MS-DOS Source Code 1.X and 2.0 http://blogs.technet.com/b/microsoft_blog/archive/2014/03/25/microsoft-makes-source-code-for-ms-dos-and-word-for-windows-available-to-public.aspx
Released into Github for the public to see.
Early MS-DOS source code that Microsoft has released to an open source license.
For educational use only.