In a rather unexpected move, Microsoft and IBM have jointly decided to open source MS-DOS 4.0, a historic command-line operating system that they collaborated on over four decades ago. This surprising release follows Microsoft’s earlier open-sourcing of two earlier versions of MS-DOS, namely 1.25 and 2.0, back in 2019.

The announcement, made by Microsoft’s Jeff Wilcox and Scott Hanselman, sheds light on the intriguing history behind the MS-DOS 4.0 project. As the post explains, Microsoft and IBM’s partnership led to the creation of various DOS versions, including a lesser-known Multitasking DOS branch that never saw wide distribution. The code from MS-DOS 4.0, dating back nearly 45 years, holds a significant place in the evolution of operating systems, especially considering it was entirely written in the 8086 assembly code.

The journey to open-source MS-DOS 4.0 involved some curious discoveries and collaborations. Scott Hanselman’s recent inquiry about 5.25-inch disk drives on Facebook was not just idle curiosity; it was part of a mission to work with enthusiast Jeff Sponaugle in imaging original MS-DOS 4.0 disks unearthed by researcher Connor Hyde in England. The story deepens further with involvement from former Microsoft Chief Technical Officer Ray Ozzie and others, adding layers to the intriguing history behind MS-DOS.

Hyde, the English researcher, plans to delve into the connections between DOS 4, MT-DOS, and the eventual evolution into OS/2, promising to unearth more fascinating insights into the software’s lineage.

The source code for MS-DOS 4.0 now joins its predecessors, MS-DOS 1.25 and 2.0, on GitHub, with each release licensed under the MIT license for broader accessibility. Microsoft’s experiments have successfully demonstrated MS-DOS 4.0 running on emulators like PCem and 86box, and remarkably, on both an original IBM PC XT and a modern Pentium-based PC.

This unexpected move by Microsoft and IBM to open-source MS-DOS 4.0 not only preserves a crucial piece of computing history but also invites enthusiasts and historians alike to explore the origins of this iconic operating system. With the code now freely accessible, it’s poised to inspire a new wave of exploration and understanding in the realm of vintage computing.