Microsoft and IBM release MS-DOS 4.00 source code

In a celebration of innovation and preserving computing history, Microsoft has released the source code for MS-DOS 4.00 in partnership with IBM. The release – available under the permissive MIT license – includes the operating system’s source code, binary files, and original documentation.

The decision to open source MS-DOS 4.00 stems from recent correspondence between a researcher named Connor “Starfrost” Hyde and former Microsoft CTO Ray Ozzie. While exploring Ozzie’s collection of old floppy disks, Hyde discovered unreleased beta binaries of DOS 4.0 that Ozzie had received during his time at Lotus Software.

Intrigued by this finding, Hyde reached out to Microsoft’s Open Source Programs Office (OSPO) to explore the possibility of releasing the DOS 4 source code. His aim was to document the relationship between DOS 4, Microsoft’s Multitasking DOS (MT-DOS), and the eventual development of IBM’s OS/2 operating system.

Scott Hanselman, with the assistance of internet archivist Jeff Sponaugle, imaged the original disks and scanned the accompanying printed documents from what became known as the “Ozzie Drop.” While unable to locate the full source code for MT-DOS, Microsoft and IBM recognized the historical significance of DOS 4.00 and decided to release it to the public.

“This code holds an important place in history and is a fascinating read of an operating system that was written entirely in 8086 assembly code nearly 45 years ago,” Microsoft stated in a blog post announcing the release.

The release includes the MS-DOS 4.00 source code, binaries, PDFs of the original documentation, and disk images. Microsoft has also pledged to continue exploring its archives and may update the release if additional relevant materials are discovered.

The story behind MS-DOS 4.00 is a complex and fascinating one. In the 1980s, Microsoft partnered with IBM for portions of the DOS code but also created a separate branch called Multitasking DOS, which never saw widespread release.
While later versions of MT-DOS binaries can be found online, the newly released Ozzie beta binaries appear to be much earlier and unreleased versions, including the ibmbio.com source code.

For those interested in exploring this historic software, Microsoft has confirmed successful runs on an original IBM PC XT, a newer Pentium system, and within the open source PCem and 86box emulators.

The release represents a remarkable collaborative effort between Microsoft, IBM, and the broader community of digital archeologists and preservationists. The source code is now available on GitHub for public perusal and use, ensuring that this slice of computer history remains accessible and influential for both today’s users and future generations.