Part III

You are to design a circuit that adds the two BCD digits. The inputs to your circuit are the numbers X and Y, plus a carry-in, cin. You should use the four-bit ripple-carry adder/subtractor (RCAS4) circuit in Figure 4 below to produce a four-bit sum and carry-out for the operation X + Y. This result is to be displayed on 7-segment displays as a two-digit sum S1S0. For a sum equal to zero you would display S1S0 = 00, for a sum of one S1S0 = 01, for nine S1S0 = 09, for ten S1S0 = 10, and so on. Note that the inputs X and Y are assumed to be decimal digits, which means that the largest sum that needs to be handled by this circuit is S1S0 = 9 + 9 + 1 = 19.