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5-44 Vol. 3
PROTECTION
5.13.2 Execute-Disable Page Protection
The execute-disable bit in the paging structures enhances page protection for data
pages. Instructions cannot be fetched from a memory page if IA32_EFER.NXE =1
and the execute-disable bit is set in any of the paging-structure entries used to map
the page.
Table 5-5 lists the valid usage of a page in relation to the value of execute-
disable bit (bit 63) of the corresponding entry in each level of the paging structures.
Execute-disable protection can be activated using the execute-disable bit at any level
of the paging structure, irrespective of the corresponding entry in other levels. When
execute-disable protection is not activated, the page can be used as code or data.
In legacy PAE-enabled mode, Table 5-6 and Table 5-7 show the effect of setting the
execute-disable bit for code and data pages.
Table 5-5. IA-32e Mode Page Level Protection Matrix
with Execute-Disable Bit Capability
Execute Disable Bit Value (Bit 63) Valid Usage
PML4 PDP PDE PTE
Bit 63 = 1 * * * Data
* Bit 63 = 1 * * Data
* * Bit 63 = 1 * Data
* * * Bit 63 = 1 Data
Bit 63 = 0 Bit 63 = 0 Bit 63 = 0 Bit 63 = 0 Data/Code
NOTES:
* Value not checked.