And with this setup, the pictures above show three very simple examples of register machines with two registers. The programs for each of the machines are given at the top, with ► representing an increment instruction, and ◄ a decrement-jump. The successive steps in the evolution of each machine are shown on successive lines down the page. The instruction being executed is indicated at each step by the position of the dot on the left, while the numbers in each of the two registers are indicated by the gray blocks on the right.
All the register machines shown start by executing the first instruction in their programs. And with the particular programs used here, the machines are then set up just to execute all the other instructions in their programs in turn, jumping back to the beginning of their programs whenever they reach the end.
Both registers in each machine are initially zero. And in the first machine, the first register alternates between 0 and 1, while the second remains zero. In the second machine, however, the first register again