Figure 1.1 depicts the high-level architecture of the UNIX system. The hardware at the center of the diagram provides the operating system with basic services that. The operating system interacts directly with the hardware, providing common services to programs and insulating them from hardware idiosyncrasies.
Viewing the system as a set of layers, the operating system is commonly called the system kernel, or just the kernel, emphasizing its isolation from user programs.
Programs such as the shell and editors (ed and w) shown in the outer layers interact with the kernel by invoking a well-defined set of system calls. The system calls instruct the kernel to do various operations for the calling program and exchange data between the kernel and the program.
Several programs shown in the figure are in standard system configurations and are known as commands, but private user programs may also exist in this layer as indicated by the program whose name is a.out, the standard name for executable files produced by the C compiler.
Other application programs can build on top of lower-level programs, hence the existence of the outermost layer in the figure. For example, the standard C compiler, cc, is in the outermost layer of the figure: it invokes a C preprocessor, two-pass compiler, assembler, and loader (link-editor), all separate lower-level programs. Although the figure depicts a two-level hierarchy of application programs, users can extend the hierarchy to whatever levels are appropriate. Indeed, the style of programming favored by the UNIX system encourages the combination of existing programs to accomplish a task.
Many application subsystems and programs that provide a high-level view of th system such as the shell, editors, SCCS (Source Code Control System), and document preparation packages, have gradually become synonymous with the name "UNIX system."