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Vol. 3 15-27
MACHINE-CHECK ARCHITECTURE
15.6.4 UCR Error Overwrite Rules
In general, the overwrite rules are as follows:
• UCR errors will overwrite corrected errors.
• Uncorrected (PCC=1) errors overwrite UCR (PCC=0) errors.
• UCR errors are not written over previous UCR errors.
• Corrected errors do not write over previous UCR errors.
Regardless of whether the 1st error is retained or the 2nd error is over-
written over the 1st error, the OVER flag in the IA32_MCi_STATUS register
will be set to indicate an overflow condition. As the S flag and AR flag in the
IA32_MCi_STATUS register are defined to be sticky flags, a second event
cannot clear these 2 flags once set, however the MC bank information may
be filled in for the 2nd error. The table below shows the overwrite rules and
how to treat a second error if the first event is already logged in a MC bank
along with the resulting bit setting of the UC, PCC, and AR flags in the
IA32_MCi_STATUS register. As UCNA and SRA0 errors do not require
recovery action from system software to continue program execution, a
system reset by system software is not required unless the AR flag or PCC
flag is set for the UCR overflow case (OVER=1, VAL=1, UC=1, PCC=0).
Table 15-7 lists overwrite rules for uncorrected errors, corrected errors, and
UCR errors.
Table 15-7. Overwrite Rules for UC, CE, and UCR Errors
First Event Second Event UC PCC S AR MCA Bank Reset System
CE UCR 1 0 0 if UCNA,
else 1
1 if SRAR,
else 0
second yes, if AR=1
UCR CE 1 0 0 if UCNA,
else 1
1 if SRAR,
else 0
first yes, if AR=1
UCNA UCNA 1 0 0 0 first no
UCNA SRAO 1 0 1 0 first no
UCNA SRAR 1 0 1 1 first yes
SRAO UCNA 1 0 1 0 first no
SRAO SRAO 1 0 1 0 first no
SRAO SRAR 1 0 1 1 first yes
SRAR UCNA 1 0 1 1 first yes
SRAR SRAO 1 0 1 1 first yes
SRAR SRAR 1 0 1 1 first yes
UCR UC 1 1 undefined undefined second yes
UC UCR 1 1 undefined undefined first yes