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,
    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;
    wire [4:0] add5 = {1'b0,a} + {1'b0,b};
    wire [4:0] sub5 = {1'b0,a} + {1'b0,~b} + 5'b1;
    always @(*) begin
        y = 4'b0000;
        cf = 1'b0;
        case (op)
            ADD : begin
                y = add5 [3:0];
                cf = add5[4];
            end
            SUB : begin
                y = sub5[3:0];
                cf = ~sub5[4];
            end
            AND : y = a & b; 
            OR  : y = a | b;
            XOR : y = a ^ b;
            default : begin 
                y = 4'b0000;
                cf = 0;
            end
        endcase
    end
endmodule