While Intel and AMD prepare their slates of new CPU releases and transitions from 0.13-micron to 90-nanometer process manufacturing, two other architectural changes await the PC in 2004. One is the long-awaited arrival of the PCI Express bus as a replacement for today's PCI and AGP alike (to be discussed in a future article); the other involves the world of system memory -- all signs point to the new year being the start of the DDR-2 revolution.

Actually, "evolution" would be a better word; just as double-data-rate (DDR) replaced standard SDRAM over the past two years, DDR-2 is the next step in the chain, anointed by industry support and economics as well as technological advances. Instead of the usual case of "bigger, better, faster" at work, DDR-2 is more of a slight improvement on current DDR design that brings a combination of higher performance and lower cost, while trying to sidestep a few potholes along the way.

In case your own memory needs refreshing (see November 2002's "Making Sense of System Memory" -- Ed.), the move from SDRAM to DDR consisted of doubling the data rate by transferring data on both the rising and falling ends of each clock cycle. This resulted in a doubling of the base clock speed, with the 133MHz of PC133 SDRAM becoming the 266MHz of DDR266 (a.k.a. PC2100 for its 2.1GB/sec transfer bandwidth).

DDR-2 changes the basic specifications once again, while maintaining backwards compatibility with DDR commands -- the new memory shares the same basic design and features, while increasing the actual data fetch rate.

Instead of current DDR's fetch rate of 2 bits per clock cycle, DDR-2 memory can fetch 4 bits per clock. This means that, with equivalent core speeds, DDR-2 offers theoretically double the data throughput of DDR or quadruple that of conventional SDRAM.

DDR-2 also lowers the voltage requirement to 1.8V, compared to standard DDR's 2.5V (some of today's ultra-high-end DDR modules require up to 2.75V to hit their top-rated speeds). DDR-2 also incorporates some recent chipset features, such as programmable on-die termination (for better signals at high speeds), along with other memory-timing and -configuration controls.

Clock Speeds and other Fairy Tales

If DDR worked some sleight-of-hand to achieve its rated speeds, DDR-2 is a master magician. Today's mainstream desktops' DDR400 memory modules perform at an effective 400MHz, transferring data on both ends of the clock with an actual core speed of 200MHz. That's quite a feat, but DDR-2/400 doubles the fetch rate to achieve the same theoretical performance while actually running at a mere 100MHz.

We say "theoretical" because the requirement of 4 bits per clock cycle tends to introduce additional latencies; if the DDR-2 memory is transferring fewer than 4 bits of random data per clock, the additional data fetch rate goes unused. Exactly how this affects performance will depend on the application, but in testing 3D graphics cards, we've found that DDR-2 had to be clocked slightly higher than DDR memory to make up for latencies.

As they ramp up for a 2004 release, memory manufacturers seem to be targeting the DDR-2/400 through DDR-2/800 ranges -- running, respectively, at 200MHz with 3.2GB/sec of data bandwidth and 400MHz with 6.4GB/sec of bandwidth. It's easy to see why memory makers are jumping on the DDR-2 bandwagon, as transitioning today's DDR400 (PC3200) technology into DDR-2/800 lets them advertise double the bandwidth, without a big increase in production costs. That also means higher profits, since the faster memory will obviously command a premium price.