65. Ripple Carry Adder

// Half Adder (to be used by the 1-bit Full Adder)
module half_adder (
    input  a, b,
    output sum, carry
);
    // Write code here
    assign sum = a ^ b;
    assign carry = a & b;
endmodule

// 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)
    wire sum0, c0, c1;
    half_adder ha0 (.a(a), .b(b), .sum(sum0) , .carry(c0));
    half_adder ha1 (.a(sum0), .b(cin), .sum(sum) , .carry(c1));
    assign cout = c0 | c1;
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 c0,c1,c2;

    // TODO: instantiate 4 full adders and chain carries
    full_adder_1bit fa0 (.a(a[0]), .b(b[0]), .cin(cin), .sum(sum[0]), .cout(c0));
    full_adder_1bit fa1 (.a(a[1]), .b(b[1]), .cin(c0),  .sum(sum[1]), .cout(c1));
    full_adder_1bit fa2 (.a(a[2]), .b(b[2]), .cin(c1),  .sum(sum[2]), .cout(c2));
    full_adder_1bit fa3 (.a(a[3]), .b(b[3]), .cin(c2),  .sum(sum[3]), .cout(cout));

    // TODO: drive cout


endmodule