65. Ripple Carry Adder

// 1-bit Full Adder (to be used by the 4-bit RCA)
module full_adder_1bit (
    input  a, b, cin,
    output sum, cout
);
    wire sum1 = (a ^ b);

    assign sum = sum1 ^ cin;
    assign cout = (a & b) | (sum1 & cin);

endmodule

// 4-bit Ripple Carry Adder – chain 4 full adders
module rca4_chain (
    input  [3:0] a,
    input  [3:0] b,
    input        cin,
    output [3:0] sum,
    output       cout
);
    wire carry0, carry1, carry2, carry3;

    // TODO: Declare internal ripple carries


    // TODO: instantiate 4 full adders and chain carries
    full_adder_1bit fa_inst0 (a[0], b[0], cin, sum[0], carry0);
    full_adder_1bit fa_inst1 (a[1], b[1], carry0, sum[1], carry1);
    full_adder_1bit fa_inst2 (a[2], b[2], carry1, sum[2], carry2);
    full_adder_1bit fa_inst3 (a[3], b[3], carry2, sum[3], carry3);

    // TODO: drive cout
    assign cout = carry3;


endmodule