An anonymous reader quotes a report from Ars Technica: 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. [...] This source code is old enough that it hadn't been stored digitally.…