Now so far the computing registers in Simon are a mystery. We have said that _C_1, _C_2, and _C_3 take in numbers 00, 01, 10, 11, that _C_4 takes in an operation 00, 01, 10, 11, and that _C_5 holds the result. What process does Simon use so that he has the correct result in register _C_5? Let us take the simplest computing operation first and see what sort of a circuit using relays will give us the result. The simplest computing operation is _negation_. In negation, a number 00, 01, 10, 11 goes into the _C_1 register, and the operation 01 meaning negation goes into the _C_4 register, and the correct result must be in the _C_5 register. So, first, we note the fact that the _C_4-2 relay must not be energized, since it contains 0, and that the _C_4-1 relay must be energized, since it contains 1. Now the table for negation, with _c_ =-_a_, is: _a_ | _c_ ————+———— 0 | 0 1 | 3 2 | 2 3 | 1 Negation in machine language will be: _a_ | _c_ ————+———— 00 | 00 01 | 11 10 | 10 11 | 01 Now if _a_ is in the _C_1 register and if _c_ is in the _C_5 register, then negation will be: _C_1 | _C_5 —————+————— 00 | 00 01 | 11 10 | 10 11 | 01 But each of these registers _C_1, _C_5 will be made up of two relays, the left-hand or 2 relay and the right-hand or 1 relay. So, in terms of these relays, negation will be: _C_1-2 _C_1-1 | _C_5-2 _C_5-1 ——————————————+—————————————— 0 0 | 0 0 0 1 | 1 1 1 0 | 1 0 1 1 | 0 1 Now, on examining the table, we see that the _C_5-1 relay is energized if and only if the _C_1-1 relay is energized. So, in order to energize the _C_5-1 relay, all we have to do is transfer the information from _C_1-1 to _C_5-1. This we can do by the circuit shown in Fig. 6. (In this and later diagrams, we have taken one more step in streamlining the drawing of relay contacts: the contacts are drawn, but the coils that energize them are represented only by their names.) [Illustration: FIG. 6. Negation—right-hand digit.] [Illustration: FIG. 7. Negation—left-hand digit.] Taking another look at the table, we see also that the _C_5-2 relay must be energized if and only if: _C_1-2 AND _C_1-1 HOLDS: HOLDS: 0 1 1 0 A circuit that will do this is the one shown in Fig. 7. In Fig. 8 is a circuit that will do all the desired things together: give the right information to the _C_5 relay coils if and only if the _C_4 relays hold 01. [Illustration: FIG. 8. Negation circuit.]