The facilities available to a user process are logically divided into two parts: kernel facilities directly implemented by code running in the operating system, and system facilities implemented either by the system, or in cooperation with a server process. The kernel facilities are described in section .Xr 1 .

The facilities implemented in the kernel are those which define the 4.4BSD virtual machine in which each process runs. Like many real machines, this virtual machine has memory management hardware, an interrupt facility, timers and counters. The 4.4BSD virtual machine allows access to files and other objects through a set of descriptors. Each descriptor resembles a device controller, and supports a set of operations. Like devices on real machines, some of which are internal to the machine and some of which are external, parts of the descriptor machinery are built-in to the operating system, while other parts are implemented in server processes on other machines. The facilities provided through the descriptor machinery are described in section .Xr 2 .