Microsoft has released the MS-DOS 4.00 source code, binaries, disk images, and documentation. The source code, which is nearly 36 years old (version 4.00 came out in 1988), has been released under the MIT license, allowing tinkerers free rein. MS-DOS 4.00 is quite an interesting version of this command-line-driven OS, as it was written in partnership with IBM and formed a branch of DOS called Multitasking DOS (or MT-DOS), that only had a limited release.
In its blog post about open-sourcing MS-DOS 4.00, Microsoft says this release was precipitated largely thanks to correspondence between former Microsoft Chief Technical Officer Ray Ozzie and a young computer researcher called Connor ‘Starfrost’ Hyde.
The code and materials now available on GitHub came from Ozzie. He was working at Lotus when he received some unreleased beta binaries of DOS 4, which he kept safe. VP Developer Community at Microsoft, Scott Hanselman, and internet archivist and enthusiast, Jeff Sponaugle, carefully digitally preserved the ‘Ozzie Drop.’ Then, the relevant permissions were sought and granted before this open-source release.
Vancouver Linux Users Group 