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The acronym DOS stands for disk operating system, a type of operating system for computers that provides the abstraction of a file system resident on hard disk or floppy disk secondary storage.

DOS for IBM PC compatibles

In particular, DOS refers to the family of related operating systems which dominated the IBM PC compatible market for the decade between 1985 and 1995: PC-DOS, MS-DOS, DR-DOS, FreeDOS, PTS-DOS, and several others.

MS-DOS (and the IBM PC-DOS which was licensed therefrom), and its predecessor, QDOS, was a successor to CP/M (Command Processor / (for) Microcomputers)—which was the dominant operating system for 8-bit Intel 8080 and Zilog Z80 based microcomputers.

Early versions of Microsoft Windows were programs which ran under DOS. Later versions were launched under DOS but "extended" it by going into protected mode. Still later versions of MS Windows ran independently of DOS but included much of the old code such that it could run in virtual machines under the new OS and the latest versions of MS Windows are continually dropping ever more of the DOS ancestry. Windows 98 was the last Microsoft OS to run on DOS.

Under Linux (running on x86-based systems) it's possible to run copies of DOS and many of its clones under dosemu (a Linux native virtual machine program for running real mode programs). There are a number of other emulators for running DOS and/or DOS-based software under various versions of UNIX, even on non-x86 platforms; one such emulator is DOSBox.

See also: List of DOS commands

Reserved device names

There are reserved device names in DOS that cannot be used as filenames regardless of extension; these restrictions also affect several Windows versions, in some cases causing crashes and security vulnerabilities.

A partial list of these reserved names is: AUX, COM, COM0, COM1, COM2, ..., COM8, COM9, CON, LPT1, LPT2, NUL, and PRN.

DOS for other computers

Prior to (and to some extent concurrently with) the development of the IBM PC compatible family of microcomputers, several other operating systems for other architectures were already known as DOS, notably:

• The DOS initial/simple operating system for the IBM System/360 family of mainframe computers (it later became DOS/VSE, and was eventually just called VSE).
• The DOS operating system for DEC PDP-11 minicomputers (this OS and the computers it ran on were nearly obsolete by the time PCs became common, with various descendents and other replacements).
• The DOS operating system for the Apple Computer's Apple II family of computers. This was the primary operating system for this family from 1979 with the introduction of the floppy disk drive until 1983 with the introduction of ProDOS; many people continued using it long after that date. Usually it was called Apple DOS to distinguish it from MS-DOS.
• Commodore DOS, which was used by 8-bit Commodore computers. Unlike most other DOS systems, it was integrated into the disk drives, not loaded into the computer's own memory.

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