- there's a standard way for the motherboard Bios to initialise _any_
graphics card, so it can initialise a new one it's never heard of before (and presumably LinuxBios could do the same), or:
this is what is the case.
BIOS has notion of "extensions", which just piece of firmware on each card put into extension slot (ISA,PCI,etc). bios will just jump to that (in 16 bit mode) and execute it.