... that while no memory lock was set, memory fetch followed immediately /EA, so internal/external code memory could be switched on the fly. As soon as some of the locks (I don't remember exatly which - see the "bible") got programmed, /EA was latched upon reset; yet another lock prevented external access altogether, no matter what was /EA's state.

While some of the older "straight" derivatives generally followed this scheme, the number of lockbits and their meaning often changed, so even there it was worth to study the datasheet carefully.

JW