Microsoft just released the source code for 86-DOS 1.00, made available as an open-source project on GitHub. The release marks the 45th anniversary of 86-DOS 1.00, created by Tim Paterson, which would later be licensed to IBM as PC-DOS and then later become MS-DOS.

Microsoft explains the story of early MS-DOS development in this Microsoft Open Source blog post, making it clear that these releases are intended to make historically significant system software accessible for purposes of study, preservation, and curiosity. Microsoft previously released the source for MS-DOS 1.25, 2.11, and 4.0.

https://www.pcworld.com/article/3127762/microsoft-open-sources-86-dos-1-00-the-ancestor-of-windows.html