Microsoft Open Sources Some of the Earliest MS-DOS Code

Microsoft today announced the release of the source code for he 86-DOS 1.00 kernel, which is among the earliest code for what became MS-DOS.

“We’re stoked today to showcase some newly available source code materials that provide [the earlist yet] look into the development of PC-DOS 1.00, the first release of DOS for the IBM PC,” Microsoft’s Stacey Haffner and Scott Hanselman write. “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 newly open sourced and released code is for the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and CHKDSK and other early DOS utilities.

“Not only were these assembler listings, but there were also listings of the assembler itself,” Microsoft notes. “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.”

This release follows several other similar releases, including the “re-open sourcing” of MS-DOS 1.25 and 2 in October 2018, and MS-DOS 4.0 in 2024. But this time is a bit different. In addition to the source code, Microsoft has also released hand-written notes by original MS-DOS author Tim Patterson, which Microsoft describes as “a timeline of changes, showing which features were implemented when, what errors were made, and how they were fixed.” This is a treasure trove of information about the earliest days of the PC revolution.

You can learn more about this from Yufeng Gao, Rich Cini, and Joshua Scarsbrook.