We are now about to embark on a detailed study of how operating systems are designed and constructed. The most central concept in any operating system is the process: an abstraction of a running program. Everything else hinges on this concept, and the operating system designer (and student) should have a thorough understanding of what a process is as early as possible.
Processes are one of the oldest and most important abstractions that operating systems provide. They support the ability to have (pseudo) concurrent operation even when there is only one CPU available. They turn a single CPU into multiple virtual CPUs. When there are four or eight or more CPUs (cores) available, there could be dozens or hundreds of processes running. Without the process abstraction, modern computing could not exist. In this chapter, we will go into considerable detail about processes and their first cousins, threads.