Microsoft has released what it calls "the earliest DOS source code discovered to date," going further back than any previous source code the company has open-sourced. The release includes the 86-DOS 1.00 kernel, several development snapshots of PC-DOS 1.00, and utilities like CHKDSK. This code predates the MS-DOS branding entirely.

Programmer Tim Paterson originally created 86-DOS (originally called QDOS, for "quick and dirty operating system") for an Intel 8086-based computer kit sold by Seattle Computer Products. Microsoft, needing an operating system for the upcoming IBM PC 5150, licensed 86-DOS and hired Paterson to continue developing it before buying the rights outright.

Microsoft then licensed the operating system to IBM as PC-DOS while retaining the ability to sell it to other companies. The version Microsoft sold directly became known as MS-DOS, and the explosion of IBM PC clones throughout the 1980s and 1990s made it the version most people actually used. This release gives developers and historians a rare look at the foundation of the PC era.