Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

Vol. 3A 3-39
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
36 35 32
Base
Reserved (set to 0)
Page-Directory Entry (2-MByte Page)
31 1211 9876543210
P
C
D
D
P
P
W
T
Page Base Address
Addr.
G1 A
Reserved (set to 0)
21 20
R
/
W
U
/
S
63
36 35 32
Base
Reserved (set to 0)
Page-Directory-Pointer-Table Entry
31
12
11 9
8
543
2
0
P
C
D
P
W
T
Avail.Page Directory Base Address
Addr.
Res.Reserved
Avail.
P
1
13
P
A
T