Microsoft Open-Sources the Original MS-DOS on GitHub

On September 28, 2018, Microsoft published early MS-DOS source code on GitHub under the MIT License. The contemporaneous announcement and repository README identify the releases as MS-DOS 1.25 and 2.0, with compiled binaries. MS-DOS 4.00 joined the repository in a separate Microsoft-and-IBM release on April 25, 2024; it was not part of the 2018 event.

The 2014 release exposed source under narrower terms

Microsoft first authorized the Computer History Museum to publish early DOS material on March 25, 2014. The museum described the package as MS-DOS 1.1 and 2.0 source and object code for non-commercial use. Its Microsoft Research License was more capable than “look but do not touch”: users could copy, compile, run, and create derivative works for non-commercial research, experimentation, and education.

The critical restrictions were redistribution and purpose. The license prohibited publishing or distributing the software or derivative works, prohibited commercial-service use without another agreement, and allowed only short code excerpts in papers or presentations. A derivative-work creator also granted Microsoft broad rights to that work. Those terms made the code source-available for research, not open source under the usual permission to redistribute modified versions for any field of endeavor.

The MIT license changed what recipients could do

The GitHub repository’s LICENSE applies the MIT License to the included files. That license permits use, copying, modification, merging, publication, distribution, sublicensing, and sale, provided the copyright and permission notice remain with substantial portions of the software. It also supplies the standard no-warranty disclaimer.

This was the material legal change in 2018. GitHub made browsing and linking easier, but hosting location alone does not determine whether code is open source. Replacing the research-only agreement with a permissive redistribution license did.

Microsoft called the event “re-open-sourcing” because the bytes had already been accessible through the museum. The Computer History Museum updated its page to point readers to the GitHub release and emphasized that the repository would not become an actively maintained DOS project.

What the 2018 repository contained

Microsoft’s launch post says the source was written in 8086 assembly. It dates the 1.25 material to roughly May 1983 and describes seven source files, including the command shell in COMMAND.ASM. The larger 2.0 tree dates to roughly August 1983 and contains about 100 assembly files, plus documentation and object material.

The current repository README says it contains original source and compiled binaries for 1.25 and 2.0. A 2026 Microsoft retrospective calls the second body of code 2.11, while the contemporaneous 2018 post and current top-level README still label it 2.0. A forensic account should expose that nomenclature mismatch instead of silently rewriting the 2018 release as 2.11.

The repository is intentionally static. Microsoft asks contributors not to submit source-change pull requests, but explicitly permits forks and experimentation. “No pull requests” is a project-maintenance policy, not a restriction that overrides the MIT license.

MS-DOS 4.00 arrived six years later

Microsoft and IBM announced the MS-DOS 4.00 source release on April 25, 2024. They added code, binaries, disk images, and scanned documents to the historical effort under MIT. The release involved material from Microsoft’s archives and beta disks preserved by Ray Ozzie, with work from archivists and researchers.

That history must not be backdated. A reader looking at today’s microsoft/MS-DOS repository sees 4.00 beside the older trees, but Git records and Microsoft’s announcement show that this content was added in 2024. The 2018 milestone opened the 1.25/2.0 material; the 2024 milestone expanded the collection.

The 4.00 release also notes a complicated relationship among IBM and Microsoft DOS 4 work and an unreleased Multitasking DOS branch. The published 4.00 source should not be treated as a complete archive of every DOS 4-related project Microsoft ever developed.

Historical source is not a supported operating-system release

Microsoft presents the repository for study, preservation, and experimentation. It does not offer security servicing, hardware support, or a maintained build pipeline for deploying the code as a current operating system. Successfully assembling a historical source tree is not equivalent to receiving a supported MS-DOS product.

The repository also does not open-source later standalone releases such as MS-DOS 5.0 or 6.22. The MIT license covers files actually included in the repository. It does not attach itself to disk images found elsewhere or to versions merely related by product name.

What the publication means for FreeDOS

FreeDOS began in 1994 as an independent DOS-compatible implementation, long before Microsoft applied MIT to this historical source. Publishing early MS-DOS did not replace the FreeDOS kernel, FreeCOM, drivers, installer, or utility packages, and it did not make their histories derivative of Microsoft code.

The archive is still valuable to operating-system researchers. It exposes early command parsing, filesystem behavior, system-call implementation, build conventions, and comments from the original codebase. It can help historians understand why DOS interfaces developed as they did and let experimenters compare implementations under a legally reusable license.

Reuse remains an engineering and provenance decision. A contributor must confirm that a particular file is covered by the repository’s MIT license, preserve required notices, and avoid assuming code from an unlisted version has the same terms. FreeDOS packages also carry their own licenses and review processes.

The accurate milestone

The 2018 event was not the first time the public could inspect the source, and it did not include MS-DOS 4.00. It was the moment Microsoft republished the 1.25 and 2.0 collection with a standard permissive license and an easily referenced Git history. That combination transformed restricted research access into reusable open-source material.

Six years later, Microsoft and IBM added 4.00. Keeping those two releases separate—and distinguishing repository policy from license rights—makes the open-source history considerably clearer than today’s combined directory listing.

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