Robert P. Cook

*MOD—A Language for Distributed Programming

Distributed programming is characterized by high communications costs and the inability to use shared variables and procedures for interprocessor synchronization and communication. *MOD is a high-level language system which attempts to address these problems by creating an environment conducive to efficient and reliable network software construction. Several of the *MOD distributed programming constructs are discussed as well as an interprocessor communication methodology. Examples illustrating these concepts are drawn from the areas of network communication and distributed process synchronization.

DYMOS: a dynamic modification system

Programs are frequently modified during their lifetime. Furthermore, modifications to some systems have to be made on the fly; that is, without stopping their execution. For example, changes to airline reservation systems, telephone switching systems, and on-line banking systems have to performed dynamically. Other continuously running systems, such as operating systems, may be modified on the fly to increase their availability. DYMOS is a programming system that supports a programmer modifying a StarMod program dynamically. StarMod [2] is an extension of Modula [3]. To modify procedures dynamically, the programmer modifies and recompiles the source code of the procedures to be replaced and then requests the system to change the current core image to incorporate new code and data.

A contextual analysis of Pascal programs

More than 120,000 lines of Pascal programs, written by graduate students and faculty members, have been statically analysed to provide a better understanding of how the language is ‘really’ used. The analysis was done within twelve distinct contexts to discover differences in usage patterns among the various contexts. For example, it was found that 47 per cent of the operands in arguments lists were constants. The results are displayed as tables of frequency counts which show how often each construct is used within a context. Also, we have compared our findings to the results from studies of other languages, such as FORTRAN, SAL and XPL.

An analysis of language models for high-performance communication in local-area networks

In this paper we present an empirical analysis of language models for communication in distributed systems. We consider a computing environment in which a high-level, distributed programming language kernel is sufficient support for high-performance programming applications. We propose programming language support for such an environment and present the performance results of an implementation. Using the distributed programming language StarMod as a context, we describe language constructs for message-based communication (including broadcast), remote invocation, and remote memory references. Each form of communication is integrated into StarMod in a consistent fashion maintaining the properties of transparency, modularity, and full parameter functionality. The costs and benefits associated with the various models of communication are analyzed based on the results of an implementation which runs on 8 PDP 11/23; microprocessors connected by a 1 megabit/second network.

High-Level Broadcast Communication for Local Area Networks

Even for LANs without collision-detection hardware, this implementation of StarMod offers improvements of from 1.1 to 7.8 over pointto-point message transmission-and thats the worst-case gain.

A Symbol Table Abstraction to Implement Languages with Explicit Scope Control

We are concerned with languages in which the programmer has explicit control over the referencing environment of a name. Several modern programming languages, including Ada, Euclid, Mesa, and Modula, implement these control capabilities. This paper describes a simple technique which uses the traditional concepts of a hashed symbol table and lexical level to solve many of the symbol table implemen-tation problems associated with explicit scope control. The primary ad-vantage of this technique is that a single symbol table abstraction can be used to simply and efficiently solve most problems in scope control.

An experiment to improve operand addressing

MCODE is a high-level language, stack machine designed to support strongly-typed, Pascal-based languages with a variety of data types in a modular programming environment. The instruction set, constructed for efficiency and extensibility, is based on an analysis of 120,000 lines of Pascal programs. The design was compared for efficiency with the instruction sets of the Digital Equipment PDP-11 and VAX by examining the generated code from the same compiler for all three machines. In addition, the original design choices were tested by analyzing the generated code from 19,000 lines of StarMod programs. As a result of this iterative experiment, we have summarized our observations in an efficient reorganization of the VAXs addressing modes.