You have just picked up a most interesting book. To understand how interesting it is, you have to take a short trip with me back in time.
It is September of 1968. You are in Philadelphia, and you are going to study computer science at a major university. The computer system you are going to work on costs in the neighborhood of several million dollars. The computer's time is scheduled twenty-four hours a day, for its time is even more valuable than your own. The concept of shutting the operating system down to allow you to practice writing an operating system is not even considered. If you (the lowly student or researcher) wish to practice writing an operating system, you use a hardware emulator (written in software) that runs on top of the hugely expensive computer system.
Time moves on. It is now 1977, and some of the first microcomputers are coming out. They are still relatively expensive, and they have very small memories and disks. While students can work on writing device drivers and other small additions to various "sample" operating systems, having the entire source code for an industry-capable operating system and hardware that would be capable of solving "real" problems is still not in the grasp of the average student.
Now it is 1994. "PC" hardware has become much more sophisticated, and a new operating system is beginning to make its presence known. The source code for this operating system is available over the Internet, and large numbers of people are working with it, improving it, and fashioning it for a commercial market. Yet there is still something missing, for while the hardware for "simple" tasks is within the realm of the average user (or even wealthy college student), there are a class of problems that are designated for those awesome systems known as "supercomputers" systems that still cost many millions of dollars to build, beyond the reach of all but a few of the most prestigious universities and laboratories.
