Testing For Extended Functions; Determining The Processor Signature - AMD -K6-2/450 - MHz Processor Application Note

23913A/0—November 2000

Testing For Extended Functions

Determining the Processor Signature

Embedded AMD Processor Recognition
Preliminary Information
AMD's definition for subsequent CPUID functions and the
registers returned for those functions.
Once the software identifies the processor's vendor, it knows
the definition for all the functions supplied by the CPUID
instruction. By using these functions, the software obtains the
processor information needed to properly tune its functionality
to the capabilities of the processor.
Software must test for extended functions with function
8000_0000h. The EAX register returns the largest extended
function input value defined for the CPUID instruction on the
processor. If this value is at least 8000_0001h, extended
functions are supported.
With one exception, the AMD extended feature flags include all
the information provided in the standard feature flags as well
as indicators for the additional AMD processor-specific feature
enhancements. The duplication of standard feature bits within
the extended feature bits can minimize the number of function
calls required by software. The exception is bit 11, which
indicates that the SYSENTER and SYSEXIT instructions are
supported in the standard features and that the SYSCALL and
SYSRET instructions are supported in the extended features.
Standard function 1 (EAX=1) of the CPUID instruction returns
the standard processor signature and feature bits. The standard
processor signature is returned in the EAX register and
provides information regarding the specific revision (stepping)
and model of the processor and the instruction family level
supported by the processor. The revision level can be used to
determine if the processor supports specific features. However,
it is not recommended that the revision level be used in this
manner unless this information is not available through the
standard or extended feature bits.
Embedded AMD-K6™ Processors BIOS Design Guide
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