Microsoft didn't buy DOS, they bought QDOS (Quick and Dirty Operating System) for about $50K. It was a plagiarized version of Digital's CP/M operating system. They turned around and licensed this OS to IBM, then later licensed to Compaq and other clone PC makers. In the end they wound up paying Digital damages, and paid off the Seattle man who "created" QDOS as well. Of course most of this has either been forgotten or buried...