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Research and Advances Oct 1 1969

A modular computer sharing systems

An alternative approach to the design and organization of a general purpose interactive multiterminal computing system is presented. The system organization described is a conceptually simple arrangement of a bank of interchangeable computers, each of which is a memory/proputor pair, that are assigned to process terminal jobs as they arrive. One of the computers serves as the master or control computer and supervises the collection and distribution of messages from and to the remote terminals. In the simplest form there is a disk drive for each connected terminal. A crosspoint switching network allows any such disk drive to be connected to any computer in the bank, under control of the control computer. Thus, while each active terminal user “occupies” a dedicated disk drive, he may share the computer with many other terminal users in a simple manner. The ratio of users to computers is dependent on both the size and power of the machines used and the computation requirements of the particular mix of users. This system organization is inherently a simpler and therefore more reliable approach to time-sharing computers and has the potential of a highly available system at relatively low cost. Economic configurations are possible for a range of systems sizes that span at least one order of magnitude. Finally, problem programs developed by remote terminal users can be run on a dedicated batch system if compatible computers are used.
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Research and Advances Oct 1 1969

A comment on optimal tree structures

In Y.N. Patt's paper “Variable Length Tree Structures Having Minimum Average Search Time” [Comm. ACM 12 (Feb. 1969)], the tree structures obtained are not actually optimal with respect to the two-dimensional chaining devised by Sussenguth in his 1963 paper. This note points out that the result can be described as “optimal” only under the constraint—which Patt implicity assumes—that no key be allowed to prefix another.
Research and Advances Aug 1 1969

The teachable language comprehender: a simulation program and theory of language

The Teachable Language Comprehender (TLC) is a program designed to be capable of being taught to “comprehend” English text. When text which the program has not seen before is input to it, it comprehends that text by correctly relating each (explicit or implicit) assertion of the new text to a large memory. This memory is a “semantic network” representing factual assertions about the world. The program also creates copies of the parts of its memory which have been found to relate to the new text, adapting and combining these copies to represent the meaning of the new text. By this means, the meaning of all text the program successfully comprehends is encoded into the same format as that of the memory. In this form it can be added into the memory. Both factual assertions for the memory and the capabilities for correctly relating text to the memory's prior content are to be taught to the program as they are needed. TLC presently contains a relatively small number of examples of such assertions and capabilities, but within the system, notations for expressing either of these are provided. Thus the program now corresponds to a general process for comprehending language, and it provides a methodology for adding the additional information this process requires to actually comprehend text of any particular kind. The memory structure and comprehension process of TLC allow new factual assertions and capabilities for relating text to such stored assertions to generalize automatically. That is, once such an assertion or capability is put into the system, it becomes available to help comprehend a great many other sentences in the future. Thus the addition of a single factual assertion or linguistic capability will often provide a large increment in TLC's effective knowledge of the world and in its overall ability to comprehend text. The program's strategy is presented as a general theory of
Research and Advances Aug 1 1969

A program for the syntactic analysis of English sentences

A program is described which produces syntactic analyses of English sentences with respect to a transformational grammar. The main features of the analyzer are that it uses only a limited dictionary of English words and that it pursues all analysis paths simultaneously while processing the sentence from left to right. The form of representation used for the dictionary and the grammar is indicated and an outline account is given of the analysis procedure. Techniques for keeping the size of the analysis record within reasonable limits and for avoiding the need for dynamic application of certain transformational rules are described. A number of examples of output produced by the program are given. The output includes timing information.
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Research and Advances Jul 1 1969

Automatic contour map

Some methods for contour mapping by means of a digital plotter are dicussed, and a new method is presented that is simple enough to be implemented by programs with a rather small number of instructions (about 120 FORTRAN IV instructions are required). Comparisons with some methods proposed by other authors are also performed. A FORTRAN IV program implementing the proposed method is availabel at the Istituto di Elettrotecnica ed Elettronica, Politecnico di Milano.
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Research and Advances Jun 1 1969

Simulation of traffic flows in a network

A computer simulation program which deals with traffic flows in the network of a large area is described. Each road is segmented into blocks of several ten-meter lengths and is represented by a bidirectional list in computer memory. The movement of cars, i.e. the transfer of cars from one block to the next, is expressed by a proper formula. This formula is based on the supposition that the speed of cars in a block is determined only by the density of cars in the block, and this speed-versus-density curve is empirically given the numerical values. This simulation scheme has its excellent point in that it makes it possible to trace the dynamic behavior of traffic flows in a variety of situations, some examples of which are given for an actual area of the city of Kyoto, Japan.
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Research and Advances Jun 1 1969

Three-dimensional computer display

A stereographic display terminal has been produced using the raster display (BRAD) recently developed at Brookhaven. The system uses a rotating refresh memory to feed standard television monitors. To produce a stereographic display the computer calculates the projected video images of an object, viewed from two separated points. The resulting video maps are stored on separate refresh bands of the rotating memory. The two output signals are connected to separate color guns of a color television monitor, thus creating a superimposed image on the screen. Optical separation is achieved by viewing the image through color filters. The display is interactive and can be viewed by a large group of people at the same time.
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News Jun 1 1969

A computer system for transformational grammar

A comprehensive system for transformational grammar has been designed and implemented on the IBM 360/67 computer. The system deals with the transformational model of syntax, along the lines of Chomsky's Aspects of the Theory of Syntax. The major innovations include a full, formal description of the syntax of a transformational grammar, a directed random phrase structure generator, a lexical insertion algorithm, an extended definition of analysis, and a simple problem-oriented programming language in which the algorithm for application of transformations can be expressed. In this paper we present the system as a whole, first discussing the general attitudes underlying the development of the system, then outlining the system and discussing its more important special features. References are given to papers which consider some particular aspect of the system in detail.
Research and Advances May 1 1969

Automated printed circuit routing with a stepping aperture

A computer program for routing interconnections on a two-sided printed circuit board with a regular pattern of lines, pins (terminals), and vias (feed-through holes) is described. In this program, each interconnection is given a planned routing—typically, down from the upper pin, through a via, and horizontally to the lower pin. From the top, a virtual aperture (i.e. a long horizontal slit) is stepped down the board. The planned routing is the basis for rerouting interconnections within the aperture to resolve conflicts for lines and vias below the aperture and to maximize the effective line usage. If a conflict has not been resolved before the aperture arrives at the lower pin, interconnections are deleted to resolve the conflict. Extensions of this technique to the control of crosstalk between routed interconnections and to the problem of obtaining 100 percent interconnect are also discussed.
Research and Advances May 1 1969

A note on reliable full-duplex transmission over half-duplex links

A simple procedure for achieving reliable full-duplex transmission over half-duplex links is proposed. The scheme is compared with another of the same type, which has recently been described in the literature. Finally, some comments are made on another group of related transmission procedures which have been shown to be unreliable under some circumstances.
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Research and Advances Apr 1 1969

An algorithm for solving a special class of tridiagonal systems of linear equations

An algorithm is presented for solving a system of linear equations Bu = k where B is tridiagonal and of a special form. This form arises when discretizing the equation - d/dx (p(x) du/dx) = k(x) (with appropriate boundary conditions) using central differences. It is shown that this algorithm is almost twice as fast as the Gaussian elimination method usually suggested for solving such systems. In addition, explicit formulas for the inverse and determinant of the matrix B are given.
Research and Advances Apr 1 1969

On coordination reduction and sentence analysis

A class of coordination phenomena in natural languages is considered within the framework of transformational theory. To account for these phenomena it is proposed that certain machinery be added to the syntactic component of a transformational grammar. This machinery includes certain rule schemata, the conditions under which they are to be applied, and conditions determining the sequence of subtrees on which they are to be performed. A solution to the syntactic analysis problem for this class of grammars is outlined. Precise specification of both the generative procedure of this paper and its inverse is given in the form of LISP function definitions.
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Research and Advances Apr 1 1969

An algorithm for hidden line elimination

The algorithm presented causes the elimination of hidden lines in the representation of a perspective view of concave and convex plane-faced objects on the picture plane. All the edges of the objects are considered sequentially, and all planes which hide every point of an edge are found. The computing time increases roughly as the square of the number of edges. The algorithm takes advantage of a reduced number of concave points and automatically recognizes if only one object with no concave points is considered. In this last case, the result is obtained in a much simpler way.
Research and Advances Apr 1 1969

Simulation of outpatient appointment systems

An experimental computer program is described which simulates appointment systems employed by outpatient departments of hospitals. Both major kinds of appointment systems—individual and block—can be simulated. The purpose of the Simulator is to enable the user to evaluate the effectiveness of alternative appointment systems in a given clinical environment.
Research and Advances Mar 1 1969

Description of FORMAT, a text-processing program

FORMAT is a production program which facilitates the editing and printing of “finished” documents directly on the printer of a relatively small (64k) computer system. It features good performance, totally free-form input, very flexible formatting capabilities including up to eight columns per page, automatic capitalization, aids for index construction, and a minimum of nontext items. It is written entirely in FORTRAN IV.
Research and Advances Feb 1 1969

On arithmetic expressions and trees

A description is given of how a tree representing the evaluation of an arithmetic expression can be drawn in such a way that the number of accumulators needed for the computation can be represented in a straightforward manner. This representation reduces the choice of the best order of computation to a specific problem under the theory of graphs. An algorithm to solve this problem is presented.
Research and Advances Feb 1 1969

Images from computers and microfilm plotters

Digital computers are widely used for the processing of information and data of all kinds, including the pictorial information contained in photographs and other graphical representations. Efficient conversion facilities for putting graphical information into the computer and retrieving it in graphical form are therefore much needed. One of the most commonly employed devices for obtaining permanent graphical output from digital computers is the microfilm plotter. Regrettably, present models have no provision for producing images with a continuous gray scale or “halftones.” In this note several programming techniques are described for obtaining halftone pictures from a microfilm plotter under the control of a digital computer. Illustrative examples of several methods are given.
Research and Advances Feb 1 1969

Exclusive simulation of activity in digital networks

A technique for simulating the detailed logic networks of large and active digital systems is described. Essential objectives sought are improved ease and economy in model generation, economy in execution time and space, and a facility for handling simultaneous activities. The main results obtained are a clear and useful separation of structural and behavioral model description, a reduction of manual tasks in converting Boolean logic into a structural model, the elimination of manual processes in achieving exclusive simulation of activity, an event-scheduling technique which does not deteriorate in economy as the event queue grows in length, and a simulation procedure which deals effectively with any mixture of serial and simultaneous activities. The passage of time is simulated in a precise, quantitative fashion, and systems to be simulated may be combinations of synchronous and asynchronous logic. Certain aspects of the techniques described may be used for the simulation of network structures other than digital networks.
Research and Advances Feb 1 1969

Variable length tree structures having minimum average search time

Sussenguth suggests in a paper (1963) that a file should be organized as a doubly-chained tree structure if it is necessary both to search and to update frequently. Such a structure provides a compromise between the fast search/slow update characteristics of binary searching and the slow search/fast update characteristics of serial searching. His method, however, contains the limiting restriction that all terminal nodes lie on the same level of the tree. This paper considers the effect of relaxing this restriction. First, trees which have the property that a priori the filial set of each node is well defined are studied. It is proved that coding the nodes within each filial set with respect to the number of terminal nodes reachable from each is necessary and sufficient to guarantee minimum average search time. Then the more general case (that is, where the entire structure of the tree is changeable) is treated. A procedure is developed for constructing a tree with a minimum average search time. A simple closed expression for this minimum average search time is obtained as a function of the number of terminal nodes. The storage capacity required to implement the doubly-chained tree structure on a digital computer is also determined. Finally, the total cost of the structure, using Sussenguth's cost criterion, is computed. It is shown that significant improvements in both the average search time and the total cost can be obtained by relaxing Sussenguth's restriction that all terminal nodes lie on the same level of the tree.
Research and Advances Jan 1 1969

Directed random generation of sentences

The problem of producing sentences of a transformational grammar by using a random generator to create phrase structure trees for input to the lexical insertion and transformational phases is discussed. A purely random generator will produce base trees which will be blocked by the transformations, and which are frequently too long to be of practical interest. A solution is offered in the form of a computer program which allows the user to constrain and direct the generation by the simple but powerful device of restricted subtrees. The program is a directed random generator which accepts as input a subtree with restrictions and produces around it a tree which satisfies the restrictions and is ready for the next phase of the grammar. The underlying linguistic model is that of Noam Chomsky, as presented in Aspects of the Theory of Syntax. The program is written in FORTRAN IV for the IBM 360/67 and is part of a unified computer system for transformational grammar. It is currently being used with several partial grammars of English.

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