software that controls the many different operations of a computer and directs and coordinates its processing of programs. An operating system is a remarkably complex set of instructions that schedules the series of jobs (user applications) to be performed by the computer and allocates them to the computer's various hardware systems, such as the central processing unit, main memory, and peripheral systems. The operating system directs the central processor in the loading, storage, and execution of programs and in such particular tasks as accessing files, operating software applications, controlling monitors and memory storage devices, and interpreting keyboard commands. When a computer is executing several jobs simultaneously, the operating system acts to allocate the computer's time and resources in the most efficient manner, prioritizing some jobs over others in a process called time-sharing. An operating system also governs a computer's interactions with other computers in a network.

The English mathematician Charles Babbage conceived the first set of operating instructions for a digital computer in the design of his "analytical engine" (1834), which was never built. The first operational stored-program computer was completed in 1949 at the University of Cambridge. The operating systems that came into wide use between 1950 and 1980 were developed mostly by private companies to operate proprietary mainframe computers and applications. The most popular of these systems--which are used to run mainframes built by the industry leader, International Business Machines Corporation (IBM)--include MVS, DOS/VSE, and VM.

In addition to proprietary systems, open, or portable, operating systems have been developed to run computers built by other manufacturers. Open operating systems rose to prominence during the 1980s and are now widely used to run personal computers (PCs) and workstations, which use extremely powerful PCs. The dominant operating system is the disk operating system (DOS) developed by Microsoft Corporation. Also popular is Microsoft's Windows NT, an adjunct to DOS that provides enhanced computer graphics.

Besides mainframe and PC-type operating systems, network operating systems have been developed that allow PCs and workstations to share peripheral devices and communicate with a mainframe computer or a server (i.e., a device that stores information and assists in the operation of a network of computers). Network operating systems usually act as an additional layer above a primary operating system, such as DOS. Dominant in this category are Novell Inc.'s UNIX, an operating system designed for networked workstations, and Novell's Netware, which is the leading network operating system.