Specifically, there are several characteristics of STT-RAM, PCRAM, and ReRAM that make them promising as working class memories (i.e., on-chip caches and off-chip main memories), or as storage class memories: (1) Compared to SRAM/DRAM, these emerging memories usually have much higher density, with comparable fast access time; (2) Due to the non-volatility feature, they have zero standby power, and immune to radiation-induced soft errors; (3) Compared to NAND-Flash SSD, STT-RAM/PCRAM are byte-addressable.