Microsoft releases earliest discovered DOS source code, including 86-DOS 1.00 kernel
The code, created by Tim Paterson before Microsoft acquired the rights, was painstakingly transcribed from paper printouts by the DOS Disassembly Group.
Microsoft has announced the release of what it describes as the earliest discovered DOS source code to date. This historic collection predates the MS-DOS branding and includes the 86-DOS 1.00 kernel, alongside development snapshots of the PC-DOS 1.00 kernel and utilities such as CHKDSK. The material also features documentation and notes from the original developer, offering a rare window into the software that underpinned the early personal computing revolution.
The code was originally written by programmer Tim Paterson for an Intel 8086-based computer kit sold by Seattle Computer Products, prior to Microsoft's acquisition of the rights. Microsoft subsequently licensed the operating system to develop software for the IBM PC 5150, later purchasing the rights outright. The system was licensed to IBM as PC-DOS, while Microsoft retained the right to sell it to other manufacturers under the MS-DOS name, a strategy that cemented its dominance in the consumer market during the 1980s and 1990s.
Unlike previous open-sourcing efforts of MS-DOS versions 1.25, 2.0, and 4.0, this release presents material that was never stored digitally. A dedicated team of historians and preservationists known as the DOS Disassembly Group, led by Yufeng Gao and Rich Cini, undertook the arduous task of transcribing and scanning the code from paper printouts provided by Paterson. Modern optical character recognition software struggled with the quality of the decades-old documents, necessitating manual transcription to ensure accuracy.
This announcement marks the latest in a series of open-source initiatives by the company, which has previously released early versions of the operating system and other historical projects. Earlier releases in 2014 and 2018 covered MS-DOS versions 1.25 and 2.0, followed by an oddball release of version 4.0 in 2024. These files are now available in the same GitHub repository, alongside other open-sourced projects such as the Zork game sequels and the 1995 Microsoft 3D Movie Maker.
The release is distinct from a separate rediscovery made just two years prior, when the earliest known version of 86-DOS was found and uploaded to the Internet Archive. While that earlier find provided a glimpse into the system's initial state, this new collection offers a more comprehensive view of the development process, including the kernel and specific utilities that formed the backbone of the platform. The preservation of this code ensures that the technical history of the operating system remains accessible for future study.
