We have long felt that the traditional approach to teaching about Operating Systems (OSs) was not the best approach. The purpose of this book is to support a different approach to this task. When studying any complex domain of knowledge, the order in which one learns the hierarchy of principles, laws, ideas, and concepts can make the process easier or more diffi cult. The most common technique is to partition the subject into major topics and then study each one in great detail. For OSs, this has traditionally meant that after a brief introduction to some terms and an overview, a student studied isolated topics in depth—processes and process management, then memory management, then fi le systems, and so on. We can call this a depth-oriented approach or a vertical approach. After learning a great mass of unrelated details in these isolated topic areas, the student then examined case studies, examples of real OSs, and fi nally saw how the different topics fi t together to make a real OS.

We believe that a better model is that followed by children when learning a language: learn a few words, a little grammar, a little sentence structure, and then cycle (or spiral) through; more words, more grammar, more sentence structure. By continuing to spiral through the same sequence, the complexity of the language is mastered. We can call this a breadth-oriented or spiral approach.

We have taken this approach to the subject of OSs. The fi rst few chapters give some basic background and defi nitions. We then begin to describe a very simple OS in a simple system—early PCs—and evolve toward more complex systems with more features: fi rst limited background tasks (such as simultaneous printing), then multitasking, and so on. In each case we try to show how the increasing requirements caused each system to be designed the way it was. This is not specifi cally a historical order of OS development. Rather, we choose a representative system at each complexity level in order to see how the different OS components interact with and infl uence one another. It is our belief that this approach will give the student a greater appreciation of how the various features of each level of OS were put together.

Part of the motivation for this approach has to do with why Computing Science students are told they must study OSs at all. It is highly unlikely that many of these students will work on the development of OSs. However, virtually every system that they do work on will run on top of an OS, though perhaps a very few will work on embedded systems with no OS. For the rest of them, the OS will stand between the applications and the hardware, and failure to thoroughly understand the nature of the OS will mean that these applications will be underperforming at best and hazardous at worst. We believe that our approach will lead students to a better understanding of the entire architecture of modern OSs than does the traditional approach.