Enlarge / A DOS prompt.
Microsoft has open-sourced another bit of computing history this week: The company teamed up with IBM to release the source code of 1988's MS-DOS 4.00, a version better known for its unpopularity, bugginess, and convoluted development history than its utility as a computer operating system.
The MS-DOS 4.00 code is available on Microsoft's MS-DOS GitHub page along with versions 1.25 and 2.0, which Microsoft open-sourced in cooperation with the Computer History Museum back in 2014.