71. Arithmetic Logic Unit

How do you plan to solve it?

module alu4(input [3:0]a,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;cf=a<b;end
        3'b010:begin y=a&b;cf=0;end
        3'b011:begin y=a|b;cf=0;end
        3'b100:begin y=a^b;cf=0;end
        default: begin y=0;cf=0;end
        endcase
    end
endmodule