Two different memory technologies are currently used as the memory that is accessed by the CPU, SRAM and DRAM. Both are volatile, meaning data stored will be lost when the power fails or is turned off. A new class of memory (Persistent or Storage Class Memory, SCM) is being developed that is non-volatile i.e. data does not disappear when power is lost
Memory located close to the CPU needs fast access time, or latency close to the CPU clock speed, for it to be used as CPU cache memory. The latency of both SSD and HDD data storage devices is too slow to be used. Differentiating memory from storage is key when discussing this topic (here, memory refers to bit-addressable memory utilized for registers and cache accessed by the CPU, while storage refers to devices using systems to organize and store data, and is not bit-addressable).
Some designs of non-volatile memory with low latency can compete with DRAM: these are called MRAM (Magnetoresistive Random Access Memory) or STT-MRAM (Spin Transfer Torque Magnetoresistive Random Access Memory). Other technologies that are between DRAM and SSD in latency are PCM (Phase Change Memory) or ReRAM (Resistive Random Access Memory). Western Digital has researched non-volatile memory for several years, ranging from basic materials research (including cell physics and design), to memory cell array fabrication and testing. All these efforts will be combined when a product is launched into this emerging market. These three areas are the “hardware” side of this technology. Other research targets the “software” implications of the transition to non-volatile memory.