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AMD Geode™ LX Processors Data Book 627
Instruction Set
33234H
8.2 CPUID Instruction Set
The CPUID instruction (opcode 0FA2) allows software to make processor inquiries as to the vendor, family, model, step-
ping, features, and specific cache organization information. The presence of support for the CPUID instruction is indicated
by the ability to change the value of the ID flag, bit 21, in the EFLAGS register.
The CPUID level allows the CPUID instruction to return different information in EAX, EBX, ECX, and EDX registers. The
level is determined by the initialized value of the EAX register prior to execution of the CPUID instruction.
8.2.1 Standard CPUID Levels
The standard CPUID levels are part of the standard x86 instruction set.
8.2.1.1 CPUID Instruction with EAX = 00000000h
Standard function 00000000h (EAX = 00000000h) of the CPUID instruction returns the maximum standard CPUID levels,
as well as the processor vendor string.
After the instruction is executed, the EAX register contains the maximum standard CPUID levels supported. The maximum
standard CPUID level is the highest acceptable value for the EAX register input. This does not include the extended CPUID
levels.
The EBX through EDX registers contain the vendor string of the processor as shown in Table 8-16.
8.2.1.2 CPUID Instruction with EAX = 00000001h
Standard function 00000001h (EAX = 00000001h) of the CPUID instruction returns the processor type, family, model, step-
ping information in the EAX register, and the supported standard feature flags in the EDX register. The EBX and ECX reg-
isters are reserved. Table 8-17 provides a register map.
In the EDX register, each flag refers to a specific feature. Some of these features have protection control in CR4. Before
using any of these features, the software should check the corresponding feature flag. Attempting to execute an unavailable
feature can cause exceptions and unexpected behavior. For example, software must check EDX[4] before attempting to use
the Time Stamp Counter instruction.
Table 8-18 on page 628 shows the EAX and EDX bit field formats when EAX = 00000001h and indicates if a feature is not
supported.
Table 8-16. CPUID Instruction with EAX = 00000000h
Register
(Note 1)
Note 1. The “Register” column is intentionally out of order.
Returned Contents Description Comment
EAX 00000001h Maximum Standard Level
EBX 68747541h {htuA} Vendor ID String 1
EDX 69746E65h {itne} Vendor ID String 2
ECX 444D4163h {DMAc} Vendor ID String 3
Table 8-17. CPUID Instruction with EAX = 00000001h
Register
Returned
Contents Description Comment
EAX 000005Axh Type/Family/Model/Step
EBX 00000400h Reserved
ECX 00000000h Reserved
EDX 0088A93Dh Standard Feature Flags