MIT-6.828 jos
YANG Weikun, 1100012442

important files:

lab4.md     markdown source file
lab4.html   lab4 report, better rendering effects on Chrome

Note from lab3:
1. if `make` fails, try to `make` again will solve the problem.
2. strange things happens when I run `make grade` on OSX. I've verified
   my output by hand.
3. Github's markdown syntax is slightly different from my markdown editor.
   Please see lab*.html for better effects.

Note from lab4:
1. `make grade` might timeout. gonna check that later.

Note from lab5:
1. some test programs ends with a breakpoint exception. Is that normal?
   their behaviours are correct.