Intel IA-32 - page 116

3-36 Vol. 3A
PROTECTED-MODE MEMORY MANAGEMENT
The base physical address in an entry specifies the following, depending on the type of entry:
•Page-directory-pointer-table entry — the physical address of the first byte of a
4-KByte page directory.
•Page-directory entry — the physical address of the first byte of a 4-KByte page table or a
2-MByte page.
•Page-table entry — the physical address of the first byte of a 4-KByte page.
For all table entries (except for page-directory entries that point to 2-MByte pages), the bits in
the page base address are interpreted as the 24 most-significant bits of a 36-bit physical address,
which forces page tables and pages to be aligned on 4-KByte boundaries. When a page-directory
entry points to a 2-MByte page, the base address is interpreted as the 15 most-significant bits of
a 36-bit physical address, which forces pages to be aligned on 2-MByte boundaries.
The present flag (bit 0) in the page-directory-pointer-table entries can be set to 0 or 1. If the
present flag is clear, the remaining bits in the page-directory-pointer-table entry are available to
the operating system. If the present flag is set, the fields of the page-directory-pointer-table entry
are defined in Figures 3-20 for 4-KByte pages and Figures 3-21 for 2-MByte pages.
The page size (PS) flag (bit 7) in a page-directory entry determines if the entry points to a page
table or a 2-MByte page. When this flag is clear, the entry points to a page table; when the flag
is set, the entry points to a 2-MByte page. This flag allows 4-KByte and 2-MByte pages to be
mixed within one set of paging tables.
Figure 3-21. Format of Page-Directory-Pointer-Table and Page-Directory Entries
for 2-MByte Pages with PAE Enabled
63 3635 32
Base
Reserved (set to 0)
Page-DirectoryEntry (2-MByte Page )
31 1211 9876543210
P
C
DDP
P
W
T
Page Base Address
Addr.
G1 A
Reserved (set to 0)
21 20
R
/
W
U
/
S
63 3635 32
Base
Reserved (set to 0)
Page-Directory-Pointer-TableEntry
31 1211 9 854320
P
C
D
P
W
T
Avail.Page Directory Base Address
Addr.
Res.Reserved
Avail.
P
1
13
P
A
T