80x86 paging with PAE disabled
若啟用 PAE(Physical Address Extension) 功能，則可使用 36bits的記憶體空間(即64GB)：
80x86 paging with PAE enabled
3-level paging, 4KB/page, 32-bit linear address to 36-bit physical address
80x86 paging with PAE enabled
2-level paging, 2MB/page, 32-bit linear address to 36-bit physical address
2.6 Linux paging with PAE disabled
2-level paging, 4KB/page, 32-bit linear address to 32-bit physical address
2.6 Linux paging with PAE enabled
3-level paging, 4KB/page, 32-bit linear address to 36-bit physical address
2.6 Linux paging with PAE enabled
2-level paging, 2MB/page, 32-bit linear address to 36-bit physical address
在 Kernel Source Code 中，Page-Directory 及 Page-Table 是用相同的結構
分別名命為 pgd_t 及 pte_t，定義於 include/asm-i386/page.h
若未使用 PAE，則其大小為 32bits，否則為 64bits
Page-Directory Entry with PAE disabled
4KB/page-table, 32-bit physical address
32bits 的 pgd_t 的欄位格式如下：
//2.6 Linux : Page Global Directory
typedef struct { unsigned long pgd; } pgd_t;
Page-Table Entry with PAE disabled
4KB/page, 32-bit physical address
32bits 的 pte_t 的欄位格式如下：
//2.6 Linux : Page Table Entry
typedef struct { unsigned long pte_low; } pte_t;
Page-Directory Entry with PAE enabled
2MB/page, 32-bit physical address
此時的型態為 64bits 的資料，定義如下：
//2.6 Linux : Page Table Entry
typedef struct { unsigned long pte_low, pte_high; } pte_t;
//2.6 Linux : Page Middle Directory
typedef struct { unsigned long pmd; } pmd_t;
//2.6 Linux : Page Global Directory
typedef struct { unsigned long pgd; } pgd_t;
結論：
Linux 使用 4-level paging 機制來套用到不同的硬體平台，而且可相容於 32/64-bit 的架構
但是，若硬體的 Paging Unit 其分頁方式大於 4-level，Linux 就無法支援
