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disk-operating system, often prefixed, as in MS-DOS and PC-DOS; a computer operating system
DOS(1) See denial of service.
(2) (Disk Operating System) Any operating system that supports hard drives. See operating system.
(3) (Disk Operating System) A family of IBM mainframe operating systems (DOS, DOS/370, DOS/VS, DOS/VSE). As disk storage became accepted in the late 1960s, DOS started out as a variant of IBM's Tape Operating System (TOS); however, it was always the "junior partner" to OS/360 and its progeny. See DOS/VSE.
(4) (Disk Operating System)Pronounced "dahss." A single-user operating system from Microsoft for Intel x86 personal computers. It was the first operating system for IBM PCs and IBM-compatible PCs, and it remained the underlying control program for Windows 3.1, 95, 98 and ME. Subsequent versions of Windows incorporated all DOS functionality, and most DOS commands work the same in Windows. See cmd abc's.
PC-DOS and MS-DOS
The OS in the IBM PC was PC-DOS, and the OS used by all other PC makers was MS-DOS. Except for DOS 6, which contained different utilities, PC-DOS and MS-DOS commands and system functions were the same, and all PC-DOS and MS-DOS versions were commonly called "DOS." See DOS 6.
DOS Lives On
In various incarnations, DOS is still used in embedded systems, where a small OS footprint is required. Examples are Digital Research's DR-DOS, ROM-DOS from DataLight (www.datalight.com) and the open source FreeDOS. See embedded system.