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This document describes the programmer interface to a computing machine partially implemented in software. The Extended Computer is part of NRL's Software Cost Reduction (SCR) project, to demonstrate the feasibility of applying advanced software engineering techniques to complex real-time systems in order to simplify maintenance. The Extended Computer allows code portability among avionics computers by providing extensible addressing, uniform i/o and data access, representation-independent data types, uniform event signalling, a standard subprogram invocation mechanism, and parallel process capability. The purpose of the Extended Computer is to allow the remainder of the software to remain unchanged when the host computer is changed or replaced. This report describes the modular structure of the Extended Computer, and contains the abstract interface specifications for all the facilities provided to users. It serves as development and maintenance documentation for the SCR software design, and is also intended as a model for other people interested in applying the abstract interface approach on other software projects. Additional keywords: real time, systems engineering. (Author)., Supersedes Rept. no. NRL-MR-4843, AD-A123 566.

NRL's Software Cost Reduction project is demonstrating the feasibility of applying advanced software engineering techniques to complex real time systems to simplify maintenance. To demonstrate the principles, the onboard software for the Navy's A 7E aircraft is being redesigned and reimplemented. The project is producing a set of model procedures and documents that can be followed by designers and producers of other software systems.

This document describes the programmer interface to a computing machine partially implemented in software. The Extended Computer is part of NRL's Software Cost Reduction (SCR) project, to demonstrate the feasibility of applying advanced software engineering techniques to complex real-time systems in order to simplify maintenance. The Extended Computer allows code portability among avionics computers by providing extensible addressing, uniform i/o and data access, representation-independent data types, uniform event signalling, a standard subprogram invocation mechanism, and parallel process capability. The purpose of the Extended Computer is to allow the remainder of the software to remain unchanged when the host computer is changed or replaced.

This document describes the basic organization of NRL's version of the A-7E onboard flight software. The report describes a structure in which modules have been designed in accordance with the Information Hiding Principle. Because of the larger number of modules that result when this principle is applied, the modules are arranged in a hierarchy. The hierarchy should simplify the task of a maintenance programmer assigned to make a specific modification. The document also describes the principles used in the design of the software. It is intended to be useful both as a guide to the A-7E software and as a model for those developing other software systems. (Author)

The overall objective of this effort is to discover on-line fault detection, isolation and repair techniques and measures of effectiveness which will be ultimately applicable to tactical and strategic modular digital systems., Prepared in cooperation with North Carolina Univ. at Chapel Hill, North Carolina State Univ., Raleigh and Duke Univ., Durham, NC.

The concept of type has been used without a precise definition in discussions about programming languages for 20 years. Before the concept of user-defined data types was introduced, a definition was not necessary for discussions of specific programming languages. This report discusses a definition of type in which the concept of variable is considered primitive and types are defined as various equivalence classes of variables that may be legally and meaningfully substituted for one another in different contexts. Abstraction from the differences between two variables is then achieved by allowing the substitution of one variable in an equivalence class for another within specified contexts such as parameter substitution, assignment, and data sharing. The user of a language with an abstraction facility based on the preceding idea is free to group variables into types in ways meaningful to him but must state the conditions under which members of a group are equivalent. Several kinds of types are proposed for inclusion in such a language. Some examples are types defined by identical externally visible behavior (equivalent specification), types whose members differ according to the value of one or more parameters, and types whose members have identical representations. The preceding approach to type definition has a strong effect on the possibilities for code sharing, data sharing, and parameter passing. New or unusual compiling techniques will probably be required to exploit these possibilities. (Author)

This report describes the programmer interface to a set of avionics-oriented abstract data types implemented in software. The Application Data Types module is part of NRL's Software Cost Reduction (SCR) project, to demonstrate the feasibility of applying advanced software engineering techniques to complex real-time systems to simplify maintenance. The Application Data Types module allows operations on data independent of the representation. In the case of numeric abstract types, which represent physical quantities such as speed or distance, arithmetic operations may be performed independent of the units of physical measure. This allows the rest of the application software to remain unchanged even when representation decisions change about these data. These report contains the abstract interface specifications for all the facilities provided to users by this module. Itr serves as development and maintenance documentation for the SCR software design, and it is also intended as a model for other people interested in applying the abstract interface approach on other software projects.

This is the notebook from the updated edition of the well-received course originated by the Naval Research Laboratory and taught annually for the past six years. It is a two-week technical course for personnel managing a software project or designing software. The purpose of the course is to improve the participant's ability to evaluate software requirements, specifications, design, correctness, and maintainability. Its purpose is not to transform the participant into an expert software designer. The course concentrates on technical problems of software design. It introduces generally accepted design practices, as well as software design research that may result in practical design applications in the near future. Design for ease of maintenance is emphasized. All course material is unclassified. Topics covered include information-hiding modules (modules that isolate the effects of changes), abstract interfaces (a technique for designing the interfaces of information-hiding modules), responses to undesired events, cooperating sequential processes (in real-time systems, software tasks in which scheduling and resource allocation decisions are not embedded), disciplined documentation techniques, techniques for formal specification, and designing systems with useful subsets. (Author)

The paper deals with the design of hierarchically structured programing systems. It develops a method for evaluating the cost of requiring programmers to work with an abstraction of a real machine. A number of examples from hardward and software are given as illustrations of the method.

This paper introduces the problem of finding a sequential process equivalent to a system of interacting discrete parallel processes. Under the assumption that the sequential process is to be composed exclusively of executions of the individual 'parallel' processes in a predetermined sequence, a method of deriving optimal sequential processes is presented. Applications to the design of simulation systems and picture processing programs are discussed. Examples are taken from logic design and picture processing. (Author)

The paper discusses modularization as a mechanism for improving the flexibility and comprehensibility of a system while allowing the shortening of its development time. The effectiveness of a modularization is dependent upon the criteria used in dividing the system into modules. Two system design problems are presented, and for each, both a conventional and unconventional decomposition are described. It is shown that the unconventional decompositions have distinct advantages for the goals outlined. The criteria used in arriving at the decompositions are discussed. The unconventional decomposition, if implemented with the conventional assumption that a module consists of one or more subroutines, will be less efficient in most cases. An alternative approach to implementation which does not have this effect is sketched.

The paper describes a mechanism for holding a program in syntactic form. This mechanism can be useful to any program which processes programs: in program verification automatic programming, and specialized text editing. In this case the program holder is used to form the basis for syntax-driven text editor. Formal specifications for the program holder are also given. (Author), Sponsored in part by Grant NSF-GJ-37728.

The report discusses a problem first introduced by Patil. Patil presented a proof that the problem cannot be solved using the P and V operations introduced by Dijkstra unless conditional statements are used. The report contains a solution to the problem as defined by Patil and shows that Patil's proof overlooks important capabilities for P and V which were used by Dijkstra in his original report. The report also discusses the need for the generalized operators suggested by Patil. (Author)

The paper presents a method for writing specifications of parts of software systems. The main goal is to provide specifications sufficiently precise and complete that other pieces of software can be written to interact with the piece specified without additional information. The secondary goal is to include in the specification no more information than necessary to meet the first goal. The technique is illustrated by means of a variety of examples from a tutorial system. (Author)

The role of documentation in the design and implementation of complex systems is explored, resulting in suggestions in sharp contrast with current practice. The concept of system structure is studied by examining the meaning of the phrase 'connections between modules.' It is shown that several system design goals (each suggesting a partial time ordering of the decisions) may be inconsistent. Some properties of programmers are discussed. System documentation, which makes all information accessible to anyone working on the project, is discussed. The thesis that such information 'broadcasting' is harmful, that it is helpful if most system information is hidden from most programmers, is supported by use of the above mentioned considerations as well as by examples. An information hiding technique of documentation is given in the appendix. (Author)