Microsoft open-sources MS-DOS 4.0.

And there's an interesting writeup to go with it.

Today, in partnership with IBM and in the spirit of open innovation, we’re releasing the source code to MS-DOS 4.00 under the MIT license. There’s a somewhat complex and fascinating history behind the 4.0 versions of DOS, as Microsoft partnered with IBM for portions of the code but also created a branch of DOS called Multitasking DOS that did not see a wide release. 
Open sourcing MS-DOS 4.0 - Microsoft Open Source Blog
In partnership with IBM, Microsoft is releasing the source code to MS-DOS 4.00 under the MIT license. Learn more.
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