These instructions are defined as taking a GPR register and a
coprocessor register for ISAs up to MIPS32. MIPS32 extended the
definition to allow a selector--a value from 0 to 32--to access
These instructions are now internally defined as being MIPS-I
instructions, but are rejected for pre-MIPS32 ISA's if they have
an explicit selector which is non-zero. This deviates slightly from
GAS's behaviour which rejects assembly instructions with an
explicit selector for pre-MIPS32 ISAs.
mfc0 $4, $5, 0
is rejected by GAS for MIPS-I to MIPS-V but will be accepted
with this patch for MIPS-I to MIPS-V.