Specifications

70 Product Errata
25759 Rev. 3.79 July 2009
Revision Guide for AMD Athlon™ 64 and AMD Opteron™ Processors
110 SAHF/LAHF Extended Feature Flag Defaults to Incorrect Value
Description
The Extended Feature Flag bit defined for SAHF and LAHF instruction support in 64-bit mode is
ECX[0]. However some processors that support the feature incorrectly return it as zero.
Potential Effect on System
The operating system will not see the feature flag and therefore the extended feature will not be used.
Suggested Workaround
For processors which support the feature (as determined by the processor revision ID), BIOS should
write a one to:
• MSR C001_100Dh, bit 32 for revision D silicon.
• MSR C001_1005h, bit 32 for revision E and later silicon.
This will cause the extended feature flag in ECX[0] to be set.
Fix Planned
Yes