Richard W. Conway

The role of work-in-process inventory in serial production lines

In serial production systems, storage may be provided between processes to avoid interference due to lack of synchronization. This paper investigates the behavior of lines buffered in this way and explores the distribution and quantity of work-in-process WIP inventory that accumulates. We study simple, generic production systems to gain insight into the behavior of more complex systems. The authors are surprised by the sometimes counterintuitive results, but are joined in this surprise by both academics and practitioners with whom the study has been discussed. Results are presented for: identical workstations with and without buffers; balanced lines in which variability of processing times differs between stations; unbalanced lines; and lines with unreliable workstations. In general, buffers between workstations increase system capacity but with sharply diminishing returns. Position as well as capacity of the buffers are important. These results are preliminary, to be confirmed and extended by further study-indeed, a primary purpose of this paper is to stimulate such study. However, even these preliminary results yield design guidelines that should be useful in industrial practice.

On the implementation of security measures in information systems

The security of an information system may be represented by a model matrix whose elements are decision rules and whose row and column indices are users and data items respectively. A set of four functions is used to access this matrix at translation and execution time. Distinguishing between data dependent and data independent decision rules enables one to perform much of the checking of security only once at translation time rather than repeatedly at execution time. The model is used to explain security features of several existing systems, and serves as a framework for a proposal for general security system implementation within todays languages and operating systems.

Design and implementation of a diagnostic compiler for PL/I

PL/C is a compiler for a dialect for PL/I. The design objective was to provide a maximum degree of diagnostic assistance in a batch processing environment. For the most part this assistance is implicit and is provided automatically by the compiler. The most remarkable characteristic of PL/C is its perseverance—it completes translation of every program submitted and continues execution until a user-established error limit is reached. This requires that the compiler repair errors encountered during both translation and execution, and the design of PL/C is dominated by this consideration. PL/C also introduces several explicit user-controlled facilities for program testing. To accommodate these extensions to PL/I without abandoning compatibility with the IBM compiler, PL/C permits “pseudo comments”—constructions whose contents can optionally be considered either source text or comment. In spite of the diagnostic effort PL/C is a fast and efficient processor. It effectively demonstrates that compilers can provide better diagnostic assistance than is customarily offered, even when a sophisticated source language is employed, and that this assistance need not be prohibitively costly.

Selective partial access to a database

A system to support a multi-function, shared-access database requires the capability of defining for each user an arbitrary subset of the fields of a logical record to which access is allowed. The feasibility of such a capability has already been demonstrated by several operational systems. This paper is concerned with the possibility of granting something less than complete access to a specified field of a record. The purpose would be to allow a user to perform various summary and statistical tasks over controlled fields without allowing identification of the exact value of a field in a particular record. Three different strategies are examined: 1. An arbitrary partition of values is defined for each restricted field. A user granted this type of access can determine only which class of the partition contains the field value. 2. The actual field value is distorted by a random perturbation. 3. Access to actual field values is allowed—but values are dissociated from the actual record in which they occur. The third strategy—dissociation—appears to be the most interesting, potentially useful, but potentially vulnerable. In each case, the utility of incomplete access is examined and various implementation alternatives are explored. The degree of protection against persistent assault is determined.

Tele-CUPL: a telephone time sharing system

A general purpose, remote access, computing system is described, that employs twelve-key keyboard telephones as terminals. Audio output is provided directly to the telephone terminah, but the system will normally be used in conjunction with remotely located high speed printing devices. The system is a compatible extension of an existing batch processing system. A significant element of Ihe system is a scheme for transmitting alphanumeric information by single strokes on a numeric keyboard. The programmed scanner uses context to eliminate the ambiguity in transmission.

Modeling asynchronous materials handling systems in XCELL

The XCELL+ Factory Modeling System has recently been extended with facilities to model automatic guided vehicle systems and power-and-free conveyor systems. This significantly expands the scope of problems that are susceptible to modeling in a graphical, menu-driven mode by non-simulation professionals. This paper summarizes the issues and solutions represented by these extensions to XCELL+.

XCELL: a cellular, graphical factory modelling system

XCELL is one of a growing number of new simulation tools that capitalize on the graphics capability of personal computers. It is a complete, self-contained, interactive system intended for use by non-programmers. Graphic representation is the fundamental medium in XCELL, used in the construction of the model as well as the display of results.

CLP-the Cornell list processor

Presented in this paper are the highlights of CLP, a teaching language which has been employed at Cornell University and was constructed to serve as a means of introducing simulation and other list-processing concepts. The various advantages of CLP are discussed and examples are given.

Introductory instruction in programming

Cornell, like most other large schools, has been struggling with the problem of introductory programming instruction for years. The problem is not yet solved but in the last few years we have done some experimenting and seem to have made some progress. The situation seems worth reporting so that others can perhaps benefit from what has been successful and avoid what has not. The major problems seem to be volume and variety. Pedagogical techniques that are obviously effective with small classes are sometimes impractical when faced with more than a thousand students per year. This may be largely a question of economics, since one could presumably allocate enough resources to the task to have small classes, but this is just not realistic at Cornell and the real question becomes one of finding practical large-scale techniques. For example, the central computing facility at Cornell (a 360/65 under OS/HASP) cannot support a sufficient number of interactive terminals to make that mode of instruction practical. Hence our solution must operate within the constraint of a conventional non-inter-active computer service.