三级页表

4.3  32-BIT PAGING

0:  CR3(12-31) --> PD page address

CR3(12-31) x 4K --> PD physical address  
1:  (PD physical address + level1 offset)(12-31) --> PT page address
(PD physical address + level1 offset)(12-31) x4k --> PT physical address 
2:  (PT physical address + level2 offset)(12-31) --> real page number
3: (PT physical address + level2 offset)(12-31) x4k + page offset -->  physical address 


所有页，都是4k对其的页，因此，表达一个4k页地址，只需要20位(12-31)。
表达一个4M页地址，只需要10位。


CR3--> Address of page directory (12-31) 
PD entry --> Address of page table (12-31)
PT entry --> Address of 4KB page frame (real page number, 12-31)