Microsoft releases earliest known DOS source code to GitHub
The release of the 86-DOS 1.00 kernel and PC-DOS 1.00 snapshots predates the MS-DOS brand, offering a rare glimpse into the operating system’s origins before Microsoft’s acquisition of the rights.
Microsoft has open-sourced the earliest known source code for the Disk Operating System, marking a significant milestone in the preservation of early computing history. The release includes the 86-DOS 1.00 kernel, development snapshots of PC-DOS 1.00, and utilities such as CHKDSK. This collection predates the MS-DOS branding that would later define the company’s dominance in the personal computer market.
The code was originally developed by Tim Paterson for Seattle Computer Products, where it was known as 86-DOS or QDOS. Microsoft licensed the operating system to support the IBM PC 5150, hired Paterson to refine the software, and subsequently purchased the rights outright. Microsoft then licensed the OS to IBM as PC-DOS while selling it to other manufacturers under the MS-DOS name.
Because the code dates back to a period before digital storage was standard, it was not preserved in electronic format. A team known as the DOS Disassembly Group, led by Yufeng Gao and Rich Cini, transcribed and scanned the source code from paper printouts provided by Paterson. The team noted that modern optical character recognition software struggled with the quality of the decades-old documents, necessitating manual transcription.
The source code is now available in Microsoft’s GitHub repository, alongside other early software projects the company has previously released. This follows earlier open-sourcing efforts for MS-DOS versions 1.25 and 2.0 in 2014 and 2018, as well as MS-DOS 4.0 in 2024. Other archived projects include the game Zork and Microsoft 3D Movie Maker.
Microsoft’s Stacey Haffner and Scott Hanselman announced the release in a co-authored blog post, highlighting the effort by historians and preservationists to maintain access to this foundational software. The release adds to the historical record, following the rediscovery of an earlier version of 86-DOS on the Internet Archive two years ago.
