lab1
_start --> entry
Code style fix
modified lab1
modified makefile
update lab2
lab3
merge lab1
git lab2
Minor lab3 updates
lab4
Fix the grade script to pass -gdb to QEMU again.
Pre-0.15 QEMU didn't support -D, so make its addition conditional.
Add a check for the IO memory region mapping
Use backticks intead of $().

Old versions of bash, like the one on Athena, don't parse the original
syntax correctly.
env_destroy() might race when the env to destory is currently running
on another CPU. Add an ENV_ZOMBIE state to postpone env_free() until
the next time the zombie env traps into the kernel.

(Thanks to Christopher Ryan Johnson for pointing this out.)
Generate handin tarballs via git.

Previously, the tarball rule sucked in everything in sight.  This
allows students to better control what gets handed in and code
reviewed.  The rule checks several sanity conditions before creating
the archive.
lab3 modification
bug_fix
Fudan OS lab4 modification.