Microsoft has released source material described as the earliest DOS code yet discovered, extending its public archive to software created before the MS-DOS brand existed.
According to Ars Technica's account, the release includes source for the 86-DOS 1.00 kernel, several development versions of the PC-DOS 1.00 kernel and familiar utilities including CHKDSK. Microsoft employees Stacey Haffner and Scott Hanselman characterized the collection as the earliest DOS source code found to date.
The material predates Microsoft's ownership of the operating system. Programmer Tim Paterson developed 86-DOS, which had previously been called QDOS, for an Intel 8086 computer kit sold by Seattle Computer Products. Microsoft needed an operating system for IBM's PC 5150 project, licensed 86-DOS and brought Paterson in to continue work before purchasing the rights.
That arrangement became an important part of personal-computer history. Microsoft licensed the system to IBM under the PC-DOS name while preserving the ability to supply it to other manufacturers as MS-DOS. As IBM-compatible computers multiplied through the 1980s and 1990s, Microsoft's version became the one encountered by a large share of users.
Recovering the newly released code required more than finding an old disk. The source was held in paper printouts supplied by Paterson and had not survived in readily usable digital form. A group of historians and preservationists led by Yufeng Gao and Rich Cini, working as the DOS Disassembly Group, transcribed the listings and scanned the documents. The age and condition of the printouts made modern optical character recognition unreliable, increasing the need for manual preservation work.
The release joins earlier Microsoft efforts to publish historic software. The company made MS-DOS 1.25 and 2.0 source available in 2014 and revisited that release in 2018; MS-DOS 4.0 followed in 2024. Those versions are kept in the same GitHub repository, according to the report. Other projects made public by Microsoft have included the adventure game Zork and its sequels and the source for Microsoft 3D Movie Maker.
The archive has practical value even though it is unlikely to become part of modern production software. Source listings let historians and students inspect how early PC operating systems were organized, compare development snapshots and trace the relationship between 86-DOS, PC-DOS and MS-DOS. They also preserve details that might otherwise remain inaccessible on aging paper.
The discovery is part of a continuing reassessment of early DOS history. Ars Technica notes that an early known version of 86-DOS was also rediscovered and uploaded to the Internet Archive two years earlier. This latest publication broadens that record by exposing the underlying source and its evolution during a foundational period for the IBM-compatible PC.
