From Newsgroup: nz.comp

In case anyone who reads this newsgroup has somehow missed it, Microsoft
has released the source code for MS-DOS 4.0 under the MIT license
including binaries of the tools required to build it. Some parts of Multitasking MS-DOS 4.0 are also included - the rest of it will follow
if it can be found.

https://github.com/microsoft/MS-DOS
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