Bit mapping for balanced PCM cell programming

Abstract

Write bandwidth is an inherent performance bottleneck for Phase Change Memory (PCM) for two reasons. First, PCM cells have long programming time, and second, only a limited number of PCM cells can be programmed concurrently due to programming current and write circuit constraints, For each PCM write, the data bits of the write request are typically mapped to multiple cell groups and processed in parallel. We observed that an unbalanced distribution of modified data bits among cell groups significantly increases PCM write time and hurts effective write bandwidth. To address this issue, we first uncover the cyclical and cluster patterns for modified data bits. Next, we propose double XOR mapping (D-XOR) to distribute modified data bits among cell groups in a balanced way. D-XOR can reduce PCM write service time by 45% on average, which increases PCM write throughput by 1.8x. As error correction (redundant bits) is critical for PCM, we also consider the impact of redundancy information in mapping data and error correction bits to cell groups. Our techniques lead to a 51% average reduction in write service time for a PCM main memory with ECC, which increases IPC by 12%.