Microsoft Releases MS-DOS 400 Source Code on GitHub

Microsoft has released the source code for MS-DOS 4.00, binaries, disk images, and documentation. Now the source code, which is almost 45 years old, is available under the MIT license, meaning it can be freely used, modified, and distributed.

Ten years ago, Microsoft the source code for MS-DOS 1.25 and 2.0 to the Computer History Museum, and later re-released it on GitHub. The company notes that this code holds an important place in the history of the operating system, which was entirely written in assembly language for the 8086 almost half a century ago.

Now, the source code for MS-DOS 4.00 has been unveiled and is now available under the MIT license. Microsoft highlights that MS-DOS 4.00 is a significant version, developed in collaboration with IBM. It was from MS-DOS 4.00 that the Multitasking DOS (or MT-DOS) fork was created, although it did not gain wide popularity in the end.

In their announcement regarding the release of the source code for MS-DOS 4.00, Microsoft representatives reveal that this release came about due to correspondence between former Microsoft Technical Director Ray Ozzie and young British researcher Connor Hyde, also known as Starfrost.

Many of the source code and materials now available on GitHub were provided by Ozzie, who once worked at Lotus and received several unreleased beta versions of MS-DOS 4. Since then, Ozzie has kept the disks in a safe place and recently found them at Hyde’s request.

Starfrost contacted Microsoft’s Open Source Programs Office (OSPO) to explore the possibility of releasing the DOS 4 source code. Hyde is working on documenting the relationships between DOS 4, MT-DOS, and what eventually became OS/2.

While some later versions of Multitasking DOS binaries could be found on the internet, Ozzie’s beta versions are earlier copies that were not previously released.

Ultimately, Microsoft Developer Community Vice President Scott Hanselman and internet archivist and enthusiast Jeff Sponaugle created images of these original media and scanned the provided original printed documents.

Additionally, Microsoft’s experts and OSPO delved into the company’s archives. Although they did not find the complete source code for MT-DOS, they discovered MS-DOS 4.00, now available on GitHub, along with additional beta versions of binaries, PDF documentation, and disk images. The company promises to enhance the release if they find anything else of interest.

“If you want to run this software yourself, we’ve successfully run it on the original IBM PC XT, a newer Pentium, as well as in the PCem and 86box open-source emulators,” the company’s TV notes.

In his personal blog, Connor Hyde elaborately discusses how MS-DOS 4.00 was unique, boasting certain multitasking capabilities and eventually forming the basis of OS/2.

One of the key components of the multitasking capability of MS-DOS 4.00 was the session manager (SM.EXE). Hyde explains that this system component allowed for the use of hotkeys and switching between predefined applications (up to six). However, based on the researcher’s tests, serious errors were observed in the operation of SM.EXE, making its use less practical.

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