“Jeff Wilcox and OSPO went to the Microsoft Archives, and while they were unable to find the full source code for MT-DOS, they did find MS DOS 4.00, which we’re releasing today”
Microsoft put MS-DOS 4.00 under the MIT license, with IBM’s blessing. The push came from a young researcher and beta floppies Ray Ozzie had kept since his Lotus days. Microsoft’s own archive could not produce the full Multitasking DOS source. Hobbyists preserved the company’s history better than the company did.