Intel architecture ia-32 ユーザーズマニュアル
Vol. 3A 3-27
PROTECTED-MODE MEMORY MANAGEMENT
(Page-directory entries for 4-KByte page tables) — Specifies the physical
address of the first byte of a page table. The bits in this field are interpreted as
the 20 most-significant bits of the physical address, which forces page tables to
be aligned on 4-KByte boundaries.
address of the first byte of a page table. The bits in this field are interpreted as
the 20 most-significant bits of the physical address, which forces page tables to
be aligned on 4-KByte boundaries.
(Page-directory entries for 4-MByte pages) — Specifies the physical address
of the first byte of a 4-MByte page. Only bits 22 through 31 of this field are
used (and bits 12 through 21 are reserved and must be set to 0, for IA-32
processors through the Pentium II processor). The base address bits are inter-
preted as the 10 most-significant bits of the physical address, which forces
4-MByte pages to be aligned on 4-MByte boundaries.
of the first byte of a 4-MByte page. Only bits 22 through 31 of this field are
used (and bits 12 through 21 are reserved and must be set to 0, for IA-32
processors through the Pentium II processor). The base address bits are inter-
preted as the 10 most-significant bits of the physical address, which forces
4-MByte pages to be aligned on 4-MByte boundaries.
Present (P) flag, bit 0
Indicates whether the page or page table being pointed to by the entry is
currently loaded in physical memory. When the flag is set, the page is in phys-
ical memory and address translation is carried out. When the flag is clear, the
page is not in memory and, if the processor attempts to access the page, it
generates a page-fault exception (#PF).
currently loaded in physical memory. When the flag is set, the page is in phys-
ical memory and address translation is carried out. When the flag is clear, the
page is not in memory and, if the processor attempts to access the page, it
generates a page-fault exception (#PF).
The processor does not set or clear this flag; it is up to the operating system or
executive to maintain the state of the flag.
executive to maintain the state of the flag.
If the processor generates a page-fault exception, the operating system gener-
ally needs to carry out the following operations:
ally needs to carry out the following operations:
1.
Copy the page from disk storage into physical memory.
2.
Load the page address into the page-table or page-directory entry and set
its present flag. Other flags, such as the dirty and accessed flags, may also
be set at this time.
its present flag. Other flags, such as the dirty and accessed flags, may also
be set at this time.
Figure 3-15. Format of Page-Directory Entries for 4-MByte Pages and 32-Bit Addresses
31
Available for system programmer’s use
Global page
Page size (1 indicates 4 MBytes)
Dirty
Global page
Page size (1 indicates 4 MBytes)
Dirty
12 11
9 8 7 6 5 4 3 2 1 0
P
S
S
P
C
A
D
Accessed
Cache disabled
Write-through
User/Supervisor
Read/Write
Present
Cache disabled
Write-through
User/Supervisor
Read/Write
Present
D
P
P
W
T
U
/
S
R
/
W
G
Avail.
Page Base Address
Page-Directory Entry (4-MByte Page)
22 21
Reserved
Page Table Attribute Index
P
A
T
T
13