Flip-N-Write

The phase-change random access memory (PRAM) technology is fast maturing to production levels. Main advantages of PRAM are non-volatility, byte addressability, in-place programmability, low-power operation, and higher write endurance than that of current flash memories. However, the relatively low write bandwidth and the less-than-desirable write endurance of PRAM remain room for improvement. This paper proposes and evaluates Flip-N-Write, a simple microarchitectural technique to replace a PRAM write operation with a more efficient read-modify-write operation. On a write, after quick bit-by-bit inspection of the original data word and the new data word, Flip-N-Write writes either the new data word or the "flipped" value of it. Flip-N-Write introduces a single bit associated with each PRAM word to indicate whether the PRAM word has been flipped or not. We analytically and experimentally show that the proposed technique reduces the PRAM write time by half, more than doubles the write endurance, and achieves commensurate savings in write energy under the same instantaneous write power constraint. Due to its simplicity, Flip-N-Write is straightforward to implement within a PRAM device.