This book is about the concepts, structures and mechanisms of operating systems. Its goal is to present as clearly and completely as possible the nature and characteristics of modern operating systems. This is a very difficult task for a number of reasons. Firstly, there is a huge range and variety of computers for which these operating systems are intended. These include embedded systems, smartphones, single-user workstations and personal computers, mid-sized shared systems, mainframe computers, supercomputers, and specialized machines such as real-time systems. The diversity is limited not only by the capacity and speed of the machines, but also by their areas of application and the requirements of the support system. Secondly, the rapid changes that have always characterized computer systems continue today. A number of key areas in operating system design are of very recent origin, and research into these and other new areas continues today. Despite this diversity and pace of change, some fundamental concepts are consistently applied across all operating systems. Of course, the application of these concepts depends on the current state of technology and the requirements of the particular application. The purpose of this book is to provide a thorough examination of the fundamentals of operating system design and to relate them to issues in modern operating system design and current trends in operating system development.