运行时会出错应该是正常的，因为没有实现进程的退出。

实现第二次机会页面置换算法

Makefile里的gdb和之前不一样了！要添加$(QEMUOPTS)，不然会导致没有挂载交换分区。

修复了测试代码中的bug。

The original time slice (200ms) is too large for the priority test to
generate a satisfactory result in 20s. If we only schedule 5 times a
second, there are only 100 pick_next calls to the scheduler.

I believe making scheduling more frequently does little harm to the
system. Actually more scheduling opportunities may also reveal bugs
which are not triggered previously. Adopting smaller time slices also
allows us to reduce the time spent on the priority test, which can
benefit the autotesting system.

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>

This target allows starting ucore with the specified user program in
pure terminal.

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>