Microsoft has released the MS-DOS 4.00 source code, binaries, disk images, and documentation. The source code, which is nearly 45 years old, 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.
