Search
Join ACM

Software Engineering and Programming Languages

Advertisement

Research and Advances

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. If some of the decision rules implied in the ELSE Rule occur with low frequency, then the ELSE Rule can be used to further simplify the decision table. Several objectives merit consideration in optimizing a decision table: reducing machine execution time; reducing preprocessing time; reducing required machine memory; reducing the number of decision rules. (This often improves the clarity of the decision table to a human reader.) It will be shown that objectives (3) and (4) can be furthered with the above methods. Objective (1) is also attained if overspecified decision rules are not combined. Objective (2) must be compared against the potential benefits of objectives (1), (3), and (4) in deciding whether to use the above methods.
Research and Advances

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 tools for reducing redundancy in CRWR organizations have been also developed. Redundancies obtained by the application of these theorems are compared with that of query-inverted file organizations. Some CRWR organizations with minimum redundancy have also been developed for queries which specify sets of keys.
Research and Advances

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 relative performance of first-fit and best-fit and a better understanding of the reasons underlying observed differences. The time-memory-product efficiencies of first-fit and best-fit were generally within 1 to 3 percent of each other. Except for small populations, the size of the request population had little effect on allocation efficiency. For exponential and hyperexponential distributions of requests, first-fit outperformed best-fit; but for normal and uniform distributions, and for exponential distributions distorted in various ways, best-fit out-performed first-fit. It is hypothesized that when first-fit outperforms best-fit, it does so because first-fit, by preferentially allocating toward one end of memory, encourages large blocks to grow at the other end. Sufficient contiguous space is thereby more likely to be available for relatively large requests. Results of simulation experiments supported this hypothesis and showed that the relative performance of first-fit and best-fit depends on the frequency of requests that are large compared to the average request. When the coefficient of variation of the request distribution is greater than or approximately equal to unity, first-fit outperformed best-fit.
Research and Advances

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 with rules to resolve syntactic ambiguities. It is shown how efficient LR and LL parsers can be constructed directly from certain classes of these specifications.
, , and
Research and Advances

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, is not necessarily uniquely determined by the initial state. For the formal derivation of programs expressed in terms of these constructs, a calculus will be be shown.
Research and Advances

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 coding two or three digits at a time in seven bit or ten bit fields and requires only permutations, deletions, and insertions for coding and decoding. We would like to briefly present some alternatives to their compression method and to discuss the relative advantages of these other methods.
Research and Advances

Decomposability, instabilities, and saturation in multiprogramming systems

A step-by-step approach to model the dynamic behavior and evaluate the performance of computing systems is proposed. It is based on a technique of variable aggregation and the concept of nearly decomposable systems, both borrowed from Econometrics. This approach is taken in order to identify in multiprogramming paging systems (i) unstable regimes of operations and (ii) critical computing loads which bring the system into states of saturation. This analysis leads to a more complete definition of the circumstances in which “thrashing” can set in.
Research and Advances

The restriction language for computer grammars of natural language

Over the past few years, a number of systems for the computer analysis of natural language sentences have been based on augmented context-free grammars: a context-free grammar which defines a set of parse trees for a sentence, plus a group of restrictions to which a tree must conform in order to be a valid sentence analysis. As the coverage of the grammar is increased, an efficient representation becomes essential for further development. This paper presents a programming language designed specifically for the compact and perspicuous statement of restrictions of a natural language grammar. It is based on ten years' experience parsing text sentences with the comprehensive English grammar of the N.Y.U. Linguistic String Project, and embodies in its syntax and routines the relations which were found to be useful and adequate for computerized natural language analysis. The language is used in the current implementation of the Linguistic String Parser.
and
Research and Advances

A note on hash linking

In current machine designs, a machine address gives the user direct access to a single piece of information, namely the contents of that machine word. This note is based on the observation that it is often useful to associate additional information, with some (relatively few) address locations determined at run time, without the necessity of preallocating the storage at all possible such addresses. That is, it can be useful to have an effective extra bit, field, or address in some words without every word having to contain a bit (or bits) to mark this as a special case. The key idea is that this extra associated information can be found by a table search. Although it could be found by any search technique (e.g. linear, binary sorted, etc.), we suggest that an appropriate low overhead mechanism is to use hash search on a table in which the key is the address of the cell to be augmented.
Research and Advances

A large semaphore based operating system

The paper describes the internal structure of a large operating system as a set of cooperating sequential processes. The processes synchronize by means of semaphores and extended semaphores (queue semaphores). The number of parallel processes is carefully justified, and the various semaphore constructions are explained. The system is proved to be free of “deadly embrace” (deadlock). The design principle is an alternative to Dijkstra's hierarchical structuring of operating systems. The project management and the performance are discussed, too. The operating system is the first large one using the RC 4000 multiprogramming system.
Research and Advances

Efficient string matching: an aid to bibliographic search

This paper describes a simple, efficient algorithm to locate all occurrences of any of a finite number of keywords in a string of text. The algorithm consists of constructing a finite state pattern matching machine from the keywords and then using the pattern matching machine to process the text string in a single pass. Construction of the pattern matching machine takes time proportional to the sum of the lengths of the keywords. The number of state transitions made by the pattern matching machine in processing the text string is independent of the number of keywords. The algorithm has been used to improve the speed of a library bibliographic search program by a factor of 5 to 10.
and
Research and Advances

A cost oriented algorithm for data set allocation in storage hierarchies

Data set allocation in today's multilevel storage systems is usually based on qualitative, ad hoc decisions. While it would be desirable to obtain an optimal solution to this allocation problem, it is clear that the number of parameters involved makes it intractable to straight-forward solution. In such a situation, we must find a set of assumptions which simplify the problem greatly, but which still provide a basis for considering all significant cost elements. This paper presents such a first, quantitative allocation step. It considers many of the significant detailed costs of system utilization, data storage, data staging, and data migration. Although many avenues of further improvement are available, the present algorithm seems to be usefully accurate. As such, it can aid in quantifying the problems of data set allocation, storage system configuration, and new device designs.
, , , and
Research and Advances

Analysis and performance of inverted data base structures

The need to envision and architecture data base systems in a hierarchical level by level framework is stressed. The inverted data base (file) organization is then analyzed, considering implementation oriented aspects. The inverted directory is viewed realistically as another large data base which itself is subjected to inversion. Formulations are derived to estimate average access time (read only) and storage requirements, formalizing the interaction of data base content characteristics, logical complexity of queries, and machine timing and blocking specifications identified as having a first-order effect on performance. The formulations presented are necessary to be used in conjunction with any index selection criteria to determine the optimum set of index keys.
Research and Advances

Copying cyclic list structures in linear time using bounded workspace

A bounded workspace copying algorithm for arbitrary list structures is given. This algorithm operates in linear time and does not require tag bits. The best previous bounded workspace copying algorithms achieved n2 time without tag bits and n log n time with one tag. The only restriction on the algorithm given here is that the copy must be placed into a contiguous section of memory. The method is applicable to fixed or variable size nodes.

Shape the Future of Computing

ACM encourages its members to take a direct hand in shaping the future of the association. There are more ways than ever to get involved.

Get Involved

Communications of the ACM (CACM) is now a fully Open Access publication.

By opening CACM to the world, we hope to increase engagement among the broader computer science community and encourage non-members to discover the rich resources ACM has to offer.

Learn More