We just open-sourced DOS 4 (and found binaries of Multitasking DOS 4) https://www.hanselman.com/blog/open-sourcing-dos-4
Open Sourcing DOS 4

See the canonical version of this blog post at the Microsoft Open Source Blog! ...

@shanselman Neat! Now do Office. 😉
Microsoft Word for Windows Version 1.1a Source Code

The dominant word processing program for personal computers in the 1980s was DOS-based WordPerfect. Microsoft Word for DOS, which had been released in 1983, was an also-ran.

CHM