Physical Address Extension(PAE)

Physical Address Extension(PAE)是一種在 Intel architacture 上面記憶體延伸的機制，可以從 4GB 的 physical memory 上限延伸到至多 64GB，有三個 requirement：

1. The CPU model supports Physical Address Extension(PAE).
2. The amount of RAM is larger than 4 GB.
3. The kernel is compiled with PAE support.

從 Pentium Pro 處理器之後才有的機制，但因為需要修改 process 的 page table，所以 run 在 User Mode 的 process 不能夠使用大於 4GB 的 physical memory.

the 32 bits of a linear address are interpreted in the following way：

cr3
　Points to a PDPT
bits 31-30
　Point to 1 of 4 possible entries in PDPT(page global directory in Linux)
bits 29-21
　Point to 1 of 512 possible entries in Page Directory(page middle directory in Linux)
bits 20-12
　Point to 1 of 512 possible entries in Page Table
bits 11-0
　Offset of 4-KB page