ECC Cache: A Lightweight Error Detection for Phase-Change Memory Stuck-At Faults

DRAM scaling has been slowed down. Emerging non-volatile memories (e.g., Phase-Change Memory) promises higher density, better scalability, and persistence. However, endurance is a fundamental issue that hinders the broad adoption of PCM-after repeated writes, a PCM cell can get stuck at a value and be no longer programmable. The prevalence of this stuck-at fault issue requires error detection and correction mechanisms for PCM. Existing solutions such as verify-after-write adds additional latency to PCM writes, which degrades overall system performance. Other solutions like in-memory error-correcting code (ECC) requires a high storage overhead and introduces more reliability issue because ECC bits tend to wear out faster than the protected data bits. In this paper, a novel stuck-at faults detection technique is proposed to improve performance and reliability simultaneously. Since stuck-at faults can only be detected after new writes, ECC does not need to be stored permanently and can be deleted immediately after a one-time detection, which helps to reduce ECC storage overhead. Therefore, this work proposes to use a small on-chip ECC cache to store the temporary ECC entries, which does not suffer from endurance issue. To maximize the utilization of the limited cache space, this work optimizes ECC entry insertion and deletion mechanisms and exploits memory bank-level parallelism to minimize performance impact. For the evaluated workloads, the proposed ECC cache achieves an average of 9.4% of the performance improvement over the baseline with a verify-after-write detection.