Microsoft Marks 45 Years of DOS by Open-Sourcing Its Oldest-Known Source Code

Microsoft has open-sourced the earliest known DOS source code on GitHub under the MIT license, marking the 45th anniversary of 86-DOS 1.00. The release includes the 86-DOS 1.00 kernel, PC-DOS 1.00 development snapshots, utilities like CHKDSK, and the original assembler. The code originated from physical printouts kept by Tim Paterson — the original author who wrote QDOS in 1980 — which historians had to scan and transcribe into compilable form. This continues a pattern of Microsoft releasing historical code, following MS-DOS 4.0 in 2024 and MS-DOS 1.25/2.0 in 2018.