Microsoft continues to make some of the earliest chapters of its operating system history open-source and freely available. Earlier this week, it announced that Tim Paterson's DOS listings, containing source code of the 86-DOS 1.00 kernel, various PC-DOS 1.00 pre-release kernels and utilities, and the Microsoft BASIC-86 Compiler runtime library, were available on GitHub. Microsoft VP Scott Hanselman tied the release to 86-DOS 1.00’s 45th anniversary. The exec confirmed that the code, transcribed from reams of old dot matrix printouts found in a garage, was perfect, "and recompiles byte for byte to the original binaries.”
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. Go dig in and learn how it was recovered! #DOS…April 28, 2026
If you head on over to the GitHub page to snag the code, you will see a photo of Tim Paterson standing in his garage with a pile of yellowed dot matrix printouts in the foreground. These pages contain the code for the software mentioned in the intro, and you can even see the original scans in PDF and PNG format via a link to the Internet Archive. These include the coder’s handwritten notes.
Probably more important to tinkerers, though, is the fact that the work of transcribing the printed code has been completed (for those three mentioned wares). Tips to compile and assemble the sources can also be found on Paterson’s GitHub.
From 86-DOS to MS-DOS
In case you aren’t familiar with the place of 86-DOS (or Tim Paterson) in Microsoft’s history, here's a short refresher. Microsoft took a shortcut and gained a foothold in the OS software market by purchasing 86-DOS from Seattle Computer Products and inventor Tim Paterson for a figure in the region of $75,000.
In the GitHub repository, you can see 86-DOS’s transformation into the PC-DOS 1.00 kernel, alongside code for some well-known utilities (still in use today) such as CHKDSK. As the Microsoft blog asserts, this work “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.”
So, we have another old DOS release to tinker with. In April 2024, we reported on Microsoft releasing the code for MS-DOS 4.00 under the generous MIT License, allowing tinkerers free rein. It did the same with MS‑DOS 1.25 and 2.11 in 2018. Also in 2024, we coincidentally covered a video demo featuring 86-DOS version 0.1C being taken for a test drive (via the Internet Archive), and now version 1.00 of this OS has hit GitHub, straight from the files squirreled away in Tim Paterson's (the creator’s) garage.
We’re still waiting for any version of Windows to be open-sourced. You have to dig through leaks if you are curious enough to want to investigate the source code for Windows XP, for example.
