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
);

always @(*) begin
    case (op)
        3'b000: {cf, y} = a + b;        
        3'b001: begin y=a+~b+1; cf=(a<b); end
        3'b010: begin y = a & b; cf = 1'b0; end         
        3'b011: begin y = a | b; cf = 1'b0; end         
        3'b100: begin y = a ^ b; cf = 1'b0; end         
        default: begin y = 4'b0000; cf = 1'b0; end
    endcase
end

endmodule