The program counter of my 4-bit processor consists of a pair of 74LS175 circuits to hold 8 bits of data, plus several 74LS244 circuits to control input and output to and from this register. The decoder logic ensures that the program counter latches onto data, or emits data on the correct bus at the appropriate times during instruction cycles. In terms of its function, the program counter is very similar to the accumulator.

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