We've been discussing DDR2 memory (here and here) so what, exactly, is it?

In fact, DDR2 (Double Data Rate 2) is a memory specification that follows on from the current DDR specification. The specs are set by JEDEC, the Joint Electron Device Engineering Council, to which most important chip manufacturers belong.

DDR2 paves the way to faster memory, which means better performing servers and desktops. Initially though, DDR2 devices will not be faster than current DDR chips but will become faster in future.

High speed graphics cards already use DDR2 memory and the technology is beginning to transition to servers and desktops. The specification for server and desktop DDR2 is slightly different from that for DDR2 memory used in graphics cards.

DDR chips run at one of four memory speeds, for example PC3200 memory runs at 400MHz. The technology cannot run much faster because signal quality inside the chip will degrade. DDR2 gets around this and also enables a speed increase. On-die termination is one way DDR2 keeps signal quality high, as it lets the chip absorb more signal noise.

Initially, DDR2 chips will be introduced at the same speeds as the fastest DDR chip but later versions will ratchet the speed up.

The speed increase comes because DDR2 can process twice as many signals as DDR through a differential signal technology. It has a 4-bit prefetch facility, compared to DDR's 2-bit. This means that DDR2 can read and write more data per clock cycle. In effect, the data bus speed is doubled, although the internal bus speed stays the same as that for DDR.

This gets complicated. The first DDR2 chips will run at a lower internal clock speed than DDR but, due to the prefetch advantage, the chips should perform equally; DDR2-400 will be as fast as DDR400, DDR-533 will be as fast as DDR2-533.

The internal bus speed for DDR2-400 is 100MHz. DDR2-533 chips will run at 133MHz.Other versions mentioned include DDR2-667, DDR2-800 and DDR2-1000, which should all be faster than any DDR chip.

DDR2 chip capacities of 256Mb, 512Mbit and 1BGit have been mentioned by suppliers such as Samsung and Micron. DDR2 module capacities of up to 4GB have been shipped by Micron to Intel and several PC manufacturers.

DDR and DDR2 chips both have a 64-bit wide data bus. Most DDR2 chipsets can be expected to support dual channel memory, providing a 128-bit wide data bus in practice.

DDR2 modules have 240 pins instead of DDR's 184 and cannot fit in the same socket. They also consume less power, as they run at 1.8V instead of DDR's 2.5V. As DDR2 chips get faster the power difference will diminish.

Both types of module are the same physical size. DDR2-suppporting motherboards are expected to become available throughout this year.

Gartner Dataquest thinks that the world-wide DDR2 market is going to be 10 percent of total DRAM sales this year but will increase to 66 percent by 2006.