65. Ripple Carry Adder

How do you plan to solve it?

//==================================================
// Module: rca4_chain
// Description: 4-bit Ripple Carry Adder using 1-bit full adders
//==================================================

module rca4_chain (
    input  wire [3:0] a,
    input  wire [3:0] b,
    input  wire       cin,
    output wire [3:0] sum,
    output wire       cout
);

    // Internal carry signals
    wire c1, c2, c3;

    // ------------------------------------------------
    // Instantiate 4 full adders connected in ripple form
    // ------------------------------------------------
    full_adder_1bit fa0 (.a(a[0]), .b(b[0]), .cin(cin), .sum(sum[0]), .cout(c1));
    full_adder_1bit fa1 (.a(a[1]), .b(b[1]), .cin(c1),  .sum(sum[1]), .cout(c2));
    full_adder_1bit fa2 (.a(a[2]), .b(b[2]), .cin(c2),  .sum(sum[2]), .cout(c3));
    full_adder_1bit fa3 (.a(a[3]), .b(b[3]), .cin(c3),  .sum(sum[3]), .cout(cout));

endmodule


//==================================================
// Submodule: 1-bit Full Adder
//==================================================
module full_adder_1bit (
    input  wire a,
    input  wire b,
    input  wire cin,
    output wire sum,
    output wire cout
);
    assign sum  = a ^ b ^ cin;               // Sum = XOR of all inputs
    assign cout = (a & b) | (b & cin) | (a & cin);  // Carry-out
endmodule