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Microsoft Open Sources Earliest DOS Code on Anniversary
Microsoft on Tuesday released the earliest known DOS source code materials found to date to mark the 45th anniversary of 86-DOS 1.00.
The new software preservation effort announced on the Microsoft Open Source Blog includes source listings for the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel and early utilities, including CHKDSK. The materials were preserved by original DOS author Tim Paterson and offer a glimpse into how early personal computer operating systems were built and revised.
"Today, on 86-DOS 1.00's 45th anniversary, we're continuing that tradition by preserving the earliest DOS source code discovered to date," said Microsoft, in the blog post. "These releases are about making historically important systems software available for study, preservation, and plain ol' curiosity."
The latest release builds on Microsoft's broader software preservation work. In 2018, the company made MS-DOS 1.25 and 2.11 available as open source. It followed that in 2024 with the release of MS-DOS 4.0. This week's release goes further back, reaching into the earliest stages of DOS' development.
DOS, short for Disk Operating System, became one of the central pieces of the early PC market. Its origins trace to Paterson's work at Seattle Computer Products, where he developed 86-DOS, originally known as QDOS, or Quick and Dirty Operating System, for Intel's 8086 processor.
Microsoft acquired 86-DOS in 1981 and adapted it into MS-DOS for IBM's first PC. The operating system quickly became a key part of the IBM PC-compatible ecosystem and helped establish Microsoft's position in the software market through the 1980s and early 1990s.
The new materials are not limited to source files. Microsoft said the release includes scanned listings, internal documents, assembler printouts and other dev artifacts from the period.
"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," the post said.
According to Microsoft, the listings show more than final product code. They also capture interim development states, hand-written notes and changes made over time. The company described the materials as something close to a printed commit history, showing when features were added, what errors appeared and how they were fixed.
A team led by Yufeng Gao and Rich Cini worked to locate, scan and transcribe the source listings. The original physical materials, donated by Paterson, are expected to be made available for viewing at the Interim Computer Museum.
The source code and scanned listings are now available through the DOS-History/Paterson-Listings GitHub repository under the MIT license. The release makes the materials available to researchers, hobbyists and others interested in the early history of PC operating systems.