QDOS was the forerunner of DOS (Disk Operating System), the first widely-used personal computer operating system. In 1980, when IBM was making plans to enter the personal computer market, it asked Bill Gates, the young owner of a small company called Microsoft, if they could locate an operating system for the new PC that IBM was developing. Microsoft, which had previously furnished IBM with a BASIC language product for the IBM PC, looked around and found an operating system called 86-DOS at a small company called Seattle Computer Products.
86-DOS - often referred to as QDOS, or Quick and Dirty Operating System - was written in six weeks by Tim Paterson, based on ideas in CP/M (Control Program for Microcomputers), an operating system popular with early personal computer users. 86-DOS was designed for use with Seattle Computer's Intel 8086-based computers. It contained about 4,000 lines of assembler language code. Microsoft bought 86-DOS from Seattle Computer Products for $50,000, revised it, renaming it MS-DOS, and then delivered it to IBM for its new PC.
IBM rewrote MS-DOS after finding 300 bugs in it and renamed it PC-DOS, which is why both IBM and Microsoft hold a copyright for it. Bill Gates saw the potential for MS-DOS and persuaded IBM to let Microsoft sell it separately from IBM's PC projects. The initial IBM PC actually offered the user a choice of one of three operating systems: PC-DOS, CP/M 86, and UCSD p-System, a Pascal-based system. PC-DOS, which was cheaper, proved the most popular and began to come bundled with the IBM PC in its second product release. The IBM PC brought personal computing to the business world for the first time and was successful beyond IBM's imaginings. In 18 months, IBM introduced the PC-XT, which included a hard drive loaded with a newer version of DOS. Microsoft promised a multitasking DOS, but that never happened. Instead, Microsoft developed Windows with multitasking features.