NVIDIA GeForce 64 MB

Texture Swapping: Hidden Evil

With the AGP bus came a new era in graphic card design. No longer would no longer would a video card have to swap textures, at least in theory. One of the most heavily touted features of the AGP bus was the fact that it could render by reading textures directly out of system memory. While this may be the case, one has to consider the very poor speed of the AGP bus as well as the fact that the GeForce does not seem to take advantage of this feature. Odds are that you have experienced texture swapping issues on many occasions during heavy 3D graphics use. The result of such texture swapping is quite evident to the user: the game or program will be responding speedily and then will begin to crawl upon entry into a new room or area with many textures. The response will remain sluggish as long as the path is changing or new objects are being encountered. The symptoms are alleviated quickly upon entry into a smaller or less complex room or area, where upon game speed and response time return to normal. To explain this phenomena, we once again turn to the memory bandwidth.

While at low colors, low resolutions, and/or low scene complexity, the 32 MB of on board video memory that many GeForce cards have is sufficient to render the current frame to the screen. The problem, however, arises when this 32 MB of available memory gets saturated with data. This usually occurs when in 32-bit color mode, high resolutions, and highly complicated scenes. The most drastic change is often times seen when going from 16-bit to 32-bit color because a scene rendered in 32-bit color requires twice as much memory as the same scene in 16-bit mode. The computer has to be able to deal with the fact that the video card's memory is full and it still needs more textures to render a frame.

The way that the computer handles the problem is by calling upon the system memory for help. That is right, the RAM you have installed in your computer is occasionally called upon to help render complex video scenes. Overflowing textures are stored in the system memory and pushed back out to the video card when called upon. While this would be great if the path to the system memory was fast, it turns out that the slowness in the game is caused by the slow path that the textures are forced to take. As explained in DDR versus SDR section, the peak available memory bandwidth in DDR cards is 4.8 GB/s and 2.7 GB/s in the SDR GeForce card. We also saw in this section how much of a bottleneck is created by going a 'slow' 2.7 GB/s, thus resulting in the large speed increase of DDR cards over SDR cards. Well, you thought that 2.7 GB/s was pokey? Wait until you see how much peak bandwidth the AGP bus has.

We can calculate the peak bandwidth using the same method mentioned in the DDR versus SDR section. For a system running at AGP 1x mode, we take the AGP clock speed (66 MHz) and multiply it by the AGP mode (1x), then multiply it by the AGP bus width (32-bits) then times it by 1/8 to make up for the fact that there are 8 bits in one byte. The resulting speed is a super slow 266 MB/s, a mere 5% of the DDR's on card memory bus. When running in AGP 2x mode, the results are not much better. Using the same calculation as above and replacing the AGP mode with 2x, we find that in this mode the peak available bandwidth is approximately 533 MB/s. Once again, this about 11% of the DDR's speed. Finally comes the speed of writing to the system memory at AGP 4x. This time we replace the 2x with a 4x and find that at AGP 4x mode, the memory can be written to at 1.06 GB/s. While this approaches the speed of the DDR GeForce, it still remains only 22% of the DDR's power.

We saw how much difference dropping from 4.8 GB/s to 2.7 GB/s makes when comparing DDR to SDR cards. 2.7 GB/s still remains at 50% of the DDR's power. Dropping to the speed found when using the AGP bus system is the reason that the computer slows down when under heavy textures. If we could only prevent using the system memory from rendering and rely on the fast DDR memory bus, we would have a winner on our hands. Well, NVIDIA has come up with just the solution: the 64 MB DDR GeForce.