Author: Microsoft
Ten years ago, Microsoft released the MS-DOS 1.25 and 2.0 source codes to the Computer History Museum and later republished them on GitHub for reference purposes. This code occupies a significant place in history and is a fascinating read about an operating system that was written almost 45 years ago entirely in 8086 assembly code.
explains Microsoft.
The company now, in cooperation with IBM, has released the source code of MS-DOS 4.00 under the MIT license. It is a version called Multitasking DOS, which was supposed to have a number of interesting new features, but was eventually abandoned and never saw a public release.
The source codes are again published on GitHub, where you can also find some other unreleased programs and complete documentation scanned in PDF. If you would like to run and explore MS-DOS 4.0, it is said to be possible on an original IBM PC XT, but it has also been run on a newer Pentium computer and in PCem and 86box emulators.
(Pointed out by Petr Hercík.)
Original diskettes with the beta version of MS-DOS 4.0
Author: Microsoft
