Why is DDR so important?

Memory Bandwidth

Double Data Rate(DDR) SGRAM is the key behind GeForce 256 performance. DDR effectively doubles the memory data rate by reading and writing at both the rising and falling edge of each clock cycle. Normal Single Data Rate(SDR) RAM only uses the rising edge of each clock cycle. Since DDR designs are based on SDRAM/SGRAM, DDR RAM will also be able to take advantage of the speed increases we've seen in SDRAM. 5ns DDR SDRAM/SGRAM chips should be widely available starting the first quarter of 2000.

In our Next Generation 3D Spec Shootout, we picked the Savage2000 over the GeForce because the S3 chip's rendering pipeline could pump out more texels per second. We gave memory bandwidth concerns a small mention in passing, but we neglected to explain how memory bandwidth actually affects fill rates.

There's more to fill rate than the pipeline

Okay, we already know the GeForce's theoretical peak fill rates of 480Mpixels/s and 480Mtexels/s, but once fill rates get this high, memory bandwidth becomes an issue. When Bitboys announced the new Glaze3D, the planned use of embedded DRAM raised a few eyebrows. Why go with an expensive and unproven memory when SDRAM is relatively cheap and readily available? Simple, the memory bandwidth in a normal SDRAM subsystem won't be enough to support the rendering pipeline on the Glaze3D. The same applies to the GeForce.

To use a simple Quake 3 analogy, pretend the rendering pipeline is the quad damage, and your weapon is the memory bandwidth. Your total damage dealing capability would be your fill rate. The amount of damage you can deal (fill rate) depends on the quad (rendering pipeline) and your weapon (memory bandwidth). Now, when the GeForce has normal SDRAM it's like having quad damage with a machine gun. You're still going to do a lot of damage because of the quad, but you'd be able to do a lot more damage if you had the lightning gun(DDR RAM) instead of the machine gun(SDRAM).

Okay, we left out a lot of other factors that affect framerate such as the non-graphics related load on the CPU, texture map transfers, the CPU-to-GPU data stream, and caching issues but you get the idea of how important the memory subsystem is on the graphics board.