# Microsoft Open-Sources DOS 1.0, Releasing Original Source Code and Annotations

_Tuesday, April 28, 2026 at 4:22 PM EDT · Tech & Business · Latest · Tier 1 — Major_

![Microsoft Open-Sources DOS 1.0, Releasing Original Source Code and Annotations — Primary](https://www.zdnet.com/a/img/resize/421d413bff3d911ad05d1c966ea002ac112e2a6f/2024/04/26/da42efe7-0b43-4f98-a19c-c4aa2ebfde65/ms-dos-4-23.jpg?auto=webp&fit=crop&height=675&width=1200)

Microsoft has open-sourced the original source code for PC-DOS 1.00, the operating system that launched the company toward its dominant position in personal computing.

Before Microsoft became a household name, Bill Gates wrote BASIC interpreters and the company's first shipping operating system was a Unix distribution called Xenix. In 1980, IBM needed an operating system for its planned IBM PC and turned to Gates. Microsoft's AT&T Unix license did not allow porting Xenix to the x86 architecture, so the company purchased 86-DOS, also known as QDOS, from Seattle Computer Products and inventor Tim Patterson for just under $100,000.

That purchase became the foundation for PC-DOS 1.00, which in turn led to MS-DOS and eventually Windows. The newly released code includes not only the original source but also annotations and historical context about the operating system's earliest days.

Microsoft's open-source release provides researchers with a concrete reference point for the tangle of early DOS builds, version numbers, and OEM releases like MS-DOS 1.25. The company has continued to embrace open source in recent years, and this release offers a direct look at the software that helped establish Microsoft's place in computing history.

## Sources

- [ZDNet](https://www.zdnet.com/article/microsoft-open-sources-dos-1-0-much-more-than-the-code/)

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Publisher: Tech & Business (techandbusiness.org)