Due to its low leakage power and high density, spin torque transfer RAM (STT-RAM) has become a good candidate for future on-chip cache. However, STT-RAM suffers from higher write energy compared to the SRAM. One state-of-the-art technique to alleviate this problem is read-before-write (RBW). In this paper, we study the pattern of the write accesses to the L2 cache and show that directly applying the RBW to a STT-RAM L2 cache can be problematic from energy perspective. We then propose a selective read-before-write (SRW) scheme to further reduce the dynamic write energy of the STT-RAM cache. Additional optimizations are included in the design of SRW so that it can save a considerable amount of energy at negligible overheads. The experimental results show that SRW achieves a 86.0% reduction in write energy consumption vs. a baseline without any write optimization techniques, and a 6.55% more reduction compared to the RBW scheme.
An energy efficient cache design using spin torque transfer (STT) RAM