This book tries to give students of computer science and professional programmers a general understanding of operating systems - the programs that enable people to share computers efficiently. To make the sharing of a computer tolerable, and operating system must enforce certain rules of behaviour on all its users. One would therefore expect the designers of operating systems to do their utmost to make them as simple, efficient, and realiable as as possible. A number of operating systems made in the early of 1960's had these characteristics; but in the late 1960's designers were often overambitious and built enormous systems with poor performance. I see no inherent reason why operating systems should not reach the quality of program costruction found in present compilers; this will require and understanding of the principle common to all operating systems and a consistent use of safe methods of desinging large programs. It is my hope that this book will give you a start in this direction. I assume that you are familiar with the basic structure ot computers and programming languages and have some experience in writing and testing non-trivial programs. In a few cases a knowledge of elementary calculus and probability theory is also needed.