BizHawk
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TASEmulators
Allow loading ROMs with unsupported mappers
BizHawk does not allow any GB games with mappers 0xEA to be loaded. It is not clear if this is intentional or not.
Unsupported mappers not loading is intentional (the core would reject it anyhow), someone has to add in mapper support (which if it's some bootleg mapper might not be easy or might be easy enough), what game is this?
BGB actually ignores this and proceeds to boot into the game (BizHawk does not do this). Also, the game is Sonic 3D Blast 5. The only core that does not reject it is SGB (BSNES) and SameBoy (but the game appears to restart every single frame).
Rejecting unsupported mappers will not change for Gambatte (can't speak for GBHawk). This game looks like some bootleg game, probably a simple mapper/clone of an official mapper. Probably isn't too hard to support.
That makes no sense, because as soon as we change mapper for example, to 0x01, then force-change it to 0xEA, the game will load without issues, but after some time, BizHawk runs into an invalid operation and will crash. But if we load the ROM with mapper 0xEA already, it won't load. There has to be an issue with our handler. This is the ONLY workaround here, and there is no way to load it without using cheats at all. Sonic Adventure 7, on the other hand, is hardcoded to use an official mapper instead.
"freeze it to 0xEA"
So you changed it after the mapper is set. Gambatte isn't constantly checking that address and changing the mapper midgame, that's insane. And your example would be making it use the MBC1, which is probably wrong and thus no duh it crashed.
Because of this issue, it is impossible to TAS this game, even though it is requried to beat without deaths.