Intel IA-32 - Manual (Page 101)

Vol. 3A 3-21
PROTECTED-MODE MEMORY MANAGEMENT
accessed for a long time. See Section 3.12, “Translation Lookaside Buffers (TLBs)”, for more
information on the TLBs.
3.6.1 Paging Options
Paging is controlled by three flags in the processor’s control registers:
•
PG (paging) flag. Bit 31 of CR0 (available in all IA-32 processors beginning with the
Intel386 processor).
•
PSE (page size extensions) flag. Bit 4 of CR4 (introduced in the Pentium processor).
•
PAE (physical address extension) flag. Bit 5 of CR4 (introduced in the Pentium Pro
processors).
The PG flag enables the page-translation mechanism. The operating system or executive usually
sets this flag during processor initialization. The PG flag must be set if the processor’s page-
translation mechanism is to be used to implement a demand-paged virtual memory system or if
the operating system is designed to run more than one program (or task) in virtual-8086 mode.
The PSE flag enables large page sizes: 4-MByte pages or 2-MByte pages (when the PAE flag
is set). When the PSE flag is clear, the more common page length of 4 KBytes is used. See
Section 3.7.2, “Linear Address Translation (4-MByte Pages)”, Section 3.8.3, “Linear Address
Translation With PAE Enabled (2-MByte Pages)”, and Section 3.9, “36-Bit Physical Addressing
Using the PSE-36 Paging Mechanism” for more information about the use of the PSE flag.
The PAE flag provides a method of extending physical addresses to 36 bits. This physical
address extension can only be used when paging is enabled. It relies on an additional page direc-
tory pointer table that is used along with page directories and page tables to reference physical
addresses above FFFFFFFFH. See Section 3.8, “36-Bit Physical Addressing Using the PAE
Paging Mechanism”, for more information about extending physical addresses using the PAE
flag.
When PAE is enabled and for processors that support Intel EM64T, the PAE mechanism is
enhanced to support more than 36 bits of physical addressing (if the processor’s implementation
supports more than 36 bits of physical addressing). This applies to IA-32e mode address trans-
lation (see Section 3.10, “PAE-Enabled Paging in IA-32e Mode”) and enhanced legacy PAE-
enabled address translation (see Section 3.8.1, “Enhanced Legacy PAE Paging”).
The 36-bit page size extension (PSE-36) feature provides an alternate method of extending
physical addressing to 36 bits. This paging mechanism uses the page size extension mode
(enabled with the PSE flag) and modified page directory entries to reference physical addresses
above FFFFFFFFH. The PSE-36 feature flag (bit 17 in the EDX register when the CPUID
instruction is executed with a source operand of 1) indicates the availability of this addressing
mechanism. See Section 3.9, “36-Bit Physical Addressing Using the PSE-36 Paging Mecha-
nism”, for more information about the PSE-36 physical address extension and page size exten-
sion mechanism.