[svnt]

Erasure is logically irreversible, writing a bit is not. When you erase a bit you compress the logical phase space of the closed system, which means the missing information has to go somewhere — in this case a couple of very low energy phonons into the larger environment.

[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.

[svnt]

Yes exactly, your conception of write includes their conception of erase.

Bennett’s reversible computing is essentially just the way to avoid erasure: whenever you write, keep all the previous states around.