The Sega genesis game: Joe and Mac Caveman Ninja crashing on the title screen

[TomateThree]

The game crashes on the title screen, in add files i gonna show a screen shot of the momment when this ocurrs.
https://github.com/user-attachments/assets/cad8a742-209e-4f89-a9fc-2d4c3b5a1323

[jsgroth]

I see why it's crashing but I'm going to have to dig in more to figure out the root cause. Seems like possibly a Z80 CPU bug.

Right before the crash, the Z80 executes a RET that jumps to a non-code address, and after executing some nonsense instructions it executes a HALT with interrupts disabled. The 68000 then gets into an infinite loop waiting for the Z80 to set a flag in audio RAM, which never happens because the Z80 is no longer running.

[jsgroth]

I think I figured this out. It wasn't a Z80 bug - this game seems to depend on the 68000 not being allowed to write to Z80 audio RAM unless the Z80 is running (i.e. not reset) and removed from the bus.

While the game is booting, the 68000 performs some stray writes to audio RAM without removing the Z80 from the bus, and the game depends on those writes being ignored. Otherwise the writes will corrupt the game's audio driver code which eventually causes the Z80 to halt and the game to crash. I'm surprised this never caused problems in anything else I tested...

This will be fixed in the next release, thanks for reporting this!