Specification Update

Errata
58 Specification Update
AH101. (E)CX May Get Incorrectly Updated When Performing Fast String REP
STOS with Large Data Structures
Problem: When performing Fast String REP STOS commands with data structures [(E)CX*Data
Size] larger than the supported address size structure (64 KB for 16-bit address size
and 4 GB for 32-bit address size) some addresses may be processed more than once.
After an amount of data greater than or equal to the address size structure has been
processed, external events (such as interrupts) will cause the (E)CX registers to be
incremented by a value that corresponds to 64 KB for 16-bit address size and 4 GB for
32-bit address size.
Implication: (E)CX may contain an incorrect count which may cause some of the STOS operations
to re-execute. Intel has not observed this erratum with any commercially-available
software.
Workaround: Do not use values in (E)CX that when multiplied by the data size, give values larger
than the address space size (64 KB for 16-bit address size and 4 GB for 32-bit address
size).
Status: For the steppings affected, see the Summary Tables of Changes.
AH102. Performance Monitoring Event BR_INST_RETIRED May Count CPUID
Instructions as Branches
Problem: Performance monitoring event BR_INST_RETIRED (C4H) counts retired branch
instructions. Due to this erratum, two of its sub-events mistakenly count for CPUID
instructions as well. Those sub events are: BR_INST_RETIRED.PRED_NOT_TAKEN
(Umask 01H) and BR_INST_RETIRED.ANY (Umask 00H).
Implication: The count value returned by the performance monitoring event
BR_INST_RETIRED.PRED_NOT_TAKEN or BR_INST_RETIRED.ANY may be higher than
expected. The extent of over counting depends on the occurrence of CPUID
instructions, while the counter is active.
Workaround: None Identified.
Status: For the steppings affected, see the Summary Tables of Changes.