GBA Enhanced GBC Games - Modified Boot Rom?

[Alyosha]

Hi,

I've been adding BIOS support to BizHawk's port of Gambatte, and a user pointed out that GBA enhanced GBC games no longer work after doing so. As is known, register B is supposed to be 1 instead of 0 when booting from a GBA. Presumably the GBA contains a modified GBC Boot Rom for doing this. I looked over the GBC Boot Rom code and didn't see a trivial change that would make B=1 instead of zero after processing, so the Boot Rom in the GBA must be more like a different version.

So, can this version be dumped? Has it been already (and maybe I just can't find it?) Or is something else altogether happening?

Thanks for any insights, as this seems like a loose end to me that could use some clarification.

[endrift]

You are correct that the GBA contains a modified GBC boot ROM. That said, I don't actually know if the GBA version has been dumped.

[Boured]

Due to endrift saying it contains a modified GBC boot ROM. One could theoretically make a program said bootrom if it is able to. Due to the ease of dumping the GBA BIOS this doesn't seem to me like that hard of a task, though I'm not a coder so I wouldn't know for sure.