Computer

computer, device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (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. Supercomputers 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 microprocessor, are subdivided into personal computers and workstations, the latter typically incorporating RISC processors. 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 network or by joining multiple microprocessors together in a parallel-processing system has enabled smaller systems to perform tasks once reserved for mainframes, and the techniques of grid computing have enabled computer scientists to utilize the unemployed processing power of connected computers.

Advances in the technology of integrated circuits 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 large pieces of furniture, but it has also made possible powerful, single-user personal computers and workstations that can sit on a desktop. These, because of their relatively low cost and versatility, have largely replaced typewriters in the workplace and rendered the analog computer inefficient.

Analog Computers

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 circuit); 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 amplifier, and the computer's capacity is determined by the number of amplifiers it contains (often over 100). 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.

Digital Computers

A digital computer is designed to process data in numerical form (see digital circuit); 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 system; 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 alphanumeric characters, and such an arrangement is called a "single-byte character set" (SBCS); the de facto standard for this representation is the extended ASCII 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); Unicode 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 code; information theory). 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 used for reservations systems, scientific investigation, data-processing and word-processing applications, desktop publishing, electronic games, and many other purposes.

Processing of Data

The operations of a digital computer are carried out by logic circuits, 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 oscillator, which acts as the computer's "clock." Typical computer clock rates range from several million cycles per second to several hundred million, with some of the fastest computers having clock rates of about a billion cycles per second. Operating at these speeds, digital computer circuits are capable of performing thousands to trillions of arithmetic or logic operations per second, thus permitting 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 small number of registers (storage locations that can be accessed faster than main storage 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 central processing unit is the storage unit, 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 circuits, each of which contains thousands of semiconductor 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), each semiconductor device now represents millions of bytes (or megabytes, MB) and the total memory of mainframe computers is measured in billions of bytes (or gigabytes, GB). Random-access memory (RAM), which both can be read from and written to, is lost each time the computer is turned off. 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.

Programs and data that are not currently being used in main storage can be saved on auxiliary storage, or external storage. Although punched paper tape and punched cards once served this purpose, the major materials used today are magnetic tape and magnetic disks, which can be read from and written to, and two types of optical disks, the compact disc (CD) and its successor the digital versatile disc (DVD). DVD is an improved optical storage technology capable of storing vastly greater amounts of data than the CD technology. 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), 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.

When compared to semiconductor memory, magnetic and optical storage is less expensive, is not volatile (i.e., data is not lost when the power to the computer is shut off), and provides a convenient way to transfer data from one computer to another. Thus operating instructions or data output from one computer can be stored away from the computer and then retrieved either by the same computer or another. In a system using magnetic tape the information is stored by a specially designed tape recorder somewhat similar to one used for recording sound. In magnetic and optical disk systems the principle is the same except that 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.

Removable magnetic disks are most commonly made of mylar enclosed in a paper or plastic holder. 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. Compact discs can hold many hundreds of megabytes, and are used, for example, to store the information contained in an entire multivolume encyclopedia or set of reference works, and DVD disks can hold ten times as much as that. Nonremovable disks are made of metal and arranged in spaced layers. They can hold more data and can read and write data much faster than floppies.

Data are entered into the computer and the processed data made available via input/output devices. All auxiliary storage devices are used as input/output devices. For many years, the most popular input/output medium was the punched card. Although this is still used, the most popular input device is now the computer terminal and the most popular output device is the high-speed printer. 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. Responses may be displayed on a cathode-ray tube, liquid-crystal display, or printer. The CPU, main storage, auxiliary storage, and input/output devices collectively make up a 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 sorted while editing a third in a word-processing session. Such personal computers can also be linked together in a network, where each computer is connected to others, usually by wires or coaxial cables, permitting all to share resources such as printers, modems, and hard-disk storage devices.

Computer Programs and Programming Languages

Before a computer can be used to solve a given problem, it must first be programmed, that is, prepared for solving the problem by being given 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 program). Software is of great importance; the usefulness of a highly sophisticated array of hardware can be severely compromised 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 solve the particular problem.

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 language, 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. Certain high-level programming languages are universal, varying little from machine to machine.

Development of Computers

Although the development of digital computers is rooted in the abacus and early mechanical calculating devices, Charles Babbage is credited with the design of the first modern computer, the "analytical engine," during the 1830s. American scientist Vannevar Bush built a mechanically operated device, called a differential analyzer, in 1930; it was the first general-purpose analog computer. John Atanassoff constructed the first semielectronic digital computing device in 1939.

The first fully automatic calculator was the Mark I, or Automatic Sequence Controlled Calculator, begun in 1939 at Harvard by Howard Aiken, 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; this was the first commercially available computer.

First-generation computers were supplanted by the transistorized computers (see transistor) 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 1980s 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 assistant, which ushered in a period of rapid growth in the computer industry.

Bibliography

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).

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computer

Programmable machine that can store, retrieve, and process data. Today's computers have at least one CPU that performs most calculations and includes a main memory, a control unit, and an arithmetic logic unit. Increasingly, personal computers contain specialized graphic processors, with dedicated memory, for handling the computations needed to display complex graphics, such as for three-dimensional simulations and games. Auxiliary data storage is usually provided by an internal hard disk and may be supplemented by other media such as floppy disks or CD-ROMs. Peripheral equipment includes input devices (e.g., keyboard, mouse) and output devices (e.g., monitor, printer), as well as the circuitry and cabling that connect all the components. Generations of computers are characterized by their technology. First-generation digital computers, developed mostly in the U.S. after World War II, used vacuum tubes and were enormous. The second generation, introduced c. 1960, used transistors and were the first successful commercial computers. Third-generation computers (late 1960s and 1970s) were characterized by miniaturization of components and use of integrated circuits. The microprocessor chip, introduced in 1974, defines fourth-generation computers.

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computer

A general-purpose machine that processes data according to a set of instructions that are stored internally either temporarily or permanently. The computer and all the equipment attached to it are called "hardware." The instructions that tell it what to do are called "software." A set of instructions that perform a particular task is called a "program" or "software program."

What a Computer Does
The instructions in the program direct the computer to input, process and output as follows:

Input/Output
The computer can selectively retrieve data into its main memory (RAM) from any peripheral device (magnetic disk, optical disk, etc.) or network connected to it. After processing the data internally, the computer can send a copy of the results from its memory out to any peripheral device or to the network. A system's size is based on how much memory it has. The more memory, the more programs and data it can work with at the same time.

Storage
By transferring data out to a magnetic disk, the computer is able to store data permanently and retrieve it when required. A system's size is also based on how much disk storage it has. The more disk, the more data are available.

Processing (The 3 C's)
The computer performs all processing by "calculating," "comparing" and "copying" the data stored in its memory (RAM).

Calculate
The computer can perform any mathematical operation on data by adding, subtracting, multiplying and dividing one set with another.

Compare
The computer can analyze and evaluate data by matching it with sets of known data that are included in the program or called in from storage.

Copy
The computer can move data around to create any kind of report or listing in any order.

Calculate, Compare and Copy
By calculating, comparing and copying, the computer accomplishes all forms of data processing. For example, records are sorted into a new order by comparing two records at a time and copying the record with the lower value in front of the one with the higher value.

The computer finds one customer out of thousands by comparing the requested account number to each record in the file. The query statement: SUM SALARY FOR TITLE = "NURSE" causes the computer to compare the title field in each record for NURSE and then add (calculate) the salary field for each match.

In word processing, inserting and deleting text is accomplished by copying characters from one place to another.

Remember The 3 C's
If you wonder whether the computer can solve a problem, identify your data on paper. If it can be calculated, compared and copied on paper, it can be processed in the computer.

The 3 C's - Finding Things

This example counts all California records in the database by comparing each record with "CA." Every record in the database is read into memory. The memory locations that state is written into are compared with the letters "CA" in the program. If they are equal, a "1" is added to the California counter. The second record is written into the same memory bytes as the first record, and the field is compared again. This process is performed until the last record has been examined.

The 3 C's - Displaying and Printing

Data are stored as contiguous fields in the database with no blanks in between. The data are displayed and printed the way we like to see it by writing the data into memory and copying the characters into the desired order. The date in this example is printed through a "picture," which is a set of characters that acts as a filter. Each character in the date is compared to a corresponding character in the picture, and the one copied as output is determined by the rules. Pictures can be implemented in software or in hardware.

The 3 C's - Sorting

Sorting (resequencing) data is accomplished by comparing each item of data with the others and copying it into the appropriate order. Of course, there is a ton of calculating going on to keep track of what is being compared. Years ago, when databases were stored on tape, the speed of a vendor's sort program was a powerful marketing feature. All transactions had to be sorted into account number sequence in order to be processed. In today's online systems, data are often indexed. Instead of sorting the actual data records themselves, the much smaller indexes are sorted.

The 3 C's - Editing

The magic of word processing is nothing more than copying text. In this example, in order for the "O" to be inserted into the word, the remaining characters are copied one memory location (byte) to the right to make room for it. Deleting is copying in reverse. As in all data processing, there is a whole lot of calculating and comparing going on to keep track of where the text is stored in memory.

The Stored Program Concept
The computer's ability to call in instructions and follow them is known as the "stored program concept." Instructions are copied into memory from a disk, tape or other source before any data can be processed. The computer is directed to start with the first instruction in the program. It copies the instruction from memory into its control unit circuit and matches it against its built-in set of instructions. If the instruction is valid, the processor carries it out. If not, the computer comes to an abnormal end (see abend).

The computer executes instructions sequentially until it finds a GOTO instruction that tells it to go to a different place in the program. It can execute billions of instructions per second, using the same program logic on each new set of data brought in.

Operations Overlap
Input/output and processing are made to overlap. While one program is waiting for input from one user, the operating system (master control program) directs the computer to process data in another program. Large computers allow many input/output operations to occur simultaneously with processing.

It can take hundreds of thousands of discrete machine steps to perform very routine tasks. Your computer could easily execute several million instructions to put a requested record on screen for you.

Computer Generations
First-generation computers, starting with the UNIVAC I in 1951, used vacuum tubes, and their memories were made of thin tubes of liquid mercury and magnetic drums.

Second-generation systems in the late 1950s replaced tubes with transistors and used magnetic cores for memories (IBM 1401, Honeywell 800). Size was reduced and reliability was significantly improved.

Third-generation computers, beginning in the mid-1960s, used the first integrated circuits (IBM 360, CDC 6400) and the first operating systems and DBMSs. Online systems were widely developed, although most processing was still batch oriented using punch cards and magnetic tapes.

Starting in the mid-1970s, the fourth generation brought us computers made entirely of chips. It spawned the microprocessor and personal computer. It introduced distributed processing and office automation. Query languages, report writers and spreadsheets put large numbers of people in touch with the computer for the first time. Even with the hundreds of millions of people using computers every day, we are still in the fourth generation. Some skill is still required to use the computer even if only to surf the Web and send e-mail.

The fifth generation implies faster hardware and more sophisticated software that uses artificial intelligence (AI) routinely. Natural language recognition is a major component of the fifth generation. When you can have a reasonably intelligent conversation with the average computer, you will be in the fifth generation, perhaps in the 2015-2020 time frame.

The Beginning of Commercial Computing

In the early 1950s, the UNIVAC I ushered in the computer age. This picture was taken in Frankfurt, Germany in 1956 and shows the console on the right, a little more than half the CPU on the left and the tape drives in the background.

The Installation

This picture, taken in 1956, shows half the CPU of the UNIVAC I. Imagine yourself watching this awesome sight and someone says to you, "in 20 years, everything you see being wheeled up the ramp will fit on the tip of your finger." Would you have believed it?

Types of Computers
Computers can be as small as a chip or as large as a truck. The difference is in the amount of work they do within the same time frame. Its power is based on many factors, including word size and the speed of its CPU, memory and peripherals. Following is a "rough" guide to system cost.

  Type of                    Approximate Cost
  Computer                   In 2010 US $

  Chips

  Microcontroller (MCU)
   (4, 8, 32, 16-bit)        $1 - $150

  Microprocessor chip
   (4, 8, 16, 32, 64-bit)    $5 - $1K


  Client Systems

  Desktop/laptop client
   (32, 64-bit)              $400 - $6K

  Workstation (32, 64-bit)   $5K - $25K


  Server Systems

  Low end server
   (32, 64-bit)              $2K - 5K

  Midrange server
   (32, 64-bit)              $15K - 250K

  High-end server/mainframe
   (64-bit)                  $500K - $3M

  Supercomputer (64-bit)     $1M - $5M

  K = thousand $
  M = million $

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computer

a. a device, usually electronic, that processes data according to a set of instructions. The digital computer stores data in discrete units and performs arithmetical and logical operations at very high speed. The analog computer has no memory and is slower than the digital computer but has a continuous rather than a discrete input. The hybrid computer combines some of the advantages of digital and analog computers.

b. (as modifier): computer technology

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computer [kəm′pyüd·ər]

(computer science)

A device that receives, processes, and presents data; the two types are analog and digital. Also known as computing machine.

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Computer

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

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1.	(publication)	Computer - A journal of the IEEE Computer Society.
2.	(computer)	computer - A machine that can be programmed to manipulate symbols. Computers can perform complex and repetitive procedures quickly, precisely and reliably and can quickly store and retrieve large amounts of data. 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.

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Computer 

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

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Computer 

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).