By Andrew Cunningham
Publication Date: 2026-04-30 14:20:00
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…
