71. Arithmetic Logic Unit

How do you plan to solve it?

module alu4 (
    input  [3:0] a,
    input  [3:0] b,
    input  [2:0] op,   // 000=ADD, 001=SUB, 010=AND, 011=OR, 100=XOR
    output reg [3:0] y,
    output reg       cf
);
    localparam [2:0]
        ADD = 3'b000,
        SUB = 3'b001,
        AND = 3'b010,
        OR  = 3'b011,
        XOR = 3'b100;

    reg carry;
    reg borrow;

    always @* begin
        y  = 4'b0000;
        cf = 1'b0;
        case (op)
            ADD: begin
                {carry,y }= a + b;
                cf = carry;
            end
            SUB: begin
                {borrow, y} = a + ~b +1;
                cf = borrow;
            end
            AND: y = a & b;
            OR : y = a | b;
            XOR: y = a ^ b;
            default: begin y = 4'b0000; cf = 1'b0; end
        endcase
    end
endmodule