Microsoft and IBM open source MS-DOS 4.00

Microsoft, in partnership with IBM, has released the source code of MS-DOS 4.00 under the open-source MIT license. This release comes a decade after Microsoft first made the source code for MS-DOS 1.25 and 2.0 available to the public through the Computer History Museum.

The decision to release the MS-DOS 4.00 source code was inspired by the discovery of early, unreleased beta binaries by researcher Connor “Starfrost” Hyde. Hyde’s discovery stemmed from a conversation with former Microsoft Chief Technical Officer Ray Ozzie, who unearthed the binaries among floppies sent to him during his tenure at Lotus. Intrigued by the software’s historical value, Hyde approached the Microsoft Open Source Programs Office (OSPO) to explore the possibility of making these resources publicly available.

The release includes not only the source code but also additional beta binaries, extensive documentation in PDF format, and disk images. This treasure trove of digital artifacts dates back to the mid-1980s when Microsoft collaborated with IBM on certain aspects of DOS development.

Microsoft's Vice President of Developer Community, Scott Hanselman, and internet archivist Jeff Sponaugle imaged the original disks and scanned in the accompanying documents from what has been dubbed the “Ozzie Drop.”

While searching the Microsoft Archives for more materials, Jeff Wilcox of OSPO discovered that the full source code for Multitasking DOS was unavailable. However, the team did find the complete source code for MS-DOS 4.00, which is included in the new release. Further releases of additional materials could follow.

The team behind the released code say it has been successfully tested on an array of hardware from an original IBM PC XT to a newer Pentium machine, as well as within various emulators such as PCem and 86box. You can download it from GitHub.

Image Credit: Microsoft

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