>The GBA and GBC are quite seperate pieces of hardware - different
>processors, different everything. The GBA just has a GBC built into
>it. So when the GBA detects that the ROM header is that of a GBC's, it
>enables that processor and the GBA effectively becomes a GBC.
>
>Hope that answers your question.

just a addition to your statement:
the GBA use same cartridge connector as the GB/GBC. but with some anti-piracy
chip that mirrors the memory banks or something like that. got no more info
on that as of now