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3-168 Vol. 2A CPUID—CPU Identification
INSTRUCTION SET REFERENCE, A-M
INPUT EAX = 0: Returns CPUID’s Highest Value for Basic Processor Information and
the Vendor Identification String
When CPUID executes with EAX set to 0, the processor returns the highest value the
CPUID recognizes for returning basic processor information. The value is returned in
the EAX register (see Table 3-13) and is processor specific.
A vendor identification string is also returned in EBX, EDX, and ECX. For Intel
processors, the string is “GenuineIntel” and is expressed:
EBX ← 756e6547h (* "Genu", with G in the low nibble of BL *)
EDX ← 49656e69h (* "ineI", with i in the low nibble of DL *)
ECX ← 6c65746eh (* "ntel", with n in the low nibble of CL *)
INPUT EAX = 80000000H: Returns CPUID’s Highest Value for Extended Processor
Information
When CPUID executes with EAX set to 0, the processor returns the highest value the
processor recognizes for returning extended processor information. The value is
returned in the EAX register (see Table 3-13) and is processor specific.
80000007H EAX
EBX
ECX
EDX
Reserved = 0
Reserved = 0
Reserved = 0
Reserved = 0
80000008H EAX Virtual/Physical Address size
Bits 7-0: #Physical Address Bits*
Bits 15-8: #Virtual Address Bits
Bits 31-16: Reserved = 0
EBX
ECX
EDX
Reserved = 0
Reserved = 0
Reserved = 0
NOTES:
* If CPUID.80000008H:EAX[7:0] is supported, the maximum physical
address number supported should come from this field.
Table 3-12. Information Returned by CPUID Instruction (Contd.)
Initial EAX
Value Information Provided about the Processor