69. Binary Subtractor

How do you plan to solve it?

/*Write your code here*/
// module sub4_2c(
//     input [3:0] a,b,
//     output [3:0] diff,
//     output bout
// );

// assign {bout,diff} = a + ~b + 1;

// endmodule

module sub4_2c (
    input  [3:0] a,
    input  [3:0] b,
    output [3:0] diff,
    output       bout
);
    // Width-safe 5-bit add to capture carry-out
    wire [4:0] sum5 = {1'b0, a} + {1'b0, ~b} + 5'b00001;
    assign diff = sum5[3:0];
    assign bout = ~sum5[4]; // borrow = NOT carry-out
endmodule

// module full_adder_1bit(
//     input  a, b, cin,
//     output sum, cout
// );
//     assign sum  = a ^ b ^ cin;
//     assign cout = (a & b) | (b & cin) | (a & cin);
// endmodule

// module rca4_add (
//     input  [3:0] x, y,
//     input        cin,
//     output [3:0] sum,
//     output       cout
// );
//     wire c1, c2, c3;
//     full_adder_1bit fa0(.a(x[0]), .b(y[0]), .cin(cin), .sum(sum[0]), .cout(c1));
//     full_adder_1bit fa1(.a(x[1]), .b(y[1]), .cin(c1 ), .sum(sum[1]), .cout(c2));
//     full_adder_1bit fa2(.a(x[2]), .b(y[2]), .cin(c2 ), .sum(sum[2]), .cout(c3));
//     full_adder_1bit fa3(.a(x[3]), .b(y[3]), .cin(c3 ), .sum(sum[3]), .cout(cout));
// endmodule

// module sub4_2c (
//     input  [3:0] a,
//     input  [3:0] b,
//     output [3:0] diff,
//     output       bout
// );
//     wire [3:0] b_inv = ~b;
//     wire       cout;
//     rca4_add adder(.x(a), .y(b_inv), .cin(1'b1), .sum(diff), .cout(cout));
//     assign bout = ~cout;
// endmodule