NVIDIA GeForce 7800 GS AGP
Sooooo, what's under the hood? It'll no doubt surprise you to know that it's NVIDIA's brand new 45nm test GPU, produced by ATI, called RNV717, clocked at 2000MHz. No, really, they're just marked as NVIDIA G70 to put you off the scent.A2 G70 cores that don't quite make the grade as a 7800 GT or GTX on PCI Express. Paired with NVIDIA's in-house BR2/HSI PCI Express-to-AGP bridge ASIC, G70 gets to ride the slightly short bus to school with AGP8X compatibility on the far side.
Specifications read as follows. Pass the Kleenex, my AGP-using brethren!
| NVIDIA GeForce 7800 GS Reference Specification | |
|---|---|
| GPU | NVIDIA G70 |
| Vertex Shader Units | 6 |
| Pixel Shader Units | 16 |
| ROPs | 8 |
| Core Clock | 375MHz |
| Memory Clock | 600MHz |
| Memory Bus Width | 256-bit |
| Pixel Fillrate | 3G pixels/sec |
| Texturing Fillrate | 24G samples/sec |
| PS MADD rate | 12G instructions/sec |
| Transformed vertex rate | 5.625G vertices/sec |
| Memory Bandwidth | 38.4GB/sec |
Fairly beastly numbers for a part on AGP, I think you'll agree. While the ROP count sees it give up pixel output rate figures big time, to the likes of a 6800 Ultra or X800 XT, we all know by now that ROP count rarely limits real-world performance.
GeForce 7800 GS AGP boards are specified with 256MB of GDDR3 at 600MHz, endowing the 375MHz GPU with gobs of memory bandwidth with which to get the job done. Texturing rate on NVIDIA's mainstream GeForce 7-series products is limited by memory bandwidth, so the apparent excess shouldn't go to waste.
GeForce 7-series highlights
Since it's powered by G70, GeForce 7800 GS AGP gets some excellent additions to image quality and potential performance per clock (and watt), compared to 6-series products like 6800 GS.G70 allows for transparency anti-aliasing, a rendering ability that allows the GPU to essentially push alpha textures through the AA hardware for extra Z or colour samples outside of what they'd usually get, at a sub-pixel level. The resulting extra data allows the chip to draw alpha textures at higher IQ than normal.
G70's texture cache gets tweaked for better access performance when filtering, so the hit for applying IQ enhancing texture filtering is less than 6-series hardware.
G70 is also better at issuing scalar instructions in its vertex units which I guess to be down to auto-vectorisation tweaks. Triangle setup is appreciably quicker than 6-series per clock and it's well known that the fragment hardware in G70 will do an NV35-esque double MADD issue per cycle, 4D wide.
Summary
In humerous and easy to understand terms, G70 takes a poop on NV40, clock-for-clock, in a few areas, especially pixel shading. So even moderately clocked there's a lot to be said for it.Plenty of memory bandwidth keeps it all fed and the IQ improvements - we mention TAA earlier - like gamma correct AA are all understood and very welcome.
Unlocking the masked units and clocking it a bit are left as exercises for the reader. Fancy a peek at a retail board? NVIDIA, happy little pixies that they are, didn't sample reference boards to press this time. Or at least they didn't sample to us!
BFG stood up to the plate instead, supplying HEXUS with their GeForce 7800 GS OC™. And it's quite unlike any other GeForce 6 or 7-series board yet produced.
