Search
Join ACM

Computing Applications

Advertisement

Research and Advances

Attribute based file organization in a paged memory environment

The high cost of page accessing implies a need for for more careful data organization in a paged memory than is typical of most inverted file and similar approaches to multi-key retrieval. This article analyses that cost and proposes a method called multiple key hashing which attempts to minimize it. Since this approach is not always preferable to inversion, a combined method is described. The exact specifications of this combination for a file with given data and traffic characteristics is formulated as a mathematical program. The proposed heuristic solution to this program can often improve on a simple inversion technique by a factor of 2 or 3.
and
Research and Advances

Production systems: or can we do better than BNF

Since the development of BNF, the definition of the syntax of programming languages has been almost universally associated with context-free requirements. Yet numerous interesting and difficult issues in syntax stem from the context-sensitive requirements, notably the compatibility between the declaration of an identifier and its uses, the correspondence between actual and formal parameters, and issues arising from block structure. This paper explores the use of a formal notation called Production Systems in providing a readable and complete formal definition of syntax. As a practical illustration, a small but significant subset of PL/I is considered. A more detailed presentation, as well as the application to define abstract syntax and translations between languages, is given in a previous paper by the author.
Research and Advances

A cell organized raster display for line drawings

Raster scan computer graphics displays with “real time” character generators have previously been limited to alphanumeric characters. A display is described which extends the capabilities of this organization to include general graphics. The feasibility of such a display is shown by deriving the minimum number of patterns required in the read only memory of the character generator to synthesize an arbitrary line. The synthesis process does not compromise picture quality, since the resulting dot patterns are identical with those of a conventional raster display. Furthermore, the time constraints of a raster display are shown to be satisfied for a typical design for very complex line drawings.
and
Research and Advances

The synthesis of loop predicates

Current methods for mechanical program verification require a complete predicate specification on each loop. Because this is tedious and error prone, producing a program with complete, correct predicates is reasonably difficult and would be facilitated by machine assistance. This paper discusses techniques for mechanically synthesizing loop predicates. Two classes of techniques are considered: (1) heuristic methods which derive loop predicates from boundary conditions and/or partially specified inductive assertions: (2) extraction methods which use input predicates and appropriate weak interpretations to obtain certain classes of loop predicates by an evaluation on the weak interpretation.
Research and Advances

An approximate method for generating asymmetric random variables

Tukey's lambda distribution is generalized to provide an algorithm for generating values of unimodal asymmetric random variables. This algorithm has the same advantages as the symmetric random variable generator previously given by the authors, except that the addition of another parameter complicates the problem of finding the parameter values to fit a distribution.
and
Research and Advances

A method of bivariate interpolation and smooth surface fitting based on local procedures

A method is designed for interpolating values given at points of a rectangular grid in a plane by a smooth bivariate function z = z(x, y). The interpolating function is a bicubic polynomial in each cell of the rectangular grid. Emphasis is on avoiding excessive undulation between given grid points. The proposed method is an extension of the method of univariate interpolation developed earlier by the author and is likewise based on local procedures.
Research and Advances

Reentrant polygon clipping

A new family of clipping algorithms is described. These algorithms are able to clip polygons against irregular convex plane-faced volumes in three dimensions, removing the parts of the polygon which lie outside the volume. In two dimensions the algorithms permit clipping against irregular convex windows. Polygons to be clipped are represented as an ordered sequence of vertices without repetition of first and last, in marked contrast to representation as a collection of edges as was heretofore the common procedure. Output polygons have an identical format, with new vertices introduced in sequence to describe any newly-cut edge or edges. The algorithms easily handle the particularly difficult problem of detecting that a new vertex may be required at a corner of the clipping window. The algorithms described achieve considerable simplicity by clipping separately against each clipping plane or window boundary. Code capable of clipping the polygon against a single boundary is reentered to clip against subsequent boundaries. Each such reentrant stage of clipping need store only two vertex values and may begin its processing as soon as the first output vertex from the preceeding stage is ready. Because the same code is reentered for clipping against subsequent boundaries, clipping against very complex window shapes is practical. For perspective applications in three dimensions, a six-plane truncated pyramid is chosen as the clipping volume. The two additional planes parallel to the projection screen serve to limit the range of depth preserved through the projection. A perspective projection method which provides for arbitrary view angles and depth of field in spite of simple fixed clipping planes is described. This method is ideal for subsequent hidden-surface computations.
and
Research and Advances

A fast method for solving a class of tridiagonal linear systems

The solution of linear systems having real, symmetric, diagonally dominant, tridiagonal coefficient matrices with constant diagonals is considered. It is proved that the diagonals of the LU decomposition of the coefficient matrix rapidly converge to full floating-point precision. It is also proved that the computed LU decomposition converges when floating-point arithmetic is used and that the limits of the LU diagonals using floating point are roughly within machine precision of the limits using real arithmetic. This fact is exploited to reduce the number of floating-point operations required to solve a linear system from 8n - 7 to 5n + 2k - 3, where k is much less than n, the order of the matrix. If the elements of the subdiagnals and superdiagonals are 1, then only 4n + 2k - 3 operations are needed. The entire LU decomposition takes k words of storage, and considerable savings in array subscripting are achieved. Upper and lower bounds on k are obtained in terms of the ratio of the coefficient matrix diagonal constants and parameters of the floating-point number system. Various generalizations of these results are discussed.
and
Research and Advances

Syntax-directed least-errors analysis for context-free languages: a practical approach

A least-errors recognizer is developed informally using the well-known recognizer of Earley, along with elements of Bellman's dynamic programming. The analyzer takes a general class of context-free grammars as drivers, and any finite string as input. Recognition consists of a least-errors count for a corrected version of the input relative to the driver grammar. The algorithm design emphasizes practical aspects which help in programming it.
Research and Advances

Comments on the algorithms of Verhelst for the conversion of limited-entry decision tables to flowcharts

In his interesting contribution to the study of the conversion of limited-entry decision tables to flowcharts or sequential testing procedures, Verhelst [1] presents two algorithms, one of which is claimed to produce the optimal flowchart with respect to run-time. Unfortunately this minimum run-time algorithm does not always produce the result claimed, as we illustrate by example below.
and
Research and Advances

Advanced cryptographic techniques for computer

Cryptographic techniques which can be used to maintain the confidentiality of information processed by computers are dealt with. Special emphasis is paid to the unique characteristics of computer files that make many cryptographic methods of little use. Relative security, costs, and preferred methods are included in this paper.
Research and Advances

Numerical analysis in a Ph.D. computer science program

Numerical Analysis is the study of methods and procedures used to obtain“approximate solutions” to mathematical problems. Much of the emphasis is on scientific calculation. The difficulties of education in such a broad area center around the question of background and emphasis. The Numerical Analysis program in the Computer Science Department should emphasize an awareness of the problems of computer implementation and experimental procedures. Nevertheless, there is a need for a solid background in applied mathematics.
Research and Advances

Is automatic “folding” of programs efficient enough to displace manual?

The operation of “folding” a program into the available memory is discussed. Measurements by Brawn et al. and by Nelson on an automatic folding mechanism of simple design, a demand paging unit built at the IBM Research Center by Belady, Nelson, O'Neill, and others, permitting its quality to be compared with that of manual folding, are discussed, and it is shown that given some care in use the unit performs satisfactorily under the conditions tested, even though it is operating across a memory-to-storage interface with a very large speed difference. The disadvantages of prefolding, which is required when the folding is manual, are examined, and a number of the important troubles which beset computing today are shown to arise from, or be aggravated by, this source. It is concluded that a folding mechanism will probably become a normal part of most computing systems.
Research and Advances

A case study in programming for parallel-processors

An affirmative partial answer is provided to the question of whether it is possible to program parallel-processor computing systems to efficiently decrease execution time for useful problems. Parallel-processor systems are multiprocessor systems in which several of the processors can simultaneously execute separate tasks of a single job, thus cooperating to decrease the solution time of a computational problem. The processors have independent instruction counters, meaning that each processor executes its own task program relatively independently of the other processors. Communication between cooperating processors is by means of data in storage shared by all processors. A program for the determination of the distribution of current in an electrical network was written for a parallel-processor computing system, and execution of this program was simulated. The data gathered from simulation runs demonstrate the efficient solution of this problem, typical of a large class of important problems. It is shown that, with proper programming, solution time when NP processors are applied approaches 1/NP times the solution time for a single processor, while improper programming can actually lead to an increase of solution time with the number of processors. Storage interference and other measures of performance are discussed. Stability of the method of solution was also investigated.
Research and Advances

Optimization of expressions in Fortran

A method of optimizing the computation of arithmetic and indexing expressions of a Fortran program is presented. The method is based on a linear analysis of the definition points of the variables and the branching and DO loop structure of the program. The objectives of the processing are (1) to eliminate redundant calculations when references are made to common sub-expression values, (2) to remove invariant calculations from DO loops, (3) to efficiently compute subscripts containing DO iteration variables, and (4) to provide efficient index register usage. The method presented requires at least a three-pass compiler, the second of which is scanned backward. It has been used in the development of several FORTRAN compilers that have proved to produce excellent object code without significantly reducing the compilation speed.
and
Research and Advances

Productivity of multiprogrammed computers—progress in developing an analytic prediction method

Multiprogramming as it is discussed here is a mode of computer operation in which two or more programs are concurrently in processor memory and proceeding, each using the same central processor unit (CPU) and input-output (I/O) channels. These programs are actually proceeding intermittently and singly, according to eligibility (readiness to proceed) and priority. It is useful to be able to represent them as proceeding continuously and simultaneously, each at an effective rate, which may be a fraction of that which it would enjoy in the absence of the other programs. The effective progress rate of each program is sensitive to many detailed characteristics of itself and its co-residents; and simulation has been the best available method of predicting it. This paper presents the results of progress in developing an alternative to simulation, a simulation-tested iterative computation of these rates under certain situations. The algorithm is sensitive to most of the factors that control the phenomenon, including nonquantitative or topological features of the programs' structures.
Research and Advances

Performance monitoring in a time-sharing system

A software measurement facility which is part of a general purpose time-sharing system is described. The Data Collection Facility (DCF) has been implemented in the Michigan Terminal System (MTS) for the System/360 model 67. It exists for the purpose of monitoring operating system and user program behavior and performance. The overall structure of MTS is outlined in order to explain the implementation of the DCF. Events in the system are identified and recorded from within the supervisor, and dumped to magnetic tape by an auxiliary program for off-line processing. Events in user programs which are unrelated to system actions are recorded with a supervisor call. The time of occurrence of each event is accurately recorded, and data items are further identified by job and type. The overhead associated with data collection and its interference with normal jobs is carefully analyzed, and both are shown to be minimal. Several examples are given of information obtained with the facility and of applications in which it has been useful. Some general guidelines are offered for the construction of future monitoring programs.

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