submit lab1, exercise 8.
submit lab1, exercise 11 and 12.
Merge branch 'lab1' into lab2
submit lab2, exercise 1.
submit lab2, exercise 4.
submit lab 2 execrise 5.

** NOTE **
There is a severe bug found here in the previous submit.
This is about the function of "boot_map_region". I've fixed
according to the code of someone.
Submit challenge task #1 - CGA print

I added some functions and macros to support CGA 16-color print.
Submit challenge task #2 - display page info.

Dispaly Page Infomation in a easy-to-read format.
submit challenge task #3 - virt/phys memory dump.

I filled the function of dumpmem so that I can dump it quickly.

Bug: if I look up the info at virt addr 0 - 0xf0000000, there may be a
general protection exception and automatic restart
(interrupt descriptor not built. )

Careful!
Merge branch 'lab2' into lab3
submit lab 3 exercise 1
Sumbit lab 3 exercise 2
I fixed a bug in grade file which trouble me a very long time.
submit lab 3 exercise 5 and 6
submit lab 3 exercise 7
submit lab 3 exercise 8
submit lab 3 exercise 8, 9 and 10
const changed
merge lab3 to lab4 manually
merge lab3 to lab4 manualual