The difference between doze and nap is that in nap mode
the bus interface unit is switched off; on the 970 and
970FX this means that the L2 cache has to be flushed
before entering nap mode (or otherwise snoops will go
missing in action); 

BZZZT... incorrect... the 970FX and MP behave the same here. There is
only one state you can "manually" enter and it's NAP. Transitions
between DOZE and NAP are automatic to handle snooping in both cases.
Cache flushing is never needed ... unless in our case the CPU gets
actually powered down for suspend-to-ram.

the 970MP automatically transitions
from nap to doze mode if snoops are requested (and back
to nap again), so no L2 flush is required.  The 970MP
also has a "deep nap" mode which saves more power by
automatically clocking the CPU down.

Also deep nap only really works on 970MP DD3 and later I think

One more thing -- perhaps keeping the CPU in reset
state saves even more power?

Dunno... Somebody can try... but I don't think we have access to the
reset line on 970's..

Ben.