[coreboot] why is firmware 32 bit as opposed to 64 bit

[John de la Garza]

I understand that the calling functions in 32 bit C uses the stack and
this is why coreboot needs to use cache as RAM.  Doesn't 64 bit C use
registers to pass arguments to functions?  If this is the case why not
run in 64 bit mode?

Also, even if cache as RAM is used and a stack is available, why not just
build a 64 bit binary?  What are the advantages to using a 32 bit binary?