Microsoft partners with IBM to release the MS-DOS 4.0 source code under the MIT license on GitHub

Fond memories of beige machines. — Ah, the PC in the spare room. Back when “going on the computer” was an event, not a daily necessity, I cut my teeth on MS-DOS games.

PC Gamer 2024-04-27 Andy Edser

Context & Ripple Effects

This release makes a historically important PC software artifact available through GitHub, with IBM participating and the permissive MIT license enabling inspection, reuse, and preservation. It connects directly to the continuing maintenance of MS-DOS-compatible FreeDOS rather than treating DOS solely as nostalgia.

It also extends Microsoft’s longer developer-facing shift toward open source, visible in its earlier plan to open-source Q# development tools and its GitHub-centered effort to rebuild developer goodwill.

First-order effects

Developers, historians, and emulator or compatibility-project maintainers can now inspect and legally reuse MS-DOS 4.0 code under MIT terms.
Microsoft and IBM turn a jointly associated legacy product into a public, version-controlled reference artifact on GitHub.

Second-order effects

FreeDOS and retrocomputing projects gain a more authoritative source for studying historical behavior and improving compatibility, though the release does not itself make modern software DOS-compatible.
GitHub further reinforces its role as a repository for both active development and software preservation, giving legacy-code releases a durable collaboration surface.

Third-order effects

If major vendors continue permissively releasing dormant code, preservation and compatibility work can shift from reverse engineering toward auditable source-based maintenance.
The move fits a broader transition in which software companies use open-source releases to sustain developer relationships and preserve technical heritage after a product’s commercial life ends.

The trend: Legacy software is increasingly being released as open source, turning retired codebases into shared infrastructure for preservation, education, and compatibility work.

Discussion

GamingOnLinux Liam Dawe on x
Microsoft open sources MS-DOS 4.0
Phoronix Phoronix on x
Microsoft Open-Sources MS-DOS 4.0 Under MIT License
@billhiggins Bill Higgins on x
I have such profound respect for the work @shanselman and team are doing to make these historically important artifacts to the history of computing accessible to all. (1/2)
@stevesi Steven Sinofsky on x
Open sourcing MS-DOS 4.0 // fun to see. This might underplay the confusion and lack of success of 4.0 which was a weird release that was based on the 2.0 code and didn't offer much for the 386 processor that was already working through new PCs. It was even confusing to OEMs who..…
@quinnypig Corey Quinn on x
“GitHub” is the only thing in this headline from the 21st century.
@vxunderground @vxunderground on x
MS-DOS comments 🔥🔥🔥 [image]
@ceooffuggy @ceooffuggy on x
We need more of this, there is no reason Windows 95 for example should be closed source
@gro_tsen Gro-Tsen on x
On this subject, I think there should be a legal obligation to release the source code of any piece of software (at least of the OS or firmware kind) that is no longer maintained for security. So someone else can maintain it in principle.
@tsoding @tsoding on x
Apparently MSDOS 2.X uses Arenas for memory allocation lol [image]
@alexjonesax Alex on x
Those crazy bastards finally did it https://cloudblogs.microsoft.com/ ...

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