A serial adder can be implemented from two shift registers, a full-adder, a D flip-flop, and an AND gate (below). What are the changes that should be made to the circuit in order to convert it to a serial subtractor?

Shift control ST Shift register A (Augend) so CLK Serial input So Shift register B Si (Addend) 0 Clear Shift control ST Shift register A (Augend) so CLK Serial input So Shift register B Si (Addend) 0 Clear