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`SFDC 1022
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`Oxford University Press, Walton Street, Oxford 0X2 6DP
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`Oxford is a trademark of Oxford University Press
`
`Published in the United States
`by Oxford University Press, New York
`
`C Market House Books Ltd., 1983, 1986
`
`First published 1983
`Reprinted 1983, 1984, 1985
`
`Second edition 1986
`
`All rights reserved. No part of this publication may be reproduced,
`stored in a retrieval system, or transmitted, in any form or by any means,
`electronic, mechanical, photocopying, recording, or otherwise, without
`the prior permission of Oxford University Press
`
`British Library Cataloguing in Publication Data
`Dictionary of computing. — 2nd ed.
`1. Computers — Dictionaries 2. Electronic
`data processing — Dictionaries
`004' .03'21 (cid:9)
`QA76.15
`ISBN 0 19 853913 4
`
`Text prepared for automatic typesetting by
`Market House Books Ltd, Aylesbury
`Printed and bound in Great Britain by
`Biddles Ltd, Guildford and King's Lynn
`
`

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`DUMP CHECK
`
`contents. In practice this may be difficult
`even with the assistance of dump analysis
`software.
`3. To take a dump (defs. 1 or 2).
`dump check A copy of the contents of all
`the workspace associated with a job or pro-
`cess. If the job or process subsequently
`fails, it can be restarted at the point at
`which the dump check was taken. Note
`that the status of peripheral devices allo-
`cated to the job or process must be consid-
`ered as constituting part of its workspace.
`dump point .See checkpoint.
`duplex (full duplex) Involving or denoting a
`connection between two endpoints, either
`physical or logical, over which data ma
`travel in both directions simultaneously.
`See also half duplex, simplex, return chan-
`nel.
`duty cycle For pulsed or square-wave sig-
`nals, the ratio of pulse duration to pulse
`spacing, often expressed as a percentage. A
`square wave signal normally has a 50%
`duty cycle, i.e. pulse duration is equal to
`the time between pulses.
`DX-2 An X.25-based *public packet net-
`work of Japan. It first became operational
`in 1979.
`
`dyadic Having two operands.
`dyadic operation (binary operation) defined
`on a set S. A function from the domain
`S x S into S itself. Many of the everyday
`arithmetic and algebraic operations are dy-
`adic, e.g. the addition of two integers, the
`union of two sets, and the conjunction of
`two Boolean expressions. Although basi-
`cally functions, dyadic operations are usu-
`ally represented using an infix notation, as
`in
`
`3 + 4, U U V, P A Q
`A symbol, such as can be used to repre-
`sent a generalized dyadic operation.
`When the set is finite, *Cayley tables
`and sometimes *truth tables are used to de-
`fine the meaning of the operation.
`
`Dyck language A concept used in *formal
`language theory. Let be the alphabet
`(a l , . . ,a,„b 1 , . . ,b„}
`The Dyck language over E is the set of all
`strings that can be reduced to the empty
`string A by "cancellations" of the form
`ai b; (cid:9)
`A
`
`For example,
`
`E = (0)
`gives the Dyck language of all balanced pa-
`renthesis strings. An important theorem
`characterizes the *context-free languages
`as those representable as the homomorphic
`image (see homomorphism) of the intersec-
`tion of a Dyck language and a *regular lan-
`guage._
`dynamic Caable of changing or of being
`—changed: With reference to operating sys-
`tems, the implication is that the system is
`capable of changing while it continues to
`run. As an example, the total amount of
`memory available may be defined by the
`contents of a word within the operating
`system. If this word can be altered without
`stopping the system and reloading a fresh
`copy of the operating system, then it is pos-
`sible to alter dynamically the total amount
`of memory on the system.
`With reference to programming, the ad-
`jective is applied to operations that take
`place while a program is running, as com-
`pared with those that take place during the
`compilation phase. For example, dynamic
`arrays are allocated space while the pro-
`gram is running.
`Compare static.
`dynamic allocation An allocation that is
`made dynamically, i.e. while the system is
`running, rather than statically at the time
`of first initiating the system.
`dynamic data structure A data structure
`whose organizational characteristics may
`change during its lifetime. The adaptability
`afforded by such structures, e.g. linked
`lists, is often at the expense of decreased
`efficiency in accessing elements of the
`structure. Two main features distinguish
`dynamic structures from *static data struc-
`
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