Microsoft has released the earliest known source code for 86-DOS 1.00 on GitHub under the MIT license. The release marks the 45th anniversary of the operating system on April 28. Historians transcribed the code from 1981 printouts held by its creator, Tim Paterson.
On April 28, Microsoft published a blog post announcing the public availability of the 86-DOS 1.00 source code. This version, originally developed by Tim Paterson at Seattle Computer Products in 1980 as QDOS, filled a gap for Intel's 8086 chip, which lacked support from the dominant CP/M operating system. Microsoft acquired the rights for under $100,000 and shipped it to IBM as PC DOS 1.0 in August 1981, while selling it as MS-DOS to other manufacturers. That deal propelled Microsoft to dominate personal computing for decades. Paterson preserved physical assembler printouts and continuous-feed paper from 1981, rather than digital files. Historians Yufeng Gao and Rich Cini scanned and transcribed these into compilable code. The GitHub repository includes the 86-DOS 1.00 kernel, development snapshots of the PC-DOS 1.00 kernel, utilities such as CHKDSK, and Paterson's original assembler. The code is now MIT-licensed and compilable using the ASM assembler from early 86-DOS or MS-DOS releases. Microsoft has followed a pattern of open-sourcing vintage software, including 6502 BASIC in September 2025, MS-DOS 4.0 in 2024, and MS-DOS 1.25 and 2.0 in 2018. Retro computing enthusiasts can follow instructions in the repository's README.