The original code for the MS-DOS 1.25 operating system has been released under an open source MIT license and made publicly available on GitHub. The code first came to light in 2014, when the original author Tim Paterson rediscovered it and Microsoft donated it to the Computer History Museum. Now Microsoft is letting the entire world take a look at this piece of computing history.
The operating system comprises just seven assembly files that fit into 12 KB of memory, and it includes the original MS-DOS command-line shell. MS-DOS 1.25 was released in May 1983 and was replaced by MS-DOS 2.0 in August 1983.