MS-DOS was the operating system Microsoft supplied for the IBM Personal Computer, which IBM unveiled on August 12, 1981. IBM’s own history of the machine records that the team “chose Intel’s 8088 chip” and that “Microsoft provided the OS, which would later become known as MS-DOS.” The same IBM account notes that Don Estridge unveiled the IBM PC at New York’s Waldorf hotel that day, priced at USD 1,565 with 16 kilobytes of RAM.
The software underneath was not written at Microsoft. It began as 86-DOS, also called QDOS, written by Tim Paterson at Seattle Computer Products. On his own blog, Paterson explains that he designed the system around the CP/M application interface for compatibility: “I designed DOS so the translated program would work the same as it had with CP/M — translation compatibility. The key to making this work was implementing the CP/M API.” He is firm that this was an interface, not copied code: “There is no suggestion that I copied any CP/M code when I wrote DOS.”
Internally, DOS differed from CP/M. Paterson’s account notes that DOS used the FAT (File Allocation Table) scheme for managing disk storage, a different design from CP/M’s. Microsoft first licensed 86-DOS from Seattle Computer Products and later acquired it outright, then shipped it to IBM as PC DOS and sold it to other manufacturers as MS-DOS. On the strength of the IBM PC and its many clones, MS-DOS became the dominant personal-computer operating system of the 1980s.