This is the design (done in circuit simulator) for the program counter. Essentially it adds one to the number stored in the D flip-flops every clock cycle. the number that the D flip-flops are feeding back into the half adders can be bypassed by the multiplexers, this allows the computer to feed a new address into the counter. The ability to reload the program counter makes jumps (loops) in code possible, by making a jump conditional 'if' statements can be implemented in code.

This is the implementation of the above diagram. This is only a quarter of the completed program counter. This has 29 transistors so the completed program counter will add 116 transistors to the computer. The details page has more info on the program counter.