1980, April: Tim Patterson begins writing an operating system for use with Seattle Computer Products' 8086-based computer. Seattle Computer Products decides to make their own disk operating system (DOS), due to delays by Digital Research in releasing a CP/M-86 OS.1980, August: QDOS 0.10 (Quick and Dirty OS) is shipped by Seattle Computer Products. Even though it had been created in only two man-months, the DOS worked surprisingly well.1980, September: Tim Patterson shows Microsoft his 86-DOS, written for the 8086 chip.1980, October: Microsoft's Paul Allen contacts Seattle Computer Products' Tim Patterson, asking for the rights to sell SCP's DOS to an unnamed client (IBM). Microsoft pays less than US$100,000 for the right.1980, December: Seattle Computer Products renames QDOS to 86-DOS, releasing it as version 0.3. Microsoft then bought non-exclusive rights to market 86-DOS.
1981, February: MS-DOS 1.0 runs for the first time on IBM's prototype microcomputer.1981, July: Microsoft buys all rights to DOS from Seattle Computer Products, and the name MS-DOS is adopted.1981, August: IBM announces the IBM 5150 PC Personal Computer, featuring a 4.77-MHz Intel 8088 CPU, 64KB RAM, 40KB ROM, one 5.25-inch floppy drive, and PC-DOS 1.0 (Microsoft's MS-DOS), for US$3000.
1982, May: Microsoft releases MS-DOS 1.1 to IBM, for the IBM PC. It supports 320KB double-sided floppy disk drives. Microsoft also releases MS-DOS 1.25, similar to 1.1 but for IBM-compatible computers.
1988, June: Microsoft releases MS-DOS 4.0, including a graphical/mouse interface.