Intel IA-32 - Manual (Page 625)

Vol. 3A 17-33
IA-32 ARCHITECTURE COMPATIBILITY
cache to be disabled and enabled, independently of the L1 and L2 caches (see Section 10.5.4,
“Disabling and Enabling the L3 Cache”).
17.29. PAGING
This section identifies enhancements made to the paging mechanism and implementation differ-
ences in the paging mechanism for various IA-32 processors.
17.29.1 Large Pages
The Pentium processor extended the memory management/paging facilities of the IA-32 to
allow large (4 MBytes) pages sizes (see Section 3.6.1, “Paging Options”). The first P6 family
processor (the Pentium Pro processor) added a 2 MByte page size to the IA-32 in conjunction
with the physical address extension (PAE) feature (see Section 3.8, “36-Bit Physical Addressing
Using the PAE Paging Mechanism”).
The availability of large pages on any IA-32 processor can be determined via feature bit 3
(PSE) of register EDX after the CPUID instruction has been execution with an argument of 1.
Intel processors that do not support the CPUID instruction do not support page size enhance-
ments. (See “CPUID—CPU Identification” in Chapter 3, “Instruction Set Reference, A-M,” in
the IA-32 Intel® Architecture Software Developer’s Manual, Volume 2A, and AP-485, Intel
Processor Identification and the CPUID Instruction, for more information on the CPUID
instruction.)
17.29.2 PCD and PWT Flags
The PCD and PWT flags were introduced to the IA-32 in the Intel486 processor to control the
caching of pages:
•
PCD (page-level cache disable) flag—Controls caching on a page-by-page basis.
•
PWT (page-level write-through) flag—Controls the write-through/writeback caching
policy on a page-by-page basis. Since the internal cache of the Intel486 processor is a
write-through cache, it is not affected by the state of the PWT flag.
17.29.3 Enabling and Disabling Paging
Paging is enabled and disabled by loading a value into control register CR0 that modifies the PG
flag. For backward and forward compatibility with all IA-32 processors, Intel recommends that
the following operations be performed when enabling or disabling paging:
1. Execute a MOV CR0, REG instruction to either set (enable paging) or clear (disable
paging) the PG flag.
2. Execute a near JMP instruction.