Operating Systems: Concurrent and Distributed Software Design

2.6. Operating system structure

Early operating systems were monolithic or unstructured, as were those for PCs until very recently. Such systems run without a privilege boundary, and arbitrary procedure calls, reads and writes can take place. The first step in providing structure is to use the hardware protection boundary to make the operating system privileged and applications unprivileged. The system calls can then be the hardware-enforced operating system interface to applications.

An operating system is a very large piece of software; for example, Sun's UNIX kernel (the basic functions described above and shown in Figure 2.13) occupies over a megabyte. Figure 2.13 shows some of the internal modules in a conventional closed operating system, ...