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Error Correction Code

The L2 cache supports optional ECC in most of its memories. For core instruction and data accesses resulting in an L2 cache hit, where a single-bit error is detected on the Data array, the L2 memory system supports in-line ECC correction. Uncorrected data is forwarded to the requesting unit, and in parallel, the ECC circuitry checks for accuracy. If a single-bit error is detected, any uncorrected data returned within two cycles before the error indicator must be discarded. The L2 memory system begins to stream corrected data to the requestor.

When there is no data transfers, the L2 memory system shifts back to return uncorrected data until it detects the next single-bit error. Forwarding uncorrected data can be disabled by programming bit[20] of the L2 Control Register. This avoids the requirement to flush requests associated with single-bit ECC errors on L2 cache hits, but adds an additional 2 cycles to the L2 hit latency.

For all other single-bit ECC errors detected, the request is flushed from the L2 pipeline and is forced to reissue. The tag bank where the single-bit error occurred, performs a read-modify-write sequence to correct the single-bit error in the array. The request is then reissued.

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