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.

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