Is PC DOS the predecessor to MS-DOS, made strictly for IBM? How many versions were there and when did Microsoft change it?
Yes. PC DOS 7.1 was the last version, i.e. it continued development in parallel with MS-DOS. The whole history can be found here [1]. All versions including their amazing (they're really good) manuals are available on WinWorld [2].
Yeah, just to confirm that for v7.1, we got a full source license of it for Symantec Ghost because Microsoft had withdrawn permission for us to distribute anything at all to do with MS-DOS, and we had a lot of customers still using DOS boot disks. Prior to that I believe we had a distribution license for PC-DOS but around that point IBM no longer wanted to maintain it and we had small things we needed to do, so we got the source license and just distributed builds we made as part of the Ghost Boot Wizard disk builder.
What was more interesting from a software archaeology point of view was when I found Symantec had acquired the assets of the then-defunct Quarterdeck; so the source for QEMM and Desqview eventually turned up in the corporate Perforce server and that was certainly fun to read, having made all kinds of things for QEMM back in the day, like a replacement overlay manager for Turbo Pascal (and Turbo C++) that would run overlay code segments directly out of the EMS page frame - this meant pretty much any program we built with the Borland compilers could be turned into a TSR with only the data segment resident.
The one thing that I never did find, though, was the source for the Whitewater Group's Actor. When it was announced that Symantec had acquired those assets from Borland I tried as hard as I could to see if the Actor source code was included but no-one I was ever able to find knew about that. Actor 3.0 was my first introduction to the Smalltalk way of building software, and I still sometimes fire it up in a VM to play enough with its VM format.
Massively predictable comment making noises about Symantec code-dumping QEMM, DESQview and DESQview/X :)
Besides being a lot of fun to read, the latter (which incidentally got some attention on here just a month ago - https://news.ycombinator.com/item?id=29396561) is a whole DOS-native X server. IIUC only Netware's commissioned port of XFree86 comes close to that, but that runs on top of Netware's kernel infrastructure so doesn't really count.
Naturally these sorts of situations sadly represent communicative chasms between geeks and management. Case studies in "it wasn't the end of the world" can perhaps be useful. My standard such example is the HP-20b and HP-30b calculators. On the surface they look just like other calculators HP made around the same time period (2008), but there happen to be a set of pins inside the battery compartment that allow reprogramming the non-bootloader-locked microcontroller inside. An SDK (https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv018.c...) (consisting mostly of a keypad test demo) filled in the remaining pieces (how the keyboard matrix worked, and how to get code running on the ARM CPU, basically), and the result was the WP-34S firmware that reimplemented calculator functionality completely from scratch (the code samples in the SDK provided zero such functionality).
TL;DR, this kind of thing can definitely work and be constrained to a nice little obscure niche of very happy people with low/zero external side effects.
In this case, I wouldn't even mind if the code was released under a similar license to what D used to use (https://archive.ph/20161022202138/https://github.com/dlang/d...) before Symantec relinquished control of the project (https://forum.dlang.org/thread/[email protected]) and it was made fully open-source (https://news.ycombinator.com/item?id=14060846). Microsoft licensing GW-BASIC (https://github.com/microsoft/GW-BASIC) and early versions of MS-DOS (https://github.com/microsoft/MS-DOS) under MIT is... fascinating but understandable, yet entirely unnecessary. "View only with no repercussions but not open source" works for historical interest just fine too.
What is a very good question is whether adequate precedent was set by relinquishing D that it would be tractably viable for say one or two individual campaigners to internally achieve something similar for arbitrary effectively-abandonware software that is only of historical interest.
(If this turns out to not be possible because QEMM and/or DESQview are actively being supported in some way I think the whole retrocomputing scene would be very curious to hear about that through the grapevine!)
Thanks for the interesting info about PC-DOS. That's very cool!