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% The Lion's Commentary, file preface.tex, version 1.5, 18 May 1994
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\section*{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:

\bi
\item it runs on a system which is already available to us;

\item it is compact and accessible;

\item it provides an extensive set of very
 usable facilities;

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

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 ``{\it 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 ``{\it 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 ``{\it 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.

\bigskip

{\noindent \large \bf Acknowledgments}

\smallskip

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.

\bt
\hspace{5cm} \= John Lions \\
\> Kensington, NSW \\
\> May, 1977 \\
\et