https://www.theregister.co.uk/2018/10/01/microsoft_msdos_source_on_github/

As Microsoft gears up to unleash the Windows 10 October 2018 Update, Rich Turner, guardian of the command line at Redmond, took a moment to remind us of simpler, MS-DOS-based times.

After original author Tim Paterson found the source for MS-DOS 1.25 (along with a six-inch stack of assembly print-outs), Microsoft handed the code for 1.25 and 2.0 to the Computer History Museum back in 2014.