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
);
    // TODO: implement 1-bit full adder (structural or dataflow)
    assign sum = a ^ b ^ cin;
    assign cout = (a & b) | (b & cin) | (cin & a);
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
);
    // TODO: Declare internal ripple carries
    wire [3:0]c;
    // TODO: instantiate 4 full adders and chain carries
    full_adder_1bit fa1(a[0], b[0], cin,  sum[0], c[0]);
    full_adder_1bit fa2(a[1], b[1], c[0], sum[1], c[1]);
    full_adder_1bit fa3(a[2], b[2], c[1], sum[2], c[2]);
    full_adder_1bit fa4(a[3], b[3], c[2], sum[3], c[3]);
    // TODO: drive cout
    assign cout = c[3];
endmodule