Microsoft has released MS-DOS 4.00 source code, binaries, disk images, and documentation for this operating system on GitHub. The 45-year-old code is released under the MIT license, which gives developers complete freedom of action. The fourth version of MS-DOS 4.00 is interesting because it was written in collaboration with IBM, showed the seeds of multitasking, and created a branch of DOS called Multitasking DOS (or MT-DOS) that was limited in distribution.

The release of MS-DOS 4.00 source code occurred thanks to the collaboration between former Microsoft technical director Ray Ozzie (Ray Ozzie) and computer researcher Connor Hyde (Connor Hyde). All the code and other stuff currently on GitHub was saved by Ozzy. While working at Lotus he archived several open binary beta versions of MS-DOS 4. Scott Hanselman, Microsoft’s vice president of community development, and Internet archivist and enthusiast Jeff Sponaglu also participated in preserving the legacy. Enthusiasts obtained all necessary permissions from the rights holder before releasing the source code of the operating system.

The most interesting thing about this version of MS-DOS is some of the basics of multitasking, which Hyde says later became the basis of IBM OS/2. A key part of MS-DOS 4.00’s intended multitasking workflow is the SM.EXE session manager. This system component allows hotkey switching between six preset programs. However, Hyde’s tests revealed serious flaws in his work. You can read more about multitasking in MS-DOS 4.00 and all the utilities included with this version on Hyde’s blog.

Intel 8086 MS-DOS 4.00 assembly code, binaries, disk images, and documentation are available on GitHub. The disk images are located in the v4.0-ozzie/bin directory. Enthusiasts with older hardware can run the operating system in its original form. You’ll need an emulator like PCem or the open source 86box to run it on modern PCs. By the way, the source codes for MS-DOS 1.25 and 2.0 are stored in the same GitHub repository.