Microsoft open-sources "the earliest DOS source code discovered to date"

Microsoft has open-sourced what it calls "the earliest DOS source code discovered to date," reaching further back in its history than ever before. The release includes the kernel source for 86-DOS 1.00, several development snapshots of PC-DOS 1.00, and classic utilities such as CHKDSK. This code predates the MS-DOS brand entirely—86-DOS was originally created by Tim Paterson for Seattle Computer Products before Microsoft licensed it, hired Paterson, and eventually purchased it outright to supply to IBM as PC-DOS and to other PC clone vendors as MS-DOS.

The source was not stored digitally; it existed only on paper printouts. A team of historians and preservationists—the "DOS Disassembly Group" led by Yufeng Gao and Rich Cini—painstakingly transcribed and scanned the code by hand, as modern OCR software could not reliably read the decades-old documents. Microsoft has previously open-sourced MS-DOS versions 1.25 and 2.0 (in 2014 and 2018) and the oddball MS-DOS 4.0 in 2024. All are now available in the same GitHub repository, offering a rare, granular look at the foundations of PC operating systems.