Wow nice. I only really saw 2.22 much so far, as well as 2.41 and 2.43. THere was also 0.41 beta or something. One other rare OS was 1.17, but it's for the 83+/83+SE. It never got released on TI site, I think, just on some calcs.

My TI CD has OS 1.13 on it, I believe. I don't remember what TI-73, TI-89 and 92 OS it has, though. I got TI-CD version 2.1 from March 2001.

Didn't you grab 1.50 from TI-Flash Debugger ROM then use that tool to convert it to an OS file? You had me testing it on my TI-73 a while ago. I think my calc still have it installed.

Do you mean like running a TI-83 on a TI-83+?

Yeah, I guess you could.  It would take some effort to get the OS to run properly on an 83+, but it could be done.  (Of course, some assembly programs wouldn't work properly, but many of them probably would.)

Yeah, I guess you could.  It would take some effort to get the OS to run properly on an 83+, but it could be done.  (Of course, some assembly programs wouldn't work properly, but many of them probably would.)

In theory, what would have to be changed in order to make a TI-83 OS run on a TI-83+ hardware/emulator?

It's true that memory addresses are different, but that's not the issue here, since we're talking about replacing the entire OS.

The most significant change between the 83 and 83+ is how the memory mapping is controlled.  The 83's hardware is based on the 82's, so it has a single port (port 2) that controls both the "bank A" and "bank B" memory mapping.  But since that only allows for 8 ROM pages, port 0 is used to switch between the lower and upper 128k of ROM.  In contrast, the 83+ has two separate ports (6 and 7) that independently control the mapping for banks A and B, and allow you to access all 32 pages of Flash.  So all of the code dealing with port 2 would need to be revised to use port 6 and/or 7.  (There actually isn't very much code you'd need to change.)

Second, the link port works differently; both models use port 0 to read and write the link port, but different input and output values are used.

Third, where the 83 reads port 3 to check the status of the ON key and interrupts, on the 83+ you need to use port 4 instead.  I think the bits are the same, and writing to port 3 works the same way (writing to port 4 does something completely unrelated.)

Fourth, the 83+ prevents execution on RAM page zero by default; you'd probably want to turn this protection off in order to run TI-83 assembly programs.  On an 83+ SE or 84+, this protection can't be disabled, but you could use RAM page 3 instead.

And finally, you'd need to make some small tweaks just to get the OS to boot, since 83+ OSes must contain a magic number at 0056h, and the boot code hands control to the OS at address 0053h.

That's very interesting.  I wonder if the Flash chip is actually upgradeable.  Going by this image from Datamath, it looks like the write-enable pin (pin 11) is in fact connected to the ASIC, so it's possible.  But it's hard to imagine that they would have put user-upgradeable Flash in a calculator without including an 83+-style boot code.