Microsoft Announces the Surprise Release, Under a Permissive License, of the MS-DOS 4.00 Sources

Microsoft has made an announcement this week that is surprising both for what the company is doing and what the company is doing it to: MS-DOS 4.0, released in 1988, is now an open source project.

"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," Microsoft's Scott Hanselman and Jeff Wilcox explain in the joint announcement. "If you’d like to run this software yourself and explore, we have successfully run it directly on an original IBM PC XT, a newer Pentium, and within the open source PCem and 86box emulators."

MS-DOS 4.0 is one of the lesser-known variants of Microsoft's infamous Disk Operating System — a family of operating systems targeting IBM and compatible computers that grew out of Seattle Computer Products' Quick and Dirty Operating System (QDOS) and which was undeniably inspired by Gary Kildall's CP/M. IBM's decision to license MS-DOS 1.0 for its Personal Computer project — thanks to the intervention of Microsoft co-founder Bill Gates' mother Mary — was the breakthrough the nascent company needed, putting Microsoft on an explosive growth trajectory.

MS-DOS 4.0, though, marked a shift for the company. Internally, Microsoft worked on what became known as MS-DOS 4.0 (Multitasking) which sits between MS-DOS 3.2 and MS-DOS 3.3 in the company's release timeline — while IBM worked on a separate MS-DOS 4.0, which did not feature the new multitasking capabilities but did add a graphical interface. This would be rewritten by Microsoft and released as MS-DOS 4.01, while the internally-developed multitasking version, also known as MT-DOS, would be largely forgotten.

"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," Hanselman and Wilcox explain of the timing of the new release. "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."

While Microsoft is best known for its proprietary, closed-source offerings, this isn't the first time it has given the green light to a wider sharing of source code to older software: a decade ago the company released the MS-DOS 1.25 and 2.0 source code, initially under a license limiting its use to "reference purposes."

The MS-DOS 4.00 source code, meanwhile, has been put on GitHub — alongside the earlier MS-DOS 1.25 and MS-DOS 2.0 releases plus scans of documentation supplied by Microsoft to its partners as part of the original release — under the permissive MIT license; more information is available on the Microsoft Open Source Blog.