[DAC 2016] AOS: Adaptive Overwrite Scheme for Energy Efficient MLC STT-RAM Cache

Spin-Transfer Torque Random Access Memory (STT-RAM)

has been identi ed as an advantageous candidate for on-chip

memory technology due to its high density and ultra low

leakage power. Recent research progress in Magnetic Tun-

neling Junction (MTJ) devices has developed Multi-Level

Cell (MLC) STT-RAM to further enhance cell density. To

avoid the write disturbance in MLC strategy, data stored

in the soft bit must be restored back immediately after the

hard bit switching is completed. However, frequent restores

are not only unnecessary, but also introduce a signi cant

energy consumption overhead. In this paper, we propose an

Adaptive Overwrite Scheme (AOS) which alleviates restora-

tion overhead by intentionally overwriting selected soft bits

based on RRD (Read Reuse Distance). Our experimental re-

sults show 54.6% reduction in soft bit restoration, delivering

10.8% decrease in overall energy consumption. Moreover,

AOS promotes MLC to be a preferable L2 design alterna-

tive in terms of energy, area and latency product.