design a 4-bit serial adder (VHDL code or schematic) which includes two shift registers and a single full-adder to perform the following functionality with proper timing controls. Two 4-bit inputs A and B will be set by eight switches on your Xilinx board. Inputs S will control two modes of operation, when S=0 your circuit will perform a serial addition of A and B,A+B,and when S=1 output will be subtraction of B from A,A-B When the 4-bit inputs A and B, and S switch is ready, pressing a pushbutton R will start operation and without further inputs the result will be ready and stable on one of the 7-segment displays. Changing the inputs A,B and S will not change the result on the 7-segment display until the pushbutton R is pressed again.