3D: Its Power And Functionality
The general Xbox architecture is derived from the NVIDIA nFORCE platform, or vice-versa. There is a Northbridge chip equipped with a graphics processor, such as the IGP by nFORCE, and a Southbridge chip, which also functions as the audio control (similar to the MCP 420-D).
Unlike the IGP by nFORCE, which uses a graphics processor derived from the GeForce 2 MX, the Xbox's XGPU uses a derivative of the graphics processor GeForce 3. The architecture of the NV2A is situated between the NV20 (GeForce 3) and the NV25 (GeForce 4). It actually has a four-pixel pipeline and two vertex shaders. By comparison, the NV20 offers four pixel pipelines and one vertex shader, and the NV25 also has two vertex shaders. The graphics chip probably consists of about sixty million transistors (NV 20:57 / NV 25:63). This 233 MHz Northbridge chip is connected to 64 MB of 200 MHz DDR memory. Be careful, the 64 MB has to be shared with the rest of the system, as we have already explained.
At 233 MHz, the theoretic fill rate of the NV2A increases to 933 Mpixels/s, while each pipeline can produce two textures in one pass. The total memory bandwidth amounts to 6.4 GB/sec. The Pentium III will use only a maximum of 1.06 GB/sec as a result of its 133 MHz FSB, leaving 5.34 GB/sec for the GPU. This value, however, is inferior to the memory bandwidth of a GeForce 3 Ti 500, which has 7.45 GB/sec available. Remember, however, that the games are programmed only with a resolution of 640x480x32 bits. This bandwidth will lag behind with the use of anti-aliasing, a multi-sampling mode derived from NVIDIA's Quincunx. With consoles, the anti-aliasing becomes all the more important because of TV's weak resolution.