MS-DOS

MS-DOS (short for Microsoft Disk Operating System) is an operating system commercialised by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the dominant operating system for the PC compatible platform during the 1980s. It has gradually been replaced on consumer desktop computers by various generations of the Windows operating system.
MS-DOS was originally released in 1981 and had eight major versions released before Microsoft stopped development in 2000. It was the key product in Microsoft's growth from a programming languages company to a diverse software development firm, providing the company with essential revenue and marketing resources. It also provided the platform on which early versions of Windows ran. MS-DOS began as QDOS (for Quick and Dirty Operating System), written by Tim Paterson for computer manufacturer Seattle Computer Products (SCP) in 1980. It was marketed by SCP as 86-DOS because it was designed to run on the Intel 8086 processor. 86-DOS function calls were based on the dominant CP/M-80 operating system, written by Digital Research, but it used a different file system. Paterson allegedly wrote it because he liked CP/M, but a version of CP/M didn't yet exist for the 8086.[1] In a sequence of events that would later inspire much folklore, Microsoft negotiated a license for 86-DOS from SCP in December 1980 for $25,000, then re-licensed 86-DOS to IBM. Microsoft then acquired all rights to 86-DOS for only $50,000 from SCP in July, 1981, shortly before the PC's release.
The original MS-DOS advertisement in 1981.
IBM and Microsoft both released versions of DOS; the IBM version was supplied with the IBM PC and known as PC-DOS. Originally, IBM only validated and packaged Microsoft developments, and thus IBM's versions tended to be released shortly after Microsoft's. However, MS-DOS 4.0 was actually based on IBM PC-DOS 4.0, as Microsoft was by then concentrating on OS/2 development.[citation needed] Microsoft released its versions under the name "MS-DOS", while IBM released its versions under the name "PC-DOS". Initially, when Microsoft would license their OEM version of MS-DOS, the computer manufacturer would customize its name (e.g. TandyDOS, Compaq DOS, etc). Most of these versions were identical to the official MS-DOS; however, Microsoft began to insist that OEMs start calling the product MS-DOS. Eventually, only IBM resisted this move.
Computer advertisements of this period often claimed that computers were "IBM-Compatible" or very rarely "MS-DOS compatible." The two terms were not synonyms. There were computers that used MS-DOS but could not run all the software that an IBM-Compatible machine could. An example is the Morrow Pivot, which used MS-DOS but was not IBM-Compatible.