In a significant move for tech historians and open-source enthusiasts, Microsoft has announced the release of the MS-DOS 4.00 source code under the MIT license. This release was made in partnership with IBM and marks a continuation of Microsoft’s commitment to open innovation. The news was detailed in a blog post on the Official Microsoft Open Source Blog by Scott Hanselman, Vice President of Developer Community, and Jeff Wilcox, Head of Open Source Programs Office.

“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,” the blog post remembered. The release includes not only the source code for MS-DOS 4.00 but also additional beta binaries, documentation in PDF format, and disk images. These materials have been preserved and made accessible thanks to the efforts of internet archivist Jeff Sponaugle and the guidance of former Microsoft CTO Ray Ozzie.

This version of MS-DOS, developed in collaboration with IBM, has a complex history, featuring contributions to what would eventually evolve into OS/2. Notably, this release includes early, unreleased beta binaries discovered by a young English researcher, Connor “Starfrost” Hyde, who found them among Ray Ozzie’s collection of software.

Microsoft’s Open Source Programs Office (OSPO) explored the possibility of releasing the source code for MT-DOS but eventually focused on MS-DOS 4.00. Although the full source code for MT-DOS was not found, the release of MS-DOS 4.00 represents a rich piece of computing history, showcasing an era when operating systems were written entirely in 8086 assembly code.

The released materials can run on hardware as old as an original IBM PC XT and as recent as a Pentium, and are also compatible with open-source emulators like PCem and 86box, allowing enthusiasts to explore this vintage software in a modern setting.

The initiative underscores the value of digital archaeology in preserving and understanding the technological advancements of the past. Microsoft and IBM’s collaborative effort highlights their ongoing commitment to sharing important historical artifacts with the public and contributing to the educational and technological community.

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