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Communications of the ACM

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On the external storage fragmentation produced by first-fit and best-fit allocation strategies

Published comparisons of the external fragmentation produced by first-fit and best-fit memory allocation have not been consistent. Through simulation, a series of experiments were performed in order to obtain better data on the …

Deterministic parsing of ambiguous grammars

Methods of describing the syntax of programming languages in ways that are more flexible and natural than conventional BNF descriptions are considered. These methods involve the use of ambiguous context-free grammars together …

Guarded commands, nondeterminacy and formal derivation of programs

So-called “guarded commands” are introduced as a building block for alternative and repetitive constructs that allow nondeterministic program components for which at least the activity evoked, but possibly even the final state …

Remark on stably updating mean and standard deviation of data

Although not published as a numbered algorithm, Hanson's article “Stably Updating Mean and Standard Deviation of Data” in the January, 1975, issue of Communications, [1] describes an algorithm for sequentially recomputing the …

Interactive consulting via natural language

Interactive programming systems often contain help commands to give the programmer on-line instruction regarding the use of the various systems commands. It is argued that it would be relatively easy to make these help commands …

Comments on a paper by T.C. Chen and I.T. Ho

Tien Chi Chen and Irving T. Ho in their paper “Storage-Efficient Representation of Decimal Data” [1] present a scheme for the compression of numbers stored in binary coded decimal (BCD) form. Their compression scheme involves …

Consecutive storage of relevant records with redundancy

This paper studies the properties of a new class of file organizations (CRWR) where records relevant to every query are stored in consecutive storage locations but the organizations contain redundancy. Some theorems which provide …

Multiple byte processing with full-word instructions

A method is described which allows parallel processing of packed data items using only ordinary full-word computer instructions, even though the processing requires operations whose execution is contingent upon the value of a …

Combining decision rules in a decision table

The techniques for minimizing logic circuits are applied to the simplification of decision tables by the combining of decision rules. This method is logically equivalent to the Quine-McCluskey method for finding prime implicants …