Microsoft has just Done Something Uunusual

Microsoft has just done something unusual (the last time was 10 years ago): releasing the source code of an MS-DOS version, 4.0. The 'grandfather' of our current Windows, characterized by the use of a text-mode terminal, joins Microsoft's list of open-source projects.

The Previous Version

Ten years ago, Microsoft released the source code of MS-DOS 1.25 and 2.0, a central part of computing history: written entirely in 8086 assembly code almost 45 years ago, it was based on PC-DOS 1.0, the operating system of IBM's early PCs. But a decade later, those at Redmond (together with IBM itself) have decided to also release, under the MIT open license, the source code of MS-DOS 4.0.

In doing so, Microsoft not only strengthens its recent commitment to open-source software but also ensures that this technological heritage is not lost, languishing forgotten in some drawer of the company. Thus, studying its code can become a valuable source of learning about low-level programming, memory management, and system architecture.

The importance of MS-DOS

MS-DOS, short for 'Microsoft Disk Operating System,' was the dominant operating system on personal computers during the eighties and early nineties. Originally developed in 1981, despite its apparent simplicity, MS-DOS played a crucial role in the personal computing revolution, facilitating its widespread adoption in homes and offices.

It's worth remembering that, before the release of Windows 95, Windows was little more than a graphical interface for MS-DOS... and that, until the release of Windows 2000, the operating system still used DOS to boot up.

Version 4.0 of MS-DOS included several significant improvements over its predecessors, such as enhanced memory management and advanced disk utilities... but, above all, the most peculiar aspect of MS-DOS 4.0 was the incorporation of real multitasking, a feature that IBM's lack of interest in (as the manufacturer at the time of all the hardware that could run DOS) doomed to withdrawal in subsequent releases of MS-DOS.

The story of a move to 'open source'

It all began, as narrated in a Microsoft corporate blog, when a young British researcher named Connor 'Starfrost' Hyde came into contact with Microsoft's former chief technical officer, Ray Ozzie, and the latter found, in his own software collection, unpublished DOS 4.0 beta binaries, prompting the former to request Microsoft's Open Source Programs Office (OSPO) to explore the possibility of releasing the source code of MS-DOS 4.

"At Microsoft, along with our friends at IBM, we believe this is a fascinating piece of operating system history worth sharing. Today we release MS DOS 4.00 along with those additional beta binaries, PDFs of documentation, and disk images. We'll continue to explore the files and may update this release if more is discovered."

To access the source code of MS-DOS 4.0, users can visit its repository on GitHub and download the files they wish. Although (understandably) the repository is archived and changes are not allowed, users can study and use the code for personal or educational purposes.

"If you want to run this software yourself and explore it, we've successfully run it directly on an original IBM PC XT, a newer Pentium, and in the open-source PCem and 86box emulators."