Microsoft has released the source code for MS-DOS 1.25 and 2.0 through its GitHub account. “These are the same files originally shared at the Computer History Museum on March 25th, 2014 and are being (re)published in this repo to make them easier to find, reference-to in external writing and works, and to allow exploration and experimentation for those interested in early PC Operating Systems.”
The source code was released under the MIT license and published as a historical reference. Microsoft hopes that developers will not make any changes to Pull Requests.
Exciting users can check it here.