1.X. THE GUTS OF A COMPUTERThis section is only for the very curious. You may skip it safely if do not care about the gory details of how computers are built. We start by introducing a simplified sketch for the circuit of Figure 1.1.1. Figure 1.X.1 shows on the left a version that has exactly the same functionality as the circuit of Figure 1.1.1. When switch A closes B also closes. On the right there is an inverter, if switch A closes, B opens.
Using the notation of Figure 1.X.1 we can draw the diagram for an adding circuit. This is shown in Figure 1.X.2. X and Y can have only 0 or 1 as values. The rules of binary addition are shown to the right of the diagram.
If both X and Y are 0, switches B and C will be open, so E will be 0. Switches F and G will also be open so H will be 0. If both X and Y are 1, switches A and D will be open, so E will be 0. But in this case switches F and G will be closed, so H will be 1. If X is 0 and Y is 1, switches C and D will be closed (A and B will be open) so H will be 1. In this case switch F will be open so H will be 0. The case of X equal to 1 and Y equal to 0 is handled similarly.
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