Microsoft has open-sourced MS-DOS 4.0 under the MIT license, making it nearly 45 years after it was first developed by Microsoft and IBM.

According to the Microsoft Open Source Blog, the company already open-sourced MS-DOS 1.25 and 2.0 a couple of years ago. The company has now done the same with MS-DOS 4.0 in cooperation with IBM, since both companies had a hand in its development.

Today, in partnership with IBM and in the spirit of open innovation, we’re releasing 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.

The goes on to highlight how the release was open-sourced.

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.

Scott Hanselman, with the help of internet archivist and enthusiast Jeff Sponaugle, has imaged these original disks and carefully scanned the original printed documents from this “Ozzie Drop”. Microsoft, along with our friends at IBM, think this is a fascinating piece of operating system history worth sharing.

Those interested in trying out the operating system can download it from its GitHub repo.