http://www.zhihu.com/question/22364502
http://www.zhihu.com/question/22364502/answer/21535141
http://www.ruanyifeng.com/blog/2013/02/booting.html
http://www.zhihu.com/question/22364502
http://www.zhihu.com/question/22364502/answer/21535141
http://www.ruanyifeng.com/blog/2013/02/booting.html