(1) (Disk Operating System) Pronounced "dahss." An earlier single-user operating system from Microsoft for the PC. It was the first operating system for IBM and IBM-compatible PCs and remained the underlying control program for Windows 3.1, 95, 98 and ME. In order to support DOS applications, Windows NT, 2000, XP and Vista include their own version of DOS, called "DOS emulation."

Although DOS in the IBM PC was called "PC-DOS," and the DOS used by all other vendors was called "MS-DOS," all releases of PC-DOS and MS-DOS were generally referred to as "DOS." Except for DOS 6, which contained different versions of various utilities, PC-DOS and MS-DOS commands and system functions were the same. A great number of DOS commands have been executable in all versions of Windows. See DOS abc's.

DOS Lives On
In various versions, DOS is still deployed where a small OS footprint is required, typically for embedded systems. Having gone through many incarnations, Digital Research's DR-DOS is still available (see DR-DOS). Other DOS versions are ROM-DOS from DataLight (www.datalight.com) and the open source FreeDOS (see FreeDOS).

(2) (DOS, DOS/370, DOS/VS, DOS/VSE, VSE/ESA) A series of IBM mainframe "disk operating systems" for System/360, System/370 and System/390. DOS started as a variant of TOS (Tape Operating System), but soon supplanted TOS as disk storage became accepted in the late 1960s. DOS was always the "junior partner" to OS/360 and its progeny. It continues today as VSE/ESA.

(3) (Disk Operating System) A generic term for operating system. See operating system.

(4) (DoS) See denial of service.

See forward slash.

Percent signs are used in DOS batch files to define user input. The following batch file example would copy a file and then delete it:

   copy %1 %2
   del %1

If the above file were named MOVEIT.BAT and you typed in the following:

moveit abc \hold

It would be the same as typing:

   copy abc \hold
   del abc

Double colons are used to make a comment in a DOS batch file. For example, the following line will not be processed by DOS or displayed on screen:

  :: sbl.exe is the screen blanker program

See DOS backslash.