Preface

This book is an attempt to explain in detail the nucleus of one of the most interesting computer operating systems to appear in recent years.

It is the UNIX Time-sharing System, which runs on the larger models of Digital Equipment Corporation’s PDP11 computer system, and was developed by Ken Thompson and Dennis Ritchie at Bell Laboratories. It was first announced to the world in the July, 1974 issue of the “Communications of the ACM”.

Very soon in our experience with UNIX, it suggested itself as an interesting candidate for formal study by students, for the following reasons:

• it runs on a system which is already available to us;

• it is compact and accessible;

• it provides an extensive set of very usable facilities;

  it is intrinsically interesting, and in fact breaks new ground in a number of areas.

Not least amongst the charms and virtues of the UNIX Time-sharing System is the compactness of its source code. The source code for the permanently resident “nucleus” of the system when only a small number of peripheral devices is represented, is comfortably less than 9,000 lines of code.

It has often been suggested that 1,000 lines of code represents the practical limit in size for a program which is to be understood and maintained by a single individual. Most operating systems either exceed this limit by one or even two orders of magnitude, or else offer the user a very limited set of facilities, i.e. either the details of the system are inaccessible to all but the most determined, dedicated and long-suffering student, or else the system is rather specialised and of little intrinsic interest.

There seem to be three main approaches to teaching Operating Systems. First there is the “general principles” approach, wherein fundamental principles are expounded, and illustrated by references to various existing systems, (most of which happen to be outside the students’ immediate experience). This is the approach advocated by the COSINE Committee, but in our view, many students are not mature or experienced enough to profit from it.

The second approach is the “building block” approach, wherein the students are enabled to synthesize a small scale or “toy” operating system for themselves. While undoubtedly this can be a valuable exercise, if properly organised, it cannot but fail to encompass the complexity and sophistication of real operating systems, and is usually biased towards one aspect of operating system design, such as process synchronisation.

The third approach is the “case study” approach. This is the one originally recommended for the Systems Programming course in “Curriculum ’68”, the report of the ACM Curriculum Committee on Computer Science, published in the March, 1968 issue of the “Communications of the ACM”.

Ten years ago, this approach, which advocates devoting “most of the course to the study of a single system” was unrealistic because the cost of providing adequate student access to a suitable system was simply too high.

Ten years later, the economic picture has changed significantly, and the costs are no longer a decisive disadvantage if a minicomputer system can be the subject of study. The considerable advantages of the approach which undertakes a detailed analysis of an existing system are now attainable.

In our opinion, it is highly beneficial for students to have the opportunity to study a working operating system in all its aspects.

Moreover it is undoubtedly good for students majoring in Computer Science, to be confronted at least once in their careers, with the task of reading and understanding a program of major dimensions.

In 1976 we adopted UNIX as the subject for case study in our courses in Operating Systems at the University of New South Wales. These notes were prepared originally for the assistance of students in those courses (6.602B and 6.657G).

The courses run for one semester each. Before entering either Course, students are presumed to have studied the PDP11 architecture and assembly language, and to have had an opportunity to use the UNIX operating system during exercises for earlier courses.

In general, students seem to find the new courses more onerous, but much more satisfying than the previous courses based on the “general principles” approach of the COSINE Committee.

Some mention needs to be made regarding the documentation provided by the authors of the UNIX system. As reproduced for use on our campus, this comprises two volumes of A4 size paper, with a total thickness of 3 cm, and a weight of 1250 grams.

A first observation is that the whole documentation is not unreasonably transportable in a student’s brief case. However it must not be assumed that this amount of documentation, which is written in a fresh, terse, whimsical style, is necessarily inadequate.

In fact the second observation (which is only made after considerable experience) is that for reference purposes, the documentation is remarkably comprehensive. However there is plenty of scope for additional tutorial material, one part of which, it is hoped, is satisfied by these notes.

The actual UNIX operating system source code is recorded in a separate companion volume entitled “UNIX Operating System Source Code”, which was first printed in July, 1976. This is a specially edited selection of code from the Level Six version of UNIX, as received by us in December, 1975.

During 1976, an initial version of the present notes was distributed in roneoed form, and only in the latter part of the year were the facilities of the “nroff” text formatting program exploited. The opportunity has recently been taken to revise and “nroff” the earlier material, to make some revisions and corrections, and to integrate them into their present form.

A decision had to be made quite early regarding the order of presentation of the source code. The intention was to provide a reasonably logical sequence for the student who wanted to learn the whole system. With the benefit of hindsight, a great many improvements in detail are still possible, and it is intended that these changes will be made in some future edition.

It is our hope that this book will be of interest and value to many students of the UNIX Time-sharing System. Although not prepared primarily for use as a reference work, some will wish to use it as such. The indices provided at the end should go some of the way towards satisfying the requirement for reference material at this level.

Since these notes refer to proprietary material administered by the Western Electric Company, they can only be made available to licensees of the UNIX Time-sharing System and hence are unable to be published through more usual channels.

Corrections, criticism and suggestions for improvement of these notes will be very welcome.

Acknowledgments

The preparation of these notes has been encouraged and supported by many of my colleagues and students including David Carrington, Doug Crompton, Ian Hayes, David Horsfall, Peter Ivanov, Ian Johnstone, Chris Maltby, Dave Milway, John O’Brien and Greg Rose.

Pat Mackie and Mary Powter did much of the initial typing, and Adele Green has assisted greatly in the transfer of the notes to “nroff” format.

David Millis and the Publications Section of the University of New South Wales have assisted greatly with the mechanics of publication, and Ian Johnstone and the Australian Graduate School of Management provided facilities for the preparation of the final draft.

Throughout this project, my wife Marianne has given me unfailing moral support and much practical support with proof-reading.

Finally Ken Thompson and Dennis Ritchie started it all.

To all the above, I wish to express my sincere thanks.

The co-operation of the “nroff” program must also be mentioned. Without it, these notes could never have been produced in this form. However it has yielded some of its more enigmatic secrets so reluctantly, that the author’s gratitude is indeed mixed. Certainly “nroff” itself must provide a fertile field for future practitioners of the program documenter’s art.