Operating system

A computer is a box that follows a plan, and the first program a computer runs, when it is turned on, is the operating system, the program that invokes other programs, and controls their execution.

Operating systems are written differently than user programs. They are written to specifically address the hardware of the computers they run on. User programs are written to run independently of hardware considerations, so they can be flexible, and run on different models of computer, from any manufacturer.

When programmers writing user programs want to direct their program to do something, like read or write data, they use a programming interface, that insulates them from the actual hardware. Typically they do this through system calls, which look like ordinary subroutines, but temporarily surrender control back to the operating system.

Early operating systems, like OS-360 and MS-DOS, only ran a single program, at a time. Multi-tasking operating systems, like Unix, manage the operation of multiple programs. In a multiprocessing operating system it is even more important that the operating system handle the computer's hardware.