An intrepid code archaeologist has found and uploaded an early ancestor of what became MS-DOS, which later sparked the IBM PC-compatible computer industry.

The newly recovered boot disk contains 86-DOS version 0.1-C, which is the oldest known surviving copy of Seattle Computer Product's operating system. Yes, Seattle Computer Products (SCP) - Microsoft did not write MS-DOS itself. It licensed the OS from SCP and renamed it - then licensed it out again to IBM for the new IBM PC, as we described when recounting the history of DOS on its 30th anniversary.

86-DOS 0.1 was found by flight simulator boffin Gene Buckle, who has long been a vintage code enthusiast - for instance, he also hosts the website and mailing list for FreeGEM, Digital Research's original DOS GUI, open sourced by Caldera in 1999. He told The Reg:

At the moment, he's working through the backlog of eight-inch floppies:

He told us that this wasn't the first old version he found:

And there may be more gems to come:

If the version history on Wikipedia is correct, this in-development version of 86-DOS is only the second to have a version number hung on it, and it's from right around the time SCP gave it the slightly more professional name "86-DOS." Before this, it was known as QDOS, for "Quick'n'Dirty Operating System."

Contrary to many claims, QDOS and 86-DOS were not derived from CP/M source code. Although CP/M was the industry-standard OS for the eight-bit Intel 8080 and Zilog Z80 chips in the 1970s, it didn't support the newer 16-bit Intel 8086 chip. As QDOS's original author Tim Paterson himself describes, CP/M-86 was repeatedly delayed; DR only released an 8086 version in 1982, a year after IBM launched the PC.

Multiple companies were shipping 8086 hardware before IBM entered the market, and SCP was one of them. In 1979, it was already selling an S100 bus board, the SCP-200B. Paterson wrote his OS for this hardware, from scratch, using the API reference in DR's CP/M manual. It wasn't cloned or reverse-engineered - writing code to work with existing APIs is the reason for publishing API documentation.

86-DOS was written to be similar enough to CP/M to make it easy to port 8080 CP/M programs to the 8086. Indeed, 86-DOS 1.0, which was recovered in 2008, even came with an automatic translator called TRANS.COM. 86-DOS didn't use the CP/M disk format, though. Instead, it used a 12-bit version of the eight-bit FAT disk format from Microsoft Standalone Disk BASIC, released in 1978. Appendix H of the product manual [PDF, page 169] describes this as follows:

The OS/2 Museum has an excellent history of 86-DOS called DOS Beginnings, as well as an article about getting version 1.0 running. If you want to see DOS 0.1 in action, Twitter X user NTDEV has a thread with screenshots as well as a Youtube walkthrough:

The history of the various versions of DOS is both complicated and contentious, with a lot of conflicting information (and disinformation) and legal claims and counter-claims in both directions. The recovery of these ancient releases may help to settle some doubts. ®

https://www.theregister.com//2024/01/05/oldest_ancestor_of_msdos_recovered/