https://arstechnica.com/gadgets/2024/04/microsoft-and-ibm-release-source-code-for-one-of-the-weirdest-versions-of-ms-dos/

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.