ARM r1p3, R4F manual Handling cache ECC errors, Cache ECC error behavior Value Behavior

Level One Memory System

Handling cache ECC errors

Table 8-3shows the behavior of the processor on a cache ECC error, depending on bits [5:3] of the Auxiliary Control Register, see Auxiliary Control Registers on page 4-38.

Table 8-3 Cache ECC error behavior

Value Behavior

b000 Abort on all ECC errors, enable hardware recovery

b001

b010 Abort on all ECC errors, force write-through, enable hardware recovery

b011 Reserved

b100 Disable ECC checking

b101 Enable hardware recovery, do not generate aborts on ECC errors

b110 Force write-through, enable hardware recovery, do not generate aborts on ECC errors

b111 Reserved

See Disabling or enabling error checking on page 8-32for information on how to safely change these bits.

When ECC checking is enabled, hardware recovery is always enabled. When an ECC error is detected, the processor tries to evict the cache line containing the error. If the line is clean, it is invalidated, and the correct data is reloaded from the L2 memory system. If the line is dirty, the eviction writes the dirty data out to the L2 memory system, and in the process it corrects any 1-bit errors. The corrected data is then reloaded from the L2 memory system.

If a 2-bit error is detected in a dirty line, the error is not correctable. If the 2-bit error is in the tag or dirty RAM, no data is written to the L2 memory system. If the 2-bit error is in the data RAM, the cache line is written to the L2 memory system, but the AXI master port WSTRBM signal is LOW for the data that contains the error. If an uncorrectable error is detected, an abort is always generated because data might have been lost. It is expected that such a situation can be fatal to the software process running.

If one of the force write-though settings is enabled, memory marked as write-back write-allocate behaves as write-though. This ensures that cache lines can never be dirty, therefore the error can always be recovered from by invalidating the cache line that contains the ECC error.

All detectable errors in the instruction cache can always be recovered from because the instruction cache can never contain dirty data.

ECC aborts

If aborts on ECC errors are enabled, software is notified of the error by a data abort or prefetch abort. The error is still automatically corrected by the hardware even if an abort is generated.

If abort generation is not enabled, the hardware recovery is invisible to software. If required, software can use events and the Correctable Fault Location Register to monitor the errors that are detected and corrected. See Error detection events on page 8-36and Correctable Fault Location Register on page 4-70.

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