Specification Update

Errata

AT15. Performance Monitoring Event For Number Of Reference Cycles When

The Processor Is Not Halted (3CH) Does Not Count According To The

Specification

Problem:

The CPU_CLK_UNHALTED performance monitor with mask 1 counts bus clock cycles

instead of counting the core clock cycles at the maximum possible ratio. The

maximum possible ratio is computed by dividing the maximum possible core

frequency by the bus frequency.

Implication: The CPU_CLK_UNHALTED performance monitor with mask 1 counts a value lower than

expected. The value is lower by exactly one multiple of the maximum possible ratio.

Workaround: Multiply the performance monitor value by the maximum possible ratio.

Status: For the steppings affected, see the Summary Tables of Changes.

AT16. Using 2M/4M Pages When A20M# Is Asserted May Result in Incorrect

Address Translations

Problem:

An external A20M# pin if enabled forces address bit 20 to be masked (forced to zero)

to emulates real-address mode address wraparound at 1 megabyte. However, if all of

the following conditions are met, address bit 20 may not be masked:

• Paging is enabled

• A linear address has bit 20 set

• The address references a large page

• A20M# is enabled

Implication: When A20M# is enabled and an address references a large page the resulting

translated physical address may be incorrect. This erratum has not been observed

with any commercially available operating system.

Workaround: Operating systems should not allow A20M# to be enabled if the masking of address

bit 20 could be applied to an address that references a large page. A20M# is normally

only used with the first megabyte of memory.

Status: For the steppings affected, see the Summary Tables of Changes.

AT17. Code Segment limit violation may occur on 4 Gigabyte limit check

Problem:

Code Segment limit violation may occur on 4 Gigabyte limit check when the code

stream wraps around in a way that one instruction ends at the last byte of the

segment and the next instruction begins at 0x0.

Implication: This is a rare condition that may result in a system hang. Intel has not observed this

erratum with any commercially available software, or system.

Workaround: Avoid code that wraps around segment limit.

Status: For the steppings affected, see the Summary Tables of Changes.