Chapter 6 describes how to structure and implement a system in which a limited number of physical resources such as discs and line printers can be shared among a greater number of processes, whose resource requirements vary with time. Each resource is represented as a single process. On each occasion that a resource is required by a user process, a new virtual resource is created.
A virtual resource is a process which behaves as if it were subordinate to the user process; but it also communicates with the real resource whenever required. Such communications are interleaved with those of other concurrently active virtual processes. So the real and virtual processes play the same roles as the monitors and envelopes of PASCAL PLUS. The chapter is illustrated by the modular development of a series of complete but very simple operating systems, which are the largest examples given in this book.