Microsoft just open source MS-DOS
https://github.com/microsoft/MS-DOS
Do not send pull requests please 🤣
GitHub - microsoft/MS-DOS: The original sources of MS-DOS 1.25, 2.0, and 4.0 for reference purposes

The original sources of MS-DOS 1.25, 2.0, and 4.0 for reference purposes - microsoft/MS-DOS

GitHub
@olimex and BTW freedos is wayyyy better...