Phase Change Memory (PCM)
Abbreviated as PCM, phase change memory is a type of non-volatile memory that is much faster than the common flash memory, and it also uses up to one-half the power. It works by using a semiconductor alloy that can be changed rapidly between an ordered, crystalline phase having lower electrical resistance to a disordered, amorphous phase with much higher electrical resistance. Because no electrical power is required to maintain either phase of the material, so phase-change memory is non-volatile.
This emerging non-volatile technology pioneered by Intel, Numonyx, Samsung and others — could turn out to be a low-cost, more reliable, faster, and just plain better alternative to flash memory. Some industry insiders even believe PCM has the potential to accelerate the data storage market's slow transition from hard disk drives (HDDs) to solid state drives (SSDs).
EnterpriseStorageForum.com has been covering storage networking for more than a decade. In this "Phase Change Memory: The Next Big Thing in Data Storage?" featured article, Herman Mehling offers an overview of phase change memory (PCM) and discusses its potential use in enterprise data storage environments.
What Exactly Is Phase Change Memory?
PCM offers high performance and low power consumption, combining the best attributes of NOR, NAND and RAM within a single chip, said Ed Doller, the chief technology officer of Numonyx, a joint venture created by Intel and STMicroelectronics. He said those attributes are: bit-alterable; non-volatile; fast read speed; fast write/erase speed; and good scalability.
Bit alterable: Similar to RAM or EEPROM, PCM is bit-alterable — meaning that stored information can be switched from one to zero, or zero to one, without a separate erase step. Flash memory technology requires a separate erase step in order to change information.
Non-volatile: PCM is non-volatile, as are NOR flash and NAND flash. PCM does not require a constant power supply to retain information, while RAM does.
Read performance: Similar to RAM and NOR flash memory, PCM features fast random access times. This enables the execution of code directly from the memory, without an intermediate copy to RAM. The read latency of PCM is comparable to single bit per cell NOR flash, while the read bandwidth can match DRAM.
Write/erase performance: PCM will achieve write throughput speeds faster than NAND and with lower latency. These features, when combined with a no separate erase step (bit-alterable), will deliver significant write performance improvement over NOR and NAND flash.
Scalability: Scaling is another area where PCM offers a difference. Both NOR and NAND rely on floating gate memory structures, which are difficult to shrink. As the memory cell shrinks on flash, the number of electrons stored on the floating gate shrinks. Because PCM does not store charge (electrons), it is immune to the charge storage scaling issue.
Recently, Intel and Numonyx researchers demonstrated a 64MB test chip that stacks, or places multiple layers of PCM arrays within a single die. The layering of arrays provides the scalability to reach higher memory densities while maintaining high performance rates.
So far, the cost of NAND flash technology has been driven to extremely low levels through the use of leading edge lithography, but it remains to be seen how much lower it can go. That is why many are looking for alternatives.
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