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

MS‑DOS (Microsoft Disk Operating System) was the foundational OS for early PCs and the underlying framework for the first versions of Microsoft Windows.

In recent years several legacy versions have been released as open‑source, allowing deeper insight into early computing history. Now the source code for MS‑DOS 4.0, originally released in 1988, is also available to developers.

MS‑DOS 4.0 marked a notable upgrade over earlier releases, adding support for up to 2 GB hard‑disk partitions and an experimental multitasking capability. Microsoft intended a “multitask” edition for some markets, but IBM’s differing requirements prevented widespread adoption.

Although the open‑source release preserves a piece of computing history, it is not particularly useful in practice. Most DOS‑based applications and games require MS‑DOS 5.0 or later, with MS‑DOS 6.22 being the most commonly used final version.

FreeDOS, an open‑source DOS clone under active development since 1994, can run almost all software designed for MS‑DOS.

Microsoft’s earlier releases of MS‑DOS source code were made public a few years ago, and the decision to open‑source this later version is welcomed by the community.

This also raises speculation about whether Microsoft might eventually open‑source early versions of Windows.

MS‑DOS 4.0 was released less than two years before Windows 3.0 (1990), which is often regarded as the first successful Windows release.

For those interested, the source repository is available at https://github.com/microsoft/MS-DOS .