Microsoft open-sources infamously weird, RAM-hungry MS-DOS 4.00 release

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. All open-source versions of DOS have been released under the MIT License.

Initially, MS-DOS 4.00 was slated to include new multitasking features that allow software to run in the background. This release of DOS, also sometimes called “MT-DOS” or “Mutitasking MS-DOS” to distinguish it from other releases, was only released through a few European PC OEMs and never as a standalone retail product.