Rank Modulation Hardware for Flash Memories

Flash has been widely used as nonvolatile memories, such as secondary or long-term persistent storage, but it has recently been adopted for solid-state drives in many mobile applications. Advances in flash technology, such as multi-level cells and error correction, have improved storage density and reliability, but these characteristics continue to be a challenge as the technology scales to smaller dimensions. Most notably, these techniques have been unable to directly deal with the failures caused by block erasures, except by spreading out the erasures across the entire memory through wear leveling. Rank modulation provides a new approach to multi-level flash memory cells; it uses the relative ranking of cell levels, instead of their absolute values. A flash memory with rank modulation based upon winner-take-all circuits is proposed, simulated, and fabricated. Using rank modulation, memory reliability and capacity is further improved.