Microsoft open-sources “the earliest DOS source code discovered to date”

Several times in the last couple of decades, Microsoft has released source code for the original MS-DOS operating system that kicked off its decades-long dominance of consumer PCs. This week, the company has reached further back than ever, releasing “the earliest DOS source code discovered to date” along with other documentation and notes from its developer.

Today’s source release is so old that it predates the MS-DOS branding, and it includes “sources to the 86-DOS 1.00 kernel, several development snapshots of the PC-DOS 1.00 kernel, and some well-known utilities such as CHKDSK,” write Microsoft’s Stacey Haffner and Scott Hanselman in their co-authored post about the release.

To understand the context, here’s a very brief history of what would become MS-DOS: Programmer Tim Paterson originally created 86-DOS (previously known as QDOS, for “quick and dirty operating system”) for an Intel 8086-based computer kit sold by Seattle Computer Products. Microsoft, on the hook to provide an operating system for the still-in-development IBM PC 5150, licensed 86-DOS and hired Paterson to continue developing it, later buying the rights to 86-DOS outright. Microsoft then licensed this operating system to IBM as PC-DOS while retaining the ability to sell the operating system to other companies. The version sold by Microsoft was called MS-DOS, and the proliferation of third-party IBM PC clones over the ’80s and ’90s made it the version of the operating system that most people ended up using.