Search
Join ACM

Computing Applications

Advertisement

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

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 significantly more helpful by having them accept requests in natural language. As a demonstration, Weizenbaum's ELIZA program has been provided with a script that turns it into a natural language system consultant.
and
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

Illumination for computer generated pictures

The quality of computer generated images of three-dimensional scenes depends on the shading technique used to paint the objects on the cathode-ray tube screen. The shading algorithm itself depends in part on the method for modeling the object, which also determines the hidden surface algorithm. The various methods of object modeling, shading, and hidden surface removal are thus strongly interconnected. Several shading techniques corresponding to different methods of object modeling and the related hidden surface algorithms are presented here. Human visual perception and the fundamental laws of optics are considered in the development of a shading rule that provides better quality and increased realism in generated images.
Research and Advances

Improved event-scanning mechanisms for discrete event simulation

Simulation models of large, complex “real-world” applications have occasionally earned the reputation of eating up hours of computer time. This problem may be attributed in part to difficulties such as slow stochastic convergence. However, an additional problem lies in the fact that a significant amount of bookkeeping time is required to keep future events in their proper sequence. This paper presents a method for significantly reducing the time spent scanning future events lists in discrete event simulations. Three models are presented, all of which improve in effectiveness as the events-list scan problem becomes more burdensome.

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