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 Under an OSI license, or one of MS's not actually open source licenses?
@BoydStephenSmithJr @shanselman it says MIT Licence in the GitHub repo