In March 2014, Microsoft released the source code to MS-DOS 1.25 and 2.0 via the Computer History Museum.
Today, we’re re-open-sourcing MS-DOS on GitHub. Why? Because it’s much easier to find, read, and refer to MS-DOS source files if they’re in a GitHub repo than in the original downloadable compressed archive file.
(from Re-Open-Sourcing MS-DOS 1.25 and 2.0 Rich Turner)