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PCRAM

One of the most interesting new technologies for persistent storage is phase-change random access memory (PCRAM), which is based on the same physical principles as rewritable CDs. Using this kind of material for memory was suggested back in the '60s, but it's only in the last few years that it's become feasible.

The alloys used in PCRAM are heated up and then cooled to either an amorphous or crystaline state. In rewritable optical disks, the high temperature is created by a laser. It's then read by shining a laser and measuring the difference in the optical properties of the two states. PCRAM takes advantage of the fact that, as well as different optical properties, the two states have different electrical resistance. The amorphous state has a high resistance, while the crystaline state is a good conductor.

In either state, the material is fairly stable. You can power down the machine and leave it for a long time without any state changes. When you are reading values, PCRAM is very fast. It's read simply by putting a voltage across it and seeing if any current flows, so it can be almost as fast as normal RAM.

A lot of the recent research has focused on making the write times competitive without compromising the density. Flipping a bit in a PCRAM module requires heating that bit up to about 600°C. At the same time, the adjacent bits must be kept cool enough that they don't change. This can also be a problem when, for example, changing 01 to 10. You need to heat the material to different temperatures to achieve the different states, so when toggling two adjacent bits in opposite directions you need to have two high—but different—temperatures in close proximity.

This is made even more complex by developments like IBM's multilevel PCRAM materials. Multilevel flash cells store a charge at one of (for example) four levels, giving the equivalent of two bits in a single element. Multilevel PCRAM is similar in concept. The material solidifies in different states with more than just two resistances, allowing a single element to store multiple bits. This can dramatically increase the storage density, but at the same time makes the thermal constraints a greater engineering problem.

PCRAM is shipping now, but still in very small quantities. The largest modules that you can buy are under 64MB—not yet a competitor for 64GB flash chips in the mass storage arena. More interestingly, they're about the same speed as EDO DRAM modules that were used with Pentium-class systems. This makes them interesting for low power applications, because you can use them as main memory and consume no power when not doing any processing.

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