DDR2

DDR2 RAM, also called DDR II RAM, reaches speeds four times that of equivalent SDRAM modules. To do so, it builds upon DDR technology that allows the DRAM to both read and write twice each clock cycle. (SDRAM allows either a read or a write each clock cycle.) DDR2 uses two cores to effectively double DRAM speed yet again.

In actuality, DDR2 has only one core, but because it can be accessed in parallel, the results are the equivalent of having two separate cores working simultaneously. Data buffers are also doubled to insure that 4 bits of data can be processed during each clock cycle. DDR2 is not, however, without drawbacks.

Drawbacks to DDR2

While DDR2 can process more data bits per clock cycle, there is also an increase in latency. While some low latency DDR1 modules have a latency of 2 cycles, DDR2 modules have a latency of 3 to 5 cycles. The delay before being able to access a new row of data on the module (data on RAM modules is arranged in row and columns) can be as high as 8 cycles.

Another drawback occurs when upgrading from DDR to DDR2. DDR2 is incompatible with DDR motherboards because the pin count on DDR2 DIMMs is higher than the pin count for DDR. DDR2 memory is also more expensive than DDR memory, although DDR2 modules will eventually be a better value than standard DDR memory modules due to their increased performance.