only the linker knows the details, so any change would be made to kl.
it's using the same approach for inherently 2-address FP instructions as
it does for 3 address FP/INT instructions where only two operands are specified
(set the third register to one of the others).

i know the change needed but i'm curious whether they've added a trap
check to the hardware that wasn't there before, or whether it's just
something else complaining (but the hardware still doesn't).

when i say i know the change needed, i'm assuming i'm `supposed'
to set the field to 0 (even though that's also a valid register number).
i can't find my sparc handbook today.