The two sets of D flip-flops, the 4 at the top and the for at the middle carry the two operands so you must load them first , to do so, change the bit connected to the gates of the mosfets to '1' and load your desired values from the above 4 inputs by changing the input connected to the clock input from 0-->1 (positive edge triggering), do the same thing for the lower set of 4 registers.
after you have done that set the gates of the MOSFETS to 0 and clock the flip-flops five times, then the answer will be in the bottom register.....
This circuit would operate better if gates were used instead of the MOSFETS but the workplace is so tiny.....
