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.
create new repo for MIT backup
Merge branch 'lab4' of http://pdos.csail.mit.edu/6.828/2011/jos into lab4
resolve conflict of master with lab4
resolve conflict of master with lab4
Hw5,6,7 complete for the lab4 scope
Exercise01 done
Exercise02 is done
indent done
Exercise03 is done
Exercise03:fix cpu falut bug
Exercise04 is done
Exercise05 is Done
Finish Exercise06 (PartA Done)