A8.2
Cache protection behavior
The configuration of the RAS extension that is implemented in the Cortex-A76 core includes cache
protection.
In this case, the Cortex-A76 core protects against errors that result in a RAM bitcell holding the incorrect
value.
The RAMs in the Cortex-A76 core have the following capability:
SED
Single Error Detect
. One bit of parity is applicable to the entire word. The word size is specific
for each RAM and depends on the protection granule.
Interleaved parity
One bit of parity is applicable to the even bits of the word, and one bit of parity is applicable to
the odd bits of the word.
SECDED
Single Error Correct, Double Error Detect
.
Table A8-1 Cache protection behavior
indicates which protection type is applied to
each RAM.
The core can progress and remain functionally correct when there is a single bit error in any RAM.
If there are multiple single bit errors in different RAMs, or within different protection granules within the
same RAM, then the core also remains functionally correct.
If there is a double bit error in a single RAM within the same protection granule, then the behavior
depends on the RAM:
• For RAMs with SECDED capability, the core detects and either reports or defers the error. If the error
is in a cache line containing dirty data, then that data might be lost.
• For RAMs with only SED, the core does not detect a double bit error. This might cause data
corruption.
If there are three or more bit errors within the same protection granule, then depending on the RAM and
the position of the errors within the RAM, the core might or might not detect the errors.
The cache protection feature of the core has a minimal performance impact when no errors are present.
Table A8-1 Cache protection behavior
RAM
Protection type
Protection granule
Correction behavior
L1 instruction cache tag
1 parity bit
31 bits
The line that contains the error is invalidated
from the L1 instruction cache and fetched
again from the subsequent memory system.
L1 instruction cache data
SED
72 bits
The line that contains the error is invalidated
from the L1 instruction cache and fetched
again from the subsequent memory system.
L1 BTB
None
-
-
L1 GHB
None
-
-
L1 BPIQ
None
-
-
L1 data cache tag
SECDED
34 bits + 7 bits for ECC attached to
the word.
The cache line that contains the error gets
evicted, corrected in line, and refilled to the
core.
A8 Reliability, Availability, and Serviceability (RAS)
A8.2 Cache protection behavior
100798_0300_00_en
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