Dual Channel DDR2 Memory Controller

Pentium 4 processors have always required a lot of memory bandwidth to perform at their best. Hence, Intel's initial support of the failed quad-pumped RDRAM memory system.

Eventually, Rambus' high-handed approach and market forces forced Intel to abandon RDRAM for the far more popular DDR memory. However, even DDR was not quite capable of providing enough memory bandwidth for the Pentium 4 to show its full potential.

Intel then followed NVIDIA's footsteps and introduced dual channel memory controllers in their i875/865 chipsets over a year ago. This allowed the Pentium 4 platform to maintain the performance lead over the AMD's Athlon XP platform.

To further improve memory bandwidth, Intel has introduced a new dual channel DDR2 memory controller with the i925/915 chipsets. DDR2 is supposed to allow higher memory clock speeds as well as a lower voltage and power consumption.

You may think that the new dual channel DDR2 controller will provide at least double the bandwidth of the old DDR controller in the i875/865 chipsets. However, the truth is the dual channel DDR2 memory controller will only provide a slight increase in bandwidth, and that's only due to the higher memory clock speed of 533MHz.

Don't forget, the output pins of DDR2 and DDR memory are similarly clocked. DDR2 memory running at 400MHz will have the same peak bandwidth as that of 400MHz DDR memory. Of course, DDR2 memory modules have higher latencies than DDR memory. This reduces DDR2's effective bandwidth so that DDR memory will always provide better performance than DDR2 memory at the same clock speed.

The main advantage of DDR2 memory is the fact it allows memory manufacturers to ramp up memory clock speeds using current DRAM technology. As you know, manufacturers are having trouble ramping up DRAM core speeds beyond 250MHz. Switching to DDR2 allows memory manufacturers to create faster memory modules without actually increasing the core speed.

The output buffers in DDR2 modules prefetch 4n-bits of data, instead of just 2n-bits in DDR modules. This allows manufacturers to reduce the core speed by half to create a DDR2 module that runs at the same speed as a standard DDR module. For example, the DRAM core of a 400MHz DDR module runs at 200MHz but in a DDR2 module, the DRAM core actually runs at just 100MHz! Needless to say, DDR2 allows manufacturers to ramp up clock speeds without developing even faster DRAM cores.

Incidentally, even with 533MHz DDR2 memory, the i925/915 chipsets won't see that much of an improvement in memory bandwidth compared to the i875/i865 chipsets running at just 400MHz. This is because to run at 533MHz, the i925/915's memory bus have to run asynchronously with the processor bus. This introduces even more latency and further reduces its performance.

Now that we got the new Intel i925X chipset covered, let's go take a look at the ASUS P5AD2 Premium motherboard!