NVIDIA GeForce 64 MB

DDR vs. SDR

The limitations to the GeForce product line described in the previous section were predicted by NVIDIA upon the GeForce's release. It was no secret that the GeForce would simply have problems sending out the required data on such a narrow memory bandwidth. In anticipation of this problem, NVIDIA released both SDR and DDR versions of the GeForce. What are the differences between the two varieties of GeForce cards? Both use the same processor with the same fill rate, so how can the DDR be significantly faster than the SDR? The difference comes from increasing the memory bandwidth, thus decreasing the bottleneck.

SDR, standing for Single Data Rate, is essentially the same type of memory found on pretty much all video cards up until now. What single data rate means is that for every memory clock cycle, the memory can be written to one time. This means that data can be passed from the GPU to the memory at a rate equal to the speed of the memory clock. For SDR GeForces, this speed is 166 MHz. The resulting peak available bandwidth can be calculated by multiplying the SDRAM clock speed (166 MHz) by the number of times written to per cycle (1 time) times the memory bus width (128 bits) times a conversion factor of 1/8 to convert bits to bytes. This results in 2.7 GB/s peak bandwidth for SDR GeForce cards.

DDR, standing for Double Data Rate, made its first commercial video card appearance with the GeForce. With double data rate RAM, the memory is actually written to twice per clock cycle of the memory clock. Writing on both the rising and falling edges of the cycle, the stock memory speed for DDR cards is 300 MHz, achieved by having 150 MHz RAM being written to twice per cycle. The 300 MHz rating is not the actual speed that the RAM is running at, that remains at 150 MHz, it is just that it is written to twice as fast, resulting in the '300 MHz' speed. Since memory clock speed is directly related to memory bandwidth, bandwidth is increased by having the faster memory clock. This is shown by the following peak bandwidth calculation, as described above: 150 MHz SDRAM clock * 2 writes per clock cycle (DDR) * 128 bits (memory bus width) * 1/8 (8 bits in a byte) = 4.8 GB/s peak bandwidth.

There is no question that DDR GeForce cards outperform their SDR counterparts. While theoretical fill rate remains the same in both cards, the effective fill rate is substantially greater in DDR cards due to the decreased bottleneck achieved by widening the memory bandwidth. The increase in speed proves that memory bandwidth is crucial to video card speed.