For some reason, the other two Game Gear games that I have on my computer work just fine. It's just that Aerial Assault (W) won't load. Don't know if anybody else has this problem.
As soon as I load it, I get nothing. No sound, no Sega screen at the beginning...nothing. Nothing happens. And I'm getting the checkmark in the lower-left corner, so I know it's a good dump.
Anybody else able to recreate this?
EDIT: Oddly enough, Aerial Assault (J) (Rev 1) works perfectly okay.
Ok, I was wrong about that. The game does set the 'ROM write enable' flag, but the root cause of the problem is that it doesn't set the stack pointer before starting to execute code. A 'CALL' instruction then overwrites the ROM bank. This happens since I initialize the stack pointer to zero, then it gets decremented to 0xFFFF (ROM bank register.)
I tried looking for documentation about where the stack pointer gets intiialized to, but I don't see it actually specified anywehere. 0x0 and 0xFFFF both crash the game, so it can't be those. It's possible that's it's just some unspecified value.
If this were the case, this game would function by pure chance on real hardware. The game does need to be able to return to the proper address after the call, so the stack pointer does have to be in writable memory.
This is very strange. My guess is that during development, they didn't realize the mistake they made since they left the ROM write anbel flag set, meaning the stack pointer could start anywhere and still be in writable memory on their dev boards, thus masking the problem.
Or it really is initialized to something internally and I just can't find what. The documentation does specify that the only register actually set is PC (to zero) so who knows what actually happens.
Anyway, I added a game specific hack to make the game run by putting the stack pointer in RAM initially. I don't particularly like it, but it does work now.
Ok, I was wrong about that. The game does set the 'ROM write enable' flag, but the root cause of the problem is that it doesn't set the stack pointer before starting to execute code. A 'CALL' instruction then overwrites the ROM bank. This happens since I initialize the stack pointer to zero, then it gets decremented to 0xFFFF (ROM bank register.)
I tried looking for documentation about where the stack pointer gets intiialized to, but I don't see it actually specified anywehere. 0x0 and 0xFFFF both crash the game, so it can't be those. It's possible that's it's just some unspecified value.
If this were the case, this game would function by pure chance on real hardware. The game does need to be able to return to the proper address after the call, so the stack pointer does have to be in writable memory.
This is very strange. My guess is that during development, they didn't realize the mistake they made since they left the ROM write anbel flag set, meaning the stack pointer could start anywhere and still be in writable memory on their dev boards, thus masking the problem.
Or it really is initialized to something internally and I just can't find what. The documentation does specify that the only register actually set is PC (to zero) so who knows what actually happens.
Anyway, I added a game specific hack to make the game run by putting the stack pointer in RAM initially. I don't particularly like it, but it does work now.
So it's the game doing this, and not the emulator? Well, this is interesting...
Edit: Forgot it was the game the whole time. Whoops.
http://z80.info/interrup.htm
This page says that SP = AF = 0xFFFF at reset.
Implementing this does indeed make the game work. So I guess I can remove that hack afterall, problem solved!
http://z80.info/interrup.htm
This page says that SP = AF = 0xFFFF at reset.
Implementing this does indeed make the game work. So I guess I can remove that hack afterall, problem solved!
