¹This description of the joint computation of A and B differs slightly from that presented in the textbook. In this description, we assume an environment that copies data from one machine to the other at the point of the control switch. In the textbook, it is assumed that the tapes are merged, i.e., that there is a single input tape that is shared by the two machines, and two shared communication tapes, one for messages from A to B and one for messages from B to A. Even with these shared tapes, each machine has its own read/write head for viewing the contents of the tape.