Who did Microsoft buy DOS from?

Q. Who did Microsoft buy the DOS operating system from that it used in early PCs?

Adam answers:

Microsoft bought the rights to an operating system called QDOS (Quick & Dirty Operating System) from Seattle Computer Products for about $50,000, then turned around and used a modified version of it (MS-DOS) to market to IBM to use in its personal computers. Microsoft also retained the rights to license MS-DOS to other computer makers, the start of Microsoft’s dominance of the personal computer software market.

The original QDOS was written by Tim Paterson, though some claim his QDOS was a thinly modified version of the existing CP/M operating system of the time.