Page Directory and Page Tables

Figure 12-5 on page 221 illustrates the relationship of the Page Directory, the Page Tables and the pages of information. CR3 identifies the base physical address of the Page Directory. The Paging Unit's Page Directory contains 1024d 32-bit entries (Page Directory Entries, or PDEs), each of which contains either the base address of a Page Table in memory, or its location on a mass storage device. Each entry contains a Page Table Present bit that indicates if the respective Page Table is currently present in memory.

Figure 12-5. Page Directory, Page Tables and Pages

[View full size image]

Each Page Table, in turn, contains up to 1024d 32-bit entries, each of which defines the physical location of a 4KB page of information either:

• in physical memory (by specifying its 4KB-aligned base address), or

• its location on a mass storage device.

Each Page Table Entry (PTE) contains a page Present bit to indicate whether the page of information is currently present in memory or not. If it is, the upper 20-bits of the entry contains the 4KB-aligned base address of the page in physical memory. If the page isn't present in memory, the upper part of the Page Table entry (bits [31:1]) can contain a mass storage address.