26 Specification Update
BU17. PEBS Record not Updated when in Probe Mode
Problem: When a performance monitoring counter is configured for PEBS (Precise Event Based
Sampling), overflows of the counter can result in storage of a PEBS record in the PEBS
buffer. Due to this erratum, if the overflow occurs during probe mode, it may be
ignored and a new PEBS record may not be added to the PEBS buffer.
Implication: Due to this erratum, the PEBS buffer may not be updated by overflows that occur
during probe mode.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.
BU18. MONITOR or CLFLUSH on the Local XAPIC's Address Space Results in Hang
Problem: If the target linear address range for a MONITOR or CLFLUSH is mapped to the local
xAPIC's address space, the processor will hang.
Implication: When this erratum occurs, the processor will hang. The local xAPIC's address space
must be uncached. The MONITOR instruction only functions correctly if the specified
linear address range is of the type write-back. CLFLUSH flushes data from the cache.
Intel has not observed this erratum with any commercially available software.
Workaround: Do not execute MONITOR or CLFLUSH instructions on the local xAPIC address space.
Status: For the steppings affected, see the Summary Tables of Changes.
BU19. Faulting MMX Instruction May Incorrectly Update x87 FPU Tag Word
Problem: Under a specific set of conditions, MMX stores (MOVD, MOVQ, MOVNTQ, MASKMOVQ)
which cause memory access faults (#GP, #SS, #PF, or #AC), may incorrectly update
the x87 FPU tag word register.
This erratum will occur when the following additional conditions are also met.
• The MMX store instruction must be the first MMX instruction to operate on x87 FPU
state (i.e. the x87 FP tag word is not already set to 0x0000).
• For MOVD, MOVQ, MOVNTQ stores, the instruction must use an addressing mode
that uses an index register (this condition does not apply to MASKMOVQ).
Implication: If the erratum conditions are met, the x87 FPU tag word register may be incorrectly set
to a 0x0000 value when it should not have been modified.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.
BU20. An Uncorrectable Error Logged in IA32_CR_MC2_STATUS May also Result in a System Hang
Problem: Uncorrectable errors logged in IA32_CR_MC2_STATUS MSR (409H) may also result in a
system hang causing an Internal Timer Error (MCACOD = 0x0400h) to be logged in
another machine check bank (IA32_MCi_STATUS).
Implication: Uncorrectable errors logged in IA32_CR_MC2_STATUS can further cause a system hang
and an Internal Timer Error to be logged.
Workaround: None identified.
Status: For the steppings affected, see the Summary Tables of Changes.