Microsoft open-sources “the earliest DOS source code discovered to date”

Old 86-DOS source code dates back to the time before Microsoft bought it.

Several times in the last couple of decades, Microsoft has released source code for the original MS-DOS operating system that kicked off its decades-long dominance of consumer PCs. This week, the company has reached further back than ever, releasing “the earliest DOS source code discovered to date” along with other documentation and notes from its developer.

Today’s source release is so old that it predates the MS-DOS branding, and it includes “sources to the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and some well-known utilities such as CHKDSK,” write Microsoft’s Stacey Haffner and Scott Hanselman in their co-authored post about the release.

To understand the context, here’s a very brief history of what would become MS-DOS: Programmer Tim Paterson originally created 86-DOS (previously known as QDOS, for “quick and dirty operating system”) for an Intel 8086-based computer kit sold by Seattle Computer Products. Microsoft, on the hook to provide an operating system for the still-in-development IBM PC 5150, licensed 86-DOS and hired Paterson to continue developing it, later buying the rights to 86-DOS outright. Microsoft then licensed this operating system to IBM as PC-DOS while retaining the ability to sell the operating system to other companies. The version sold by Microsoft was called MS-DOS, and the proliferation of third-party IBM PC clones over the ’80s and ’90s made it the version of the operating system that most people ended up using.

This source code is old enough that it hadn’t been stored digitally. “A dedicated team of historians and preservationists led by Yufeng Gao and Rich Cini,” calling itself the “DOS Disassembly Group,” painstakingly transcribed and scanned in code from paper printouts provided by Paterson. This process was made even more difficult because modern OCR software struggled with the quality of the decades-old printout.

Microsoft has also open-sourced several of its other early software projects. In 2014 (and again in 2018), the company open-sourced MS-DOS versions 1.25 and 2.0. It followed that up in 2024 with the oddball MS-DOS 4.0 release. Those versions are all available in the same GitHub repo. Other open-sourced projects include the game Zork and its sequels and 1995’s Microsoft 3D Movie Maker (plans to modernize this app and add new features have largely gone nowhere). The open source remake of the old MS-DOS Editor isn’t actually the same app as the old EDIT.COM, but its heart is in the right place.

For students of early PC history, this isn’t even the first piece of 86-DOS history that has been newly rediscovered this decade. Just two years ago, the earliest known version of 86-DOS was rediscovered and uploaded to the Internet Archive.

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Posted Friday 1 May 2026 at 7:29 am AEST (my time).

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