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

Vol. 3A 3-51

PROTECTED-MODE MEMORY MANAGEMENT

procedures running at privilege level of 0 can invalidate TLBs or selected TLB entries.

Whenever a page-directory or page-table entry is changed (including when the

present flag is set to zero), the operating-system must immediately invalidate the

corresponding entry in the TLB so that it can be updated the next time the entry is

referenced.

All of the (non-global) TLBs are automatically invalidated any time the CR3 register is

loaded (unless the G flag for a page or page-table entry is set, as describe later in this

section). The CR3 register can be loaded in either of two ways:

• Explicitly, using the MOV instruction, for example:

MOV CR3, EAX

where the EAX register contains an appropriate page-directory base address.

• Implicitly by executing a task switch, which automatically changes the contents

of the CR3 register.

The INVLPG instruction is provided to invalidate a specific page-table entry in the

TLB. Normally, this instruction invalidates only an individual TLB entry; however, in

some cases, it may invalidate more than the selected entry and may even invalidate

all of the TLBs. This instruction ignores the setting of the G flag in a page-directory or

page-table entry (see following paragraph).

(Introduced in the Pentium Pro processor.) The page global enable (PGE) flag in

can be used to prevent frequently used pages from being automatically invalidated in

the TLBs on a task switch or a load of register CR3. (See Section 3.7.6, “Page-Direc-

tory and Page-Table Entries”, for more information about the global flag.) When the

processor loads a page-directory or page-table entry for a global page into a TLB, the

entry will remain in the TLB indefinitely. The only ways to deterministically invalidate

global page entries are as follows:

• Clear the PGE flag; this will invalidate the TLBs.

• Execute the INVLPG instruction to invalidate individual page-directory or page-

table entries in the TLBs.

For additional information about invalidation of the TLBs, see Section 10.9, “Invali-

dating the Translation Lookaside Buffers (TLBs)”.