In the early '80s, IBM had built a stellar reputation for itself selling mainframe computers to big businesses. But the back rooms of conglomerates weren't enough for Big Blue — or as it was known back then, Medium-Sized Teal (no, not really).
Despite their previous failure to sell an affordable microcomputer with the IBM 5100, the company decided to try again after seeing the success of the Commodore PET, Apple II, Tandy's TRS-80s and the Atari 8-bit computers.
Bill Gates accepted IBM's offer, but Microsoft wasn't up to the challenge of writing an operating system itself. Microsoft instead bought the rights to QDOS (the "Quick and Dirty Operating System") for $50,000, written by Tim Paterson of Seattle Computer Products. QDOS itself was based heavily on the CP/M operating system. Paterson had bought a CP/M manual and used it as a basis to write QDOS in six weeks — with just enough changes to fly on the upside of legal.