An operating system (sometimes

abbreviated as "OS") is the program that, after

being initially loaded into the computer by a bootstrap

program, manages all the other programs in a computer. The

other programs are called applications. The applications

make use of the operating system by making

requests for services through a defined

application program interface (API).

In addition, users can

interact directly with the operating system through an

interface such as a command language.

 

An operating system performs these services for

applications:

• In multitasking operating systems where multiple programs can be running at the same time, the operating system determines which applications should run in what order and how much time should be allowed for each application before giving another application a turn.

• It manages the sharing of internal memory among multiple applications.

• It handles input and output to and from attached hardware devices, such as hard disks, printers, and dial-up ports.

• It sends messages to the applications or interactive user (or to a system operator) about the status of operation and any errors that may have occurred.

• It can offload the management of what are called batch jobs (for example, printing) so that the initiating application is freed from this work.

• On computers that can provide parallel processing, an operating system can manage how to divide the program so that it runs on more than one processor at a time.

All major computer platforms (hardware and

software) require and sometimes include an

operating system. UNIX, Windows 95,

DEC's VMS, IBM's OS/2, AIX, and OS/390 are all examples of

operating systems.