Microsoft Open Sources MS-DOS 4.0

Well, this came out of nowhere: Microsoft and IBM have open-sourced MS-DOS 4.0, a legacy command line OS the two worked on together over 40 years ago. This release follows the open-sourcing of two previous MS-DOS versions, 1.25 and 2.0, in 2019.

“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,” Microsoft’s Jeff Wilcox and Scott Hanselman write in the announcement post. “This code holds an important place in history and is a fascinating read of an operating system that was written entirely in 8086 assembly code nearly 45 years ago.”

I had seen Scott asking if anyone had 5.25-inch disk drives on Facebook a few weeks back, and now we know why: He worked with enthusiast Jeff Sponaugle to image original MS-DOS 4.0 disks that had been uncovered by Connor Hyde, a researcher in England. There’s a whole story there, and it involves former Microsoft Chief Technical Officer Ray Ozzie and others. And Hyde plans to document the relationship between DOS 4, MT-DOS, and what would eventually become OS/2. Fascinating stuff.

The source code for MS-DOS 4.0 joins the source code and compiled binaries for MS-DOS 1.25 and 2.0 on GitHub, and each is licensed under the MIT license. Microsoft notes that it has successfully run MS-DOS 4 in the PCem and 86box emulators, and—seriously—on an original IBM PC XT and a newer Pentium-based PC too.