Why Microsoft Finally Open‑Sourced MS‑DOS 4.0 After 36 Years

For decades, enthusiasts wondered why Microsoft never open‑sourced its legacy operating systems, assuming they had no commercial value. In April 2024, prompted by researcher Connor “Starfrost” Hyde, Microsoft and IBM finally released MS‑DOS 4.0 on GitHub ( https://github.com/microsoft/MS-DOS) under the MIT license, including source code, binaries, disk images and documentation.

The release stems from a correspondence between former Microsoft CTO Ray Ozzie and Hyde, who discovered unpublished DOS 4.0 test binaries on a floppy collection. After contacting Microsoft’s Open Source Program Office, the team imaged the original disks, scanned accompanying printed materials, and made the artifacts publicly available.

MS‑DOS 4.0, first launched in 1988, is nearly 36 years old. It was part of a complex history where Microsoft and IBM co‑developed code, and a “multitask DOS” (MT‑DOS) branch existed but never saw wide distribution. The newly released repository contains the complete MIT‑licensed source, allowing anyone to modify or redistribute the OS.

Microsoft demonstrated that the released OS runs on original IBM PC XT hardware, modern Pentium machines, and emulators such as PCem and 86Box. The GitHub repository has already attracted over 26 k stars and 3.6 k forks, indicating strong community interest.

This is not Microsoft’s first foray into DOS open‑source. In 2014, MS‑DOS 1.25 and 2.0 were added to the Computer History Museum, and the full MS‑DOS source was re‑released on GitHub in 2018. Other DOS variants, like PC‑MOS/386, have also been open‑sourced.

The article also revisits the broader DOS timeline: Tim Paterson’s 1980 creation of QDOS to fill a gap for the Intel 8086, its acquisition and renaming to MS‑DOS, the partnership with IBM, and subsequent versions (2.0, 3.0, 4.0). While MS‑DOS 4.0 introduced features such as expanded memory support and mouse‑enabled directories, it suffered from compatibility issues and high memory usage, leading many users to prefer later systems like DR‑DOS or the emerging Linux kernel.

Ultimately, the open‑sourcing of MS‑DOS 4.0 provides a valuable resource for OS historians, developers studying early assembly‑level code, and hobbyists exploring the roots of modern computing.