MICROSOFT has released the source code to MS-DOS 4.00 under the MIT license ...
https://cloudblogs.microsoft.com/opensource/2024/04/25/open-sourcing-ms-dos-4-0/
"Ten years ago, Microsoft released the source for MS-DOS 1.25 and 2.0 to the Computer History Museum, and then later republished them for reference purposes. This code holds an important place in history and is a fascinating read of an operating system that was written entirely in 8086 assembly code nearly 45 years ago.
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."
Jeff Wilcox & Scott Hanselman
Post 302: Microsoft, Open Source Blog, The release of the MS DOS 4.0 source code under the MIT licence, 2024.
