The operating system Microsoft shipped on the IBM PC was not originally Microsoft’s. It started as 86-DOS, also called QDOS, short for “Quick and Dirty Operating System,” written by Tim Paterson at Seattle Computer Products to give that company’s 8086 hardware a usable system.
Paterson designed it for compatibility with the reigning standard of the day. On his own blog, he explains: “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 equally clear that he reimplemented an interface rather than lifting code: “There is no suggestion that I copied any CP/M code when I wrote DOS.”
The Computer History Museum’s account of the early Digital Research source code corroborates the lineage from the other side: “Tim Paterson at Seattle Computer Products used DRI’s 1976 CP/M Interface Guide and other information to guide the development of QDOS.” Microsoft first licensed this system from Seattle Computer Products and later bought it outright, then sold it to IBM as PC DOS and to other makers as MS-DOS. The everyday operating system of millions of 1980s PCs began life as a stopgap with a self-deprecating name.