Microsoft puts MS-DOS 4.0 source code on GitHub

Microsoft and IBM have made the code for MS-DOS 4.0 open source. The source code, images and documentation are available on GitHub under an MIT license. This means that anyone who wants can share the code, modify it or use it in their own code.

In a blog post, Microsoft reports that making the code public was partly achieved thanks to the English researcher Connor 'Starfrost Hyde. To document the relationship between DOS 4 and MT-DOS, Hyde turned to former Microsoft CTO Ray Ozzie, who had floppy disks containing unreleased beta binaries of DOS 4 in his possession.

These floppies ended up at the Microsoft Open Source Programs Office, where they are read to digitize the original content. Microsoft CEO Jeff Wilcox and OSPO then turned to the Microsoft archives and eventually discovered the complete source code of MS-DOS 4.00. This code is now available on GitHub, along with the found beta binaries, documentation, and images. Since 2018, versions 1.25 and 2.0 can also be found there.

MS-DOS 4.0 is based on MS-DOS 2.0 and was announced in 1986. This version of the OS was supposed to be better suited for multitasking thanks to the using the session manager. This allows the system to switch between up to six 'active' programs via hotkeys.