April 25th, Microsoft, in partnership with IBM and in the spirit of open innovation,  released the source code to MS-DOS 4.00 under the MIT license.

There’s a somewhat complex and fascinating history behind the 4.0 versions of DOS, as Microsoft partnered with IBM for portions of the code but also created a branch of DOS called Multitasking DOS that did not see a wide release.

A young English researcher named Connor “Starfrost” Hyde recently corresponded with former Microsoft Chief Technical Officer Ray Ozzie about some of the software in his collection. Amongst the floppies, Ray found unreleased beta binaries of DOS 4.0 that he was sent while he was at Lotus.

Starfrost reached out to the Microsoft Open Source Programs Office (OSPO) to explore releasing DOS 4 source, as he is working on documenting the relationship between DOS 4, MT-DOS, and what would eventually become OS/2. Some later versions of these Multitasking DOS binaries can be found around the internet, but these new Ozzie beta binaries appear to be much earlier, unreleased, and also include the ibmbio.com source.

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 was released, alongside additional beta binaries, PDFs of the documentation, and disk images.

Read more on the Microsoft blog, from Scott Hanselman and view the source code here.