256 Bit Memory Interface
The back of the card, cooled through a large passive element.
At 256 bits, the memory bus of the NV35 is twice as wide as that of the NV30 design. Many may view the switch from DDR-II memory "back" to conventional DDR as a step backwards. Looking at the numbers, this doesn't seem to be the case, though. With its RAM running at 425 MHz, the card offers a peak memory bandwidth of 27.2 GB/s, a major improvement over the 16 GB/s of the NV30. Not even ATi's Radeon 9800 with its 21.8 GB/s can keep up in this respect. Since FSAA and anisotropic filtering put a major strain on the memory subsystem, the extra bandwidth should have a noticeable effect.
One of the more remarkable features of the FX 5800 (NV30) is the so-called "Color Compression." This compression is primarily employed when FSAA is enabled. For example, when 4xFSAA is used, four sub-pixels of one pixel are calculated and then averaged to produce the final color value. If all four of these sub-pixels lie within the polygon and not on its edge, the optimal compression ratio of 4:1 can be achieved. Of course, the real-world efficiency, i.e., the number of pixels that can be compressed, strongly depends on the textures that are used. The efficiency of this compression technique has been improved by nearly 50 percent in the new FX 5900 Ultra using HCT (high compression technology). Of course, the compression factor still remains 4:1, but that factor can now be achieved more often.
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