32-bit support for large memory capacity

 

In the traditional 32-bit protection mode, x86 processors use a two-level conversion solution. In this solution, the control register f33. it points to a page directory with a length of 4 kib, and the page directory is divided into 1024 page tables for each 4 kib ); the last page table is divided into 1024 pages with a length of 4 kib.

 

Enabling PAE (enabled by setting the 5th-bit control register CR4) will change the above scheme. By default, the size of each page is 4KiB. The table items in the page table and page Directory are expanded from 32-bit to 64-bit (8 bytes) to use the additional address bit. However, the total size of the page table and page Directory remains unchanged. Therefore, both the page table and the page Directory have only 512 table items. Because this has become half of the original solution, another level is added. Currently, Cr 3 points to the page Directory Index Table, that is, a table containing four page Directory pointers.

 

The 7th-bit PS (Page Size) of the table item in the Page Directory ). If this bit is set to 1, the table items in the page Directory do not point to the page table, but to a 2 MIB page. In the page Directory, there is another sign called the NX bit. It is 63rd bits, indicating No eXecute. Because the maximum 12 bits of a page table item are either such identifiers or operating system-related data, a maximum of 52 bits can be used in 252 bytes in the future, that is, addressing in 4pebibyte physical memory.

 

Currently, the x86 architecture only uses 36 of the 52-bit (that is, the maximum memory is 2 ^ 36 = 64 GB ). For x86-64 processors in long mode, PAE is required; where 40 of 52 bits are used.

 

CPU support for the PAE mode can be identified by the CPUID mark PAE.

 

 

Windows XP SP2 and other new versions, which run in PAE mode on processors with no-execute (NX) and execute-disable (XD) features by default to allow NX. NX (or XD) is 63rd bits in the page table. Without PAE, the page table items are only 32 bits. To use the NX feature, you must run in PAE mode. However, to be compatible with the driver, the physical address space of Windows (Windows XP, Windows Vista) on the desktop is limited to 4 Gib.