Figure 2.16 provides a general description of the classic UNIX architecture. The
underlying hardware is surrounded by the OS software. The OS is often called the
system kernel, or simply the kernel, to emphasize its isolation from the user and applications.
It is the UNIX kernel that we will be concerned with in our use of UNIX as
an example in this book. UNIX also comes equipped with a number of user services
and interfaces that are considered part of the system. These can be grouped into
the shell, other interface software, and the components of the C compiler (compiler,
assembler, loader). The layer outside of this consists of user applications and the user
interface to the C compiler.