Trident XP4 Preview - A Cheap GeForce4?

Truly Four Rendering Pipelines?

The reason the XP4 is basically half the size of a GeForce4 comes down to Trident's implementation of their 4 pixel rendering pipelines. These four pipes make up the largest single part of the GPU by far and the more pipes you have, the greater the transistor count gets. ATI and NVIDIA outfit their GPUs with multiple pixel rendering pipelines by producing one and basically duplicating the logic multiple times until they reach the desired number of pipes. For example, if one pixel rendering pipeline on the GeForce4 took 15M transistors to implement then each subsequent pipeline would take 15M transistors.

Trident took a slightly different approach with the XP4; the first rendering pipeline of the XP4 is identical to any DX8.1 pixel shader pipeline on any present-day GPU. Using the example above, we can say that this pipe would be made up of around 15M transistors. However, instead of duplicating the logic three more times to implement the remaining pipes, Trident implements a large amount of resource sharing very early on in each of the subsequent pipelines. The end result is that each pipeline after the first one is less complex and shares some of the logic that went into the first pipeline.

Four pixels can still come out of the pipes every clock but, because of the resource sharing, the performance of the solution will never be as great as four completely independent pipelines as there is a reduction in parallelism with this design.

The obvious benefit of Trident's approach is a massively reduced transistor count, making the chip much more affordable to make. To give you an idea of the level of transistor savings, if you think of the first pipe requiring 15M transistors then the second would require only 7.5M, the third 3.8M and the fourth around 2M transistors. These are obviously rough estimates but they can give you a good idea of the transistor savings the XP4 is able to enjoy because of this technology.

The big question here is at what performance cost, which we won't be able to find out until we're able to do some extensive testing on the first XP4 solutions in a few weeks.