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Inside the Xbox GPU - tech
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| (hx) 06:21 PM CEST - May,23 2001 |
Microsoft has often emphasized the power of the Xbox's Nvidia-designed graphics chip, code-named the NV2A. However, there have been varying reports as to whether the Xbox chip is derived from the Nvidia's most recent PC graphics chip, the GeForce3 (known internally as the NV20), or a future design, the NV25. Microsoft has previously stated that the NV2A is closely related to the NV25, but Nvidia representatives close to the engineering teams have told GameSpot otherwise.
In fact, the NV2A is a faster GeForce3 with one major addition to the hardware: a second vertex shader pipeline. Since the chip design was recently finalized for production, Nvidia could confirm that it will run at the faster speed of 250MHz. This puts the pixel fill rate at 1,000 megapixels and the texture fill rate at 2,000 megatexels--much lower than the Microsoft's original official spec of over 4,000 megapixels and 4,000 megatexels, but fill rate isn't the most important spec. Fill rate is not a determining factor because, as a console, the Xbox is naturally destined to run at the low resolution of consumer TVs. Also, most of the Xbox's special capabilities, such as vertex shaders or full-scene antialiasing, occur at different points in the pipeline and are not at all dependent on fill rate. More important is the NV2A's bandwidth to the 64MB of shared memory. The memory in the console is 200MHz double-data rate (DDR) SDRAM, which provides a maximum of 6.4GB per second of throughput to both the CPU and graphics chip. The custom CPU uses up to 1GB per second of this shared bandwidth. This leaves less headroom than the GeForce3 has with its 7.36GB per second bandwidth, but again television's lower display resolution means this should be plenty. The shared memory design also avoids the bottlenecks PC developers see when sending a lot of data from a PC's main memory over the AGP bus to the graphics card.
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