Softcover reprint ofthe hardcover 1st edition 1986978-0-333-39083-2
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First published 1986 by THE MACMILLAN PRESS LID London and Basingstoke
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British Library Cataloguing in Publication Data
Longley, Dennis Macmillan dictionary of personal computing and communications. 1. Microcomputers - Dictionaries I. Title 11. Shain, Michael OO~.16'03'21 QA 76.5 ISBN 978-0-333-42170-3 ISBN 978-1-349-18341-8 (eBook) DOI 10.1007/978-1-349-18341-8
The paperback edition of this book is sold subject to the condition that It shall not, by way of trade of otherwise, be lent, resold, hired out, or otherwise circulated without the publisher's prior consent in any form ofbinding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser.
abbreviated addressing In programming, a process that enables a user to employ an address having fewer characters than the full address. lt provides a faster me ans of pro­ cessing data because the shorter address requires less time to decode. See ADDRESS­
ING.
ABEND In operations , an ABnormal END of a task prior to its completion on a mainframe because of an error. A micro­ computer in emulation mode should be programmed to handle this in a controlled manner. See TERMINAL EMULATION .
abort In operations, to terminate, in a controlled manner, a processing activity in a computer system because it is impossible or undesirable for the activity to proceed.
A
abscissa In mathematics, the horizontal axis of a coordinate graph. Compare ORDINATE. See CARTESIAN COORDINATES, co­ ORDINATE GRAPH.
absolute address In programming, (1) an address in a computer language that identi­ fies a storage location or a device without the use of any intermediate reference, (2) an address that is permanently assigned by the machine designer to a storage location, (3) a pattern of characters that identifies a unique storage location or device without further modification. See ADDRESS.
absolute assembler In programming, a specific type of assembly language program designed to produce binary programs con­ taining only absolute addresses and address references. See ABSOLUTE ADDRESS, ASSEM­
BLING.
absolute code In programming, a code that uses computer instructions with absolute addresses. See ABSOLUTE ADDRESS.
absolute loader In programming, a routine that reads a computer program into main memory, beginning at the assembled origin. See ASSEMBLING, LOADER , MAIN MEMORY.
absolute value In mathematics, the value of a number regardless of a prefixed plus or minus sign, i.e. the absolute value of -5 is 5.
abstract data type In data structures, a data type that is defined solely in terms of the operations that can be performed on objects of that type and the range of values that it can take, without regard to the method of representation of the value. See TYPE.
AC See CURRENT.
ALTERNATING
acceleration potential In electronics, the voltage between the cathode and the face of the tube which attracts the beam of focused electrons, causing them to impinge on the phosphor dots. See CRT, PHOSPHOR DOTS.
acceptance testing In operations, aseries of tests designed to demonstrate the functional capabilities of a new computer system. lt is usually conducted by the manufacturer to show the customer that the system is in working order.
ACCESS US Army Automated Catalog of Computer Equipment and Software Systems.
access In programming, the manner in which files or data sets are referred to by the computer. See DIRECT ACCESS, RANDOM
ACCESS, SEQUENTIAL ACCESS.
access arm In backing storage, a mech­ anical device in a disk drive that positions the reading and writing mechanisms. See DISK DRIVE, HEAD.
2 access barred
access barred In data communications, a data faciJity that permits a terminal instal­ lation to make outgoing, or receive incom­ ing, calls but not both.
access control (1) In computer networks, the control of system usage, imposed by hardware, software and administrative con­ trols. Such controls include system monitor­ ing, user identification, ensuring data in­ tegrity, recording system access and changes and methods for granting user access. See HARDWARE, SOFTWARE. (2) In databases, the control of the use of the database information. Access to a particular user may be restricted both in terms of the data items that may be made available and the operations that may be performed on it, e.g. read, upqate etc.
access line In data communications, a tele­ communication line that continuously con­ nects a remote station to a DSE. A tele­ phone number is associated with such lines. See DSE.
access time In backing storage, the time interval between the instant that data is requested from the storage device to the instant that it is delivered to the CPU, and vice versa. The average access time for magnetic disk systems is usually based upon the assumption that the read/write heads will have to move over one-third of the maxi­ mum number of tracks in a random seek operation. Compare SEEKS PER HOUR. See CPU, MAGNETIC DISK, READ/WRITE HEAD,
TRACK.
accidental destruction In data security, the unintentional overwriting or deletion of data, e.g. by faulty hardware or software. Backup is needed for recovery. See BACKUP
COPY.
accordion fold In pnntmg, a method of folding paper in which each fold is in the opposite direction to the previous one. A printer can be fed with accordion-folded paper without continuous operator inter­ vention.
accumulator In architecture, a device that functions as a holding register for arithmetic, logical and input/output operations. In some CPUs, data words fetched from memory are loaded into the accumulator and words to be stored into memory are first loaded into this register. See CPU, INPUT/OUTPUT, REGISTER.
accuracy In mathematics, the degree of exactness of an approximation or measure­ ment. It denotes the absolute quaJity of the result with respect to its true value, as compared with precision which is concerned with the amount of detail used in specifying a result. Thus a two-digit result may be more accurate than an incorrect three-digit result, but it will be less precise. See PRECISION.
Aces In computer languages, a continuous simulation language. See CONTINUOUS SIMU­
LATION.
achromatic color In computer graphics, an intermediate gray level in the monochro­ matic gray scale. See GRAY SCALE.
AC IA See ASYNCHRONOUS COMMUNICA-
TIONS INTERFACE ADAPTOR.
access time
Memory cost/performance trade off.
0.0001
acknowledge character In data com­ munications, a character transmitted by a station as an affirmative response to the station with wh ich the connection has been set up. Compare NEGATIVE ACKNOWLEDGE­
MENT. See ACKNOWLEDGEMENT, STATION.
acknowledgement In data communica­ tions, the transmission by a receiver of acknowledge characters as a response to a sender. See AFFIRMATIVE ACKNOWLEDGE­
MENT, NEGATIVE ACKNOWLEDGEMENT.
ACM See ASSOCIATION FOR COMPUTING MACHINERY.
acoustic coupler In data communications, a device to interface an item of equipment, producing digital signals, to a telephone network. Sound transducers in the acoustic coupler produce sound tones corresponding to the digital signals; a telephone handset is placed in contact with the sound transducers so that these tones can be input into the telephone network. See MODEM, TRANS­ DUCER.
action message In operations, a message issued because of a condition that requires an operator response.
activation In computer networks, the process by which a component of anode is made ready to perform the functions for which it was designed. See NODE.
aetive device In e1ectronics, a circuit that contains an amplifier providing gain. Com­ pare PASSIVE DEVICE. See AMPLIFIER, GAIN.
active file In data structures, a permanent or temporary file, having an expiration date that is later than the job date. See JOB.
active state In microelectronics, the digital state that causes a given action to occur. It may be either the high state or low state, depending on the circuit and pin in question.
aetive wire concentrator In computer net­ works, cabinets with star connection to individual nodes. In some local area net­ works, they are connected in a ring, thus providing a combined ring/star configur­ ation. See LOCAL AREA NETWORK, NODE, RING, STAR.
aetivity In data processing, the percentage of records in a file that are processed in a run. See VOLATILITY.
Ada 3
activity loading In data processing, a method of storing records on a file in which the most frequently processed records can be located most readily. See RECORD.
activity ratio In data processing, the ratio of the number of records in a file that are in use to the total number of records in that file. See RECORD.
ACTSU US Association of Computer Time Sharing Users.
aetual data transfer rate In data com­ munications, the average number of bits, characters or blocks, per unit time, trans­ ferred from a data source and received by a data sink. See DATA SINK, SOURCE.
aetuator In hardware, a device that is capable of mechanical action under the control of a signal. See INDUSTRIAL ROBOTS.
ACU See AUTOMATIC CALLING UNIT.
Ada A trademark, in programming, a language named after Augusta Ada, Coun­ tess of Lovelace who assisted Babbage.
(a) Main applications. Ada was developed for use in computer control and communica­ tion systems, where instruments or systems are monitored or governed by a program. Typical uses include factory production lines, data recording in laboratories, naviga­ ti on al systems, networking and interfacing of multiple processors. Ada is aimed at installations with a long lifespan where software modification and maintenance are of major concern.
(b) Originated in the US Department of Defense in the late 1970s.
(c) Facilities. Ada is essentially derived from Pascal. This entry will therefore con­ centrate upon those additional features that are unique to it. Heavy emphasis is placed upon program readability; it is considered that it is more important for the program to be easily understood than rapidly written. In many applications the cost of maintenance can be an order of magnitude greater than that of the original production. The langu­ age is therefore rigidly structured and data typing is strongly enforced; even simple operations require a substantial block of code. Machine independence is assisted by the modular nature of Ada, ensuring that
4 ADAPSO
implementation details are confined to a few specialized interface packages.
An unusual feature of this language lies in its provision for programmable exception handling. Whereas most languages assurne the correctness of finished programs, and exceptional conditions are detected and handled by the operating system, Ada recognizes that the nature of realtime opera­ tion, hardware communication and large sections of code render total reliability unattainable, and so allows for the software detection and correction of errors without recourse to human intervention. In many control applications, e.g. those involving heavy machinery, it is vital that the advent of a run time fault does not cause the software to cease to operate.
Concurrent programming is supported to permit the control of parallel processes, through the declaration of tasks whose execution is interleaved. Tasks may co m­ municate, usually by means of a rendezvous, and nondeterministic operation is permitted. Ada programs may not be necessarily sup­ ported by an operating system in the host computer, and they must therefore take command of the necessary input/output and housekeeping operations. Hence a special­ ized support environment for developing and testing code has been investigated for the language.
In Ada the major units of program struc­ ture are the package and the procedure. Packages may be separately compiled but where this is done the package must come in two parts: the specification part, which defines the package interface; and the body or implementation part. The purpose of the separation is to permit the compiler to check for valid interface usage in the same way as languages such as Pascal do within a single compilation unit. The package constructed thus assists in the correct implementation of very large programs and provides a frame­ work for the creation of abstract data types.
(d) Syntax. Ada is a block-structured language that centers upon the nesting of statement sequences, successive statements being separated by semicolons. A substan­ tial amount of data and control declarations are required by the compiler, and, as for Pascal, the precise rules are described by a complex set of graphie syntax diagrams.
(e) Sampie program. The following com-
plete program declares two integers, reads them as input and prints out the largest, having invoked the package TEXTIO to perform input/output operations. with TEXT 10; use TEXT 10; procedure DEMO is pragma main; P,Q: integer range 0 .. 100; begin GET(P);GET(Q); NEW UNE; if P>Q then PUT(P); else PUT(Q); end if; end DEMO;
(f) Comments. Ada is a relatively new language whose real worth may not become apparent for some years . While it has many advantages in the programming of control computers and other real time activities, with associated ease of software maintenance, it is highly verbose and unlikely to compete with the established block-structured langu­ ages for more mundane tasks.
(g) Similar to Pascal, C, PLil. see ABSTRACT DATA TYPE, BLOCK STRUCTURE, C, COMPILER, CONCURRENT PROGRAMMING, DATA ACQUISITION, DATA TYPE, DECLARATION, HIS­ TORY OF COMPUTING, HOST COMPUTER, HOUSE­ KEEPING, NEST, PARALLEL PROCESSING, PASCAL, PLI, REALTIME, RENDEZVOUS, SOFT­ WARE MAINTENANCE, SYNTAX.
ADAPSO US and Canada Association of Data Processing Service Organizations.
adaptive channel allocation In communica­ tions, a method of multiplexing where chan­ nels are allocated according to demand rather than on a fixed, predetermined plan. See FREQUENCY DIVISION MULTIPLE ACCESS, MULTIPLEXING, TIME DIVISION MULTIPLE ACCESS.
adaptive routing In data communications, a routing scheme for packets or messages in which the behavior adapts to network changes such as line failures or variation of the traffic pattern. See MESSAGE SWITCHING, PACKET SWITCHING, ROUTING.
adaptive systems Systems that display the ability to learn to change, alter their state or otherwise react to a stimulus.
ADC See ANALOG TO DIGITAL CONVERTER.
ADCCP See ADVANCED DATA COMMUNICA­
TIONS CONTROL PROCEDURE.
VERTER.
addend In mathematics, the operand of the addition operation, the number added to the augend to form a sumo See AUGEND,
OPERAND.
adder In architecture, a device that forms an output resulting from the sum of Mo or more numbers presented as inputs. See FULL
ADDER, HALF ADDER.
add-in In hardware, an expansion card that slots into a microcomputer to provide ad­ ditional facilities. This is a very simple method of enhancing a microcomputer. The boards available allow for additional RAM, additional operating facilities, particularly CP/M, enhanced graphics, modems, instru­ mentation etc. See CP/M, EXPANSION CARD,
MODEM, RAM.
address In programming, (1) a character or group of characters that identifies a register, a particular part of storage, or some other data source or destination, (2) to refer to a device or an item of data by its address. (3) In communications, the part of the selection signals that indicates the destination of a call. (4) In word processing, the location, identified by an address code, of a specific section of the recording medium or storage.
addressability In computer graphics, the number of addressable points within a speci­ fied display space or image space. See DISPLAY SPACE, PICTURE ELEMENT.
address bus In buses, a unidirectional bus over wh ich digital information is transmitted to identify either a particular memory loca­ tion or a particular input/output device. Compare CONTROL BUS, DATA BUS. See BUS,
INPUT/OUTPUT DEVICE, MICROCOMPUTER.
addressing mode 5
address fleld In programming, the specific portion of a computer word that contains either the address of the operand or the information necessary to derive that ad­ dress. See OPERAND, WORD.
address format In programming, the arrangement of the parts of a simple address, such as those required for identify­ ing a channel, module or track on a magnetic disko See ADDRESS, MAGNETIC DISK, TRACK.
addressing (1) In programming, the assign­ ment of addresses to the instructions of a program. (2) In communications, the means whereby the originator or control station selects the unit to which it is going to send a message. See STATION.
addressing mode In programming, the method of specifying the location of data during the execution of a machine code instruction. At the simplest level the operand may contain the actual address of the data or, in the immediate mode, the da ta itself.
Locating the absolute address in the operand, however, is uneconomic, inflexible and inefficient for many operations. Un­ economic because with al-Mbyte address space each operand of each instruction would need to be 20 bits long. Inflexible because there would be no straightforward method of using the same instruction to access successive locations in successive in­ vocations, e.g. in a loop performing a table lookup. Inefficient because every data operation would involve transfer of data between main memory and the CPU. The major dis advantage and restriction to this mode, however, is that the program can only be executed from one position in memory.
With relative addressing the operand con­ tains a quantity that represents a displace­ ment from the current value of the instruc­ tion pointer. This is often used in jump instructions and has both the advantage that the displacement requires fewer bits than the absolute address and that the instruction will function correctly if the program is relo­ cated.
Register addressing is employed to en­ hance the power of instruction sets. Register addresses may be implicit with the instruc-
6 address modification
Installed .&.ddress rnerTIOfy
AAMoo rnolher· 000'"
RAMon molher· _d
address spaces in an IBM-pe, XT and AT
tion or a particular register may be specified with a few bits of the instruction. Data moves from one register to another are fast and efficient because they require no traffic on the data bus. More importantly, how­ ever, register addressing provides both high flexibility in programming and facilitates memory management. Indexed addressing provides a data address by combining the contents of an index register with the fixed value of an operand; this technique is essen­ tial to update addresses in loop operations.
Indirect addressing implies that the memory location accessed contains not the required information but the address of that information; this technique provides for considerable flexibility in programming be­ cause the location of the requisite informa­ tion can be changed during program execu­ tion. 16-bit microcomputers provide for very sophisticated addressing modes; the Intel 8088/8086 has 25 such modes. A range of registers is employed to facilitate seg-
mentation, program relocation and the accessing of sophisticated data structures. For example, an address in an 8088/8086 instruction may involve the contents of segment register, a base register, an index register and a displacement. See ABSOLUTE ADDRESS, ADDRESS, BIT, BYTE, CPU, DATA BUS, DATA STRUCTURE, IMMEDIATE ADDRESS, INDEXED ADDRESS, INDEX REGISTER, INDIRECT ADDRESS, INSTRUCTION COUNTER, LOOKUP TABLE, LOOP, MACHINE CODE INSTRUCTION, MEMORY MANAGEMENT, MEMORY ORGANIZA­ TION.
address modification In programming, an action that causes an address to be alte red in a prescribed way by an arithmetic, syntactic or logic operation. See ADDRESSING MODE.
address register In architecture, a special register used by the CPU to store the address of data to be fetched from, or stored in, the computer memory. See CPU, REGISTER.
address space In architecture, the nu mb er of memory cells that may be accessed by the CPU. If an 8-bit processor employs a 2-byte address then the address space is 65,536 cells, and each cell normally holds 1 byte. With 16-bit microprocessors the address bus normally has 20 lines giving an address space in excess of 1 Mbyte. Bank-switching tech­ niques can be employed to reduce the disadvantages of a sm all address space. See BANK SWITCHING, BYTE, CPU, 8-BIT MICRO­ PROCESSOR, I6-BIT MICROPROCESSOR.
address track In backing storage, a track on a magnetic disk containing the addresses of files, records etc. stored on other tracks of the same device. See MAGNETIC DISK, TRACK.
add time In performance, the time re­ quired by a particular CPU to add two multidigit numbers, not including the time taken to read the numbers or store the result. Microcomputers are often rated by comparing add times as a criterion of their relative speed. See MICROCOMPUTER.
ADIS Automatic System.
Data Interchange
adjacency In character recognition, a con­ dition in which the character spacing refer­ ence lines, of two consecutively printed characters on the same line, are separated by less than a specified distance.
adjacent channel In communications, the next channel, or the one in elose proximity, either physically or electrically, to the one in current use. See CHANNEL.
adjust In word processing, an editing feature in which the system automatically adjusts the right-hand margin for insertion or deletion of copy during playback. Word, and sometimes page, wraparound is auto­ matically performed as needed. See WRAP­
AROUND.
ADP See AUTOMATIC DATA PROCESSING.
ADSR In man-machine interfaces, Attack Decay Sustain Release; the shape of the envelope that modulates the amplitude of a periodic waveform such as a sawtooth or sine wave. The sound generator chip of a microcomputer can be programmed to pro­ duce music by varying the ADSR and the modulating waveform. See MODULATION,
MUSIC SYNTHESIZER, SAWTOOTH, SINE WAVE.
~ ~.l!- Sustain ~ . " 0.6 Decay
ADSR Typical amplitude envelope shape for a musical note.
advanced data communications control pro­ cedure In data communications, pertaining to the operation of a data link using an advanced (SDLC, HDLC) protocol. See HDLC, PROTOCOL, SDLC.
advanced data link control In data com­ munications, a link protocol used in HDLC and SDLC systems. See HDLC, SDLC.
AECT US Association for Educational Communication and Technology.
air gap 7
affine c10sure In mathematics, a repre­ sentation of infinity in which the positive and negative infinities are considered to be at opposite 'ends' of a line of all real numbers. Compare PROJECTIVE CLOSURE. See INFINITY, REAL NUMBER.
affirmative acknowledgement In data communications, the replies ACKO and ACK1 in binary synchronous transmission indicate that the previous transmission block was accepted by the receiver and that it is ready to accept the next block. ACKO and ACK1 sent altemately provide sequential checking for aseries of replies. ACKO is also used as an affirmative reply to station se1ection signal in a multidrop circuit, or to an initialization sequence in a point to point operation. See BINARY SYNCHRONOUS COM­
MUNICATIONS, MULTIDROP CIRCUIT, POINT TO POINT.
AFIPS American Federation of Informa­ tion Processing Inc. A federation founded in 1961 which ineludes the American Society for Information Science, American Statis­ tical Association, Association for Comput­ ing Machinery, Association for Education Data Systems, Data Processing Manage­ ment Association, IEEE Computer Society, Instrument Society of America, Society for Computer Simulation, Society for Industrial and Applied Mathematics, Society for In­ formation Display. It is the US member of IFIP. See IFIP.
AFNOR Association Fran~aise de NOR­ malisation - the French standards organisa­ tion.
afterglow Synonymous with PERSISTENCE.
after image In databases, a copy of a record after it has been modified by a user, or program. If there is a system failure the after images can be used to update the database from a previous dump. Compare BEFORE IMAGE. See DUMP, TRANSACTION .
AI See ARTIFICIAL INTELLIGENCE.
air gap In backing storage, the very narrow gap between the two elements of a magnetic recording or playback head. See HEAD.
8 alarm
alarm In operations, a visual or audio signal to signify that an error has arisen or an abnormal situation exists.
ALGOL In programming, ALGOrithmic Language, an early block-structured langu­ age providlng many elegant features that were lacking in other early high-level langu­ ages. It has been largely superseded by Pascal. See HIGH-LEVEL LANGUAGE, PASCAL, PROGRAMMING.
algorithm In progamming, a finite set of well-defined rules for the solution of a problem in a finite number of steps; for example, a precise description of the steps involved in determining the record with the highest value of a specified numerical attri­ bute. See ATIRIBUTE, PROGRAMMING , RECORD.
algorithmic language A computer language designed for expressing algorithms. See ALGOL.
algorithmic pattern generation In memory systems, a method of testing RAM in which a binary pattern is written and the RAM is subsequently read to verify that it was written correctly, and that the writing into one area of RAM did not affect the data stored in another section. See FIXED PATIERN TESTING, GALLOPING PATIERN TESTING, RAM.
alias In operating systems, a Unix shell command that enables the user to set up another name for a Unix program, e.g. the user could replace 'ls' with the more mnem­ onic term 'list directory'. See LS, SHELL, UNIX.
aliasing (1) An effect that occurs when a signal is sampled at a rate less than twice the highest frequency present in it. When a subsequent signal is recovered from the sampies it will not contain the high-frequ­ ency component of the original signal, and it will instead display a false low-frequency signal. See NYQUIST SAMPLING THEOREM. (2) In computer graphics, an effect that occurs when a computer attempts to handle detail of a diagram which exceeds the basic resolving power of the system, e.g. the staircase effect produced when a
low-resolution computer graphic system attempts to displaya diagonalline. Compare ANTI-ALIASING.
aligning edge In optical character recogni­ tion, the edge of a form which, in conjunc­ tion with the leading edge, serves to position correctly the document that is to be scanned. See SCAN.
alignment pin In electronics, any pin or device that will ensure the correct mating of two components designed to be connected.
allocate In operating systems, (1) to assign aresource, such as a disk or a diskette file , to a specific task. See FILE, MAGNETIC D1SK, TASK. (2) To assign memory allocations to main routines and subroutines. See SUB-ROUTINE.
allophone In man-machine interfaces, a manifestation of a phoneme in a speech signal. A phoneme may be acoustically different depending upon word position and an allophone is a positional variant of the same phoneme. See PHONEME, SPEECH SYNTHESIZER.
Aloha In computer networks, a packet­ switched system at the University of Hawaii which uses radio broadcast techniques.
alphabet (1) An ordered set of all the letters and associated marks used in a language or work. (2) An ordered set of letters used in a code language, e.g. the 128 characters of the ASCII alphabet. See ASCII.
alpha beta technique In artificial intelli­ gence, a technique used in game-playing routines to determine the best set of moves for a given player. The player will pick the set of moves to maximize his score while the adversary will always attempt to select moves that will minimize his losses. The successive set of moves can be represented by a tree structure, one player having the choice of branches from one level and the adversary the choice at the next level. The alpha beta technique eliminates subtrees to be searched from the tree, on the basis that both players using optimal strategies would never employ such subtrees, thus reducing
the effort of searching for optimum moves. See TREE STRUCTURE.
alphabetic character set A character set that contains letters but not digits. The set may contain control characters, special characters and the space character. Compare ALPHANUMERIC. See CONTROL CHARACTER.
a1phabetic shirt In peripherals, a control for selecting the alphabetic character set in an alphanumeric keyboard printer.
alphabetic string A character string con­ sisting of letters from the same alphabet. See STRING.
a1phageometric In computer graphics, a standard in which the codes can instruct the terminal to produce line drawings, fill areas with color etc. in addition to normal charac­ ter display modes. Compare ALPHAMOSAIC. See NAPLPS, VIDEOTEX.
alphamosaic In computer graphics, a standard in which the codes determine the alphanumeric character or mosaic pattern to be displayed in a character space. Compare ALPHAGEOMETRIC. See ALPHANUMERIC, CHAR­ ACTER SPACE, VIDEOTEX.
a1phanumeric Pertaining to a character set that contains letters, digits and usually other characters, e.g. punctuation marks.
a1phanumeric character set A character set that contains both letters and digits and may contain control characters, special charac­ ters and the space character. See ALPHA­ NUMERIC.
a1phanumeric data Data represented by letters and digits and perhaps special charac­ ters and the space character. See ALPHANU­ MERIC.
a1phanumeric display device Synony- mous with CHARACTER DISPLAY DEVICE.
a1phanumeric keyboard In videotex, a keyboard used for ente ring letters, numbers and special characters . It is required by IPs for creating frames. Users with this key­ board can send messages via electronic mail or fill response frames with alphanumeric
Alvey 9
information. Users who are only equipped with numeric keypads enter such informa­ tion in a format determined by a predefined menu selection. See ALPHANUMERIC, ELEC­ TRONIC MAlL, FRAME, IP, KEYPAD, RESPONSE FRAME.
alphanumerics mode In videotex, the dis­ play mode in which the display characters are those of the alphanumerics set. Compare GRAPHICS MODE. See DISPLAY MODE.
alphanumerics set In videotex, the set of 96 display characters comprising all the alpha­ numerics characters. See DISPLAY CHAR­ ACTER.
alphaphotographic In videotex, a method of displaying alphanumeric characters and picture quality graphics from individually transmitted and stored picture elements. See PICTURE PRESTEL.
alpha testing In programming, the in­ house testing of a package by a software house prior to beta testing. There are a wide variety of conditions under which a software package is used, and the testing phase of product development can be as much as three times as expensive as the development phase. Compare BETA TESTING. See SOFT­ WARE HOUSE.
Altair bus Synonymous with SIOO.
alternate route In communications, a secondary or backup route that is used if normal routing is not possible.
alternate track In backing storage, a track on a magnetic disk or other storage device, which is automatically substituted for a damaged track. See MAGNETIC DISK, TRACK.
alternating current In electronics, electric power supply in the form of a sine wave, normally a frequency of 60 Hz in the USA, and 50 Hz in the UK. Compare DIRECT CURRENT. See Hz.
ALU See ARITHMETIC LOGIC UNIT.
Alvey A programme, named after Mr John Alvey, of pre-competitive research in ad-
10 AM
vanced information technology costing some BOO million over five years. See ESPRIT.
AM See AMPLITUDE MODULATION.
ambient noise level In electronics, random, uncontrollable and irreducible noise level at a location or circuit. See NOISE.
American National Standards Institute A body that organizes committees formed of computer users, manufacturers etc., to develop and publish industry standards; e.g. ANSI FORTRAN, ANSI Standard Code for Periodical Identification.
ampere In electronics, the basic unit of electrical current. Compare VOLT. See CURRENT.
amplification In electronics, (1) the strengthening of a weak signal, (2) the ratio between so me measure of the output signal and the input signal of a device. Compare ATTENUATION.
amplifier In electronics, a normally unidi­ rectional device that increases the power or amplitude of an electrical signal. See AMPLI­ TUDE.
amplitude In electronics, the magnitude of the greatest deviation from the midpoint value of a periodic signal or phenomenon. See FREQUENCY, WAVELENGTH.
amplitude modulation In communications, a form of modulation in which the amplitude of the carrier signal is varied in accordance with the amplitude of the modulating signal. Compare FREQUENCY MODULATION, PHASE
MODULATION. See CARRIER, MODULATION.
analog In computing and communications, pertaining to the form of continuously vari­ able physical quantities. Compare DIGITAL.
See ANALOG SIGNAL, TRANSDUCER.
analog channel In communications, a data channel in which the amplitude of the signal transmitted can take any value between the limits defined for the channel. Voice-grade channels are analog channels. See ANALOG.
analog computer A device that performs mathematical functions on continuous vari­ ables, usually voltages, and produces a solution in the form of an analog signal. They were used extensively in the study of dynamic systems and as simulators. The basic building block of the analog computer was the operation al amplifier, used to pro­ duce the mathematical functions of addition, subtraction, integration and multiplication. Non-linear effects, e.g. saturation, were also effected with special diode units. Unlike digital computers they were parallel in operation, easy to program for their dass of problems and relatively fast. They did not benefit, however, from the rapid advances in digital technology, and they are now obso­ lete except for special purpose applications. See DIGITAL COMPUTER, HYBRID COMPUTER,
OPERATIONAL AMPLIFIER, SIMULATION.
analog flux copier Synonymous with BIT
COPIER.
analog signal A signal that varies contin­ uously according to the information in trans­ mission, e.g. sound waves. Compare DIGITAL
SIGNAL.
analog to digital converter In control and instrumentation, a device that accepts a continuous analog signal and produces a stream of digital signals corresponding to values of the analog signal at sampling instants. The speed of conversion of the sampled signal may vary from less than 1 microsecond to up to 100 milliseconds. The converter may operate by successive ap­ proximation or integration.
The successive approximation method re­ quires aperiod of 1 to 25 microseconds; the converter successively approximates the digital bits until an internally genera ted digital to analog converter output equals the analog input. The analog input must be held constant during the conversion period by a sampie and hold device. This type of analog to digital converter is used for high sampling rates where the noise rejection is not a major problem.
The integration converter is used when the analog signal is subject to noise and high
sampling rates (i.e. greater than 1000 read­ ings per second) are not required. The analog to digital conversion integrates the analog signalover aperiod of several milli­ seconds, thus averaging out the effects of noise. Compare DIGITAL TO ANALOG CON­ VERTER. See COMPUTERIZED INSTRUMENTA­ TION, SAMPLE AND HOLD CIRCUIT.
analog transmission In communications, the transmission of information by analog signal. See ANALOG SIGNAL.
analysis The methodical investigation of a problem, and the separation of the problem into smaller related units for further detailed study. Compare SYNTHESIS. See SYSTEMS ANALYSIS.
analyst A person who defines problems and develops algorithms and procedures for their solution. See SYSTEMS ANALYSIS.
ancillary equipment In communications, equipment located in a subscriber's pre­ mises, e.g. answering devices, automatic dialers, to provide a greater utility of a communications channel for individual sub­ scribers.
AND A logical operation, A AND B has the result true only if both of the logical variables A and Bare true. The correspond­ ing truth table is:
A B AANDB 000 100 o 1 0 1 1 1
Compare OR. See BOOLEAN ALGEBRA, TRUTH TABLE.
AND gate In electronics, a logic unit that produces an output signal that is the logical AND of the input signals . Compare OR GATE. See AND.
angstrom A unit of measurement equal to 10 to the power of minus 10 meters, i.e. one ten-millionth of a millimeter. Commonly used in the measurement of wavelengths of light.
animation In video and computer graphics, the projection of a continuous sequence of related images, at a speed that matches the
ANSI-SPARC 11
human eye's inherent persistence of vision, to create a flicker-free image. At the average cinematic screen illumination the human eye detects no flicker for frequencies of inter­ mission above apl?roximately 16 per second. American televIsion generates images at 30 frames per second, while the European standard is 25 frames per second. Conflicts arise in the television and video recording of films employing 24 frames per second. American television uses three two puH­ down. British television transmits the film at 25 frames per second giving a speed increase of 4 percent and a rise in the sound pitch. See PERSISTENCE OF VISION, THREE TWO PULL­ DOWN.
anisochronous signal In electronics, a sig­ nal that is not related to any dock and in wh ich transitions could occur at any instant. See ANISOCHRONOUS TRANSMISSION, CLOCK.
anisochronous transmission In data com­ munications, a form of data transmission, similar to asynchronous transmission, but in which there can be variable time intervals between the bits within a character as weil as between the characters themselves. Com­ pare ISOCHRONOUS TRANSMISSION. See ASYNCHRONOUS TRANSMISSION, BIT.
A/N See ALPHANUMERIC.
annotation symbol In programming, a symbol used to add messages or notes to a flowchart. See FLOWCHART.
anode In electronics, the positive terminal of a device. Compare CATHODE.
ANS I See AMERICAN NATIONAL STANDARDS INSTITUTE.
ANSI MCll-1975 In buses, another designation for GPIB. See ANSI, GPIB.
ANSI-SPARC In databases, pertaining to schemata developed as a general standard for the description of database management systems. The standard defines three schemata: (a) a conceptual schema which provides a logical description of the data; (b) one or more user schemata which represent the users' views of the data; and (c) a physical schema which describes the mann er in which the data is stored in the computer. See SCHEMA.
12 ANS I X3H3
ANSI X3H3 In computer graphics, the ANSI Technical Committee on Computing Graphics Programming Languages. See ANSI.
answer back In data communications, a signal sent by a receiving unit to the sending station for identification or to indicate it is ready for transmission. See VOlCE ANSWER
BACK.
answering In data communications, the process of responding to a calling station to complete the establishment of a connection between data stations. See STATION.
answering time The time between the appearance of a signal and the response made to it.
anti-aliasing In computer graphics, a method of disguising the aliasing errors introduced by low-resolution graphics dis­ plays. The jagged edges produced by boun­ dary pixels may be softened by adjusting the shading intensities to create a smoother transition of color changes. Compare ALiASING. See PIXEL.
anticipatory staging In operating systems, a technique in which blocks of data are moved from one storage device to another, with a shorter access time, in anticipation that they will be required by the program and before the program actually requests them. Compare DEMAND STAGING.
anticoincidence circuit In hardware, a logic circuit that only provldes an output if dif­ ferent signals are received on the input lines. Compare COINCIDENCE CIRCUIT.
Antiope The French standard for character coding and display for videotex terminals.
APA In computer graphics, All Points Addressable. Synonymous with PIXEL.
APD See AVALANCHE PHOTODIODE.
aperture In architecture, apart of a mask that permits retention of the corresponding portions of data. See MASK.
APL In programming, A Programming Language.
(a) Main applications. APL is an unusual language in that it began life as a functional notation to express mathematical algorithms and was subsequently adapted for use as a programming language. It is extremely con­ cise and has found favour mainly in mathe­ matical applications, although it has also been used for tasks such as data retrieval, teaching and simulation. As an interactive, and interpreted, language it is more suited to one-off jobs and experimental tasks than large program construction.
(b) Originated from IBM in the late 1960s from Iverson's notation.
(c) Facilities. APL allows the expression of complex mathematical functions in a very compact manner, employing a special char­ acter set to this end. Whereas most langu­ ages can only operate directly upon single variables, APL handles complete data struc­ tures (arrays) with its primitive operations and thus offers a great saving of effort in many situations. Indeed, the whole language is based around the use of numeric and character arrays alone, providing simplicity at the cost of more elegant data structures.
APL provides a powerful range of mathe­ matical and logical array functions, gener­ ator operations wh ich permit sophisticated array manipulation and rather limited input/ output facilities; its mathematical derivation gives rise to certain peculiarities such as right to left evaluation of expressions. The pro­ gram constructs are rather weak; they are centered on the GO TO statement, but provision is made for recursive subprogram calls with parameter passing and local vari­ ables.
The interactive operation of APL is one of its most important features, the language is well suited to the rapid development of small programs for experimentation, or single runs of data processing. The user is allocated a temporary workspace for the creation of programs and variable storage, wh ich can be loaded from, or stored into, a system library. The user is also afforded intimate control of code execution within this environment, to the extent that there is no real concept of a main program but rather a collection of subprograms which can be called into action, interrupted and rearanged at will.
(d) Syntax. The syntax is straightforward and homogeneous. A piece of code com­ prises a set of independent subprograms,
each composed 01 statements numbered by their order of execution. A statement may only be a single expression, although the variety is somewhat bewildering due to the many operators available.
(e) Typical program . An APL program for operations on the rows of a two dimensional array would have more the appearance of a complex mathematical expression than a statement of computer instructions.
(f) Comments. APL may perhaps be better described as a programming environ­ ment, than a language, since it is ill­ equipped for the production of finished programs intended for intensive use. The dense code allows good execution speeds but also permits the construction of single state­ ments that are alm ost unintelligible even to the highly experienced programmer. The impoverished program and data structures also restrict its range of applications, where­ as the requirement of a special character set restricts its use on many computers. See ARRAY, DATA STRUCTURE, GO Ta, INTER­ PRETER, LOCAL VARIABLE, PARAMETER PASS­ ING, RECURSIVE ROUTINE, SYNTAX, SYSTEM LIBRARY, VECTOR, WORKSPACE. application interface - In distributed pro­ cessing, a routine that enables a program in a microcomputer to communicate with the microprocessor on an emulator expansion card. This routine could be a sequence of operator commands for automating file up­ loading to the host. See EXPANSION CARD, HOST COMPUTER, SCRIPT, TERMINAL EMULA­ TION , UPLOAD. application layer In data communications, the topmost layer in the ISO Open Systems Interconnections model. The content of this layer is left to the users, and it is expected that standard protocols for specific indus­ tries will be developed. Cornpare DATA LINK LAYER, NETWORK LAYER, PHYSICAL LAYER, PRESENTATION LAYER, SESSION LAYER, TRANS­ PORT LAYER. See OPEN SYSTEMS INTERCON­ NECTION. application-oriented language In pro­ gramming, a language that has facilities or notations useful for solving problems in one or more specific classes of applications, e.g. numerical analysis, business data processing, simulation. application program In programming, a program, usually written in-house, for a
architecture 13
specific user application, e.g. payroll. Corn­ pare OPERATING SYSTEM, SOFTWARE PACKAGE, SOFTWARE TOOL. See IN-HOUSE.
architecture The specification of the re­ lationships between parts of a computer system. All generations of computers prior to the fifth generation were based upon Von Neumann architecture. This architecture was the turning point in computer design because it incorporated the concept of stored pro gram control, thus decoupling the hardware design from the detailed consider­ ation of algorithms relating to applications and providing the incredible flexibility of modern computers. The traditional Von Neumann computer contains: (a) a single computing element incorporating a proces­ sor, communications and memory; (b) a linear organization of fixed-sized memory cells; (c) a one-Ievel address space of memory cells; (d) a low-Ievel machine language; (e) sequential centralized control of computation; and (f) a primitive input/ output capability.
The Von Neumann architecture was the keystone of computer development for over 40 years, but it was designed in an era when electronic devices were expensive, large, relatively unreliable and dissipated consider­ able thermal power. Moreover the computer applications in the 1950s and 1960s were relatively straightforward and amenable to design by conventional high-Ievellanguages. The situation changed during the late 1970s; the software crisis can be, at least in part, attributed to the fact that relatively simple architectures can only be induced to perform complex processes by the design of sophis­ ticated software. Moreover the massive advances in microelectronics have provided architecture designers with powerful, cheap, fast processors and memories. The wheel has thus undergone a complete revolution, and it is now more economical to design hardware modules for classes of problems.
The perceived shortcomings of current architectures are given below.
(a) They are not equipped with the necessary functions to process non-numeric data: sentences, symbols, speech, graphics and images. These function as are essential for user friendly man-machine interfaces.
(b) They are unable to provide the level of performance demanded by a wide range of
14 archiving
and difficult to implement. (d) System performance with conven­
tional architectures appears to be elose to a plateau.
(e) It is not elear that VLSI technology can be efficiently exploited with multimicro­ computer architecture.
Von Neumann architectures operate upon the cyele: control unit sends one instruction at a time to the ALU, the ALU performs one instruction at a time on a single piece of data extracted from memory for that pur­ pose. This mechanism is very inefficient for modern applications, e.g. controlling the multifarious aspects of a sophisticated robot, analyzing images from TV cameras. Parallel processing is essential for these and many other modern areas of computer application. Attempts to develop parallel processor architecture have been made, e.g. ICL distributed array processor and London University College CLIP-4 machine. With the ever-decreasing cost of processors, multiprocessor architectures providing true parallelism, i.e. operating on multiple in­ struction streams and multiple data streams, with possibly hundreds or more processors, can be expected. Such machines could pro­ vide the high degree of redundancy essential for realtime processing.
New forms of architecture have moved away from the control flow characteristics of Von Neumann machines. Thus in data flow architecture, instructions are actuated when the required input data is available and are weIl geared to parallel processing. Dis­ tributed function architectures are designed with modular facilities allowing specialized machines to perform their specific functions, e.g. database management, computation, man-machine interface operations, network control, with high efficiency. The fifth gen­ eration of computers will necessarily em­ body novel architecture to achieve their objective. See ALU, DATABASE MANAGE­
MENT SYSTEM, DATA FLOW, FIFTH-GENERATION
COMPUTER, HISTORY OF COMPUTING, INDUS­
TRIAL ROBOTS, LOW-LEVEL LANGUAGE, MAN­
MACHINE INTERFACE, MULTIPROCESSING,
PARALLEL PROCESSING, PROCESSOR, VLSI.
archiving In operations, the storage of backup files and associated journals, usually
for a given period of time. See FILE, JOUR­
NAL.
ARDIS US Army Research and Develop­ ment Information System.
area In databases, the CODASYL defi­ nition of an area is a named subdivision of the addressable storage space in the data­ base which may contain occurrences of records and sets, or parts of sets of various types. See CODASYL, RECORD, SET.
area infill In computer graphics, a tech­ nique used with pixel-based systems in which a defined area of the screen is filled with a specified color or pattern. See PIXEL.
areal density In backing storage, the num­ ber of bits per unit area that can be stored on a recording device. In magnetic and optical disk systems, it is equal to the product of bits per inch and tracks per inch. See BITS PER
INCH, MAGNETIC DISK, OPTICAL DISK STORAGE,
TRACKS PER INCH.
argument (1) Any value of an independent variable. (2) In programming, a parameter passed between a calling program and a subprogram or statement function. See SUB­ ROUTINE.
arithmetic (1) The branch of mathematics concerned with the study of the positive real numbers and zero. (2) In programming, the operations of addition, subtraction, multi­ plication and division.
arithmetic capability In word processing, the ability of a system to be used as a calculator or adding machine.
arithmetic instruction In programming, an instruction in which the operation part specifies an operation that follows the rules of arithmetic. See INSTRUCTION.
arithmetic logic unit In architecture, the unit in which arithmetic, logic and related operations are performed. See CENTRAL PRO­
CESSING UNIT.
arithmetic mean In mathematics, the aver­ age value of a number of values of a variable. It is calculated by summing all the
component values and dividing the result by the number of values.
arithmetic overflow See OVERFLOW.
arithmetic shift In programming, (1) a shift that does not affect the sign position, (2) a shift that is equivalent to the multiplication of a number by a positive or negative integral power of the radix. Compare CIRCU­ LAR SHIFI, LOGICAL SHlFI. See RADIX, SHIFI.
arithmetic unit See ARITHMETIC LOGIC UNIT.
arm stealing In backing storage, the rapid movement of read/write heads, in a multi­ user environment, when the system is re­ sponding to a succession of enquiries from different users. See MAGNETIC DISK, READ/ WRITE HEAD.
ASCII 15
decay envelope. See ANALOG SIGNAL, MUSIC SYNTHESIZER.
artificial intelIigence (1) The science of making computers do things that would require intelligence if done by man. (2) That branch of computer science that studies how to make computers smarter. The term has been employed to cover a wide variety of computer developments, e.g. efficient repre­ sentation of knowledge, reasoning, deduc­ tion, problem solving and heuristic search. These capabilities have been applied in game playing, automatic theorem proving, automatic computer programming, robots, machine vision, natural language systems and information processing. See EXPERT SYSTEMS, MACHINE TRANSLATION.
ARPA Advanced Research ARU Audio Response Unit. See AUDIO
Projects RESPONSE TERMINAL. Agency. See D ARP A.
ARQ See AUTOMATIC RETRANSMISSION RE­ QUEST.
array In da ta structures, an ordered arrangement or pattern of items or numbers, e.g. table of numbers. The use of arrays can substantially reduce programming effort in the processing of repetitive operations. For example, if the same operation is to be performed on variables Al, A2, ... , AlOO, then the redefinition of these variables into an array A with 100 members enables the process to be defined as a loop operation. See LOOP, VECTOR.
array processor In architecture, a com­ puter system that is designed to perform identical operations, on elements of an array, in parallel. It may be a self-contained unit or attached to a mainframe computer via an internal bus or input/output port. See ARRAY, INPUT/OUTPUT PORT, PARALLEL PRO­ CESSING.
articulator In man-machine interfaces, a component of a music synthesizer that causes the sound to decay according to the characteristics of a particular instrument. The circuit effectively multiplies the analog signal of the sound by the approximate
ASA American Standards Association; a body with groups responsible for the estab­ lishment of data-processing standards.
ascender In printing, the portion of a lower case character, such as '1', 'h', etc. that rises above the upper level of letters such as· '0', 'a' etc. Compare DESCENDER. See LOWER CASE.
ASCII American Standard Code for Infor­ mation Interchange, pronounced ASKEE. A standard data transmission code that was introduced to achieve compatibility between data devices. It consists of seven information bits and one parity bit for error-checking purposes, thus allowing 128 code combin­ ations. Of these, 32 are used for upper case characters and a few punctuation marks, another group of 32 characters is used for numbers, spacing and additional punctua­ tion symbols, a third group of 32 characters is assigned to lower case characters and some rarely used punctuation symbols. The last set of 32 characters is allocated to machine and control commands, e.g. line feed, carriage return. See BIT, CARRIAGE CONTROL, INTERNATIONAL ALPHABET NUM­ BER 5, LOWER CASE, PARITY CHECKING, UPPER CASE.
>­ '" ~ - - C O
N T
R O
L C
H A
R A
C T
E R
ASIS American Society for Information Science.
aspect ratio In television, the ratio of the dimensions of a TV screen, normally 4:3.
assemble In programming, (1) to translate a source program using an assembler, (2) to in te grate subroutines into the main pro­ gram. See ASSEMBLING, SOURCE PROGRAM,
SUBROUTINE, TRANSLATOR.
assembler In programming, a program that translates a source pro gram written in a low­ level language to machine code. Compare COMPILER, INTERPRETER. See ASSEMBLING,
LOW-LEVEL LANGUAGE, MACHINE CODE,
SOURCE PROGRAM, TRANSLATOR.
assembling In programming, the processes involved in translating a low-Ievel pro gram into an object code program. The procedure commences with a source pro gram produced with a text editor. The source code instruc­ tions are generally arranged under four columns: label, operation code, source code and comment. Individual instructions do not necessarily contain entries in all the c?lumns, e.g. not all instructions require elther labels, operands or comments, and in some cases only the operation code entry is completed. Generally, but not invariably, one source code instruction will correspond to one machine code instruction. The excep­ tions are macros, where one source code instruction is translated into a predeter­ mined sequence of machine code instruc­ tions, and assembly directives. The latter do not produce any machine code instructions but simply provide commands to the assembler, e.g. to reserve storage space for data entries.
The source code file is processed by the assembler to produce the object code file. The assembler performs this task in two passes. One of the functions of the assembler is to replace mnemonics in the source code program by corresponding machine code addresses (in absolute or relative terms). In scanning through the program, however, the assembler will often encounter a reference to a mnemonic be fore it is defined, e.g. in a jump to a label instruction when the label is attached to an instruction further down the source code
assembly directive 17
program. The first pass is therefore required to construct the symbol table of all mnem­ onic references to data, instructions or addresses. All internal references, termed local references, can be handled at the end of the first pass. However external refer­ ences, to code in other modules or to library subroutines, must be handled in a special manner at a later stage . Moreover the programmer mayaiso specify that certain labels are to be referenced by external modules. Such references are dec\ared to be public, or global, and are stored in global reference tables to be accessed in the later linking stage of producing the executable machine code.
In the second pass, the assembler can produce the necessary object code file. Operation code mnemonics are replaced by their machine code equivalents, the address­ ing modes for operands are determined and the symbols for internal references are replaced by machine code addresses. The assembler will also check the syntax of the source code instructions and report ' errors. A listing file, showing the source code, corresponding machine code and error mess­ ages, is often genera ted during this pass.
In some cases, the second pass will produce absolute code that can be loaded into the computer, or microprocessor system, and run. In other cases, the object code is in the form of relocatable code. This is the form used when the program is to be run in conjunction with other programs, or library subroutines, and the operating system requires the facility to decide upon the location of the program in main memory. Relocatable code will be incom­ plete in the sense that the assembler was unable to provide machine code addresses to external references. The linker is employed to complete the process of producing the executable machine code program. Compare COMPILER, INTERPRETER . See ADDRESSING
MODE, ASSEMBLY D1RECTIVE, COMMENT, EX­
TERNAL REFERENCE, INTERNAL REFERENCE,
REFERENCE, MACHINE CODE INSTRUCTION,
CODE, SYMBOL TABLE, SYNTAX.
assembly directive In programming, an in­ struction in a low-Ievel language that teils the assembler to perform certain actions,
18 assembly language
e.g. assign memory space to a variable or constant, but does not generate any machine code instructions. See ASSEMBLING, MACHINE
CODE INSTRUCTION.
assembly language In programming, a language that allows a programmer to de­ velop a machine code program using sym­ bols and mnemonics for storage locations and operations. This language greatly im­ proves the comprehension of the program and enables modifications to be more readily incorporated. Compare HIGH-LEVEL LANGU­
AGE. See ASSEMBLING, LOW-LEVEL LANGUAGE,
MACHINE CODE, TRANSLATOR.
assembly Iisting In programming, the printed list produced by the assembler giving details of any syntax errors, a listing of the source program and, normally, an associated listing of the corresponding machine code program. See ASSEMBLING, MACHINE CODE.
assembly time In programming, the time at wh ich an assembler translates the source program into the corresponding object code. See ASSEMBLING, OBJECT CODE, SOURCE PRO­
GRAM.
assertion (1) In programming, a boolean expression that is stated to be true. In proof of program correctness assertions are used to make statements about program states. See PROOF OF PROGRAM CORRECTNESS. (2) In artificial intelligence, a hypothesis about the problem to be solved. The likelihood of an assertion is established by asking the user questions or, alternatively, rules may be used to deduce the likelihood from other assertions or stored data. Compare OBJECT.
See EXPERT SYSTEMS, RULE.
Assign In operating systems, a command commonly employed to reassign a disk drive. H is employed when applications pro grams specifically address one disk drive and the user wishes to employ a different drive. See APPLICATION PROGRAM, D1SK
DRIVE.
assignment statement In programming, a statement that results in the assignment of a value to a variable. The expression on the right-hand side of the statement is evalu-
ated, and this value is assigned to the variable named on the left-hand side. See EXPRESSION, STATEMENT, VARIABLE.
Association for Computing Machinery US professional computer science organization. Hs objective is to advance all aspects of information processing and to promote the interchange of such techniques between computer specialists and users.
associative processor In architecture, a device using associative storage methods, i.e. data is accessed by reading keys and comparing their values with those that identify the item sought. See ASSOCIATIVE
STORAGE.
associative storage In memory systems, a storage device in which the user identifies data by apart of its conte nt rather than by its physical location. It provides a fast method of searching for data with certain keys. The computer system mayaiso rearrange its storage of data without affecting the user's application programs. See APPLICATION PRO­
GRAM, KEY.
async In data communications, an abbre­ viation for asynchronous communication. See ASYNCHRONOUS TRANSMISSION.
asynchronous Pertaining to actions and events that are not correlated with some reference time. Compare SYNCHRONOUS.
asynchronous access In memory systems, a method of accessing the dynamic RAM for refresh operation. Requests for refresh access are genera ted at a fixed rate indepen­ dent of the state of the microprocessor. Compare SYNCHRONOUS ACCESS. See DYN­
AMIC MOS RAM, REFRESH CONTROL.
asynchronous communications interface adaptor In hardware, a device performing a similar function to a UART. See UART.
asynchronous computer A computer in which each operation is initiated as a result of a signal genera ted by the completion of the previous operation or by the availability of the equipment required for the next operation. Compare SYNCHRONOUS COM­
PUTER.
2 3 S
1 2 3 4 S 7 8 Stop Stop
Mark Mark Mark
asyncbronous transmission
asynchronous transmission In data com­ munications, a form of data transmission in which there can be variable time intervals between characters but the bits within a character are sent with fixed time intervals. Start and stop elements are used to indicate the beginning and end of characters. Com­ pare SYNCHRONOUS TRANSMISSION.
ATM See AUTOMATED TELLER MECHANISM.
ATN In buses, ATtentioN, a command signal on the GPIB indicating whether the information on the data bus is to be inter­ preted as an interface control message or device-dependent data. See GPIB.
atom In programming, a value that cannot be decomposed further. In LlSP-like langu­ ages, an atom is an arbitrary string of characters. In languages operating entity­ attribute structures, an atom would corre­ spond to the set of characters assigned to an entity name or attribute. See ATIRIBUTE,
ENTITY, LlSP.
attacker In data security, an intruder who attempts to decipher the ciphertext message. See CIPHERTEXT, INTRUDER.
AT&T American Telephone & Telegraph Co.
attended trail printer In word processing, a trail printer that has no paper-handling facilities and so requires operator interven­ tion before and after the printing of each page. See TRAIL PRINTER.
attention interruption In peripherals, an input/output interruption caused by a ter­ minal user pressing an attention key, or its equivalent. See INTERRUPT, INPUT/OUTPUT.
attention key In peripherals, a key on a terminal that, when pressed, causes an input/output interruption in the processing unit. See INPUT/OUTPUT, INTERRUPT.
attenuation In electronics and communica­ tions, the reduction in strength of an elec­ trical signal as it passes through a circuit or an electromagnetic wave as it propagates through a transmission medium. Compare AMPLIFJCATION.
attribute In databases, a field that contains information ab out an entity, e.g. in a per­ sonnel database 'horne address' would be an attribute of entity employee. Compare ENTITY, RELATIONSHIP. See DISPLAY ATIRI­ BUTE, FJELD.
audio cassette recorder In backing storage, a recorder designed for use with compact cassettes.
20 audio cassette recorder interface
audio cassette recorder interface In back­ ing storage, an interface unit that enables an ordinary cassette tape recorder to be used as an input/output device for a microcomputer.
audio frequency The frequency of an audible sound wave; for normal hearing the range of frequencies lies between 20 and 20,000 Hz. See Hz.
audio inquiry See VOICE ANSWER BACK.
audio response terminal In peripherals, a terminal that receives spoken information from a computer. Digitized speech in the form of words/phrases or phonemes is accessed by the program, and converted from digital to analog form for transmission. If the receiving unit is a push button telephone, with multifrequncy tones, the user can send inquiries via the push button. See PHONEME, SPEECH SYNTHESIZER, VOICE
ANSWER BACK.
BACK.
audit traU In programming, a clerical or automated method for tracing the trans­ actions affecting the contents of arecord. See RECORD.
augend The number to which an addend is added to produce the sum in an arithmetic operation. See ADDEND.
authentication In data security, processes that ensure everything ab out a teleprocess­ ing transaction is genuine and that the message has not been alte red or corrupted in transmission. The parties to the trans action must identify each other reliably, know that each message they receive comes from the other party and has not been modified, or stored earlier and replayed, by a third party. Acheck field can be added to the message such that the calculation of the check field depends upon the contents of the whole message, and the calculation involves a secret key. Additional information is re­ quired in the message to guard against replay of earlier messages; time/date fields are sometimes employed for this purpose. Public key cryptosystems can be used for authentication. In this case the message is
encrypted with the private key and de­ crypted with the public key. Such messages can be decrypted by an attacker, but only the holder of the private key is able to create a meaningful message. See CRYPTOGRAPHY,
MAC, PUBLIC KEY CRYPTOSYSTEM, REPLAY,
WIRE TAPPING.
authentication of users In data communica­ tions, the verification that the user at the terminal corresponds to his claimed identity.
author The writer of books, articles, com­ puter software, computer-assisted leaming packages etc.
authoring In interactive video disk, a struc­ tured approach to developing all elements.of an interactive video disk program wlth emphasis on preproduction. See PREPRODUC­
TION.
authoring system In computer-aided leam­ ing, a computer system capable of executing an author language. See AUTHOR LANGUAGE.
authorization In operations, the right given to a user to communicate with or make use of a computer system or stored data.
authorization code In operations, a code used to protect against unauthorized access to da ta and system facilities. The code normally consists of a user id (identification) and password. See PASSWORD, USER ID.
author language In computer-aided leam­ ing, a programming language used for de­ signing instructional programs for computer­ assisted instruction and computer-based training systems. See COMPUTER-ASSISTED
LEARNING, COMPUTER-BASED TRAINING.
ANSWERING.
autodialer In computing and videotex, a device that automatically dials a prerecorded telephone number for connection to a host computer.
Autoexec See AUTOMATIC PROGRAM EXECU­
TION.
autoidentifier In computing and videotex, a device by which a terminal automatically identifies itself to a computer.
autokeying In peripherals, the ability to record frequently used groups of keystrokes and to play them back at designated loca­ tions on the screen.
automata theory A mathematical study of the systems that receive discrete inputs, change their internal states according to the input, and their current states, and deliver outputs according to their internal states and inputs. See Tu RING MACHINE.
automated teller meehanism In peri­ pherals, a device that provides for cash withdrawals, payment of bills, account balance inquiries and transfers of funds between accounts. See SELF-BANKING.
automatie Pertaining to a process or device that, under specified conditions, functions without intervention by a human operator.
automatie answering In data communica­ tions, a system in wh ich the called station automatically responds to the calling signal; the call may be established whether or not the called station is attended.
automatie eaIling In communications, a machine feature that allows astation to initiate a call automatically over a switched line. See STATION.
automatie eaIling unit In data communica­ tions, a device that enables a business machine to dial calls automatically over a network. See AUTODIALER.
automatie earriage return In word process­ ing, the automatic performance of a carriage return when the last word that will fit onto a line of print is typed. A system that has this facility will usually employ a buffer to hold the word currently being typed until it can decide whether to place the word on the CUTrent line, or to wrap it onto the next line. See CARRIAGE CONTROL, WRAPAROUND.
automatie eentering In word processing, the automatie ability to center a word or portion of text.
automatie data proeessing Data processing performed by computer systems as com­ pared with manual systems. See ELECTRONIC
DATA PROCESSING.
automatie letter writing 21
automatie decimal alignment In word pro­ cessing, the feature of a machine that enables numbers to be aligned automatically on either side of a decimal marker.
automatie decimal tab Synonymous with AUTOMATIC DECIMAL ALIGNMENT.
automatie dialer See AUTODIALER.
automatie dietionary (1) In machine trans­ lation, a database that provides a word for word substitution from one language to another. See TERM BANK. (2) In information retrieval, a system that substitutes codes for words or phrases in the encoding operation.
automatie file seleet In word processing, a facility for making a selection from a data file based on the characters that appear in a specified data field. For example, using a zip code field, the system can select all the addresses with a 12480 zip code for one letter, and print a different letter for all other codes etc.
automatie file sort In word processing, a facility for performing sorts on files in alphabetical or other order. This feature is useful for manipulating address lists so that changes need not be performed in alphabet i­ calorder.
automatie footnote tie in In word process­ ing, a system that ties a footnote to the appropriate text segment. If the text seg­ ment is moved to another page or document, the foot note will travel with it.
automatie headers/footers In word process­ ing, the ability to place header/footer text at the top or bottom of each page of a multi page document. The operator specifies the text once, and the header/footer (usually document title, company name or con­ fidentiality requirements) is automatically added during print out.
automatie letter writing In word process­ ing, the ability to produce a standard docu­ ment as though it were typed specially for the recipient.
22 automatie line/paragraph numbering
automatie line/paragraph numbering In word processing, a facility whereby the system automatica11y supplies an identifying number for each li ne or paragraph during input for use in defining locations during sub se quent editing. The line/paragraph numbers are automatica11y deleted during final print out.
automatie line spacing In word processing, the ability of a printer to perform different line spacings without the need for operator intervention.
automatie loader In programming, a loader program implemented in a special ROM that a110ws loading of the first re cord or sector of a backing storage device. See BACKING STOR­
AGE, LOADER, ROM.
automatie logging In word processing, a facility in wh ich a system automatica11y records titles and log numbers with a11 documents. Thus data can be played back separately when required. See LOG.
automatie margin adjust In word process­ ing, a facility to change margins with a single command. Line endings are adjusted with­ out further intervention.
automatie message-switehing center In data communications, a location at wh ich mess­ ages are automatically routed according to the information they contain. See MESSAGE
SWITCHING.
automatie page numbering In word pro­ cessing, a facility to generate automatically page numbers within documents. When text is rearranged and page numbers change, the system can generate a new set of correct page numbers. See AUTOMATIC PAGINATION.
automatie pagination In word processing, a facility to take a multi page document and divide it into pages of a specified length in terms of line numbers. Often, this feature is joined with the capability to generate auto­ matica11y page numbers. See AUTOMATIC
PAGE NUMBERING.
automatie poIling In computing and data communications, a feature of a transmission control unit that enables it to handle ne ga-
tive responses to polling without interrupt­ ing the CPU. See CPU, POLLING.
automatie program exeeution In operating systems, a facility that automatica11y starts a specific program every time the operating system is loaded.
automatie programming In programming, the use of a computer to convert a program written in a convenient language for the programm er into a set of machine code instructions that can be executed by the computer. See HIGH-LEVEL LANGUAGE,
TRANSLATOR.
automatie restart In operations, a facility to perform automatically the initialization functions necessary to resurne operation following an equipment or power failure. See INITIALIZATION.
automatie retransmission request In data communications, a technique to ensure accurate transmission of data. Data to be transmitted is held in a buffer until the communication link is ready to deal with it. The data is then despatched and a copy made at the same time. The copy is de­ posited in the buffer and erased when the sending device receives acknowledgement of correct receipt, as verified by CRC. If the receiving device detects an error in the data it informs the sending device, which then retransmits the buffered copy. See CRC.
automatie tab memory In word processing, a facility of a system that enables it to store a format of tab settings to be restored auto­ matica11y to the typewriter at the time of printing.
automatie volume switehing In backing storage, access to a sequential data set that extends across two or more volumes, and to concatenated data sets stored on different volumes. See CONCATENATE, SEQUENTIAL
DATA SET, VOLUME.
automatie widow adjust In word process­ ing, a facility that prevents the first line of a paragraph, title or heading from being the last line on a page. It mayaIso prevent the last line from being the first line on a new page. See WIDOW.
automatie word wraparound In word pro­ cessing, the automatie placing of a word onto the next line if it does not fit onto the line being typed; frequently combined with the automatie carriage return feature. Also it is used to denote systems that can wrap words during margin adjust procedures. See AUTOMATIC CARRIAGE RETURN.
automation The technology concerned with the design and development of pro­ cesses and systems that minimize the neces­ sity of human intervention in their oper­ ation. See FEEDBACK.
auxiliaryequipment In architecture, equip­ ment not under the direct control of the CPU. See CPU.
auxiliary storage In backing storage, data storage other than the main memory, usually with slower access, e.g. magnetic tape or direct access devices. See D1RECT ACCESS
STORAGE DEVICE.
availability In operations, the degree to wh ich a system or resource is ready when needed to process data. See UPTIME.
available point In computer graphics, an addressable point at which a characteristic, such as color, intensity or on/off condition, may be specified. See PIXEL.
azerty keyboard 23
available time Synonymous with UPTIME.
avalanche photodiode In electronics, a photodiode operated with a high reverse voltage. Hole electron pairs are produced by incident infrared or light energy, and these carriers are swept to the appropriate elec­ trode. The electron carriers can collide with other atoms releasing more electrons, hence increasing the sensitivity of the device. Compare PIN PHOTODIODE. See PHOTODIODE,
SEMICONDUCTOR.
average Synonymous with ARITHMETIC MEAN.
average access time In backing storage, the average time between the instant of a re­ quest for data and its delivery from a storage device.
average delay In communications, the average time that a caller must wait for access to a communication facility.
AVIP See BAVIP.
A WC See ACTIVE WIRE CONCENTRATOR.
azerty keyboard A keyboard arranged as on the standard typewriters of continental Europe, with the keys a, z, e, r, t, y on the upper left-hand side. Compare DVORAK KEY­
BOARD, MALTRON KEYBOARD, QWERTY KEY­
BOARD. See KEYBOARD.
B babble In communications, the aggregate cross talk from a number of interfering sourees. See CROSS TALK.
backend In programming, a functional unit of a compiler. See COMPILER.
back faces In computer graphics, the facets of a three-dimensional object that cannot be seen from the current position of the obser­ ver. Deletion of current back faces can reduce the effort in computing the screen display of the object. See CULUNG, HIDDEN UNE.
background See BACKGROUND PROGRAM.
background color In videotex, the color filling the parts of the character rectangle not occupied by the character itself. The background color may be black or one of the seven display colors. It may be changed within a row by control characters. See CHARACTER RECTANGLE, CONTROL CHARACTER,
DISPLAY COLOR.
background ink In optical character recog­ nition, a type of ink that is not detected by the scan head because of its high reflective characteristics. It is used for print location guides, logotypes, instructions and any other desired preprinting that would otherwise interfere with the scan head reading.
background job In operations, a job of relatively low priority in a multitasking environment; computer resources are only allocated to it when they are not required for higher-priority foreground tasks. Compare FOREGROUND PROGRAM. See MULTITASKING.
background processing (1) In operations, the execution of lower-priority computer programs when .higher-priority programs do not require any system resource. (2) In word processing, the execution of an operator's request such as printing a document while the operator is performing other tasks. See BACKGROUND PROGRAM, PRIORITY.
24
background program In operating sys­ tems, a program that does not involve interactions with a user and is run with a low priority in a multiprogramming environ­ ment. Compare FOREGROUND PROGRAM. See BATCH PROCESSING, MULTIPROGRAMMING.
background region In operations, a region in main memory to which a background job is assigned. See BACKGROUND JOB, MAIN
MEMORY.
backing storage In peripherals, an inter­ mediate storage medium, e.g. magnetic tape, magnetic disk etc., on to which data is entered for later processing by the central computer. Ideally computers would be pro­ vided with a single nonvolatile store that had random access, extremely low access times, of the order of nanoseconds, and massive capacity , of the order of gigabytes. However fast, random access storage is expensive, and economical high-capacity storage devices have slow access, of the order of milli­ seconds, or even seconds, and provide only se rial or direct access. In practice therefore, computers are provided with hierarchies of memory: sm all volatile cache memories for nanosecond access by the CPU; volatile random access memories with capacities of the order of kilobytes, or more, with access times of the order of microseconds; and nonvolatile backing stores of magnetic disk, magnetic tape, optical digital disk, magnetic bubble etc. with storage capacities in the hundreds of kilobytes to gigabyte range. Data is accessed from backing storage in either serial or direct access mode; in the first case all stored data must be read until the required item is attained, whereas in direct access it is possible to jump to a required block of data and the individual data items are then read in aserial mode.
The programmer must arrange for the transfer of data to and from backing storage in such a manner to ensure that program execution is not unnecessarily delayed by such accesses; data that will be required
after the computer has been switched off must be transferred to backing storage before the end of program execution. Back­ ing storage is also employed to hold soft­ ware. See BUBBLE MEMORY, CACHE MEMORY,
CPU, DIRECT ACCESS, GIGABYTE, KILOBYTE,
MAGNETIC DlSK, MAGNETIC TAPE , MEGABYTE,
NANOSECOND, OPTICAL DlSK, RANDOM ACCESS ,
RANDOM ACCESS MEMORY, SERIAL ACCESS ,
VOLATILE STORAGE.
backlog In operations, application pro­ grams that have been authorized but not written. See APPLICATION PROGRAM.
backplane In hardware, the connector blocks and wiring units that provide the me ans of interconnection between a com­ puter and its peripherals. It normally com­ prises aseries of multiway sockets connected to the internal bus of the computer. See BUS,
MOTHERBOARD .
back porch In television, a picture signal that lies between the trailing edge of the line sync pulse and the trailing edge of the corresponding blanking pulse. Compare FRONT PORCH . See BLANKING .
backspace In peripherals, to move the printing head on a printer back one charac­ ter position, or the cursor on a VDU back by the same amount. See VDU.
backspace character In printing, a control character that causes the print or display position to move one position backward along the line without printing or displaying any graphic character. See CONTROL CHAR­
ACTER.
backtracking In programming, a technique of searching for a goal which allows for the possibility that a given search path may prove to be a dead end. Algorithms using this method retreat from such dead ends and recommence the search along a different path that was not subjected to an earlier search. See PROLOG.
Backup In operating systems, a command commonly employed to backup one or more files from a fixed disk to floppy disks. See FILE, FlXED DlSK STORAGE, FLOPPY DlSK.
backupIrestore 25
backup In reliability, pertaining to a sys­ tem, device, file or facility that can be brought into action in the event of a mal­ function or loss of data. See BACKUP/
RESTORE.
backup copy In operations and word pro­ cessing, a copy of a file or data set that is kept for reference in case the original file or data set is destroyed. See BACKUP/RESTORE,
DATA SET, FILE.
backup diskette In backing storage, a disk­ ette that contains information copied from another diskette . It is used in case the original information is unintentionally altered or destroyed. See BACKUP/RESTORE.
backupIrestore In backing storage, the actions involved in transferring data from magnetic disk to tape, or disk, for backup, and the subsequent action of restoring the data to disko Winchester disk drives have fixed disk systems, and their data must be periodically transferred to another medium to provide insurance against data corruption or a drive failure , which would render the disk data inaccessible. The available tech­ niques for backup are floppy disks, magnetic tape (1/2 or 1/4 inch) and removable disks.
Floppy disk backup suffers from the rela­ tively low storage capacity of diskettes, compared with Winchester disks , and their comparatively low data transfer rates. Back­ up operations should be convenient to the operator, otherwise there exists the danger that they will not be performed with suf­ ficient regularity. Backup onto floppy disks often involves tens of minutes, and contin­ uous operator attention , as new disks are inserted and backup instructions given for the various sets of disk files.
Tape streamers using 1/2- or 114-inch magnetic tape were developed for backup operations; such streamers are designed for the continuous flow of data to and from a computer. They are simpler mechanically, and hence cheaper than conventional start/ stop magnetic tape devices. These devices require a continuous flow of data to, or from, the read/write heads for efficient operation, and the software and interface systems must provide this continuous flow, or the repositioning cycles of the streamer will seriously extend the total time of the
26 Backus Naur Form
backup operations. Start/stop magnetic tape drives are akin to the magnetic tape trans­ ports used in mainframe systems and, as the name implies, are designed to cope with more intermittent data flows. They are more sophisticated mechanical devices, and hence more expensive, than tape streamers, but provide for greater flexibility in the b