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Research and Advances

Performing remote operations efficiently on a local computer network

A communication model is described that can serve as a basis for a highly efficient communication subsystem for local networks. The model contains a taxonomy of communication instructions that can be implemented efficiently and can be a good basis for interprocessor communication. These communication instructions, called remote references, cause an operation to be performed by a remote process and, optionally, cause a value to be returned. This paper also presents implementation considerations for a communication system based upon the model and describes an experimental communication subsystem that provides one class of remote references. These remote references take about 150 microseconds or 50 average instruction times to perform on Xerox Alto computers connected by a 2.94 megabit Ethernet.
Research and Advances

A technique for testing command and control software

A technique for testing embedded-microprocessor command and control programs is described. The continuity inherent in functions computed by programs which monitor natural phenomena is exploited by a simple difference equation-based algorithm to predict a program's next output from its preceding ones. The predicted output is compared with the actual output while indexing through the program's domain. Outputs which cannot be predicted are flagged as potential errors. Data are presented which show that this technique can be a sensitive measure of a program's correctness.
Research and Advances

The “worm” programs—early experience with a distributed computation

The “worm” programs were an experiment in the development of distributed computations: programs that span machine boundaries and also replicate themselves in idle machines. A “worm” is composed of multiple “segments,” each running on a different machine. The underlying worm maintenance mechanisms are responsible for maintaining the worm—finding free machines when needed and replicating the program for each additional segment. These techniques were successfully used to support several real applications, ranging from a simple multimachine test program to a more sophisticated real-time animation system harnessing multiple machines.
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Research and Advances

The future of programming

The nature of programming is changing. These changes will accelerate as improved software development practices and more sophisticated development tools and environments are produced. This paper surveys the most likely changes in the programming task and in the nature of software over the short term, the medium term, and the long term. In the short term, the focus is on gains in programmer productivity through improved tools and integrated development environments. In the medium term, programmers will be able to take advantage of libraries of software components and to make use of packages that generate programs automatically for certain kinds of common systems. Over the longer term, the nature of programming will change even more significantly as programmers become able to describe desired functions in a nonprocedural way, perhaps through a set of rules or formal specification languages. As these changes occur, the job of the application programmer will become increasingly analysis-oriented and software developers will be able to attack a large number of application areas which could not previously be addressed effectively.
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Research and Advances

Some practical experience with a software quality assurance program

Within a production programming environment, a software quality assurance program (QAP) was instituted to produce standards, conventions, and methodologies for all phases of the software development process. Software language standards and several support processors, in turn, developed. The authors offer a plan which may help others avoid some of the pitfalls they experienced while attempting to construct a meaningful software QAP.
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Research and Advances

Control flow and data structure documentation: two experiments

Two experiments were carried out to assess the utility of external documentation aids such as macro flowcharts, pseudocode, data structure diagrams, and data structure descriptions. A 223 line Pascal program which manipulates four arrays was used. The program interactively handles commands that allow the user to manage five lists of items. A comprehension test was given to participants along with varying kinds of external documentation. The results indicate that for this program the data structure information was more helpful than the control flow information, independently of whether textual or graphic formats were used.
Research and Advances

Reciprocal hashing: a method for generating minimal perfect hashing functions

A method is presented for building minimal perfect hash functions, i.e., functions which allow single probe retrieval from minimally sized tables of identifier sets. A proof of existence for minimal perfect hash functions of a special type (reciprocal type) is given. Two algorithms for determining hash functions of reciprocal type are presented and their practical limitations are discussed. Further, some application results are given and compared with those of earlier approaches for perfect hashing.
Research and Advances

A two-list synchronization procedure for discrete event simulation

The traditional mechanism for maintaining a list of pending events in a discrete event simulation is the simple linked list. However, in large scale simulations this list often becomes cumbersome to maintain since the number of pending events may become quite large. As a result, the execution time required by the simple linked list is often a significant portion of total simulation time. Several papers have been published suggesting improved synchronization procedures. The most efficient procedures reported are the time-indexed procedure and the two-level procedure. Both methodologies are much more efficient than simple linked lists; however, neither has been adopted by a general purpose simulation language. Further, both procedures require external parameter definition, which is a major handicap to their adoption by a general purpose language. This paper introduces a new sychronization procedure, the two-list procedure, which is much faster than simple linked lists for large pending event files. This procedure was designed for implementation in Fortran, and properly implemented it is transparent to the user. Thus it is ideal for adoption by general purpose simulation languages.
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Research and Advances

A generalized user interface for applications programs

A general method for using disk files (instead of a more conventional parameter-passing mechanism) to transfer control information from a user interface to a set of related applications programs is described. This technique effectively moves much of the user interface, including command decoding and limited parameter checking, from the applications programs to a table-driven executive.
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Research and Advances

Microcomputers as remote nodes of a distributed system

A time-sharing system may be extended into a general-purpose distributed computing facility through the use of remote microcomputer systems. In such an extension the microcomputer system is not merely a remote terminal but a complete computer system. It becomes part of an integrated network of computers, each with significant local computational power with access to host system data resources. The implementation described emphasizes ease-of-use by minimizing any differences between local and remote data use.
Research and Advances

Problems in application software maintenance

The problems of application software maintenance in 487 data processing organizations were surveyed. Factor analysis resulted in the identification of six problem factors: user knowledge, programmer effectiveness, product quality, programmer time availability, machine requirements, and system reliability. User knowledge accounted for about 60 percent of the common problem variance, providing new evidence of the importance of the user relationship for system success or failure. Problems of programmer effectiveness and product quality were greater for older and larger systems and where more effort was spent in corrective maintenance. Larger scale data processing environments were significantly associated with greater problems or programmer effectiveness, but with no other problem factor. Product quality was seen as a lesser problem when certain productivity techniques were used in development.
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Research and Advances

Authentication of signatures using public key encryption

One of Needham and Schroeder's proposed signature authentication protocols is shown to fail when there is a possibility of compromised keys: this invalidates one of the applications of their technique. A more elaborate mechanism is proposed which does not require a network clock, but does require a third party to the transaction. The latter approach is shown to be reliable in a fairly strong sense.
Research and Advances

The evolution of user behavior in a computerized conferencing system

Data from 18-month operational trials of the EIES system indicate that the range of features considered valuable in a computer-based communication system increases with the amount of experience gained by using this medium of communication. Simple message systems alone are not likely to satisfy the communications needs of long term, regular users of computerized communications systems. Among the capabilities which long term, regular users find valuable are group conferences, notebooks for text composition, and self-defined commands.
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Research and Advances

Documentation for a model: a hierarchical approach

A set of documents and their organization according to functional requirements in order to produce information that will facilitate the use of models are described. The authors discuss the role of models in the policy process and of documentation in the assessment of such models.

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