We can view an operating system from several vantage points. One view
focuses on the services that the system provides; another, on the interface that
it makes available to users and programmers; a third, on its components and
their interconnections. In this chapter, we explore all three aspects of operating
systems, showin.g the viewpoints of users, programmers, and operating-system designers. We consider what services an operating system provides,how they are debugged, and what the various methodologies are for designing such systems. Finally, we describe how operating systems are created and how a computer starts its operating system.