Emulator

Do the exception handlers from normal games work for this? Are they always the same, or are there differences? Could you dump those exception handlers when running any gamecube game, and use them for the Datel-AGP stuff?

The ones that regular games install with the SDK are the same as the ones that the IPL installs.
Just ripping the ones from the games/IPL don't seem to work, but they do cause a change in behaviour. Also we wouldn't be able to package them in Dolphin.

Creating a default exception handler that saves the required information to the exception frame is fairly easy to do.

So, the exception handlers are only half of the issue?

And i wasn't thinking about packing the exception handlers in Dolphin, more like dumping them when running a gamecube game. And then add like a 3rd bios/ipl option that copies those exception handlers to memory.

Well, it seems like somebody needs to debug this to find out what exactly is going on

I investigated this further, and it does seem like they do install exception handlers; they just do it in a somewhat broken way.

The function that copies the exception handlers (or more precisely the function that does dcbst after they've been copied) contains a sc instruction instruction, which jumps to the interrupt handler at 00000c00, which is odd because they also are writing the exception handler at 00000c00 (they copy a single exception handler to each of the exception handlers). The sc instruction corresponds to ppcsync() in libogc, which ends up calling this for them. (Libogc's version doesn't exactly match; the official one doesn't save r9 and r10)

After they've set all of the exception handlers once, they overwrite 00000c00 with a separate one that syncs the instruction cache. Using pure interpreter with a Datel title, you can see some icache-related warnings about that address when booting normally, so I think it only works due to icache.

The IPL also does the same thing where it copies a default exception handler over all of the exception handlers (I think; I haven't checked too closely) and then special-cases 00000c00 afterwards. However, they don't use the sc instruction to jump to 00000c00; instead, they use sync and isync directly.

The same applies to retail titles (for both GameCube and Wii); OSInit calls OSExceptionInit. For each address in __OSExceptionLocations, that memcpys OSExceptionVector to the address, then calls DCFlushRangeNoSync, _sync() [i.e. the sync instruction], and ICInvalidateRange. Then OSInit calls __OSInitSystemCall, which special-cases 00000c00 with SystemCallVector. Datel's equivalent to OSExceptionInit instead calls DCFlushRange and doesn't call ICInvalidateRange.

I think that if we provide a default interrupt handler at 00000c00 that does the expected stuff, everything should work fine. And, it would be only 7 instructions, so I don't think it's as big of a deal as needing to provide all of the default exception handlers. I haven't gotten it to work yet though.

After investigating it further, I noticed that the icache message also appears on the jit... but for both the jit and the pure interpreter, it didn't appear when booting via emulated BS2. As it turns out, icache starts disabled when booting booting via emulated BS2, but it's normally enabled when booting from the IPL. Most titles enable icache on startup, but Datel doesn't.

Adding HID0.ICE = true into CBoot::EmulatedBS2_GC is enough to get Datel titles to start! Unfortunately they still don't work quite right; textures are completely missing: https://i.imgur.com/BK0Daad.png (except for titles such as "Ultimate Cheats for use with Enter the Matrix" that do all of their rendering in software, which render fine). So, we don't need to provide a default system call interrupt after all (other than the existing rfi that makes it immediately return)... which I guess makes sense because we ignore the sync and isync instructions and the HID0 ABE (address broadcast enable) bit.

That's weird that they're missing textures now. This is seriously confusing.

Here's a detailed writeup of how Datel's code was broken and the above change fixes it: https://gist.github.com/Pokechu22/abed8faefa0afc6dd881a8958e2407fe