is set in the RPB's rpb_variation field. This fixes a bug where machines
(e.g. the 3000/900) would see that they could touch memory where a built-in
PMAGB-BA, assume that it was there, and panic later because the memory they
were accessing didn't look like PMAGB-BA registers (because it wasn't).
handling code so that if a given interrupt is disabled (and therefore
can't have caused the actual I/O interrupt), its handler won't be
called even if its bit is set in the interrupt register.
cgd