Comment by griffzhowl

Ah, I thought writing a bit was irreversible, because after writing say 1, the previous state could have been a 0 or a 1. But in fact writing a bit should be thought of as the whole process "0 to 1" or "1 to 1", including the initial bit, so that the process is logically reversible. Is that right? Then what I had in mind as an irreversible process of writing would be equivalent to first erasing the bit and then writing the new one.