This is a pretty poor description of the problem if you've never heard of it before. The salient information is that there is a single binary switch in the room that can be turned to an 'on' or 'off' position. The prisoners entering the room can observe what the state of the switch is when they enter, and they have the option to flip the switch or not.

Also, one of the prisoners is not a 100-bit accumulator. He or she would be a 7-bit accumulator (128 > 100), but this is a very tortured way to say that this one prisoner can count to 100.