Microsoft open-sources MS-DOS 4.00, releases early beta of MS-DOS 4.0 (multitasking)

Today, in partnership with IBM and in the spirit of open innovation, we’re releasing the source code to MS-DOS 4.00 under the MIT license. There’s a somewhat complex and fascinating history behind the 4.0 versions of DOS, as Microsoft partnered with IBM for portions of the code but also created a branch of DOS called Multitasking DOS that did not see a wide release.
↫ Scott Hanselman

Not only did they release the source code to MS-DOS 4.00, they also released disk images of a very early version of Multitasking DOS, which did not see a wide release, as the article states. I’ve only vaguely heard of MT-DOS over the decades, so I had to do some minor reading and research to untangle what, exactly, MT-DOS really is. Much of this information is probably table stakes for the many older readers we have, but bear with me.

MT-DOS, which has the official name MS-DOS 4.0 (often further specified by adding “multitasking” in brackets after the version number) was a version of MS-DOS developed by Microsoft based on MS-DOS 2.0, whose headlining feature was pre-emptive multitasking, which allowed specifically written applications to continue to run in a special background mode. Interestingly enough, it had to perform this multitasking with the same 640k memory limitation as other versions of DOS. Very few OEMs ended up licensing it, and most notably IBM wasn’t interested, so after one or two more OEM-specific versions, it was quickly abandoned by Microsoft.

MS-DOS 4.0 (multitasking) is entirely unrelated to the “real” versions 4 of MS-DOS that followed later. The actual version 4 was called MS-DOS 4.00, and it’s the source code to this specific version that’s being released as open source today. MS-DOS 4.00 was quickly followed by 4.01 and 4.01a, but apparently OEMs would confusingly still label 4.01 disks as “MS-DOS 4.0”. The whole MS-DOS 4 saga is quite convoluted and messy, and I’m probably oversimplifying a great deal.

Regardless, this code joins the open source releases of MS-DOS 1.25 and 2.0 that Microsoft released years ago.