Microsoft has published the source code MS-DOS 4.00, binaries, disk images and documentation. The nearly 45-year-old source code is now available under the MIT license, meaning it can be freely manipulated, modified, and redistributed.
Ten years ago, Microsoft already distributed the source code for MS-DOS 1.25 and 2.0 Computer History Museumand then re-published them on GitHub. The company notes that this code occupies an important place in the history of the operating system, which was written entirely in assembly language for the 8086 almost half a century ago.
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The source code for MS-DOS 4.00 has now been made public and is now available under the MIT license. Microsoft reminds us that MS-DOS 4.00 is a rather interesting version, written jointly with IBM. In the past, it was from MS-DOS 4.00 that a branch of Multitasking DOS (or MT-DOS) was formed, which ultimately did not become widespread.
In his message about the open source code of MS-DOS 4.00, Microsoft representatives say that this release was released thanks to correspondence between former Microsoft technical director Ray Ozzie and young British researcher Connor Hyde, also known under the nickname Starfrost.
The fact is that many of the sources and materials now available on GitHub were provided by Ozzie, who once upon a time worked for Lotus, and was sent several unreleased beta versions of MS-DOS 4. Since then, Ozzie has stored the floppy disks in a secure place, and recently found them at Hyde’s request.
As a result, Starfrost contacted Microsoft's open source program office (OSPO) to explore the possibility of publishing DOS 4 source code. After all, Hyde himself is working on documenting the relationships between DOS 4, MT-DOS and what eventually became OS/2.
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It is noted that some later versions of Multitasking DOS binaries could be found on the Internet, but Ozzy's beta versions are earlier copies that were not previously released.
As a result, Microsoft Developer Community Vice President Scott Hanselman and Internet archivist and enthusiast Jeff Sponaugle created images of these original media and scanned the original printed documents provided.
In addition, Microsoft and OSPO specialists delved into the company's archives. While they were unable to find complete MT-DOS source code, they did find MS-DOS 4.00, which is now available on GitHub, as well as additional beta binaries, PDF documentation, and disk images. The company promises that they will update the release if they discover anything else interesting.
“If you want to run this software yourself, we have successfully run it on the original IBM PC XT, the newer Pentium, as well as the open source PCem and 86box emulators,” the TV company notes.
Connor Hyde's personal blog details tells that MS-DOS 4.00 was unique and, for example, had certain capabilities for multitasking, and ultimately formed the basis of OS/2.
Thus, one of the key components of MS-DOS 4.00 multitasking was the session manager (SM.EXE). Hyde explains that this component of the system allowed the use of hot keys and switching between predefined applications (there could be up to six of them). However, judging by the researcher's own tests, there were serious errors in the operation of SM.EXE, so its use was not very advisable.