How is this accomplished? In the first place, Simon is a machine, and he behaves during time. He does different things from time to time. His behavior is organized in _cycles_. He repeats a cycle of behavior every second or so. In each cycle of Simon, he listens to or reads the input tape once and he listens to or reads the program tape twice. Every complete instruction that goes on the program tape tells Simon a register from which information is to be sent and a register in which information is to be received. The first time that he reads the program tape he gets the name of the register that is to receive certain information, the _receiving register_. The second time he reads the program tape he gets the name of the register from which information is to be sent, the _sending register_. He finishes each cycle of behavior by transferring information from the sending register to the receiving register. For example, suppose that we want to get an answer out of Simon’s computer into Simon’s output lights. We put down the instruction Send information from _C_5 into _O_ or, more briefly, _C_5 → _O_ But he does not understand this language. We must translate into machine language, in this case punched holes in the program tape. Naturally, the punched holes in the program tape must be able to specify any sending register and any receiving register. There are 15 registers, and so we give them punched hole _codes_ as follows: REGISTER CODE REGISTER CODE _I_ 0001 _C_1 1010 _S_1 0010 _C_2 1011 _S_2 0011 _C_3 1100 _S_3 0100 _C_4 1101 _S_4 0101 _C_5 1110 _S_5 0110 _O_ 1111 _S_6 0111 _S_7 1000 _S_8 1001 To translate the direction of transfer of information, which we showed as an arrow, we put on the program tape the code for the receiving register first—in this case, output, _O_, 1111—and the code for the sending register second—in this case, _C_5, 1110. The instruction becomes 1111, 1110. The first time in any cycle that Simon listens with his right ear, he knows that what he hears is the name of the receiving register; and the second time that he listens, he knows that what he hears is the name of the sending register. One reason for this sequence is that any person or machine has to be prepared beforehand to absorb or take in any information. Now how do we tell Simon to add 1 and 2? On the input tape, we put: Add 00 1 01 2 10 On the program tape, we need to put: _I_ → _C_4 _I_ → _C_1 _I_ → _C_2 _C_5 → _O_ which becomes: 1101, 0001; 1010, 0001; 1011, 0001; 1111, 1110 THE USEFULNESS OF SIMON Thus we can see that Simon can do such a problem as: