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4-6 Vol. 3
PAGING
4.1.4 Enumeration of Paging Features by CPUID
Software can discover support for different paging features using the CPUID instruc-
tion:
• PSE: page-size extensions for 32-bit paging.
If CPUID.01H:EDX.PSE [bit 3] = 1, CR4.PSE may be set to 1, enabling support
for 4-MByte pages with 32-bit paging (see Section 4.3).
• PAE: physical-address extension.
If CPUID.01H:EDX.PAE [bit 6] = 1, CR4.PAE may be set to 1, enabling PAE
paging (this setting is also required for IA-32e paging).
• PGE: global-page support.
If CPUID.01H:EDX.PGE [bit 13] = 1, CR4.PGE may be set to 1, enabling the
global-page feature (see Section 4.10.1.4).
• PAT: page-attribute table.
If CPUID.01H:EDX.PAT [bit 16] = 1, the 8-entry page-attribute table (PAT) is
supported. When the PAT is supported, three bits in certain paging-structure
entries select a memory type (used to determine type of caching used) from the
PAT (see Section 4.9).
• PSE-36: 36-Bit page size extension.
If CPUID.01H:EDX.PSE-36 [bit 17] = 1, the PSE-36 mechanism is supported,
indicating that translations using 4-MByte pages with 32-bit paging may produce
physical addresses with more than 32 bits (see Section 4.3).
• NX: execute disable.
If CPUID.80000001H:EDX.NX [bit 20] = 1, IA32_EFER.NXE may be set to 1,
allowing PAE paging and IA-32e paging to disable execute access to selected
pages (see Section 4.6). (Processors that do not support CPUID function
80000001H do not allow IA32_EFER.NXE to be set to 1.)
• LM: IA-32e mode support.
If CPUID.80000001H:EDX.LM [bit 29] = 1, IA32_EFER.LME may be set to 1,
enabling IA-32e paging. (Processors that do not support CPUID function
80000001H do not allow IA32_EFER.LME to be set to 1.)
• CPUID.80000008H:EAX[7:0] reports the physical-address width supported by
the processor. (For processors that do not support CPUID function 80000008H,
the width is generally 36 if CPUID.01H:EDX.PAE [bit 6] = 1 and 32 otherwise.)
This width is referred to as MAXPHYADDR. MAXPHYADDR is at most 52.
• CPUID.80000008H:EAX[15:8] reports the linear-address width supported by the
processor. Generally, this value is 48 if CPUID.80000001H:EDX.LM [bit 29] = 1
and 32 otherwise. (Processors that do not support CPUID function 80000008H,
support a linear-address width of 32.)