Microsoft releases source code for MS-DOS 4.0

Together with IBM, Microsoft has released another version of its old operating system, MS-DOS, under an open-source license. The source code for MS-DOS 4.0 is now available as a repository on GitHub under an MIT license. MS-DOS versions 1.25 and 2.0 have already been available there for six years. Microsoft and IBM believe that this is a "fascinating piece of operating system history".

Originally, an English researcher named Connor "Starfrost" Hyde wanted to talk to former Microsoft CTO Ray Ozzie, who was working for Lotus at the time, about software from his collection, write Microsoft employees Scott Hanselman and Jeff Wilcox. Hyde was actually interested in the relationship between MS-DOS 4, its multitasking version (MT) and OS/2, and his research ultimately led to this publication.

MT-DOS: Multitasking DOS

The beta editions of the MT version of the operating system from 1984 come from Ozzie's collection, which Hanselman secured with the help of Internet archivist Jeff Sponaugle. They have also scanned the corresponding original documents. These can be found as PDF files together with the MT-DOS beta on GitHub in a separate folder. Wilcox was unable to find the source code for MT-DOS in his company's archive, but was able to find the source code for MS-DOS 4.0.

Hanselman and Wilcox promise to continue looking for the archives and update the repository if necessary. They also mention that some of the programmers who wrote the source code for MS-DOS 4.0 in the mid-1980s are still working at Microsoft or IBM today.

As on previous occasions, Microsoft points out that it has only released the source code for historical reasons and that the files are static; pull requests are ignored. If you want to experiment and program with it, please fork the code. The beta version of MT-DOS available on GitHub can be executed, Microsoft assures. The company has used an original IBM Personal Computer XT and the PCem and 86Box emulators for this purpose.

(anw)