https://purengom.com/2009/09/14/%ec%a7%80%eb%8a%94-pda-%eb%9c%a8%eb%8a%94-%ec%8a%a4%eb%a7%88%ed%8a%b8%ed%8f%b0-%ed%8a%b8%eb%a0%8c%eb%93%9c%ea%b0%80-%eb%b0%94%eb%80%8c%ec%97%88%eb%82%98/