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Microsoft Releases Source Code for DOS 1.0

Catwell

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Developing a DOS operating system in the early days. (Image Credit: Rich Cini/Microsoft)

Microsoft has been making its early operating system code available to the public over the past few years. In 2018, the tech giant open-sourced MS-DOS 1.25 and 2.0 before allowing public access to MS-DOS 4.0 in 2024. More recently, Microsoft released the source code for the 86-DOS 1.00 to mark its 45th anniversary. Microsoft and the Computer History Museum say the release preserves an important part of computing history and is intended for those who want to learn about it.

Even then, the archive goes further than the code itself. We can also see software history in scanned listings, internal documents, assembler printouts, and analog artifacts that shaped operating systems from the late 1970s and early 1980s. The MS-DOS 1.25 and 2.0 releases remind us that there’s more to the story than just code, as context matters when exploring the evolution of modern platforms.

Yufeng Gao and Rich Cini’s team of historians and Preservationists found, scanned, and transcribed a stack of DOS-era source listings from Tim Paterson, the original author of 86-DOS. Those listings are loaded with development snapshots, kernel source, CHKDSK and other utilities, and assembler listings. It also included the assembly, giving us a glimpse into how MS-DOS and PC-DOS took shape.

Microsoft’s releases aren’t meant for real-world use. But anyone interested in how operating systems were structured on 1st-gen 8086 hardware can still learn from them. Since DOS 1.0 is so small and limited, it works like a traceable system that can be understood from top to bottom.  

“It’s also worth noting that these materials aren’t just operating system releases in the traditional sense. In several cases, the listings represent point‑in‑time working states and hand-written notes, preserved by Tim Paterson himself. Think of them as a printed commit history of a Git repository. They create a timeline of changes, showing which features were implemented when, what errors were made, and how they were fixed. Soon you’ll be able to visit these living artifacts at the Interim Computer Museum as they’ve been generously donated by Tim Paterson,” Microsoft wrote in its blog post.

I hope this leads to another path for the embedded OS space.

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  • in reply to bradfordmiller

    I think there were two reasons.

    One is the cost of creating a new OS for a project that IBM did not see a lot of return for. They just wanted to sell the hardware, they did not see a return for software.

    Second, they originally wanted to use CPM as it dominated the current PC market at the time.

    After Kildall blew them off, they went to MS as the new emerging OS and Gates was more than happy to sign the contract.

    PC's took off and became a much larger market than IBM had predicted and hardware competition soon pushed IBM out of the business, but almost everyone stayed with DOS, so Microsoft profited nicely and expanded their list of products as the market place went to the 100's of million units.

    At that time, UNIX costs about 800 USD for each computer, which put it out of home PC market.

    Had it not been for Linux, it would not have seen a resurgence for home PC's

  • in reply to DAB

    It's always been a mystery why IBM would essentially commodify their own hardware as well, handing a monopoly over to M$. But history is a strange beast: who would have guessed that one of the worst OSes of that time would end up being the foundation of pretty much everything today? (And like the ICE, increasingly exponential amounts of labor to address fundamental flaws to maintain market dominance.) Though to be fair, UNIX was once one of the smallest OSes as well, not the behemoth we use today.

  • It is hard to believe that it has been 45 years since DOS 1.0.

    I always thought that Bill Gates should have sent a check to Gary Kildall every year thanking him for being an idiot so that IBM went to Seattle and gave Microsoft the contract.