DICTIONARY OF
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`Comcast, Ex. 1027
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`A Dictionary of
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`Computing
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`FOURTH EDITION
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`Oxford New York
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`OXFORD UNIVERSITY PRESS
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`Oxford University Press, Great Clarendon Street, Oxford OX2 GDP
`Oxford New York
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`Oxford is a registered trade mark of Oxford University Press
`© Market House Books Ltd. 1983, 1986, 1990, 1996
`First published 1983
`Second published 1986
`Third edition 1990
`Fourth edition I 996
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`British Library Cataloguing in Publication Data
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`Library of Congress Cataloging in Publication Data
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`ISBN 0—19—280046—9
`7 9 10 8
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`Printed in Great Britain by
`Cox Cg Wyman Ltd
`Reading, Berkshire
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`BLENDING
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`blending The process of constructing
`*blends.
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`blink An attribute that causes a character to be
`intermittently displayed on a screen at a reg—
`ular rate, usually in the range 1—10 Hz. Com—
`pare flicker.
`blobby modeI A model where objects have a
`basic shape that can be perturbed by interac—
`tions with neighbors or the environment.
`Such objects are described as so]? and include
`muscles and the human body. See also meta—
`balls.
`
`block I. A collection of data units such as
`words, characters, or records (generally more
`than a single word) that are stored in adjacent
`physical positions in memory or on a periph—
`eral storage device. A block can therefore be
`treated as a single unit whereby data can be
`(and usually is) transferred between storage
`device and memory, using one instruction.
`Blocks may be fixed or variable in size.
`A stream of data to be recorded on mag-
`netic tape is divided into blocks for conveni—
`ence of handling and particularly of *error
`recovery. (The equivalent on disks is *sec—
`tors.) Successive blocks are usually separated
`by interblock gaps and often also by control
`signals introduced by the magnetic tape sub—
`system and invisible to the host (see tape for—
`mat). It is usual but not essential for the block
`' length to be the same for all blocks of data
`within a volume or at
`least within a file,
`though this may not apply to *labels; where
`the end of a file occurs partway through a
`block,
`the remainder of the block may be
`filled with padding drummers.
`The choice of block length is largely
`dependent on *error management consider-
`ations. The minimum length of
`the
`interblock gap is defined by the standard for
`the tape format in use; the maximum length
`is usually undefined, except that a very long
`stretch (typically 25 feet) of blank tape is
`taken to mean that there is no more data on
`the volume. To avoid wastage of tape the gap
`written is usually fairly close to the minimum
`but it may be elongated in some circum—
`stances, e.g. by error recovery actions or to
`leave space for the editing of a file (which in
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`this context means its replacement by a new
`version of the same length).
`In conventional magnetic tape subsystems
`the division of data into blocks is carried out
`by the host. However some buffered tape
`subsystems, particularly streaming cartridge
`tape, accept a continuous data stream from
`the host, and the subsystem itself divides the
`data into blocks (in this case often called
`Malcolm) in a manner that is not visible to
`the host. In these subsystems the interblock
`gap may be very short or absent.
`2. In coding theory, an ordered set ofsymbols,
`usually of a fixed length. The term is gener-
`ally synonymous with word or string, but
`with the impliution of fixed length.
`3. See block—structured languages.
`1. In parallel programming, to prevent fur—
`ther execution of one sequence of instruc—
`tions until another sequence has done what—
`ever is necessary to unblock it. See also
`blocked process.
`block cipher A cipher in which a fixed-length
`block of data is encrypted, or decrypted, at
`each iteration of the algorithm: each block is
`input, encrypted, and output, with no mem—
`ory (to retain message-dependent informa—
`tion) between blocks
`(compare
`stream
`cipher). Nevertheless, it is possible to use a
`block cipher as a component within a more
`complex system that effects a stream cipher.
`Ste alto cryptography.
`block code A type of *error—correcting or
`*error—detecting code in which a fixed num—
`ber (conventionally k) of digits are taken into
`the encoder at a time and then output in the
`form of a codeword consisting of a greater
`number (conventionally n) of digits. It
`is
`often specified as an (n, k) code, with block
`length Ia and codeword length n. The corre—
`sponding decoder takes in 71 digits, and out—
`puts Ie digits, at a time. Since the codewords
`are longer than the input words, the possible
`received words are no more numerous. The
`codewords are only a selection of all possible
`words of their length: the selection method
`gives any code its particular properties. See
`also code.
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`block compaction Another namefor memory
`compaction.
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`LIKELIHOOD
`
`being displayed to determine the position of
`the light pen.
`The light pen is used to draw items with
`the aid of a tracking cross. As the light pen is
`moved across the screen,
`the part of the
`tracking cross sensed changes thus allowing
`the direction of movement of the light pen to
`be ascertained. The tracking cross can be
`redrawn to locate it at the expected new cen—
`ter of the light pen’s position and thus
`appears to follow the light pen.
`Ilkellhood The *probability that an observa-
`tion belongs to a *probability distribution
`with *parameters 0, considered as a function
`of the parameters rather than of the observa—
`tion.
`The method of maximum likelihood, origi—
`nated by R. A. Fisher, estimates *parameters
`in statistical models by maximizing the likeli—
`hood ofobserving the data with respect to the
`parameters ofthe model. The values taken by
`the parameters at the maximum are known as
`maximum likelihood estimates. This method is
`computationally equivalent to the method of
`*least squares when the distribution of the
`observations about their theoretical means is
`the *normal distribution.
`Ilrnited license A license granted to a cus-
`tomer that limits the use that is made of a
`computer program. Under *copyright law,
`limited licenses restrict customers from
`legally being entitled to use several copies of
`individual programs at the same time or use
`copies on anything except a particular com-
`puter.
`Linda A particular model for *disu-ibuted pro-
`cessing in which the *prooesses communi-
`mte by inserting, examining, and deleting
`objects in a common *bag. It is architecture—
`independent.
`. Llndenmeyer system See L—system.
`linear algebraic equatlons (slmultaneous
`equations) A problem in *numerical linear
`algebra that requires the solution of n equa—
`tions in the unknowns x1, rpm, an of the
`form
`Ax = b
`where A is a square an matrix. The solution
`obtained by computing the inverse matrix
`and forming A411 is
`less accurate and
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`requires more arithmetical operatiOns than
`elimination methods. In Gaussian eliminatin
`multiples of successive equations are adde;
`to all succeeding ones to eliminate the
`unknowns x], xz,..., xH In turn. Properly .
`used, with row interchanges to avoid large
`multiples, this leads to a solution that 5mg~
`fies exactly a system close to the one gm,“
`relative to the machine precision. The ace“:
`racy of the solution, which can be Chaply
`estimated, depends on the *condition num.
`ber of the problem.
`Many other methods are used to deal with
`matrices of special form. Very large Systems
`where the matrix A has predominantly Zero
`entries occur in the solution of *pardal dif—
`ferential equations. Elimination methods
`tend to fill in the zeros causing storage prob.
`lems and *iterative methods are often pre—
`ferred for such problems.
`linear array Another namefor one-dimension—
`al array, i.e. for a *vector. See alto array.
`"near-bounded automaton (LBA) A
`*Turing machine M such that the number of
`tape cells visited by M is bounded by Some
`linear function of the length of the input
`string. 0f equivalent power is the smaller
`class of Turing machines that visit only the
`cells bearing the input string. The *context—
`sensitive languages are precisely those recog—
`nized by such Turing machines.
`linear channel A transmission channel in
`which the information *signal and the *noise
`signal combine addiuvely to form the output
`signal. In a *q—ary linear channel, with a
`finite number, 4, of amplitudes, the signals
`add modulo-q; in the binary use (4 = 2), this
`has the same effect as an *exclusive—OR
`operation between the signals.
`llnear codes In coding theory, codes whose
`encoding and decoding operations may be
`expressed in terms of linear operations. The
`term is usually applied to certain *error—
`correcting codes in which the encoding oper-
`ation involves a generator matrix and the
`decoding operation involves a parity—cheek
`matrix. Linear codes are,
`therefore, also
`called parity-cheek codes. A particular linear
`code forms a commutative *group that has
`the zero codeword as its identity.
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