7800 GT versus 7800 GTX
HEXUS showed you the technical details of G70 here, outlining its reference clocks for the 7800 GTX SKU. 7800 GT differs only in its fragment quad count, vertex unit count and clock frequencies.| 7800 GT | 7800 GTX | |
| Fragment units | 20 | 24 |
| Vertex units | 7 | 8 |
| Core clock | 400MHz | 430MHz |
| Memory clock | 500MHz | 600MHz |
So one less fragment quad (group of four fragment units that process pixel fragments before output), one less vertex unit and slightly slower clocks. Since it's the same G70 chip that powers the GTX, NVIDIA mask off a quad and vertex unit to achieve the GT's configuration. Yeilds likely play a large part in why they chose that way of creating the new configuration, rather than simply lowering clocks on a fully working chip.
Not all G70 processors come back from the fab with all processing units functional. Infact, it's a fairly low proportion that come back fully functional. The chips are tested prior to packaging for functionality and the ones that have a single non-working fragment quad and vertex unit are marked as such and used for the GT. The other side of that coin is demand for the GT part is likely to be much higher than the GTX, due to price. It stands to reason that fully working G70s could therefore also be used to create GTs, depending on demand and what's coming out of the fab at the time, as defect rates drop over time.
The GT variant of G70 still has 16 ROP units to output fully resolved pixels from, leaving it still able to match the GTX's pixel output rate provided both are at the same frequencies.
So with the information above, we can then calculate the GT's deficits in relation to the GTX in terms of theoretical shader rates, pixel output rate and memory bandwidth. The GT ends up with around 77% of the fragment processing ability, around 81% of its vertex processing ability, and around 83% of the memory bandwidth, compared to the GTX.
Given the base performance of the GTX, giving up those numbers is likely to be no bad thing. Of course, being a G70-based product, 7800 GT inherits Shader Model 3.0 capability, the ability to filter and blend FP16 surfaces, SLI support for running two 7800 GT graphics cards in the same system to increase performance, and all the other basic attributes that make the 7800 GTX so attractive for the well-heeled enthusiast.
Let's look at a pseudo-reference board to see form factor and other physical attributes.
