For full backwards compatibility EA might be a problem a bit, as it is sampled at reset or at runtime (depending on state of fuses). Even if it would be sampled only at reset, for using it as a general input, the would-be input should be gated during reset; for using it as a general output it could not drive a base of the transistor directly as this would sample as low at reset.

PSEN is less of a problem as it is of the open-drain type and it won't pull down unless external cycle occurs.

It would be possible to configure both (and also ALE; and eventually RESET/XTALs if internal POR/BOD/WD and RC oscillator is present and used) by fuses at programming time, though.

That would leave only the power pins unusable for I/O :-D

Another entry for the "kitchen sink" list :-)

Jan Waclawek

PS. Speaking of "kitchen sinks", I would like to see a WAIT input for external memory access...