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1,701 | 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 co... |
1,702 | 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 swit... |
1,703 | 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.... |
1,704 | 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 increase... |
1,705 | 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 we... |
1,706 | 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 ... |
1,707 | 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 ... |
1,708 | 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... |
1,709 | 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 minicom... |
1,710 | 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 singl... |
1,711 | 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 abou... |
1,712 | 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 ... |
1,713 | 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 instructio... |
1,714 | 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 "f... |
1,715 | 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 instruc... |
1,716 | 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. |
1,717 | 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 a... |
1,718 | 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 microprogr... |
1,719 | 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 acti... |
1,720 | 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 subseq... |
1,721 | 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... |
1,722 | 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. B... |
1,723 | 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. |
1,724 | 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 contr... |
1,725 | 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 ... |
1,726 | 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 rese... |
1,727 | Modern CPUs typically contain more than one ALU to improve performance. |
1,728 | 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 cycle... |
1,729 | 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 a... |
1,730 | 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 ... |
1,731 | 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. |
1,732 | 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 indepe... |
1,733 | 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 ... |
1,734 | Other types of caches exist , such as the translation lookaside buffer that is part of the memory management unit that most CPUs have. |
1,735 | 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. |
1,736 | 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 ... |
1,737 | 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 ri... |
1,738 | 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 u... |
1,739 | 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 ... |
1,740 | 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 c... |
1,741 | 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 perf... |
1,742 | 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. |
1,743 | 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 repres... |
1,744 | 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 C... |
1,745 | 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 additio... |
1,746 | 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, howev... |
1,747 | 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 instructio... |
1,748 | 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 . |
1,749 | 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... |
1,750 | 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: |
1,751 | 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. |
1,752 | 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... |
1,753 | 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 nece... |
1,754 | 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 ... |
1,755 | 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 require... |
1,756 | 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. In... |
1,757 | 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... |
1,758 | 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 . |
1,759 | 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 avoidin... |
1,760 | 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 t... |
1,761 | 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 Penti... |
1,762 | 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. |
1,763 | 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 POWE... |
1,764 | 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 process... |
1,765 | 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 floati... |
1,766 | 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-or... |
1,767 | 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. |
1,768 | 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... |
1,769 | Most modern CPUs have privileged modes to support operating systems and virtualization. |
1,770 | Cloud computing can use virtualization to provide virtual central processing units for separate users. |
1,771 | 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. Re... |
1,772 | 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 wi... |
1,773 | 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, on... |
1,774 | 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 lo... |
1,775 | 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 foot... |
1,776 | 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 programmin... |
1,777 | 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 pan... |
1,778 | 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. |
1,779 | 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: |
1,780 | 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, inc... |
1,781 | Most assemblers provide macros to generate common sequences of instructions. |
1,782 | Example: The same Fibonacci number calculator as above, but in x86-64 assembly language using AT&T syntax: |
1,783 | 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 ... |
1,784 | Compare this with the same function in C: |
1,785 | This code is similar in structure to the assembly language example but there are significant differences in terms of abstraction: |
1,786 | 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. |
1,787 | 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 th... |
1,788 | Parse the source code and perform its behavior directly; |
1,789 | Translate source code into some efficient intermediate representation or object code and immediately execute that; |
1,790 | Explicitly execute stored precompiled bytecode made by a compiler and matched with the interpreter's Virtual Machine. |
1,791 | 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 independ... |
1,792 | 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 implementa... |
1,793 | 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 ... |
1,794 | 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 int... |
1,795 | 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 garba... |
1,796 | 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. |
1,797 | 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. Compi... |
1,798 | 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... |
1,799 | 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 t... |
1,800 | 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 o... |
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