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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.
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"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.
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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.
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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.
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Many field buses are serial data buses , several of which use the RS-485 electrical characteristics and then specify their own protocol and connector:
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Other serial buses include:
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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.
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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 .
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Direct memory access is a means for devices to transfer large chunks of data to and from memory independently of the CPU.
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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
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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".
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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.
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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.
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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.
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Data are encoded by assigning a bit pattern to each character, digit, or multimedia object. Many standards exist for encoding .
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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.
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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.
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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.
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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.
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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 .
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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.
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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.
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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.
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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.
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As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:
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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.
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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 .
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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.
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Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Tertiary storage is also known as nearline storage because it is "near to online". The formal distinction between online, nearline, and offline storage is:
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Utilities such as hdparm and sar can be used to measure IO performance in Linux.
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Full disk encryption, volume and virtual disk encryption, andor file/folder encryption is readily available for most storage devices.
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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.
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Distinct types of data storage have different points of failure and various methods of predictive failure analysis.
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Vulnerabilities that can instantly lead to total loss are head crashing on mechanical hard drives and failure of electronic components on flash storage.
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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.
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Flash storage may experience downspiking transfer rates as a result of accumulating errors, which the flash memory controller attempts to correct.
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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.
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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.
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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.
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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.
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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.
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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:
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In early computers, magnetic storage was also used as:
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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.
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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:
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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.
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3D optical data storage has also been proposed.
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Light induced magnetization melting in magnetic photoconductors has also been proposed for high-speed low-energy consumption magneto-optical storage.
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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.
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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.
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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:
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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 .
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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.
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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.
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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.
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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.
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This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 22 January 2022.
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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.
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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.
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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.
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The control unit may include a binary counter to tell the control unit's logic what step it should do.
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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.
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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.
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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.
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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.
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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.
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Many medium-complexity computers pipeline instructions. This design is popular because of its economy and speed.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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