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computer,device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculatorcalculator
or calculating machine,
device for performing numerical computations; it may be mechanical, electromechanical, or electronic. The electronic computer is also a calculator but performs other functions as well.
..... Click the link for more information. , by being able to store a computer programcomputer program,
a series of instructions that a computer can interpret and execute; programs are also called software to distinguish them from hardware, the physical equipment used in data processing.
..... Click the link for more information. (so that it can repeat its operations and make logical decisions), by the number and complexity of the operations it can perform, and by its ability to process, store, and retrieve data without human intervention. Computers developed along two separate engineering paths, producing two distinct types of computer—analog and digital. An analog computer operates on continuously varying data; a digital computer performs operations on discrete data.
Computers are categorized by both size and the number of people who can use them concurrently. Supercomputerssupercomputer,
a state-of-the-art, extremely powerful computer capable of manipulating massive amounts of data in a relatively short time. Supercomputers are very expensive and are employed for specialized scientific and engineering applications that must handle very large
..... Click the link for more information. are sophisticated machines designed to perform complex calculations at maximum speed; they are used to model very large dynamic systems, such as weather patterns. Mainframes, the largest and most powerful general-purpose systems, are designed to meet the computing needs of a large organization by serving hundreds of computer terminals at the same time. Minicomputers, though somewhat smaller, also are multiuser computers, intended to meet the needs of a small company by serving up to a hundred terminals. Microcomputers, computers powered by a microprocessormicroprocessor,
integrated circuit containing the arithmetic, logic, and control circuitry required to interpret and execute instructions from a computer program. When combined with other integrated circuits that provide storage for data and programs, often on a single
..... Click the link for more information. , are subdivided into personal computerspersonal computer
(PC), small but powerful computer primarily used in an office or home without the need to be connected to a larger computer. PCs evolved after the development of the microprocessor made possible the hobby-computer movement of the late 1970s, when some computers
..... Click the link for more information. and workstations, the latter typically incorporating RISC processorsRISC processor
[Reduced Instruction Set Computer], computer arithmetic-logic unit that uses a minimal instruction set, emphasizing the instructions used most often and optimizing them for the fastest possible execution.
..... Click the link for more information. . Although microcomputers were originally single-user computers, the distinction between them and minicomputers has blurred as microprocessors have become more powerful. Linking multiple microcomputers together through a local area networklocal area network
(LAN), a computer network dedicated to sharing data among several single-user workstations or personal computers, each of which is called a node. A LAN can have from two to several hundred such nodes, each separated by distances of several feet to as much as a
..... Click the link for more information. or joining multiple microprocessors together in a parallel-processingparallel processing,
the concurrent or simultaneous execution of two or more parts of a single computer program, at speeds far exceeding those of a conventional computer.
..... Click the link for more information. system has enabled smaller systems to perform tasks once reserved for mainframes, and the techniques of grid computinggrid computing,
the concurrent application of the processing and data storage resources of many computers in a network to a single problem. It also can be used for load balancing as well as high availability by employing multiple computers—typically personal computers and
..... Click the link for more information. have enabled computer scientists to utilize the unemployed processing power of computers connected over a network or the Internet.
Advances in the technology of integrated circuitsintegrated circuit
(IC), electronic circuit built on a semiconductor substrate, usually one of single-crystal silicon. The circuit, often called a chip, is packaged in a hermetically sealed case or a nonhermetic plastic capsule, with leads extending from it for input, output,
..... Click the link for more information. have spurred the development of smaller and more powerful general-purpose digital computers. Not only has this reduced the size of the large, multi-user mainframe computers—which in their early years were large enough to walk through—to that of pieces of furniture, but it has also made possible powerful, single-user personal computers and workstations that can sit on a desktop or be easily carried. These, because of their relatively low cost and versatility, have replaced typewriters in the workplace and rendered the analog computer inefficient. The reduced size of computer components has also led to the development of thin, lightweight notebook computers and even smaller computer tablets and smartphones that have much more computing and storage capacity than that of the desktop computers that were available in the early 1990s.
An analog computer represents data as physical quantities and operates on the data by manipulating the quantities. It is designed to process data in which the variable quantities vary continuously (see analog circuitanalog circuit,
electronic circuit that operates with currents and voltages that vary continuously with time and have no abrupt transitions between levels. Generally speaking, analog circuits are contrasted with digital circuits, which function as though currents or voltages
..... Click the link for more information. ); it translates the relationships between the variables of a problem into analogous relationships between electrical quantities, such as current and voltage, and solves the original problem by solving the equivalent problem, or analog, that is set up in its electrical circuits. Because of this feature, analog computers were especially useful in the simulation and evaluation of dynamic situations, such as the flight of a space capsule or the changing weather patterns over a certain area. The key component of the analog computer is the operational amplifieroperational amplifier,
amplifier whose output voltage is proportional to the negative of its input voltage and that boosts the amplitude of an input signal many times, i.e., has a very high gain. It is usually connected so that part of the output is fed back to the input.
..... Click the link for more information. , and the computer's capacity is determined by the number of amplifiers it contains. Although analog computers are commonly found in such forms as speedometers and watt-hour meters, they largely have been made obsolete for general-purpose mathematical computations and data storage by digital computers.
A digital computer is designed to process data in numerical form (see digital circuitdigital circuit,
electronic circuit that can take on only a finite number of states. That is contrasted with analog circuits, whose voltages or other quantities vary in a continuous manner. Binary (two-state) digital circuits are the most common.
..... Click the link for more information. ); its circuits perform directly the mathematical operations of addition, subtraction, multiplication, and division. The numbers operated on by a digital computer are expressed in the binary systembinary system,
numeration system based on powers of 2, in contrast to the familiar decimal system, which is based on powers of 10. In the binary system, only the digits 0 and 1 are used.
..... Click the link for more information. ; binary digits, or bits, are 0 and 1, so that 0, 1, 10, 11, 100, 101, etc., correspond to 0, 1, 2, 3, 4, 5, etc. Binary digits are easily expressed in the computer circuitry by the presence (1) or absence (0) of a current or voltage. A series of eight consecutive bits is called a "byte"; the eight-bit byte permits 256 different "on-off" combinations. Each byte can thus represent one of up to 256 alphanumericalphanumeric
, the set of letters and numbers. When used in reference to computer input and output, the set usually is expanded to include the upper- and lower-case alphabetic characters (A–Z, a–z), the numeric characters (0–9), and
..... Click the link for more information. characters, and such an arrangement is called a "single-byte character set" (SBCS); the de facto standard for this representation is the extended ASCIIASCII
or American Standard Code for Information Interchange,
a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers.
..... Click the link for more information. character set. Some languages, such as Japanese, Chinese, and Korean, require more than 256 unique symbols. The use of two bytes, or 16 bits, for each symbol, however, permits the representation of up to 65,536 characters or ideographs. Such an arrangement is called a "double-byte character set" (DBCS); UnicodeUnicode
, set of codes used to represent letters, numbers, control characters, and the like, designed for use internationally in computers. It has been expanded to include such items as scientific, mathematical, and technical symbols, and even musical notation.
..... Click the link for more information. is the international standard for such a character set. One or more bytes, depending on the computer's architecture, is sometimes called a digital word; it may specify not only the magnitude of the number in question, but also its sign (positive or negative), and may also contain redundant bits that allow automatic detection, and in some cases correction, of certain errors (see codecode,
in communications, set of symbols and rules for their manipulation by which the symbols can be made to carry information. By this extended definition all written and spoken languages are codes.
..... Click the link for more information. ; information theoryinformation theory
or communication theory,
mathematical theory formulated principally by the American scientist Claude E. Shannon to explain aspects and problems of information and communication.
..... Click the link for more information. ). A digital computer can store the results of its calculations for later use, can compare results with other data, and on the basis of such comparisons can change the series of operations it performs. Digital computers are now used for a wide range of personal, business, scientific, and government purposes, from electronic gameselectronic game,
device or computer program that provides entertainment by challenging a person's eye-hand coordination or mental abilities. Made possible by the development of the microprocessor, electronic games are marketed in various formats, such as hand-held one-player
..... Click the link for more information. , e-mail, social networking, and data- and word-processing applications to desktop publishingdesktop publishing,
system for producing printed materials that consists of a personal computer or computer workstation, a high-resolution printer (usually a laser printer), and a computer program that allows the user to select from a variety of type fonts and sizes, column
..... Click the link for more information. , video conferencing, weather forecasting, simulated nuclear weapons testing, cryptography, and many other purposes.
Processing of Data
The operations of a digital computer are carried out by logic circuitslogic circuit,
electric circuit whose output depends upon the input in a way that can be expressed as a function in symbolic logic; it has one or more binary inputs (capable of assuming either of two states, e.g., "on" or "off") and a single binary output.
..... Click the link for more information. , which are digital circuits whose single output is determined by the conditions of the inputs, usually two or more. The various circuits processing data in the computer's interior must operate in a highly synchronized manner; this is accomplished by controlling them with a very stable oscillatoroscillator, electronic
, electronic circuit that produces an output signal of a specific frequency. An oscillator generally consists of an amplifier having part of its output returned to the input by means of a feedback loop; the necessary and sufficient condition for
..... Click the link for more information. , which acts as the computer's "clock." Typical personal computer clock rates now range from several hundred million cycles per second to several billion. Operating at these speeds, digital computer circuits are capable of performing hundred of billions of of arithmetic or logic operations per second, but supercomputers are capable of performing more than 1 million times faster; such speeds permit the rapid solution of problems that would be impossible for a human to solve by hand. In addition to the arithmetic and logic circuitry and a number of registers (storage locations that can be accessed faster than main storage, or memory, and are used to hold the intermediate results of calculations), the heart of the computer—called the central processing unit, or CPU—contains the circuitry that decodes the set of instructions, or program, and causes it to be executed.
Storage and Retrieval of Data
Associated with the CPU is the main storage, or memory, where results or other data are stored for periods of time ranging from a small fraction of a second to days or weeks before being retrieved for further processing. Once made up of vacuum tubes and later of small doughnut-shaped ferromagnetic cores strung on a wire matrix, main storage now consists of integrated circuitsintegrated circuit
(IC), electronic circuit built on a semiconductor substrate, usually one of single-crystal silicon. The circuit, often called a chip, is packaged in a hermetically sealed case or a nonhermetic plastic capsule, with leads extending from it for input, output,
..... Click the link for more information. , each of may contain billions of semiconductorsemiconductor,
solid material whose electrical conductivity at room temperature is between that of a conductor and that of an insulator (see conduction; insulation). At high temperatures its conductivity approaches that of a metal, and at low temperatures it acts as an insulator.
..... Click the link for more information. devices. Where each vacuum tube or core represented one bit and the total memory of the computer was measured in thousands of bytes (or kilobytes, KB), modern computer memory chips represent hundreds of millions of bytes (or megabytes, MB) and the total memory of both personal and mainframe computers is measured in billions of bytes (gigabytes, GB) or more. Read-only memory (ROM), which cannot be written to, maintains its content at all times and is used to store the computer's control information. Random-access memory (RAM), which both can be read from and written to, is lost each time the computer is turned off. Modern computers now include cache memory, which the CPU can access faster than RAM but slower than the registers; data in cache memory also is lost when the computer is turned off.
Programs and data that are not currently being used in main storage can be saved on auxiliary or secondary storage. Although punched paper tape and punched cards once served this purpose, the major materials used today are magnetic tape and disks and flash memory devices, all of which can be read from and written to, and two types of optical disksoptical disk,
any of a variety of information storage disks that are played or read using a laser. Optical disks include compact discs (CDs and CD-ROMs), laser discs (see videodisc), and digital versatile discs (or digital video discs; DVDs and DVD-ROMs).
..... Click the link for more information. , the compact disccompact disc
(CD), a small plastic disc used for the storage of digital data. As originally developed for audio systems, the sound signal is sampled at a rate of 44,100 times a second, then each sample is measured and digitally encoded on the 4 3-4 in (12 cm) disc as a series of
..... Click the link for more information. (CD) and its successor the digital versatile discdigital versatile disc
or digital video disc
(DVD), a small plastic disc used for the storage of digital data. The successor media to the compact disc (CD), a DVD can have more than 100 times the storage capacity of a CD.
..... Click the link for more information. (DVD). When compared to RAM, these are less expensive (though flash memory is more expensive than the other two), are not volatile (i.e., data is not lost when the power to the computer is shut off), and can provide a convenient way to transfer data from one computer to another. Thus operating instructions or data output from one computer can be stored and be used later either by the same computer or another.
In a system using magnetic tape the information is stored by a specially designed tape recordertape recorder,
device for recording and replaying of sound, video, and digital information on plastic (usually polyester) or paper tape. The tape is coated with fine particles of a magnetic substance, usually an oxide of iron, cobalt, or chromium.
..... Click the link for more information. somewhat similar to one used for recording sound. Magnetic tape is now largely used for offsite storage of large volumes of data or major systems backups. In magnetic and optical disk systems the principle is the same; the magnetic or optical medium lies in a path, or track, on the surface of a disk. The disk drive also contains a motor to spin the disk and a magnetic or optical head or heads to read and write the data to the disk. Drives take several forms, the most significant difference being whether the disk can be removed from the drive assembly. Flash memory devices, such as USB flash drives, flash memory cards, and solid-state drives, use nonvolatile memory that can be erased and reprogrammed in blocks.
Removable magnetic disks made of mylar enclosed in a plastic holder (older versions had paper holders) are now largely outdated. These floppy disks have varying capacities, with very high density disks holding 250 MB—more than enough to contain a dozen books the size of Tolstoy's Anna Karenina. Internal and external magnetic hard disks, or hard drives, are made of metal and arranged in spaced layers. They can hold vastly more data than floppies or optical disks, and can read and write data much faster than floppies. As hard disks dropped in price, they became increasingly included as a component of personal computers and replaced floppy disks as the standard media for the storage of operating systems, programs, and data.
Compact discs can hold hundreds of megabytes, and have been used, for example, to store the information contained in an entire multivolume encyclopedia or set of reference works. DVD is an improved optical storage technology capable of storing as much as ten times the data that CD technology can store. CD–Read-Only Memory (CD-ROM) and DVD–Read-Only Memory (DVD-ROM) disks can only be read—the disks are impressed with data at the factory but once written cannot be erased and rewritten with new data. The latter part of the 1990s saw the introduction of new optical storage technologies: CD-Recordable (CD-R) and DVD-Recordable (DVD-R, DVD+R), optical disks that can be written to by the computer to create a CD-ROM or DVD-ROM, but can be written to only once; and CD-ReWritable (CD-RW), DVD-ReWritable (DVD-RW and DVD+RW), and DVD–Random Access Memory (DVD-RAM), disks that can be written to multiple times.
Flash memory devices, a still more recent development, are an outgrowth of electrically erasible programmable read-only memory. Although more expensive than magnetic and optical storage technologies, flash memory can be read and written to much faster, permitting shorter boot times and quicker data access and storage. Because flash memory also is resistant to mechanical shock and has become increasingly compact, a USB flash drive allows for the easy, portable external storage of large quantities of data. Solid-state drives are more easily accessed and written to than magnetic hard drives and use less power, and have become common in high-end, lightweight notebook computers and in high-performance computers. Flash memory is also used in computer tablets and smartphones. Hybrid drives, which combine a smaller amount of flash memory with a large magnetic hard drive, permit the economical storage of large amounts of data while benefitting from a more responsive access to frequently used but only occasionally changed operating system and program files.
Data are entered into the computer and the processed data made available via input/output devices, also called peripherals. All auxiliary storage devices are used as input/output devices. For many years, the most popular input/output medium was the punched card. The most popular input devices are the computer terminalcomputer terminal,
a device that enables a computer to receive or deliver data. Computer terminals vary greatly depending on the format of the data they handle. For example, a simple early terminal comprised a typewriter keyboard for input and a typewriter printing element for
..... Click the link for more information. and internal magnetic hard drives, and the most popular output devices are the computer display screen associated with a terminal (typically displaying output that has been processed by a graphics processing unit) and the printerprinter,
device that reproduces text, images, or other data from a computer, digital camera, smartphone, or the like on paper or another medium.
Impact printers, which mostly have been superseded by ink-jet and laser printers, use a mechanical hammering device to produce
..... Click the link for more information. . Human beings can directly communicate with the computer through computer terminals, entering instructions and data by means of keyboards much like the ones on typewriters, by using a pointing device such as a mouse, trackball, or touchpad, or by speaking into a microphone that is connected to computer running voice-recognition software. The result of the input may be displayed on a liquid-crystalliquid crystal,
liquid whose component particles, atoms or molecules, tend to arrange themselves with a degree of order far exceeding that found in ordinary liquids and approaching that of solid crystals.
..... Click the link for more information. , light-emitting diode, or cathode-ray tubecathode-ray tube
(CRT), special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube.
..... Click the link for more information. screen or on printer output. Another important input/output device in modern computers is the network card, which allows the computer to connect to a computer network and the Internet using a wired or radio (wireless) connection. The CPU, main storage, auxiliary storage, and input/output devices collectively make up a cumputer system.
Sharing the Computer's Resources
Generally, the slowest operations that a computer must perform are those of transferring data, particularly when data is received from or delivered to a human being. The computer's central processor is idle for much of this period, and so two similar techniques are used to use its power more fully.
Time sharing, used on large computers, allows several users at different terminals to use a single computer at the same time. The computer performs part of a task for one user, then suspends that task to do part of another for another user, and so on. Each user only has the computer's use for a fraction of the time, but the task switching is so rapid that most users are not aware of it. Most of the tens of millions of computers in the world are stand-alone, single-user devices known variously as personal computers or workstations. For them, multitasking involves the same type of switching, but for a single user. This permits a user, for example, to have one file printed and another uploaded to an Internet website while editing a third in a word-processing session and listening to a recording streamed over the Internet. Personal computers can also be linked together in a network, where each computer is connected to others, usually by network, coaxial, or fiber-optic cable or by radio signals (wireless), permitting all to share resources such as printers, hard-disk storage devices, and an Internet connection. Cloud computingcloud computing,
the delivery of shared computing resources over a network in a manner that makes accessing and configuring those resources convenient and largely independent of the use of a required location, device, and the like.
..... Click the link for more information. is another form of resource sharing. Delivering access to both hardware and software over a network, most often the Internet, cloud computing is designed to allow many individuals and organizations using a wide range of devices both ease of access to computing resources and flexibility in changing the type and volume of the resources to which they have access.
Computer Programs and Programming Languages
Before a computer can be used for a given purpose, it must first be programmed, that is, prepared for use by loading a set of instructions, or program. The various programs by which a computer controls aspects of its operations, such as those for translating data from one form to another, are known as software, as contrasted with hardware, which is the physical equipment comprising the installation. In most computers the moment-to-moment control of the machine resides in a special software program called an operating system, or supervisor. Other forms of software include assemblers and compilers for programming languages and applications for business and home use (see computer programcomputer program,
a series of instructions that a computer can interpret and execute; programs are also called software to distinguish them from hardware, the physical equipment used in data processing.
..... Click the link for more information. ). Software is of great importance; the usefulness of a highly sophisticated array of hardware can be limited by the lack of adequate software.
Each instruction in the program may be a simple, single step, telling the computer to perform some arithmetic operation, to read the data from some given location in the memory, to compare two numbers, or to take some other action. The program is entered into the computer's memory exactly as if it were data, and on activation, the machine is directed to treat this material in the memory as instructions. Other data may then be read in and the computer can carry out the program to complete the particular task.
Since computers are designed to operate with binary numbers, all data and instructions must be represented in this form; the machine language, in which the computer operates internally, consists of the various binary codes that define instructions together with the formats in which the instructions are written. Since it is time-consuming and tedious for a programmer to work in actual machine language, a programming languageprogramming language,
syntax, grammar, and symbols or words used to give instructions to a computer. Development of Low-Level Languages
All computers operate by following machine language programs, a long sequence of instructions called machine code that is
..... Click the link for more information. , or high-level language, designed for the programmer's convenience, is used for the writing of most programs. The computer is programmed to translate this high-level language into machine language and then solve the original problem for which the program was written. Many high-level programming languages are now universal, varying little from machine to machine.
Development of Computers
Although the development of digital computers is rooted in the abacusabacus
, in mathematics, simple device for performing arithmetic calculations. The type of abacus now best known is represented by a frame with sliding counters. An elementary abacus might have ten parallel wires strung between two boards on a frame, with nine beads on each wire.
..... Click the link for more information. and early mechanical calculating devices, Charles BabbageBabbage, Charles
, 1792–1871, English mathematician and inventor. He devoted most of his life and expended much of his private fortune and a government subsidy in an attempt to perfect a mechanical calculating machine that foreshadowed present-day machines.
..... Click the link for more information. is credited with the design of the first modern computer, the "analytical engine," during the 1830s. Vannevar BushBush, Vannevar
, 1890–1974, American electrical engineer and physicist, b. Everett, Mass., grad. Tufts College (B.S., 1913). He went to Massachusetts Institute of Technology (MIT) in 1919; there he was professor (1923–32) and vice president and dean of engineering
..... Click the link for more information. built a mechanically operated device, called a differential analyzer, in 1930; it was the first general-purpose analog computer. John AtanasoffAtanasoff, John Vincent,
1903–1995, inventor of the digital computer, b. Hamilton, N.Y., grad. Univ. of Florida (B.S., 1925), Iowa State College (M.S., 1926), Univ. of Wisconsin (Ph.D., 1930).
..... Click the link for more information. constructed the first electronic digital computing device in 1939; a full-scale version of the prototype was completed in 1942 at Iowa State College (now Iowa State Univ.). In 1943 Conrad Zuse built the Z3, a fully operational electromechanical computer.
During World War II, the Colossus was developed for British codebreakers; it was the first programmable electronic digital computer. The Mark I, or Automatic Sequence Controlled Calculator, completed in 1944 at Harvard by Howard Aiken, was the first machine to execute long calculations automatically, while the first all-purpose electronic digital computer, ENIAC (Electronic Numerical Integrator And Calculator), which used thousands of vacuum tubes, was completed in 1946 at the Univ. of Pennsylvania. UNIVAC (UNIVersal Automatic Computer) became (1951) the first computer to handle both numeric and alphabetic data with equal facility; intended for business and government use, this was the first widely sold commercial computer.
First-generation computers were supplanted by the transistorized computers (see transistortransistor,
three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all common applications.
..... Click the link for more information. ) of the late 1950s and early 60s, second-generation machines that were smaller, used less power, and could perform a million operations per second. They, in turn, were replaced by the third-generation integrated-circuit machines of the mid-1960s and 1970s that were even smaller and were far more reliable. The 1970s, 80s, and 90s were characterized by the development of the microprocessor and the evolution of increasingly smaller but powerful computers, such as the personal computer and personal digital assistantpersonal digital assistant
(PDA), lightweight, hand-held computer designed for use as a personal organizer with communications capabilities; also called a handheld. A typical PDA has no keyboard, relying instead on special hardware and pen-based computer software to enable the
..... Click the link for more information. (PDA), which ushered in a period of rapid growth in the computer industry.
The World Wide WebWorld Wide Web
(WWW or W3), collection of globally distributed text and multimedia documents and files and other network services linked in such a way as to create an immense electronic library from which information can be retrieved quickly by intuitive searches.
..... Click the link for more information. was unveiled in 1990, and with the development of graphical web browser programs in succeeding years the Web and the Internet spurred the growth of general purpose home computing and the use of computing devices as a means of social interaction. Smartphones, which integrate a range of computer software with a cellular telephonecellular telephone
or cellular radio,
telecommunications system in which a portable or mobile radio transmitter and receiver, or "cellphone," is linked via microwave radio frequencies to base transmitter and receiver stations that connect the user to a conventional
..... Click the link for more information. that now typically has a touchscreen interface, date to 2000 when a PDA was combined with a cellphone. Although computer tablets date to the 1990s, they only succeeded commercially in 2010 with the introduction of Apple's iPad, which built on software developed for smartphones. The increasing screen size on some smartphones has made them the equivalent of smaller computer tablets, leading some to call them phablets.
See S. G. Nash, A History of Scientific Computing (1990); D. I. A. Cohen, Introduction to Computer Theory (2d ed. 1996); P. Norton, Peter Norton's Introduction to Computers (2d ed. 1996); A. W. Biermann, Great Ideas in Computer Science: A Gentle Introduction (2d ed. 1997); R. L. Oakman, The Computer Triangle: Hardware, Software, People (2d ed. 1997); R. Maran, Computers Simplified (4th ed. 1998); A. S. Tanenbaum and J. R. Goodman. Structured Computer Organization (4th ed. 1998).
a device or set of devices for the mechanization and automation of data processing (computations).
Modern computers are subdivided into three classes according to the method of representing data: (1) analog computers, in which data are represented in the form of continuously changing variables expressed by physical quantities (the angle of rotation of a shaft, the intensity of an electrical current, voltage, and so on); (2) digital computers, in which data are represented in the form of discrete values of variables (numbers) expressed by a combination of discrete values of some physical quantity; and (3) hybrid computers, various units of which represent data by one or the other method.
Historically, digital computing devices appeared first—for example, abacuses and their numerous precursors. In the 17th century the French scientist B. Pascal, and later the German mathematician G. W. von Leibniz, built the first digital computers. The first computer suitable for practical use was Thomas de Colmar’s adding machine (1820). V. T. Odhner’s adding machine, which became very widespread, was built in 1874. In the early 20th century tabulators appeared for performing various statistical, bookkeeping, and financial-banking operations.
The idea of building a general-purpose digital computer belongs to Professor C. Babbage of Cambridge University. In 1833 he designed a computer whose features are close to those of modern devices. The plan was ahead of the needs of the day and technical possibilities for realization.
The development of the theory of relay-switching circuits and experience in using telephone and punched card equipment made it possible during the 1930’s to undertake the development of a computer with program control. At first electromagnetic relays were used. The first such machine, the Mark I, was built in 1944 in the United States. The first electronic digital computer, the ENIAC (Electronic Numerical Integrator and Computer) was built in 1946, also in the United States.
In the Soviet Union, the MESM (Small Electronic Calculator) electronic digital computer was developed in 1950 at the Academy of Sciences of the Ukrainian SSR under the direction of Academician S. A. Lebedev. The MESM marked the beginning of work in the field of mathematical electronic machine building in the USSR. In subsequent years a number of digital computers that differed in productivity and technical concept were built in the USSR to satisfy the needs of the national economy (for example, the BESM, Strela, M-20, M-220, Minsk, Ural, and Mir).
The first continuous-operation devices appeared in the 16th and 17th centuries. These include the slide rule and nomograms for navigational calculations. In the mid-19th century, very simple mechanical integrators appeared. Work on analog computers developed significantly at the turn of the 20th century. Machines for solving differential equations, electromechanical integrating machines, and others were developed. In the USSR, the beginning of analog computer development dates back to 1927; it is associated with the work of S. A. Gershgorin, M. V. Kirpichev, I. S. Bruk, V. S. Luk’ianov, and others. During the 1950’s and 1960’s, several types of analog computers were built, many of which have found wide application.
The development of electronic computers is closely linked to achievements in the field of electronics. The first electronic computers used vacuum tubes; it is customary to call these computers first-generation machines. The development of semiconductor radio electronics made possible the transition to designing second- and third-generation computers. They are characterized by a more complex logic diagram and by software, which is a programmed extension of the hard-ware. The technology for manufacturing second-generation computers differed little from that used for the manufacture of first-generation computers; vacuum tubes were replaced by semiconductor triodes (transistors) and diodes. Third-generation computers are built with integrated circuits, which contain dozens of transistors, resistors, and diodes in a single module. The transition to producing computers with integrated circuits demanded an almost complete revision of the technology of production of electronic computers.
The theory of mathematical modeling is the basis for constructing analog computers. Using analogies among phenomena that differ in their physical nature, the analog computer models the processes being calculated. A large part of the analog computer equipment consists of linear and non-linear decision elements. In electronic analog computers these are operational DC amplifiers (integrator, amplifier, inverter), units of coefficients, typical nonlinearities, delays, and so on. To solve a specific problem, the units of an analog computer are interconnected in the necessary combinations. Output data of the analog computer are obtained from readings on the displays at the terminal points of the circuit. The analog computer is characterized by high speed of operation, simplicity in the linkage with the object being investigated, the possibility of easily changing the parameters of the problem under investigation both during its preparation and during the solution process, comparatively low precision, and a limited class of solvable problems.
Solving a problem on digital computers involves the sequential performance of arithmetic operations on numbers that correspond to quantities representing the initial data. The numbers are usually represented in the form of an aggre-gate of mechanical, pneumatic, or electrical pulses and are fixed by elements that can each assume several stable states that correspond strictly to one digit of the number. Before solution on the digital computer, the problem is broken down into a series of simple sequential operations and their order is set—that is, a program of computations is drawn up.
Digital computers are subdivided into three classes according to their method of control: those with manual control, those with a rigid program; and general-purpose types. Digital computers with manual control include keyboard computers, adding machines, and lever-operated computers. Modern desk digital computers are manufactured almost entirely with electronic components. The computing process is controlled manually, which results in low computation speed. The digital computer with manual control is a means of mechanizing calculation operations and is only suitable for solving very simple problems with a limited number of computations.
Rigid-program digital computers include tabulators, specialized machines oriented to solving a narrow range of problems (for example, on-board computers), and so on. In these computers the computing process is controlled automatically by a program set up on a switchboard or permanently built into the machine. The digital computer with a switchable program is a means for partial automation of the computing process and is rapidly being replaced by general-purpose digital computers. Computers with built-in programs are used in cases where simplicity, reliability, low cost, and small size and weight are needed, primarily for one-time use (for example, in missiles).
General-purpose computers with automatic program control are the most refined means of automating the labor-consuming processes of human mental activity. Modern general-purpose digital computers are a complicated automatic computing complex that includes a processor, an immediate-access memory, one or several large-capacity external memories, and data input-output units. The process of computation is controlled by the control device and the pro-gram of computations located in the computer memory. The loading of particular units, the coordination of their operation, and the control of the sequence of solving problems are done by program equipment. The set of programs that per-forms these and many other functions is called software. Algorithmic languages (ALGOL, FORTRAN, COBOL, and others) are used to describe problem-solving. The input of initial data and programs and the output of results in the form most convenient for the user are accomplished by the set of input-output devices that are part of the general-purpose digital computer. Initial data may be given in the form of graphs, digital and textual documents, images of the object being calculated (for example, an overall view of a building, a cross section of an airplane wing, and so on), audiovisual display, and others.
Digital computers are characterized by high productivity and precision in the results obtained and by algorithmic universality, which results from the fact that the rearrangement of the digital computer to solve a new problem involves only a change in the computation program and the initial data stored in the computer memory, without changing the design of the machine itself.
Hybrid computer systems consist of organically linked analog and digital computers. Data exchange between analog-action and discrete-action computers is accomplished through special convertors. A division of functions between machines is typical for a combined system: the analog computer is used to reproduce fast-occurring processes with limited precision of variables, and the digital computer is used for computations with greater precision and for statistical processing of results. The hybrid computing system combines high precision and speed, which are more difficult to obtain by means of just one of the computers.
A. N. MIAMLIN
the term adopted in foreign literature (chiefly English-language) for equipment that operates automatically either according to a preassigned program or according to sequential instructions to solve mathematical problems and problems of economics, statistics, production planning, management, and so forth. The term usually refers to electronic computers (in Russian, elektronnye vychislitel’nye mashiny).
What does it mean when you dream about a computer?
For someone who works in front of a computer every day, this could simply be a reflection of daily life in a dream. A computer can also indicate research, analysis, or communication, depending on how one uses a computer. We sometimes say that something doesn’t “compute” as a way of saying we don’t understand, so a broken computer in a dream could represent confusion.
A device that receives, processes, and presents information. The two basic types of computers are analog and digital. Although generally not regarded as such, the most prevalent computer is the simple mechanical analog computer, in which gears, levers, ratchets, and pawls perform mathematical operations—for example, the speedometer and the watt-hour meter (used to measure accumulated electrical usage). The general public has become much more aware of the digital computer with the rapid proliferation of the hand-held calculator and a large variety of intelligent devices and especially with exposure to the Internet and the World Wide Web. See Internet
An analog computer uses inputs that are proportional to the instantaneous value of variable quantities, combines these inputs in a predetermined way, and produces outputs that are a continuously varying function of the inputs and the processing. These outputs are then displayed or connected to another device to cause action, as in the case of a speed governor or other control device. Small electronic analog computers are frequently used as components in control systems. If the analog computer is built solely for one purpose, it is termed a special-purpose electronic analog computer. In any analog computer the key concepts involve special versus general-purpose computer designs, and the technology utilized to construct the computer itself, mechanical or electronic. See Analog computer
In contrast, a digital computer uses symbolic representations of its variables. The arithmetic unit is constructed to follow the rules of one (or more) number systems. Further, the digital computer uses individual discrete states to represent the digits of the number system chosen. A digital computer can easily store and manipulate numbers, letters, images, sounds, or graphical information represented by a symbolic code. Through the use of the stored program, the digital computer achieves a degree of flexibility unequaled by any other computing or data-processing device.
The advent of the relatively inexpensive and readily available personal computer, and the combination of the computer and communications, such as by the use of networks, have dramatically expanded computer applications. The most common application now is probably text and word processing, followed by electronic mail. See Electronic mail, Local-area networks, Microcomputer
Computers have begun to meet the barrier imposed by the speed of light in achieving higher speeds. This has led to research and development in the areas of parallel computers (in order to accomplish more in parallel rather than by serial computation) and distributed computers (taking advantage of network connections to spread the work around, thus achieving more parallelism). Continuing demand for more processing power has led to significant changes in computer hardware and software architectures, both to increase the speed of basic operations and to reduce the overall processing time. See Computer systems architecture, Concurrent processing, Distributed systems (computers), Multiprocessing, Supercomputer
The physical components from which a computer is constructed (electronic circuits and input/output devices) are known as "hardware". Most computers have four types of hardware component: CPU, input, output and memory. The CPU (central processing unit) executes programs ("software") which tell the computer what to do. Input and output (I/O) devices allow the computer to communicate with the user and the outside world. There are several kinds of memory - fast, expensive, short term memory (e.g. RAM) to hold intermediate results, and slower, cheaper, long-term memory (e.g. magnetic disk and magnetic tape) to hold programs and data between jobs.
See also analogue computer.
computerA general-purpose machine that processes data according to a set of instructions stored internally. The computer and all the equipment attached to it are "hardware." The instructions that tell it what to do are "software."
A set of instructions that perform a particular task is called a "program," "software program," "application" or "app." See how to select a computer, stored program concept and CPU. For a summary of the generations of computers since the 1940s, see computer generations.
What a Computer Does
The instructions in the program direct the computer to input, process and output as follows:
The computer can selectively retrieve data into its main memory (RAM) from any drive (optical, magnetic, solid state or USB) or from a network. After processing the data internally, the computer can send a copy of the results from its memory out to any drive or network. The more memory, the more programs and data it can work with at the same time. An entry level computer generally has at least 2 gigabytes of memory.
By transferring data to a magnetic or solid state drive, data are stored permanently for retrieval later. The more storage, the more data can be accumulated. An entry level computer typically has at least 500 gigabytes of storage.
Processing (The 3 C's)
The computer performs all processing by "calculating," "comparing" and "copying" the data stored in its memory (RAM).
Calculate - Compute Amounts and Keep Track
A computer can add, subtract, multiply and divide numbers to compute money amounts as well as geometric measurements of all variety. The computer's calculating capability enables it to keep track of its own internal iterations for myriad tasks.
Compare - Match One Set With Another
The computer can look at two sets of data and determine whether they are equal or not or which one is higher or lower in value. Comparing is performed for searching, analyzing and evaluating data for countless purposes.
Copy - From One Place to Another
The computer can rearrange data for organizing and reporting by copying data from one area in memory to another.
|The 3 C's - Find Things|
|This example counts all California records in the database by reading every record into memory. The memory bytes that hold the state are compared with "CA," and if equal, a "1" is added to the California counter. Each record is written into the same memory bytes (memory buffer) and compared until the last record has been examined.|
|The 3 C's - Display and Print|
|Data are stored as contiguous fields in a database but are displayed and printed by copying the characters into the desired order. The date is rendered through a mask (see picture).|
|The 3 C's - Sort|
|Resequencing data is accomplished by comparing each item with the others and copying it into the appropriate order. There is also significant calculating going on to keep track of what goes where. Data records are generally indexed, and instead of sorting the actual records, the much smaller indexes are sorted (see index).|
|The 3 C's - Edit|
|The magic of word processing is copying. In this example, in order to insert the "O", the remaining characters are copied one memory byte to the right to make room. This is a very simplified example. The way it is actually rearranged is more complex but is nevertheless accomplished by copying text and calculating beginning and ending locations.|
|The UNIVAC I (Frankfurt Germany, 1956)|
|Imagine being here watching this UNIVAC CPU being pushed up the ramp and someone says "you know some day all of that will fit on the head of a pin." See UNIVAC I.|
|How About the Tip of a Pencil?|
|In fact, these PICmicro microcontrollers from Microchip (www.microchip.com), are a whole lot faster than the UNIVAC I. See microcontroller.|