`
`I
`
`A
`
`The Facts On File
`DICTIONARY
`of
`COMPUTER SCIENCE
`
`RPX-1009, p.1
`
`
`
`
`
`The Facts On File Dictionary of Computer Science
`
`Fourth Edition
`
`Copyright © 2001 by Market House Books Ltd
`
`All rights reserved. No part of this book may be reproduced or utilized in any
`form or by any means, electronic or mechanical, including photocopying,
`recording, or by any information storage or retrieval systems, without
`permission in writing from the publisher. For information contact:
`
`Checkmark Books
`An imprint of Facts On File, Inc
`1 1 Penn Plaza
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`
`II Daintith,
`
`
`
`Library of Congress Cataloging-in-Publication Data
`The Facts on File dictionary of computer science. / [edited by]
`Valerie Illingworth and John Daintith.
`p.
`cm.
`: acid—free paper)
`ISBN 0-8160-4285-3 (hc.
`2 ‘acid—free paper)
`ISBN 0-8160-4286-1 (pbk.
`1. Computer Science—Dictionaries.
`I. Illingworth, Valerie.
`John.
`III. Facts on File, Inc.
`IV. Title: Dictionary
`of computer science.
`QA76.15 F345 2000
`004'.03—dc21
`
`00—055555
`
`Checkmark Books are available at special discounts when purchased in bulk
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`Compiled and typeset by Market House Books Ltd, Aylesbury, UK
`
`Cover design by Cathy Rincon
`
`Printed in the United States of America
`
`1 1
`
`MP 10987 6543
`(pbk)109876543 [\)[\J
`
`This book is printed on acid—free paper
`
`RPX-1009, p.2
`
`
`
`E1“
`
`Iponent in a system is waiting
`component. For example, in
`a on a disk, latency is the time
`vosition the sector under the
`:ad. See access time.
`'king, the amount of time it
`:t to travel from source to des-
`;ether latency and BANDWIDTH
`e speed and capacity of the
`
`typesetting system based on
`gramming language for type-
`)vides for higher level macros
`king it easier to format docu-
`doing so loses some flexibil-
`
`start a program.
`
`el in a program or system that
`Irticular function or functions
`iined interface for passing re-
`the layers next to it. OSI is an
`layered system.
`
`vord processing and desktop
`fers to the arrangement of the
`hics on a page.
`
`crystal display) A device
`digital watches, calculators,
`ers, etc., to display numbers,
`Iometimes other characters.
`‘s are formed from groups of
`dots. The segments or dots
`id that is usually transparent.
`;ments or dots can be dark-
`r
`(by applying an electric
`1 the shape of a particular
`)te that LCDs do not emit
`only be seen because light re-
`rehind the display makes the
`
`
`
`
`
`dark characters stand out. LCD screens
`can now display all the characters available
`on VDUs. A simple SEVEN-SEGMENT DISPLAY
`can produce the numbers 0-9 and some let-
`ters. LCDs require less electrical power
`than LED DISPLAYS.
`
`LDAP (lightweight directory access proto-
`col) A significantly simpler version of the
`X.50O standard for a set of protocols for
`accessing information directories. Unlike
`X.50O it supports TCP/IP and is an open
`protocol, which means that applications
`do not need to know about the type of
`server hosting the directory.
`
`leaf An item at the very bottom of a hi-
`erarchical tree structure. In hierarchical file
`systems, files are leaves whereas directories
`are NODES.
`
`is
`leased line A telephone line that
`rented for exclusive use by the customer
`from one location to another. Typically
`leased lines are used by businesses to con-
`nect geographically distant sites for high-
`speed data transfer.
`
`least significant digit (lsd) The digit in
`the least significant, i.e. rightmost, position
`in the representation of a number, and thus
`making the smallest contribution to the
`value of the number. With a binary num-
`ber, this digit is referred to as the least sig-
`nificant bit (lsb).
`
`LED display A device used in some cal-
`culators, digital clocks, etc.,
`to display
`numbers and letters. The characters are
`formed from groups of segments. The seg-
`ments are small electronic components
`called LEDs, i.e. light-emitting diodes. In-
`dividual LEDs can be made to emit light
`(usually red) so as to form the shape of a
`particular character. A simple SEVEN-SEG-
`MENT DISPLAY can display the numbers 0-9
`and some letters. LED displays require
`more electrical power than LCDS.
`
`left—justified
`
`See justify.
`
`legacy system A computer, software,
`operating system, network, or other com-
`
`‘I11
`
`light-emitting diode
`
`puter equipment that exists before a new
`system is installed, and is still required af-
`terward. BACKWARD COMPATIBILITY is an
`important part of the design and imple-
`mentation of new systems. Currently many
`companies are migrating their legacy appli-
`cations to new programming languages
`and operating systems that follow open or
`standard programming interfaces, which
`should help to make it easier to update ap-
`plications in the future and will enable the
`application to run on any operating sys-
`tem.
`
`LEO Lyons Electronic Office. See first
`generation computers.
`
`LET See Basic.
`
`letter quality A quality of printing that
`is as good as a top-quality typewriter. The
`term is becoming less common with the
`increased use of laser printers, which gen-
`erally give better quality than electric type-
`writers.
`
`The initial phase in the
`lexical analysis
`COMPILATION of a program during which
`the program is split up into meaningful
`units. These units could, for example, be
`NAMES, CONSTANTS, RESERVED WORDS, OI‘
`OPERATORS. The part of the compiler pro-
`gram that does this analysis is called a lex-
`ical analyzer. The units recognized by the
`analyzer are known as tokens. They are
`output in some conveniently coded form
`for further processing by the compiler. See
`also syntax analysis.
`
`library
`library.
`
`See program library; subroutine
`
`library program See program library.
`
`license agreement An agreement defin-
`ing terms for the use of hardware or soft-
`ware by a user.
`
`LIFO (last in first out)
`
`See stack.
`
`light-emitting diode
`
`See LED display.
`
`
`
`RPX-1009, p.3
`
`
`
`
`
`thrashing
`
`storage, input/output, and other resources
`became vested in the operating system or
`the machine itself. In addition new pro-
`gramming languages were
`introduced,
`such as COBOL, and later versions of exist-
`ing languages, such as FORTRAN, came into
`use.
`
`See also first, fourth, fifth generation
`computers.
`
`1. Excessive hard-disk use
`thrashing
`that occurs when a large number of short
`files scattered over the disk are being re-
`trieved. Performance will be degraded.
`2. The situation in which a virtual memory
`system is continually swapping pages in
`and out of memory rather than running ap-
`plications. Very little progress is made
`under these circumstances as memory or
`other resources have become exhausted or
`too limited to perform the necessary oper-
`ations.
`
`thread On the Internet, a sequence of
`postings relating to an original newsgroup
`or forum message that continue on from
`each other. To continue the thread as a
`user a reference topic is specified as part of
`the user’s message posting.
`
`throughput A measure of the overall
`performance of a computer system, i.e. of
`the amount of work performed in a given
`period. It can, for example, be measured in
`terms of JOBS per day.
`
`thumbnail A small version of an image
`or document page, which is much quicker
`
`to display on a screen or load from a Web
`site. Thumbnails make it quicker and eas-
`ier to View or manage a group of larger im-
`ages. They are often used in image
`catalogs.
`
`thunk A code in an operating system
`that allows 16-bit code to call 32-bit code
`and vice versa. For example, Widows 95
`can run programs written in both 16-bit
`and 32-bit instruction sets.
`
`TIFF (tagged image file format) A com-
`mon format for scanning, storing, and in-
`terchanging bitmapped images. The files
`have the file extension .tif.
`
`time division multiplexing (TDM)
`multiplexer.
`
`See
`
`timeout A condition that occurs when a
`process waiting for either an external event
`or the expiry of a preset
`time interval
`reaches the end of the time interval before
`the external event has been detected. If, for
`example, the process has sent a message
`and no ACKNOWLEDGMENT has been de-
`tected at the end of the preset time period,
`then the process may take appropriate ac-
`tion, such as retransmitting the message.
`
`timer
`
`See counter.
`
`time sharing A technique whereby the
`time of a computer can be shared among
`several jobs, a brief period being allocated
`(by the OPERATING SYSTEM) to each job in
`turn. During such a period — known as a
`time slice — the job is permitted to use the
`resources of the computer, i.e. the proces-
`sor, main store, etc. A MULTIACCESS SYSTEM
`relies on time sharing.
`
`time slice
`
`See time sharing.
`
`TLU (table look-up)
`
`See table.
`
`token
`
`See lexical analysis.
`
`token ring network A LAN in which
`the nodes are connected together in a ring
`in such a way that each node can only com-
`municate with its neighbors on either side,
`
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`
`204
`
`RPX-1009, p.4
`
`
`
`vances in electronic technology now made
`it possible to design a computer to suit the
`requirements of the tasks envisaged for the
`machine: the concept of computer ARCm-
`TECTURE thus became a reality with this
`generation. Most manufacturers
`intro-
`duced at least three members of a computer
`family, architecturally similar but differing
`in price and performance. The IBM 360 se-
`ries, first introduced 1964, is an important
`example.
`Comprehensive OPERATING SYSTEMS be-
`came, more or less, part of third generation
`machines. MULTIPROGRAMMING was facili-
`tated and much of the task of control of