Microsoft opens door to the past by releasing 86-DOS and PC-DOS 1.00

Back to a time when source repositories were printouts and commits were hand-written notes

Antiques code show Microsoft has released the source for another of its relics. This time, it's 86-DOS 1.00 getting the open source treatment, and a whole lot more for retro enthusiasts.

The release comes a year after Microsoft set the source for MS-DOS 4.0 free, but it includes a lot more than just the bones of an old operating system.

According to a post by Microsoft's Stacey Haffner and Scott Hanselman, "The listings include 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."

"Not only were these assembler listings, but there were also listings of the assembler itself!"

It is quite the treasure trove of information, and a throwback to an era when printouts were your friend for storing source code. "Software history lives in code, yes," acknowledged the pair, "but also in scanned listings, internal documents, assembler printouts, and the sometimes wonderfully analog artifacts of how operating systems came together in the late 1970s and early 1980s."

In a post on Bluesky, Hanselman wrote, "The earliest DOS source code was found on printer paper in Tim Paterson's garage."

The printout predates tools like GitHub by decades. Where a Git repository might have a commit history, the printouts "create a timeline of changes, showing which features were implemented when, what errors were made, and how they were fixed."

86-DOS was released in 1980 and was the brainchild of Tim Paterson. Microsoft licensed it and eventually purchased all rights in 1981, shortly before the release of the IBM PC. Various lawsuits were launched in subsequent years over the deal, and some alleged similarities to CP/M, but the key thing here is the insight the material provides into the development of PC-DOS 1.00.

Hanselman, Haffner, and the rest of the team behind the release are to be commended for their effort in dusting off the source and painstakingly scanning in and transcribing a stack of Paterson's printouts and notes.

How much practical use the first version of DOS will have outside the retro enthusiast community is debatable, though it is a fascinating insight into how the operating system came to be and highlights a time when code was crammed into the smallest space possible, while retaining utility and efficiency.

Lessons for modern programmers, then, as well as an invaluable resource for IT historians. ®