first commit
Merge http://pdos.csail.mit.edu/6.828/2012/jos
修改QEMU参数
Lab1, Exercise 8:print octal numbers using patterns of the form "%o".
Lab1: Exercise 10,12: 实现backtrace
Merge branch 'lab1' into lab2
Changes to lab2 after handin
Merge branch 'lab2' into lab3
merge lab2 to lab3