Microsoft has announced the release of the sources of versions 1.25 and 2.0 of its MS-DOS operating system, which were made available on the GitHub website, under the MIT free / open source license. In 2014 the same sources were released to the Computer History Museum but with a restrictive license, for historical reasons. The released versions are very old and Microsoft will not accept pull requests to modify them sent through the GitHub system but anyone can freely not only read the assembly code but also experiment with it, perhaps on a virtual machine.