2.1.1 The Kernel

The kernel is the part of the system that runs in protected mode and mediates access by all user programs to the underlying hardware (e.g., CPU, disks, terminals, network links) and software constructs (e.g., filesystem, network protocols). The kernel provides the basic system facilities; it creates and manages processes, and provides functions to access the filesystem and communication facilities. These functions, called system calls appear to user processes as library subroutines. These system calls are the only interface that processes have to these facilities. Details of the system-call mechanism are given in Chapter 3, as are descriptions of several kernel mechanisms that do not execute as the direct result of a process doing a system call.

A kernel in traditional operating-system terminology, is a small nucleus of software that provides only the minimal facilities necessary for implementing additional operating-system services. In contemporary research operating systems -- such as Chorus Rozier et al, 1988, Mach Accetta et al, 1986, Tunis Ewens et al, 1985, and the V Kernel Cheriton, 1988 -- this division of functionality is more than just a logical one. Services such as filesystems and networking protocols are implemented as client application processes of the nucleus or kernel.

The 4.4BSD kernel is not partitioned into multiple processes. This basic design decision was made in the earliest versions of UNIX. The first two implementations by Ken Thompson had no memory mapping, and thus made no hardware-enforced distinction between user and kernel space Ritchie, 1988. A message-passing system could have been implemented as readily as the actually implemented model of kernel and user processes. The monolithic kernel was chosen for simplicity and performance. And the early kernels were small; the inclusion of facilities such as networking into the kernel has increased its size. The current trend in operating-systems research is to reduce the kernel size by placing such services in user space.

Users ordinarily interact with the system through a command-language interpreter, called a shell, and perhaps through additional user application programs. Such programs and the shell are implemented with processes. Details of such programs are beyond the scope of this book, which instead concentrates almost exclusively on the kernel.

Sections 2.3 and 2.4 describe the services provided by the 4.4BSD kernel, and give an overview of the latter's design. Later chapters describe the detailed design and implementation of these services as they appear in 4.4BSD.