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.…