https://rein.kr/posts/2008-01-17-os-%EC%9E%AC%EC%84%A4%EC%B9%98-%EC%A4%91%EC%97%90-%EC%A0%9C%EC%9D%BC-%EB%AC%B4%EC%84%9C%EC%9A%B4-%EA%B3%BC%EC%A0%95/