Microsoft open-sources 86-DOS 1.00, the ancestor of Windows

Microsoft just released the source code for 86-DOS 1.00, made available as an open-source project on GitHub. The release marks the 45th anniversary of 86-DOS 1.00, created by Tim Paterson, which would later be licensed to IBM as PC-DOS and then later become MS-DOS.

Microsoft explains the story of early MS-DOS development in this Microsoft Open Source blog post, making it clear that these releases are intended to make historically significant system software accessible for purposes of study, preservation, and curiosity. Microsoft previously released the source for MS-DOS 1.25, 2.11, and 4.0.

In the blog post, Microsoft continues:

But [this] work doesn’t end with a GitHub repo. Software history lives in code, yes, but also in scanned listings, internal documents, assembler printouts, and the sometimes wonderfully analog artifacts of how operating systems came together in the late 1970s and early 1980s. If you read the original announcement around re‑open sourcing MS‑DOS 1.25 and 2.0 on the Windows Command Line blog, you’ll know how much context matters when trying to understand where today’s platforms came from.

For IT historians, this collection is likely to be a real treasure trove:

We’re stoked today to showcase some newly available source code materials that provide an even earlier look into the development of PC-DOS 1.00, the first release of DOS for the IBM PC. A dedicated team of historians and preservationists led by Yufeng Gao and Rich Cini has worked to locate, scan, and transcribe the stack of DOS-era source listings from Tim Paterson, the author of DOS.

It continues:

The listings include 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. Not only were these assembler listings, but there were also listings of the assembler itself! 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.

What Microsoft fails to mention is that Bill Gates didn’t develop MS-DOS 1.00 entirely on his own. Rather, Gates—who in 1980 urgently needed a functioning operating system for the IBM PC—purchased 86-DOS (also known as QDOS) from Seattle Computer Products and its founder Tim Patterson for about $75,000. After Gates made some adjustments to 86-DOS, PC-DOS 1.0 was ready in August 1981.

However, Microsoft retained the rights to this DOS and continued to market it as MS-DOS for other IBM-compatible computers. This laid the foundation for Microsoft’s legendary rise in personal computing. You can read more about this in our history of Windows.

Further reading: How to run DOS classics on Windows 11