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

Microsoft has just released the source code for 86-DOS 1.00, made available as an open source project on GitHub. This 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 later became MS-DOS.

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

In the blog post, Microsoft continues:

But [this] the work doesn’t end with a GitHub repository. The history of software lies in the code, yes, but also in the scanned lists, internal documents, assembler printouts, and sometimes wonderfully analog artifacts of how operating systems were created in the late 1970s and early 1980s. If you read the original announcement about the reopening of MS‑DOS 1.25 and 2.0 on the Windows Command Line blog, you’ll know how much context there is important when trying to understand where current platforms come from.

For computer historians, this collection is likely to constitute a real treasure:

Today we are excited to introduce some newly available source code documents that provide an even earlier look at the development of PC-DOS 1.00, the first version of DOS for the IBM PC. A dedicated team of historians and curators led by Yufeng Gao and Rich Cini worked to locate, digitize, and transcribe the stack of DOS-era source lists from Tim Paterson, the author of DOS.

He continues:

The lists include the 86-DOS 1.00 kernel sources, several PC-DOS 1.00 kernel development snapshots, and some well-known utilities such as CHKDSK. Not only were these lists of assemblers, but there were also lists of the assembler itself! This work provides a rare insight into the birth of MS-DOS/PC-DOS and how operating system development was carried out at the time, not as it was later reconstructed.

What Microsoft fails to mention is that Bill Gates did not develop MS-DOS 1.00 entirely alone. Gates, who in 1980 urgently needed a working operating system for his IBM PC, instead purchased 86-DOS (also known as QDOS) from Seattle Computer Products and its founder Tim Patterson for approximately $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 Windows History.

Further reading: How to Run DOS Classics on Windows 11