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Earliest 86-DOS and PC-DOS code released as open source
April 29, 2026
Posted 2 hours ago by
Microsoft is continuing its efforts to release early versions of DOS as open source, and today we’ve got a special one. We’re stoked today to showcase some newly available source code materials that provide an even earlier look into the development of PC-DOS 1.00, the first release of DOS for the IBM PC. A dedicated team of historians and preservationists led by Yufeng Gao and Rich Cini has worked to locate, scan, and transcribe the stack of DOS-era source listings from Tim Paterson, the author of DOS.
The listings include sources to the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and some well-known utilities such as CHKDSK. Not only were these assembler listings, but there were also listings of the assembler itself! This work offers rare insight into how MS-DOS/PC-DOS came to be, and how operating system development was done at the time, not as it was later reconstructed. Stacey Haffner and Scott Hanselman It’s wild that the source code had to be transcribed from paper, including notes and changes. You can find more information about the process on Gao’s website and Cini’s website.
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