It has long being rumored over the Web that the new generation of graphics processors from the leading developers would have an architecture completely different from everything we’ve ever seen on the market as yet. Today we’ve got a chance to give a scrutinizing eye to the earliest representative of the new generation, the Nvidia G80 processor, and to the GeForce 8800 GTX graphics card based on that GPU.
Dawn of Unified Architecture
Graphics processors have come a long way since their origin. Their evolution started with rather simple devices like the GeForce 256 that had a modest selection of fixed-function capabilities. Such chips could not even be called processors in the true sense of the word because they were unable to execute unique program code. It was the Nvidia GeForce 3 (NV20) that became the first truly programmable GPU. It could run pixel and vertex shaders described in the Direct X 8.0 specification.
Later, the graphics processor was evolving in terms of programmability, so that it could execute ever more complex shader code. The GPU eventually transformed into an almost all-purpose computing device with tremendous calculating power and capable of visualizing the most sophisticated special effect the game developer’s imagination could bring forth. With some reservations, it came to be not unlike an ordinary CPU in terms of performance and universality: the maximum length and complexity of shader programs grew up with every new version of DirectX until became virtually infinite in Shader Model 3.0. But all GPUs have had one fundamental limitation until now: their execution units were divided into those that ran pixel and those that ran vertex shaders. So, each graphics processor had to contain two separate sets of units to process each kind of shaders.
This division, although it had a number of advantages, had a negative effect on the overall GPU efficiency. For example, in a pixel shader heavy scene the available pixel processors may turn to lack performance whereas the computational resources of the vertex processors remain uncalled for, or vice versa. Thus, the next step in the evolution of the GPU was evident. The described misbalance problem could only be solved by unifying the shader processors so that the overall load could be distributed among them dynamically depending on the specifics of the processed scene. The new product from Nvidia, the GeForce 8800 (G80) GPU, is the realization of that concept.
DirectX 10 provides more resources to the GPU, improving 3D performance. The key differences in
GPU resources between DirectX 9 and DirectX 10 you can see on the table below.
Resources DirectX 9 DirectX 10
Temporary Registers 32 4,096
Constant Registers 256 16 x 4,096
Textures 16 128
Render Targets 4 8
Maximum Texture Size 4,048 x 4,048 8,096 x 8,096
On the table below you see a comparison between shader models 1.0 (DirectX 8.1), 2.0 (DirectX 9.0), 3.0 (DirectX 9.0c) and 4.0 (DirectX 10).
Shader 1.x Shader 2.0 Shader 3.0 Shader 4.0
Vertex Instructions 128 256 512 65,536 *
Pixel Instructions 4+8 32+64 512 65,536 *
Vertex Constants 96 256 256 16 x 4,096 *
Pixel Constants 8 32 224 16 x 4,096 *
Vertex Temps 16 16 16 4,096 *
Pixel Temps 2 12 32 4,096 *
Vertex Inputs 16 16 16 16
Pixel Inputs 4+2 8+2 10 32
Render Targets 1 4 4 8
Vertex Textures - - 4 128 *
Pixel Textures 8 16 16 128 *
2D Texture Size - - 2,048 x 2,048 8,192 x 8,192
Int Ops - - - Yes
Load Ops - - - Yes
Derivatives - - Yes Yes
Vertex Flow Control - Static Static/Dynamic Dynamic *
Pixel Flow Control - - Static/Dynamic Dynamic *
The GeForce 8800 GTX and GeForce 8800 GTS, both targeted to PCI Express x16. GeForce 8800 GTX requires two auxiliary power connectors and has two SLI connectors (we are asking ourselves if this isn’t a future support for SLI mode with four video cards).
As we mentioned before, the shader engines use a different clock rate from the rest of the GPU.
We list below the main specs for these two new video cards.
GeForce 8800 GTX GeForce 8800 GTS
Core clock 575 MHz 500 MHz
Streaming Processors (Shader Engines) 128 96
Streaming Processors Clock 1.35 GHz 1.2 GHz
Memory Clock 1.8 GHz 1.6 GHz
Memory Capacity 768 MB 640 MB
Memory Interface 384-bit 320-bit
We can’t make any conclusions about the GeForce 8800 GTX as a gaming card until we’ve finished our gaming tests, but the new architecture from Nvidia boasts an impressive potential, beyond any doubt. This is obvious even from the theoretical tests where the GeForce 8800 GTX is the absolute winner. Moreover, we suspect that our tests cannot fully show the advantages of the new graphics cards from Nvidia over its last-generation opponents.
On one hand, we often see the performance of the GeForce 8800 GTX being limited by the bandwidth of its TMUs or onboard memory, which conceals the huge difference between the GeForce 8800 GTX and the Radeon X1950 XTX in raw computing power. On the other hand, the unified shader architecture is a priori better in synthetic benchmarks because the whole computing power of the GPU is allotted for one task.
Nvidia also took care about the image quality provided by its new flagship: the new FSAA and anisotropic filtering algorithms are indeed an improvement over the previous products. This is most important for the GeForce series that used to lag behind its opponents from the Radeon X1000 series in terms of image quality.
That said, the true potential of the GeForce 8 will only be revealed after the release of Windows Vista and DirectX 10 which promise us a lot of improvements in future games in terms of speed as well as quality.
The triumph came to Nvidia at a price: the 0.09-micron G80 is very large and uneconomical chip with high heat dissipation. Combined with the wider, 384-bit memory bus, this made the PCB of the new card very large and expensive, too. So, there were some problems as a consequence. As you may know, the senior GeForce 8800 model was withdrawn from sales channels not long before the announcement due to technical issues that made the first series of the GeForce 8800 GTX inoperable. Fortunately, that problem doesn’t concern the GeForce 8800 GTS, yet the delay with the shipments of the world’s fastest graphics card is no good for Nvidia. We hope this problem will be solved in near future and the GeForce 8800 GTX will be available for everyone who wants it. Nvidia says that GeForce 8800 GTX, even though in limited quantities, will indeed be available right after its announcement.