DOS (/dɒs/, /dɔːs/[1]) is a platform-independent acronym for disk operating system which later became a common shorthand for disk-based operating systems on IBM PC compatibles.[2] DOS primarily consists of Microsoft's MS-DOS and a rebranded version under the name IBM PC DOS, both of which were introduced in 1981. Later compatible systems from other manufacturers include DR DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998). MS-DOS dominated the IBM PC compatible market between 1981 and 1995.
Dozens of other operating systems also use the acronym "DOS", beginning with the mainframe DOS/360 from 1966. Others include Apple DOS, Apple ProDOS, Atari DOS, Commodore DOS, TRSDOS, and AmigaDOS.
IBM PC DOS (and the separately sold MS-DOS) and its predecessor, 86-DOS, was based on Digital Research's CP/M—the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 microcomputers—but instead ran on Intel 8086 16-bit processors.
When IBM introduced the IBM PC, built with the Intel 8088 microprocessor, they needed an operating system. Seeking an 8088-compatible build of CP/M, IBM initially approached Microsoft CEO Bill Gates (possibly believing that Microsoft owned CP/M due to the Microsoft Z-80 SoftCard, which allowed CP/M to run on an Apple II).[3] IBM was sent to Digital Research, and a meeting was set up. However, the initial negotiations for the use of CP/M broke down; Digital Research wished to sell CP/M on a royalty basis, while IBM sought a single license, and to change the name to "PC DOS". Digital Research founder Gary Kildall refused, and IBM withdrew.[3][4]
IBM again approached Bill Gates. Gates in turn approached Seattle Computer Products. There, programmer Tim Paterson had developed a variant of CP/M-80, intended as an internal product for testing SCP's new 16-bit Intel 8086 CPU card for the S-100 bus. The system was initially named QDOS (Quick and Dirty Operating System), before being made commercially available as 86-DOS. Microsoft purchased 86-DOS, allegedly for US$50,000. This became Microsoft Disk Operating System, MS-DOS, introduced in 1981.[5]Within a year Microsoft licensed MS-DOS to over 70 other companies,[6] which supplied the operating system for their own hardware, sometimes under their own names. Microsoft later required the use of the MS-DOS name, with the exception of the IBM variant. IBM continued to develop their version, PC DOS, for the IBM PC.[5] Digital Research became aware that an operating system similar to CP/M was being sold by IBM (under the same name that IBM insisted upon for CP/M), and threatened legal action. IBM responded by offering an agreement: they would give PC consumers a choice of PC DOS or CP/M-86, Kildall's 8086 version. Side-by-side, CP/M cost US$200 more than PC DOS, and sales were low. CP/M faded, with MS-DOS and PC DOS becoming the marketed operating system for PCs and PC compatibles.[3]
Microsoft originally sold MS-DOS only to original equipment manufacturers (OEMs). One major reason for this was that not all early PCs were 100% IBM PC compatible. DOS was structured such that there was a separation between the system specific device driver code (IO.SYS) and the DOS kernel (MSDOS.SYS). Microsoft provided an OEM Adaptation Kit (OAK) which allowed OEMs to customize the device driver code to their particular system. By the early 1990s, most PCs adhered to IBM PC standards so Microsoft began selling MS-DOS in retail with MS-DOS 5.0.