Microsoft has made available the earliest-known version of the DOS operating system via an open-source GitHub repository. This follows the open-sourcing of MS-DOS 1.25 and 2.x in 2018, as well as the release of the MS-DOS 4.0 source in 2024. The version being issued in 2026 actually predates the MS-DOS label — it is 86-DOS 1.00, and its release has been coordinated with the software’s 45th anniversary. This difference in naming suggests that Microsoft did not create MS-DOS entirely independently — instead, it acquired the rights to 86-DOS (initially known as “QD-DOS,” which stands for “Quick and Dirty”) and brought on board its developer, Tim Paterson, during this acquisition.
Upon acquiring the rights to 86-DOS, Microsoft rebranded the software, leading to the establishment of MS-DOS in July 1981. This became the foundation for PC DOS 1.0, which Microsoft modified for the IBM Personal Computer 5150 that launched in August 1981. Subsequently, Microsoft began licensing MS-DOS to other manufacturers of IBM-compatible PCs such as Compaq and Zenith. These iterations of DOS (disk operating system) preceded Windows and functioned as the core operating system until Windows XP, effectively running atop MS-DOS or PC DOS and providing a graphical user interface for system functions instead of relying solely on the command line.
As noted by Scott Hanselman, who facilitated the release of these vintage operating systems for public experimentation, the “earliest DOS source code” was discovered in Paterson’s garage. These printouts featured handwritten annotations, offering a glimpse into the development processes of the 1980s, prior to Microsoft features becoming prevalent. In theory, one could rewrite all of this in assembly language and create a personal version of DOS.