Hardware Behind the Consoles - Part I: Microsoft's Xboxby Anand Lal Shimpi on November 21, 2001 3:47 AM EST
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nForce in Action - The X-IGP
Although the chipset made its "debut" at Computex 2001 we are just now seeing NVIDIA's nForce appear on retail boards most of which won't ship for a few more weeks. In spite of NVIDIA's disappointing launch on the PC side, the nForce platform has played a huge role in the development of the Xbox.
Months ago NVIDIA told us that it was the nForce design that won them Xbox; the PC derivative came afterwards. It was no surprise then to find out that the nForce chipset could work just as fine as a Pentium III solution or an Athlon solution. Of course us PC users get to see nForce paired with an Athlon, but because of Microsoft's licensing of the P6 bus from Intel the world is able to see the Pentium III on the nForce platform as well.
The chipset behind the Xbox does veer slightly from the nForce 420-D design for the PC. The Integrated Graphics Processor (IGP) still functions as the effective North Bridge for the platform however it features a much more powerful graphics core than the PC's nForce IGP. Whereas the nForce IGP on the PC features a GeForce2 MX (NV11) core, the Xbox IGP features a custom designed core internally known as the NV2A. You can take the codename to mean that the integrated graphics offers performance and features somewhere in-between the currently available NV20 (GeForce3) core and the upcoming NV25 core.
The NV2A features the same 4 pixel pipelines of the GeForce3 core and operates at 233MHz. This puts the fill rate of the NV2A in between that of the GeForce3 and the GeForce3 Ti 500. Naturally the NV2A features the same DirectX 8.1 pixel and vertex shader support that was introduced with the GeForce3 with one major modification - the NV2A has two vertex shaders. The addition of a second vertex shader is a huge performance gain for games the reason being that most instructions that will be sent to the vertex shader require at least two instructions to execute. Providing the GPU with dual vertex shaders will tremendously increase the throughput of these vertex programs allowing some operations to be completed in a single clock cycle. Considering that a very impressive feature of the vertex shader is the ability to do the setup for DOT3 bump-mapping, the dual vertex shaders will definitely help performance tremendously in titles that make extensive use of per-pixel DOT3 bump-mapping.
The core also supports the same multisampling AA formats as the GeForce3 including Quincunx. This is actually a very important feature since most games are rendered and displayed at 640 x 480 where aliasing is especially bad.
The rest of the features of the Xbox IGP are identical to that of the nForce IGP which we've covered extensively here.