This repo contains the original source-code and compiled binaries for MS-DOS v1.25 and MS-DOS v2.0, plus the source-code for MS-DOS v4.00 jointly developed by IBM and Microsoft. https://github.com/microsoft/MS-DOS #opensource
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

@nixCraft This is awesome. I met up with a guy in Miami who worked on versions of DOS -- Tony I. Just spent a little time trolling through the files trying to see if any of his comments were there, but it may be a little before that time.

Writing (and debugging) assembler was not for the faint-of-heart.