and its effect on the economy, industry, and lifestyle of the United Kingdom."
+6421,"The BBC wanted to base its project on a microcomputer capable of performing various tasks which they could then demonstrate in the TV series The Computer Programme. The list of topics included programming, graphics, sound and music, teletext, controlling external hardware, and artificial intelligence. It developed an ambitious specification for a BBC computer, and discussed the project with several companies including Acorn Computers, Sinclair Research, Newbury Laboratories, Tangerine Computer Systems, and Dragon Data."
+6422,"The introduction of a specific microcomputer to a more general computer literacy initiative was a topic of controversy, however, with criticism aimed at the BBC for promoting a specific commercial product and for going beyond the ""traditional BBC pattern"" of promoting existing information networks of training and education providers. Accusations were even levelled at the Department of Industry for making the BBC ""an arm of Government industrial policy"" and using the Computer Literacy Project as a way of ""funding industry through the back door"", obscuring public financial support on behalf of a government that was ostensibly opposed to subsidising industry."
+6423,"The Acorn team had already been working on a successor to their existing Atom microcomputer. Known as the Proton, it included better graphics and a faster 2 MHz MOS Technology 6502 central processing unit. The machine was only at the design stage at the time, and the Acorn team, including Steve Furber and Sophie Wilson, had one week to build a working prototype from the sketched designs. The team worked through the night to get a working Proton together to show the BBC. Although the BBC expected a computer with the Zilog Z80 CPU and CP/M operating system, not the Proton's 6502 CPU and proprietary operating system, the Proton was the only machine to match the BBC's specification; it also exceeded the specification in nearly every parameter. Based on the Proton prototype, the BBC signed a contract with Acorn as early as February 1981; by June the BBC Micro's specifications and pricing were decided. As a concession to the BBC's expectation of ""industry standard"" compatibility with CP/M, apparently under the direction of John Coll, the Tube interface was incorporated into the design, enabling a Z80 second processor to be added. A new contract between Acorn and BBC Enterprises was agreed in 1984 for another four-year term, with other manufacturers having tendered for the deal. An Acorn representative admitted that the BBC Model B would not be competitive throughout the term of the renewed contract, and that a successor would emerge."
+6424,The OS ROM v1.0 contains the following ASCII credits string :
+6425,"Additionally, the last bytes of the BASIC read-only memory  include the word ""Roger"", which is a reference to Sophie Wilson whose name at the time was Roger Wilson."
+6426,"The machine was released as the BBC Microcomputer on 1 December 1981, although production problems pushed delivery of the majority of the initial run into 1982. Nicknamed ""the Beeb"", it was popular in the UK, especially in the educational market; about 80% of British schools had a BBC microcomputer."
+6427,"Byte called the BBC Micro Model B ""a no-compromise computer that has many uses beyond self-instruction in computer technology"". It called the Tube interface ""the most innovative feature"" of the computer, and concluded that ""although some other British microcomputers offer more features for a given price, none of them surpass the BBC ... in terms of versatility and expansion capability"". As with Sinclair Research's ZX Spectrum and Commodore International's Commodore 64, both released the next year, in 1982, demand greatly exceeded supply. For some months, there were long delays before customers received the machines they had ordered."
+6428,"The involvement of the BBC in microcomputing also initiated tentative plans by the independent television companies of the ITV network to introduce their own initiative and rival computing system, with a CP/M-based system proposed by Transam Computers under consideration for such an initiative by the Independent Television Companies Association at a late 1983 meeting. The proposed machine would have been priced at £399, matching that of the BBC Model B, and was reported as offering 64 KB of RAM, a disc interface, and serial and parallel interfaces, itself being a ""low-cost development"" of an existing machine, the Transam Tuscan, which included dual floppy drives and cost £1,700."
+6429,"This proposal was voted down by the ITV companies, citing a possible contravention of the companies' obligations under broadcasting regulations prohibiting sponsorship, along with concerns about a conflict of interest with advertisers of computer products. Despite denials of involvement with ITV from Prism Microproducts, the company had already been pursuing a joint venture with Transam on a product rumoured to be under consideration by the broadcasting group. This product, a business system subsequently known as the Wren, had reportedly been positioned as such an ""ITV Micro"" towards the end of 1983, also to be offered in a home variant with ORACLE teletext reception capabilities. However, not all ITV franchise holders were equally enthusiastic about scheduling programmes related to microcomputing or about pursuing a computer retailing strategy."
+6430,"Efforts were made to market the machine in the United States and West Germany. Acorn's strategy in the US focused on the education market, worth a reported $700 million, by offering the BBC Micro in an upgraded form of the Model B with an expanded ROM, speech synthesis hardware, and built-in Econet interface for a price of $995, complementing this with the provision of software and materials designed to support teaching and to encourage adoption by teachers ""fearful"" of computers or skeptical of the role of computers in the curriculum."
+6431,"By October 1983, the US operation reported that American schools had placed orders with it totalling $21 million. In one deployment in Lowell, Massachusetts valued at $177,000, 138 BBC Micros were installed in eight of the 27 schools in the city, with the computer's networking capabilities, educational credentials, and the availability of software with ""high education quality"" accompanied by ""useful lesson plans and workbooks"" all given as reasons for selecting Acorn's machine in preference to the competition from IBM, Apple, and Commodore. Another deployment in Phoenix, Arizona valued at $174,697 saw 175 BBC Micros installed, with the local Acorn dealer predicting sales worth $2 million in the next two years, of which around 85 to 90 percent would be made into education, the remainder going to the small business market. In early 1984, Acorn claimed a US network of more than 1,000 dealers, also reporting ""over $50 million worth of education orders for the BBC Micro system"" for which 200 educational titles were being offered."
+6432,"In October 1984, while preparing a major expansion of its US dealer network, Acorn claimed sales of 85 per cent of the computers in British schools, and delivery of 40,000 machines per month. That December, Acorn stated its intention to become the market leader in US educational computing. The New York Times considered the inclusion of local area networking to be of prime importance to teachers. The operation resulted in advertisements by at least one dealer in Interface Age magazine, but ultimately the attempt failed."
+6433,"The success of the machine in the UK was due largely to its acceptance as an ""educational"" computer – UK schools used BBC Micros to teach computer literacy, information technology skills. Acorn became more known for its BBC Model B computer than for its other products. Some Commonwealth countries, including India, started their own computer literacy programmes around 1984. Intending to avoid ""re-inventing the wheel"", such efforts adopted the BBC Micro in order to take immediate advantage of the extensive range of software already developed under the United Kingdom's own literacy initiative, proposing that software tailored for local requirements would ultimately also be developed.: 71–72  A clone of the BBC Micro was produced by Semiconductor Complex Limited and named the SCL Unicorn.  Another Indian computer manufacturer, Hope Computers Pvt Ltd, made a BBC Micro clone called the Dolphin. Unlike the original BBC Micro, the Dolphin featured blue function keys."
+6434,"Production agreements were made with both SCL in India and distributor Harry Mazal in Mexico for the assembly of BBC Micro units from kits of parts, leading to full-scale manufacturing, with SCL also planning to fabricate the 6502 CPU under licence from Rockwell. According to reporting from early 1985, ""several thousand Beebs a month"" were being produced in India. Meanwhile, the eventual production arrangement in Mexico involved local manufacturer Datum , aiming to assemble 2000 units per month by May 1985, with the initial assembly intended to lead to the manufacture of all aspects of the machines apart from Acorn's proprietary ULA components. Such machines were intended for the Mexican and South American markets, potentially also appealing to those south-western states of the US having large Spanish-speaking populations. Ultimately, upon Acorn's withdrawal from the US in 1986, Datum would continue manufacturing at a level of 7000 to 8000 Spanish-language machines per year for the North and South American markets."
+6435,"The initial strategy for the BBC's computer literacy endeavour involved the marketing of the ""Acorn Proton-based BBC microcomputer for less than £200"". The Model A and the Model B were initially priced at £235 and £335 respectively, but increased almost immediately to £299 and £399 due to higher costs. The Model B price of nearly £400 was roughly £1200  in 2011 prices – thirty years after its launch – or around £1600 today. Acorn anticipated total sales to be around 12,000 units, but eventually more than 1.5 million BBC Micros were sold. The cost of the BBC Models was high compared to competitors such as the ZX Spectrum and the Commodore 64, and from 1983 on, Acorn attempted to counter this by producing a simplified but largely compatible version intended for home use, complementing the use of the BBC Micro in schools: the 32K Acorn Electron."
+6436,"A key feature of the BBC Micro's design is the high-performance random-access memory  it is equipped with. A common design note in 6502-based computers of the era was to run the RAM at twice the clock rate as the CPU. This allowed a separate video display controller to access memory while the CPU was busy processing the data just read. In this way, the CPU and graphics driver could share access to RAM through careful timing. This technique was used, for example, on the Apple II Plus and the early Commodore models."
+6437,"The BBC machine, however, was designed to run at the faster CPU speed, 2 MHz, double that of these earlier machines. In this case, bus contention is normally an issue, as there is not enough time for the CPU to access the memory during the period when the video hardware is idle. Some machines of the era accept the inherent performance hit, as was the case for the Amstrad CPC, Atari 8-bit family, and to a lesser extent the ZX Spectrum. Others, like the MSX systems, used entirely separate pools of memory for the CPU and video, slowing access between the two."
+6438,"Furber believed that the Acorn design should have a flat memory model and allow the CPU and video system to access the bus without interfering with each other. To do so, the RAM had to allow four million access cycles per second. Hitachi was the only company considering a DRAM which ran at that speed, the HM4816. To equip the prototype machine, the only four 4816s in the country were hand-carried by the Hitachi representative to Acorn."
+6439,"The National Semiconductor 81LS95 multiplexer was needed for the high memory speed. Furber recalled that competitors came to Acorn offering to replace the component with their own, but ""none of them worked. And we never knew why. Which of course, means we didn't know why the National Semiconductor one did work correctly. And a million and a half BBC Micros later, it was still working, and I still didn't know why"". Another mystery was the 6502's data bus. The prototype BBC Micro exceeded the CPU's specifications, causing it to fail. The designers found that putting a finger on a certain place on the motherboard caused the prototype to work. Acorn put a resistor pack across the data bus, which Furber described as ""'the engineer's finger' and again, we have no idea why it's necessary, and a million and a half machines later it's still working, so nobody asked any questions""."
+6440,"The Model A shipped with 16 KB of user RAM, while the Model B had 32 KB. Extra ROMs could be fitted  and accessed via paged memory."
+6441,"The machines included three video ports, one with an RF modulator sending out a signal in the UHF band, another sending composite video suitable for connection to computer monitors, and a separate RGB video port. The separate RGB video out socket was an engineering requirement from the BBC to allow the machine to directly output a broadcast quality signal for use within television programming; it was used on episodes of The Computer Programme and Making the Most of the Micro."
+6442,"The computer included several input/output  interfaces: serial and parallel printer ports, an 8-bit general purpose digital I/O port, a port offering four analogue inputs, a light pen input, and an expansion connector  which enabled other hardware to be connected. An Econet network interface and a disk drive interface were available as options. All motherboards had space for the electronic components, but Econet was rarely installed."
+6443,"Additionally, an Acorn proprietary interface named the ""Tube"" allowed a second processor to be added. Three models of second processor were offered by Acorn, based on the 6502, Z80 and 32016 CPUs. The Tube was used for third-party add-ons, including a Z80 board and hard disk drive from Torch that allowed the BBC machine to run CP/M programs."
+6444,"Separate pages, each with a codename, are used to control the access to the I/O:"
+6445,"The Tube interface allowed Acorn to use BBC Micros with ARM CPUs as software development machines when creating the Acorn Archimedes. This resulted in the ARM development kit for the BBC Micro in 1986, priced at around £4000. From 2006, a kit with an ARM7TDMI CPU running at 64 MHz, with as much as 64 MB of RAM, was released for the BBC Micro and Master, using the Tube interface to upgrade the 8-bit micros to 32-bit RISC machines. Among the software that operated on the Tube are an enhanced version of the Elite video game and a computer-aided design system that required a second 6502 CPU and a 3-dimensional joystick named a ""Bitstik""."
+6446,"The Model A and the Model B were built on the same printed circuit board , and a Model A can be upgraded to a Model B. Users wishing to operate Model B software need to add the extra RAM and the user/printer MOS Technology 6522 VIA  and snip a link, a task that can be achieved without soldering. To do a full upgrade with all the external ports requires soldering the connectors to the motherboard. The original machines shipped with ""OS 0.1"", with later updates advertised in magazines, supplied as a clip-in integrated circuit, with the last official version being ""OS 1.2"". Variations in the Acorn OS exist as a result of home-made projects and modified machines can still be bought on Internet auction sites such as eBay as of 2011."
+6447,"The BBC Model A was phased out of production with the introduction of the Acorn Electron, with chairman Chris Curry stating at the time that Acorn ""would no longer promote it"" ."
+6448,"Early BBC Micros used linear power supplies at the insistence of the BBC, which, as a broadcaster, was cautious about electromagnetic interference. The supplies were unreliable, and after a few months the BBC allowed switched-mode units."
+6449,An apparent oversight in the manufacturing process resulted in many Model Bs producing a constant buzzing noise from the built-in speaker. This fault can be rectified partly by soldering a resistor across two pads.
+6450,"There are five developments of the main BBC Micro circuit board that addressed various issues through the model's production, from 'Issue 1' through to 'Issue 7' with variants 5 and 6 not being released. The 1985 'BBC Microcomputer Service Manual' from Acorn documents the details of the technical changes."
+6451,Per Watford Electronics comments in their '32K Ram Board Manual':
+6452,"Early issue BBCs  are notorious for out of specification timings. If problems occur with this sort of machine, the problem can generally be cured by the use of either a Rockwell 6502A CPU chip, or by replacing IC14  with either another 74LS245 or the faster 74ALS245."
+6453,"Two export models were developed: one for the US, with Econet and speech hardware as standard; the other for West Germany. Despite concerns of unsuitability for the Australian market, with the design failing at temperatures above 35 °C , the machine was still ""widely used in Australian schools"". Export models were fitted with radio frequency shielding as required by the respective countries. From June 1983 the name was always spelled out completely – ""British Broadcasting Corporation Microcomputer System"" – to avoid confusion with Brown, Boveri & Cie in international markets, after warnings from the Swiss multinational not to market the computer with the BBC label in West Germany, thus forcing Acorn to relabel ""hundreds of machines"" to comply with these demands."
+6454,"US models include the BASIC III ROM chip, modified to accept the American spelling of COLOR, but the height of the graphics display was reduced to 200 scan lines to suit NTSC TVs, severely affecting applications written for British computers. After the failed US marketing campaign, the unwanted machines were remanufactured for the British market and sold, resulting in a third export variant."
+6455,"In October 1984, the Acorn Business Computer /Acorn Cambridge Workstation range of machines was announced, based primarily on BBC hardware."
+6456,"In mid-1985, Acorn introduced the Model B+ which increased the total RAM to 64 KB. This had a modest market impact and received a rather unsympathetic reception, with one reviewer's assessment being that the machine was ""18 months too late"" and that it ""must be seen as a stop gap"", and others criticising the elevated price of £500  in the face of significantly cheaper competition providing as much or even twice as much memory. The extra RAM in the Model B+ is assigned as two blocks, a block of 20 KB dedicated solely for screen display  and a block of 12 KB of special sideways RAM. The B+128, introduced towards the end of 1985, comes with an additional 64 KB  to give a total RAM of 128 KB."
+6457,"The B+ is incapable of operating some original BBC B programs and games, such as the very popular Castle Quest. A particular problem is the replacement of the Intel 8271 floppy-disk controller with the Western Digital 1770: not only was the new controller mapped to different addresses, it is fundamentally incompatible, and the 8271 emulators that existed were necessarily imperfect for all but basic operation. Software that use copy protection techniques involving direct access to the controller do not operate on the new system. Acorn attempted to alleviate this, starting with version 2.20 of the 1770 DFS, via an 8271-backward- compatible Ctrl+Z+Break option."
+6458,"There is also a long-running problem late in the B/B+'s commercial life infamous amongst B+ owners, when Superior Software released Repton Infinity, which did not run on the B+. A series of unsuccessful replacements were issued before one compatible with both was finally released."
+6459,"During 1986, Acorn followed up with the BBC Master, which offers memory sizes from 128 KB and many other refinements which improved upon the 1981 original. It has essentially the same 6502-based BBC architecture, with many of the upgrades that the original design intentionally makes possible  now included on the circuit board as internal plug-in modules."
+6460,The BBC Micro platform amassed a large software base of both games and educational programs for its two main uses as a home and educational computer. Notable examples of each include the original release of Elite and Granny's Garden. Programming languages and some applications were supplied on ROM chips to be installed on the motherboard. These load instantly and leave the RAM free for programs or documents.
+6461,"Although appropriate content was little-supported by television broadcasters, telesoftware could be downloaded via the optional Teletext Adapter and the third-party teletext adaptors that emerged."
+6462,"The built-in operating system, Acorn MOS, provides an extensive API to interface with all standard peripherals, ROM-based software, and the screen. Features specific to some versions of BASIC, like vector graphics, keyboard macros, cursor-based editing, sound queues, and envelopes, are in the MOS ROM and made available to any application. BBC BASIC itself, being in a separate ROM, can be replaced with another language."
+6463,"BASIC, other languages, and utility ROM chips reside in any of four 16 KB paged ROM sockets, with OS support for sixteen sockets via expansion hardware. The five  sockets are located partly obscured under the keyboard, with the leftmost socket hard-wired for the OS. The intended purpose for the perforated panel on the left of the keyboard was for a Serial ROM or Speech ROM. The paged ROM system is essentially modular. A language-independent system of star commands, prefixed with an asterisk, provides the ability to select a language , a filing system , change settings , or carry out ROM-supplied tasks  from the command line. The MOS recognises certain built-in commands, and polls the paged ROMs in descending order for service otherwise; if none of them claims the command, then the OS returns a Bad command error. Suitable ROM  images could be written and provide functions without requiring RAM for the code itself."
+6464,"Not all ROMs offer star commands , but any ROM can ""hook"" into vectors to enhance the system's functionality. Often the ROM is a device driver for mass storage combined with a filing system, starting with Acorn's 1982 Disc Filing System  which API became the de facto standard for floppy-disc access. The Acorn Graphics Extension ROM  expands the VDU routines to draw geometric shapes, flood fills, and sprites. During 1985, Micro Power designed and marketed a Basic Extension ROM, introducing statements such as WHILE, ENDWHILE, CASE, WHEN, OTHERWISE, ENDCASE, and direct mode commands including VERIFY."
+6465,"Acorn strongly discouraged programmers from directly accessing the system variables and hardware, favouring official system calls. This was ostensibly to make sure programs keep working when migrated to coprocessors that utilise the Tube interface, but it also makes BBC Micro software more portable across the Acorn range. Whereas untrappable PEEKs and POKEs are used by other computers to reach the system elements, programs in either machine code or BBC BASIC instead pass parameters to an operating system routine. In this way, the 6502 can translate the request for the local machine or send it across the Tube interface, as direct access is impossible from the coprocessor. Published programs largely conform to the API except for games, which routinely engage with the hardware for greater speed, and require a particular Acorn model."
+6466,"Many schools and universities employed the machines in Econet networks, and so networked multiplayer games were possible. Few became popular, due to the limited number of machines aggregated in one place. A relatively late but well documented example can be found in a dissertation based on a ringed RS-423 interconnect."
+6467,"The built-in ROM-resident BBC BASIC programming language interpreter realised the system's educational emphasis and was key to its success; it is the most comprehensive BASIC compared to other contemporary implementations, and runs very efficiently. Advanced programs can be written without resorting to non-structured programming or machine code. Should one want or need to program in assembly language, BBC BASIC has a built-in assembler that allows a mix of BASIC and assembly language for the processor on which a particular implementation of BASIC is running."
+6468,"When the BBC Micro was released, many competing home computers used Microsoft BASIC, or variants typically designed to resemble it. Compared to Microsoft BASIC, BBC BASIC features IF...THEN...ELSE, REPEAT...UNTIL, and named procedures and functions, but retains GOTO and GOSUB for compatibility. It also supports high-resolution graphics, four-channel sound, pointer-based memory access , and rudimentary macro assembly. Long variable names are accepted and distinguished completely, not just by the first two characters."
+6469,"Acorn had made a point of not just supporting BBC Basic but also supporting a number of contemporary languages, some of which were supplied as ROM chips to fit the spare sideways ROM sockets on the motherboard. Other languages were supplied on tape or disk."
+6470,Programming languages from Acornsoft included:
+6471,"Many of these languages were also provided under the Panos environment for the 32016 Second Processor. As the Z80 Second Processor provided a CP/M environment, languages available for CP/M were supportable via this route. For example, Turbo Pascal was regarded in one instance as ""by far the best version of Pascal"" for a BBC Micro with Z80 second processor. DOS-based language implementations such as Turbo C and Turbo Pascal could also be run by computers expanded with the Master 512 board, this being fitted to BBC Micro machines using a Universal Second Processor unit. Torch Computers' Z80 expansions supported the UCSD p-System, and the Torch Unicorn expansion provided a Unix environment that featured a C compiler and other utilities, with Pascal and Fortran 77 implementations also advertised.: 7"
+6472,"Acornsoft C did not run on the original BBC Micro models, requiring the extra resources provided by the B+ and Master series machines. Beebug C did, however, run on the standard Model B and later machines. Both of these implementations provided compilers producing interpreted ""p-code"" as opposed to machine code, similar to Acornsoft's ISO Pascal and BCPL compilers. A Small-C implementation was also made available by Mijas."
+6473,"For a BBC Micro without a second processor, Acornsoft's ISO Pascal primarily saw competition from Oxford Pascal. A Forth-based Pascal implementation from HCCS known as Pascal T was regarded as being ""intended primarily for learning Pascal, rather than using it seriously"", putting it in the same category as Acornsoft's S-Pascal, described as ""a simple subset of Pascal for teaching the language and structured programming""."
+6474,"As a computer aimed at the education market, the BBC Micro was supported by several implementations of Logo: Acornsoft Logo competing with Logo products from Logotron, LSL, and the Open University. These products provided either one or two 16 KB ROM chips for fitting inside the machine, although the Open University's Open Logo provided a second processor implementation that was loaded from disc. Acornsoft's Logo was itself written in the BCPL language whereas other implementations had been assembled to machine code. Compatibility with Logo dialects and standards varied, with Logotron Logo claiming a level of adherence to the broader Logo Computer Systems Inc.  industry standard, and the level of provided functionality differed between the BBC Micro implementations, with Acornsoft Logo providing the most comprehensive set of primitives in many areas. Support for various peripherals and accessories was advertised, floor turtles having particular significance, but hardware extensions offering sprite capabilities were also developed for use with certain implementations, notably Logotron Logo. Logotron Logo was eventually judged to be the most popular product and was bundled with the Master Compact by Acorn.: 103"
+6475,"In line with its ethos of expandability, Acorn produced its own range of peripherals for the BBC Micro, including:"
+6476,"Various products from other manufacturers competed directly with Acorn's expansions. For example, companies such as Torch Computers and Cambridge Microprocessor Systems offered second processor solutions for the BBC Micro."
+6477,"A large number of third-party suppliers also produced an abundance of add-on hardware, some of the most common being:"
+6478,"Acorn produced their own 32-bit Reduced Instruction Set Computing  CPU during 1985, the ARM1. Furber composed a reference model of the processor on the BBC Micro with 808 lines of BASIC, and Arm Ltd. retains copies of the code for intellectual property purposes. The first prototype ARM platforms, the ARM Evaluation System and the A500 workstation, functioned as second processors attached to the BBC Micro's Tube interface. Acorn staff developed the A500's operating system in situ through the Tube until, one by one, the on-board I/O ports were enabled and the A500 ran as a stand-alone computer. With an upgraded processor, this was eventually released during 1987 as four models in the Archimedes series, the lower-specified two models  continuing the BBC Microcomputer brand with the distinctive red function keys. Although the Archimedes ultimately was not a major success, the ARM family of processors has become the dominant processor architecture in mobile embedded consumer devices, particularly mobile telephones."
+6479,"Acorn's last BBC-related model, the BBC A3000, was released in 1989. It was essentially a 1 MB Archimedes back in a single case form factor."
+6480,"Furber said in 2015 that he was amazed that the BBC Micro ""established this reputation for being reliable, because lots of it was finger-in-the-air engineering"". As of 2018, thanks to its ready expandability and I/O functions, there are still numbers of BBC Micros in use, and a retrocomputing community of dedicated users finding new tasks for the old hardware. They still survive in a few interactive displays in museums across the United Kingdom, and the Jodrell Bank observatory was reported using a BBC Micro to steer its 42 ft radio telescope in 2004. Furber said that although ""the  margins on the Beeb were very, very small"", when he asked BBC owners at a retrocomputing meeting what components had failed after 30 years, they said ""you have to replace the capacitors in the power supply but everything else still works"". The Archimedes came with 65Arthur, an emulator which Byte stated ""lets many programs for the BBC Micro run"". Other emulators exist for many operating systems, including Beebdroid for Android and JS Beeb for the web."
+6481,"In March 2008, the creators of the BBC Micro met at the Science Museum in London. There was to be an exhibition about the computer and its legacy during 2009."
+6482,The UK National Museum of Computing at Bletchley Park uses BBC Micros as part of a scheme to educate school children about computer programming.
+6483,"In March 2012, the BBC and Acorn teams responsible for the BBC Micro and Computer Literacy Project met for a 30th anniversary party, entitled ""Beeb@30"". This was held at Arm's offices in Cambridge and was co-hosted by the Centre for Computing History."
+6484,"Long after the ""venerable old Beeb"" was superseded, additional hardware and software has been developed. Such developments have included Sprow's 1999 zip compression utility and a ROM Y2K bugfix for the BBC Master."
+6485,There are also a number of websites still supporting both hardware and software development for the BBC Micros and Acorn in general.
+6486,"Like the IBM PC with the contemporary Color Graphics Adapter, the video output of the BBC Micro could be switched by software between a number of display modes. These varied between 20 and 40-column text suitable for a domestic TV and 80-column text best viewed with a high-quality RGB-connected monitor; the latter mode was often too blurred to view when using a domestic TV via the UHF output. The variety of modes offered applications a flexible compromise between colour depth, resolution and memory economy. In the first models, the OS and applications were left with the RAM left over from the display mode."
+6487,"Mode 7 was a Teletext mode, extremely economical on memory and an original requirement due to the BBC's own use of broadcast teletext . It also made the computer useful as a Prestel terminal. The teletext characters were generated using an SAA5050 chip, for use with monitors and TV sets without a Teletext receiver. Mode 7 used only 1 KB for video RAM by storing each character as its ASCII code, rather than its bitmap image as was needed for the other modes."
+6488,"Modes 0 to 6 could display colours from a logical palette of sixteen: the eight basic colours at the vertices of the RGB colour cube and eight flashing colours made by alternating the basic colour with its inverse. The palette could be freely reprogrammed without touching display memory. Modes 3 and 6 were special text-only modes that used less RAM by reducing the number of text rows and inserting blank scan lines below each row. Mode 6 was the smallest, allocating 8 KB as video memory. Modes 0 to 6 could show diacritics and other user defined characters. All modes except mode 7 supported bitmapped graphics, but graphics commands such as DRAW and PLOT had no effect in the text-only modes."
+6489,"The BBC B+ and the later Master provided 'shadow modes', where the 1–20 KB frame buffer was stored in an alternative RAM bank, freeing the main memory for user programs. This feature was requested by setting bit 7 of the mode variable, i.e. by requesting modes 128–135."
+6490,"A speech synthesis upgrade based on the Texas Instruments TMS5220 featured sampled words spoken by BBC newscaster Kenneth Kendall. This speech system was standard on the US model where it had an American vocabulary. The Computer Concepts Speech ROM also made use of the TMS5220 speech processor but not the speech ROMs, instead driving the speech processor directly. The speech upgrade sold poorly and was largely superseded by Superior Software's software-based synthesiser using the standard sound hardware."
+6491,"The speech upgrade also added two empty sockets next to the keyboard, intended for 16 KB serial ROM cartridges containing either extra speech phoneme data beyond that held in the speech paged ROM or general software accessed through the ROM Filing System. The original plan was that some games would be released on cartridges, but due to the limited sales of the speech upgrade combined with economic and other viability concerns, little or no software was ever produced for these sockets. The cut-out space next to the keyboard  was more commonly used to install other upgrades, such as a ZIF socket for conventional paged ROMs."
+6492,"The BBC Domesday Project, a pioneering multimedia experiment, was based on a modified version of the BBC Micro's successor, the BBC Master."
+6493,"Musician Vince Clarke of the British synth pop bands Depeche Mode, Yazoo, and Erasure used a BBC Micro  with the UMI music sequencer to compose many hits. In music videos from the 1980s featuring Vince Clarke, a BBC Micro is often present or provides text and graphics such as a clip for Erasure's ""Oh L'Amour"". The musical group Queen used the UMI Music Sequencer on their record A Kind of Magic. The UMI is also mentioned in the CD booklet. Other bands who have used the BBC Micro for making music are A-ha and the reggae band Steel Pulse. Paul Ridout is credited as ""UMI programmer"" on Cars' bassist/vocalist Benjamin Orr's 1986 solo album, The Lace. Other UMI users included Blancmange, Alan Parsons, and Mutt Lange. Black Uhuru used the Envelope Generator from SYSTEM software  running on a BBC Micro, to create some of the electro-dub sounds on Try It ."
+6494,"The BBC Micro was used extensively to provide graphics and sound effects for many early 1980s BBC TV shows. These included, notably, series 3 and 4 of The Adventure Game; the children's quiz game ""First Class"" ; and numerous 1980s episodes of Doctor Who including ""Castrovalva"", ""The Five Doctors"", and ""The Twin Dilemma""."
+6495,"In 2013, NESTA released a report into the legacy of The BBC Micro, looking at the history and impact of the machine and The BBC Computer Literacy project. In June 2018, the BBC released its archives of the Computer Literacy Project."
+6496,"The BBC Micro had a lasting technological impact on the education market by introducing an informal educational standard around the hardware and software technologies employed by the range, particularly the use of BBC BASIC, and by establishing a considerable investment by schools in software for the machine. Consequently, manufacturers of rival systems such as IBM PC compatibles , the Apple Macintosh, Commodore Amiga, and Acorn as manufacturer of the BBC Micro's successor, the Archimedes, were compelled to provide a degree of compatibility with the large number of machines already deployed in schools."
+6497,BBC portal
+6498,1980s portal
+6499,"Punched tape or perforated paper tape is a form of data storage device that consists of a long strip of paper through which small holes are punched. It was developed from and was subsequently used alongside punched cards, the difference being that the tape is continuous."
+6500,"Punched cards, and chains of punched cards, were used for control of looms in the 18th century. Use for telegraphy systems started in 1842. Punched tapes were used throughout the 19th and for much of the 20th centuries for programmable looms, teleprinter communication, for input to computers of the 1950s and 1960s, and later as a storage medium for minicomputers and CNC machine tools. During the Second World War, high-speed punched tape systems using optical readout methods were used in code breaking systems. Punched tape was used to transmit data for manufacture of read-only memory chips."
+6501,"Perforated paper tapes were first used by Basile Bouchon in 1725 to control looms. However, the paper tapes were expensive to create, fragile, and difficult to repair. By 1801, Joseph Marie Jacquard had developed machines to create paper tapes by tying punched cards in a sequence for Jacquard looms. The resulting paper tape, also called a ""chain of cards"", was stronger and simpler both to create and to repair. This led to the concept of communicating data not as a stream of individual cards, but as one ""continuous card"" . Paper tapes constructed from punched cards were widely used throughout the 19th century for controlling looms. Many professional embroidery operations still refer to those individuals who create the designs and machine patterns as punchers even though punched cards and paper tape were eventually phased out in the 1990s."
+6502,"In 1842, a French patent by Claude Seytre described a piano playing device that read data from perforated paper rolls. By 1900, wide perforated music rolls for player pianos were used to distribute popular music to mass markets."
+6503,"In 1846, Alexander Bain used punched tape to send telegrams. This technology was adopted by Charles Wheatstone in 1857 for the Wheatstone system used for the automated preparation, storage and transmission of data in telegraphy."
+6504,"In the 1880s, Tolbert Lanston invented the Monotype typesetting system, which consisted of a keyboard and a composition caster. The tape, punched with the keyboard, was later read by the caster, which produced lead type according to the combinations of holes in up to 31 positions. The tape reader used compressed air, which passed through the holes and was directed into certain mechanisms of the caster. The system went into commercial use in 1897 and was in production well into the 1970s, undergoing several changes along the way."
+6505,"In the 21st century, punched tape is obsolete except among hobbyists. In computer numerical control  machining applications, though paper tape has been superseded by digital memory, some modern systems still measure the size of stored CNC programs in feet or meters, corresponding to the equivalent length if the data were actually punched on paper tape."
+6506,"Data was represented by the presence or absence of a hole at a particular location. Tapes originally had five rows of holes for data across the width of the tape. Later tapes had more rows. A 1944 electro-mechanical programmable calculating machine, the Automatic Sequence Controlled Calculator or Harvard Mark I, used paper tape with 24 rows. Australia's 1951 electronic computer, CSIRAC, used 3-inch  wide paper tape with twelve rows."
+6507,"A row of smaller sprocket holes was always punched to be used to synchronize tape movement. Originally, this was done using a wheel with radial teeth called a sprocket wheel. Later, optical readers made use of the sprocket holes to generate timing pulses. The sprocket holes were slightly closer to one edge of the tape, dividing the tape into unequal widths, to make it unambiguous which way to orient the tape in the reader. The bits on the narrower width of the tape were generally the least significant bits when the code was represented as numbers in a digital system."
+6508,"Many early machines used oiled paper tape, which was pre-impregnated with a light machine oil, to lubricate the reader and punch mechanisms. The oil impregnation usually made the paper somewhat translucent and slippery, and excess oil could transfer to clothing or any surfaces it contacted. Later optical tape readers often specified non-oiled opaque paper tape, which was less prone to depositing oily debris on the optical sensors and causing read errors. Another innovation was fanfold paper tape, which was easier to store compactly and less prone to tangling, as compared to rolled paper tape."
+6509,"For heavy-duty or repetitive use, polyester Mylar tape was often used. This tough, durable plastic film was usually thinner than paper tapes, but could still be used in many devices originally designed for paper media. The plastic tape was sometimes transparent, but usually was aluminized to make it opaque enough for use in high-speed optical readers."
+6510,"Tape for punching was usually 0.00394 inches  thick. The two most common widths were 11⁄16 inch  for five bit codes, and 1 inch  for tapes with six or more bits. Hole spacing was 0.1 inches  in both directions. Data holes were 0.072 inches  in diameter; sprocket feed holes were 0.046 inches ."
+6511,"Most tape-punching equipment used solid circular punches to create holes in the tape. This process created ""chad"", or small circular pieces of paper. Managing the disposal of chad was an annoying and complex problem, as the tiny paper pieces had a tendency to escape containment and to interfere with the other electromechanical parts of the teleprinter equipment. Chad from oiled paper tape was particularly problematic, as it tended to clump and build up, rather than flowing freely into a collection container."
+6512,"A variation on the tape punch was a device called a Chadless Printing Reperforator. This machine would punch a received teleprinter signal into tape and print the message on it at the same time, using a printing mechanism similar to that of an ordinary page printer. The tape punch, rather than punching out the usual round holes, would instead punch little U-shaped cuts in the paper, so that no chad would be produced; the ""hole"" was still filled with a little paper trap-door. By not fully punching out the hole, the printing on the paper remained intact and legible. This enabled operators to read the tape without having to decipher the holes, which would facilitate relaying the message on to another station in the network. Also, there was no ""chad box"" to empty from time to time."
+6513,"A disadvantage to this technology was that, once punched, chadless tape did not roll up well for storage, because the protruding flaps of paper would catch on the next layer of tape so it could not be coiled up tightly. Another disadvantage that emerged in time, was that there was no reliable way to read chadless tape in later high-speed readers which used optical sensing. However, the mechanical tape readers used in most standard-speed equipment had no problem with chadless tape, because they sensed the holes by means of blunt spring-loaded mechanical sensing pins, which easily pushed the paper flaps out of the way."
+6514,"Text was encoded in several ways. The earliest standard character encoding was Baudot, which dates back to the 19th century and had five holes. The Baudot code was superseded by modified five-hole codes such as the Murray code  which was developed into the Western Union code which was further developed into the International Telegraph Alphabet No. 2 , and a variant called the American Teletypewriter code . Other standards, such as Teletypesetter , FIELDATA and Flexowriter, had six holes. In the early 1960s, the American Standards Association led a project to develop a universal code for data processing, which became the American Standard Code for Information Interchange . This seven-level code was adopted by some teleprinter users, including AT&T . Others, such as Telex, stayed with the earlier codes."
+6515,"Punched tape was used as a way of storing messages for teletypewriters. Operators typed in the message to the paper tape, and then sent the message at the maximum line speed from the tape. This permitted the operator to prepare the message ""off-line"" at the operator's best typing speed, and permitted the operator to correct any error prior to transmission. An experienced operator could prepare a message at 135 words per minute  or more for short periods."
+6516,"The line typically operated at 75 WPM, but it operated continuously. By preparing the tape ""off-line"" and then sending the message with a tape reader, the line could operate continuously rather than depending on continuous ""on-line"" typing by a single operator. Typically, a single 75 WPM line supported three or more teletype operators working offline. Tapes punched at the receiving end could be used to relay messages to another station. Large store and forward networks were developed using these techniques."
+6517,"Paper tape could be read into computers at up to 1,000 characters per second. In 1963, a Danish company called Regnecentralen introduced a paper tape reader called RC 2000 that could read 2,000 characters per second; later they increased the speed further, up to 2,500 cps. As early as World War II, the Heath Robinson tape reader, used by Allied codebreakers, was capable of 2,000 cps while Colossus could run at 5,000 cps using an optical tape reader designed by Arnold Lynch."
+6518,"When the first minicomputers were being released, most manufacturers turned to the existing mass-produced ASCII teleprinters  as a low-cost solution for keyboard input and printer output. The commonly specified Model 33 ASR included a paper tape punch/reader, where ASR stands for ""Automatic Send/Receive"" as opposed to the punchless/readerless KSR – Keyboard Send/Receive and RO – Receive Only models. As a side effect, punched tape became a popular medium for low-cost minicomputer data and program storage, and it was common to find a selection of tapes containing useful programs in most minicomputer installations. Faster optical readers were also common."
+6519,"Binary data transfer to or from these minicomputers was often accomplished using a doubly encoded technique to compensate for the relatively high error rate of punches and readers. The low-level encoding was typically ASCII, further encoded and framed in various schemes such as Intel Hex, in which a binary value of ""01011010"" would be represented by the ASCII characters ""5A"". Framing, addressing and checksum  information helped with error detection. Efficiencies of such an encoding scheme are on the order of 35–40% ."
+6520,"In the 1970s, computer-aided manufacturing equipment often used paper tape. A paper tape reader was smaller and less expensive than Hollerith card or magnetic tape readers, and the medium was reasonably reliable in a manufacturing environment. Paper tape was an important storage medium for computer-controlled wire-wrap machines, for example."
+6521,"Premium black waxed and lubricated long-fiber papers, and Mylar film tape were developed so that heavily used production tapes would last longer."
+6522,"In the 1970s through the early 1980s, paper tape was commonly used to transfer binary data for incorporation in either mask-programmable read-only memory  chips or their erasable counterparts EPROMs. A significant variety of encoding formats were developed for use in computer and ROM/EPROM data transfer. Encoding formats commonly used were primarily driven by those formats that EPROM programming devices supported and included various ASCII hex variants as well as a number of proprietary formats."
+6523,"A much more primitive as well as a much longer high-level encoding scheme was also used, BNPF , also written as BPNF . In BNPF encoding, a single byte  would be represented by a highly redundant character framing sequence starting with a single uppercase ASCII ""B"", eight ASCII characters where a ""0"" would be represented by a ""N"" and a ""1"" would be represented by a ""P"", followed by an ending ASCII ""F"". These ten-character ASCII sequences were separated by one or more whitespace characters, therefore using at least eleven ASCII characters for each byte stored . The ASCII ""N"" and ""P"" characters differed in four bit positions, providing excellent protection from single punch errors. Alternative schemes named BHLF  and B10F  were also available where either ""L"" and ""H"" or ""0"" and ""1"" were also available to represent data bits, but in both of these encoding schemes, the two data-bearing ASCII characters differ in only one bit position, providing very poor single punch error detection."
+6524,"NCR of Dayton, Ohio, made cash registers around 1970 that would punch paper tape. Sweda made similar cash registers around the same time. The tape could then be read into a computer and not only could sales information be summarized, billings could be done on charge transactions. The tape was also used for inventory tracking, recording department and class numbers of items sold."
+6525,"Punched paper tape was used by the newspaper industry until the mid-1970s or later. Newspapers were typically set in hot lead by devices like Linotype machines. With the wire services coming into a device that would punch paper tape, rather than the Linotype operator having to retype all the incoming stories, the paper tape could be put into a paper tape reader on the Linotype and it would create the lead slugs without the operator re-typing the stories. This also allowed newspapers to use devices, such as the Friden Flexowriter, to convert typing to lead type via tape. Even after the demise of Linotype and hot lead typesetting, many early phototypesetter devices utilized paper tape readers."
+6526,"If an error was found at one position on the six-level tape, that character could be turned into a null character to be skipped by punching out the remaining non-punched positions with what was known as a “chicken plucker"". It looked like a strawberry stem remover that, pressed with thumb and forefinger, could punch out the remaining positions, one hole at a time."
+6527,"Vernam ciphers were invented in 1917 to encrypt teleprinter communications using a key stored on paper tape. During the last third of the 20th century, the National Security Agency  used punched paper tape to distribute cryptographic keys. The eight-level paper tapes were distributed under strict accounting controls and read by a fill device, such as the hand held KOI-18, that was temporarily connected to each security device that needed new keys. NSA has been trying to replace this method with a more secure electronic key management system , but as of 2016, paper tape was apparently still being employed. The paper tape canister is a tamper-resistant container that contains features to prevent undetected alteration of the contents."
+6528,"Acid-free paper or Mylar tapes can be read many decades after manufacture, in contrast with magnetic tape that can deteriorate and become unreadable with time. The hole patterns of punched tape can be decoded by eye if necessary, and even editing of a tape is possible by manual cutting and splicing. Unlike magnetic tape, magnetic fields such as produced by electric motors cannot alter the punched data. In cryptography applications, a punched tape used to distribute a key can be rapidly and completely destroyed by burning, preventing the key from falling into the hands of an enemy."
+6529,"Reliability of paper tape punching operations was a concern, so that for critical applications a new punched tape could be read after punching to verify the correct contents. Rewinding a tape required a takeup reel or other measures to avoid tearing or tangling the tape.  In some uses, ""fan fold"" tape simplified handling as the tape would refold into a ""takeup tank"" ready to be re-read. The information density of punched tape was low compared with magnetic tape, making large datasets clumsy to handle in punched tape form."
+6530,A punched card  is a piece of card stock that stores digital data using punched holes. Punched cards were once common in data processing and the control of automated machines.
+6531,"Punched cards were widely used in the 20th century, where unit record machines, organized into data processing systems, used punched cards for data input, output, and storage. The IBM 12-row/80-column punched card format came to dominate the industry. Many early digital computers used punched cards as the primary medium for input of both computer programs and data."
+6532,Data can be entered onto a punched card using a keypunch.
+6533,"While punched cards are now obsolete as a storage medium, as of 2012, some voting machines still used punched cards to record votes. Punched cards also had a significant cultural impact in the 20th century."
+6534,The idea of control and data storage via punched holes was developed independently on several occasions in the modern period. In most cases there is no evidence that each of the inventors was aware of the earlier work.
+6535,"Basile Bouchon developed the control of a loom by punched holes in paper tape in 1725. The design was improved by his assistant Jean-Baptiste Falcon and by Jacques Vaucanson. Although these improvements controlled the patterns woven, they still required an assistant to operate the mechanism."
+6536,In 1804 Joseph Marie Jacquard demonstrated a mechanism to automate loom operation. A number of punched cards were linked into a chain of any length. Each card held the instructions for shedding  and selecting the shuttle for a single pass.
+6537,Semyon Korsakov was reputedly the first to propose punched cards in informatics for information store and search. Korsakov announced his new method and machines in September 1832.
+6538,"Charles Babbage proposed the use of ""Number Cards"", ""pierced with certain holes and stand opposite levers connected with a set of figure wheels ... advanced they push in those levers opposite to which there are no holes on the cards and thus transfer that number together with its sign"" in his description of the Calculating Engine's Store. There is no evidence that he built a practical example."
+6539,"In 1881 Jules Carpentier developed a method of recording and playing back performances on a harmonium using punched cards. The system was called the Mélographe Répétiteur and ""writes down ordinary music played on the keyboard dans le langage de Jacquard"", that is as holes punched in a series of cards. By 1887 Carpentier had separated the mechanism into the Melograph which recorded the player's key presses and the Melotrope which played the music."
+6540,"At the end of the 1800s Herman Hollerith invented the recording of data on a medium that could then be read by a machine, developing punched card data processing technology for the 1890 U.S. census. His tabulating machines read and summarized data stored on punched cards and they began use for government and commercial data processing."
+6541,"Initially, these electromechanical machines only counted holes, but by the 1920s they had units for carrying out basic arithmetic operations.: 124 
+Hollerith founded the Tabulating Machine Company  which was one of four companies that were amalgamated via stock acquisition to form a fifth company, Computing-Tabulating-Recording Company  in 1911, later renamed International Business Machines Corporation  in 1924. Other companies entering the punched card business included The Tabulator Limited , Deutsche Hollerith-Maschinen Gesellschaft mbH  , Powers Accounting Machine Company , Remington Rand , and H.W. Egli Bull . These companies, and others, manufactured and marketed a variety of punched cards and unit record machines for creating, sorting, and tabulating punched cards, even after the development of electronic computers in the 1950s."
+6542,"Both IBM and Remington Rand tied punched card purchases to machine leases, a violation of the US 1914 Clayton Antitrust Act. In 1932, the US government took both to court on this issue. Remington Rand settled quickly. IBM viewed its business as providing a service and that the cards were part of the machine. IBM fought all the way to the Supreme Court and lost in 1936; the court ruled that IBM could only set card specifications.: 300–301"
+6543,"""By 1937... IBM had 32 presses at work in Endicott, N.Y., printing, cutting and stacking five to 10 million punched cards every day."" Punched cards were even used as legal documents, such as U.S. Government checks and savings bonds."
+6544,"During World War II punched card equipment was used by the Allies in some of their efforts to decrypt Axis communications. See, for example, Central Bureau in Australia. At Bletchley Park in England, ""some 2 million punched cards a week were being produced, indicating the sheer scale of this part of the operation"". In Nazi Germany, punched cards were used for the censuses of various regions and other purposes ."
+6545,"Punched card technology developed into a powerful tool for business data-processing. By 1950 punched cards had become ubiquitous in industry and government. ""Do not fold, spindle or mutilate,"" a warning that appeared on some punched cards distributed as documents such as checks and utility bills to be returned for processing, became a motto for the post-World War II era."
+6546,"In 1956 IBM signed a consent decree requiring, amongst other things, that IBM would by 1962 have no more than one-half of the punched card manufacturing capacity in the United States. Tom Watson Jr.'s decision to sign this decree, where IBM saw the punched card provisions as the most significant point, completed the transfer of power to him from Thomas Watson, Sr."
+6547,"The Univac UNITYPER introduced magnetic tape for data entry in the 1950s. During the 1960s, the punched card was gradually replaced as the primary means for data storage by magnetic tape, as better, more capable computers became available. Mohawk Data Sciences introduced a magnetic tape encoder in 1965, a system marketed as a keypunch replacement which was somewhat successful. Punched cards were still commonly used for entering both data and computer programs until the mid-1980s when the combination of lower cost magnetic disk storage, and affordable interactive terminals on less expensive minicomputers made punched cards obsolete for these roles as well.: 151  However, their influence lives on through many standard conventions and file formats. The terminals that replaced the punched cards, the IBM 3270 for example, displayed 80 columns of text in text mode, for compatibility with existing software. Some programs still operate on the convention of 80 text columns, although fewer and fewer do as newer systems employ graphical user interfaces with variable-width type fonts."
+6548,"The terms punched card, punch card, and punchcard were all commonly used, as were IBM card and Hollerith card . IBM used ""IBM card"" or, later, ""punched card"" at first mention in its documentation and thereafter simply ""card"" or ""cards"". Specific formats were often indicated by the number of character positions available, e.g. 80-column card. A sequence of cards that is input to or output from some step in an application's processing is called a card deck or simply deck. The rectangular, round, or oval bits of paper punched out were called chad  or chips . Sequential card columns allocated for a specific use, such as names, addresses, multi-digit numbers, etc., are known as a field. The first card of a group of cards, containing fixed or indicative information for that group, is known as a master card. Cards that are not master cards are detail cards."
+6549,"The Hollerith punched cards used for the 1890 U.S. census were blank. Following that, cards commonly had printing such that the row and column position of a hole could be easily seen. Printing could include having fields named and marked by vertical lines, logos, and more. ""General purpose"" layouts  were also available. For applications requiring master cards to be separated from following detail cards, the respective cards had different upper corner diagonal cuts and thus could be separated by a sorter. Other cards typically had one upper corner diagonal cut so that cards not oriented correctly, or cards with different corner cuts, could be identified."
+6550,"Herman Hollerith was awarded three patents in 1889 for electromechanical tabulating machines. These patents described both paper tape and rectangular cards as possible recording media. The card shown in U.S. patent 395,781 of January 8 was printed with a template and had hole positions arranged close to the edges so they could be reached by a railroad conductor's ticket punch, with the center reserved for written descriptions. Hollerith was originally inspired by railroad tickets that let the conductor encode a rough description of the passenger:"
+6551,"I was traveling in the West and I had a ticket with what I think was called a punch photograph...the conductor...punched out a description of the individual, as light hair, dark eyes, large nose, etc. So you see, I only made a punch photograph of each person.: 15"
+6552,"When use of the ticket punch proved tiring and error-prone, Hollerith developed the pantograph ""keyboard punch"". It featured an enlarged diagram of the card, indicating the positions of the holes to be punched. A printed reading board could be placed under a card that was to be read manually.: 43"
+6553,"Hollerith envisioned a number of card sizes. In an article he wrote describing his proposed system for tabulating the 1890 U.S. census, Hollerith suggested a card 3 by 5+1⁄2 inches  of Manila stock ""would be sufficient to answer all ordinary purposes."" The cards used in the 1890 census had round holes, 12 rows and 24 columns. A reading board for these cards can be seen at the Columbia University Computing History site. At some point, 3+1⁄4 by 7+3⁄8 inches  became the standard card size. These are the dimensions of the then-current paper currency of 1862–1923. This size was needed in order to use available banking-type storage for the 60,000,000 punched cards to come nationwide.
+Hollerith's original system used an ad hoc coding system for each application, with groups of holes assigned specific meanings, e.g. sex or marital status. His tabulating machine had up to 40 counters, each with a dial divided into 100 divisions, with two indicator hands; one which stepped one unit with each counting pulse, the other which advanced one unit every time the other dial made a complete revolution. This arrangement allowed a count up to 9,999. During a given tabulating run counters were assigned specific holes or, using relay logic, combination of holes."
+6554,"Later designs led to a card with ten rows, each row assigned a digit value, 0 through 9, and 45 columns. 
+This card provided for fields to record multi-digit numbers that tabulators could sum, instead of their simply counting cards. Hollerith's 45 column punched cards are illustrated in Comrie's The application of the Hollerith Tabulating Machine to Brown's Tables of the Moon."
+6555,"By the late 1920s, customers wanted to store more data on each punched card. Thomas J. Watson Sr., IBM's head, asked two of his top inventors, Clair D. Lake and J. Royden Pierce, to independently develop ways to increase data capacity without increasing the size of the punched card. Pierce wanted to keep round holes and 45 columns but to allow each column to store more data, Lake suggested rectangular holes, which could be spaced more tightly, allowing 80 columns per punched card, thereby nearly doubling the capacity of the older format. Watson picked the latter solution, introduced as The IBM Card, in part because it was compatible with existing tabulator designs and in part because it could be protected by patents and give the company a distinctive advantage."
+6556,"This IBM card format, introduced in 1928, has rectangular holes, 80 columns, and 10 rows. Card size is 7+3⁄8 by 3+1⁄4 inches . The cards are made of smooth stock, 0.007 inches  thick. There are about 143 cards to the inch . In 1964, IBM changed from square to round corners. They come typically in boxes of 2000 cards or as continuous form cards. Continuous form cards could be both pre-numbered and pre-punched for document control ."
+6557,"Initially designed to record responses to yes–no questions, support for numeric, alphabetic and special characters was added through the use of columns and zones. The top three positions of a column are called zone punching positions, 12 , 11, and 0 . For decimal data the lower ten positions are called digit punching positions, 0  through 9. An arithmetic sign can be specified for a decimal field by overpunching the field's rightmost column with a zone punch: 12 for plus, 11 for minus . For Pound sterling pre-decimalization currency a penny column represents the values zero through eleven; 10 , 11, then 0 through 9 as above. An arithmetic sign can be punched in the adjacent shilling column.: 9  Zone punches had other uses in processing, such as indicating a master card."
+6558,"Diagram: Note: The 11 and 12 zones were also called the X and Y zones, respectively."
+6559,"In 1931, IBM began introducing upper-case letters and special characters . The 26 letters have two punches . The languages of Germany, Sweden, Denmark, Norway, Spain, Portugal and Finland require up to three additional letters; their punching is not shown here.: 88–90  Most special characters have two or three punches ; a few special characters were exceptions: ""&"" is 12 only, ""-"" is 11 only, and ""/"" is 0 + 1). The Space character has no punches.: 38  The information represented in a column by a combination of zones  and digits  is dependent on the use of that column. For example, the combination ""12-1"" is the letter ""A"" in an alphabetic column, a plus signed digit ""1"" in a signed numeric column, or an unsigned digit ""1"" in a column where the ""12"" has some other use. The introduction of EBCDIC in 1964 defined columns with as many as six punches . IBM and other manufacturers used many different 80-column card character encodings. A 1969 American National Standard defined the punches for 128 characters and was named the Hollerith Punched Card Code , honoring Hollerith.: 7"
+6560,"For some computer applications, binary formats were used, where each hole represented a single binary digit , every column  is treated as a simple bit field, and every combination of holes is permitted."
+6561,"For example, on the IBM 701 and IBM 704, card data was read, using an IBM 711, into memory in row binary format. For each of the twelve rows of the card, 72 of the 80 columns would be read into two 36-bit words; a control panel was used to select the 72 columns to be read. Software would translate this data into the desired form. One convention was to use columns 1 through 72 for data, and columns 73 through 80 to sequentially number the cards, as shown in the picture above of a punched card for FORTRAN. Such numbered cards could be sorted by machine so that if a deck was dropped the sorting machine could be used to arrange it back in order. This convention continued to be used in FORTRAN, even in later systems where the data in all 80 columns could be read."
+6562,"As an aid to humans who had to deal with the punched cards, the IBM 026 and later 029 and 129 key punch machines could print human-readable text above each of the 80 columns."
+6563,"As a prank, punched cards could be made where every possible punch position had a hole. Such ""lace cards"" lacked structural strength, and would frequently buckle and jam inside the machine."
+6564,"The IBM 80-column punched card format dominated the industry, becoming known as just IBM cards, even though other companies made cards and equipment to process them."
+6565,"One of the most common punched card formats is the IBM 5081 card format, a general purpose layout with no field divisions. This format has digits printed on it corresponding to the punch positions of the digits in each of the 80 columns. Other punched card vendors manufactured cards with this same layout and number."
+6566,"Long cards were available with a scored stub on either end which, when torn off, left an 80 column card. The torn off card is called a stub card."
+6567,"80-column cards were available scored, on either end, creating both a short card and a stub card when torn apart. Short cards can be processed by other IBM machines. A common length for stub cards was 51 columns. Stub cards were used in applications requiring tags, labels, or carbon copies."
+6568,"According to the IBM Archive: IBM's Supplies Division introduced the Port-A-Punch in 1958 as a fast, accurate means of manually punching holes in specially scored IBM punched cards. Designed to fit in the pocket, Port-A-Punch made it possible to create punched card documents anywhere. The product was intended for ""on-the-spot"" recording operations��such as physical inventories, job tickets and statistical surveys—because it eliminated the need for preliminary writing or typing of source documents."
+6569,"In 1969 IBM introduced a new, smaller, round-hole, 96-column card format along with the IBM System/3 low-end business computer. These cards have tiny, 1 mm diameter circular holes, smaller than those in paper tape. Data are stored in 6-bit BCD, with three rows of 32 characters each, or 8-bit EBCDIC. In this format, each column of the top tiers are combined with two punch rows from the bottom tier to form an 8-bit byte, and the middle tier is combined with two more punch rows, so that each card contains 64 bytes of 8-bit-per-byte binary coded data. As in the 80 column card, readable text was printed in the top section of the card. There was also a 4th row of 32 characters that could be printed. This format was never widely used; it was IBM-only, but they did not support it on any equipment beyond the System/3, where it was quickly superseded by the 1973 IBM 3740 Data Entry System using 8-inch floppy disks."
+6570,"The Powers/Remington Rand card format was initially the same as Hollerith's; 45 columns and round holes. In 1930, Remington Rand leap-frogged IBM's 80 column format from 1928 by coding two characters in each of the 45 columns – producing what is now commonly called the 90-column card.: 142  There are two sets of six rows across each card. The rows in each set are labeled 0, 1/2, 3/4, 5/6, 7/8 and 9. The even numbers in a pair are formed by combining that punch with a 9 punch. Alphabetic and special characters use 3 or more punches."
+6571,"The British Powers-Samas company used a variety of card formats for their unit record equipment. They began with 45 columns and round holes. Later 36, 40 and 65 column cards were provided. A 130 column card was also available – formed by dividing the card into two rows, each row with 65 columns and each character space with 5 punch positions. A 21 column card was comparable to the IBM Stub card.: 47–51"
+6572,"Mark sense  cards, developed by Reynold B. Johnson at IBM, have printed ovals that could be marked with a special electrographic pencil. Cards would typically be punched with some initial information, such as the name and location of an inventory item. Information to be added, such as quantity of the item on hand, would be marked in the ovals. Card punches with an option to detect mark sense cards could then punch the corresponding information into the card."
+6573,"Aperture cards have a cut-out hole on the right side of the punched card. A piece of 35 mm microfilm containing a microform image is mounted in the hole. Aperture cards are used for engineering drawings from all engineering disciplines. Information about the drawing, for example the drawing number, is typically punched and printed on the remainder of the card."
+6574,"IBM's Fred M. Carroll developed a series of rotary presses that were used to produce punched cards, including a 1921 model that operated at 460 cards per minute . In 1936 he introduced a completely different press that operated at 850 cpm. Carroll's high-speed press, containing a printing cylinder, revolutionized the company's manufacturing of punched cards. It is estimated that between 1930 and 1950, the Carroll press accounted for as much as 25 percent of the company's profits."
+6575,"Discarded printing plates from these card presses, each printing plate the size of an IBM card and formed into a cylinder, often found use as desk pen/pencil holders, and even today are collectible IBM artifacts ."
+6576,"In the mid-1930s a box of 1,000 cards cost $1.05 ."
+6577,"While punched cards have not been widely used for generations, the impact was so great for most of the 20th century that they still appear from time to time in popular culture. For example:"
+6578,"A common example of the requests often printed on punched cards which were to be individually handled, especially those intended for the public to use and return is ""Do Not Fold, Spindle or Mutilate"" .: 43–55  Coined by Charles A. Phillips, it became a motto for the post–World War II era , and was widely mocked and satirized. Some 1960s students at Berkeley wore buttons saying: ""Do not fold, spindle or mutilate. I am a student"". The motto was also used for a 1970 book by Doris Miles Disney with a plot based around an early computer dating service and a 1971 made-for-TV movie based on that book, and a similarly titled 1967 Canadian short film, Do Not Fold, Staple, Spindle or Mutilate."
+6579,"ANSI INCITS 21-1967 , Rectangular Holes in Twelve-Row Punched Cards ) Specifies the size and location of rectangular holes in twelve-row 3+1⁄4-inch-wide  punched cards."
+6580,ANSI X3.11-1990 American National Standard Specifications for General Purpose Paper Cards for Information Processing
+6581,ANSI X3.26-1980  Hollerith Punched Card Code
+6582,ISO 1681:1973 Information processing – Unpunched paper cards – Specification
+6583,"ISO 6586:1980 Data processing – Implementation of the ISO 7- bit and 8- bit coded character sets on punched cards. Defines ISO 7-bit and 8-bit character sets on punched cards as well as the representation of 7-bit and 8-bit combinations on 12-row punched cards. Derived from, and compatible with, the Hollerith Code, ensuring compatibility with existing punched card files."
+6584,"Processing of punched cards was handled by a variety of machines, including:"
+6585,"The term ""batch processing"" originates in the traditional classification of methods of production as job production , batch production , and flow production ."
+6586,"Early computers were capable of running only one program at a time. Each user had sole control of the machine for a scheduled period of time. They would arrive at the computer with program and data, often on punched paper cards and magnetic or paper tape, and would load their program, run and debug it, and carry off their output when done."
+6587,"As computers became faster the setup and takedown time became a larger percentage of available computer time. Programs called monitors, the forerunners of operating systems, were developed which could process a series, or ""batch"", of programs, often from magnetic tape prepared offline. The monitor would be loaded into the computer and run the first job of the batch. At the end of the job it would regain control and load and run the next until the batch was complete. Often the output of the batch would be written to magnetic tape and printed or punched offline. Examples of monitors were IBM's Fortran Monitor System, SOS , and finally IBSYS for IBM's 709x systems in 1960."
+6588,"Third-generation computers capable of multiprogramming began to appear in the 1960s. Instead of running one batch job at a time, these systems can have multiple batch programs running at the same time in order to keep the system as busy as possible. One or more programs might be awaiting input, one actively running on the CPU, and others generating output. Instead of offline input and output, programs called spoolers read jobs from cards, disk, or remote terminals and place them in a job queue to be run. In order to prevent deadlocks the job scheduler needs to know each job's resource requirements—memory, magnetic tapes, mountable disks, etc., so various scripting languages were developed to supply this information in a structured way. Probably the most well-known is IBM's Job Control Language . Job schedulers select jobs to run according to a variety of criteria, including priority, memory size, etc.  Remote batch is a procedure for submitting batch jobs from remote terminals, often equipped with a punch card reader and a line printer. Sometimes asymmetric multiprocessing is used to spool batch input and output for one or more large computers using an attached smaller and less-expensive system, as in the IBM System/360 Attached Support Processor."
+6589,"The first general purpose time sharing system, Compatible Time-Sharing System , was compatible with batch processing.  This facilitated transitioning from batch processing to interactive computing."
+6590,"From the late 1960s onwards, interactive computing such as via text-based computer terminal interfaces , and later graphical user interfaces became common. Non-interactive computation, both one-off jobs such as compilation, and processing of multiple items in batches, became retrospectively referred to as batch processing, and the term batch job  became common. Early use is particularly found at the University of Michigan, around the Michigan Terminal System .
+"
+6591,"Although timesharing did exist, its use was not robust enough for corporate data processing; none of this was related to the earlier unit record equipment, which was human-operated."
+6592,"Non-interactive computation remains pervasive in computing, both for general data processing and for system ""housekeeping"" tasks . A high-level program  is today most often called a script, and written in scripting languages, particularly shell scripts for system tasks; in IBM PC DOS and MS-DOS this is instead known as a batch file. That includes UNIX-based computers, Microsoft Windows, macOS , and even smartphones. A running script, particularly one executed from an interactive login session, is often known as a job, but that term is used very ambiguously."
+6593,"""There is no direct counterpart to z/OS batch processing in PC or UNIX systems. Batch jobs are typically executed at a scheduled time or on an as-needed basis. Perhaps the closest comparison is with processes run by an at or cron command in UNIX, although the differences are significant."""
+6594,"Batch applications are still critical in most organizations in large part because many common business processes are amenable to batch processing. While online systems can also function when manual intervention is not desired, they are not typically optimized to perform high-volume, repetitive tasks. Therefore, even new systems usually contain one or more batch applications for updating information at the end of the day, generating reports, printing documents, and other non-interactive tasks that must complete reliably within certain business deadlines."
+6595,"Some applications are amenable to flow processing, namely those that only need data from a single input at once : start the next step for each input as it completes the previous step. In this case flow processing lowers latency for individual inputs, allowing them to be completed without waiting for the entire batch to finish. However, many applications require data from all records, notably computations such as totals. In this case the entire batch must be completed before one has a usable result: partial results are not usable."
+6596,"Modern batch applications make use of modern batch frameworks such as Jem The Bee, Spring Batch or implementations of JSR 352 written for Java, and other frameworks for other programming languages, to provide the fault tolerance and scalability required for high-volume processing.  In order to ensure high-speed processing, batch applications are often integrated with grid computing solutions to partition a batch job over a large number of processors, although there are significant programming challenges in doing so. High volume batch processing places particularly heavy demands on system and application architectures as well. Architectures that feature strong input/output performance and vertical scalability, including modern mainframe computers, tend to provide better batch performance than alternatives."
+6597,Scripting languages became popular as they evolved along with batch processing.
+6598,"A batch window is ""a period of less-intensive online activity"", when the computer system is able to run batch jobs without interference from, or with, interactive online systems."
+6599,"A bank's end-of-day  jobs require the concept of cutover, where transaction and data are cut off for a particular day's batch activity ."
+6600,"As requirements for online systems uptime expanded to support globalization, the Internet, and other business needs, the batch window shrank and increasing emphasis was placed on techniques that would require online data to be available for a maximum amount of time."
+6601,The batch size refers to the number of work units to be processed within one batch operation. Some examples are:
+6602,"The IBM mainframe z/OS operating system or platform has arguably the most highly refined and evolved set of batch processing facilities owing to its origins, long history, and continuing evolution. Today such systems commonly support hundreds or even thousands of concurrent online and batch tasks within a single operating system image. Technologies that aid concurrent batch and online processing include Job Control Language , scripting languages such as REXX, Job Entry Subsystem , Workload Manager , Automatic Restart Manager , Resource Recovery Services , IBM Db2 data sharing, Parallel Sysplex, unique performance optimizations such as HiperDispatch, I/O channel architecture, and several others."
+6603,"The Unix programs cron, at, and batch  allow for complex scheduling of jobs. Windows has a job scheduler. Most high-performance computing clusters use batch processing to maximize cluster usage."
+6604,"Computer science education is essential to preparing students for the 21st century workforce. As technology becomes increasingly integrated into all aspects of society, the demand for skilled computer scientists is growing. According to the Bureau of Labor Statistics, employment of computer and information technology occupations is projected to ""grow 21 percent from 2021 to 2031"", much faster than the average for all occupations."
+6605,"In addition to preparing students for careers in the technology industry, computer science education also promotes computational thinking skills, which are valuable in many fields, including business, healthcare, and education. By learning to think algorithmically and solve problems systematically, students can become more effective problem solvers and critical thinkers."
+6606,"The history of computer science education can be traced back to the early days of computing, when programming was primarily done by scientists and mathematicians. As computers became more widely used in industry and government, the need for skilled programmers grew, and universities began to offer courses in computer science."
+6607,"In comparison to  science education and mathematics education, computer science  education is a much younger field. In the history of computing, digital computers were only built from around the 1940s – although computation has been around for centuries since the invention of analog computers."
+6608,"Another differentiator of computer science education is that it has primarily only been taught at university level until recently, with some notable exceptions in Israel, Poland and the United Kingdom with the BBC Micro in the 1980s as part of Computer science education in the United Kingdom. Computer science has been a part of the school curricula from age 14 or age 16 in a few countries for a few decades, but has typically as an elective subject."
+6609,"Primary and secondary computer science education is relatively new in the United States with many K-12 CS teachers facing obstacles to integrating CS instruction such as professional isolation, limited CS professional development resources, and low levels of CS teaching self-efficacy. According to a 2021 report, only 51% of high schools in the US offer computer science.
+Elementary CS teachers in particular have lower CS teaching efficacy and have fewer chances to implement CS into their instruction than their middle and high school peers. Connecting CS teachers to resources and peers using methods such as Virtual Communities of Practice has been shown to help CS and STEM teachers improve their teaching self-efficacy and implement CS topics into student instruction."
+6610,"The curriculum for computer science education varies depending on the level of education and country. At the elementary and middle school level, computer science education usually focuses on block or visual programming languages such as Scratch or python using basic programming concepts, such as loops, conditionals, and variables. At the high school level, students may learn more advanced programming concepts and algorithms, as well as web development, networking, and data analysis."
+6611,"In college and graduate school, computer science education may include courses in topics such as artificial intelligence, machine learning, data science, and computer graphics. Many computer science programs also offer courses in computer architecture, operating systems, and computer networks."
+6612,"Teaching methods in computer science education vary depending on the level of education and the goals of the program. At the elementary and middle school level, computer science education may focus on interactive games and puzzles to teach programming concepts. In high school and college, computer science education may involve lectures, labs, and hands-on projects that allow students to apply their knowledge to real-world problems."
+6613,"Online learning platforms and coding bootcamps have also become popular methods of teaching computer science skills. These programs offer self-paced learning and can be accessed from anywhere with an internet connection, as computer science is more about practicality and real-world application."
+6614,"Computing education research  or Computer science education research is an interdisciplinary field that focuses on studying the teaching and learning of computer science. It is a subfield of both computer science and education research, and is concerned with understanding how computer science is taught, learned, and assessed in a variety of settings, such as K-12 schools, colleges and universities, and online learning environments."
+6615,"Computer science education research emerged as a field of study in the 1970s, when researchers began to explore the effectiveness of different approaches to teaching computer programming. Since then, the field has grown to encompass a wide range of topics related to computer science education, including curriculum design, assessment, pedagogy, and diversity and inclusion."
+6616,"One of the primary goals of computer science education research is to improve the teaching and learning of computer science. To this end, researchers study a variety of topics, including:"
+6617,"Researchers in computer science education seek to design curricula that are effective and engaging for students. This may involve studying the effectiveness of different programming languages, or developing new pedagogical approaches that promote active learning."
+6618,"Computer science education researchers are interested in developing effective ways to assess student learning outcomes. This may involve developing new measures of student knowledge or skills, or evaluating the effectiveness of different assessment methods."
+6619,"Researchers in computer science education are interested in exploring different teaching methods and instructional strategies. This may involve studying the effectiveness of lectures, online tutorials, or peer-to-peer learning."
+6620,"Computer science education researchers are interested in promoting diversity and inclusion in computer science education. This may involve studying the factors that contribute to under representation of certain groups in computer science, and developing interventions to promote inclusivity and equity."
+6621,"The Association for Computing Machinery  runs a Special Interest Group  on Computer science education known as SIGCSE which celebrated its 50th anniversary in 2018, making it one of the oldest and longest running ACM Special Interest Groups. An outcome of computing education research are Parsons problems."
+6622,"In many countries, there is a significant gender gap in computer science education. In 2015, 15.3% of computer science students graduating from non-doctoral granting institutions in the US were women while at doctoral granting institutions, the figure was 16.6%. The number of female PhD recipients in the US was 19.3% in 2018. In almost everywhere in the world, less than 20% of the computer science graduates are female."
+6623,"This problem mainly arises due to the lack of interests of girls in computing starting from the primary level. Despite numerous efforts by programs specifically designed to increase the ratio of women in this field, no significant improvement has been observed. Furthermore, a declining trend has been noticed in the involvement of women in past decades."
+6624,"The main reason for the failure of these programs is because almost all of them focused on girls in high school or higher levels of education. Researchers argue that by then women have already made up their mind and stereotypes start to form about computer scientists. Computer Science is perceived as a male dominated field, pursued by people who are nerdy and lack social skills. All these characteristics seem to be more damaging for a woman as compared to a man. Therefore, in order to break these stereotypes and to engage more women in computer science, it is crucial that there are special outreach programs designed to develop interest in girls starting at the middle school level and prepare them for a academic track towards the hard sciences."
+6625,"Evidently, there are a few countries in Asia and Africa where these stereotypes do not exist and women are encouraged for a career in science starting at the primary level, thus resulting in a gender gap that is virtually nonexistent. In 2011, women earned half of the computer science degrees in Malaysia. In 2001, 55 percent of computer science graduates in Guyana were women."
+6626,"A less common, broader meaning of proceedings are the acts and happenings of an academic field, a learned society. For example, the title of the Acta Crystallographica journals is Neo-Latin for ""Proceedings in Crystallography""; the Proceedings of the National Academy of Sciences of the United States of America is the main journal of that academy. Scientific journals whose ISO 4 title abbreviations start with Proc, Acta, or Trans are journals of the proceedings  of a field or of an organization concerned with it, in that secondary meaning of the word."
+6627,"Selecting and collecting papers for conferences is organized by one or more persons, who form the editorial team. The quality of the papers is typically ensured by having external people read the papers before they are accepted in the proceedings. The level of quality control varies considerably from conference to conference: some have only a binary accept/reject decision, others go through more thorough feedback and revisions cycles . Depending on the level of the conference, this process can take up to a year. The editors decide about the composition of the proceedings, the order of the papers, and produce the preface and possibly other pieces of text. Although most changes in papers occur on basis of consensus between editors and authors, editors can also single-handedly make changes in papers."
+6628,"Since the collection of papers comes from individual researchers, the character of proceedings is distinctly different from an educational textbook. Each paper typically is quite isolated from the other papers in the proceedings. Mostly there is no general argument leading from one contribution to the next."
+6629,"In some cases, the editors of the proceedings may decide to further develop the proceedings into a textbook. This may even be a goal at the outset of the conference."
+6630,"Conference proceedings are published in-house by the organizing institution of the conference or via an academic publisher. For example, the Lecture Notes in Computer Science by Springer take much of their input from proceedings. Conference proceedings also get published through dedicated proceedings series as an edited volume where all their inputs comes from the conference papers. For example, AIJR Proceedings series published by academic publisher AIJR. Publication of proceedings as edited volume in such series are different from publishing conference paper in the journals; also known as conference issue. Increasingly, proceedings are published in electronic format via the internet or on CD, USB, etc."
+6631,"In the sciences, the quality of publications in conference proceedings is usually not as high as that of international scientific journals. However, in computer science, papers published in conference proceedings are accorded a higher status than in other fields, due to the fast-moving nature of the field."
+6632,"A number of full-fledged academic journals unconnected to particular conferences also use the word ""proceedings"" as part of their name, for example, Proceedings of the National Academy of Sciences of the United States of America."
+6633,"Conference proceedings may be published as a book or book series, in a journal, or otherwise as a serial publication . In many cases, impact factors are not available, although other journal metrics  might exist. Bibliographic indexing often is done in separate bibliographic databases and citation indexes, e.g., Conference Proceedings Citation Index instead of Science Citation Index."
+6634,"Conferences accepting a broad range of topics from theoretical computer science, including algorithms, data structures, computability, computational complexity, automata theory and formal languages:"
+6635,"Conferences whose topic is algorithms and data structures considered broadly, but that do not include other areas of theoretical computer science such as computational complexity theory:"
+6636,"Conferences on computational geometry, graph drawing, and other application areas of geometric computing:"
+6637,GD – International Symposium on Graph Drawing
+6638,SoCG – Symposium on Computational Geometry
+6639,"Conferences on programming languages, programming language theory and compilers:"
+6640,Conferences on software engineering:
+6641,"Conferences on formal methods in software engineering, including formal specification, formal verification, and static code analysis:"
+6642,"Conferences on concurrent, distributed, and parallel computing, fault-tolerant systems, and dependable systems:"
+6643,"Conferences on high-performance computing, cluster computing, and grid computing:"
+6644,"Conferences on operating systems, storage systems and middleware:"
+6645,Conferences on computer architecture:
+6646,Conferences on computer-aided design and electronic design automation:
+6647,Conferences on computer networking:
+6648,"Wireless networks and mobile computing, including ubiquitous and pervasive computing, wireless ad hoc networks and wireless sensor networks:"
+6649,Conferences on computer security and privacy:
+6650,Cryptography conferences:
+6651,"Conferences on databases, information systems, information retrieval, data mining and the World Wide Web:"
+6652,Conferences on artificial intelligence and machine learning:
+6653,Conferences on Evolutionary computation.
+6654,Conferences on automated reasoning:
+6655,Conferences on computer vision  and pattern recognition:
+6656,Conferences on computational linguistics and natural language processing:
+6657,"Conferences on computer graphics, geometry processing, image processing, and multimedia:"
+6658,Conferences on human–computer interaction and user interfaces:
+6659,Conferences on bioinformatics and computational biology:
+6660,"An in-memory service in SOP can be transparently externalized as a web service operation. Due to language and platform independent Web Service standards, SOP embraces all existing programming paradigms, languages and platforms.  In SOP, the design of the programs pivot around the semantics of service calls, logical routing and data flow description across well-defined service interfaces.  All SOP program modules are encapsulated as services and a service can be composed of other nested services in a hierarchical manner with virtually limitless depth to this service stack hierarchy. A composite service can also contain programming constructs some of which are specific and unique to SOP. A service can be an externalized system component that is accessed via any proprietary API or web service standards utilizing an in-memory plug-in technique."
+6661,"While SOP supports the basic programming constructs for sequencing, selection and iteration, it is differentiated with a slew of new programming constructs that provide built-in native ability geared towards data list manipulation, data integration, automated multithreading of service modules, declarative context management and synchronization of services.  SOP design enables programmers to semantically synchronize the execution of services in order to guarantee that it is correct, or to declare a service module as a transaction boundary with automated commit/rollback behavior."
+6662,"Semantic design tools and runtime automation platforms can be built to support the fundamental concepts of SOP.  For example, a service virtual machine  that automatically creates service objects as units of work and manages their context can be designed to run based on the SOP program metadata stored in XML and created by a design-time automation tool. In SOA terms, the SVM is both a service producer and a service consumer."
+6663,SOP concepts provide a robust base for a semantic approach to programming integration and application logic.  There are three significant benefits to this approach:
+6664,The following are some of the key concepts of SOP:
+6665,"In SOP, in-memory software modules are strictly encapsulated through well-defined service interfaces that can be externalized on-demand as web service operations. This minimal unit of encapsulation maximizes the opportunities for reusability within other in-memory service modules as well as across existing and legacy software assets.  By using service interfaces for information hiding, SOP extends the service-oriented design principles used in SOA to achieve separation of concerns across in-memory service modules."
+6666,"A service interface in SOP is an in-memory object that describes a well-defined software task with well-defined input and output data structures. Service interfaces can be grouped into packages. An SOP service interface can be externalized as a WSDL operation and a single service or a package of services can be described using WSDL. Furthermore, service interfaces can be assigned to one or many service groups based on shared properties."
+6667,"In SOP, runtime properties stored on the service interface metadata serve as a contract with the service virtual machine .  One example for the use of runtime properties is that in declarative service synchronization. A service interface can be declared as a fully synchronized interface, meaning that only a single instance of that service can run at any given time. Or, it can be synchronized based on the actual value of key inputs at runtime, meaning that no two service instances of that service with the same value for their key input data can run at the same time. Furthermore, synchronization can be declared across services interfaces that belong to the same service group. For example, if two services, 'CreditAccount"" and 'DebitAccount"", belong to the same synchronization service group and are synchronized on the accountName input field, then no two instances of 'CreditAccount"" and 'DebitAccount"" with the same account name can execute at the same time."
+6668,"A service invoker makes service requests. It is a pluggable in-memory interface that abstracts the location of a service producer as well as the communication protocol, used between the consumer and producer when going across computer memory, from the SOP runtime environment such as an SVM.  The producer can be in-process , outside the process on the same server machine, or virtualized across a set of networked server machines.  The use of a service invoker in SOP is the key to location transparency and virtualization.  Another significant feature of the service invoker layer is the ability to optimize bandwidth and throughput when communicating across machines.  For example, a ""SOAP Invoker"" is the default service invoker for remote communication across machines using the web service standards. This invoker can be dynamically swapped out if, for example, the producer and consumer wish to communicate through a packed proprietary API for better security and more efficient use of bandwidth."
+6669,"A service listener receives service requests. It is a pluggable in-memory interface that abstracts the communication protocol for incoming service requests made to the SOP runtime environment such as the SVM. Through this abstract layer, the SOP runtime environment can be virtually embedded within the memory address of any traditional programming environment or application service."
+6670,"In SOP, a service module can be either implemented as a Composite or Atomic service. It is important to note that Service modules built through the SOP paradigm have an extroverted nature and can be transparently externalized through standards such as SOAP or any proprietary protocol."
+6671,"One of the most important characteristic of SOP is that it can support a fully semantic-based approach to programming. Furthermore, this semantic-based approach can be layered into a visual environment built on top of a fully metadata-driven layer for storing the service interface and service module definitions. Furthermore, if the SOP runtime is supported by a SVM capable of interpreting the metadata layer, the need for automatic code generation can be eliminated.  The result is tremendous productivity gain during development, ease of testing and significant agility in deployment."
+6672,"A composite service implementation is the semantic definition of a service module based on SOP techniques and concepts.  If you look inside of a black-boxed interface definition of a composite service, you may see other service interfaces connected to each other and connected to SOP programming constructs. A Composite service has a recursive definition meaning that any service inside  may be another atomic or composite service. An inner service may be a recursive reference to the same containing composite service."
+6673,"SOP supports the basic programming constructs for sequencing, selection and iteration as well as built-in, advance behavior.  Furthermore, SOP supports semantic constructs for automatic data mapping, translation, manipulation and flow across inner services of a composite service."
+6674,"A service inside of the definition of a composite service  is implicitly sequenced through the semantic connectivity of built-in success or failure ports of other inner services with its built-in activation port.  When an inner service runs successfully, all the inner services connected to its success port will run next. If an inner service fails, all the services connected to its failure port will run next."
+6675,"Logical selection is accomplished through data-driven branching constructs and other configurable constructs.  In general, configurable constructs are services built into the SOP platform with inputs and outputs that can assume the input/output shape of other connected services.  For example, a configurable construct used for filtering output data of services can take a list of Sales orders, Purchase orders or any other data structure, and filter its data based on user declared filter properties stored on the interface of that instance of the filter construct.  In this example, the structure to be filtered becomes the input of the particular instance of the filter construct and the same structure representing the filtered data becomes the output of the configurable construct."
+6676,"A composite service can be declared to loop.  The loop can be bound by a fixed number of iterations with an optional built-in delay between iterations and it can dynamically terminate using a ""service exit with success"" or ""service exit with failure"" construct inside of the looping composite service.  Furthermore, any service interface can automatically run in a loop or ""foreach"" mode, if it is supplied with two or more input components upon automatic preparation. This behavior is supported at design-time when a data list structure from one service is connected to a service that takes a single data structure  as its input. If a runtime property of the composite service interface is declared to support ""foreach"" in parallel, then the runtime automation environment can automatically multi-thread the loop and run it in parallel. This is an example of how SOP programming constructs provide built-in advanced functionality."
+6677,"Data mapping, translation, and transformation constructs enable automatic transfer of data across inner services. An inner-service is prepared to run, when it is activated and all of its input dependencies are resolved. All the prepared inner-services within a composite service run in a parallel burst called a ""hypercycle"". This is one of the means by which automatic parallel-processing is supported in SOP. The definition of a composite service contains an implicit directed graph of inner service dependencies.  The runtime environment for SOP can create an execution graph based on this directed graph by automatically instantiating and running inner services in parallel whenever possible."
+6678,"Exception handling is a run-time error in Java. Exception handling in SOP is simply accomplished by connecting the failure port of inner services to another inner service, or to a programming construct. ""Exit with failure"" and ""exit with success"" constructs are examples of constructs used for exception handling.  If no action is taken on the failure port of a service, then the outer  service will automatically fail and the standard output messages from the failed inner service will automatically bubble up to the standard output of the parent."
+6679,A composite service can be declared as a transaction boundary. The runtime environment for SOP automatically creates and manages a hierarchical context for composite service objects which are used as a transaction boundary. This context automatically commits or rollbacks upon the successful execution of the composite service.
+6680,"Special composite services, called compensation services, can be associated with any service within SOP.  When a composite service that is declared as a transaction boundary fails without an exception handling routing, the SOP runtime environment automatically dispatches the compensation services associated with all the inner services which have already executed successfully."
+6681,"An atomic service is an in-memory extension of the SOP runtime environment through a service native interface  it is essentially a plug-in mechanism. For example, if SOP is automated through an SVM, a service plug-in is dynamically loaded into the SVM when any associated service is consumed. An example of a service plug-in would be a SOAP communicator plug-in that can on-the-fly translate any in-memory service input data to a Web Service SOAP request, post it to a service producer, and then translate the corresponding SOAP response to in-memory output data on the service.  Another example of a service plug-in is a standard database SQL plug-in that supports data access, modification and query operations.  A further example that can help establish the fundamental importance of atomic services and service plug-ins is using a service invoker as a service plug-in to transparently virtualize services across different instances of an SOP platform.  This unique, component-level virtualization is termed ""service grid virtualization"" in order to distinguish it from traditional application, or process-level virtualization."
+6682,SOP presents significant opportunities to support cross-cutting concerns for all applications built using the SOP technique. The following sections define some of these opportunities:
+6683,"The SOP runtime environment can systematically provide built-in and optimized profiling, logging and metering for all services in real-time."
+6684,"Based on declared key input values of a service instance, the outputs of a non time-sensitive inner service can be cached by the SOP runtime environment when running in the context of a particular composite service.  When a service is cached for particular key input values, the SOP runtime environment fetches the cached outputs corresponding to the keyed inputs from its service cache instead of consuming the service.  Availability of this built-in mechanism to the SOP application developer can significantly reduce the load on back-end systems."
+6685,"SOP provides a mechanism for associating a special kind of composite service, trigger service, to any other service.  When that service is consumed, the SOP platform automatically creates and consumes an instance of the associated trigger service with an in-memory copy of the inputs of the triggering service.  This consumption is non-intrusive to the execution of the triggering service.  A service trigger can be declared to run upon activation, failure or success completion of the triggering service."
+6686,"In addition to the ability to call any service, Service Request Events and Shared Memory are two of the SOP built-in mechanisms provided for inter-service communication. The consumption of a service is treated as an Event in SOP. SOP provides a correlation-based event mechanism that results in the pre-emption of a running composite that has declared, through a ""wait"" construct, the need to wait for one or more other service consumption events to happen with specified input data values. The execution of the composite service continues when services are consumed with specific correlation key inputs associated with the wait construct.  SOP also provides a shared memory space with access control where services can access and update a well-defined data structure that is similar to the input/output structure of services.  The shared memory mechanism within SOP can be programmatically accessed through service interfaces."
+6687,"In SOP, customizations are managed through an inventive feature called Service Overrides. Through this feature, a service implementation can be statically or dynamically overridden by one of many possible implementations at runtime.  This feature is analogous to polymorphism in object-oriented programming. Each possible override implementation can be associated to one or more override configuration portfolios in order to manage activation of groups of related overrides throughout different SOP application installations at the time of deployment."
+6688,"Select services can be deployed securely for external programmatic consumption by a presentation  layer, or other applications.  Once service accounts are defined, the SOP runtime environment automatically manages access through consumer account provisioning mechanisms."
+6689,"The SOP runtime environment can systematically provide built-in authentication and service authorization. For the purpose of authorization, SOP development projects, consumer accounts, packages and services are treated as resources with access control.  In this way, the SOP runtime environment can provide built-in authorization. Standards or proprietary authorization and communication security is customized through service overrides, plug-in invoker and service listener modules."
+6690,"Since all artifacts of SOP are well-encapsulated services and all SOP mechanisms, such as shared memory, can be provided as distributable services, large-scale virtualization can be automated by the SOP runtime environment.  Also, the hierarchical service stack of a composite service with the multiple execution graphs associated to its inner services, at each level, provides tremendous opportunities for automated multi-threading to the SOP runtime environment."
+6691,"The term service-oriented programming was first published in 2002 by Alberto Sillitti, Tullio Vernazza and Giancarlo Succi in a book called ""Software Reuse: Methods, Techniques, and Tools.""  SOP, as described above, reflects some aspects of the use of the term proposed by  Sillitti, Vernazza and Succi."
+6692,"Today, the SOP paradigm is in the early stages of mainstream adoption. There are four market drivers fueling this adoption:"
+6693,Service-Oriented Programming: A New Paradigm of Software Reuse
+6694,Grid Virtualization and Itanium®-based Solutions
+6695,"Object-oriented programming  is a programming paradigm based on the concept of objects, which can contain data and code: data in the form of fields , and code in the form of procedures . In OOP, computer programs are designed by making them out of objects that interact with one another."
+6696,"Many of the most widely used programming languages  are multi-paradigm and they support object-oriented programming to a greater or lesser degree, typically in combination with imperative programming, procedural programming and functional programming."
+6697,"Significant object-oriented languages include Ada, ActionScript, C++, Common Lisp, C#, Dart, Eiffel, Fortran 2003, Haxe, Java, JavaScript, Kotlin, Logo, MATLAB, Objective-C, Object Pascal, Perl, PHP, Python, R, Raku, Ruby, Scala, SIMSCRIPT, Simula, Smalltalk, Swift, Vala and Visual Basic.NET."
+6698,"Terminology invoking ""objects"" in the modern sense of object-oriented programming made its first appearance at the artificial intelligence group at MIT in the late 1950s and early 1960s. ""Object"" referred to LISP atoms with identified properties . Another early MIT example was Sketchpad created by Ivan Sutherland in 1960–1961; in the glossary of the 1963 technical report based on his dissertation about Sketchpad, Sutherland defined notions of ""object"" and ""instance"" , albeit specialized to graphical interaction. Also, in 1968, an MIT ALGOL version, AED-0, established a direct link between data structures  and procedures, prefiguring what were later termed ""messages"", ""methods"", and ""member functions"". Topics such as data abstraction and modular programming were common points of discussion at this time."
+6699,"Independently of later MIT work such as AED, Simula was developed during the years 1961–1967. Simula introduced important concepts that are today an essential part of object-oriented programming, such as class and object, inheritance, and dynamic binding. The object-oriented Simula programming language was used mainly by researchers involved with physical modelling, such as models to study and improve the movement of ships and their content through cargo ports."
+6700,"Influenced by the work at MIT and the Simula language, in November 1966 Alan Kay began working on ideas that would eventually be incorporated into the Smalltalk programming language. Kay used the term ""object-oriented programming"" in conversation as early as 1967. Although sometimes called ""the father of object-oriented programming"", Alan Kay has differentiated his notion of OO from the more conventional abstract data type notion of object, and has implied that the computer science establishment did not adopt his notion. A 1976 MIT memo co-authored by Barbara Liskov lists Simula 67, CLU, and Alphard as object-oriented languages, but does not mention Smalltalk."
+6701,"In the 1970s, the first version of the Smalltalk programming language was developed at Xerox PARC by Alan Kay, Dan Ingalls and Adele Goldberg. Smalltalk-72 included a programming environment and was dynamically typed, and at first was interpreted, not compiled. Smalltalk became noted for its application of object orientation at the language-level and its graphical development environment. Smalltalk went through various versions and interest in the language grew. While Smalltalk was influenced by the ideas introduced in Simula 67 it was designed to be a fully dynamic system in which classes could be created and modified dynamically."
+6702,"During the late 1970s and 1980s, object-oriented programming rose to prominence. The Flavors object-oriented Lisp was developed starting 1979, introducing multiple inheritance and mixins. In 1981, Goldberg edited the August issue of Byte Magazine, introducing Smalltalk and object-oriented programming to a wide audience. LOOPS, the object system for Interlisp-D, was influenced by Smalltalk and Flavors, and a paper about it was published in 1982. In 1986, the Association for Computing Machinery organized the first Conference on Object-Oriented Programming, Systems, Languages, and Applications , which was attended by 1,000 people. Among other developments was the Common Lisp Object System, which integrates functional programming and object-oriented programming and allows extension via a Meta-object protocol. In the 1980s, there were a few attempts to design processor architectures that included hardware support for objects in memory but these were not successful. Examples include the Intel iAPX 432 and the Linn Smart Rekursiv."
+6703,"In the mid-1980s Objective-C was developed by Brad Cox, who had used Smalltalk at ITT Inc.. Bjarne Stroustrup, who had used Simula for his PhD thesis, created the object-oriented C++. In 1985, Bertrand Meyer also produced the first design of the Eiffel language. Focused on software quality, Eiffel is a purely object-oriented programming language and a notation supporting the entire software lifecycle. Meyer described the Eiffel software development method, based on a small number of key ideas from software engineering and computer science, in Object-Oriented Software Construction. Essential to the quality focus of Eiffel is Meyer's reliability mechanism, Design by Contract, which is an integral part of both the method and language."
+6704,"In the early and mid-1990s object-oriented programming developed as the dominant programming paradigm when programming languages supporting the techniques became widely available. These included Visual FoxPro 3.0, C++, and Delphi. Its dominance was further enhanced by the rising popularity of graphical user interfaces, which rely heavily upon object-oriented programming techniques. An example of a closely related dynamic GUI library and OOP language can be found in the Cocoa frameworks on Mac OS X, written in Objective-C, an object-oriented, dynamic messaging extension to C based on Smalltalk. OOP toolkits also enhanced the popularity of event-driven programming ."
+6705,"At ETH Zürich, Niklaus Wirth and his colleagues investigated the concept of type checking across module boundaries. Modula-2  included this concept, and their succeeding design, Oberon, included a distinctive approach to object orientation, classes, and such. Inheritance is not obvious in Wirth's design since his nomenclature looks in the opposite direction: It is called type extension and the viewpoint is from the parent down to the inheritor."
+6706,"Object-oriented features have been added to many previously existing languages, including Ada, BASIC, Fortran, Pascal, and COBOL. Adding these features to languages that were not initially designed for them often led to problems with compatibility and maintainability of code."
+6707,"More recently, some languages have emerged that are primarily object-oriented, but that are also compatible with procedural methodology. Two such languages are Python and Ruby. Probably the most commercially important recent object-oriented languages are Java, developed by Sun Microsystems, as well as C# and Visual Basic.NET , both designed for Microsoft's .NET platform. Each of these two frameworks shows, in its way, the benefit of using OOP by creating an abstraction from implementation. VB.NET and C# support cross-language inheritance, allowing classes defined in one language to subclass classes defined in the other language."
+6708,"Object-oriented programming uses objects, but not all of the associated techniques and structures are supported directly in languages that claim to support OOP. The features listed below are common among languages considered to be strongly class- and object-oriented , with notable exceptions mentioned. Christopher J. Date stated that critical comparison of OOP to other technologies, relational in particular, is difficult because of lack of an agreed-upon and rigorous definition of OOP."
+6709,Modular programming support provides the ability to group procedures into files and modules for organizational purposes. Modules are namespaced so identifiers in one module will not conflict with a procedure or variable sharing the same name in another file or module.
+6710,"Objects sometimes correspond to things found in the real world. For example, a graphics program may have objects such as ""circle"", ""square"", and ""menu"". An online shopping system might have objects such as ""shopping cart"", ""customer"", and ""product"". Sometimes objects represent more abstract entities, like an object that represents an open file, or an object that provides the service of translating measurements from U.S. customary to metric. Objects are accessed somewhat like variables with complex internal structures, and in many languages are effectively pointers, serving as actual references to a single instance of said object in memory within a heap or stack. Procedures are known as methods; variables are also known as fields, members, attributes, or properties."
+6711,"Objects can contain other objects in their instance variables; this is known as object composition. For example, an object in the Employee class might contain  an object in the Address class, in addition to its own instance variables like ""first_name"" and ""position"". Object composition is used to represent ""has-a"" relationships: every employee has an address, so every Employee object has access to a place to store an Address object . Date and Darwen have proposed a theoretical foundation that uses OOP as a kind of customizable type system to support RDBMS, but it forbids object pointers."
+6712,"The OOP paradigm has been criticized for overemphasizing the use of objects for software design and modeling at the expense of other important aspects . For example, Rob Pike has said that OOP languages frequently shift the focus from data structures and algorithms to types. Steve Yegge noted that, as opposed to functional programming:"
+6713,Object Oriented Programming puts the nouns first and foremost. Why would you go to such lengths to put one part of speech on a pedestal? Why should one kind of concept take precedence over another? It's not as if OOP has suddenly made verbs less important in the way we actually think. It's a strangely skewed perspective.
+6714,"Rich Hickey, creator of Clojure, described object systems as overly simplistic models of the real world. He emphasized the inability of OOP to model time properly, which is getting increasingly problematic as software systems become more concurrent."
+6715,Alexander Stepanov compares object orientation unfavourably to generic programming:
+6716,I find OOP technically unsound. It attempts to decompose the world in terms of interfaces that vary on a single type. To deal with the real problems you need multisorted algebras — families of interfaces that span multiple types. I find OOP philosophically unsound. It claims that everything is an object. Even if it is true it is not very interesting — saying that everything is an object is saying nothing at all.
+6717,"OOP languages are diverse, but typically OOP languages allow inheritance for code reuse and extensibility in the form of either classes or prototypes. These forms of inheritance are significantly different, but analogous terminology is used to define the concepts of object and instance."
+6718,"In class-based programming, the most popular style, each object is required to be an instance of a particular class. The class defines the data format or type  and available procedures  for a given type or class of object. Objects are created by calling a special type of method in the class known as a constructor.  Classes may inherit from other classes, so they are arranged in a hierarchy that represents ""is-a-type-of"" relationships. For example, class Employee might inherit from class Person. All the data and methods available to the parent class also appear in the child class with the same names. For example, class Person might define variables ""first_name"" and ""last_name"" with method ""make_full_name"". These will also be available in class Employee, which might add the variables ""position"" and ""salary"". It is guaranteed that all instances of class Employee will have the same attributes, such as the name, position, and salary. Procedures and variables can be specific to either the class or the instance; this leads to the following terms:"
+6719,"Depending on the definition of the language, subclasses may or may not be able to override the methods defined by superclasses. Multiple inheritance is allowed in some languages, though this can make resolving overrides complicated. Some languages have special support for other concepts like traits and mixins, though, in any language with multiple inheritance, a mixin is simply a class that does not represent an is-a-type-of relationship. Mixins are typically used to add the same methods to multiple classes. For example, class UnicodeConversionMixin might provide a method unicode_to_ascii when included in class FileReader and class WebPageScraper, which do not share a common parent."
+6720,"Abstract classes cannot be instantiated into objects; they exist only for inheritance into other ""concrete"" classes that can be instantiated. In Java, the final keyword can be used to prevent a class from being subclassed."
+6721,"In contrast, in prototype-based programming, objects are the primary entities. Generally, the concept of a ""class"" does not even exist. Rather, the prototype or parent of an object is just another object to which the object is linked. In Self, an object may have multiple or no parents, but in the most popular prototype-based language, Javascript, every object has one prototype link . New objects can be created based on already existing objects chosen as their prototype. You may call two different objects apple and orange a fruit if the object fruit exists, and both apple and orange have fruit as their prototype. The idea of the fruit class does not exist explicitly, but can be modeled as the equivalence class of the objects sharing the same prototype, or as the set of objects satisfying a certain interface . Unlike class-based programming, it is typically possible in prototype-based languages to define attributes and methods not shared with other objects; for example, the attribute sugar_content may be defined in apple but not orange."
+6722,"The doctrine of composition over inheritance advocates implementing has-a relationships using composition instead of inheritance. For example, instead of inheriting from class Person, class Employee could give each Employee object an internal Person object, which it then has the opportunity to hide from external code even if class Person has many public attributes or methods. Some languages like Go do not support inheritance at all. Go states that it is object-oriented, and Bjarne Stroustrup, author of C++, has stated that it is possible to do OOP without inheritance. Delegation is another language feature that can be used as an alternative to inheritance. Rob Pike has called object-oriented programming ""the Roman numerals of computing"" and cites an instance of a Java professor whose ""idiomatic"" solution to a problem was to create six new classes, rather than to simply use a lookup table."
+6723,"Bob Martin states that because they are software, related classes do not necessarily share the relationships of the things they represent."
+6724,"It is the responsibility of the object, not any external code, to select the procedural code to execute in response to a method call, typically by looking up the method at run time in a table associated with the object. This feature is known as dynamic dispatch. If the call variability relies on more than the single type of the object on which it is called , one speaks of multiple dispatch. A method call is also known as message passing. It is conceptualized as a message  being passed to the object for dispatch."
+6725,"Dispatch interacts with inheritance; if a method is not present in a given object or class, the dispatch is delegated to its parent object or class, and so on, going up the chain of inheritance."
+6726,"Data abstraction is a design pattern in which data are visible only to semantically related functions, to prevent misuse. The success of data abstraction leads to frequent incorporation of data hiding as a design principle in object-oriented and pure functional programming. Similarly, encapsulation prevents external code from being concerned with the internal workings of an object. This facilitates code refactoring, for example allowing the author of the class to change how objects of that class represent their data internally without changing any external code . It also encourages programmers to put all the code that is concerned with a certain set of data in the same class, which organizes it for easy comprehension by other programmers. Encapsulation is a technique that encourages decoupling."
+6727,"In object oriented programming, objects provide a layer which can be used to separate internal from external code and implement abstraction and encapsulation. External code can only use an object by calling a specific instance method with a certain set of input parameters, reading an instance variable, or writing to an instance variable. A program may create many instances of objects as it runs, which operate independently. This technique, it is claimed, allows easy re-use of the same procedures and data definitions for different sets of data, in addition to potentially mirroring real-world relationships intuitively. Rather than utilizing database tables and programming subroutines, the developer utilizes objects the user may be more familiar with: objects from their application domain. These claims that the OOP paradigm enhances reusability and modularity have been criticized."
+6728,"If a class does not allow calling code to access internal object data and permits access through methods only, this is also a form of information hiding. Some languages  let classes enforce access restrictions explicitly, for example, denoting internal data with the private keyword and designating methods intended for use by code outside the class with the public keyword. Methods may also be designed public, private, or intermediate levels such as protected . In other languages  this is enforced only by convention . In C#, Swift & Kotlin languages, internal keyword permits access only to files present in the same assembly, package, or module as that of the class."
+6729,"In programming languages, particularly object-oriented ones, the emphasis on abstraction is vital. Object-oriented languages extend the notion of type to incorporate data abstraction, highlighting the significance of restricting access to internal data through methods. Eric S. Raymond has written that object-oriented programming languages tend to encourage thickly layered programs that destroy transparency. Raymond compares this unfavourably to the approach taken with Unix and the C programming language."
+6730,"The ""open/closed principle"" advocates that classes and functions ""should be open for extension, but closed for modification"". Luca Cardelli has claimed that OOP languages have ""extremely poor modularity properties with respect to class extension and modification"", and tend to be extremely complex. The latter point is reiterated by Joe Armstrong, the principal inventor of Erlang, who is quoted as saying:"
+6731,The problem with object-oriented languages is they've got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.
+6732,Leo Brodie has suggested a connection between the standalone nature of objects and a tendency to duplicate code in violation of the don't repeat yourself principle of software development.
+6733,"Subtyping – a form of polymorphism – is when calling code can be independent of which class in the supported hierarchy it is operating on – the parent class or one of its descendants. Meanwhile, the same operation name among objects in an inheritance hierarchy may behave differently."
+6734,"For example, objects of the type Circle and Square are derived from a common class called Shape. The Draw function for each type of Shape implements what is necessary to draw itself while calling code can remain indifferent to the particular type of Shape being drawn."
+6735,This is another type of abstraction that simplifies code external to the class hierarchy and enables strong separation of concerns.
+6736,"A common feature of objects is that methods are attached to them and can access and modify the object's data fields. In this brand of OOP, there is usually a special name such as this or self used to refer to the current object. In languages that support open recursion, object methods can call other methods on the same object  using this name. This variable is late-bound; it allows a method defined in one class to invoke another method that is defined later, in some subclass thereof."
+6737,"Simula  is generally accepted as being the first language with the primary features of an object-oriented language. It was created for making simulation programs, in which what came to be called objects were the most important information representation. Smalltalk  is another early example and the one with which much of the theory of OOP was developed. Concerning the degree of object orientation, the following distinctions can be made:"
+6738,"Many widely used languages, such as C++, Java, and Python, provide object-oriented features. Although in the past object-oriented programming was widely accepted, more recently essays criticizing object-oriented programming and recommending the avoidance of these features  have been very popular in the developer community. Paul Graham has suggested that OOP's popularity within large companies is due to ""large  groups of mediocre programmers"". According to Graham, the discipline imposed by OOP prevents any one programmer from ""doing too much damage"". Eric S. Raymond, a Unix programmer and open-source software advocate, has been critical of claims that present object-oriented programming as the ""One True Solution""."
+6739,"Richard Feldman argues that these languages may have improved their modularity by adding OO features, but they became popular for reasons other than being object-oriented. In an article, Lawrence Krubner claimed that compared to other languages  OOP languages have no unique strengths, and inflict a heavy burden of unneeded complexity. A study by Potok et al. has shown no significant difference in productivity between OOP and procedural approaches. Luca Cardelli has claimed that OOP code is ""intrinsically less efficient"" than procedural code and that OOP can take longer to compile."
+6740,"In recent years, object-oriented programming has become especially popular in dynamic programming languages. Python, PowerShell, Ruby and Groovy are dynamic languages built on OOP principles, while Perl and PHP have been adding object-oriented features since Perl 5 and PHP 4, and ColdFusion since version 6."
+6741,"The Document Object Model of HTML, XHTML, and XML documents on the Internet has bindings to the popular JavaScript/ECMAScript language. JavaScript is perhaps the best known prototype-based programming language, which employs cloning from prototypes rather than inheriting from a class . Another scripting language that takes this approach is Lua."
+6742,"The messages that flow between computers to request services in a client-server environment can be designed as the linearizations of objects defined by class objects known to both the client and the server. For example, a simple linearized object would consist of a length field, a code point identifying the class, and a data value. A more complex example would be a command consisting of the length and code point of the command and values consisting of linearized objects representing the command's parameters. Each such command must be directed by the server to an object whose class  recognizes the command and can provide the requested service. Clients and servers are best modeled as complex object-oriented structures. Distributed Data Management Architecture  took this approach and used class objects to define objects at four levels of a formal hierarchy:"
+6743,"Fields defining the data values that form messages, such as their length, code point and data values."
+6744,Objects and collections of objects similar to what would be found in a Smalltalk program for messages and parameters.
+6745,"Managers similar to IBM i Objects, such as a directory to files and files consisting of metadata and records. Managers conceptually provide memory and processing resources for their contained objects."
+6746,"A client or server consisting of all the managers necessary to implement a full processing environment, supporting such aspects as directory services, security, and concurrency control."
+6747,The initial version of DDM defined distributed file services. It was later extended to be the foundation of Distributed Relational Database Architecture .
+6748,"Challenges of object-oriented design are addressed by several approaches. The most common is known as the design patterns codified by Gamma et al.. More broadly, the term ""design patterns"" can be used to refer to any general, repeatable, solution pattern to a commonly occurring problem in software design. Some of these commonly occurring problems have implications and solutions particular to object-oriented development."
+6749,"It is intuitive to assume that inheritance creates a semantic ""is a"" relationship, and thus to infer that objects instantiated from subclasses can always be safely used instead of those instantiated from the superclass. This intuition is unfortunately false in most OOP languages, in particular in all those that allow mutable objects. Subtype polymorphism as enforced by the type checker in OOP languages  cannot guarantee behavioral subtyping in any context. Behavioral subtyping is undecidable in general, so it cannot be implemented by a program . Class or object hierarchies must be carefully designed, considering possible incorrect uses that cannot be detected syntactically. This issue is known as the Liskov substitution principle."
+6750,"Design Patterns: Elements of Reusable Object-Oriented Software is an influential book published in 1994 by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, often referred to humorously as the ""Gang of Four"". Along with exploring the capabilities and pitfalls of object-oriented programming, it describes 23 common programming problems and patterns for solving them."
+6751,The book describes the following patterns:
+6752,"Both object-oriented programming and relational database management systems  are extremely common in software today. Since relational databases do not store objects directly , there is a general need to bridge the two worlds. The problem of bridging object-oriented programming accesses and data patterns with relational databases is known as object-relational impedance mismatch. There are some approaches to cope with this problem, but no general solution without downsides. One of the most common approaches is object-relational mapping, as found in IDE languages such as Visual FoxPro and libraries such as Java Data Objects and Ruby on Rails' ActiveRecord."
+6753,"There are also object databases that can be used to replace RDBMSs, but these have not been as technically and commercially successful as RDBMSs."
+6754,"OOP can be used to associate real-world objects and processes with digital counterparts. However, not everyone agrees that OOP facilitates direct real-world mapping  or that real-world mapping is even a worthy goal; Bertrand Meyer argues in Object-Oriented Software Construction that a program is not a model of the world but a model of some part of the world; ""Reality is a cousin twice removed"". At the same time, some principal limitations of OOP have been noted.
+For example, the circle-ellipse problem is difficult to handle using OOP's concept of inheritance."
+6755,"However, Niklaus Wirth  said of OOP in his paper, ""Good Ideas through the Looking Glass"", ""This paradigm closely reflects the structure of systems 'in the real world', and it is therefore well suited to model complex systems with complex behaviors"" ."
+6756,Steve Yegge and others noted that natural languages lack the OOP approach of strictly prioritizing things  before actions . This problem may cause OOP to suffer more convoluted solutions than procedural programming.
+6757,"OOP was developed to increase the reusability and maintainability of source code. Transparent representation of the control flow had no priority and was meant to be handled by a compiler. With the increasing relevance of parallel hardware and multithreaded coding, developing transparent control flow becomes more important, something hard to achieve with OOP."
+6758,"Responsibility-driven design defines classes in terms of a contract, that is, a class should be defined around a responsibility and the information that it shares. This is contrasted by Wirfs-Brock and Wilkerson with data-driven design, where classes are defined around the data-structures that must be held. The authors hold that responsibility-driven design is preferable."
+6759,SOLID is a mnemonic invented by Michael Feathers which spells out five software engineering design principles:
+6760,GRASP  is another set of guidelines advocated by Craig Larman.
+6761,"Objects are the run-time entities in an object-oriented system. They may represent a person, a place, a bank account, a table of data, or any item that the program has to handle."
+6762,There have been several attempts at formalizing the concepts used in object-oriented programming. The following concepts and constructs have been used as interpretations of OOP concepts:
+6763,"Attempts to find a consensus definition or theory behind objects have not proven very successful , and often diverge widely. For example, some definitions focus on mental activities, and some on program structuring. One of the simpler definitions is that OOP is the act of using ""map"" data structures or arrays that can contain functions and pointers to other maps, all with some syntactic and scoping sugar on top. Inheritance can be performed by cloning the maps ."
+6764,"The term is often used in contrast to declarative programming, which focuses on what the program should accomplish without specifying all the details of how the program should achieve the result."
+6765,"Procedural programming is a type of imperative programming in which the program is built from one or more procedures . The terms are often used as synonyms, but the use of procedures has a dramatic effect on how imperative programs appear and how they are constructed. Heavy procedural programming, in which state changes are localized to procedures or restricted to explicit arguments and returns from procedures, is a form of structured programming. Since the 1960s, structured programming and modular programming in general have been promoted as techniques to improve the maintainability and overall quality of imperative programs. The concepts behind object-oriented programming attempt to extend this approach."
+6766,"Procedural programming could be considered a step toward declarative programming. A programmer can often tell, simply by looking at the names, arguments, and return types of procedures , what a particular procedure is supposed to do, without necessarily looking at the details of how it achieves its result. At the same time, a complete program is still imperative since it fixes the statements to be executed and their order of execution to a large extent."
+6767,"The programming paradigm used to build programs for almost all computers typically follows an imperative model. Digital computer hardware is designed to execute machine code, which is native to the computer and is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as lisp machines."
+6768,"From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer. Higher-level imperative languages use variables and more complex statements, but still follow the same paradigm. Recipes and process checklists, while not computer programs, are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state. Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style."
+6769,"Assignment statements, in imperative paradigm, perform an operation on information located in memory and store the results in memory for later use. High-level imperative languages, in addition, permit the evaluation of complex expressions, which may consist of a combination of arithmetic operations and function evaluations, and the assignment of the resulting value to memory. Looping statements  allow a sequence of statements to be executed multiple times. Loops can either execute the statements they contain a predefined number of times, or they can execute them repeatedly until some condition is met. Conditional branching statements allow a sequence of statements to be executed only if some condition is met. Otherwise, the statements are skipped and the execution sequence continues from the statement following them. Unconditional branching statements allow an execution sequence to be transferred to another part of a program. These include the jump , switch, and the subprogram, subroutine, or procedure call ."
+6770,"Early in the development of high-level programming languages, the introduction of the block enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside the introduction of subroutines, enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures."
+6771,Many imperative programming languages  are abstractions of assembly language.
+6772,"The earliest imperative languages were the machine languages of the original computers. In these languages, instructions were very simple, which made hardware implementation easier but hindered the creation of complex programs. FORTRAN, developed by John Backus at International Business Machines  starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs. FORTRAN was a compiled language that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages. The next two decades saw the development of many other major high-level imperative programming languages. In the late 1950s and 1960s, ALGOL was developed in order to allow mathematical algorithms to be more easily expressed and even served as the operating system's target language for some computers. MUMPS  carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to the extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $TEST. COBOL  and BASIC  were both attempts to make programming syntax look more like English. In the 1970s, Pascal was developed by Niklaus Wirth, and C was created by Dennis Ritchie while he was working at Bell Laboratories. Wirth went on to design Modula-2 and Oberon. For the needs of the United States Department of Defense, Jean Ichbiah and a team at Honeywell began designing Ada in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995, 2005, and 2012."
+6773,"The 1980s saw a rapid growth in interest in object-oriented programming. These languages were imperative in style, but added features to support objects. The last two decades of the 20th century saw the development of many such languages. Smalltalk-80, originally conceived by Alan Kay in 1969, was released in 1980, by the Xerox Palo Alto Research Center . Drawing from concepts in another object-oriented language—Simula —Bjarne Stroustrup designed C++, an object-oriented language based on C. Design of C++ began in 1979 and the first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were Perl, released by Larry Wall in 1987; Python, released by Guido van Rossum in 1990; Visual Basic and Visual C++  2.0), released by Microsoft in 1991 and 1993 respectively; PHP, released by Rasmus Lerdorf in 1994; Java, by James Gosling  in 1995, JavaScript, by Brendan Eich , and Ruby, by Yukihiro ""Matz"" Matsumoto, both released in 1995. Microsoft's .NET Framework  is imperative at its core, as are its main target languages, VB.NET and C# that run on it; however Microsoft's F#, a functional language, also runs on it."
+6774,"FORTRAN  was unveiled as ""The IBM Mathematical FORmula TRANslating system."" It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and statements, it supported:"
+6775,It succeeded because:
+6776,"However, non IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute  developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:"
+6777,"COBOL  stands for ""COmmon Business Oriented Language."" Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced. The US Department of Defense influenced COBOL's development, with Grace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal."
+6778,"COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming."
+6779,"ALGOL  stands for ""ALGOrithmic Language."" It had a profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like:"
+6780,"block structure, where variables were local to their block"
+6781,arrays with variable bounds
+6782,"""for"" loops"
+6783,functions
+6784,recursion
+6785,"Algol's direct descendants include Pascal, Modula-2, Ada, Delphi and Oberon on one branch. On another branch there's C, C++ and Java."
+6786,"BASIC  stands for ""Beginner's All Purpose Symbolic Instruction Code."" It was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language."
+6787,Basic pioneered the interactive session. It offered operating system commands within its environment:
+6788,"However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface."
+6789,"C programming language  got its name because the language BCPL was replaced with B, and AT&T Bell Labs called the next version ""C."" Its purpose was to write the UNIX operating system. C is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but uses a high-level syntax. It added advanced features like:"
+6790,C allows the programmer to control in which region of memory data is to be stored. Global variables and static variables require the fewest clock cycles to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc function.
+6791,"In the 1970s, software engineers needed language support to break large projects down into modules. One obvious feature was to decompose large projects physically into separate files. A less obvious feature was to decompose large projects logically into abstract datatypes. At the time, languages supported concrete  datatypes like integer numbers, floating-point numbers, and strings of characters. Concrete datatypes have their representation as part of their name. Abstract datatypes are structures of concrete datatypes — with a new name assigned. For example, a list of integers could be called integer_list."
+6792,"In object-oriented jargon, abstract datatypes are called classes. However, a class is only a definition; no memory is allocated. When memory is allocated to a class, it's called an object."
+6793,"Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming. A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method, member function, or operation. Object-oriented programming is executing operations on objects."
+6794,"Object-oriented languages support a syntax to model subset/superset relationships. In set theory, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. Object-oriented languages model subset/superset relationships using inheritance. Object-oriented programming became the dominant language paradigm by the late 1990s."
+6795,"C++  was originally called ""C with Classes."" It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula."
+6796,An object-oriented module is composed of two files. The definitions file is called the header file. Here is a C++ header file for the GRADE class in a simple school application:
+6797,A constructor operation is a function with the same name as the class name. It is executed when the calling operation executes the new statement.
+6798,A module's other file is the source file. Here is a C++ source file for the GRADE class in a simple school application:
+6799,Here is a C++ header file for the PERSON class in a simple school application:
+6800,Here is a C++ source file for the PERSON class in a simple school application:
+6801,Here is a C++ header file for the STUDENT class in a simple school application:
+6802,Here is a C++ source file for the STUDENT class in a simple school application:
+6803,Here is a driver program for demonstration:
+6804,Here is a makefile to compile everything:
+6805,"In functional programming, functions are treated as first-class citizens, meaning that they can be bound to names , passed as arguments, and returned from other functions, just as any other data type can. This allows programs to be written in a declarative and composable style, where small functions are combined in a modular manner."
+6806,"Functional programming is sometimes treated as synonymous with purely functional programming, a subset of functional programming which treats all functions as deterministic mathematical functions, or pure functions. When a pure function is called with some given arguments, it will always return the same result, and cannot be affected by any mutable state or other side effects. This is in contrast with impure procedures, common in imperative programming, which can have side effects . Proponents of purely functional programming claim that by restricting side effects, programs can have fewer bugs, be easier to debug and test, and be more suited to formal verification."
+6807,"Functional programming has its roots in academia, evolving from the lambda calculus, a formal system of computation based only on functions. Functional programming has historically been less popular than imperative programming, but many functional languages are seeing use today in industry and education, including Common Lisp, Scheme, Clojure, Wolfram Language, Racket, Erlang, Elixir, OCaml, Haskell, and F#. Functional programming is also key to some languages that have found success in specific domains, like JavaScript in the Web, R in statistics, J, K and Q in financial analysis, and XQuery/XSLT for XML. Domain-specific declarative languages like SQL and Lex/Yacc use some elements of functional programming, such as not allowing mutable values. In addition, many other programming languages support programming in a functional style or have implemented features from functional programming, such as C++11, C#, Kotlin, Perl, PHP, Python, Go, Rust, Raku, Scala, and Java ."
+6808,"The lambda calculus, developed in the 1930s by Alonzo Church, is a formal system of computation built from function application. In 1937 Alan Turing proved that the lambda calculus and Turing machines are equivalent models of computation, showing that the lambda calculus is Turing complete. Lambda calculus forms the basis of all functional programming languages. An equivalent theoretical formulation, combinatory logic, was developed by Moses Schönfinkel and Haskell Curry in the 1920s and 1930s."
+6809,"Church later developed a weaker system, the simply-typed lambda calculus, which extended the lambda calculus by assigning a data type to all terms. This forms the basis for statically typed functional programming."
+6810,"The first high-level functional programming language, Lisp, was developed in the late 1950s for the IBM 700/7000 series of scientific computers by John McCarthy while at Massachusetts Institute of Technology . Lisp functions were defined using Church's lambda notation, extended with a label construct to allow recursive functions. Lisp first introduced many paradigmatic features of functional programming, though early Lisps were multi-paradigm languages, and incorporated support for numerous programming styles as new paradigms evolved. Later dialects, such as Scheme and Clojure, and offshoots such as Dylan and Julia, sought to simplify and rationalise Lisp around a cleanly functional core, while Common Lisp was designed to preserve and update the paradigmatic features of the numerous older dialects it replaced."
+6811,"Information Processing Language , 1956, is sometimes cited as the first computer-based functional programming language. It is an assembly-style language for manipulating lists of symbols. It does have a notion of generator, which amounts to a function that accepts a function as an argument, and, since it is an assembly-level language, code can be data, so IPL can be regarded as having higher-order functions. However, it relies heavily on the mutating list structure and similar imperative features."
+6812,"Kenneth E. Iverson developed APL in the early 1960s, described in his 1962 book A Programming Language . APL was the primary influence on John Backus's FP. In the early 1990s, Iverson and Roger Hui created J. In the mid-1990s, Arthur Whitney, who had previously worked with Iverson, created K, which is used commercially in financial industries along with its descendant Q."
+6813,"In the mid-1960s, Peter Landin invented SECD machine, the first abstract machine for a functional programming language, described a correspondence between ALGOL 60 and the lambda calculus, and proposed the ISWIM programming language."
+6814,"John Backus presented FP in his 1977 Turing Award lecture ""Can Programming Be Liberated From the von Neumann Style? A Functional Style and its Algebra of Programs"". He defines functional programs as being built up in a hierarchical way by means of ""combining forms"" that allow an ""algebra of programs""; in modern language, this means that functional programs follow the principle of compositionality. Backus's paper popularized research into functional programming, though it emphasized function-level programming rather than the lambda-calculus style now associated with functional programming."
+6815,"The 1973 language ML was created by Robin Milner at the University of Edinburgh, and David Turner developed the language SASL at the University of St Andrews. Also in Edinburgh in the 1970s, Burstall and Darlington developed the functional language NPL. NPL was based on Kleene Recursion Equations and was first introduced in their work on program transformation. Burstall, MacQueen and Sannella then incorporated the polymorphic type checking from ML to produce the language Hope. ML eventually developed into several dialects, the most common of which are now OCaml and Standard ML."
+6816,"In the 1970s, Guy L. Steele and Gerald Jay Sussman developed Scheme, as described in the Lambda Papers and the 1985 textbook Structure and Interpretation of Computer Programs. Scheme was the first dialect of lisp to use lexical scoping and to require tail-call optimization, features that encourage functional programming."
+6817,"In the 1980s, Per Martin-Löf developed intuitionistic type theory , which associated functional programs with constructive proofs expressed as dependent types. This led to new approaches to interactive theorem proving and has influenced the development of subsequent functional programming languages."
+6818,"The lazy functional language, Miranda, developed by David Turner, initially appeared in 1985 and had a strong influence on Haskell. With Miranda being proprietary, Haskell began with a consensus in 1987 to form an open standard for functional programming research; implementation releases have been ongoing as of 1990."
+6819,"More recently it has found use in niches such as parametric CAD in the OpenSCAD language built on the CGAL framework, although its restriction on reassigning values  has led to confusion among users who are unfamiliar with functional programming as a concept."
+6820,Functional programming continues to be used in commercial settings.
+6821,"A number of concepts and paradigms are specific to functional programming, and generally foreign to imperative programming . However, programming languages often cater to several programming paradigms, so programmers using ""mostly imperative"" languages may have utilized some of these concepts."
+6822,"Higher-order functions are closely related to first-class functions in that higher-order functions and first-class functions both allow functions as arguments and results of other functions. The distinction between the two is subtle: ""higher-order"" describes a mathematical concept of functions that operate on other functions, while ""first-class"" is a computer science term for programming language entities that have no restriction on their use ."
+6823,"Higher-order functions enable partial application or currying, a technique that applies a function to its arguments one at a time, with each application returning a new function that accepts the next argument. This lets a programmer succinctly express, for example, the successor function as the addition operator partially applied to the natural number one."
+6824,"Pure functions  have no side effects . This means that pure functions have several useful properties, many of which can be used to optimize the code:"
+6825,"While most compilers for imperative programming languages detect pure functions and perform common-subexpression elimination for pure function calls, they cannot always do this for pre-compiled libraries, which generally do not expose this information, thus preventing optimizations that involve those external functions. Some compilers, such as gcc, add extra keywords for a programmer to explicitly mark external functions as pure, to enable such optimizations. Fortran 95 also lets functions be designated pure. C++11 added constexpr keyword with similar semantics."
+6826,"Iteration  in functional languages is usually accomplished via recursion. Recursive functions invoke themselves, letting an operation be repeated until it reaches the base case. In general, recursion requires maintaining a stack, which consumes space in a linear amount to the depth of recursion. This could make recursion prohibitively expensive to use instead of imperative loops. However, a special form of recursion known as tail recursion can be recognized and optimized by a compiler into the same code used to implement iteration in imperative languages. Tail recursion optimization can be implemented by transforming the program into continuation passing style during compiling, among other approaches."
+6827,"The Scheme language standard requires implementations to support proper tail recursion, meaning they must allow an unbounded number of active tail calls. Proper tail recursion is not simply an optimization; it is a language feature that assures users that they can use recursion to express a loop and doing so would be safe-for-space. Moreover, contrary to its name, it accounts for all tail calls, not just tail recursion. While proper tail recursion is usually implemented by turning code into imperative loops, implementations might implement it in other ways. For example, Chicken intentionally maintains a stack and lets the stack overflow. However, when this happens, its garbage collector will claim space back, allowing an unbounded number of active tail calls even though it does not turn tail recursion into a loop."
+6828,"Common patterns of recursion can be abstracted away using higher-order functions, with catamorphisms and anamorphisms  being the most obvious examples. Such recursion schemes play a role analogous to built-in control structures such as loops in imperative languages."
+6829,"Most general purpose functional programming languages allow unrestricted recursion and are Turing complete, which makes the halting problem undecidable, can cause unsoundness of equational reasoning, and generally requires the introduction of inconsistency into the logic expressed by the language's type system. Some special purpose languages such as Coq allow only well-founded recursion and are strongly normalizing . As a consequence, these languages fail to be Turing complete and expressing certain functions in them is impossible, but they can still express a wide class of interesting computations while avoiding the problems introduced by unrestricted recursion. Functional programming limited to well-founded recursion with a few other constraints is called total functional programming."
+6830,"Functional languages can be categorized by whether they use strict  or non-strict  evaluation, concepts that refer to how function arguments are processed when an expression is being evaluated. The technical difference is in the denotational semantics of expressions containing failing or divergent computations. Under strict evaluation, the evaluation of any term containing a failing subterm fails. For example, the expression:"
+6831,"fails under strict evaluation because of the division by zero in the third element of the list. Under lazy evaluation, the length function returns the value 4 , since evaluating it does not attempt to evaluate the terms making up the list. In brief, strict evaluation always fully evaluates function arguments before invoking the function. Lazy evaluation does not evaluate function arguments unless their values are required to evaluate the function call itself."
+6832,"The usual implementation strategy for lazy evaluation in functional languages is graph reduction. Lazy evaluation is used by default in several pure functional languages, including Miranda, Clean, and Haskell."
+6833,"Hughes 1984 argues for lazy evaluation as a mechanism for improving program modularity through separation of concerns, by easing independent implementation of producers and consumers of data streams. Launchbury 1993  describes some difficulties that lazy evaluation introduces, particularly in analyzing a program's storage requirements, and proposes an operational semantics to aid in such analysis. Harper 2009 proposes including both strict and lazy evaluation in the same language, using the language's type system to distinguish them."
+6834,"Especially since the development of Hindley–Milner type inference in the 1970s, functional programming languages have tended to use typed lambda calculus, rejecting all invalid programs at compilation time and risking false positive errors, as opposed to the untyped lambda calculus, that accepts all valid programs at compilation time and risks false negative errors, used in Lisp and its variants , as they reject all invalid programs at runtime when the information is enough to not reject valid programs. The use of algebraic datatypes makes manipulation of complex data structures convenient; the presence of strong compile-time type checking makes programs more reliable in absence of other reliability techniques like test-driven development, while type inference frees the programmer from the need to manually declare types to the compiler in most cases."
+6835,"Some research-oriented functional languages such as Coq, Agda, Cayenne, and Epigram are based on intuitionistic type theory, which lets types depend on terms. Such types are called dependent types. These type systems do not have decidable type inference and are difficult to understand and program with. But dependent types can express arbitrary propositions in higher-order logic. Through the Curry–Howard isomorphism, then, well-typed programs in these languages become a means of writing formal mathematical proofs from which a compiler can generate certified code. While these languages are mainly of interest in academic research , they have begun to be used in engineering as well. Compcert is a compiler for a subset of the C programming language that is written in Coq and formally verified."
+6836,"A limited form of dependent types called generalized algebraic data types  can be implemented in a way that provides some of the benefits of dependently typed programming while avoiding most of its inconvenience. GADT's are available in the Glasgow Haskell Compiler, in OCaml and in Scala, and have been proposed as additions to other languages including Java and C#."
+6837,"Functional programs do not have assignment statements, that is, the value of a variable in a functional program never changes once defined. This eliminates any chances of side effects because any variable can be replaced with its actual value at any point of execution. So, functional programs are referentially transparent."
+6838,"Consider C assignment statement x=x * 10, this changes the value assigned to the variable x. Let us say that the initial value of x was 1, then two consecutive evaluations of the variable x yields 10 and 100 respectively. Clearly, replacing x=x * 10 with either 10 or 100 gives a program a different meaning, and so the expression is not referentially transparent. In fact, assignment statements are never referentially transparent."
+6839,"Now, consider another function such as int plusone {return x+1;} is transparent, as it does not implicitly change the input x and thus has no such side effects.
+Functional programs exclusively use this type of function and are therefore referentially transparent."
+6840,"Purely functional data structures are often represented in a different way to their imperative counterparts. For example, the array with constant access and update times is a basic component of most imperative languages, and many imperative data-structures, such as the hash table and binary heap,  are based on arrays. Arrays can be replaced by maps or random access lists, which admit purely functional implementation, but have logarithmic access and update times. Purely functional data structures have persistence, a property of keeping previous versions of the data structure unmodified. In Clojure, persistent data structures are used as functional alternatives to their imperative counterparts. Persistent vectors, for example, use trees for partial updating. Calling the insert method will result in some but not all nodes being created."
+6841,"Functional programming is very different from imperative programming. The most significant differences stem from the fact that functional programming avoids side effects, which are used in imperative programming to implement state and I/O. Pure functional programming completely prevents side-effects and provides referential transparency."
+6842,"Higher-order functions are rarely used in older imperative programming. A traditional imperative program might use a loop to traverse and modify a list. A functional program, on the other hand, would probably use a higher-order ""map"" function that takes a function and a list, generating and returning a new list by applying the function to each list item."
+6843,"The following two examples  achieve the same effect: they multiply all even numbers in an array by 10 and add them all, storing the final sum in the variable ""result""."
+6844,Traditional Imperative Loop:
+6845,Functional Programming with higher-order functions:
+6846,"There are tasks  that often seem most naturally implemented with state. Pure functional programming performs these tasks, and I/O tasks such as accepting user input and printing to the screen, in a different way."
+6847,"The pure functional programming language Haskell implements them using monads, derived from category theory. Monads offer a way to abstract certain types of computational patterns, including  modeling of computations with mutable state  in an imperative manner without losing purity. While existing monads may be easy to apply in a program, given appropriate templates and examples, many students find them difficult to understand conceptually, e.g., when asked to define new monads ."
+6848,"Functional languages also simulate states by passing around immutable states. This can be done by making a function accept the state as one of its parameters, and return a new state together with the result, leaving the old state unchanged."
+6849,"Impure functional languages usually include a more direct method of managing mutable state. Clojure, for example, uses managed references that can be updated by applying pure functions to the current state. This kind of approach enables mutability while still promoting the use of pure functions as the preferred way to express computations."
+6850,Alternative methods such as Hoare logic and uniqueness have been developed to track side effects in programs. Some modern research languages use effect systems to make the presence of side effects explicit.
+6851,"Functional programming languages are typically less efficient in their use of CPU and memory than imperative languages such as C and Pascal.  This is related to the fact that some mutable data structures like arrays have a very straightforward implementation using present hardware. Flat arrays may be accessed very efficiently with deeply pipelined CPUs, prefetched efficiently through caches , or handled with SIMD instructions.  It is also not easy to create their equally efficient general-purpose immutable counterparts. For purely functional languages, the worst-case slowdown is logarithmic in the number of memory cells used, because mutable memory can be represented by a purely functional data structure with logarithmic access time . However, such slowdowns are not universal. For programs that perform intensive numerical computations, functional languages such as OCaml and Clean are only slightly slower than C according to The Computer Language Benchmarks Game. For programs that handle large matrices and multidimensional databases, array functional languages  were designed with speed optimizations."
+6852,"Immutability of data can in many cases lead to execution efficiency by allowing the compiler to make assumptions that are unsafe in an imperative language, thus increasing opportunities for inline expansion."
+6853,"Lazy evaluation may also speed up the program, even asymptotically, whereas it may slow it down at most by a constant factor . Launchbury 1993 discusses theoretical issues related to memory leaks from lazy evaluation, and O'Sullivan et al. 2008 give some practical advice for analyzing and fixing them.
+However, the most general implementations of lazy evaluation making extensive use of dereferenced code and data perform poorly on modern processors with deep pipelines and multi-level caches  ."
+6854,"It is possible to use a functional style of programming in languages that are not traditionally considered functional languages. For example, both D and Fortran 95 explicitly support pure functions."
+6855,"JavaScript, Lua, Python and Go had first class functions from their inception. Python had support for ""lambda"", ""map"", ""reduce"", and ""filter"" in 1994, as well as closures in Python 2.2, though Python 3 relegated  ""reduce"" to the functools standard library module. First-class functions have been introduced into other mainstream languages such as PHP 5.3, Visual Basic 9, C# 3.0, C++11, and Kotlin."
+6856,"In PHP, anonymous classes, closures and lambdas are fully supported. Libraries and language extensions for immutable data structures are being developed to aid programming in the functional style."
+6857,"In Java, anonymous classes can sometimes be used to simulate closures; however, anonymous classes are not always proper replacements to closures because they have more limited capabilities. Java 8 supports lambda expressions as a replacement for some anonymous classes."
+6858,"In C#, anonymous classes are not necessary, because closures and lambdas are fully supported. Libraries and language extensions for immutable data structures are being developed to aid programming in the functional style in C#."
+6859,"Many object-oriented design patterns are expressible in functional programming terms: for example, the strategy pattern simply dictates use of a higher-order function, and the visitor pattern roughly corresponds to a catamorphism, or fold."
+6860,"Similarly, the idea of immutable data from functional programming is often included in imperative programming languages, for example the tuple in Python, which is an immutable array, and Object.freeze in JavaScript."
+6861,"Logic programming can be viewed as a generalisation of functional programming, in which functions are a special case of relations.
+For example, the function, mother = Y,  can be represented by the relation mother. Whereas functions have a strict input-output pattern of arguments, relations can be queried with any pattern of inputs and outputs. Consider the following logic program:"
+6862,"The program can be queried, like a functional program, to generate mothers from children:"
+6863,"But it can also be queried backwards, to generate children:"
+6864,It can even be used to generate all instances of the mother relation:
+6865,"Compared with relational syntax, functional syntax is a more compact notation for nested functions. For example, the definition of maternal grandmother in functional syntax can be written in the nested form:"
+6866,The same definition in relational notation needs to be written in the unnested form:
+6867,"Here :- means if and  , means and."
+6868,"However, the difference between the two representations is simply syntactic. In Ciao Prolog, relations can be nested, like functions in functional programming:"
+6869,Ciao transforms the function-like notation into relational form and executes the resulting logic program using the standard Prolog execution strategy.
+6870,"Spreadsheets can be considered a form of pure, zeroth-order, strict-evaluation functional programming system. However, spreadsheets generally lack higher-order functions as well as code reuse, and in some implementations, also lack recursion. Several extensions have been developed for spreadsheet programs to enable higher-order and reusable functions, but so far remain primarily academic in nature."
+6871,"Functional programming is an active area of research in the field of programming language theory. There are several peer-reviewed publication venues focusing on functional programming, including the International Conference on Functional Programming, the Journal of Functional Programming, and the Symposium on Trends in Functional Programming."
+6872,"Functional programming has been employed in a wide range of industrial applications. For example, Erlang, which was developed by the Swedish company Ericsson in the late 1980s, was originally used to implement fault-tolerant telecommunications systems, but has since become popular for building a range of applications at companies such as Nortel, Facebook, Électricité de France and WhatsApp. Scheme, a dialect of Lisp, was used as the basis for several applications on early Apple Macintosh computers and has been applied to problems such as training-simulation software and telescope control. OCaml, which was introduced in the mid-1990s, has seen commercial use in areas such as financial analysis, driver verification, industrial robot programming and static analysis of embedded software. Haskell, though initially intended as a research language, has also been applied in areas such as aerospace systems, hardware design and web programming."
+6873,"Other functional programming languages that have seen use in industry include Scala, F#, Wolfram Language, Lisp, Standard ML and Clojure."
+6874,"Functional ""platforms"" have been popular in finance for risk analytics . Risk factors are coded as functions that form interdependent graphs  to measure correlations in market shifts, similar in manner to Gröbner basis optimizations but also for regulatory frameworks such as Comprehensive Capital Analysis and Review. Given the use of OCaml and Caml variations in finance, these systems are sometimes considered related to a categorical abstract machine. Functional programming is heavily influenced by category theory."
+6875,Many universities teach functional programming. Some treat it as an introductory programming concept while others first teach imperative programming methods.
+6876,"Outside of computer science, functional programming is used to teach problem-solving, algebraic and geometric concepts. It has also been used to teach classical mechanics, as in the book Structure and Interpretation of Classical Mechanics."
+6877,"Abelson, Hal; Sussman, Gerald Jay . Structure and Interpretation of Computer Programs. MIT Press."
+6878,"Cousineau, Guy and Michel Mauny. The Functional Approach to Programming. Cambridge, UK: Cambridge University Press, 1998."
+6879,"Curry, Haskell Brooks and Feys, Robert and Craig, William. Combinatory Logic. Volume I. North-Holland Publishing Company, Amsterdam, 1958."
+6880,"Curry, Haskell B.; Hindley, J. Roger; Seldin, Jonathan P. . Combinatory Logic. Vol. II. Amsterdam: North Holland. ISBN 978-0-7204-2208-5."
+6881,"Dominus, Mark Jason. Higher-Order Perl. Morgan Kaufmann. 2005."
+6882,"Felleisen, Matthias; Findler, Robert; Flatt, Matthew; Krishnamurthi, Shriram . How to Design Programs. MIT Press."
+6883,"Graham, Paul. ANSI Common LISP. Englewood Cliffs, New Jersey: Prentice Hall, 1996."
+6884,"MacLennan, Bruce J. Functional Programming: Practice and Theory. Addison-Wesley, 1990."
+6885,"Michaelson, Greg . An Introduction to Functional Programming Through Lambda Calculus. Courier Corporation. ISBN 978-0-486-28029-5."
+6886,"O'Sullivan, Brian; Stewart, Don; Goerzen, John . Real World Haskell. O'Reilly."
+6887,"Pratt, Terrence W. and Marvin Victor Zelkowitz. Programming Languages: Design and Implementation. 3rd ed. Englewood Cliffs, New Jersey: Prentice Hall, 1996."
+6888,"Salus, Peter H. Functional and Logic Programming Languages. Vol. 4 of Handbook of Programming Languages. Indianapolis, Indiana: Macmillan Technical Publishing, 1998."
+6889,"Thompson, Simon. Haskell: The Craft of Functional Programming. Harlow, England: Addison-Wesley Longman Limited, 1996."
+6890,"Paradigms are separated along and described by different dimensions of programming. Some paradigms are about implications of the execution model, such as allowing side effects, or whether the sequence of operations is defined by the execution model. Other paradigms are about the way code is organized, such as grouping into units that include both state and behavior. Yet others are about syntax and grammar."
+6891,Some common programming paradigms include :
+6892,"Programming paradigms come from computer science research into existing practices of software development. The findings allow for describing and comparing programming practices and the languages used to code programs. For perspective, other research studies 
+software engineering processes and describes various methodologies to describe and compare them."
+6893,"A programming language can be described in terms of paradigms. Some languages support only one paradigm. For example, Smalltalk supports object-oriented and Haskell supports functional. Most languages support multiple paradigms. For example, a program written in C++, Object Pascal or PHP can be purely procedural, purely object-oriented, or can contain aspects of both – or other paradigms."
+6894,"When using a language that supports multiple paradigms, the developer chooses which paradigm elements to use. But, this choice may not involve considering paradigms per se. The developer often uses the features of a language as the language provides them and to the extent that the developer knows them. Categorizing the resulting code by paradigm is often an academic activity done in retrospect."
+6895,"Languages categorized as imperative paradigm have two main features: they state the order in which operations occur, with constructs that explicitly control that order, and they allow side effects, in which state can be modified at one point in time, within one unit of code, and then later read at a different point in time inside a different unit of code.  The communication between the units of code is not explicit."
+6896,"In contrast, languages in the declarative paradigm do not state the order in which to execute operations. Instead, they supply a number of available operations in the system, along with the conditions under which each is allowed to execute.  The implementation of the language's execution model tracks which operations are free to execute and chooses the order independently. More at  Comparison of multi-paradigm programming languages."
+6897,"In object-oriented programming, code is organized into objects that contain state that is owned by and  controlled by the code of the object.  Most object-oriented languages are also imperative languages."
+6898,"In object-oriented programming, programs are treated as a set of interacting objects. In functional programming, programs are treated as a sequence of stateless function evaluations. When programming computers or systems with many processors, in process-oriented programming, programs are treated as sets of concurrent processes that act on a logical shared data structures."
+6899,"Many programming paradigms are as well known for the techniques they forbid as for those they support. For instance, pure functional programming disallows side-effects, while structured programming disallows the goto construct. Partly for this reason, new paradigms are often regarded as doctrinaire or overly rigid by those accustomed to older ones. Yet, avoiding certain techniques can make it easier to understand program behavior, and to prove theorems about program correctness."
+6900,"Programming paradigms can also be compared with programming models, which allows invoking an execution model by using only an API. Programming models can also be classified into paradigms based on features of the execution model."
+6901,"For parallel computing, using a programming model instead of a language is common.  The reason is that details of the parallel hardware leak into the abstractions used to program the hardware.  This causes the programmer to have to map patterns in the algorithm onto patterns in the execution model .  As a consequence, no one parallel programming language maps well to all computation problems.  Thus, it is more convenient to use a base sequential language and insert API calls to parallel execution models via a programming model.  Such parallel programming models can be classified according to abstractions that reflect the hardware, such as shared memory, distributed memory with message passing, notions of place visible in the code, and so forth.  These can be considered flavors of programming paradigm that apply to only parallel languages and programming models."
+6902,"Some programming language researchers criticise the notion of paradigms as a classification of programming languages, e.g. Harper, and Krishnamurthi.  They argue that many programming languages cannot be strictly classified into one paradigm, but rather include features from several paradigms. See Comparison of multi-paradigm programming languages."
+6903,"Different approaches to programming have developed over time. Classification of each approach was either described at the time the approach was first developed, but often not until some time later, retrospectively. An early approach consciously identified as such is structured programming, advocated since the mid 1960s. The concept of a programming paradigm as such dates at least to 1978, in the Turing Award lecture of Robert W. Floyd, entitled The Paradigms of Programming, which cites the notion of paradigm as used by Thomas Kuhn in his The Structure of Scientific Revolutions . Early programming languages did not have clearly defined programming paradigms and sometimes programs made extensive use of goto statements. Liberal use of which lead to spaghetti code which is difficult to understand and maintain. This led to the development of structured programming paradigms that disallowed the use of goto statements; only allowing the use of more structured programming constructs."
+6904,Machine code is the lowest-level of computer programming as it is machine instructions that define behavior at the lowest level of abstract possible for a computer. As it is the most prescriptive way to code it is classified as imperative.
+6905,It is sometimes called the first-generation programming language.
+6906,Assembly language introduced mnemonics for machine instructions and memory addresses. Assembly is classified as imperative and is sometimes called the second-generation programming language.
+6907,"In the 1960s, assembly languages were developed to support library COPY and quite sophisticated conditional macro generation and preprocessing abilities, CALL to subroutine, external variables and common sections , enabling significant code re-use and isolation from hardware specifics via the use of logical operators such as READ/WRITE/GET/PUT. Assembly was, and still is, used for time-critical systems and often in embedded systems as it gives the most control of what the machine does."
+6908,"Procedural languages, also called the third-generation programming languages are the first described as high-level languages. They support vocabulary related to the problem being solved. For example,"
+6909,These languages are classified as procedural paradigm. They directly control the step by step process that a computer program follows. The efficacy and efficiency of such a program is therefore highly dependent on the programmer's skill.
+6910,"In attempt to improve on procedural languages, object-oriented programming  languages were created, such as Simula, Smalltalk, C++, Eiffel, Python, PHP, Java, and C#. In these languages, data and methods to manipulate the data are in the same code unit called an object. This encapsulation ensures that the only way that an object can access data is via methods of the object that contains the data. Thus, an object's inner workings may be changed without affecting code that uses the object."
+6911,"There is controversy raised by Alexander Stepanov, Richard Stallman and other programmers, concerning the efficacy of the OOP paradigm versus the procedural paradigm. The need for every object to have associative methods leads some skeptics to associate OOP with software bloat; an attempt to resolve this dilemma came through polymorphism."
+6912,"Although most OOP languages are third-generation, it is possible to create an object-oriented assembler language. High Level Assembly  is an example of this that fully supports advanced data types and object-oriented assembly language programming –  despite its early origins. Thus, differing programming paradigms can be seen rather like motivational memes of their advocates, rather than necessarily representing progress from one level to the next. Precise comparisons of competing paradigms' efficacy are frequently made more difficult because of new and differing terminology applied to similar entities and processes together with numerous implementation distinctions across languages."
+6913,"A declarative programming program describes what the problem is, not how to solve it. The program is structured as a set of properties to find in the expected result, not as a procedure to follow. Given a database or a set of rules, the computer tries to find a solution matching all the desired properties. An archetype of a declarative language is the fourth generation language SQL, and the family of functional languages and logic programming."
+6914,"Functional programming is a subset of declarative programming. Programs written using this paradigm use functions, blocks of code intended to behave like mathematical functions. Functional languages discourage changes in the value of variables through assignment, making a great deal of use of recursion instead."
+6915,"The logic programming paradigm views computation as automated reasoning over a body of knowledge. Facts about the problem domain are expressed as logic formulas, and programs are executed by applying inference rules over them until an answer to the problem is found, or the set of formulas is proved inconsistent."
+6916,"Symbolic programming is a paradigm that describes programs able to manipulate formulas and program components as data.  Programs can thus effectively modify themselves, and appear to ""learn"", making them suited for applications such as artificial intelligence, expert systems, natural-language processing and computer games.  Languages that support this paradigm include Lisp and Prolog."
+6917,"Differentiable programming structures programs so that they can be differentiated throughout, usually via automatic differentiation."
+6918,"Literate programming, as a form of imperative programming, structures programs as a human-centered web, as in a hypertext essay: documentation is integral to the program, and the program is structured following the logic of prose exposition, rather than compiler convenience."
+6919,"Symbolic techniques such as reflection, which allow the program to refer to itself, might also be considered as a programming paradigm. However, this is compatible with the major paradigms and thus is not a real paradigm in its own right."
+6920,Classification of the principal programming paradigms
+6921,How programming paradigms evolve and get adopted?
+6922,"The structured chart subject to these constraints, particularly the loop constraint implying a single exit , may however use additional variables in the form of bits  in order to keep track of information that the original program represents by the program location. The construction was based on Böhm's programming language P′′."
+6923,"The theorem forms the basis of structured programming, a programming paradigm which eschews goto commands and exclusively uses subroutines, sequences, selection and iteration."
+6924,"The theorem is typically credited: 381  to a 1966 paper by Corrado Böhm and Giuseppe Jacopini. David Harel wrote in 1980 that the Böhm–Jacopini paper enjoyed ""universal popularity"",: 381  particularly with proponents of structured programming. Harel also noted that ""due to its rather technical style  is apparently more often cited than read in detail"": 381  and, after reviewing a large number of papers published up to 1980, Harel argued that the contents of the Böhm–Jacopini proof were usually misrepresented as a folk theorem that essentially contains a simpler result, a result which itself can be traced to the inception of modern computing theory in the papers of von Neumann and Kleene.: 383"
+6925,"Harel also writes that the more generic name was proposed by H.D. Mills as ""The Structure Theorem"" in the early 1970s.: 381"
+6926,"This version of the theorem replaces all the original program's control flow with a single global while loop that simulates a program counter going over all possible labels  in the original non-structured program. Harel traced the origin of this folk theorem to two papers marking the beginning of computing. One is the 1946 description of the von Neumann architecture, which explains how a program counter operates in terms of a while loop. Harel notes that the single loop used by the folk version of the structured programming theorem basically just provides operational semantics for the execution of a flowchart on a von Neumann computer.: 383  Another, even older source that Harel traced the folk version of the theorem is Stephen Kleene's normal form theorem from 1936.: 383"
+6927,"Donald Knuth criticized this form of the proof, which results in pseudocode like the one below, by pointing out that the structure of the original program is completely lost in this transformation.: 274  Similarly, Bruce Ian Mills wrote about this approach that ""The spirit of block structure is a style, not a language. By simulating a Von Neumann machine, we can produce the behavior of any spaghetti code within the confines of a block-structured language. This does not prevent it from being spaghetti."""
+6928,"The proof in Böhm and Jacopini's paper proceeds by induction on the structure of the flow chart.: 381  Because it employed pattern matching in graphs, the proof of Böhm and Jacopini's was not really practical as a program transformation algorithm, and thus opened the door for additional research in this direction."
+6929,"The Reversible Structured Program Theorem is an important concept in the field of reversible computing. It posits that any computation achievable by a reversible program can also be accomplished through a reversible program using only a structured combination of control flow constructs such as sequences, selections, and iterations. Any computation achievable by a traditional, irreversible program can also be accomplished through a reversible program, but with the additional constraint that each step must be reversible and some extra output.  Furthermore, any reversible unstructured program can also be accomplished through a structured reversible program with only one iteration without any extra output. This theorem lays the foundational principles for constructing reversible algorithms within a structured programming framework."
+6930,"For the Structured Program Theorem, both local and global methods of proof are known. However, for its reversible version, while a global method of proof is recognized, a local approach similar to that undertaken by Böhm and Jacopini is not yet known. This distinction is an example that underscores the challenges and nuances in establishing the foundations of reversible computing compared to traditional computing paradigms."
+6931,"The Böhm–Jacopini proof did not settle the question of whether to adopt structured programming for software development, partly because the construction was more likely to obscure a program than to improve it. On the contrary, it signalled the beginning of the debate. Edsger Dijkstra's famous letter, ""Go To Statement Considered Harmful,"" followed in 1968."
+6932,"Some academics took a purist approach to the Böhm–Jacopini result and argued that even instructions like break and return from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini proof, and thus they advocated that all loops should have a single exit point. This purist approach is embodied in the Pascal programming language , which up to the mid-1990s was the preferred tool for teaching introductory programming classes in academia."
+6933,"Edward Yourdon notes that in the 1970s there was even philosophical opposition to transforming unstructured programs into structured ones by automated means, based on the argument that one needed to think in structured programming fashion from the get go. The pragmatic counterpoint was that such transformations benefited a large body of existing programs. Among the first proposals for an automated transformation was a 1971 paper by Edward Ashcroft and Zohar Manna."
+6934,"The direct application of the Böhm–Jacopini theorem may result in additional local variables being introduced in the structured chart, and may also result in some code duplication. The latter issue is called the loop and a half problem in this context. Pascal is affected by both of these problems and according to empirical studies cited by Eric S. Roberts, student programmers had difficulty formulating correct solutions in Pascal for several simple problems, including writing a function for searching an element in an array. A 1980 study by Henry Shapiro cited by Roberts found that using only the Pascal-provided control structures, the correct solution was given by only 20% of the subjects, while no subject wrote incorrect code for this problem if allowed to write a return from the middle of a loop."
+6935,"In 1973, S. Rao Kosaraju proved that it's possible to avoid adding additional variables in structured programming, as long as arbitrary-depth, multi-level breaks from loops are allowed. Furthermore, Kosaraju proved that a strict hierarchy of programs exists, nowadays called the Kosaraju hierarchy, in that for every integer n, there exists a program containing a multi-level break of depth n that cannot be rewritten as program with multi-level breaks of depth less than n . Kosaraju cites the multi-level break construct to the BLISS programming language. The multi-level breaks, in the form a leave label keyword were actually introduced in the BLISS-11 version of that language; the original BLISS only had single-level breaks. The BLISS family of languages didn't provide an unrestricted goto. The Java programming language would later follow this approach as well.: 960–965"
+6936,"A simpler result from Kosaraju's paper is that a program is reducible to a structured program  if and only if it does not contain a loop with two distinct exits. Reducibility was defined by Kosaraju, loosely speaking, as computing the same function and using the same ""primitive actions"" and predicates as the original program, but possibly using different control flow structures.  Inspired by this result, in section VI of his highly-cited paper that introduced the notion of cyclomatic complexity, Thomas J. McCabe described an analogue of Kuratowski's theorem for the control-flow graphs  of non-structured programs, which is to say, the minimal subgraphs that make the CFG of a program non-structured. These subgraphs have a very good description in natural language. They are:"
+6937,"McCabe actually found that these four graphs are not independent when appearing as subgraphs, meaning that a necessary and sufficient condition for a program to be non-structured is for its CFG to have as subgraph one of any subset of three of these four graphs. He also found that if a non-structured program contains one of these four sub-graphs, it must contain another distinct one from the set of four. This latter result helps explain how the control flow of non-structured program becomes entangled in what is popularly called ""spaghetti code"". McCabe also devised a numerical measure that, given an arbitrary program, quantifies how far off it is from the ideal of being a structured program; McCabe called his measure essential complexity."
+6938,"McCabe's characterization of the forbidden graphs for structured programming can be considered incomplete, at least if the Dijkstra's D structures are considered the building blocks.: 274–275 "
+6939,"Up to 1990 there were quite a few proposed methods for eliminating gotos from existing programs, while preserving most of their structure. The various approaches to this problem also proposed several notions of equivalence, which are stricter than simply Turing equivalence, in order to avoid output like the folk theorem discussed above. The strictness of the chosen notion of equivalence dictates the minimal set of control flow structures needed. The 1988 JACM paper by Lyle Ramshaw surveys the field up to that point, as well proposing its own method. Ramshaw's algorithm was used for example in some Java decompilers because the Java virtual machine code has branch instructions with targets expressed as offsets, but the high-level Java language only has multi-level break and continue statements. Ammarguellat  proposed a transformation method that goes back to enforcing single-exit."
+6940,"In the 1980s IBM researcher Harlan Mills oversaw the development of the COBOL Structuring Facility, which applied a structuring algorithm to COBOL code. Mills's transformation involved the following steps for each procedure."
+6941,This construction can be improved by converting some cases of the selection statement into subprocedures.
+6942,Material not yet covered above:
+6943,"The machine operates on an infinite memory tape divided into discrete cells, each of which can hold a single symbol drawn from a finite set of symbols called the alphabet of the machine. It has a ""head"" that, at any point in the machine's operation, is positioned over one of these cells, and a ""state"" selected from a finite set of states. At each step of its operation, the head reads the symbol in its cell. Then, based on the symbol and the machine's own present state, the machine writes a symbol into the same cell, and moves the head one step to the left or the right, or halts the computation. The choice of which replacement symbol to write, which direction to move the head, and whether to halt is based on a finite table that specifies what to do for each combination of the current state and the symbol that is read. Like a real computer program, it is possible for a Turing machine to go into an infinite loop which will never halt."
+6944,"The Turing machine was invented in 1936 by Alan Turing, who called it an ""a-machine"" . It was Turing's doctoral advisor, Alonzo Church, who later coined the term ""Turing machine"" in a review.  With this model, Turing was able to answer two questions in the negative:"
+6945,"Thus by providing a mathematical description of a very simple device capable of arbitrary computations, he was able to prove properties of computation in general—and in particular, the uncomputability of the Entscheidungsproblem ."
+6946,"Turing machines proved the existence of fundamental limitations on the power of mechanical computation. While they can express arbitrary computations, their minimalist design makes them too slow for computation in practice: real-world computers are based on different designs that, unlike Turing machines, use random-access memory."
+6947,Turing completeness is the ability for a computational model or a system of instructions to simulate a Turing machine. A programming language that is Turing complete is theoretically capable of expressing all tasks accomplishable by computers; nearly all programming languages are Turing complete if the limitations of finite memory are ignored.
+6948,"A Turing machine is an idealised model of a central processing unit  that controls all data manipulation done by a computer, with the canonical machine using sequential memory to store data. Typically, the sequential memory is represented as a tape of infinite length on which the machine can perform read and write operations."
+6949,"In the context of formal language theory, a Turing machine  is capable of enumerating some arbitrary subset of valid strings of an alphabet. A set of strings which can be enumerated in this manner is called a recursively enumerable language. The Turing machine can equivalently be defined as a model that recognises valid input strings, rather than enumerating output strings."
+6950,"Given a Turing machine M and an arbitrary string s, it is generally not possible to decide whether M will eventually produce s. This is due to the fact that the halting problem is unsolvable, which has major implications for the theoretical limits of computing."
+6951,"The Turing machine is capable of processing an unrestricted grammar, which further implies that it is capable of robustly evaluating first-order logic in an infinite number of ways. This is famously demonstrated through lambda calculus."
+6952,"A Turing machine that is able to simulate any other Turing machine is called a universal Turing machine . Another mathematical formalism, lambda calculus, with a similar ""universal"" nature was introduced by Alonzo Church. Church's work intertwined with Turing's to form the basis for the Church–Turing thesis. This thesis states that Turing machines, lambda calculus, and other similar formalisms of computation do indeed capture the informal notion of effective methods in logic and mathematics and thus provide a model through which one can reason about an algorithm or ""mechanical procedure"" in a mathematically precise way without being tied to any particular formalism. Studying the  abstract properties of Turing machines has yielded many insights into computer science, computability theory, and complexity theory."
+6953,"In his 1948 essay, ""Intelligent Machinery"", Turing wrote that his machine consisted of:"
+6954,"...an unlimited memory capacity obtained in the form of an infinite tape marked out into squares, on each of which a symbol could be printed. At any moment there is one symbol in the machine; it is called the scanned symbol. The machine can alter the scanned symbol, and its behavior is in part determined by that symbol, but the symbols on the tape elsewhere do not affect the behavior of the machine. However, the tape can be moved back and forth through the machine, this being one of the elementary operations of the machine. Any symbol on the tape may therefore eventually have an innings."
+6955,"The Turing machine mathematically models a machine that mechanically operates on a tape.  On this tape are symbols, which the machine can read and write, one at a time, using a tape head. Operation is fully determined by a finite set of elementary instructions such as ""in state 42, if the symbol seen is 0, write a 1; if the symbol seen is 1, change into state 17; in state 17, if the symbol seen is 0, write a 1 and change to state 6;"" etc. In the original article , Turing imagines not a mechanism, but a person whom he calls the ""computer"", who executes these deterministic mechanical rules slavishly ."
+6956,"More explicitly, a Turing machine consists of:"
+6957,"In the 4-tuple models, erasing or writing a symbol  and moving the head left or right  are specified as separate instructions. The table tells the machine to  erase or write a symbol or  move the head left or right, and then  assume the same or a new state as prescribed, but not both actions  and  in the same instruction. In some models, if there is no entry in the table for the current combination of symbol and state, then the machine will halt; other models require all entries to be filled."
+6958,"Every part of the machine  and its actions  is finite, discrete and distinguishable; it is the unlimited amount of tape and runtime that gives it an unbounded amount of storage space."
+6959,The 7-tuple for the 3-state busy beaver looks like this :
+6960,"In the words of van Emde Boas , p. 6: ""The set-theoretical object  provides only partial information on how the machine will behave and what its computations will look like."""
+6961,"For instance,"
+6962,"The most common convention represents each ""Turing instruction"" in a ""Turing table"" by one of nine 5-tuples, per the convention of Turing/Davis  in The Undecidable, p. 126–127 and Davis  p. 152):"
+6963,"Other authors  p. 119, Hopcroft and Ullman  p. 158, Stone  p. 9) adopt a different convention, with new state qm listed immediately after the scanned symbol Sj:"
+6964,"For the remainder of this article ""definition 1""  will be used."
+6965,"In the following table, Turing's original model allowed only the first three lines that he called N1, N2, N3 . He allowed for erasure of the ""scanned square"" by naming a 0th symbol S0 = ""erase"" or ""blank"", etc. However, he did not allow for non-printing, so every instruction-line includes ""print symbol Sk"" or ""erase"" , The Undecidable, p. 300). The abbreviations are Turing's . Subsequent to Turing's original paper in 1936–1937, machine-models have allowed all nine possible types of five-tuples:"
+6966,"Any Turing table  can be constructed from the above nine 5-tuples. For technical reasons, the three non-printing or ""N"" instructions  can usually be dispensed with. For examples see Turing machine examples."
+6967,"Less frequently the use of 4-tuples are encountered: these represent a further atomization of the Turing instructions , Boolos & Jeffrey , Davis-Sigal-Weyuker ); also see more at Post–Turing machine."
+6968,"The word ""state"" used in context of Turing machines can be a source of confusion, as it can mean two things. Most commentators after Turing have used ""state"" to mean the name/designator of the current instruction to be performed—i.e. the contents of the state register. But Turing  made a strong distinction between a record of what he called the machine's ""m-configuration"", and the machine's  ""state of progress"" through the computation—the current state of the total system. What Turing called ""the state formula"" includes both the current instruction and all the symbols on the tape:"
+6969,"Thus the state of progress of the computation at any stage is completely determined by the note of instructions and the symbols on the tape. That is, the state of the system may be described by a single expression  consisting of the symbols on the tape followed by Δ  and then by the note of instructions. This expression is called the ""state formula""."
+6970,"Earlier in his paper Turing carried this even further: he gives an example where he placed a symbol of the current ""m-configuration""—the instruction's label—beneath the scanned square, together with all the symbols on the tape ; this he calls ""the complete configuration"" . To print the ""complete configuration"" on one line, he places the state-label/m-configuration to the left of the scanned symbol."
+6971,"A variant of this is seen in Kleene  where Kleene shows how to write the Gödel number of a machine's ""situation"": he places the ""m-configuration"" symbol q4 over the scanned square in roughly the center of the 6 non-blank squares on the tape  and puts it to the right of the scanned square. But Kleene refers to ""q4"" itself as ""the machine state"" . Hopcroft and Ullman call this composite the ""instantaneous description"" and follow the Turing convention of putting the ""current state""  to the left of the scanned symbol , that is, the instantaneous description is the composite of non-blank symbols to the left, state of the machine, the current symbol scanned by the head, and the non-blank symbols to the right."
+6972,"Example: total state of 3-state 2-symbol busy beaver after 3 ""moves"" :"
+6973,"This means: after three moves the tape has ... 000110000 ... on it, the head is scanning the right-most 1, and the state is A. Blanks  can be part of the total state as shown here: B01; the tape has a single 1 on it, but the head is scanning the 0  to its left and the state is B."
+6974,"""State"" in the context of Turing machines should be clarified as to which is being described:  the current instruction, or the list of symbols on the tape together with the current instruction, or  the list of symbols on the tape together with the current instruction placed to the left of the scanned symbol or to the right of the scanned symbol."
+6975,Turing's biographer Andrew Hodges  has noted and discussed this confusion.
+6976,"To the right: the above table as expressed as a ""state transition"" diagram."
+6977,"Usually large tables are better left as tables . They are more readily simulated by computer in tabular form . However, certain concepts—e.g. machines with ""reset"" states and machines with repeating patterns —can be more readily seen when viewed as a drawing."
+6978,Whether a drawing represents an improvement on its table must be decided by the reader for the particular context.
+6979,"The reader should again be cautioned that such diagrams represent a snapshot of their table frozen in time, not the course  of a computation through time and space. While every time the busy beaver machine ""runs"" it will always follow the same state-trajectory, this is not true for the ""copy"" machine that can be provided with variable input ""parameters""."
+6980,"The diagram ""progress of the computation"" shows the three-state busy beaver's ""state""  progress through its computation from start to finish. On the far right is the Turing ""complete configuration""  at each step. If the machine were to be stopped and cleared to blank both the ""state register"" and entire tape, these ""configurations"" could be used to rekindle a computation anywhere in its progress  The Undecidable, pp. 139–140)."
+6981,"Many machines that might be thought to have more computational capability than a simple universal Turing machine can be shown to have no more power ). They might compute faster, perhaps, or use less memory, or their instruction set might be smaller, but they cannot compute more powerfully . "
+6982,"A Turing machine is equivalent to a single-stack pushdown automaton  that has been made more flexible and concise by relaxing the last-in-first-out  requirement of its stack. In addition, a Turing machine is also equivalent to a two-stack PDA with standard LIFO  semantics, by using one stack to model the tape left of the head and the other stack for the tape to the right."
+6983,"At the other extreme, some very simple models turn out to be Turing-equivalent, i.e. to have the same computational power as the Turing machine model."
+6984,"Common equivalent models are the multi-tape Turing machine, multi-track Turing machine, machines with input and output, and the non-deterministic Turing machine  as opposed to the deterministic Turing machine  for which the action table has at most one entry for each combination of symbol and state."
+6985,"Read-only, right-moving Turing machines are equivalent to DFAs ."
+6986,For practical and didactical intentions the equivalent register machine can be used as a usual assembly programming language.
+6987,"A relevant question is whether or not the computation model represented by concrete programming languages is Turing equivalent. While the computation of a real computer is based on finite states and thus not capable to simulate a Turing machine, programming languages themselves do not necessarily have this limitation. Kirner et al., 2009 have shown that among the general-purpose programming languages some are Turing complete while others are not. For example, ANSI C is not Turing-equivalent, as all instantiations of ANSI C  imply a finite-space memory. This is because the size of memory reference data types, called pointers, is accessible inside the language. However, other programming languages like Pascal do not have this feature, which allows them to be Turing complete in principle.
+It is just Turing complete in principle, as memory allocation in a programming language is allowed to fail, which means the programming language can be Turing complete when ignoring failed memory allocations, but the compiled programs executable on a real computer cannot."
+6988,"Early in his paper  Turing makes a distinction between an ""automatic machine""—its ""motion ... completely determined by the configuration"" and a ""choice machine"":"
+6989,"...whose motion is only partially determined by the configuration ... When such a machine reaches one of these ambiguous configurations, it cannot go on until some arbitrary choice has been made by an external operator. This would be the case if we were using machines to deal with axiomatic systems."
+6990,"Turing  does not elaborate further except in a footnote in which he describes how to use an a-machine to ""find all the provable formulae of the  calculus"" rather than use a choice machine. He ""suppose that the choices are always between two possibilities 0 and 1. Each proof will then be determined by a sequence of choices i1, i2, ..., in , and hence the number 2n + i12n-1 + i22n-2 + ... +in completely determines the proof. The automatic machine carries out successively proof 1, proof 2, proof 3, ..."" "
+6991,This is indeed the technique by which a deterministic  Turing machine can be used to mimic the action of a nondeterministic Turing machine; Turing solved the matter in a footnote and appears to dismiss it from further consideration.
+6992,"An oracle machine or o-machine is a Turing a-machine that pauses its computation at state ""o"" while, to complete its calculation, it ""awaits the decision"" of ""the oracle""—an entity unspecified by Turing ""apart from saying that it cannot be a machine"" , The Undecidable, p. 166–168)."
+6993,"As Turing wrote in The Undecidable, p. 128 :"
+6994,"It is possible to invent a single machine which can be used to compute any computable sequence. If this machine U is supplied with the tape on the beginning of which is written the string of quintuples separated by semicolons of some computing machine M, then U will compute the same sequence as M."
+6995,"This finding is now taken for granted, but at the time  it was considered astonishing. The model of computation that Turing called his ""universal machine""—""U"" for short—is considered by some ) to have been the fundamental theoretical breakthrough that led to the notion of the stored-program computer."
+6996,"Turing's paper ... contains, in essence, the invention of the modern computer and some of the programming techniques that accompanied it."
+6997,"In terms of computational complexity, a multi-tape universal Turing machine need only be slower by logarithmic factor compared to the machines it simulates. This result was obtained in 1966 by F. C. Hennie and R. E. Stearns. "
+6998,"It is often believed that Turing machines, unlike simpler automata, are as powerful as real machines, and are able to execute any operation that a real program can. What is neglected in this statement is that, because a real machine can only have a finite number of configurations, it is nothing but a finite-state machine, whereas a Turing machine has an unlimited amount of storage space available for its computations."
+6999,There are a number of ways to explain why Turing machines are useful models of real computers:
+7000,"A limitation of Turing machines is that they do not model the strengths of a particular arrangement well. For instance, modern stored-program computers are actually instances of a more specific form of abstract machine known as the random-access stored-program machine or RASP machine model. Like the universal Turing machine, the RASP stores its ""program"" in ""memory"" external to its finite-state machine's ""instructions"". Unlike the universal Turing machine, the RASP has an infinite number of distinguishable, numbered but unbounded ""registers""—memory ""cells"" that can contain any integer , Hartmanis , and in particular Cook-Rechow ; references at random-access machine). The RASP's finite-state machine is equipped with the capability for indirect addressing ; thus the RASP's ""program"" can address any register in the register-sequence. The upshot of this distinction is that there are computational optimizations that can be performed based on the memory indices, which are not possible in a general Turing machine; thus when Turing machines are used as the basis for bounding running times, a ""false lower bound"" can be proven on certain algorithms' running times . An example of this is binary search, an algorithm that can be shown to perform more quickly when using the RASP model of computation rather than the Turing machine model."
+7001,"In the early days of computing, computer use was typically limited to batch processing, i.e., non-interactive tasks, each producing output data from given input data. Computability theory, which studies computability of functions from inputs to outputs, and for which Turing machines were invented, reflects this practice."
+7002,"Since the 1970s, interactive use of computers became much more common.  In principle, it is possible to model this by having an external agent read from the tape and write to it at the same time as a Turing machine, but this rarely matches how interaction actually happens; therefore, when describing interactivity, alternatives such as I/O automata are usually preferred."
+7003,The arithmetic model of computation differs from the Turing model in two aspects: : 32
+7004,Some algorithms run in polynomial time in one model but not in the other one. For example:
+7005,"However, if an algorithm runs in polynomial time in the arithmetic model, and in addition, the binary length of all involved numbers is polynomial in the length of the input, then it is always polynomial-time in the Turing model. Such an algorithm is said to run in strongly polynomial time."
+7006,"Robin Gandy —a student of Alan Turing , and his lifelong friend—traces the lineage of the notion of ""calculating machine"" back to Charles Babbage  and actually proposes ""Babbage's Thesis"":"
+7007,That the whole of development and operations of analysis are now capable of being executed by machinery.
+7008,Gandy's analysis of Babbage's analytical engine describes the following five operations :
+7009,"Gandy states that ""the functions which can be calculated by , , and  are precisely those which are Turing computable."" . He cites other proposals for ""universal calculating machines"" including those of Percy Ludgate , Leonardo Torres Quevedo , Maurice d'Ocagne , Louis Couffignal , Vannevar Bush , Howard Aiken . However:"
+7010,… the emphasis is on programming a fixed iterable sequence of arithmetical operations. The fundamental importance of conditional iteration and conditional transfer for a general theory of calculating machines is not recognized…
+7011,"With regard to Hilbert's problems posed by the famous mathematician David Hilbert in 1900, an aspect of problem #10 had been floating about for almost 30 years before it was framed precisely. Hilbert's original expression for No. 10 is as follows:"
+7012,"10. Determination of the solvability of a Diophantine equation. Given a Diophantine equation with any number of unknown quantities and with rational integral coefficients: To devise a process according to which it can be determined in a finite number of operations whether the equation is solvable in rational integers.
+The Entscheidungsproblem  is solved when we know a procedure that allows for any given logical expression to decide by finitely many operations its validity or satisfiability ... The Entscheidungsproblem must be considered the main problem of mathematical logic."
+7013,"By 1922, this notion of ""Entscheidungsproblem"" had developed a bit, and H. Behmann stated that"
+7014,... most general form of the Entscheidungsproblem  as follows:
+7015,A quite definite generally applicable prescription is required which will allow one to decide in a finite number of steps the truth or falsity of a given purely logical assertion ...
+7016,Behmann remarks that ... the general problem is equivalent to the problem of deciding which mathematical propositions are true.
+7017,"If one were able to solve the Entscheidungsproblem then one would have a ""procedure for solving many  mathematical problems""."
+7018,"By the 1928 international congress of mathematicians, Hilbert ""made his questions quite precise. First, was mathematics complete ... Second, was mathematics consistent ... And thirdly, was mathematics decidable?"" . The first two questions were answered in 1930 by Kurt Gödel at the very same meeting where Hilbert delivered his retirement speech ; the third—the Entscheidungsproblem—had to wait until the mid-1930s."
+7019,"The problem was that an answer first required a precise definition of ""definite general applicable prescription"", which Princeton professor Alonzo Church would come to call ""effective calculability"", and in 1928 no such definition existed. But over the next 6–7 years Emil Post developed his definition of a worker moving from room to room writing and erasing marks per a list of instructions , as did Church and his two students Stephen Kleene and J. B. Rosser by use of Church's lambda-calculus and Gödel's recursion theory . Church's paper  showed that the Entscheidungsproblem was indeed ""undecidable"" and beat Turing to the punch by almost a year . In the meantime, Emil Post submitted a brief paper in the fall of 1936, so Turing at least had priority over Post. While Church refereed Turing's paper, Turing had time to study Church's paper and add an Appendix where he sketched a proof that Church's lambda-calculus and his machines would compute the same functions."
+7020,"But what Church had done was something rather different, and in a certain sense weaker. ... the Turing construction was more direct, and provided an argument from first principles, closing the gap in Church's demonstration."
+7021,"And Post had only proposed a definition of calculability and criticised Church's ""definition"", but had proved nothing."
+7022,"In the spring of 1935, Turing as a young Master's student at King's College, Cambridge, took on the challenge; he had been stimulated by the lectures of the logician M. H. A. Newman ""and learned from them of Gödel's work and the Entscheidungsproblem ... Newman used the word 'mechanical' ... In his obituary of Turing 1955 Newman writes:"
+7023,"To the question 'what is a ""mechanical"" process?' Turing returned the characteristic answer 'Something that can be done by a machine' and he embarked on the highly congenial task of analysing the general notion of a computing machine."
+7024,Gandy states that:
+7025,"I suppose, but do not know, that Turing, right from the start of his work, had as his goal a proof of the undecidability of the Entscheidungsproblem. He told me that the 'main idea' of the paper came to him when he was lying in Grantchester meadows in the summer of 1935. The 'main idea' might have either been his analysis of computation or his realization that there was a universal machine, and so a diagonal argument to prove unsolvability."
+7026,"While Gandy believed that Newman's statement above is ""misleading"", this opinion is not shared by all. Turing had a lifelong interest in machines: ""Alan had dreamt of inventing typewriters as a boy;  Mrs. Turing had a typewriter; and he could well have begun by asking himself what was meant by calling a typewriter 'mechanical'"" . While at Princeton pursuing his PhD, Turing built a Boolean-logic multiplier . His PhD thesis, titled ""Systems of Logic Based on Ordinals"", contains the following definition of ""a computable function"":"
+7027,"It was stated above that 'a function is effectively calculable if its values can be found by some purely mechanical process'. We may take this statement literally, understanding by a purely mechanical process one which could be carried out by a machine. It is possible to give a mathematical description, in a certain normal form, of the structures of these machines. The development of these ideas leads to the author's definition of a computable function, and to an identification of computability with effective calculability. It is not difficult, though somewhat laborious, to prove that these three definitions  are equivalent."
+7028,"Alan Turing invented the ""a-machine""  in 1936. Turing submitted his paper on 31 May 1936 to the London Mathematical Society for its Proceedings , but it was published in early 1937 and offprints were available in February 1937  It was Turing's doctoral advisor, Alonzo Church, who later coined the term ""Turing machine"" in a review.  With this model, Turing was able to answer two questions in the negative:"
+7029,"Thus by providing a mathematical description of a very simple device capable of arbitrary computations, he was able to prove properties of computation in general—and in particular, the uncomputability of the Entscheidungsproblem ."
+7030,"When Turing returned to the UK he ultimately became jointly responsible for breaking the German secret codes created by encryption machines called ""The Enigma""; he also became involved in the design of the ACE , "" ACE proposal was effectively self-contained, and its roots lay not in the EDVAC , but in his own universal machine"" . Arguments still continue concerning the origin and nature of what has been named by Kleene  Turing's Thesis. But what Turing did prove with his computational-machine model appears in his paper ""On Computable Numbers, with an Application to the Entscheidungsproblem"" :"
+7031," the Hilbert Entscheidungsproblem can have no solution ... I propose, therefore to show that there can be no general process for determining whether a given formula U of the functional calculus K is provable, i.e. that there can be no machine which, supplied with any one U of these formulae, will eventually say whether U is provable."
+7032,"Turing's example : If one is to ask for a general procedure to tell us: ""Does this machine ever print 0"", the question is ""undecidable""."
+7033,"In 1937, while at Princeton working on his PhD thesis, Turing built a digital  multiplier from scratch, making his own electromechanical relays . ""Alan's task was to embody the logical design of a Turing machine in a network of relay-operated switches ..."" . While Turing might have been just initially curious and experimenting, quite-earnest work in the same direction was going in Germany ), and in the United States  and George Stibitz ; the fruits of their labors were used by both the Axis and Allied militaries in World War II . In the early to mid-1950s Hao Wang and Marvin Minsky reduced the Turing machine to a simpler form ; simultaneously European researchers were reducing the new-fangled electronic computer to a computer-like theoretical object equivalent to what was now being called a ""Turing machine"". In the late 1950s and early 1960s, the coincidentally parallel developments of Melzak and Lambek , Minsky , and Shepherdson and Sturgis  carried the European work further and reduced the Turing machine to a more friendly, computer-like abstract model called the counter machine; Elgot and Robinson , Hartmanis , Cook and Reckhow  carried this work even further with the register machine and random-access machine models—but basically all are just multi-tape Turing machines with an arithmetic-like instruction set."
+7034,"Today, the counter, register and random-access machines and their sire the Turing machine continue to be the models of choice for theorists investigating questions in the theory of computation. In particular, computational complexity theory makes use of the Turing machine:"
+7035,"Depending on the objects one likes to manipulate in the computations , two models have obtained a dominant position in machine-based complexity theory:"
+7036,"the off-line multitape Turing machine..., which represents the standard model for string-oriented computation, and
+the random access machine  as introduced by Cook and Reckhow ..., which models the idealised Von Neumann-style computer."
+7037,Only in the related area of analysis of algorithms this role is taken over by the RAM model.
+7038,"Digital physics suggests that there exists, at least in principle, a program for a universal computer that computes the evolution of the universe. The computer could be, for example, a huge cellular automaton."
+7039,"Extant models of digital physics are incompatible with the existence of several continuous characters of physical symmetries, e.g., rotational symmetry, translational symmetry, Lorentz symmetry, and the Lie group gauge invariance of Yang–Mills theories, all central to current physical theory. Moreover, extant models of digital physics violate various well-established features of quantum physics, belonging to the class of theories with local hidden variables that have so far been ruled out experimentally using Bell's theorem."
+7040,"However, covariant discrete theories can be formulated that preserve the aforementioned symmetries."
+7041,"Rapaport has done research and written extensively on intentionality and artificial intelligence. He has research interests in computer science, artificial intelligence , computational linguistics, cognitive science, logic and mathematics, and published many scientific articles on them."
+7042,"While a philosophy graduate student at Indiana University in 1972, he concocted the sentence: ""Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo"". Throughout his career he developed this theme, and discussed it extensively."
+7043,His early work on nonexistent objects was influenced by Alexius Meinong.
+7044,"Rapaport has written on the field of intentionality, influencing scientists and writers including Daniel Dennett, Héctor-Neri Castañeda  and John Searle .
+Rapaport is interested in science educational theory, and received the New York Chancellor's Award for Excellence in Teaching."
+7045,"In June 1988, Rapaport compiled a list of restaurants in the Buffalo area for attendees of an ACL meeting at SUNY Buffalo. The list was continued, becoming interactive, with user reviews of restaurants."
+7046,"Rapaport and his wife Mary, with whom he has a son Michael, are the principal donors to the Lucille Ball-Desi Arnaz Center in Jamestown, NY. The Desilu Playhouse, located in the Rapaport Center, contains memorabilia and other vintage I Love Lucy items. He and his wife have also purchased and renovated Lucille Ball's childhood home in Celoron, New York."
+7047,"Broadly, query languages can be classified according to whether they are database query languages or information retrieval query languages. The difference is that a database query language attempts to give factual answers to factual questions, while an information retrieval query language attempts to find documents containing information that is relevant to an area of inquiry. Other types of query languages include:"
+7048,Common logical data models for databases include:
+7049,An object–relational database combines the two related structures.
+7050,Physical data models include:
+7051,Other models include:
+7052,Multidimensional model
+7053,Multivalue model
+7054,Semantic model
+7055,XML database
+7056,Named graph
+7057,Triplestore
+7058,"A given database management system may provide one or more models. The optimal structure depends on the natural organization of the application's data, and on the application's requirements, which include transaction rate , reliability, maintainability, scalability, and cost.  Most database management systems are built around one particular data model, although it is possible for products to offer support for more than one model."
+7059,"Various physical data models can implement any given logical model. Most database software will offer the user some level of control in tuning the physical implementation, since the choices that are made have a significant effect on performance."
+7060,"A model is not just a way of structuring data: it also defines a set of operations that can be performed on the data. The relational model, for example, defines operations such as select  and join. Although these operations may not be explicit in a particular query language, they provide the foundation on which a query language is built."
+7061,"The flat  model consists of a single, two-dimensional array of data elements, where all members of a given column are assumed to be similar values, and all members of a row are assumed to be related to one another. For instance, columns for name and password that might be used as a part of a system security database. Each row would have the specific password associated with an individual user. Columns of the table often have a type associated with them, defining them as character data, date or time information, integers, or floating point numbers.  This tabular format is a precursor to the relational model."
+7062,"These models were popular in the 1960s, 1970s, but nowadays can be found primarily in old legacy systems. They are characterized primarily by being navigational with strong connections between their logical and physical representations, and deficiencies in data independence."
+7063,"In a hierarchical model, data is organized into a tree-like structure, implying a single parent for each record. A sort field keeps sibling records in a particular order. Hierarchical structures were widely used in the early mainframe database management systems, such as the Information Management System  by IBM, and now describe the structure of XML documents. This structure allows one-to-many relationship between two types of data. This structure is very efficient to describe many relationships in the real world; recipes, table of contents, ordering of paragraphs/verses, any nested and sorted information."
+7064,"This hierarchy is used as the physical order of records in storage. Record access is done by navigating downward through the data structure using pointers combined with sequential accessing.  Because of this, the hierarchical structure is inefficient for certain database operations when a full path  is not also included for each record.  Such limitations have been compensated for in later IMS versions by additional logical hierarchies imposed on the base physical hierarchy."
+7065,"The network model expands upon the hierarchical structure, allowing many-to-many relationships in a tree-like structure that allows multiple parents.  It was most popular before being replaced by the relational model, and is defined by the CODASYL specification."
+7066,"The network model organizes data using two fundamental concepts, called records and sets. Records contain fields . Sets  define one-to-many relationships between records: one owner, many members. A record may be an owner in any number of sets, and a member in any number of sets."
+7067,"A set consists of circular linked lists where one record type, the set owner or parent, appears once in each circle, and a second record type, the subordinate or child, may appear multiple times in each circle. In this way a hierarchy may be established between any two record types, e.g., type A is the owner of B. At the same time another set may be defined where B is the owner of A. Thus all the sets comprise a general directed graph , or network construct. Access to records is either sequential  or by navigation in the circular linked lists."
+7068,"The network model is able to represent redundancy in data more efficiently than in the hierarchical model, and there can be more than one path from an ancestor node to a descendant.  The operations of the network model are navigational in style: a program maintains a current position, and navigates from one record to another by following the relationships in which the record participates. Records can also be located by supplying key values."
+7069,"Although it is not an essential feature of the model, network databases generally implement the set relationships by means of pointers that directly address the location of a record on disk. This gives excellent retrieval performance, at the expense of operations such as database loading and reorganization."
+7070,"Popular DBMS products that utilized it were Cincom Systems' Total and Cullinet's IDMS. IDMS gained a considerable customer base; in the 1980s, it adopted the relational model and SQL in addition to its original tools and languages."
+7071,"Most object databases  use the navigational concept to provide fast navigation across networks of objects, generally using object identifiers as ""smart"" pointers to related objects. Objectivity/DB, for instance, implements named one-to-one, one-to-many, many-to-one, and many-to-many named relationships that can cross databases. Many object databases also support SQL, combining the strengths of both models."
+7072,"In an inverted file or inverted index, the  contents of the data are used as keys in a lookup table, and the values in the table are pointers to the location of each instance of a given content item.  This is also the logical structure of contemporary database indexes, which might only use the contents from a particular columns in the lookup table.  The inverted file data model can put indexes in a set of files next to existing flat database files, in order to efficiently directly access needed records in these files."
+7073,"Notable for using this data model is the ADABAS DBMS of Software AG, introduced in 1970. ADABAS has gained considerable customer base and exists and supported until today. In the 1980s it has adopted the relational model and SQL in addition to its original tools and languages."
+7074,Document-oriented database Clusterpoint uses inverted indexing model to provide fast full-text search for XML or JSON data objects for example.
+7075,"The relational model was introduced by E.F. Codd in 1970 as a way to make database management systems more independent of any particular application. It is a mathematical model defined in terms of predicate logic and set theory, and implementations of it have been used by mainframe, midrange and microcomputer systems."
+7076,"The products that are generally referred to as relational databases in fact implement a model that is only an approximation to the mathematical model defined by Codd. Three key terms are used extensively in relational database models: relations, attributes, and domains. A relation is a table with columns and rows. The named columns of the relation are called attributes, and the domain is the set of values the attributes are allowed to take."
+7077,"The basic data structure of the relational model is the table, where information about a particular entity  is represented in rows  and columns. Thus, the ""relation"" in ""relational database"" refers to the various tables in the database; a relation is a set of tuples. The columns enumerate the various attributes of the entity , and a row is an actual instance of the entity  that is represented by the relation. As a result, each tuple of the employee table represents various attributes of a single employee."
+7078,"All relations  in a relational database have to adhere to some basic rules to qualify as relations. First, the ordering of columns is immaterial in a table. Second, there can not be identical tuples or rows in a table. And third, each tuple will contain a single value for each of its attributes."
+7079,"A relational database contains multiple tables, each similar to the one in the ""flat"" database model. One of the strengths of the relational model is that, in principle, any value occurring in two different records , implies a relationship among those two records.  Yet, in order to enforce explicit integrity constraints,  relationships between records in tables can also be defined explicitly, by identifying or non-identifying parent-child relationships characterized by assigning cardinality 1:M, M:M). Tables can also have a designated single attribute or a set of attributes that can act as a ""key"", which can be used to uniquely identify each tuple in the table."
+7080,"A key that can be used to uniquely identify a row in a table is called a primary key. Keys are commonly used to join or combine data from two or more tables. For example, an Employee table may contain a column named Location which contains a value that matches the key of a Location table. Keys are also critical in the creation of indexes, which facilitate fast retrieval of data from large tables. Any column can be a key, or multiple columns can be grouped together into a compound key. It is not necessary to define all the keys in advance; a column can be used as a key even if it was not originally intended to be one."
+7081,"A key that has an external, real-world meaning  is sometimes called a ""natural"" key. If no natural key is suitable , an arbitrary or surrogate key can be assigned . In practice, most databases have both generated and natural keys, because generated keys can be used internally to create links between rows that cannot break, while natural keys can be used, less reliably, for searches and for integration with other databases. "
+7082,The most common query language used with the relational model is the Structured Query Language .
+7083,"The dimensional model is a specialized adaptation of the relational model used to represent data in data warehouses in a way that data can be easily summarized using online analytical processing, or OLAP queries. In the dimensional model, a database schema consists of a single large table of facts that are described using dimensions and measures. A dimension provides the context of a fact  and is used in queries to group related facts together. Dimensions tend to be discrete and are often hierarchical; for example, the location might include the building, state, and country. A measure is a quantity describing the fact, such as revenue. It is important that measures can be meaningfully aggregated—for example, the revenue from different locations can be added together."
+7084,"In an OLAP query, dimensions are chosen and the facts are grouped and aggregated together to create a summary."
+7085,"The dimensional model is often implemented on top of the relational model using a star schema, consisting of one highly normalized table containing the facts, and surrounding denormalized tables containing each dimension. An alternative physical implementation, called a snowflake schema, normalizes multi-level hierarchies within a dimension into multiple tables."
+7086,"A data warehouse can contain multiple dimensional schemas that share dimension tables, allowing them to be used together. Coming up with a standard set of dimensions is an important part of dimensional modeling."
+7087,Its high performance has made the dimensional model the most popular database structure for OLAP.
+7088,"Products offering a more general data model than the relational model are sometimes classified as post-relational. Alternate terms include ""hybrid database"", ""Object-enhanced RDBMS"" and others. The data model in such products incorporates relations but is not constrained by E.F. Codd's Information Principle, which requires that"
+7089,all information in the database must be cast explicitly in terms of values in relations and in no other way
+7090,"Some of these extensions to the relational model integrate concepts from technologies that pre-date the relational model. For example, they allow representation of a directed graph with trees on the nodes. The German company sones implements this concept in its GraphDB."
+7091,"Some post-relational products extend relational systems with non-relational features. Others arrived in much the same place by adding relational features to pre-relational systems. Paradoxically, this allows products that are historically pre-relational, such as PICK and MUMPS, to make a plausible claim to be post-relational."
+7092,The resource space model  is a non-relational data model based on multi-dimensional classification.
+7093,Graph databases allow even more general structure than a network database; any node may be connected to any other node.
+7094,"Multivalue databases are ""lumpy"" data, in that they can store exactly the same way as relational databases, but they also permit a level of depth which the relational model can only approximate using sub-tables. This is nearly identical to the way XML expresses data, where a given field/attribute can have multiple right answers at the same time. Multivalue can be thought of as a compressed form of XML."
+7095,"An example is an invoice, which in either multivalue or relational data could be seen as  Invoice Header Table - one entry per invoice, and  Invoice Detail Table - one entry per line item. In the multivalue model, we have the option of storing the data as on table, with an embedded table to represent the detail:  Invoice Table - one entry per invoice, no other tables needed."
+7096,"The advantage is that the atomicity of the Invoice  and the Invoice  are one-to-one. This also results in fewer reads, less referential integrity issues, and a dramatic decrease in the hardware needed to support a given transaction volume."
+7097,"In the 1990s, the object-oriented programming paradigm was applied to database technology, creating a new database model known as object databases.  This aims to avoid the object–relational impedance mismatch – the overhead of converting information between its representation in the database  and its representation in the application program .  Even further, the type system used in a particular application can be defined directly in the database, allowing the database to enforce the same data integrity invariants.  Object databases also introduce the key ideas of object programming, such as encapsulation and polymorphism, into the world of databases."
+7098,"A variety of these ways have been tried for storing objects in a database. Some products have approached the problem from the application programming end, by making the objects manipulated by the program persistent. This typically requires the addition of some kind of query language, since conventional programming languages do not have the ability to find objects based on their information content. Others have attacked the problem from the database end, by defining an object-oriented data model for the database, and defining a database programming language that allows full programming capabilities as well as traditional query facilities."
+7099,"Object databases suffered because of a lack of standardization: although standards were defined by ODMG, they were never implemented well enough to ensure interoperability between products. Nevertheless, object databases have been used successfully in many applications: usually specialized applications such as engineering databases or molecular biology databases rather than mainstream commercial data processing. However, object database ideas were picked up by the relational vendors and influenced extensions made to these products and indeed to the SQL language."
+7100,An alternative to translating between objects and relational databases is to use an object–relational mapping  library.
+7101,"The term ""data mining"" is a misnomer because the goal is the extraction of patterns and knowledge from large amounts of data, not the extraction  of data itself. It also is a buzzword and is frequently applied to any form of large-scale data or information processing  as well as any application of computer decision support system, including artificial intelligence  and business intelligence. Often the more general terms  data analysis and analytics—or, when referring to actual methods, artificial intelligence and machine learning—are more appropriate."
+7102,"The actual data mining task is the semi-automatic or automatic analysis of large quantities of data to extract previously unknown, interesting patterns such as groups of data records , unusual records , and dependencies . This usually involves using database techniques such as spatial indices. These patterns can then be seen as a kind of summary of the input data, and may be used in further analysis or, for example, in machine learning and predictive analytics. For example, the data mining step might identify multiple groups in the data, which can then be used to obtain more accurate prediction results by a decision support system. Neither the data collection, data preparation, nor result interpretation and reporting is part of the data mining step, although they do belong to the overall KDD process as additional steps."
+7103,"The difference between data analysis and data mining is that data analysis is used to test models and hypotheses on the dataset, e.g., analyzing the effectiveness of a marketing campaign, regardless of the amount of data. In contrast, data mining uses machine learning and statistical models to uncover clandestine or hidden patterns in a large volume of data."
+7104,"The related terms data dredging, data fishing, and data snooping refer to the use of data mining methods to sample parts of a larger population data set that are  too small for reliable statistical inferences to be made about the validity of any patterns discovered. These methods can, however, be used in creating new hypotheses to test against the larger data populations."
+7105,"In the 1960s, statisticians and economists used terms like data fishing or data dredging to refer to what they considered the bad practice of analyzing data without an a-priori hypothesis. The term ""data mining"" was used in a similarly critical way by economist Michael Lovell in an article published in the Review of Economic Studies in 1983. Lovell indicates that the practice ""masquerades under a variety of aliases, ranging from ""experimentation""  to ""fishing"" or ""snooping"" ."
+7106,"The term data mining appeared around 1990 in the database community, with generally positive connotations. For a short time in 1980s, the phrase ""database mining""™, was used, but since it was trademarked by HNC, a San Diego-based company, to pitch their Database Mining Workstation; researchers consequently turned to data mining. Other terms used include data archaeology, information harvesting, information discovery, knowledge extraction, etc. Gregory Piatetsky-Shapiro coined the term ""knowledge discovery in databases"" for the first workshop on the same topic  and this term became more popular in the AI and machine learning communities. However, the term data mining became more popular in the business and press communities. Currently, the terms data mining and knowledge discovery are used interchangeably."
+7107,"The manual extraction of patterns from data has occurred for centuries. Early methods of identifying patterns in data include Bayes' theorem  and regression analysis . The proliferation, ubiquity and increasing power of computer technology have dramatically increased data collection, storage, and manipulation ability. As data sets have grown in size and complexity, direct ""hands-on"" data analysis has increasingly been augmented with indirect, automated data processing, aided by other discoveries in computer science, specially in the field of machine learning, such as neural networks, cluster analysis, genetic algorithms , decision trees and decision rules , and support vector machines . Data mining is the process of applying these methods with the intention of uncovering hidden patterns. in large data sets. It bridges the gap from applied statistics and artificial intelligence  to database management by exploiting the way data is stored and indexed in databases to execute the actual learning and discovery algorithms more efficiently, allowing such methods to be applied to ever-larger data sets."
+7108,The knowledge discovery in databases  process is commonly defined with the stages:
+7109,"It exists, however, in many variations on this theme, such as the Cross-industry standard process for data mining  which defines six phases:"
+7110,"or a simplified process such as  Pre-processing,  Data Mining, and  Results Validation."
+7111,"Polls conducted in 2002, 2004, 2007 and 2014 show that the CRISP-DM methodology is the leading methodology used by data miners."
+7112,"The only other data mining standard named in these polls was SEMMA. However, 3–4 times as many people reported using CRISP-DM. Several teams of researchers have published reviews of data mining process models, and Azevedo and Santos conducted a comparison of CRISP-DM and SEMMA in 2008."
+7113,"Before data mining algorithms can be used, a target data set must be assembled. As data mining can only uncover patterns actually present in the data, the target data set must be large enough to contain these patterns while remaining concise enough to be mined within an acceptable time limit. A common source for data is a data mart or data warehouse. Pre-processing is essential to analyze the multivariate data sets before data mining. The target set is then cleaned. Data cleaning removes the observations containing noise and those with missing data."
+7114,Data mining involves six common classes of tasks:
+7115,"Data mining can unintentionally be misused, producing results that appear to be significant but which do not actually predict future behavior and cannot be reproduced on a new sample of data, therefore bearing little use. This is sometimes caused by investigating too many hypotheses and not performing proper statistical hypothesis testing. A simple version of this problem in machine learning is known as overfitting, but the same problem can arise at different phases of the process and thus a train/test split—when applicable at all—may not be sufficient to prevent this from happening."
+7116,"The final step of knowledge discovery from data is to verify that the patterns produced by the data mining algorithms occur in the wider data set. Not all patterns found by the algorithms are necessarily valid. It is common for data mining algorithms to find patterns in the training set which are not present in the general data set. This is called overfitting. To overcome this, the evaluation uses a test set of data on which the data mining algorithm was not trained. The learned patterns are applied to this test set, and the resulting output is compared to the desired output. For example, a data mining algorithm trying to distinguish ""spam"" from ""legitimate"" e-mails would be trained on a training set of sample e-mails. Once trained, the learned patterns would be applied to the test set of e-mails on which it had not been trained. The accuracy of the patterns can then be measured from how many e-mails they correctly classify. Several statistical methods may be used to evaluate the algorithm, such as ROC curves."
+7117,"If the learned patterns do not meet the desired standards, it is necessary to re-evaluate and change the pre-processing and data mining steps. If the learned patterns do meet the desired standards, then the final step is to interpret the learned patterns and turn them into knowledge."
+7118,"The premier professional body in the field is the Association for Computing Machinery's  Special Interest Group  on Knowledge Discovery and Data Mining . Since 1989, this ACM SIG has hosted an annual international conference and published its proceedings, and since 1999 it has published a biannual academic journal titled ""SIGKDD Explorations""."
+7119,Computer science conferences on data mining include:
+7120,"Data mining topics are also present in many data management/database conferences such as the ICDE Conference, SIGMOD Conference and International Conference on Very Large Data Bases."
+7121,"There have been some efforts to define standards for the data mining process, for example, the 1999 European Cross Industry Standard Process for Data Mining  and the 2004 Java Data Mining standard . Development on successors to these processes  was active in 2006 but has stalled since. JDM 2.0 was withdrawn without reaching a final draft."
+7122,"For exchanging the extracted models—in particular for use in predictive analytics—the key standard is the Predictive Model Markup Language , which is an XML-based language developed by the Data Mining Group  and supported as exchange format by many data mining applications. As the name suggests, it only covers prediction models, a particular data mining task of high importance to business applications. However, extensions to cover  subspace clustering have been proposed independently of the DMG."
+7123,"Data mining is used wherever there is digital data available. Notable examples of data mining can be found throughout business, medicine, science, finance, construction, and surveillance."
+7124,"While the term ""data mining"" itself may have no ethical implications, it is often associated with the mining of information in relation to user behavior ."
+7125,"The ways in which data mining can be used can in some cases and contexts raise questions regarding privacy, legality, and ethics. In particular, data mining government or commercial data sets for national security or law enforcement purposes, such as in the Total Information Awareness Program or in ADVISE, has raised privacy concerns."
+7126,"Data mining requires data preparation which uncovers information or patterns which compromise confidentiality and privacy obligations. A common way for this to occur is through data aggregation. Data aggregation involves combining data together  in a way that facilitates analysis . This is not data mining per se, but a result of the preparation of data before—and for the purposes of—the analysis. The threat to an individual's privacy comes into play when the data, once compiled, cause the data miner, or anyone who has access to the newly compiled data set, to be able to identify specific individuals, especially when the data were originally anonymous."
+7127,It is recommended to be aware of the following before data are collected:
+7128,"Data may also be modified so as to become anonymous, so that individuals may not readily be identified. However, even ""anonymized"" data sets can potentially contain enough information to allow identification of individuals, as occurred when journalists were able to find several individuals based on a set of search histories that were inadvertently released by AOL."
+7129,"The inadvertent revelation of personally identifiable information leading to the provider violates Fair Information Practices.   This indiscretion can cause financial,
+emotional, or bodily harm to the indicated individual.  In one instance of privacy violation, the patrons of Walgreens filed a lawsuit against the company in 2011 for selling
+prescription information to data mining companies who in turn provided the data
+to pharmaceutical companies."
+7130,"Europe has rather strong privacy laws, and efforts are underway to further strengthen the rights of the consumers. However, the U.S.–E.U. Safe Harbor Principles, developed between 1998 and 2000, currently effectively expose European users to privacy exploitation by U.S. companies. As a consequence of Edward Snowden's global surveillance disclosure, there has been increased discussion to revoke this agreement, as in particular the data will be fully exposed to the National Security Agency, and attempts to reach an agreement with the United States have failed."
+7131,In the United Kingdom in particular there have been cases of corporations using data mining as a way to target certain groups of customers forcing them to pay unfairly high prices. These groups tend to be people of lower socio-economic status who are not savvy to the ways they can be exploited in digital market places.
+7132,"In the United States, privacy concerns have been addressed by the US Congress via the passage of regulatory controls such as the Health Insurance Portability and Accountability Act . The HIPAA requires individuals to give their ""informed consent"" regarding information they provide and its intended present and future uses. According to an article in Biotech Business Week, ""'n practice, HIPAA may not offer any greater protection than the longstanding regulations in the research arena,' says the AAHC. More importantly, the rule's goal of protection through informed consent is approach a level of incomprehensibility to average individuals."" This underscores the necessity for data anonymity in data aggregation and mining practices."
+7133,U.S. information privacy legislation such as HIPAA and the Family Educational Rights and Privacy Act  applies only to the specific areas that each such law addresses. The use of data mining by the majority of businesses in the U.S. is not controlled by any legislation.
+7134,"Under European copyright database laws, the mining of in-copyright works  without the permission of the copyright owner is not legal. Where a database is pure data in Europe, it may be that there is no copyright—but database rights may exist, so data mining becomes subject to intellectual property owners' rights that are protected by the Database Directive. On the recommendation of the Hargreaves review, this led to the UK government to amend its copyright law in 2014 to allow content mining as a limitation and exception. The UK was the second country in the world to do so after Japan, which introduced an exception in 2009 for data mining. However, due to the restriction of the Information Society Directive , the UK exception only allows content mining for non-commercial purposes. UK copyright law also does not allow this provision to be overridden by contractual terms and conditions.
+Since 2020 also Switzerland has been regulating data mining by allowing it in the research field under certain conditions laid down by art. 24d of the Swiss Copyright Act. This new article entered into force on 1 April 2020."
+7135,"The European Commission facilitated stakeholder discussion on text and data mining in 2013, under the title of Licences for Europe. The focus on the solution to this legal issue, such as licensing rather than limitations and exceptions, led to representatives of universities, researchers, libraries, civil society groups and open access publishers to leave the stakeholder dialogue in May 2013."
+7136,"US copyright law, and in particular its provision for fair use, upholds the legality of content mining in America, and other fair use countries such as Israel, Taiwan and South Korea. As content mining is transformative, that is it does not supplant the original work, it is viewed as being lawful under fair use. For example, as part of the Google Book settlement the presiding judge on the case ruled that Google's digitization project of in-copyright books was lawful, in part because of the transformative uses that the digitization project displayed—one being text and data mining."
+7137,The following applications are available under free/open-source licenses. Public access to application source code is also available.
+7138,Carrot2: Text and search results clustering framework.
+7139,Chemicalize.org: A chemical structure miner and web search engine.
+7140,ELKI: A university research project with advanced cluster analysis and outlier detection methods written in the Java language.
+7141,GATE: a natural language processing and language engineering tool.
+7142,"KNIME: The Konstanz Information Miner, a user-friendly and comprehensive data analytics framework."
+7143,Massive Online Analysis : a real-time big data stream mining with concept drift tool in the Java programming language.
+7144,MEPX: cross-platform tool for regression and classification problems based on a Genetic Programming variant.
+7145,mlpack: a collection of ready-to-use machine learning algorithms written in the C++ language.
+7146,NLTK : A suite of libraries and programs for symbolic and statistical natural language processing  for the Python language.
+7147,OpenNN: Open neural networks library.
+7148,Orange: A component-based data mining and machine learning software suite written in the Python language.
+7149,PSPP: Data mining and statistics software under the GNU Project similar to SPSS
+7150,"R: A programming language and software environment for statistical computing, data mining, and graphics. It is part of the GNU Project."
+7151,scikit-learn: An open-source machine learning library for the Python programming language;
+7152,Torch: An open-source deep learning library for the Lua programming language and scientific computing framework with wide support for machine learning algorithms.
+7153,"UIMA: The UIMA  is a component framework for analyzing unstructured content such as text, audio and video – originally developed by IBM."
+7154,Weka: A suite of machine learning software applications written in the Java programming language.
+7155,The following applications are available under proprietary licenses.
+7156,"For more information about extracting information out of data , see:"
+7157,"The history of garbled circuits is complicated. The invention of garbled circuit was credited to Andrew Yao, as Yao introduced the idea in the oral presentation of a paper in FOCS'86. This was documented by Oded Goldreich in 2003. The first written document about this technique was by Goldreich, Micali, and
+Wigderson in STOC'87. The term ""garbled circuit"" was first used by Beaver, Micali, and Rogaway in STOC'90. Yao's protocol solving Yao's Millionaires' Problem was the beginning example of secure computation, yet it is not directly related to garbled circuits."
+7158,The oblivious transfer can be built using asymmetric cryptography like the RSA cryptosystem.
+7159,The protocol consists of 6 steps as follows:
+7160,The Boolean circuit for small functions can be generated by hand. It is conventional to make the circuit out of 2-input XOR and AND gates. It is important that the generated circuit has the minimum number of AND gates . There are methods that generate the optimized circuit in term of number of AND gates using logic synthesis technique. The circuit for the Millionaires' Problem is a digital comparator circuit . A full adder circuit can be implemented using only one AND gate and some XOR gates. This means the total number of AND gates for the circuit of the Millionaires' Problem is equal to the bit-width of the inputs.
+7161,Alice replaced 0 and 1 with the corresponding labels:
+7162,"After this, Alice randomly permutes the table such that the output value cannot be determined from the row. The protocol's name, garbled, is derived from this random permutation."
+7163,"This optimization reduces the size of garbled tables from 4 rows to 3 rows. Here, instead of generating a label for the output wire of a gate randomly, Alice generates it using a function of the input labels. She generates the output labels such that the first entry of the garbled table becomes all 0 and no longer needs to be sent:"
+7164,"Free XOR optimization implies an important point that the amount of data transfer  and number of encryption and decryption  of the garbled circuit protocol relies only on the number of AND gates in the Boolean circuit not the XOR gates. Thus, between two Boolean circuits representing the same function, the one with the smaller number of AND gates is preferred."
+7165,"This optimization reduce the size of garbled table for AND gates from 3 row in Row Reduction to 2 rows. It is shown that this is the theoretical minimum for the number of rows in the garbled table, for a certain class of garbling techniques."
+7166,"The Yao's Garbled Circuit is secure against a semi-honest adversary. This type of adversary follows the protocol and does not do any malicious behavior, but it tries to violate the privacy of the other party's input by scrutinizing the messages transmitted in the protocol."
+7167,"It is more challenging to make this protocol secure against a malicious adversary that deviates from the protocol. One of the first solutions to make the protocol secure against malicious adversary is to use zero-knowledge proof to prevent malicious activities during the protocol. For years, this approach was considered more as theoretical solution than a practical solution because of complexity overheads of it. But, it is shown that it is possible to use it with just a small overhead. Another approach is using several GC for a circuit and verifying the correctness of a subset of them and then using the rest for the computation with the hope that if the garbler was malicious, it would be detected during the verification phase. Another solution is to make the garbling scheme authenticated such that the evaluator can verify the garbled circuit."
+7168,"In light of the fact that one should be able to generate a proof of some statement only when in possession of certain secret information connected to the statement, the verifier, even after having become convinced of the statement's truth, should nonetheless remain unable to prove the statement to third parties."
+7169,"In the plain model, nontrivial zero-knowledge proofs  demand interaction between the prover and the verifier. This interaction usually entails the selection of one or more random challenges by the verifier; the random origin of these challenges, together with the prover's successful responses to them notwithstanding, jointly convince the verifier that the prover does possess the claimed knowledge. If interaction weren't present, then the verifier, having obtained the protocol's execution transcript—that is, the prover's one and only message—could replay that transcript to a third party, thereby convincing the third party that the verifier too possessed the secret information."
+7170,"In the common random string and random oracle models, non-interactive zero-knowledge proofs exist, in light of the Fiat–Shamir heuristic. These proofs, in practice, rely on computational assumptions ."
+7171,"There is a well-known story presenting the fundamental ideas of zero-knowledge proofs, first published in 1990 by Jean-Jacques Quisquater and others in their paper ""How to Explain Zero-Knowledge Protocols to Your Children"". The two parties in the zero-knowledge proof story are Peggy as the prover of the statement, and Victor, the verifier of the statement."
+7172,"In this story, Peggy has uncovered the secret word used to open a magic door in a cave. The cave is shaped like a ring, with the entrance on one side and the magic door blocking the opposite side. Victor wants to know whether Peggy knows the secret word; but Peggy, being a very private person, does not want to reveal her knowledge  to Victor or to reveal the fact of her knowledge to the world in general."
+7173,"They label the left and right paths from the entrance A and B. First, Victor waits outside the cave as Peggy goes in. Peggy takes either path A or B; Victor is not allowed to see which path she takes. Then, Victor enters the cave and shouts the name of the path he wants her to use to return, either A or B, chosen at random. Providing she really does know the magic word, this is easy: she opens the door, if necessary, and returns along the desired path."
+7174,"However, suppose she did not know the word. Then, she would only be able to return by the named path if Victor were to give the name of the same path by which she had entered. Since Victor would choose A or B at random, she would have a 50% chance of guessing correctly. If they were to repeat this trick many times, say 20 times in a row, her chance of successfully anticipating all of Victor's requests would be reduced to 1 in 220, or 9.56 × 10−7."
+7175,"Thus, if Peggy repeatedly appears at the exit Victor names, he can conclude that it is extremely probable that Peggy does, in fact, know the secret word."
+7176,"One side note with respect to third-party observers: even if Victor is wearing a hidden camera that records the whole transaction, the only thing the camera will record is in one case Victor shouting ""A!"" and Peggy appearing at A or in the other case Victor shouting ""B!"" and Peggy appearing at B. A recording of this type would be trivial for any two people to fake . Such a recording will certainly never be convincing to anyone but the original participants. In fact, even a person who was present as an observer at the original experiment would be unconvinced, since Victor and Peggy might have orchestrated the whole ""experiment"" from start to finish."
+7177,"Further, if Victor chooses his A's and B's by flipping a coin on-camera, this protocol loses its zero-knowledge property; the on-camera coin flip would probably be convincing to any person watching the recording later. Thus, although this does not reveal the secret word to Victor, it does make it possible for Victor to convince the world in general that Peggy has that knowledge—counter to Peggy's stated wishes. However, digital cryptography generally ""flips coins"" by relying on a pseudo-random number generator, which is akin to a coin with a fixed pattern of heads and tails known only to the coin's owner. If Victor's coin behaved this way, then again it would be possible for Victor and Peggy to have faked the experiment, so using a pseudo-random number generator would not reveal Peggy's knowledge to the world in the same way that using a flipped coin would."
+7178,"Notice that Peggy could prove to Victor that she knows the magic word, without revealing it to him, in a single trial. If both Victor and Peggy go together to the mouth of the cave, Victor can watch Peggy go in through A and come out through B. This would prove with certainty that Peggy knows the magic word, without revealing the magic word to Victor. However, such a proof could be observed by a third party, or recorded by Victor and such a proof would be convincing to anybody. In other words, Peggy could not refute such proof by claiming she colluded with Victor, and she is therefore no longer in control of who is aware of her knowledge."
+7179,"Imagine your friend ""Victor"" is red-green colour-blind  and you have two balls: one red and one green, but otherwise identical. To Victor, the balls seem completely identical.  Victor is skeptical that the balls are actually distinguishable. You want to prove to Victor that the balls are in fact differently coloured, but nothing else.  In particular, you do not want to reveal which ball is the red one and which is the green."
+7180,"Here is the proof system. You give the two balls to Victor and he puts them behind his back. Next, he takes one of the balls and brings it out from behind his back and displays it. He then places it behind his back again and then chooses to reveal just one of the two balls, picking one of the two at random with equal probability. He will ask you, ""Did I switch the ball?"" This whole procedure is then repeated as often as necessary."
+7181,"By looking at the balls' colours, you can, of course, say with certainty whether or not he switched them. On the other hand, if the balls were the same colour and hence indistinguishable, there is no way you could guess correctly with probability higher than 50%."
+7182,"Since the probability that you would have randomly succeeded at identifying each switch/non-switch is 50%, the probability of having randomly succeeded at all switch/non-switches approaches zero . If you and your friend repeat this ""proof"" multiple times , your friend should become convinced  that the balls are indeed differently coloured."
+7183,"The above proof is zero-knowledge because your friend never learns which ball is green and which is red; indeed, he gains no knowledge about how to distinguish the balls."
+7184,"One well-known example of a zero-knowledge proof is the ""Where's Waldo"" example. In this example, the prover wants to prove to the verifier that they know where Waldo is on a page in a Where's Waldo? book, without revealing his location to the verifier."
+7185,"The prover starts by taking a large black board with a small hole in it, the size of Waldo. The board is twice the size of the book in both directions, so the verifier cannot see where on the page the prover is placing it. The prover then places the board over the page so that Waldo is in the hole."
+7186,"The verifier can now look through the hole and see Waldo, but they cannot see any other part of the page. Therefore, the prover has proven to the verifier that they know where Waldo is, without revealing any other information about his location."
+7187,"This example is not a perfect zero-knowledge proof, because the prover does reveal some information about Waldo's location, such as his body position. However, it is a decent illustration of the basic concept of a zero-knowledge proof."
+7188,A zero-knowledge proof of some statement must satisfy three properties:
+7189,The first two of these are properties of more general interactive proof systems. The third is what makes the proof zero-knowledge.
+7190,We can apply these ideas to a more realistic cryptography application. Peggy wants to prove to Victor that she knows the discrete log of a given value in a given group.
+7191,"Thus, a cheating prover has a 0.5 probability of successfully cheating in one round. By executing a large enough number of rounds, the probability of a cheating prover succeeding can be made arbitrarily low."
+7192,Peggy proves to know the value of x .
+7193,The following scheme is due to Manuel Blum.
+7194,"In this scenario, Peggy knows a Hamiltonian cycle for a large graph G. Victor knows G but not the cycle  Finding a Hamiltonian cycle given a large graph is believed to be computationally infeasible, since its corresponding decision version is known to be NP-complete. Peggy will prove that she knows the cycle without simply revealing it ."
+7195,"To show that Peggy knows this Hamiltonian cycle, she and Victor play several rounds of a game:"
+7196,"It is important that the commitment to the graph be such that Victor can verify, in the second case, that the cycle is really made of edges from H. This can be done by, for example, committing to every edge  separately."
+7197,"If Peggy does know a Hamiltonian cycle in G, she can easily satisfy Victor's demand for either the graph isomorphism producing H from G  or a Hamiltonian cycle in H ."
+7198,"Peggy's answers do not reveal the original Hamiltonian cycle in G.  Each round, Victor will learn only H's isomorphism to G or a Hamiltonian cycle in H.  He would need both answers for a single H to discover the cycle in G, so the information remains unknown as long as Peggy can generate a distinct H every round.  If Peggy does not know of a Hamiltonian cycle in G, but somehow knew in advance what Victor would ask to see each round then she could cheat.  For example, if Peggy knew ahead of time that Victor would ask to see the Hamiltonian cycle in H then she could generate a Hamiltonian cycle for an unrelated graph.  Similarly, if Peggy knew in advance that Victor would ask to see the isomorphism then she could simply generate an isomorphic graph H .  Victor could simulate the protocol by himself  because he knows what he will ask to see.  Therefore, Victor gains no information about the Hamiltonian cycle in G from the information revealed in each round."
+7199,"If Peggy does not know the information, she can guess which question Victor will ask and generate either a graph isomorphic to G or a Hamiltonian cycle for an unrelated graph, but since she does not know a Hamiltonian cycle for G she cannot do both. With this guesswork, her chance of fooling Victor is 2−n, where n is the number of rounds. For all realistic purposes, it is infeasibly difficult to defeat a zero-knowledge proof with a reasonable number of rounds in this way."
+7200,"Different variants of zero-knowledge can be defined by formalizing the intuitive concept of what is meant by the output of the simulator ""looking like"" the execution of the real proof protocol in the following ways:"
+7201,"Research in zero-knowledge proofs has been motivated by authentication systems where one party wants to prove its identity to a second party via some secret information  but doesn't want the second party to learn anything about this secret. This is called a ""zero-knowledge proof of knowledge"".  However, a password is typically too small or insufficiently random to be used in many schemes for zero-knowledge proofs of knowledge. A zero-knowledge password proof is a special kind of zero-knowledge proof of knowledge that addresses the limited size of passwords."
+7202,"In April 2015, the one-out-of-many proofs protocol  was introduced. In August 2021, Cloudflare, an American web infrastructure and security company decided to use the one-out-of-many proofs mechanism for private web verification using vendor hardware."
+7203,"One of the uses of zero-knowledge proofs within cryptographic protocols is to enforce honest behavior while maintaining privacy.  Roughly, the idea is to force a user to prove, using a zero-knowledge proof, that its behavior is correct according to the protocol. Because of soundness, we know that the user must really act honestly in order to be able to provide a valid proof.  Because of zero knowledge, we know that the user does not compromise the privacy of its secrets in the process of providing the proof."
+7204,"In 2016, the Princeton Plasma Physics Laboratory and Princeton University demonstrated a technique that may have applicability to future nuclear disarmament talks. It would allow inspectors to confirm whether or not an object is indeed a nuclear weapon without recording, sharing or revealing the internal workings which might be secret."
+7205,"Zero-knowledge proofs were applied in the Zerocoin and Zerocash protocols, which culminated in the birth of Zcoin  and Zcash cryptocurrencies in 2016. Zerocoin has a built-in mixing model that does not trust any peers or centralised mixing providers to ensure anonymity. Users can transact in a base currency and can cycle the currency into and out of Zerocoins. The Zerocash protocol uses a similar model  except that it can obscure the transaction amount, while Zerocoin cannot. Given significant restrictions of transaction data on the Zerocash network, Zerocash is less prone to privacy timing attacks when compared to Zerocoin. However, this additional layer of privacy can cause potentially undetected hyperinflation of Zerocash supply because fraudulent coins cannot be tracked."
+7206,"In 2018, Bulletproofs were introduced. Bulletproofs are an improvement from non-interactive zero-knowledge proof where trusted setup is not needed. It was later implemented into the Mimblewimble protocol  and Monero cryptocurrency. In 2019, Firo implemented the Sigma protocol, which is an improvement on the Zerocoin protocol without trusted setup. In the same year, Firo introduced the Lelantus protocol, an improvement on the Sigma protocol, where the former hides the origin and amount of a transaction."
+7207,"Zero-knowledge proofs by its nature can enhance privacy in identity sharing systems, which are vulnerable to data breaches and identity theft. When integrated to a decentralized identifier system, ZKPs add an extra layer of encryption on DID documents."
+7208,"Zero-knowledge proofs were first conceived in 1985 by Shafi Goldwasser, Silvio Micali, and Charles Rackoff in their paper ""The Knowledge Complexity of Interactive Proof-Systems"". This paper introduced the IP hierarchy of interactive proof systems  and conceived the concept of knowledge complexity, a measurement of the amount of knowledge about the proof transferred from the prover to the verifier. They also gave the first zero-knowledge proof for a concrete problem, that of deciding quadratic nonresidues mod m. Together with a paper by László Babai and Shlomo Moran, this landmark paper invented interactive proof systems, for which all five authors won the first Gödel Prize in 1993."
+7209,"In their own words, Goldwasser, Micali, and Rackoff say:"
+7210,"The quadratic nonresidue problem has both an NP and a co-NP algorithm, and so lies in the intersection of NP and co-NP. This was also true of several other problems for which zero-knowledge proofs were subsequently discovered, such as an unpublished proof system by Oded Goldreich verifying that a two-prime modulus is not a Blum integer."
+7211,"Oded Goldreich, Silvio Micali, and Avi Wigderson took this one step further, showing that, assuming the existence of unbreakable encryption, one can create a zero-knowledge proof system for the NP-complete graph coloring problem with three colors. Since every problem in NP can be efficiently reduced to this problem, this means that, under this assumption, all problems in NP have zero-knowledge proofs. The reason for the assumption is that, as in the above example, their protocols require encryption. A commonly cited sufficient condition for the existence of unbreakable encryption is the existence of one-way functions, but it is conceivable that some physical means might also achieve it."
+7212,"On top of this, they also showed that the graph nonisomorphism problem, the complement of the graph isomorphism problem, has a zero-knowledge proof. This problem is in co-NP, but is not currently known to be in either NP or any practical class. More generally, Russell Impagliazzo and Moti Yung as well as Ben-Or et al. would go on to show that, also assuming one-way functions or unbreakable encryption, that there are zero-knowledge proofs for all problems in IP = PSPACE, or in other words, anything that can be proved by an interactive proof system can be proved with zero knowledge."
+7213,"Not liking to make unnecessary assumptions, many theorists sought a way to eliminate the necessity of one way functions. One way this was done was with multi-prover interactive proof systems , which have multiple independent provers instead of only one, allowing the verifier to ""cross-examine"" the provers in isolation to avoid being misled. It can be shown that, without any intractability assumptions, all languages in NP have zero-knowledge proofs in such a system."
+7214,"It turns out that in an Internet-like setting, where multiple protocols may be executed concurrently, building zero-knowledge proofs is more challenging.  The line of research investigating concurrent zero-knowledge proofs was initiated by the work of Dwork, Naor, and Sahai. One particular development along these lines has been the development of witness-indistinguishable proof protocols. The property of witness-indistinguishability is related to that of zero-knowledge, yet witness-indistinguishable protocols do not suffer from the same problems of concurrent execution."
+7215,"Another variant of zero-knowledge proofs are non-interactive zero-knowledge proofs. Blum, Feldman, and Micali showed that a common random string shared between the prover and the verifier is enough to achieve computational zero-knowledge without requiring interaction."
+7216,"The most popular interactive or non-interactive zero-knowledge proof  protocols can be broadly categorized in the following four categories: Succinct Non-Interactive ARguments of Knowledge , Scalable Transparent ARgument of Knowledge , Verifiable Polynomial Delegation , and Succinct Non-interactive ARGuments . A list of zero-knowledge proof protocols and libraries is provided below along with comparisons based on transparency, universality, plausible post-quantum security, and programming paradigm. A transparent protocol is one that does not require any trusted setup and uses public randomness. A universal protocol is one that does not require a separate trusted setup for each circuit. Finally, a plausibly post-quantum protocol is one that is not susceptible to known attacks involving quantum algorithms."
+7217,"A blockchain is a distributed ledger with growing lists of records  that are securely linked together via cryptographic hashes. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data . Since each block contains information about the previous block, they effectively form a chain , with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks."
+7218,"Blockchains are typically managed by a peer-to-peer  computer network for use as a public distributed ledger, where nodes collectively adhere to a consensus algorithm protocol to add and validate new transaction blocks. Although blockchain records are not unalterable, since blockchain forks are possible, blockchains may be considered secure by design and exemplify a distributed computing system with high Byzantine fault tolerance."
+7219,"A blockchain was created by a person  using the name  Satoshi Nakamoto in 2008 to serve as the public distributed ledger for bitcoin cryptocurrency transactions, based on previous work by Stuart Haber, W. Scott Stornetta, and Dave Bayer. The implementation of the blockchain within bitcoin made it the first digital currency to solve the double-spending problem without the need for a trusted authority or central server. The bitcoin design has inspired other applications and blockchains that are readable by the public and are widely used by cryptocurrencies. The blockchain may be considered a type of payment rail."
+7220,"Private blockchains have been proposed for business use. Computerworld called the marketing of such privatized blockchains without a proper security model ""snake oil""; however, others have argued that permissioned blockchains, if carefully designed, may be more decentralized and therefore more secure in practice than permissionless ones."
+7221,"Cryptographer David Chaum first proposed a blockchain-like protocol in his 1982 dissertation ""Computer Systems Established, Maintained, and Trusted by Mutually Suspicious Groups."" Further work on a cryptographically secured chain of blocks was described in 1991 by Stuart Haber and W. Scott Stornetta. They wanted to implement a system wherein document timestamps could not be tampered with. In 1992, Haber, Stornetta, and Dave Bayer incorporated Merkle trees into the design, which improved its efficiency by allowing several document certificates to be collected into one block. Under their company Surety, their document certificate hashes have been published in The New York Times every week since 1995."
+7222,"The first decentralized blockchain was conceptualized by a person  known as Satoshi Nakamoto in 2008. Nakamoto improved the design in an important way using a Hashcash-like method to timestamp blocks without requiring them to be signed by a trusted party and introducing a difficulty parameter to stabilize the rate at which blocks are added to the chain. The design was implemented the following year by Nakamoto as a core component of the cryptocurrency bitcoin, where it serves as the public ledger for all transactions on the network."
+7223,"In August 2014, the bitcoin blockchain file size, containing records of all transactions that have occurred on the network, reached 20 GB . In January 2015, the size had grown to almost 30 GB, and from January 2016 to January 2017, the bitcoin blockchain grew from 50 GB to 100 GB in size. The ledger size had exceeded 200 GB by early 2020."
+7224,"The words block and chain were used separately in Satoshi Nakamoto's original paper, but were eventually popularized as a single word, blockchain, by 2016."
+7225,"According to Accenture, an application of the diffusion of innovations theory suggests that blockchains attained a 13.5% adoption rate within financial services in 2016, therefore reaching the early adopters' phase. Industry trade groups joined to create the Global Blockchain Forum in 2016, an initiative of the Chamber of Digital Commerce."
+7226,"In May 2018, Gartner found that only 1% of CIOs indicated any kind of blockchain adoption within their organisations, and only 8% of CIOs were in the short-term ""planning or  active experimentation with blockchain"". For the year 2019 Gartner reported 5% of CIOs believed blockchain technology was a 'game-changer' for their business."
+7227,"A blockchain is a decentralized, distributed, and often public, digital ledger consisting of records called blocks that are used to record transactions across many computers so that any involved block cannot be altered retroactively, without the alteration of all subsequent blocks. This allows the participants to verify and audit transactions independently and relatively inexpensively. A blockchain database is managed autonomously using a peer-to-peer network and a distributed timestamping server. They are authenticated by mass collaboration powered by collective self-interests. Such a design facilitates robust workflow where participants' uncertainty regarding data security is marginal. The use of a blockchain removes the characteristic of infinite reproducibility from a digital asset. It confirms that each unit of value was transferred only once, solving the long-standing problem of double-spending. A blockchain has been described as a value-exchange protocol. A blockchain can maintain title rights because, when properly set up to detail the exchange agreement, it provides a record that compels offer and acceptance."
+7228,"Logically, a blockchain can be seen as consisting of several layers:"
+7229,"Blocks hold batches of valid transactions that are hashed and encoded into a Merkle tree. Each block includes the cryptographic hash of the prior block in the blockchain, linking the two. The linked blocks form a chain. This iterative process confirms the integrity of the previous block, all the way back to the initial block, which is known as the genesis block . To assure the integrity of a block and the data contained in it, the block is usually digitally signed."
+7230,"Sometimes separate blocks can be produced concurrently, creating a temporary fork. In addition to a secure hash-based history, any blockchain has a specified algorithm for scoring different versions of the history so that one with a higher score can be selected over others. Blocks not selected for inclusion in the chain are called orphan blocks. Peers supporting the database have different versions of the history from time to time. They keep only the highest-scoring version of the database known to them. Whenever a peer receives a higher-scoring version  they extend or overwrite their own database and retransmit the improvement to their peers. There is never an absolute guarantee that any particular entry will remain in the best version of history forever. Blockchains are typically built to add the score of new blocks onto old blocks and are given incentives to extend with new blocks rather than overwrite old blocks. Therefore, the probability of an entry becoming superseded decreases exponentially as more blocks are built on top of it, eventually becoming very low.: ch. 08  For example, bitcoin uses a proof-of-work system, where the chain with the most cumulative proof-of-work is considered the valid one by the network. There are a number of methods that can be used to demonstrate a sufficient level of computation. Within a blockchain the computation is carried out redundantly rather than in the traditional segregated and parallel manner."
+7231,"The block time is the average time it takes for the network to generate one extra block in the blockchain. By the time of block completion, the included data becomes verifiable. In cryptocurrency, this is practically when the transaction takes place, so a shorter block time means faster transactions. The block time for Ethereum is set to between 14 and 15 seconds, while for bitcoin it is on average 10 minutes."
+7232,"For example, Ethereum was hard forked in 2016 to ""make whole"" the investors in The DAO, which had been hacked by exploiting a vulnerability in its code. In this case, the fork resulted in a split creating Ethereum and Ethereum Classic chains. In 2014 the Nxt community was asked to consider a hard fork that would have led to a rollback of the blockchain records to mitigate the effects of a theft of 50 million NXT from a major cryptocurrency exchange. The hard fork proposal was rejected, and some of the funds were recovered after negotiations and ransom payment. Alternatively, to prevent a permanent split, a majority of nodes using the new software may return to the old rules, as was the case of bitcoin split on 12 March 2013."
+7233,"By storing data across its peer-to-peer network, the blockchain eliminates some risks that come with data being held centrally. The decentralized blockchain may use ad hoc message passing and distributed networking."
+7234,"In a so-called ""51% attack"" a central entity gains control of more than half of a network and can then manipulate that specific blockchain record at will, allowing double-spending."
+7235,Blockchain security methods include the use of public-key cryptography.: 5  A public key  is an address on the blockchain. Value tokens sent across the network are recorded as belonging to that address. A private key is like a password that gives its owner access to their digital assets or the means to otherwise interact with the various capabilities that blockchains now support. Data stored on the blockchain is generally considered incorruptible.
+7236,"Every node in a decentralized system has a copy of the blockchain. Data quality is maintained by massive database replication and computational trust. No centralized ""official"" copy exists and no user is ""trusted"" more than any other. Transactions are broadcast to the network using the software. Messages are delivered on a best-effort basis. Early blockchains rely on energy-intensive mining nodes to validate transactions, add them to the block they are building, and then broadcast the completed block to other nodes.: ch. 08  Blockchains use various time-stamping schemes, such as proof-of-work, to serialize changes. Later consensus methods include proof of stake. The growth of a decentralized blockchain is accompanied by the risk of centralization because the computer resources required to process larger amounts of data become more expensive."
+7237,"Finality is the level of confidence that the well-formed block recently appended to the blockchain will not be revoked in the future  and thus can be trusted. Most distributed blockchain protocols, whether proof of work or proof of stake, cannot guarantee the finality of a freshly committed block, and instead rely on ""probabilistic finality"": as the block goes deeper into a blockchain, it is less likely to be altered or reverted by a newly found consensus."
+7238,"Byzantine fault tolerance-based proof-of-stake protocols purport to provide so called ""absolute finality"": a randomly chosen validator proposes a block, the rest of validators vote on it, and, if a supermajority decision approves it, the block is irreversibly committed into the blockchain. A modification of this method, an ""economic finality"", is used in practical protocols, like the Casper protocol used in Ethereum: validators which sign two different blocks at the same position in the blockchain are subject to ""slashing"", where their leveraged stake is forfeited."
+7239,"Open blockchains are more user-friendly than some traditional ownership records, which, while open to the public, still require physical access to view. Because all early blockchains were permissionless, controversy has arisen over the blockchain definition. An issue in this ongoing debate is whether a private system with verifiers tasked and authorized  by a central authority should be considered a blockchain. Proponents of permissioned or private chains argue that the term ""blockchain"" may be applied to any data structure that batches data into time-stamped blocks. These blockchains serve as a distributed version of multiversion concurrency control  in databases. Just as MVCC prevents two transactions from concurrently modifying a single object in a database, blockchains prevent two transactions from spending the same single output in a blockchain.: 30–31  Opponents say that permissioned systems resemble traditional corporate databases, not supporting decentralized data verification, and that such systems are not hardened against operator tampering and revision. Nikolai Hampton of Computerworld said that ""many in-house blockchain solutions will be nothing more than cumbersome databases,"" and ""without a clear security model, proprietary blockchains should be eyed with suspicion."""
+7240,"An advantage to an open, permissionless, or public, blockchain network is that guarding against bad actors is not required and no access control is needed. This means that applications can be added to the network without the approval or trust of others, using the blockchain as a transport layer."
+7241,"Bitcoin and other cryptocurrencies currently secure their blockchain by requiring new entries to include proof of work. To prolong the blockchain, bitcoin uses Hashcash puzzles. While Hashcash was designed in 1997 by Adam Back, the original idea was first proposed by Cynthia Dwork and Moni Naor and Eli Ponyatovski in their 1992 paper ""Pricing via Processing or Combatting Junk Mail""."
+7242,"In 2016, venture capital investment for blockchain-related projects was weakening in the USA but increasing in China. Bitcoin and many other cryptocurrencies use open  blockchains. As of April 2018, bitcoin has the highest market capitalization."
+7243,"Permissioned blockchains use an access control layer to govern who has access to the network. It has been argued that permissioned blockchains can guarantee a certain level of decentralization, if carefully designed, as opposed to permissionless blockchains, which are often centralized in practice."
+7244,"Nikolai Hampton argued in Computerworld that ""There is also no need for a '51 percent' attack on a private blockchain, as the private blockchain  already controls 100 percent of all block creation resources. If you could attack or damage the blockchain creation tools on a private corporate server, you could effectively control 100 percent of their network and alter transactions however you wished."" This has a set of particularly profound adverse implications during a financial crisis or debt crisis like the financial crisis of 2007–08, where politically powerful actors may make decisions that favor some groups at the expense of others, and ""the bitcoin blockchain is protected by the massive group mining effort. It's unlikely that any private blockchain will try to protect records using gigawatts of computing power — it's time-consuming and expensive."" He also said, ""Within a private blockchain there is also no 'race'; there's no incentive to use more power or discover blocks faster than competitors. This means that many in-house blockchain solutions will be nothing more than cumbersome databases."""
+7245,"The analysis of public blockchains has become increasingly important with the popularity of bitcoin, Ethereum, litecoin and other cryptocurrencies. A blockchain, if it is public, provides anyone who wants access to observe and analyse the chain data, given one has the know-how. The process of understanding and accessing the flow of crypto has been an issue for many cryptocurrencies, crypto exchanges and banks. The reason for this is accusations of blockchain-enabled cryptocurrencies enabling illicit dark market trade of drugs, weapons, money laundering, etc. A common belief has been that cryptocurrency is private and untraceable, thus leading many actors to use it for illegal purposes. This is changing and now specialised tech companies provide blockchain tracking services, making crypto exchanges, law-enforcement and banks more aware of what is happening with crypto funds and fiat-crypto exchanges. The development, some argue, has led criminals to prioritise the use of new cryptos such as Monero."
+7246,"In April 2016, Standards Australia submitted a proposal to the International Organization for Standardization to consider developing standards to support blockchain technology. This proposal resulted in the creation of ISO Technical Committee 307, Blockchain and Distributed Ledger Technologies. The technical committee has working groups relating to blockchain terminology, reference architecture, security and privacy, identity, smart contracts, governance and interoperability for blockchain and DLT, as well as standards specific to industry sectors and generic government requirements. More than 50 countries are participating in the standardization process together with external liaisons such as the Society for Worldwide Interbank Financial Telecommunication , the European Commission, the International Federation of Surveyors, the International Telecommunication Union  and the United Nations Economic Commission for Europe ."
+7247,"Many other national standards bodies and open standards bodies are also working on blockchain standards. These include the National Institute of Standards and Technology , the European Committee for Electrotechnical Standardization , the Institute of Electrical and Electronics Engineers , the Organization for the Advancement of Structured Information Standards , and some individual participants in the Internet Engineering Task Force ."
+7248,"Although most of blockchain implementation are decentralized and distributed, Oracle launched a centralized blockchain table feature in Oracle 21c database. The Blockchain Table in Oracle 21c database is a centralized blockchain which provide immutable feature. Compared to decentralized blockchains, centralized blockchains normally can provide a higher throughput and lower latency of transactions than consensus-based distributed blockchains."
+7249,"Currently, there are at least four types of blockchain networks — public blockchains, private blockchains, consortium blockchains and hybrid blockchains."
+7250,"A public blockchain has absolutely no access restrictions. Anyone with an Internet connection can send transactions to it as well as become a validator . Usually, such networks offer economic incentives for those who secure them and utilize some type of a proof-of-stake or proof-of-work algorithm."
+7251,"Some of the largest, most known public blockchains are the bitcoin blockchain and the Ethereum blockchain."
+7252,"A private blockchain is permissioned. One cannot join it unless invited by the network administrators. Participant and validator access is restricted. To distinguish between open blockchains and other peer-to-peer decentralized database applications that are not open ad-hoc compute clusters, the terminology Distributed Ledger  is normally used for private blockchains."
+7253,A hybrid blockchain has a combination of centralized and decentralized features. The exact workings of the chain can vary based on which portions of centralization and decentralization are used.
+7254,A sidechain is a designation for a blockchain ledger that runs in parallel to a primary blockchain. Entries from the primary blockchain  can be linked to and from the sidechain; this allows the sidechain to otherwise operate independently of the primary blockchain .
+7255,"A consortium blockchain is a type of blockchain that combines elements of both public and private blockchains. In a consortium blockchain, a group of organizations come together to create and operate the blockchain, rather than a single entity. The consortium members jointly manage the blockchain network and are responsible for validating transactions. Consortium blockchains are permissioned, meaning that only certain individuals or organizations are allowed to participate in the network. This allows for greater control over who can access the blockchain and helps to ensure that sensitive information is kept confidential."
+7256,"Consortium blockchains are commonly used in industries where multiple organizations need to collaborate on a common goal, such as supply chain management or financial services. One advantage of consortium blockchains is that they can be more efficient and scalable than public blockchains, as the number of nodes required to validate transactions is typically smaller. Additionally, consortium blockchains can provide greater security and reliability than private blockchains, as the consortium members work together to maintain the network. Some examples of consortium blockchains include Quorum and Hyperledger."
+7257,"Blockchain technology can be integrated into multiple areas. The primary use of blockchains is as a distributed ledger for cryptocurrencies such as bitcoin; there were also a few other operational products that had matured from proof of concept by late 2016. As of 2016, some businesses have been testing the technology and conducting low-level implementation to gauge blockchain's effects on organizational efficiency in their back office."
+7258,"In 2019, it was estimated that around $2.9 billion were invested in blockchain technology, which represents an 89% increase from the year prior. Additionally, the International Data Corp has estimated that corporate investment into blockchain technology will reach $12.4 billion by 2022. Furthermore, According to PricewaterhouseCoopers , the second-largest professional services network in the world, blockchain technology has the potential to generate an annual business value of more than $3 trillion by 2030. PwC's estimate is further augmented by a 2018 study that they have conducted, in which PwC surveyed 600 business executives and determined that 84% have at least some exposure to utilizing blockchain technology, which indicates a significant demand and interest in blockchain technology."
+7259,"In 2019, the BBC World Service radio and podcast series Fifty Things That Made the Modern Economy identified blockchain as a technology that would have far-reaching consequences for economics and society. The economist and Financial Times journalist and broadcaster Tim Harford discussed why the underlying technology might have much wider applications and the challenges that needed to be overcome. His first broadcast was on June 29, 2019."
+7260,The number of blockchain wallets quadrupled to 40 million between 2016 and 2020.
+7261,A paper published in 2022 discussed the potential use of blockchain technology in sustainable management.
+7262,"Most cryptocurrencies use blockchain technology to record transactions. For example, the bitcoin network and Ethereum network are both based on blockchain."
+7263,"The criminal enterprise Silk Road, which operated on Tor, utilized cryptocurrency for payments, some of which the US federal government has seized through research on the blockchain and forfeiture."
+7264,"Governments have mixed policies on the legality of their citizens or banks owning cryptocurrencies. China implements blockchain technology in several industries including a national digital currency which launched in 2020. To strengthen their respective currencies, Western governments including the European Union and the United States have initiated similar projects."
+7265,"Blockchain-based smart contracts are contracts that can be partially or fully executed or enforced without human interaction. One of the main objectives of a smart contract is automated escrow. A key feature of smart contracts is that they do not need a trusted third party  to act as an intermediary between contracting entities — the blockchain network executes the contract on its own. This may reduce friction between entities when transferring value and could subsequently open the door to a higher level of transaction automation. An IMF staff discussion from 2018 reported that smart contracts based on blockchain technology might reduce moral hazards and optimize the use of contracts in general. But ""no viable smart contract systems have yet emerged."" Due to the lack of widespread use, their legal status was unclear."
+7266,"According to Reason, many banks have expressed interest in implementing distributed ledgers for use in banking and are cooperating with companies creating private blockchains, and according to a September 2016 IBM study, this is occurring faster than expected."
+7267,"Banks are interested in this technology not least because it has the potential to speed up back office settlement systems. Moreover, as the blockchain industry has reached early maturity institutional appreciation has grown that it is, practically speaking, the infrastructure of a whole new financial industry, with all the implications which that entails."
+7268,Banks such as UBS are opening new research labs dedicated to blockchain technology in order to explore how blockchain can be used in financial services to increase efficiency and reduce costs.
+7269,"Berenberg, a German bank, believes that blockchain is an ""overhyped technology"" that has had a large number of ""proofs of concept"", but still has major challenges, and very few success stories."
+7270,"The blockchain has also given rise to initial coin offerings  as well as a new category of digital asset called security token offerings , also sometimes referred to as digital security offerings . STO/DSOs may be conducted privately or on public, regulated stock exchange and are used to tokenize traditional assets such as company shares as well as more innovative ones like intellectual property, real estate, art, or individual products. A number of companies are active in this space providing services for compliant tokenization, private STOs, and public STOs."
+7271,"Blockchain technology, such as cryptocurrencies and non-fungible tokens , has been used in video games for monetization. Many live-service games offer in-game customization options, such as character skins or other in-game items, which the players can earn and trade with other players using in-game currency. Some games also allow for trading of virtual items using real-world currency, but this may be illegal in some countries where video games are seen as akin to gambling, and has led to gray market issues such as skin gambling, and thus publishers typically have shied away from allowing players to earn real-world funds from games. Blockchain games typically allow players to trade these in-game items for cryptocurrency, which can then be exchanged for money."
+7272,"The first known game to use blockchain technologies was CryptoKitties, launched in November 2017, where the player would purchase NFTs with Ethereum cryptocurrency, each NFT consisting of a virtual pet that the player could breed with others to create offspring with combined traits as new NFTs. The game made headlines in December 2017 when one virtual pet sold for more than US$100,000. CryptoKitties also illustrated scalability problems for games on Ethereum when it created significant congestion on the Ethereum network in early 2018 with approximately 30% of all Ethereum transactions being for the game."
+7273,"By the early 2020s, there had not been a breakout success in video games using blockchain, as these games tend to focus on using blockchain for speculation instead of more traditional forms of gameplay, which offers limited appeal to most players. Such games also represent a high risk to investors as their revenues can be difficult to predict. However, limited successes of some games, such as Axie Infinity during the COVID-19 pandemic, and corporate plans towards metaverse content, refueled interest in the area of GameFi, a term describing the intersection of video games and financing typically backed by blockchain currency, in the second half of 2021. Several major publishers, including Ubisoft, Electronic Arts, and Take Two Interactive, have stated that blockchain and NFT-based games are under serious consideration for their companies in the future."
+7274,"In October 2021, Valve Corporation banned blockchain games, including those using cryptocurrency and NFTs, from being hosted on its Steam digital storefront service, which is widely used for personal computer gaming, claiming that this was an extension of their policy banning games that offered in-game items with real-world value. Valve's prior history with gambling, specifically skin gambling, was speculated to be a factor in the decision to ban blockchain games. Journalists and players responded positively to Valve's decision as blockchain and NFT games have a reputation for scams and fraud among most PC gamers, and Epic Games, which runs the Epic Games Store in competition to Steam, said that they would be open to accepted blockchain games in the wake of Valve's refusal."
+7275,There have been several different efforts to employ blockchains in supply chain management.
+7276,"There are several different efforts to offer domain name services via the blockchain. These domain names can be controlled by the use of a private key, which purports to allow for uncensorable websites. This would also bypass a registrar's ability to suppress domains used for fraud, abuse, or illegal content."
+7277,"Namecoin is a cryptocurrency that supports the "".bit"" top-level domain . Namecoin was forked from bitcoin in 2011. The .bit TLD is not sanctioned by ICANN, instead requiring an alternative DNS root. As of 2015, .bit was used by 28 websites, out of 120,000 registered names. Namecoin was dropped by OpenNIC in 2019, due to malware and potential other legal issues. Other blockchain alternatives to ICANN include The Handshake Network, EmerDNS, and Unstoppable Domains."
+7278,"Specific TLDs include "".eth"", "".luxe"", and "".kred"", which are associated with the Ethereum blockchain through the Ethereum Name Service . The .kred TLD also acts as an alternative to conventional cryptocurrency wallet addresses as a convenience for transferring cryptocurrency."
+7279,"Blockchain technology can be used to create a permanent, public, transparent ledger system for compiling data on sales, tracking digital use and payments to content creators, such as wireless users or musicians. The Gartner 2019 CIO Survey reported 2% of higher education respondents had launched blockchain projects and another 18% were planning academic projects in the next 24 months. In 2017, IBM partnered with ASCAP and PRS for Music to adopt blockchain technology in music distribution. Imogen Heap's Mycelia service has also been proposed as a blockchain-based alternative ""that gives artists more control over how their songs and associated data circulate among fans and other musicians."""
+7280,"New distribution methods are available for the insurance industry such as peer-to-peer insurance, parametric insurance and microinsurance following the adoption of blockchain. The sharing economy and IoT are also set to benefit from blockchains because they involve many collaborating peers. The use of blockchain in libraries is being studied with a grant from the U.S. Institute of Museum and Library Services."
+7281,"Other blockchain designs include Hyperledger, a collaborative effort from the Linux Foundation to support blockchain-based distributed ledgers, with projects under this initiative including Hyperledger Burrow  and Hyperledger Fabric . Another is Quorum, a permissioned private blockchain by JPMorgan Chase with private storage, used for contract applications."
+7282,Oracle introduced a blockchain table feature in its Oracle 21c database.
+7283,Blockchain is also being used in peer-to-peer energy trading.
+7284,"Lightweight blockchains, or simplified blockchains, are more suitable for internet of things  applications than conventional blockchains. One experiment suggested that a lightweight blockchain-based network could accommodate up to 1.34 million authentication processes every second, which could be sufficient for resource-constrained IoT networks."
+7285,"Blockchain could be used in detecting counterfeits by associating unique identifiers to products, documents and shipments, and storing records associated with transactions that cannot be forged or altered. It is however argued that blockchain technology needs to be supplemented with technologies that provide a strong binding between physical objects and blockchain systems, as well as provisions for content creator verification ala KYC standards. The EUIPO established an Anti-Counterfeiting Blockathon Forum, with the objective of ""defining, piloting and implementing"" an anti-counterfeiting infrastructure at the European level. The Dutch Standardisation organisation NEN uses blockchain together with QR Codes to authenticate certificates."
+7286,"Beijing and Shanghai are among the cities designated by China to trial blockchain applications as January 30, 2022. In Chinese legal proceedings, blockchain technology was first accepted as a method for authenticating internet evidence by the Hangzhou Internet Court in 2019 and has since been accepted by other Chinese courts.: 123–125"
+7287,"With the increasing number of blockchain systems appearing, even only those that support cryptocurrencies, blockchain interoperability is becoming a topic of major importance. The objective is to support transferring assets from one blockchain system to another blockchain system. Wegner stated that ""interoperability is the ability of two or more software components to cooperate despite differences in language, interface, and execution platform"". The objective of blockchain interoperability is therefore to support such cooperation among blockchain systems, despite those kinds of differences."
+7288,"There are already several blockchain interoperability solutions available. They can be classified into three categories: cryptocurrency interoperability approaches, blockchain engines, and blockchain connectors."
+7289,Several individual IETF participants produced the draft of a blockchain interoperability architecture.
+7290,"Some cryptocurrencies use blockchain mining — the peer-to-peer computer computations by which transactions are validated and verified. This requires a large amount of energy. In June 2018, the Bank for International Settlements criticized the use of public proof-of-work blockchains for their high energy consumption."
+7291,"Early concern over the high energy consumption was a factor in later blockchains such as Cardano , Solana  and Polkadot  adopting the less energy-intensive proof-of-stake model. Researchers have estimated that Bitcoin consumes 100,000 times as much energy as proof-of-stake networks."
+7292,"In 2021, a study by Cambridge University determined that Bitcoin  used more electricity than Argentina  and the Netherlands . According to Digiconomist, one bitcoin transaction required 708 kilowatt-hours of electrical energy, the amount an average U.S. household consumed in 24 days."
+7293,"In February 2021, U.S. Treasury secretary Janet Yellen called Bitcoin ""an extremely inefficient way to conduct transactions"", saying ""the amount of energy consumed in processing those transactions is staggering"". In March 2021, Bill Gates stated that ""Bitcoin uses more electricity per transaction than any other method known to mankind"", adding ""It's not a great climate thing."""
+7294,"Nicholas Weaver, of the International Computer Science Institute at the University of California, Berkeley, examined blockchain's online security, and the energy efficiency of proof-of-work public blockchains, and in both cases found it grossly inadequate. The 31TWh-45TWh of electricity used for bitcoin in 2018 produced 17-23 million tonnes of CO2. By 2022, the University of Cambridge and Digiconomist estimated that the two largest proof-of-work blockchains, Bitcoin and Ethereum, together used twice as much electricity in one year as the whole of Sweden, leading to the release of up to 120 million tonnes of CO2 each year."
+7295,Some cryptocurrency developers are considering moving from the proof-of-work model to the proof-of-stake model.
+7296,"In October 2014, the MIT Bitcoin Club, with funding from MIT alumni, provided undergraduate students at the Massachusetts Institute of Technology access to $100 of bitcoin. The adoption rates, as studied by Catalini and Tucker , revealed that when people who typically adopt technologies early are given delayed access, they tend to reject the technology. Many universities have founded departments focusing on crypto and blockchain, including MIT, in 2017. In the same year, Edinburgh became ""one of the first big European universities to launch a blockchain course"", according to the Financial Times."
+7297,"Motivations for adopting blockchain technology  have been investigated by researchers. For example, Janssen, et al. provided a framework for analysis, and Koens & Poll pointed out that adoption could be heavily driven by non-technical factors. Based on behavioral models, Li has discussed the differences between adoption at the individual level and organizational levels."
+7298,"Scholars in business and management have started studying the role of blockchains to support collaboration. It has been argued that blockchains can foster both cooperation  and coordination . Thanks to reliability, transparency, traceability of records, and information immutability, blockchains facilitate collaboration in a way that differs both from the traditional use of contracts and from relational norms. Contrary to contracts, blockchains do not directly rely on the legal system to enforce agreements. In addition, contrary to the use of relational norms, blockchains do not require a trust or direct connections between collaborators."
+7299,"The need for internal audits to provide effective oversight of organizational efficiency will require a change in the way that information is accessed in new formats. Blockchain adoption requires a framework to identify the risk of exposure associated with transactions using blockchain. The Institute of Internal Auditors has identified the need for internal auditors to address this transformational technology. New methods are required to develop audit plans that identify threats and risks. The Internal Audit Foundation study, Blockchain and Internal Audit, assesses these factors. The American Institute of Certified Public Accountants has outlined new roles for auditors as a result of blockchain."
+7300,"In September 2015, the first peer-reviewed academic journal dedicated to cryptocurrency and blockchain technology research, Ledger, was announced. The inaugural issue was published in December 2016. The journal covers aspects of mathematics, computer science, engineering, law, economics and philosophy that relate to cryptocurrencies. The journal encourages authors to digitally sign a file hash of submitted papers, which are then timestamped into the bitcoin blockchain. Authors are also asked to include a personal bitcoin address on the first page of their papers for non-repudiation purposes."
+7301,Changelog – a record of all notable changes made to a project
+7302,Checklist – an informational aid used to reduce failure
+7303,Economics of digitization
+7304,List of blockchains
+7305,Privacy and blockchain
+7306,Version control – a record of all changes  in a form of a graph
+7307,"Many key exchange systems have one party generate the key, and simply send that key to the other party—the other party has no influence on the key.
+Using a key-agreement protocol avoids some of the key distribution problems associated with such systems."
+7308,Protocols where both parties influence the final derived key are the only way to implement perfect forward secrecy.
+7309,"The first publicly known public-key agreement protocol that meets the above criteria was the Diffie–Hellman key exchange, in which two parties jointly exponentiate a generator with random numbers, in such a way that an eavesdropper cannot feasibly determine what the resultant value used to produce a shared key is."
+7310,Exponential key exchange in and of itself does not specify any prior agreement or subsequent authentication between the participants.  It has thus been described as an anonymous key agreement protocol.
+7311,"Anonymous key exchange, like Diffie–Hellman, does not provide authentication of the parties, and is thus vulnerable to man-in-the-middle attacks."
+7312,"A wide variety of cryptographic authentication schemes and protocols have been developed to provide authenticated key agreement to prevent man-in-the-middle and related attacks. These methods generally mathematically bind the agreed key to other agreed-upon data, such as the following:"
+7313,"A widely used mechanism for defeating such attacks is the use of digitally signed keys that must be integrity-assured: if Bob's key is signed by a trusted third party vouching for his identity, Alice can have considerable confidence that a signed key she receives is not an attempt to intercept by Eve. When Alice and Bob have a public-key infrastructure, they may digitally sign an agreed Diffie–Hellman key, or exchanged Diffie–Hellman public keys. Such signed keys, sometimes signed by a certificate authority, are one of the primary mechanisms used for secure web traffic . Other specific examples are MQV, YAK and the ISAKMP component of the IPsec protocol suite for securing Internet Protocol communications. However, these systems require care in endorsing the match between identity information and public keys by certificate authorities in order to work properly."
+7314,"Hybrid systems use public-key cryptography to exchange secret keys, which are then used in a symmetric-key cryptography systems. Most practical applications of cryptography use a combination of cryptographic functions to implement an overall system that provides all of the four desirable features of secure communications ."
+7315,"Password-authenticated key agreement protocols require the separate establishment of a password  in a manner that is both private and integrity-assured. These are designed to resist man-in-the-middle and other active attacks on the password and the established keys. For example, DH-EKE, SPEKE, and SRP are password-authenticated variations of Diffie–Hellman."
+7316,"If one has an integrity-assured way to verify a shared key over a public channel, one may engage in a Diffie–Hellman key exchange to derive a short-term shared key, and then subsequently authenticate that the keys match. One way is to use a voice-authenticated read-out of the key, as in PGPfone. Voice authentication, however, presumes that it is infeasible for a man-in-the-middle to spoof one participant's voice to the other in real-time, which may be an undesirable assumption. Such protocols may be designed to work with even a small public value, such as a password. Variations on this theme have been proposed for Bluetooth pairing protocols."
+7317,"In an attempt to avoid using any additional out-of-band authentication factors, Davies and Price proposed the use of the interlock protocol of Ron Rivest and Adi Shamir, which has been subject to both attack and subsequent refinement."
+7318,"Secret-key  cryptography requires the initial exchange of a shared key in a manner that is private and integrity-assured. When done right, man-in-the-middle attack is prevented. However, without the use of public-key cryptography, one may be left with undesirable key-management problems."
+7319,"Cryptographic hash functions have many information-security applications, notably in digital signatures, message authentication codes , and other forms of authentication. They can also be used as ordinary hash functions, to index data in hash tables, for fingerprinting, to detect duplicate data or uniquely identify files, and as checksums to detect accidental data corruption. Indeed, in information-security contexts, cryptographic hash values are sometimes called  fingerprints, checksums, or just hash values, even though all these terms stand for more general functions with rather different properties and purposes."
+7320,"Non-cryptographic hash functions are used in hash tables and to detect accidental errors, their construction frequently provides no resistance to a deliberate attack. For example, a denial-of-service attack on hash tables is possible if the collisions are easy to find, like in the case of linear cyclic redundancy check  functions."
+7321,Most cryptographic hash functions are designed to take a string of any length as input and produce a fixed-length hash value.
+7322,"A cryptographic hash function must be able to withstand all known types of cryptanalytic attack. In theoretical cryptography, the security level of a cryptographic hash function has been defined using the following properties:"
+7323,"Collision resistance implies second pre-image resistance but does not imply pre-image resistance. The weaker assumption is always preferred in theoretical cryptography, but in practice, a hash-function which is only second pre-image resistant is considered insecure and is therefore not recommended for real applications."
+7324,"Informally, these properties mean that a malicious adversary cannot replace or modify the input data without changing its digest. Thus, if two strings have the same digest, one can be very confident that they are identical. Second pre-image resistance prevents an attacker from crafting a document with the same hash as a document the attacker cannot control. Collision resistance prevents an attacker from creating two distinct documents with the same hash."
+7325,"A function meeting these criteria may still have undesirable properties. Currently, popular cryptographic hash functions are vulnerable to length-extension attacks: given hash and len but not m, by choosing a suitable m′ an attacker can calculate hash, where ∥ denotes concatenation. This property can be used to break naive authentication schemes based on hash functions. The HMAC construction works around these problems."
+7326,"In practice, collision resistance is insufficient for many practical uses. In addition to collision resistance, it should be impossible for an adversary to find two messages with substantially similar digests; or to infer any useful information about the data, given only its digest. In particular, a hash function should behave as much as possible like a random function  while still being deterministic and efficiently computable. This rules out functions like the SWIFFT function, which can be rigorously proven to be collision-resistant assuming that certain problems on ideal lattices are computationally difficult, but, as a linear function, does not satisfy these additional properties."
+7327,"Checksum algorithms, such as CRC32 and other cyclic redundancy checks, are designed to meet much weaker requirements and are generally unsuitable as cryptographic hash functions. For example, a CRC was used for message integrity in the WEP encryption standard, but an attack was readily discovered, which exploited the linearity of the checksum."
+7328,"In cryptographic practice, ""difficult"" generally means ""almost certainly beyond the reach of any adversary who must be prevented from breaking the system for as long as the security of the system is deemed important"". The meaning of the term is therefore somewhat dependent on the application since the effort that a malicious agent may put into the task is usually proportional to their expected gain. However, since the needed effort usually multiplies with the digest length, even a thousand-fold advantage in processing power can be neutralized by adding a dozen bits to the latter."
+7329,"For messages selected from a limited set of messages, for example passwords or other short messages, it can be feasible to invert a hash by trying all possible messages in the set. Because cryptographic hash functions are typically designed to be computed quickly, special key derivation functions that require greater computing resources have been developed that make such brute-force attacks more difficult."
+7330,"In some theoretical analyses ""difficult"" has a specific mathematical meaning, such as ""not solvable in asymptotic polynomial time"". Such interpretations of difficulty are important in the study of provably secure cryptographic hash functions but do not usually have a strong connection to practical security. For example, an exponential-time algorithm can sometimes still be fast enough to make a feasible attack. Conversely, a polynomial-time algorithm  may be too slow for any practical use."
+7331,"An illustration of the potential use of a cryptographic hash is as follows: Alice poses a tough math problem to Bob and claims that she has solved it. Bob would like to try it himself, but would yet like to be sure that Alice is not bluffing. Therefore, Alice writes down her solution, computes its hash, and tells Bob the hash value . Then, when Bob comes up with the solution himself a few days later, Alice can prove that she had the solution earlier by revealing it and having Bob hash it and check that it matches the hash value given to him before. "
+7332,"An important application of secure hashes is the verification of message integrity. Comparing message digests  calculated before, and after, transmission can determine whether any changes have been made to the message or file."
+7333,"MD5, SHA-1, or SHA-2 hash digests are sometimes published on websites or forums to allow verification of integrity for downloaded files, including files retrieved using file sharing such as mirroring. This practice establishes a chain of trust as long as the hashes are posted on a trusted site – usually the originating site – authenticated by HTTPS. Using a cryptographic hash and a chain of trust detects malicious changes to the file. Non-cryptographic error-detecting codes such as cyclic redundancy checks only prevent against non-malicious alterations of the file, since an intentional spoof can readily be crafted to have the colliding code value."
+7334,"Almost all digital signature schemes require a cryptographic hash to be calculated over the message. This allows the signature calculation to be performed on the relatively small, statically sized hash digest. The message is considered authentic if the signature verification succeeds given the signature and recalculated hash digest over the message. So the message integrity property of the cryptographic hash is used to create secure and efficient digital signature schemes."
+7335,"Password verification commonly relies on cryptographic hashes. Storing all user passwords as cleartext can result in a massive security breach if the password file is compromised. One way to reduce this danger is to only store the hash digest of each password. To authenticate a user, the password presented by the user is hashed and compared with the stored hash. A password reset method is required when password hashing is performed; original passwords cannot be recalculated from the stored hash value."
+7336,"However, use of standard cryptographic hash functions, such as the SHA series, is no longer considered safe for password storage.: 5.1.1.2  These algorithms are designed to be computed quickly, so if the hashed values are compromised, it is possible to try guessed passwords at high rates. Common graphics processing units can try billions of possible passwords each second. Password hash functions that perform key stretching – such as PBKDF2, scrypt or Argon2 – commonly use repeated invocations of a cryptographic hash to increase the time  required to perform brute-force attacks on stored password hash digests. For details, see § Attacks on hashed passwords."
+7337,"A password hash also requires the use of a large random, non-secret salt value which can be stored with the password hash. The salt is hashed with the password, altering the password hash mapping for each password, thereby making it infeasible for an adversary to store tables of precomputed hash values to which the password hash digest can be compared or to test a large number of purloined hash values in parallel."
+7338,"A proof-of-work system  is an economic measure to deter denial-of-service attacks and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer. A key feature of these schemes is their asymmetry: the work must be moderately hard  on the requester side but easy to check for the service provider. One popular system – used in Bitcoin mining and Hashcash – uses partial hash inversions to prove that work was done, to unlock a mining reward in Bitcoin, and as a good-will token to send an e-mail in Hashcash.  The sender is required to find a message whose hash value begins with a number of zero bits. The average work that the sender needs to perform in order to find a valid message is exponential in the number of zero bits required in the hash value, while the recipient can verify the validity of the message by executing a single hash function. For instance, in Hashcash, a sender is asked to generate a header whose 160-bit SHA-1 hash value has the first 20 bits as zeros. The sender will, on average, have to try 219 times to find a valid header."
+7339,"A message digest can also serve as a means of reliably identifying a file; several source code management systems, including Git, Mercurial and Monotone, use the sha1sum of various types of content  to uniquely identify them. Hashes are used to identify files on peer-to-peer filesharing networks. For example, in an ed2k link, an MD4-variant hash is combined with the file size, providing sufficient information for locating file sources, downloading the file, and verifying its contents. Magnet links are another example. Such file hashes are often the top hash of a hash list or a hash tree which allows for additional benefits."
+7340,"One of the main applications of a hash function is to allow the fast look-up of data in a hash table. Being hash functions of a particular kind, cryptographic hash functions lend themselves well to this application too."
+7341,"However, compared with standard hash functions, cryptographic hash functions tend to be much more expensive computationally. For this reason, they tend to be used in contexts where it is necessary for users to protect themselves against the possibility of forgery  by potentially malicious participants."
+7342,"CAS systems work by passing the content of the file through a cryptographic hash function to generate a unique key, the ""content address"". The file system's directory stores these addresses and a pointer to the physical storage of the content. Because an attempt to store the same file will generate the same key, CAS systems ensure that the files within them are unique, and because changing the file will result in a new key, CAS systems provide assurance that the file is unchanged."
+7343,"There are several methods to use a block cipher to build a cryptographic hash function, specifically a one-way compression function."
+7344,"The methods resemble the block cipher modes of operation usually used for encryption. Many well-known hash functions, including MD4, MD5, SHA-1 and SHA-2, are built from block-cipher-like components designed for the purpose, with feedback to ensure that the resulting function is not invertible. SHA-3 finalists included functions with block-cipher-like components  though the function finally selected, Keccak, was built on a cryptographic sponge instead."
+7345,"A standard block cipher such as AES can be used in place of these custom block ciphers; that might be useful when an embedded system needs to implement both encryption and hashing with minimal code size or hardware area. However, that approach can have costs in efficiency and security. The ciphers in hash functions are built for hashing: they use large keys and blocks, can efficiently change keys every block, and have been designed and vetted for resistance to related-key attacks. General-purpose ciphers tend to have different design goals. In particular, AES has key and block sizes that make it nontrivial to use to generate long hash values; AES encryption becomes less efficient when the key changes each block; and related-key attacks make it potentially less secure for use in a hash function than for encryption."
+7346,"A hash function must be able to process an arbitrary-length message into a fixed-length output. This can be achieved by breaking the input up into a series of equally sized blocks, and operating on them in sequence using a one-way compression function. The compression function can either be specially designed for hashing or be built from a block cipher. A hash function built with the Merkle–Damgård construction is as resistant to collisions as is its compression function; any collision for the full hash function can be traced back to a collision in the compression function."
+7347,"The last block processed should also be unambiguously length padded; this is crucial to the security of this construction. This construction is called the Merkle–Damgård construction. Most common classical hash functions, including SHA-1 and MD5, take this form."
+7348,"A straightforward application of the Merkle–Damgård construction, where the size of hash output is equal to the internal state size , results in a narrow-pipe hash design. This design causes many inherent flaws, including length-extension, multicollisions, long message attacks, generate-and-paste attacks, and also cannot be parallelized. As a result, modern hash functions are built on wide-pipe constructions that have a larger internal state size – which range from tweaks of the Merkle–Damgård construction to new constructions such as the sponge construction and HAIFA construction. None of the entrants in the NIST hash function competition use a classical Merkle–Damgård construction."
+7349,"Meanwhile, truncating the output of a longer hash, such as used in SHA-512/256, also defeats many of these attacks."
+7350,"Hash functions can be used to build other cryptographic primitives. For these other primitives to be cryptographically secure, care must be taken to build them correctly."
+7351,Message authentication codes   are often built from hash functions. HMAC is such a MAC.
+7352,"Just as block ciphers can be used to build hash functions, hash functions can be used to build block ciphers.  Luby-Rackoff constructions using hash functions can be provably secure if the underlying hash function is secure.  Also, many hash functions  are built by using a special-purpose block cipher in a Davies–Meyer or other construction.  That cipher can also be used in a conventional mode of operation, without the same security guarantees; for example, SHACAL, BEAR and LION."
+7353,Pseudorandom number generators  can be built using hash functions.  This is done by combining a  random seed with a counter and hashing it.
+7354,"Some hash functions, such as Skein, Keccak, and RadioGatún, output an arbitrarily long stream and can be used as a stream cipher, and stream ciphers can also be built from fixed-length digest hash functions. Often this is done by first building a cryptographically secure pseudorandom number generator and then using its stream of random bytes as keystream. SEAL is a stream cipher that uses SHA-1 to generate internal tables, which are then used in a keystream generator more or less unrelated to the hash algorithm.  SEAL is not guaranteed to be as strong  as SHA-1. Similarly, the key expansion of the HC-128 and HC-256 stream ciphers makes heavy use of the SHA-256 hash function."
+7355,"Concatenating outputs from multiple hash functions provide collision resistance as good as the strongest of the algorithms included in the concatenated result.  For example, older versions of Transport Layer Security  and Secure Sockets Layer  used concatenated MD5 and SHA-1 sums. This ensures that a method to find collisions in one of the hash functions does not defeat data protected by both hash functions."
+7356,"For Merkle–Damgård construction hash functions, the concatenated function is as collision-resistant as its strongest component, but not more collision-resistant. Antoine Joux observed that 2-collisions lead to n-collisions: if it is feasible for an attacker to find two messages with the same MD5 hash, then they can find as many additional messages with that same MD5 hash as they desire, with no greater difficulty. Among those n messages with the same MD5 hash, there is likely to be a collision in SHA-1. The additional work needed to find the SHA-1 collision  requires only polynomial time."
+7357,There are many cryptographic hash algorithms; this section lists a few algorithms that are referenced relatively often. A more extensive list can be found on the page containing a comparison of cryptographic hash functions.
+7358,"MD5 was designed by Ronald Rivest in 1991 to replace an earlier hash function, MD4, and was specified in 1992 as RFC 1321. Collisions against MD5 can be calculated within seconds which makes the algorithm unsuitable for most use cases where a cryptographic hash is required. MD5 produces a digest of 128 bits ."
+7359,"SHA-1 was developed as part of the U.S. Government's Capstone project. The original specification – now commonly called SHA-0 – of the algorithm was published in 1993 under the title Secure Hash Standard, FIPS PUB 180, by U.S. government standards agency NIST . It was withdrawn by the NSA shortly after publication and was superseded by the revised version, published in 1995 in FIPS  PUB 180-1 and commonly designated SHA-1. Collisions against the full SHA-1 algorithm can be produced using the shattered attack and the hash function should be considered broken. SHA-1 produces a hash digest of 160 bits ."
+7360,"Documents may refer to SHA-1 as just ""SHA"", even though this may conflict with the other Secure Hash Algorithms such as SHA-0, SHA-2, and SHA-3."
+7361,"RIPEMD  is a family of cryptographic hash functions developed in Leuven, Belgium, by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel at the COSIC research group at the Katholieke Universiteit Leuven, and first published in 1996. RIPEMD was based upon the design principles used in MD4 and is similar in performance to the more popular SHA-1. RIPEMD-160 has, however, not been broken.  As the name implies, RIPEMD-160 produces a hash digest of 160 bits ."
+7362,"Whirlpool is a cryptographic hash function designed by Vincent Rijmen and Paulo S. L. M. Barreto, who first described it in 2000. Whirlpool is based on a substantially modified version of the Advanced Encryption Standard . Whirlpool produces a hash digest of 512 bits ."
+7363,"SHA-2  is a set of cryptographic hash functions designed by the United States National Security Agency , first published in 2001. They are built using the Merkle–Damgård structure, from a one-way compression function itself built using the Davies–Meyer structure from a  specialized block cipher."
+7364,SHA-2 basically consists of two hash algorithms: SHA-256 and SHA-512. SHA-224 is a variant of SHA-256 with different starting values and truncated output. SHA-384 and the lesser-known SHA-512/224 and SHA-512/256 are all variants of SHA-512. SHA-512 is more secure than SHA-256 and is commonly faster than SHA-256 on 64-bit machines such as AMD64.
+7365,"The output size in bits is given by the extension to the ""SHA"" name, so SHA-224 has an output size of 224 bits ; SHA-256, 32 bytes; SHA-384, 48 bytes; and SHA-512, 64 bytes."
+7366,"SHA-3  was released by NIST on August 5, 2015. SHA-3 is a subset of the broader cryptographic primitive family Keccak. The Keccak algorithm is the work of Guido Bertoni, Joan Daemen, Michael Peeters, and Gilles Van Assche. Keccak is based on a sponge construction which can also be used to build other cryptographic primitives such as a stream cipher. SHA-3 provides the same output sizes as SHA-2: 224, 256, 384, and 512 bits."
+7367,Configurable output sizes can also be obtained using the SHAKE-128 and SHAKE-256 functions. Here the -128 and -256 extensions to the name imply the security strength of the function rather than the output size in bits.
+7368,"BLAKE2, an improved version of BLAKE, was announced on December 21, 2012. It was created by Jean-Philippe Aumasson, Samuel Neves, Zooko Wilcox-O'Hearn, and Christian Winnerlein with the goal of replacing the widely used but broken MD5 and SHA-1 algorithms. When run on 64-bit x64 and ARM architectures, BLAKE2b is faster than SHA-3, SHA-2, SHA-1, and MD5. Although BLAKE and BLAKE2 have not been standardized as SHA-3 has, BLAKE2 has been used in many protocols including the Argon2 password hash, for the high efficiency that it offers on modern CPUs. As BLAKE was a candidate for SHA-3, BLAKE and BLAKE2 both offer the same output sizes as SHA-3 – including a configurable output size."
+7369,"BLAKE3, an improved version of BLAKE2, was announced on January 9, 2020. It was created by Jack O'Connor, Jean-Philippe Aumasson, Samuel Neves, and Zooko Wilcox-O'Hearn. BLAKE3 is a single algorithm, in contrast to BLAKE and BLAKE2, which are algorithm families with multiple variants. The BLAKE3 compression function is closely based on that of BLAKE2s, with the biggest difference being that the number of rounds is reduced from 10 to 7. Internally, BLAKE3 is a Merkle tree, and it supports higher degrees of parallelism than BLAKE2."
+7370,"There is a long list of cryptographic hash functions but many have been found to be vulnerable and should not be used. For instance, NIST selected 51 hash functions as candidates for round 1 of the SHA-3 hash competition, of which 10 were considered broken and 16 showed significant weaknesses and therefore did not make it to the next round; more information can be found on the main article about the NIST hash function competitions."
+7371,"Even if a hash function has never been broken, a successful attack against a weakened variant may undermine the experts' confidence. For instance, in August 2004 collisions were found in several then-popular hash functions, including MD5. These weaknesses called into question the security of stronger algorithms derived from the weak hash functions – in particular, SHA-1 , RIPEMD-128, and RIPEMD-160 ."
+7372,"On August 12, 2004, Joux, Carribault, Lemuel, and Jalby announced a collision for the full SHA-0 algorithm. Joux et al. accomplished this using a generalization of the Chabaud and Joux attack. They found that the collision had complexity 251 and took about 80,000 CPU hours on a supercomputer with 256 Itanium 2 processors – equivalent to 13 days of full-time use of the supercomputer."
+7373,"In February 2005, an attack on SHA-1 was reported that would find collision in about 269 hashing operations, rather than the 280 expected for a 160-bit hash function. In August 2005, another attack on SHA-1 was reported that would find collisions in 263 operations. Other theoretical weaknesses of SHA-1 have been known: and in February 2017 Google announced a collision in SHA-1. Security researchers recommend that new applications can avoid these problems by using later members of the SHA family, such as SHA-2, or using techniques such as randomized hashing that do not require collision resistance."
+7374,"A successful, practical attack broke MD5 used within certificates for Transport Layer Security in 2008."
+7375,"Many cryptographic hashes are based on the Merkle–Damgård construction. All cryptographic hashes that directly use the full output of a Merkle–Damgård construction are vulnerable to length extension attacks. This makes the MD5, SHA-1, RIPEMD-160, Whirlpool, and the SHA-256 / SHA-512 hash algorithms all vulnerable to this specific attack. SHA-3, BLAKE2, BLAKE3, and the truncated SHA-2 variants are not vulnerable to this type of attack."
+7376,"Rather than store plain user passwords, controlled access system frequently store the hash of each user's password in a file or database. When someone requests access, the password they submit is hashed and compared with the stored value. If the database is stolen , the thief will only have the hash values, not the passwords."
+7377,"Passwords may still be retrieved by an attacker from the hashes, because most people choose passwords in predictable ways. Lists of common passwords are widely circulated and many passwords are short enough that even all possible combinations may be tested if calculation of the hash does not take too much time."
+7378,"The use of cryptographic salt prevents some attacks, such as building files of precomputing hash values, e.g. rainbow tables. But searches on the order of 100 billion tests per second are possible with high-end graphics processors, making direct attacks possible even with salt.
+
+The United States National Institute of Standards and Technology recommends storing passwords using special hashes called key derivation functions  that have been created to slow brute force searches.: 5.1.1.2   Slow hashes include pbkdf2, bcrypt, scrypt, argon2, Balloon and some recent modes of Unix crypt. For KDFs that perform multiple hashes to slow execution, NIST recommends an iteration count of 10,000 or more.: 5.1.1.2"
+7379,"Digital signatures are a standard element of most cryptographic protocol suites, and are commonly used for software distribution, financial transactions, contract management software, and in other cases where it is important to detect forgery or tampering."
+7380,"Digital signatures are often used to implement electronic signatures, which include any electronic data that carries the intent of a signature, but not all electronic signatures use digital signatures. Electronic signatures have legal significance in some countries, including Brazil,  Canada, South Africa, the United States, Algeria, Turkey, India,  Indonesia, Mexico, Saudi Arabia, Uruguay, Switzerland, Chile and the countries of the European Union."
+7381,"Digital signatures employ asymmetric cryptography. In many instances, they provide a layer of validation and security to messages sent through a non-secure channel: Properly implemented, a digital signature gives the receiver reason to believe the message was sent by the claimed sender. Digital signatures are equivalent to traditional handwritten signatures in many respects, but properly implemented digital signatures are more difficult to forge than the handwritten type. Digital signature schemes, in the sense used here, are cryptographically based, and must be implemented properly to be effective. They can also provide non-repudiation, meaning that the signer cannot successfully claim they did not sign a message, while also claiming their private key remains secret. Further, some non-repudiation schemes offer a timestamp for the digital signature, so that even if the private key is exposed, the signature is valid. Digitally signed messages may be anything representable as a bitstring: examples include electronic mail, contracts, or a message sent via some other cryptographic protocol."
+7382,A digital signature scheme typically consists of three algorithms:
+7383,Two main properties are required:
+7384,"First, the authenticity of a signature generated from a fixed message and fixed private key can be verified by using the corresponding public key."
+7385,"Secondly, it should be computationally infeasible to generate a valid signature for a party without knowing that party's private key.
+A digital signature is an authentication mechanism that enables the creator of the message to attach a code that acts as a signature.
+The Digital Signature Algorithm , developed by the National Institute of Standards and Technology, is one of many examples of a signing algorithm."
+7386,"In the following discussion, 1n refers to a unary number."
+7387,"Formally, a digital signature scheme is a triple of probabilistic polynomial time algorithms, , satisfying:"
+7388,"For correctness, S and V must satisfy"
+7389,"A digital signature scheme is secure if for every non-uniform probabilistic polynomial time adversary, A"
+7390,"where AS denotes that A has access to the oracle, S, Q denotes the set of the queries on S made by A, which knows the public key, pk, and the security parameter, n, and x ∉ Q denotes that the adversary may not directly query the string, x, on S."
+7391,"In 1976, Whitfield Diffie and Martin Hellman first described the notion of a digital signature scheme, although they only conjectured that such schemes existed based on functions that are trapdoor one-way permutations.  Soon afterwards, Ronald Rivest, Adi Shamir, and Len Adleman invented the RSA algorithm, which could be used to produce primitive digital signatures . The first widely marketed software package to offer digital signature was Lotus Notes 1.0, released in 1989, which used the RSA algorithm."
+7392,"Other digital signature schemes were soon developed after RSA, the earliest being Lamport signatures, Merkle signatures , and Rabin signatures."
+7393,"In 1988, Shafi Goldwasser, Silvio Micali, and Ronald Rivest became the first to rigorously define the security requirements of digital signature schemes. They described a hierarchy of attack models for signature schemes, and also presented the GMR signature scheme, the first that could be proved to prevent even an existential forgery against a chosen message attack, which is the currently accepted security definition for signature schemes.  The first such scheme which is not built on trapdoor functions but rather on a family of function with a much weaker required property of one-way permutation was presented by Moni Naor and Moti Yung."
+7394,"One digital signature scheme  is based on RSA. To create signature keys, generate an RSA key pair containing a modulus, N, that is the product of two random secret distinct large primes, along with integers, e and d, such that e d ≡ 1 ), where φ is Euler's totient function. The signer's public key consists of N and e, and the signer's secret key contains d."
+7395,"Used directly, this type of signature scheme is vulnerable to key-only existential forgery attack. To create a forgery, the attacker picks a random signature σ and uses the verification procedure to determine the message, m, corresponding to that signature. In practice, however, this type of signature is not used directly, but rather, the message to be signed is first hashed to produce a short digest, that is then padded to larger width comparable to N, then signed with the reverse trapdoor function. This forgery attack, then, only produces the padded hash function output that corresponds to σ, but not a message that leads to that value, which does not lead to an attack. In the random oracle model, hash-then-sign , this form of signature is existentially unforgeable, even against a chosen-plaintext attack."
+7396,There are several reasons to sign such a hash  instead of the whole document.
+7397,"As organizations move away from paper documents with ink signatures or authenticity stamps, digital signatures can provide added assurances of the evidence to provenance, identity, and status of an electronic document as well as acknowledging informed consent and approval by a signatory.  The United States Government Printing Office  publishes electronic versions of the budget, public and private laws, and congressional bills with digital signatures.  Universities including Penn State, University of Chicago, and Stanford are publishing electronic student transcripts with digital signatures."
+7398,Below are some common reasons for applying a digital signature to communications:
+7399,"A message may have letterhead or a handwritten signature identifying its sender, but letterheads and handwritten signatures can be copied and pasted onto forged messages.
+Even legitimate messages may be modified in transit."
+7400,"If a bank's central office receives a letter claiming to be from a branch office with instructions to change the balance of an account, the central bankers need to be sure, before acting on the instructions, that they were actually sent by a branch banker, and not forged—whether a forger fabricated the whole letter, or just modified an existing letter in transit by adding some digits."
+7401,"With a digital signature scheme, the central office can arrange beforehand to have a public key on file whose private key is known only to the branch office.
+The branch office can later sign a message and the central office can use the public key to verify the signed message was not a forgery before acting on it.
+A forger who doesn't know the sender's private key can't sign a different message, or even change a single digit in an existing message without making the recipient's signature verification fail."
+7402,"Encryption can hide the content of the message from an eavesdropper, but encryption on its own may not let recipient verify the message's authenticity, or even detect selective modifications like changing a digit—if the bank's offices simply encrypted the messages they exchange, they could still be vulnerable to forgery.
+In other applications, such as software updates, the messages are not secret—when a software author publishes an patch for all existing installations of the software to apply, the patch itself is not secret, but computers running the software must verify the authenticity of the patch before applying it, lest they become victims to malware."
+7403,"Replays.
+A digital signature scheme on its own does not prevent a valid signed message from being recorded and then maliciously reused in a replay attack.
+For example, the branch office may legitimately request that bank transfer be issued once in a signed message.
+If the bank doesn't use a system of transaction ids in their messages to detect which transfers have already happened, someone could illegitimately reuse the same signed message many times to drain an account."
+7404,"Uniqueness and malleability of signatures.
+A signature itself cannot be used to uniquely identify the message it signs—in some signature schemes, every message has a large number of possible valid signatures from the same signer, and it may be easy, even without knowledge of the private key, to transform one valid signature into another.
+If signatures are misused as transaction ids in an attempt by a bank-like system such as a Bitcoin exchange to detect replays, this can be exploited to replay transactions."
+7405,"Authenticating a public key.
+Prior knowledge of a public key can be used to verify authenticity of a signed message, but not the other way around—prior knowledge of a signed message cannot be used to verify authenticity of a public key.
+In some signature schemes, given a signed message, it is easy to construct a public key under which the signed message will pass verification, even without knowledge of the private key that was used to make the signed message in the first place."
+7406,"Non-repudiation, or more specifically non-repudiation of origin, is an important aspect of digital signatures. By this property, an entity that has signed some information cannot at a later time deny having signed it. Similarly, access to the public key only does not enable a fraudulent party to fake a valid signature."
+7407,"Note that these authentication, non-repudiation etc. properties rely on the secret key not having been revoked prior to its usage. Public revocation of a key-pair is a required ability, else leaked secret keys would continue to implicate the claimed owner of the key-pair. Checking revocation status requires an ""online"" check; e.g., checking a certificate revocation list or via the Online Certificate Status Protocol.  Very roughly this is analogous to a vendor who receives credit-cards first checking online with the credit-card issuer to find if a given card has been reported lost or stolen. Of course, with stolen key pairs, the theft is often discovered only after the secret key's use, e.g., to sign a bogus certificate for espionage purpose."
+7408,"In their foundational paper, Goldwasser, Micali, and Rivest lay out a hierarchy of attack models against digital signatures:"
+7409,They also describe a hierarchy of attack results:
+7410,"The strongest notion of security, therefore, is security against existential forgery under an adaptive chosen message attack."
+7411,"All public key / private key cryptosystems depend entirely on keeping the private key secret. A private key can be stored on a user's computer, and protected by a local password, but this has two disadvantages:"
+7412,"A more secure alternative is to store the private key on a smart card. Many smart cards are designed to be tamper-resistant . In a typical digital signature implementation, the hash calculated from the document is sent to the smart card, whose CPU signs the hash using the stored private key of the user, and then returns the signed hash. Typically, a user must activate their smart card by entering a personal identification number or PIN code . It can be arranged that the private key never leaves the smart card, although this is not always implemented. If the smart card is stolen, the thief will still need the PIN code to generate a digital signature. This reduces the security of the scheme to that of the PIN system, although it still requires an attacker to possess the card. A mitigating factor is that private keys, if generated and stored on smart cards, are usually regarded as difficult to copy, and are assumed to exist in exactly one copy. Thus, the loss of the smart card may be detected by the owner and the corresponding certificate can be immediately revoked. Private keys that are protected by software only may be easier to copy, and such compromises are far more difficult to detect."
+7413,"Entering a PIN code to activate the smart card commonly requires a numeric keypad. Some card readers have their own numeric keypad. This is safer than using a card reader integrated into a PC, and then entering the PIN using that computer's keyboard. Readers with a numeric keypad are meant to circumvent the eavesdropping threat where the computer might be running a keystroke logger, potentially compromising the PIN code. Specialized card readers are also less vulnerable to tampering with their software or hardware and are often EAL3 certified."
+7414,"Smart card design is an active field, and there are smart card schemes which are intended to avoid these particular problems, despite having few security proofs so far."
+7415,"One of the main differences between a digital signature and a written signature is that the user does not ""see"" what they sign. The user application presents a hash code to be signed by the digital signing algorithm using the private key. An attacker who gains control of the user's PC can possibly replace the user application with a foreign substitute, in effect replacing the user's own communications with those of the attacker. This could allow a malicious application to trick a user into signing any document by displaying the user's original on-screen, but presenting the attacker's own documents to the signing application."
+7416,"To protect against this scenario, an authentication system can be set up between the user's application  and the signing application. The general idea is to provide some means for both the user application and signing application to verify each other's integrity. For example, the signing application may require all requests to come from digitally signed binaries."
+7417,"One of the main differences between a cloud based digital signature service and a locally provided one is risk.  Many risk averse companies, including governments, financial and medical institutions,  and payment processors require more secure standards, like FIPS 140-2 level 3 and FIPS 201 certification, to ensure the signature is validated and secure."
+7418,"Technically speaking, a digital signature applies to a string of bits, whereas humans and applications ""believe"" that they sign the semantic interpretation of those bits. In order to be semantically interpreted, the bit string must be transformed into a form that is meaningful for humans and applications, and this is done through a combination of hardware and software based processes on a computer system. The problem is that the semantic interpretation of bits can change as a function of the processes used to transform the bits into semantic content. It is relatively easy to change the interpretation of a digital document by implementing changes on the computer system where the document is being processed. From a semantic perspective this creates uncertainty about what exactly has been signed. WYSIWYS  means that the semantic interpretation of a signed message cannot be changed. In particular this also means that a message cannot contain hidden information that the signer is unaware of, and that can be revealed after the signature has been applied. WYSIWYS is a requirement for the validity of digital signatures, but this requirement is difficult to guarantee because of the increasing complexity of modern computer systems. The term WYSIWYS was coined by Peter Landrock and Torben Pedersen to describe some of the principles in delivering secure and legally binding digital signatures for Pan-European projects."
+7419,"An ink signature could be replicated from one document to another by copying the image manually or digitally, but to have credible signature copies that can resist some scrutiny is a significant manual or technical skill, and to produce ink signature copies that resist professional scrutiny is very difficult."
+7420,"Digital signatures cryptographically bind an electronic identity to an electronic document and the digital signature cannot be copied to another document. Paper contracts sometimes have the ink signature block on the last page, and the previous pages may be replaced after a signature is applied.  Digital signatures can be applied to an entire document, such that the digital signature on the last page will indicate tampering if any data on any of the pages have been altered, but this can also be achieved by signing with ink and numbering all pages of the contract."
+7421,RSA
+7422,DSA
+7423,ECDSA
+7424,EdDSA
+7425,RSA with SHA
+7426,ECDSA with SHA
+7427,"ElGamal signature scheme as the predecessor to DSA, and variants Schnorr signature and Pointcheval–Stern signature algorithm"
+7428,Rabin signature algorithm
+7429,Pairing-based schemes such as BLS
+7430,NTRUSign is an example of a digital signature scheme based on hard lattice problems
+7431,Undeniable signatures
+7432,"Aggregate signatureru – a signature scheme that supports aggregation: Given n signatures on n messages from n users, it is possible to aggregate all these signatures into a single signature whose size is constant in the number of users. This single signature will convince the verifier that the n users did indeed sign the n original messages. A scheme by Mihir Bellare and Gregory Neven may be used with Bitcoin."
+7433,Signatures with efficient protocols – are signature schemes that facilitate efficient cryptographic protocols such as zero-knowledge proofs or secure computation.
+7434,Most digital signature schemes share the following goals regardless of cryptographic theory or legal provision:
+7435,"Only if all of these conditions are met will a digital signature actually be any evidence of who sent the message, and therefore of their assent to its contents. Legal enactment cannot change this reality of the existing engineering possibilities, though some such have not reflected this actuality."
+7436,"Legislatures, being importuned by businesses expecting to profit from operating a PKI, or by the technological avant-garde advocating new solutions to old problems, have enacted statutes and/or regulations in many jurisdictions authorizing, endorsing, encouraging, or permitting digital signatures and providing for  their legal effect. The first appears to have been in Utah in the United States, followed closely by the states Massachusetts and California. Other countries have also passed statutes or issued regulations in this area as well and the UN has had an active model law project for some time. These enactments  vary from place to place, have typically embodied expectations at variance  with the state of the underlying cryptographic engineering, and have had the net effect of confusing potential users and specifiers, nearly all of whom are not cryptographically knowledgeable."
+7437,"Adoption of technical standards for digital signatures have lagged behind much of the legislation, delaying a more or less unified engineering position on interoperability, algorithm choice, key lengths, and so on what the engineering is attempting to provide."
+7438,Some industries have established common interoperability standards for the use of digital signatures between members of the industry and with regulators.  These include the Automotive Network Exchange for the automobile industry and the SAFE-BioPharma Association for the healthcare industry.
+7439,"In several countries, a digital signature has a status somewhat like that of a traditional pen and paper signature, as in the
+1999 EU digital signature directive and 2014 EU follow-on legislation. Generally, these provisions mean that anything digitally signed legally binds the signer of the document to the terms therein. For that reason, it is often thought best to use separate key pairs for encrypting and signing. Using the encryption key pair, a person can engage in an encrypted conversation , but the encryption does not legally sign every message he or she sends. Only when both parties come to an agreement do they sign a contract with their signing keys, and only then are they legally bound by the terms of a specific document. After signing, the document can be sent over the encrypted link.  If a signing key is lost or compromised, it can be revoked to mitigate any future transactions.  If an encryption key is lost, a backup or key escrow should be utilized to continue viewing encrypted content.  Signing keys should never be backed up or escrowed unless the backup destination is securely encrypted."
+7440,"For technical reasons, an encryption scheme usually uses a pseudo-random encryption key generated by an algorithm. It is possible to decrypt the message without possessing the key but, for a well-designed encryption scheme, considerable computational resources and skills are required. An authorized recipient can easily decrypt the message with the key provided by the originator to recipients but not to unauthorized users."
+7441,"Historically, various forms of encryption have been used to aid in cryptography. Early encryption techniques were often used in military messaging. Since then, new techniques have emerged and become commonplace in all areas of modern computing. Modern encryption schemes use the concepts of public-key and symmetric-key. Modern encryption techniques ensure security because modern computers are inefficient at cracking the encryption."
+7442,"One of the earliest forms of encryption is symbol replacement, which was first found in the tomb of Khnumhotep II, who lived in 1900 BC Egypt. Symbol replacement encryption is “non-standard,” which means that the symbols require a cipher or key to understand. This type of early encryption was used throughout Ancient Greece and Rome for military purposes.  One of the most famous military encryption developments was the Caesar Cipher, which was a system in which a letter in normal text is shifted down a fixed number of positions down the alphabet to get the encoded letter. A message encoded with this type of encryption could be decoded with the fixed number on the Caesar Cipher."
+7443,"Around 800 AD, Arab mathematician Al-Kindi developed the technique of frequency analysis – which was an attempt to systematically crack Caesar ciphers. This technique looked at the frequency of letters in the encrypted message to determine the appropriate shift. This technique was rendered ineffective after the creation of the polyalphabetic cipher by Leon Battista Alberti in 1465, which incorporated different sets of languages. In order for frequency analysis to be useful, the person trying to decrypt the message would need to know which language the sender chose."
+7444,"Around 1790, Thomas Jefferson theorized a cipher to encode and decode messages in order to provide a more secure way of military correspondence. The cipher, known today as the Wheel Cipher or the Jefferson Disk, although never actually built, was theorized as a spool that could jumble an English message up to 36 characters. The message could be decrypted by plugging in the jumbled message to a receiver with an identical cipher."
+7445,"A similar device to the Jefferson Disk, the M-94, was developed in 1917 independently by US Army Major Joseph Mauborne. This device was used in U.S. military communications until 1942."
+7446,"In World War II, the Axis powers used a more advanced version of the M-94 called the Enigma Machine. The Enigma Machine was more complex because unlike the Jefferson Wheel and the M-94, each day the jumble of letters switched to a completely new combination. Each day's combination was only known by the Axis, so many thought the only way to break the code would be to try over 17,000 combinations within 24 hours. The Allies used computing power to severely limit the number of reasonable combinations they needed to check every day, leading to the breaking of the Enigma Machine."
+7447,"Today, encryption is used in the transfer of communication over the Internet for security and commerce. As computing power continues to increase, computer encryption is constantly evolving to prevent eavesdropping attacks. With one of the first ""modern"" cipher suites, DES, utilizing a 56-bit key with 72,057,594,037,927,936 possibilities being able to be cracked in 22 hours and 15 minutes by EFF's DES cracker in 1999, which used a brute-force method of cracking. Modern encryption standards often use stronger key sizes often 256, like AES, TwoFish, ChaCha20-Poly1305, Serpent. Cipher suites utilizing a 128-bit or higher key, like AES, will not be able to be brute-forced due to the total amount of keys of 3.4028237e+38 possibilities. The most likely option for cracking ciphers with high key size is to find vulnerabilities in the cipher itself, like inherent biases and backdoors. For example, RC4, a stream cipher, was cracked due to inherent biases and vulnerabilities in the cipher."
+7448,"In the context of cryptography, encryption serves as a mechanism to ensure confidentiality. Since data may be visible on the Internet, sensitive information such as passwords and personal communication may be exposed to potential interceptors. The process of encrypting and decrypting messages involves keys. The two main types of keys in cryptographic systems are symmetric-key and public-key ."
+7449,Many complex cryptographic algorithms often use simple modular arithmetic in their implementations.
+7450,"In symmetric-key schemes, the encryption and decryption keys are the same. Communicating parties must have the same key in order to achieve secure communication. The German Enigma Machine utilized a new symmetric-key each day for encoding and decoding messages."
+7451,"In public-key encryption schemes, the encryption key is published for anyone to use and encrypt messages. However, only the receiving party has access to the decryption key that enables messages to be read. Public-key encryption was first described in a secret document in 1973; beforehand, all encryption schemes were symmetric-key .: 478  Although published subsequently, the work of Diffie and Hellman was published in a journal with a large readership, and the value of the methodology was explicitly described. The method became known as the Diffie-Hellman key exchange."
+7452,"RSA  is another notable public-key cryptosystem. Created in 1978, it is still used today for applications involving digital signatures. Using number theory, the RSA algorithm selects two prime numbers, which help generate both the encryption and decryption keys."
+7453,"A publicly available public-key encryption application called Pretty Good Privacy  was written in 1991 by Phil Zimmermann, and distributed free of charge with source code. PGP was purchased by Symantec in 2010 and is regularly updated."
+7454,"Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. For example, the Computer Security Institute reported that in 2007, 71% of companies surveyed utilized encryption for some of their data in transit, and 53% utilized encryption for some of their data in storage. Encryption can be used to protect data ""at rest"", such as information stored on computers and storage devices . In recent years, there have been numerous reports of confidential data, such as customers' personal records, being exposed through loss or theft of laptops or backup drives; encrypting such files at rest helps protect them if physical security measures fail. Digital rights management systems, which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering , is another somewhat different example of using encryption on data at rest."
+7455,"Encryption is also used to protect data in transit, for example data being transferred via networks , mobile telephones, wireless microphones, wireless intercom systems, Bluetooth devices and bank automatic teller machines. There have been numerous reports of data in transit being intercepted in recent years. Data should also be encrypted when transmitted across networks in order to protect against eavesdropping of network traffic by unauthorized users."
+7456,"Conventional methods for permanently deleting data from a storage device involve overwriting the device's whole content with zeros, ones, or other patterns – a process which can take a significant amount of time, depending on the capacity and the type of storage medium. Cryptography offers a way of making the erasure almost instantaneous. This method is called crypto-shredding. An example implementation of this method can be found on iOS devices, where the cryptographic key is kept in a dedicated 'effaceable storage'. Because the key is stored on the same device, this setup on its own does not offer full privacy or security protection if an unauthorized person gains physical access to the device."
+7457,"Encryption is used in the 21st century to protect digital data and information systems. As computing power increased over the years, encryption technology has only become more advanced and secure. However, this advancement in technology has also exposed a potential limitation of today's encryption methods."
+7458,"The length of the encryption key is an indicator of the strength of the encryption method. For example, the original encryption key, DES , was 56 bits, meaning it had 2^56 combination possibilities. With today's computing power, a 56-bit key is no longer secure, being vulnerable to brute force attacks."
+7459,"Quantum computing utilizes properties of quantum mechanics in order to process large amounts of data simultaneously. Quantum computing has been found to achieve computing speeds thousands of times faster than today's supercomputers. This computing power presents a challenge to today's encryption technology. For example, RSA encryption utilizes the multiplication of very large prime numbers to create a semiprime number for its public key. Decoding this key without its private key requires this semiprime number to be factored, which can take a very long time to do with modern computers. It would take a supercomputer anywhere between weeks to months to factor in this key. However, quantum computing can use quantum algorithms to factor this semiprime number in the same amount of time it takes for normal computers to generate it. This would make all data protected by current public-key encryption vulnerable to quantum computing attacks. Other encryption techniques like elliptic curve cryptography and symmetric key encryption are also vulnerable to quantum computing."
+7460,"While quantum computing could be a threat to encryption security in the future, quantum computing as it currently stands is still very limited. Quantum computing currently is not commercially available, cannot handle large amounts of code, and only exists as computational devices, not computers. Furthermore, quantum computing advancements will be able to be utilized in favor of encryption as well. The National Security Agency  is currently preparing post-quantum encryption standards for the future. Quantum encryption promises a level of security that will be able to counter the threat of quantum computing."
+7461,"Encryption is an important tool but is not sufficient alone to ensure the security or privacy of sensitive information throughout its lifetime. Most applications of encryption protect information only at rest or in transit, leaving sensitive data in clear text and potentially vulnerable to improper disclosure during processing, such as by a cloud service for example. Homomorphic encryption and secure multi-party computation are emerging techniques to compute encrypted data; these techniques are general and Turing complete but incur high computational and/or communication costs."
+7462,"In response to encryption of data at rest, cyber-adversaries have developed new types of attacks. These more recent threats to encryption of data at rest include cryptographic attacks, stolen ciphertext attacks, attacks on encryption keys, insider attacks, data corruption or integrity attacks, data destruction attacks, and ransomware attacks. Data fragmentation and active defense data protection technologies attempt to counter some of these attacks, by distributing, moving, or mutating ciphertext so it is more difficult to identify, steal, corrupt, or destroy."
+7463,"The question of balancing the need for national security with the right to privacy has been debated for years, since encryption has become critical in today's digital society. The modern encryption debate started around the '90s when US government tried to ban cryptography because, according to them, it would threaten national security. The debate is polarized around two opposing views. Those who see strong encryption as a problem making it easier for criminals to hide their illegal acts online and others who argue that encryption keep digital communications safe. The debate heated up in 2014, when Big Tech like Apple and Google set encryption by default in their devices. This was the start of a series of controversies that puts governments, companies and internet users at stake."
+7464,"Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code  or a digital signature usually done by a hashing algorithm or a PGP signature. Authenticated encryption algorithms are designed to provide both encryption and integrity protection together. Standards for cryptographic software and hardware to perform encryption are widely available, but successfully using encryption to ensure security may be a challenging problem. A single error in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See for example traffic analysis, TEMPEST, or Trojan horse."
+7465,"Integrity protection mechanisms such as MACs and digital signatures must be applied to the ciphertext when it is first created, typically on the same device used to compose the message, to protect a message end-to-end along its full transmission path; otherwise, any node between the sender and the encryption agent could potentially tamper with it. Encrypting at the time of creation is only secure if the encryption device itself has correct keys and has not been tampered with. If an endpoint device has been configured to trust a root certificate that an attacker controls, for example, then the attacker can both inspect and tamper with encrypted data by performing a man-in-the-middle attack anywhere along the message's path. The common practice of TLS interception by network operators represents a controlled and institutionally sanctioned form of such an attack, but countries have also attempted to employ such attacks as a form of control and censorship."
+7466,"Even when encryption correctly hides a message's content and it cannot be tampered with at rest or in transit, a message's length is a form of metadata that can still leak sensitive information about the message. For example, the well-known CRIME and BREACH attacks against HTTPS were side-channel attacks that relied on information leakage via the length of encrypted content. Traffic analysis is a broad class of techniques that often employs message lengths to infer sensitive implementation about traffic flows by aggregating information about a large number of messages."
+7467,"Padding a message's payload before encrypting it can help obscure the cleartext's true length, at the cost of increasing the ciphertext's size and introducing or increasing bandwidth overhead. Messages may be padded randomly or deterministically, with each approach having different tradeoffs. Encrypting and padding messages to form padded uniform random blobs or PURBs is a practice guaranteeing that the cipher text leaks no metadata about its cleartext's content, and leaks asymptotically minimal 
+  
+    
+      
+        O
+        
+      
+    
+    {\displaystyle O}
+  
+ information via its length."
+7468,"Read/write conflicts, commonly termed interlocking in accessing the same shared memory location simultaneously are resolved by one of the following strategies:"
+7469,"Here, E and C stand for 'exclusive' and 'concurrent' respectively. The read causes no discrepancies while the concurrent write is further defined as:"
+7470,Several simplifying assumptions are made while considering the development of algorithms for PRAM. They are:
+7471,"These kinds of algorithms are useful for understanding the exploitation of concurrency, dividing the original problem into similar sub-problems and solving them in parallel. The introduction of the formal 'P-RAM' model in Wyllie's 1979 thesis had the aim of quantifying analysis of parallel algorithms in a way analogous to the Turing Machine. The analysis focused on a MIMD model of programming using a CREW model but showed that many variants, including implementing a CRCW model and implementing on an SIMD machine, were possible with only constant overhead."
+7472,"PRAM algorithms cannot be parallelized with the combination of CPU and dynamic random-access memory  because DRAM does not allow concurrent access to a single bank ; but they can be implemented in hardware or read/write to the internal static random-access memory  blocks of a field-programmable gate array , it can be done using a CRCW algorithm."
+7473,"However, the test for practical relevance of PRAM  algorithms depends on whether their cost model provides an effective abstraction of some computer; the structure of that computer can be quite different than the abstract model. The knowledge of the layers of software and hardware that need to be inserted is beyond the scope of this article. But, articles such as Vishkin  demonstrate how a PRAM-like abstraction can be supported by the explicit multi-threading  paradigm and articles such as Caragea & Vishkin  demonstrate that a PRAM algorithm for the maximum flow problem can provide strong speedups relative to the fastest serial program for the same problem. The article Ghanim, Vishkin & Barua  demonstrated that PRAM algorithms as-is can achieve competitive performance even without any additional effort to cast them as multi-threaded programs on XMT."
+7474,"This is an example of SystemVerilog code which finds the maximum value in the array in only 2 clock cycles. It compares all the combinations of the elements in the array at the first clock, and merges the result at the second clock. It uses CRCW memory; m <= 1 and maxNo <= data are written concurrently. The concurrency causes no conflicts because the algorithm guarantees that the same value is written to the same memory. This code can be run on FPGA hardware."
+7475,"While the variety of existing process calculi is very large , there are several features that all process calculi have in common:"
+7476,"To define a process calculus, one starts with a set of names  whose purpose is to provide means of communication.  In many implementations, channels have rich internal structure to improve efficiency, but this is abstracted away in  most theoretic models.  In addition to names, one needs a means to form new processes from old ones. The basic operators, always present in some form or other, allow:"
+7477,"Parallel composition of two processes 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ and 
+  
+    
+      
+        
+          
+            Q
+          
+        
+      
+    
+    {\displaystyle {\mathit {Q}}}
+  
+, usually written 
+  
+    
+      
+        P
+        |
+        Q
+      
+    
+    {\displaystyle P\vert Q}
+  
+, is the key primitive distinguishing the process calculi from sequential models of computation. Parallel composition allows computation in 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ and 
+  
+    
+      
+        
+          
+            Q
+          
+        
+      
+    
+    {\displaystyle {\mathit {Q}}}
+  
+ to proceed simultaneously and independently. But it also allows interaction, that is synchronisation and flow of information from 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ to 
+  
+    
+      
+        
+          
+            Q
+          
+        
+      
+    
+    {\displaystyle {\mathit {Q}}}
+  
+  on a channel shared by both. Crucially, an agent or process can be connected to more than one channel at a time."
+7478,"Channels may be synchronous or asynchronous. In the case of a synchronous channel, the agent sending a message waits until another agent has received the message. Asynchronous channels do not require any such synchronization.  In some process calculi  channels themselves can be sent in messages through  channels, allowing the topology of process interconnections to change. Some process calculi also allow channels to be created during the execution of a computation."
+7479,"Interaction can be  a directed flow of information.  That is, input and output can be distinguished as dual interaction primitives. Process calculi that make such distinctions typically define an input operator 
+      
+    
+    {\displaystyle x}
+  
+) and an output operator , both of which name an interaction point  that is used to synchronise with a dual interaction primitive."
+7480,"Should information be exchanged, it will flow from the outputting to the inputting process. The output primitive will specify the data to be sent. In 
+  
+    
+      
+        x
+        ⟨
+        y
+        ⟩
+      
+    
+    {\displaystyle x\langle y\rangle }
+  
+, this data is 
+  
+    
+      
+        y
+      
+    
+    {\displaystyle y}
+  
+.  Similarly, if an input expects to receive data, one or more bound variables will act as place-holders to be substituted by data, when it arrives. In 
+  
+    
+      
+        x
+        
+      
+    
+    {\displaystyle x}
+  
+, 
+  
+    
+      
+        v
+      
+    
+    {\displaystyle v}
+  
+ plays that role. The choice of the kind of data that can be exchanged in an interaction is one of the key features that distinguishes different process calculi."
+7481,"Sometimes interactions must be temporally ordered. For example, it might be desirable to specify algorithms such as: first receive some data on 
+  
+    
+      
+        
+          
+            x
+          
+        
+      
+    
+    {\displaystyle {\mathit {x}}}
+  
+ and then send that data on 
+  
+    
+      
+        
+          
+            y
+          
+        
+      
+    
+    {\displaystyle {\mathit {y}}}
+  
+.  Sequential composition can be used for such purposes. It is well known from other models of computation. In process calculi, the sequentialisation operator is usually integrated with input or output, or both. For example, the process 
+  
+    
+      
+        x
+        
+        ⋅
+        P
+      
+    
+    {\displaystyle x\cdot P}
+  
+ will wait for an input on 
+  
+    
+      
+        
+          
+            x
+          
+        
+      
+    
+    {\displaystyle {\mathit {x}}}
+  
+. Only when this input has occurred will the process 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ be activated, with the received data through 
+  
+    
+      
+        
+          
+            x
+          
+        
+      
+    
+    {\displaystyle {\mathit {x}}}
+  
+ substituted for identifier 
+  
+    
+      
+        
+          
+            v
+          
+        
+      
+    
+    {\displaystyle {\mathit {v}}}
+  
+."
+7482,"The key operational reduction rule, containing the computational essence of process calculi, can be given solely in terms of parallel composition, sequentialization, input, and output. The details of this reduction vary among the calculi, but the essence remains roughly the same. The reduction rule is:"
+7483,The interpretation to this reduction rule is:
+7484,"The class of processes that 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ is allowed to range over as the continuation of the output operation substantially influences the properties of the calculus."
+7485,"Processes do not limit the number of connections that can be made at a given interaction point. But interaction points allow interference . For the
+synthesis of compact, minimal and compositional systems, the ability to restrict interference is crucial. Hiding operations allow control of the connections made between interaction points when composing
+agents in parallel. Hiding can be denoted in a variety of ways. For example, in the π-calculus the hiding of a name 
+  
+    
+      
+        
+          
+            x
+          
+        
+      
+    
+    {\displaystyle {\mathit {x}}}
+  
+ in 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ can be expressed as 
+  
+    
+      
+        
+        P
+      
+    
+    {\displaystyle P}
+  
+, while in CSP it might be written as 
+  
+    
+      
+        P
+        ∖
+        {
+        x
+        }
+      
+    
+    {\displaystyle P\setminus \{x\}}
+  
+."
+7486,"The operations presented so far describe only finite interaction and are consequently insufficient for full computability, which includes non-terminating behaviour. Recursion and replication are operations that allow finite descriptions of infinite behaviour.  Recursion is well known from the sequential world. Replication 
+  
+    
+      
+        !
+        P
+      
+    
+    {\displaystyle !P}
+  
+ can be understood as abbreviating the parallel composition of a countably infinite number of 
+  
+    
+      
+        
+          
+            P
+          
+        
+      
+    
+    {\displaystyle {\mathit {P}}}
+  
+ processes:"
+7487,Process calculi generally also include a null process  which has no interaction points. It is utterly inactive and its sole purpose is to act as the inductive anchor on top of which more interesting processes can be generated.
+7488,Process algebra has been studied for discrete time and continuous time .
+7489,"In the first half of the 20th century, various formalisms were proposed to capture the informal concept of a computable function, with μ-recursive functions, Turing machines  and the lambda calculus possibly being the best-known  examples today.  The surprising fact that they are essentially equivalent, in the sense that they are all encodable into each other, supports the Church-Turing thesis.  Another shared feature is more rarely commented on: they all are most readily understood as models of sequential computation. The subsequent consolidation of computer science required a more subtle formulation of the notion of computation, in particular explicit representations of concurrency and communication. Models of concurrency such as the process calculi, Petri nets in 1962, and the actor model in 1973 emerged from this line of inquiry."
+7490,"Research on process calculi began in earnest with Robin Milner's seminal work on the Calculus of Communicating Systems  during the period from 1973 to 1980. C.A.R. Hoare's Communicating Sequential Processes  first appeared in 1978, and was subsequently developed into a full-fledged process calculus during the early 1980s. There was much cross-fertilization of ideas between CCS and CSP as they developed. In 1982 Jan Bergstra and Jan Willem Klop began work on what came to be known as the Algebra of Communicating Processes , and introduced the term process algebra to describe their work. CCS, CSP, and ACP constitute the three major branches of the process calculi family: the majority of the other process calculi can trace their roots to one of these three calculi."
+7491,Various process calculi have been studied and not all of them fit the paradigm sketched here. The most prominent example may be the ambient calculus. This is to be expected as process calculi are an active field of study. Currently research on process calculi focuses on the following problems.
+7492,The ideas behind process algebra have given rise to several tools including:
+7493,CADP
+7494,Concurrency Workbench
+7495,mCRL2 toolset
+7496,"The history monoid is the free object that is generically able to represent the histories of individual communicating processes. A process calculus is then a formal language imposed on a history monoid in a consistent fashion. That is, a history monoid can only record a sequence of events, with synchronization, but does not specify the allowed state transitions. Thus, a process calculus is to a history monoid what a formal language is to a free monoid ."
+7497,"The use of channels for communication is one of the features distinguishing the process calculi from other models of concurrency, such as Petri nets and the actor model . One of the fundamental motivations for including channels in the process calculi was to enable certain algebraic techniques, thereby making it easier to reason about processes algebraically."
+7498,"Like industry standards such as UML activity diagrams, Business Process Model and Notation, and event-driven process chains, Petri nets offer a graphical notation for stepwise processes that include choice, iteration, and concurrent execution. Unlike these standards, Petri nets have an exact mathematical definition of their execution semantics, with a well-developed mathematical theory for process analysis."
+7499,"The German computer scientist Carl Adam Petri, after whom such structures are named, analyzed Petri nets extensively in his 1962 dissertation Kommunikation mit Automaten."
+7500,"A Petri net consists of places, transitions, and arcs. Arcs run from a place to a transition or vice versa, never between places or between transitions. The places from which an arc runs to a transition are called the input places of the transition; the places to which arcs run from a transition are called the output places of the transition."
+7501,"Graphically, places in a Petri net may contain a discrete number of marks called tokens.  Any distribution of tokens over the places will represent a configuration of the net called a marking.  In an abstract sense relating to a Petri net diagram, a transition of a Petri net may fire if it is enabled, i.e. there are sufficient tokens in all of its input places; when the transition fires, it consumes the required input tokens, and creates tokens in its output places. A firing is atomic, i.e. a single non-interruptible step."
+7502,"Unless an execution policy  is defined, the execution of Petri nets is nondeterministic: when multiple transitions are enabled at the same time, they will fire in any order."
+7503,"Since firing is nondeterministic, and multiple tokens may be present anywhere in the net , Petri nets are well suited for modeling the concurrent behavior of distributed systems."
+7504,Petri nets are state-transition systems that extend a class of nets called elementary nets.
+7505,"Definition 1. A net is a tuple 
+  
+    
+      
+        N
+        =
+        
+      
+    
+    {\displaystyle N=}
+  
+ where"
+7506,"Definition 2. Given a net N = , a configuration is a set C so that C ⊆ P."
+7507,Definition 3. An elementary net is a net of the form EN =  where
+7508,"Definition 4. A Petri net is a net of the form PN = , which extends the elementary net so that"
+7509,"If a Petri net is equivalent to an elementary net, then Z can be the countable set {0,1} and those elements in P that map to 1 under M form a configuration. Similarly, if a Petri net is not an elementary net, then the multiset M can be interpreted as representing a non-singleton set of configurations. In this respect, M extends the concept of configuration for elementary nets to Petri nets."
+7510,"In the diagram of a Petri net , places are conventionally depicted with circles, transitions with long narrow rectangles and arcs as one-way arrows that show connections of places to transitions or transitions to places. If the diagram were of an elementary net, then those places in a configuration would be conventionally depicted as circles, where each circle encompasses a single dot called a token. In the given diagram of a Petri net , the place circles may encompass more than one token to show the number of times a place appears in a configuration. The configuration of tokens distributed over an entire Petri net diagram is called a marking."
+7511,"In the top figure , the place p1 is an input place of transition t; whereas, the place p2 is an output place to the same transition. Let PN0  be a Petri net with a marking configured M0, and PN1  be a Petri net with a marking configured M1. The configuration of PN0 enables transition t through the property that all input places have sufficient number of tokens  ""equal to or greater"" than the multiplicities on their respective arcs to t. Once and only once a transition is enabled will the transition fire. In this example, the firing of transition t generates a map that has the marking configured M1 in the image of M0 and results in Petri net PN1, seen in the bottom figure. In the diagram, the firing rule for a transition can be characterised by subtracting a number of tokens from its input places equal to the multiplicity of the respective input arcs and accumulating a new number of tokens at the output places equal to the multiplicity of the respective output arcs."
+7512,"Remark 1. The precise meaning of ""equal to or greater"" will depend on the precise algebraic properties of addition being applied on Z in the firing rule, where subtle variations on the algebraic properties can lead to other classes of Petri nets; for example, algebraic Petri nets."
+7513,The following formal definition is loosely based on .  Many alternative definitions exist.
+7514,"A Petri net graph  is a 3-tuple 
+  
+    
+      
+        
+      
+    
+    {\displaystyle }
+  
+, where"
+7515,"The flow relation is the set of arcs: 
+  
+    
+      
+        F
+        =
+        {
+        
+        ∣
+        W
+        
+        >
+        0
+        }
+      
+    
+    {\displaystyle F=\{\mid W>0\}}
+  
+.  In many textbooks, arcs can only have multiplicity 1. These texts often define Petri nets using F instead of W.  When using this convention, a Petri net graph is a bipartite directed graph 
+  
+    
+      
+        
+      
+    
+    {\displaystyle }
+  
+ with node partitions S and T."
+7516,"The preset of a transition t is the set of its input places: 
+  
+    
+      
+        
+          
+
+          
+          
+            ∙
+          
+        
+        t
+        =
+        {
+        s
+        ∈
+        S
+        ∣
+        W
+        
+        >
+        0
+        }
+      
+    
+    {\displaystyle {}^{\bullet }t=\{s\in S\mid W>0\}}
+  
+;
+its postset is the set of its output places: 
+  
+    
+      
+        
+          t
+          
+            ∙
+          
+        
+        =
+        {
+        s
+        ∈
+        S
+        ∣
+        W
+        
+        >
+        0
+        }
+      
+    
+    {\displaystyle t^{\bullet }=\{s\in S\mid W>0\}}
+  
+. Definitions of pre- and postsets of places are analogous."
+7517,"A marking of a Petri net  is a multiset of its places, i.e., a mapping 
+  
+    
+      
+        M
+        :
+        S
+        →
+        
+          N
+        
+      
+    
+    {\displaystyle M:S\to \mathbb {N} }
+  
+.  We say the marking assigns to each place a number of tokens."
+7518,"A Petri net  is a 4-tuple 
+  
+    
+      
+        
+      
+    
+    {\displaystyle }
+  
+, where"
+7519,In words
+7520,We are generally interested in what may happen when transitions may continually fire in arbitrary order.
+7521,"We say that a marking M' is reachable from a marking M in one step if 
+  
+    
+      
+        M
+        
+          
+            ⟶
+            G
+          
+        
+        
+          M
+          ′
+        
+      
+    
+    {\displaystyle M{\underset {G}{\longrightarrow }}M'}
+  
+; we say that it is reachable from M if 
+  
+    
+      
+        M
+        
+          
+            
+              ⟶
+              G
+            
+            ∗
+          
+        
+        
+          M
+          ′
+        
+      
+    
+    {\displaystyle M{\overset {*}{\underset {G}{\longrightarrow }}}M'}
+  
+, where 
+  
+    
+      
+        
+          
+            
+              ⟶
+              G
+            
+            ∗
+          
+        
+      
+    
+    {\displaystyle {\overset {*}{\underset {G}{\longrightarrow }}}}
+  
+ is the reflexive transitive closure of 
+  
+    
+      
+        
+          
+            ⟶
+            G
+          
+        
+      
+    
+    {\displaystyle {\underset {G}{\longrightarrow }}}
+  
+; that is, if it is reachable in 0 or more steps."
+7522,"For a  Petri net 
+  
+    
+      
+        N
+        =
+        
+      
+    
+    {\displaystyle N=}
+  
+, we are interested in the firings that can be performed starting with the initial marking 
+  
+    
+      
+        
+          M
+          
+            0
+          
+        
+      
+    
+    {\displaystyle M_{0}}
+  
+.  Its set of reachable markings is the set
+
+  
+    
+      
+        R
+        
+         
+        
+          
+            
+              
+                =
+              
+              
+                D
+              
+            
+          
+        
+         
+        
+          {
+          
+            
+              M
+              ′
+            
+            
+              
+                |
+              
+            
+            
+              M
+              
+                0
+              
+            
+            
+              
+                →
+                
+                  
+                    
+                  
+                
+                
+                  ∗
+                
+              
+            
+            
+              M
+              ′
+            
+          
+          }
+        
+      
+    
+    {\displaystyle R\ {\stackrel {D}{=}}\ \left\{M'{\Bigg |}M_{0}{\xrightarrow{*}}M'\right\}}"
+7523,"The reachability graph of N is the transition relation 
+  
+    
+      
+        
+          
+            ⟶
+            G
+          
+        
+      
+    
+    {\displaystyle {\underset {G}{\longrightarrow }}}
+  
+ restricted to its reachable markings 
+  
+    
+      
+        R
+        
+      
+    
+    {\displaystyle R}
+  
+.  It is the state space of the net."
+7524,"A firing sequence for a Petri net with graph G and initial marking 
+  
+    
+      
+        
+          M
+          
+            0
+          
+        
+      
+    
+    {\displaystyle M_{0}}
+  
+ is a sequence of transitions 
+  
+    
+      
+        
+          
+            
+              σ
+              →
+            
+          
+        
+        =
+        ⟨
+        
+          t
+          
+            1
+          
+        
+        ⋯
+        
+          t
+          
+            n
+          
+        
+        ⟩
+      
+    
+    {\displaystyle {\vec {\sigma }}=\langle t_{1}\cdots t_{n}\rangle }
+  
+ such that 
+  
+    
+      
+        
+          M
+          
+            0
+          
+        
+        
+          
+            ⟶
+            
+              G
+              ,
+              
+                t
+                
+                  1
+                
+              
+            
+          
+        
+        
+          M
+          
+            1
+          
+        
+        ∧
+        ⋯
+        ∧
+        
+          M
+          
+            n
+            −
+            1
+          
+        
+        
+          
+            ⟶
+            
+              G
+              ,
+              
+                t
+                
+                  n
+                
+              
+            
+          
+        
+        
+          M
+          
+            n
+          
+        
+      
+    
+    {\displaystyle M_{0}{\underset {G,t_{1}}{\longrightarrow }}M_{1}\wedge \cdots \wedge M_{n-1}{\underset {G,t_{n}}{\longrightarrow }}M_{n}}
+  
+.  The set of firing sequences is denoted as 
+  
+    
+      
+        L
+        
+      
+    
+    {\displaystyle L}
+  
+."
+7525,"A common variation is to disallow arc multiplicities and replace the bag of arcs W with a simple set, called the flow relation, 
+  
+    
+      
+        F
+        ⊆
+        
+        ∪
+        
+      
+    
+    {\displaystyle F\subseteq \cup }
+  
+.
+This does not limit expressive power as both can represent each other."
+7526,"Another common variation, e.g. in Desel and Juhás , is to allow capacities to be defined on places. This is discussed under extensions below."
+7527,"The markings of a Petri net 
+  
+    
+      
+        
+      
+    
+    {\displaystyle }
+  
+ can be regarded as vectors of non-negative integers of length 
+  
+    
+      
+        
+          |
+        
+        S
+        
+          |
+        
+      
+    
+    {\displaystyle |S|}
+  
+."
+7528,Then their difference
+7529,It must be required that w is a firing sequence; allowing arbitrary sequences of transitions will generally produce a larger set.
+7530,Meseguer and Montanari considered a kind of symmetric monoidal categories known as Petri categories.
+7531,"One thing that makes Petri nets interesting is that they provide a balance between modeling power and analyzability: many things one would like to know about concurrent systems can be automatically determined for Petri nets, although some of those things are very expensive to determine in the general case. Several subclasses of Petri nets have been studied that can still model interesting classes of concurrent systems, while these determinations become easier."
+7532,"An overview of such decision problems, with decidability and complexity results for Petri nets and some subclasses, can be found in Esparza and Nielsen ."
+7533,"It is a matter of walking the reachability-graph defined above, until either the requested-marking is reached or it can no longer be found. This is harder than it may seem at first: the reachability graph is generally infinite, and it isn't easy to determine when it is safe to stop."
+7534,"In fact, this problem was shown to be EXPSPACE-hard years before it was shown to be decidable at all . Papers continue to be published on how to do it efficiently. In 2018, Czerwiński et al. improved the lower bound and showed that the problem is not ELEMENTARY. In 2021, this problem was shown to be non-primitive recursive, independently by Jerome Leroux 
+and by Wojciech Czerwiński and Łukasz Orlikowski. These results thus close the long-standing complexity gap."
+7535,"While reachability seems to be a good tool to find erroneous states, for practical problems the constructed graph usually has far too many states to calculate. To alleviate this problem, linear temporal logic is usually used in conjunction with the tableau method to prove that such states cannot be reached. Linear temporal logic uses the semi-decision technique to find if indeed a state can be reached, by finding a set of necessary conditions for the state to be reached then proving that those conditions cannot be satisfied."
+7536,"A place in a Petri net is called k-bound if it does not contain more than k tokens in all reachable markings, including the initial marking; it is said to be safe if it is 1-bounded; it is bounded if it is k-bounded for some k."
+7537,"A  Petri net is called k-bounded, safe, or bounded when all of its places are.
+A Petri net  is called  bounded if it is bounded for every possible initial marking."
+7538,A Petri net is bounded if and only if its reachability graph is finite.
+7539,"Boundedness is decidable by looking at covering, by constructing the Karp–Miller Tree."
+7540,"It can be useful to explicitly impose a bound on places in a given net.
+This can be used to model limited system resources."
+7541,"For example, if in the net N, both places are assigned capacity 2, we obtain a Petri net with place capacities, say N2; its reachability graph is displayed on the right."
+7542,"Alternatively, places can be made bounded by extending the net. To be exact,
+a place can be made k-bounded by adding a ""counter-place"" with flow opposite to that of the place, and adding tokens to make the total in both places k."
+7543,"As well as for discrete events, there are Petri nets for continuous and hybrid discrete-continuous processes that are useful in discrete, continuous and hybrid control theory, and related to discrete, continuous and hybrid automata."
+7544,"There are many extensions to Petri nets. Some of them are completely backwards-compatible  with the original Petri net, some add properties that cannot be modelled in the original Petri net formalism . Although backwards-compatible models do not extend the computational power of Petri nets, they may have more succinct representations and may be more convenient for modeling. Extensions that cannot be transformed into Petri nets are sometimes very powerful, but usually lack the full range of mathematical tools available to analyse ordinary Petri nets."
+7545,"The term  high-level Petri net is used for many Petri net formalisms that extend the basic P/T net formalism; this includes coloured Petri nets, hierarchical Petri nets such as Nets within Nets, and all other extensions sketched in this section. The term is also used specifically for the type of coloured nets supported by CPN Tools."
+7546,A short list of possible extensions follows:
+7547,"There are many more extensions to Petri nets, however, it is important to keep in mind, that as the complexity of the net increases in terms of extended properties, the harder it is to use standard tools to evaluate certain properties of the net. For this reason, it is a good idea to use the most simple net type possible for a given modelling task."
+7548,"Instead of extending the Petri net formalism, we can also look at restricting it, and look at particular types of Petri nets, obtained by restricting the syntax in a particular way.  Ordinary Petri nets are the nets where all arc weights are 1.  Restricting further, the following types of ordinary Petri nets are commonly used and studied:"
+7549,"Workflow nets  are a subclass of Petri nets intending to model the workflow of process activities. 
+The WF-net transitions are assigned to tasks or activities, and places are assigned to the pre/post conditions.
+The WF-nets have additional structural and operational requirements, mainly the addition of a single input  place with no previous transitions, and output place  with no following transitions. Accordingly, start and termination markings can be defined that represent the process status."
+7550,"WF-nets have the soundness property, indicating that a process with a start marking of k tokens in its source place, can reach the termination state marking with k tokens in its sink place . Additionally, all the transitions in the process could fire . 
+A general sound  WF-net is defined as being k-sound for every k > 0."
+7551,A directed path in the Petri net is defined as the sequence of nodes  linked by the directed arcs. An elementary path includes every node in the sequence only once.
+7552,"A well-handled Petri net is a net in which there are no fully distinct elementary paths between a place and a transition , i.e., if there are two paths between the pair of nodes then these paths share a node.
+An acyclic well-handled WF-net is sound ."
+7553,Extended WF-net is a Petri net that is composed of a WF-net with additional transition t . The sink place is connected as the input place of transition t and the source place as its output place. Firing of the transition causes iteration of the process .
+7554,"WRI  WF-net, is an extended acyclic well-handled WF-net.  
+WRI-WF-net can be built as composition of nets, i.e., replacing a transition within a WRI-WF-net with a subnet which is a WRI-WF-net. The result is also WRI-WF-net. WRI-WF-nets are G-sound, therefore by using only WRI-WF-net building blocks, one can get WF-nets that are G-sound by construction."
+7555,"The design structure matrix  can model process relations, and be utilized for process planning. The DSM-nets are realization of DSM-based plans into workflow processes by Petri nets, and are equivalent to WRI-WF-nets. The DSM-net construction process ensures the soundness property of the resulting net."
+7556,"Other ways of modelling concurrent computation have been proposed, including vector addition systems, communicating finite-state machines, Kahn process networks, process algebra, the actor model, and trace theory. Different models provide tradeoffs of concepts such as compositionality, modularity, and locality."
+7557,An approach to relating some of these models of concurrency is proposed in the chapter by Winskel and Nielsen.
+7558,"The components of a distributed system communicate and coordinate their actions by passing messages to one another in order to achieve a common goal. Three significant challenges of distributed systems are: maintaining concurrency of components, overcoming the lack of a global clock, and managing the independent failure of components. When a component of one system fails, the entire system does not fail. Examples of distributed systems vary from SOA-based systems to massively multiplayer online games to peer-to-peer applications."
+7559,"A computer program that runs within a distributed system is called  a distributed program, and distributed programming is the process of writing such programs. There are many different types of implementations for the message passing mechanism, including pure HTTP, RPC-like connectors and message queues."
+7560,"Distributed computing also refers to the use of distributed systems to solve computational problems. In distributed computing, a problem is divided into many tasks, each of which is solved by one or more computers, which communicate with each other via message passing."
+7561,"The word distributed in terms such as ""distributed system"", ""distributed programming"", and ""distributed algorithm"" originally referred to computer networks where individual computers were physically distributed within some geographical area. The terms are nowadays used in a much wider sense, even referring to autonomous processes that run on the same physical computer and interact with each other by message passing."
+7562,"While there is no single definition of a distributed system, the following defining properties are commonly used as:"
+7563,"A distributed system may have a common goal, such as solving a large computational problem; the user then perceives the collection of autonomous processors as a unit. Alternatively, each computer may have its own user with individual needs, and the purpose of the distributed system is to coordinate the use of shared resources or provide communication services to the users."
+7564,Other typical properties of distributed systems include the following:
+7565,"Distributed systems are groups of networked computers which share a common goal for their work.
+The terms ""concurrent computing"", ""parallel computing"", and ""distributed computing"" have much overlap, and no clear distinction exists between them. The same system may be characterized both as ""parallel"" and ""distributed""; the processors in a typical distributed system run concurrently in parallel. Parallel computing may be seen as a particularly tightly coupled form of distributed computing, and distributed computing may be seen as a loosely coupled form of parallel computing. Nevertheless, it is possible to roughly classify concurrent systems as ""parallel"" or ""distributed"" using the following criteria:"
+7566,"The figure on the right illustrates the difference between distributed and parallel systems. Figure  is a schematic view of a typical distributed system; the system is represented as a network topology in which each node is a computer and each line connecting the nodes is a communication link. Figure  shows the same distributed system in more detail: each computer has its own local memory, and information can be exchanged only by passing messages from one node to another by using the available communication links. Figure  shows a parallel system in which each processor has a direct access to a shared memory."
+7567,"The situation is further complicated by the traditional uses of the terms parallel and distributed algorithm that do not quite match the above definitions of parallel and distributed systems . Nevertheless, as a rule of thumb, high-performance parallel computation in a shared-memory multiprocessor uses parallel algorithms while the coordination of a large-scale distributed system uses distributed algorithms."
+7568,"The use of concurrent processes which communicate through message-passing has its roots in operating system architectures studied in the 1960s. The first widespread distributed systems were local-area networks such as Ethernet, which was invented in the 1970s."
+7569,"ARPANET, one of the predecessors of the Internet, was introduced in the late 1960s, and ARPANET e-mail was invented in the early 1970s. E-mail became the most successful application of ARPANET, and it is probably the earliest example of a large-scale distributed application. In addition to ARPANET , other early worldwide computer networks included Usenet and FidoNet from the 1980s, both of which were used to support distributed discussion systems."
+7570,"The study of distributed computing became its own branch of computer science in the late 1970s and early 1980s. The first conference in the field, Symposium on Principles of Distributed Computing , dates back to 1982, and its counterpart International Symposium on Distributed Computing  was first held in Ottawa in 1985 as the International Workshop on Distributed Algorithms on Graphs."
+7571,"Various hardware and software architectures are used for distributed computing. At a lower level, it is necessary to interconnect multiple CPUs with some sort of network, regardless of whether that network is printed onto a circuit board or made up of loosely coupled devices and cables. At a higher level, it is necessary to interconnect processes running on those CPUs with some sort of communication system."
+7572,Whether these CPUs share resources or not determines a first distinction between three types of architecture:
+7573,Shared memory
+7574,Shared disk
+7575,Shared nothing.
+7576,"Distributed programming typically falls into one of several basic architectures: client–server, three-tier, n-tier, or peer-to-peer; or categories: loose coupling, or tight coupling."
+7577,"Another basic aspect of distributed computing architecture is the method of communicating and coordinating work among concurrent processes. Through various message passing protocols, processes may communicate directly with one another, typically in a main/sub relationship. Alternatively, a ""database-centric"" architecture can enable distributed computing to be done without any form of direct inter-process communication, by utilizing a shared database. Database-centric architecture in particular provides relational processing analytics in a schematic architecture allowing for live environment relay. This enables distributed computing functions both within and beyond the parameters of a networked database."
+7578,Reasons for using distributed systems and distributed computing may include:
+7579,Examples of distributed systems and applications of distributed computing include the following:
+7580,"Many tasks that we would like to automate by using a computer are of question–answer type: we would like to ask a question and the computer should produce an answer. In theoretical computer science, such tasks are called computational problems. Formally, a computational problem consists of instances together with a solution for each instance. Instances are questions that we can ask, and solutions are desired answers to these questions."
+7581,"Theoretical computer science seeks to understand which computational problems can be solved by using a computer  and how efficiently . Traditionally, it is said that a problem can be solved by using a computer if we can design an algorithm that produces a correct solution for any given instance. Such an algorithm can be implemented as a computer program that runs on a general-purpose computer: the program reads a problem instance from input, performs some computation, and produces the solution as output. Formalisms such as random-access machines or universal Turing machines can be used as abstract models of a sequential general-purpose computer executing such an algorithm."
+7582,"The field of concurrent and distributed computing studies similar questions in the case of either multiple computers, or a computer that executes a network of interacting processes: which computational problems can be solved in such a network and how efficiently? However, it is not at all obvious what is meant by ""solving a problem"" in the case of a concurrent or distributed system: for example, what is the task of the algorithm designer, and what is the concurrent or distributed equivalent of a sequential general-purpose computer?"
+7583,"The discussion below focuses on the case of multiple computers, although many of the issues are the same for concurrent processes running on a single computer."
+7584,Three viewpoints are commonly used:
+7585,"In the case of distributed algorithms, computational problems are typically related to graphs. Often the graph that describes the structure of the computer network is the problem instance. This is illustrated in the following example."
+7586,Consider the computational problem of finding a coloring of a given graph G. Different fields might take the following approaches:
+7587,"While the field of parallel algorithms has a different focus than the field of distributed algorithms, there is much interaction between the two fields. For example, the Cole–Vishkin algorithm for graph coloring was originally presented as a parallel algorithm, but the same technique can also be used directly as a distributed algorithm."
+7588,"Moreover, a parallel algorithm can be implemented either in a parallel system  or in a distributed system . The traditional boundary between parallel and distributed algorithms  does not lie in the same place as the boundary between parallel and distributed systems ."
+7589,"In parallel algorithms, yet another resource in addition to time and space is the number of computers. Indeed, often there is a trade-off between the running time and the number of computers: the problem can be solved faster if there are more computers running in parallel . If a decision problem can be solved in polylogarithmic time by using a polynomial number of processors, then the problem is said to be in the class NC. The class NC can be defined equally well by using the PRAM formalism or Boolean circuits—PRAM machines can simulate Boolean circuits efficiently and vice versa."
+7590,"In the analysis of distributed algorithms, more attention is usually paid on communication operations than computational steps. Perhaps the simplest model of distributed computing is a synchronous system where all nodes operate in a lockstep fashion. This model is commonly known as the LOCAL model. During each communication round, all nodes in parallel  receive the latest messages from their neighbours,  perform arbitrary local computation, and  send new messages to their neighbors. In such systems, a central complexity measure is the number of synchronous communication rounds required to complete the task."
+7591,"This complexity measure is closely related to the diameter of the network. Let D be the diameter of the network. On the one hand, any computable problem can be solved trivially in a synchronous distributed system in approximately 2D communication rounds: simply gather all information in one location , solve the problem, and inform each node about the solution ."
+7592,"On the other hand, if the running time of the algorithm is much smaller than D communication rounds, then the nodes in the network must produce their output without having the possibility to obtain information about distant parts of the network. In other words, the nodes must make globally consistent decisions based on information that is available in their local D-neighbourhood. Many distributed algorithms are known with the running time much smaller than D rounds, and understanding which problems can be solved by such algorithms is one of the central research questions of the field. Typically an algorithm which solves a problem in polylogarithmic time in the network size is considered efficient in this model."
+7593,"Another commonly used measure is the total number of bits transmitted in the network . The features of this concept are typically captured with the CONGEST model, which is similarly defined as the LOCAL model, but where single messages can only contain B bits."
+7594,"Traditional computational problems take the perspective that the user asks a question, a computer  processes the question, then produces an answer and stops. However, there are also problems where the system is required not to stop, including the dining philosophers problem and other similar mutual exclusion problems. In these problems, the distributed system is supposed to continuously coordinate the use of shared resources so that no conflicts or deadlocks occur."
+7595,"There are also fundamental challenges that are unique to distributed computing, for example those related to fault-tolerance. Examples of related problems include consensus problems, Byzantine fault tolerance, and self-stabilisation."
+7596,Much research is also focused on understanding the asynchronous nature of distributed systems:
+7597,"Coordinator election  is the process of designating a single process as the organizer of some task distributed among several computers . Before the task is begun, all network nodes are either unaware which node will serve as the ""coordinator""  of the task, or unable to communicate with the current coordinator. After a coordinator election algorithm has been run, however, each node throughout the network recognizes a particular, unique node as the task coordinator."
+7598,"The network nodes communicate among themselves in order to decide which of them will get into the ""coordinator"" state. For that, they need some method in order to break the symmetry among them. For example, if each node has unique and comparable identities, then the nodes can compare their identities, and decide that the node with the highest identity is the coordinator."
+7599,"The definition of this problem is often attributed to LeLann, who formalized it as a method to create a new token in a token ring network in which the token has been lost."
+7600,"Coordinator election algorithms are designed to be economical in terms of total bytes transmitted, and time. The algorithm suggested by Gallager, Humblet, and Spira  for general undirected graphs has had a strong impact on the design of distributed algorithms in general, and won the Dijkstra Prize for an influential paper in distributed computing."
+7601,"Many other algorithms were suggested for different kinds of network graphs, such as undirected rings, unidirectional rings, complete graphs, grids, directed Euler graphs, and others. A general method that decouples the issue of the graph family from the design of the coordinator election algorithm was suggested by Korach, Kutten, and Moran."
+7602,"In order to perform coordination, distributed systems employ the concept of coordinators. The coordinator election problem is to choose a process from among a group of processes on different processors in a distributed system to act as the central coordinator. Several central coordinator election algorithms exist."
+7603,So far the focus has been on designing a distributed system that solves a given problem. A complementary research problem is studying the properties of a given distributed system.
+7604,"The halting problem is an analogous example from the field of centralised computation: we are given a computer program and the task is to decide whether it halts or runs forever. The halting problem is undecidable in the general case, and naturally understanding the behaviour of a computer network is at least as hard as understanding the behaviour of one computer."
+7605,"However, there are many interesting special cases that are decidable. In particular, it is possible to reason about the behaviour of a network of finite-state machines. One example is telling whether a given network of interacting  finite-state machines can reach a deadlock. This problem is PSPACE-complete, i.e., it is decidable, but not likely that there is an efficient  algorithm that solves the problem in the case of large networks."
+7606,"According to Rob Pike, concurrency is the composition of independently executing computations, and concurrency is not parallelism: concurrency is about dealing with lots of things at once but parallelism is about doing lots of things at once. Concurrency is about structure, parallelism is about execution, concurrency provides a way to structure a solution to solve a problem that may  be parallelizable."
+7607,"A number of mathematical models have been developed for general concurrent computation including Petri nets, process calculi, the parallel random-access machine model, the actor model and the Reo Coordination Language."
+7608,"As Leslie Lamport  notes, ""While concurrent program execution had been considered for years, the computer science of concurrency began with Edsger Dijkstra's seminal 1965 paper that introduced the mutual exclusion problem. ... The ensuing decades have seen a huge growth of interest in concurrency—particularly in distributed systems. Looking back at the origins of the field, what stands out is the fundamental role played by Edsger Dijkstra""."
+7609,"Because computations in a concurrent system can interact with each other while being executed, the number of possible execution paths in the system can be extremely large, and the resulting outcome can be indeterminate. Concurrent use of shared resources can be a source of indeterminacy leading to issues such as deadlocks, and resource starvation."