Computing with REAL-TIME SYSTEMS Volume 2 Proceedings of the Second European Seminar University of Erlangen - Nürnberg Computing with REAL-TIME SYSTEMS Volume 2 Proceedings of the Second European Seminar University of Erlangen - Nürnberg Edited by 1.C. PYLE AERE Harwe/1 and P. ELZER University of Erlangen TRANSCRIPTA BOOKS 30 Crr1ven Street, Strand, London WC2 (t b) Publisher: S. Chomet © Copyright P. ELZER and 1. C. PYLE where not otherwise noted Printed in the United Kingdom by Shortlands Press Ltd, London 2 CONTENTS 5 PREFACE GENERAL CONSIDERATIONS ON REAL-TIME PROGRAMMING 9 Experiences with languages for real-time programming D.R.FROST 15 Standard software versus high-level languages R. C.HUTTY /P.W. HEYWOOD (Speaker) 19 Which programming languages for minicomputers in process control? V.HAASE 23 The influence of the structure of high-level languages on the efficiency of object code G.MUSSTOPF 29 Efficiency of programming in higher-level languages H.MITTENOORF 34 Workshop on general considerations on real-time programming BASIC DESIGN PRINCIPLES OF OPERATING SYSTEMS 39 Adaptive operating systems H.WETTSTEIN 43 Description of operating systems in terms of a virtual machine H-P.ROST 46 Interrupt handling in real-time control systems R.BAUMANN 53 Choosing a standard high-level language for real-time programming M.G. SCHOMBERG, and P.WARD 57 Use of high- and low-level languages in a real-time system J.STENSON 61 Workshop on basic design principles of operating systems COMPONENTS OF REAL-TIME SYSTEMS 65 Basic supervisor facilities for real-time I.C.PYLE 3 70 Software aspects of the UMIST hybrid J. N. HAMBURY 75 Process scheduling by output considerations G. McC. HAWORTH 81 CALAS70 - a real-time operating system based on pseudo­ processors G.HEPKE 84 A dynamic adaptive scheduling scheme for a real-time operating system H. HERBSTREITH 87 Workshop on components of real-time systems SPECIAL SYSTEMS AND SYSTEM FEATURES 91 Simulation du deroulement logique de programmes de commande de spectrometres h. neutrons Ph. BLANCHARD, Ph. LEDEBT, and M.TAESCHNER 96 An implementation of a virtual CAMAC processor A. LANGSFORD 99 An implementation of the assembly language and program assembler for a virtual CAMAC processor V.J.HOWARD 106 System software real-time testing aids W.E. QUILLIN 112 Testing and diagnostic aids for real-time programming E.C.SEDMAN 115 Real-time programming using a real-time language of intermediate level B.EICHENAUER 121 Workshop on special systems and system features 123 CLOSING ADDRESS 125 AUTHOR INDEX 4 PREFACE Following the First European Seminar on Real-Time Programming at Harwell in April 1971, this Second Seminar was held at Uie University of Erlangen, Nürnberg, in April 1972. lt was organised by P. Elzer of the Physikalisches Institut of the University, assisted by Mrs Brehm, with further help from K. Pelz and P. Holleczek. As at the first seminar, special periods were allocated for 'workshop' discussion sessions covering particular areas, and notes on these discussions are included. (The absence of identification of the speakers in these discussions is deliberate, to emphasise that speakers' views are personal and do not necessarily represent those of their institutions.) The papers presented and workshop notes were prepared for publica­ tion by P. Elzer and I. C. Pyle. 5 GENERAL CONSIDERATIONS ON REAL-TIME PROGRAMMING Chairman: Prof. Dr R. LAUBER 7 EDITORS IAN C. PYLE, MA, PhD {Cantab) has been working with computer software since 1956, particularly concerned with programming systems (Hartran on the ICL Atlas Computer), multi-access sys­ tems (HUW on the IBM 360 computer at Harwell), and multi-computer command and control systems. He is now Professor of Computer Science at the University of York. PETER ELZER, MA {Erlangen) has been working with computers since 1966, at the Physics Institute of the University of Erlangen. His early work concerned the development of a programming system for application to nuclear physics experiments. He was one of the initiators of the PEARL project and is now leader of a PEARL implement team at the Physics Institute. Since 1971 he has now been involved with the LTP� project, of which he is now the European Chairman. TRANSCRIPTA BOOKS Experiences with languages for real-time programming D.R. FROST General Electric Co, Phoenix, Arizona, USA Publication rights reserved to D.R. Frost and the General Electric Company At General Electric's Process Computer Opera­ A good programming tool: tion in Phoenix our product consists of real-time 1. Gives the programmer a vocabulary that process data acquisition and control systems, matches his application. with applications including the automation of power 2. Takes care of hardware plants, power distribution, refineries, . chemical. - storage allocation plants; such processes as discrete parts manu­ - controlling peripherals. facturing, steel making; production of cement, 3. Provides non-ferrous metals, paper, textiles, and others. - standard algorithms For these automation systems we provide the - data formats computer and process input;output. We frequently - character codes. provide analysis and programming as well. Since 4. Promotes good design, and allows good economics dictate that we will get no more money design to be maintained as the system for this service than it is worth to our customers, gradually evolves. profitability depends upon our skill in keeping 5. Has self-contained debugging aids, which programming costs down. This paper is about - make mistakes less likely our attempts to lower costs through the use of - make error correction more reliable. higher level programming languages and applica­ 6. Gets rid of 'patches' (machine-oriented tion packages. I will discuss: program corrections] and the dependence 1. What we expect from higher level languages on knowledge of the computer which 'patch­ and application packages. ing' requires. 2. Our specific experiences with these various 7. Aids maintenance of a design as the applica­ types of languages and application packages tion changes. and how successful they have been. 8. Aids maintenance of code as errors are 3. Where we are headed. found and corrected. 4. Lessons we have learned. Good design is probably the biggest cost saver in software production, and good tools encourage good design. An excellent example is the suc­ Expectations cessful language COBOL, which forces the pro­ grammer to define and document his data base. The whole rationale for using higher level (1 suspect that this is actually more a key to languages and application packages is to sa ve sys­ COBOL's success than is its natural English­ tem costs through lowering programming costs. language-like syntax.) There have been many successful tools for various Successful languages and application packages other computer applications: FORTRAN, ALGOL, such as the ones mentioned exist in the compara­ COBOL, LISP, various report generators, sort ti vely orderly world of scientific and business packages, and simulation languages are just a few. data processing. They fit their applications very All of these tools have a common objective: to well. FORTRAN is a good language for mathema­ produce machine-independent source programs. tics and logic. The vocabulary of COBOL Machine independence is often equated with consists of business data processing terms in portability between computers. Yet the truly common use lang before computers. The environ­ important facet of machine independence is not so ment of a sort generator package is orderly files much portability; of more importance, it allows of data. But the term 'real-time programming' the user to implement his solution without all the causes many of us to think simplistically. lt computer-related clutter that gets in his way. makes us believe we will find a single, ultimate To a problem solver this clutter is overhead, with tool for 'real-time programming'. But real-time all of that word's connotation of cost - and he is applications are just as diverse as those for right. He wants independence from the computer. which we use LISP, FORTRAN, and report 9 generators. As a result, we are finding and will In spite of these shortcomings, FORTRAN is in continue to find a multiplicity of 'best' tools for use today. In some companies (and some applica­ real-time systems, depending on the particular tion areas) it is used a great deal. This is applications. possible because of various attempts which have been made to remedy FORTRAN's shortcomings. Good tools for real-time programming must These various attempts ha ve resulted in a provide the same advantages as do existing proliferation of FORTRAN dialects and extensions. successful tools for other applications: We have two ourselves, one predating the 1. Independence from the computer. FORTRAN ANSI standard, the other based on it. 2. A problem-oriented vocabulary. 3. Encouragement of good software design. The real-time problem is solved through provid­ ing a real-time operating system outside of Let us now look at some of the tools that have FORTRAN. System functions or subroutines are been used by employees and customers of General made available to the programmer for interroga­ Electric's Process Computer Operation. ting time, which is kept by the operating system. Data types are augmented by allowing various forms of bit arrays and providing operations to Our experiences work on them. Data communication problems are also handled FORTRAN - a semi-success through various extensions to the language. In one case we changed the meaning of labelled Soon after FORTRAN swept to supremacy as the COMMON to allow more than one common area. earliest and most used mathematical program­ Named system variables are another useful ming language in the United States our customers technique. wanted to use it to program their process appli­ cations. In the early days there was considerable Program organisation is dependent on the avail­ naivete about the usefulness of FORTRAN for able operating system.