86-DOS 1.00 Source Code Found in a Garage and Open-Sourced on Its 45th Anniversary
This is the kind of software archaeology that makes you stop and appreciate just how far we’ve come. Microsoft has open-sourced the earliest DOS source code — 86-DOS 1.00 — and the story behind it is almost as interesting as the code itself.
The Discovery
Tim Paterson, the inventor of 86-DOS, had a stack of yellowed dot-matrix printouts of the original source code sitting in his garage. Not backed up to cloud storage, not preserved in some corporate archive — literally printed on paper, sitting in a garage for 45 years. When Microsoft VP Scott Hanselman got hold of them, he confirmed that the transcribed code “is perfect and recompiles byte for byte to the original binaries.”
Hanselman put it perfectly on Microsoft’s blog: “The earliest DOS source code was found on printer paper in Tim Paterson’s garage so we’ve open-sourced it on 86-DOS 1.00’s 45th anniversary! This is next-level software archaeology for study, preservation, and plain ol’ curiosity.”
What’s in the Repository
The GitHub repository includes:
- The 86-DOS 1.00 kernel — the original code before Microsoft even touched it
- PC-DOS 1.00 pre-release kernels and utilities — showing Microsoft’s early modifications
- The Microsoft BASIC-86 Compiler runtime library — the development toolchain from the era
- Scans of the original printouts in PDF and PNG format via the Internet Archive, including Paterson’s handwritten notes in the margins
You can literally watch the code evolve from Tim Paterson’s 86-DOS into Microsoft’s PC-DOS 1.00, including the early versions of utilities like CHKDSK that we still use today.
The Price of Revolution
Here’s the bit that still blows my mind: Microsoft purchased 86-DOS from Seattle Computer Products and Tim Paterson for approximately $75,000. That’s the entire operating system that would become the foundation of the PC industry. Adjusted for inflation, that’s roughly $350,000 today — less than the cost of a single developer’s salary at Microsoft in 2026.
Context in the DOS Open-Sourcing Story
This isn’t the first time Microsoft has gone back and open-sourced DOS. MS-DOS 1.25 and 2.11 were released to GitHub in 2018, and MS-DOS 4.00 followed under the MIT License in April 2024. But 86-DOS 1.00 is the original — the code as it existed before Microsoft even owned it.
The Microsoft blog notes that this “offers rare insight into how MS-DOS/PC-DOS came to be, and how operating system development was done at the time, not as it was later reconstructed.”
What Hasn’t Been Released
Despite this generous streak with DOS, Microsoft has not open-sourced any version of Windows. The only Windows XP source code available to the public came through leaks, not official channels. So while the DOS legacy is now fully documented and preserved, the Windows story remains locked behind corporate walls.
Why This Matters
Beyond the nostalgia factor (and there’s plenty of that), this is a genuine piece of computing history. The 86-DOS source code shows how a two-pass assembler and a handful of C routines built the operating system that launched the IBM PC revolution. Every modern operating system owes something to the design decisions made in those few thousand lines of code.
The fact that it survived on dot-matrix printouts in a garage is a reminder that software preservation is still an unsolved problem. We worry about Bitrot on cloud storage, but the original source code for the most influential operating system in history was on printer paper.
Worth a browse through the repo if you’ve got an hour — the handwritten notes in the margins are a particularly nice touch.