> ## Content Index > Fetch the complete content index at: https://itsfoss.com/llms.txt > Use this file to discover other available public pages before exploring further. # Microsoft Marks 45 Years of DOS by Open-Sourcing Its Oldest-Known Source Code - URL: https://itsfoss.com/news/ms-dos-early-code-open-sourced/ - Published: 2026-04-30T15:43:47.000Z - Updated: 2026-04-30T15:43:47.000Z - Description: Tim Paterson's 1981 assembler printouts are now transcribed, compilable, and MIT-licensed. - Author: Sourav Rudra - Tags: News Before Microsoft became the company that shipped Windows to corporate desks around the world, it had to start somewhere. That somewhere was a scrappy little operating system written by one guy at [Seattle Computer Products](https://en.wikipedia.org/wiki/Seattle%5FComputer%5FProducts?ref=itsfoss.com). [Tim Paterson](https://en.wikipedia.org/wiki/Tim%5FPaterson?ref=itsfoss.com) built what he initially called [QDOS](https://en.wikipedia.org/wiki/86-DOS?ref=itsfoss.com), short for *Quick and Dirty Operating System*, in 1980\. Intel's 8086 chip was out, but [CP/M](https://en.wikipedia.org/wiki/CP/M?ref=itsfoss.com), the dominant OS of the time, had no 8086 support. He wrote something to fill that gap, modeling the CP/M API so existing software would run on it. Microsoft bought the rights to [86-DOS](https://en.wikipedia.org/wiki/86-DOS?ref=itsfoss.com) for just under $100,000, shipped it to IBM as **PC DOS 1.0** in August 1981, and retained the rights to sell the same OS to other PC manufacturers as MS-DOS. That single deal set Microsoft on the path to dominating personal computing for the next two decades. ## Fast forward to now ![a cropped screenshot that shows paterson listings on github with a picture of tim paterson visible in the middle](https://itsfoss.com/content/images/2026/04/Screenshot-From-2026-04-30-14-44-23.png) On April 28, the 45th anniversary of 86-DOS 1.00, Microsoft published [a blog post](https://opensource.microsoft.com/blog/2026/04/28/continuing-the-story-of-early-dos-development/?ref=itsfoss.com) announcing that **the earliest known DOS source code is now publicly available on GitHub**, under the MIT license. And the story behind it is an interesting one. Tim did not hand over a tidy source archive; instead, what he kept were physical assembler printouts and stacks of continuous-feed paper from 1981 that he had held onto over the decades. Getting those into usable shape took effort, with historians [Yufeng Gao](https://thebrokenpipe.com/blog/86-dos-0-11-from-scratch/?ref=itsfoss.com) and [Rich Cini](http://cini.classiccmp.org/recoveryblog.htm?ref=itsfoss.com) having to locate, scan, and transcribe the DOS-related portions into compilable code. What's included are the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, utilities like [CHKDSK](https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/chkdsk?ref=itsfoss.com), and the assembler Paterson used to write the OS itself. ## Who's this for? Honestly, **seeing Microsoft open up old code is not that surprising anymore**. [6502 BASIC](https://itsfoss.com/news/basic-6502-open-source/) went open source in September 2025\. [MS-DOS 4.0](https://opensource.microsoft.com/blog/2024/04/25/open-sourcing-ms-dos-4-0/?ref=itsfoss.com) in 2024\. [MS-DOS 1.25 and 2.0](https://devblogs.microsoft.com/commandline/re-open-sourcing-ms-dos-1-25-and-2-0/?ref=itsfoss.com) back in 2018\. There is a clear pattern at this point. If you are into [retro computing](https://itsfoss.com/tag/retro/) or low-level systems work, this is genuinely worth digging into. The source code is compilable, and you will need a copy of Seattle Computer Products' ASM assembler, which you can pull from any 86-DOS or early MS-DOS release. The [GitHub](https://github.com/DOS-History/Paterson-Listings?ref=itsfoss.com) repository's *README* has the necessary steps for you to follow. [DOS 1.00](https://github.com/DOS-History/Paterson-Listings?ref=itsfoss.com) --- **Suggested Read 📖:** [*Someone Turned a PS5 Into a Linux Gaming PC*](https://itsfoss.com/news/run-linux-on-ps5/) [Someone Turned a PS5 Into a Linux Gaming PC, and It Actually WorksIt runs Linux, plays Steam games, and only time will tell how long before Sony DMCAs it.![](https://itsfoss.com/content/images/icon/android-chrome-512x512-395.png)It's FOSSSourav Rudra![](https://itsfoss.com/content/images/thumbnail/linux-on-ps5-banner.png)](https://itsfoss.com/news/run-linux-on-ps5/)