Texturing Units and RBE

Nvidia G00 texturing processors haven’t changed that much since G84/G92, although Nvidia claims that they improved the control logic, which sped up texture sampling. There are 8 texturing processors with two addressing and two texture filtering units assigned to each of the 10 computational clusters of the chip.

Theoretically, this architecture of texture processors allows processing up to 80 pixels per clock, but with enabled anisotropic filtering or floating-point textures, this number drops down to 40. In other words, 80 texturing processors in reality often turn into 40. This Nvidia’s passion for growing texturing performance at any rate cost them a significant number of transistors.

Although computational part occupies a bigger area, texture processors till take up a lot of space. As a result, the G200 die got almost 600sq.mm big, and it became so complex that the developers had to move display controllers into a separate chip once again, the same way they did with G80 back in the days.

The basic functionality of G200 raster processors remained the same, however, the number of render back ends (RBE) increased to 8. In other words, the new chip has 32 ROP and can release up to 32 pixels per clock.

This is another way of using extensive approach to increasing performance. However, intensive approach is present too: RBE can shift pixels in RGBA8 format at full speed, compared with half the speed by G80/G92.

On the one hand, 8 RBE units translate into 8 64-bit memory controllers and 512-bit external access interface with unmatched bandwidth. But on the other hand, 32 RBE demanded their piece of transistor pie, which turned out not that small at all, as you can see on the diagram above. Moreover, the number of RBE is rarely the main factor limiting the gaming performance, although 32 raster processors may prove pretty handy in resolutions over 1920x1200 with enabled full-screen anti-aliasing.  We are going to discuss it in detail in our Nvidia GeForce GTX 200 gaming performance review that will follow shortly.

I have to say that betting on multi-GPU solutions seems to make much more sense here: dual-GPU ATI solutions as well as Nvidia GeForce 9800 GX2 also have 32 RBE, but they do not require a lot of time and resources for designing a new high-end monolithic GPU.