71. Arithmetic Logic Unit

How do you plan to solve it?

/*Write your code here*/
module alu4 (
    input [3:0] a, b,
    input [2:0] op,
    output reg cf,
    output reg [3:0] y
);
    always @* begin
        cf=0;
        case (op)
            3'b000 : {cf,y} = {1'b0,a} + {1'b0,b}; 
            3'b001 : begin
                y = ( a + (~b+1));
                cf = (a < b);
            end
            3'b010 : y = a & b;
            3'b011 : y = a | b;
            3'b100 : y = a ^ b;
            default : y = 4'b0000;
        endcase
    end
endmodule