Microsoft releases earliest 86-DOS source code recovered from paper printouts

Microsoft has released the earliest DOS source code ever discovered, marking a milestone in the company's effort to preserve the history of its foundational operating system. The release, announced on the 45th anniversary of 86-DOS 1.00, includes source code that predates the MS-DOS branding altogether, making it older than any previous open-source DOS release by the company.

The newly available materials include sources to the 86-DOS 1.00 kernel, multiple development snapshots of the PC-DOS 1.00 kernel, and utilities such as CHKDSK, according to Microsoft developers Stacey Haffner and Scott Hanselman. The release also includes listings of the assembler itself, providing rare insight into how operating system development was actually conducted during that era, not as it was later reconstructed. This follows Microsoft's previous releases of MS-DOS 1.25 and 2.11 in 2018, and MS-DOS 4.0 in 2024.