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4-36 Vol. 3
PAGING
Page-fault exceptions occur only due to an attempt to use a linear address. Failures
to load the PDPTE registers with PAE paging (see Section 4.4.1) cause general-
protection exceptions (#GP(0)) and not page-fault exceptions.
4.8 ACCESSED AND DIRTY FLAGS
For any paging-structure entry that is used during linear-address translation, bit 5 is
the accessed flag.
1
For paging-structure entries that map a page (as opposed to
referencing another paging structure), bit 6 is the dirty flag. These flags are
provided for use by memory-management software to manage the transfer of pages
and paging structures into and out of physical memory.
Whenever the processor uses a paging-structure entry as part of linear-address
translation, it sets the accessed flag in that entry (if it is not already set).
Whenever there is a write to a linear address, the processor sets the dirty flag (if it is
not already set) in the paging-structure entry that identifies the final physical
address for the linear address (either a PTE or a PDE in which the PS flag is 1).
Memory-management software may clear these flags when a page or a paging struc-
ture is initially loaded into physical memory. These flags are “sticky,” meaning that,
once set, the processor does not clear them; only software can clear them.
A processor may cache information from the paging-structure entries in TLBs and
paging-structure caches (see Section 4.10). This fact implies that, if software
changes an accessed flag or a dirty flag from 1 to 0, the processor might not set the
corresponding bit in memory on a subsequent access using an affected linear
address (see Section 4.10.3.3). See Section 4.10.3.2 for how software can ensure
that these bits are updated as desired.
NOTE
The accesses used by the processor to set these flags may or may not
be exposed to the processor’s self-modifying code detection logic. If
the processor is executing code from the same memory area that is
being used for the paging structures, the setting of these flags may
or may not result in an immediate change to the executing code
stream.
1. With PAE paging, the PDPTEs are not used during linear-address translation but only to load the
PDPTE registers for some executions of the MOV CR instruction (see Section 4.4.1). For this rea-
son, the PDPTEs do not contain accessed flags with PAE paging.