Rethinking DRAM's Page Mode With STT-MRAM

Abstract

STT-MRAM is a promising candidate for drop-in replacement for DRAM-based main memory because of its higher energy efficiency and similar latency to DRAM. However, simply replacing DRAM with STT-MRAM without optimizations severely limits STT-MRAM from exploiting its full potential. STT-MRAM employs costly sense amplifiers that demand an order of magnitude more area and power than DRAM. To manage the high cost, STT-MRAM shares one sense amplifier across multiple bit-lines, exploiting the non-destructive nature of its read operation. This sense amplifier sharing reduces the size of row buffers; as a result, it incurs higher activation energy and lower performance. Other issues arise if STT-MRAM is required to be compatible with DRAM interfaces and policies. To address these challenges in a cost-effective manner, we propose SMART that, unlike DRAM and conventional STT-MRAM, waits to do bit-line sensing until after receiving a column access command instead of a row activation command. This results in several benefits: larger pages, fewer sense amplifiers, lower activation power, higher bank-level parallelism, shorter latency, fewer address pins, and more efficient repairing of defective columns than conventional STT-MRAM. Our evaluation shows that SMART consumes lower energy while providing higher performance than conventional STT-MRAM and DRAM. Additionally, SMART consumes less area compared to conventional STT-MRAM.