ASUS GeForce EN8800GTX/HTDP/768M Video Card Review :: GeForce 8800 Architecture

11-09-2006 · Category: Hardware - Video Cards

By Benjamin Sun

NVIDIA decided to go with a Unified Shader Architecture with the G80 chip. The benefits of Unified versus discrete Pixel and Vertex Shader units is that the unified shader can be allocated to either Pixel, Vertex, Geometry or Physics shaders as needed by the application. In a scene where more Vertex Shading is required, the G80 can allocate more of its Stream Processors to Vertex Shading.


G80 architecture diagram


ASUS GeForce EN8800GTX/HTDP/768M Video Card Review

The 8800GTX has 128 individual Stream processors. Each processor is capable of being used for pixel, vertex, geometry or physics operations. The classical graphics card had many pipeline stages for each of the major stages. The pixel shader alone on the GeForce 7 required over 200 pipeline stages. The 8800GTX streamlines the number of stages because of the nature of the Unified Shader, meaning that the data will move from the shader core for processing to the top of the shader core to continue processing until all shader operations are done and the fragment is sent to the ROP to write to the memory. Each Stream Processor is fully decoupled, scalar can dual-issue a MUL and a MAD (Multiply and Multiply Add) and supports IEEE754 floating-point precision.

Each Stream Processor is clocked at 1.35 GHz. The GeForce 8800GTX has three different clock domains, the core clock, the memory clock and the shader clock. The 8800GTX does approximately 520 Gigaflops of shader power, but is much more efficient with its floating point operations than a traditional architecture with instructional issue limitations like the x1950 which can only issue 3+1 instructions in a clock.

In gaming situations, there generally has been more need for pixels than vertices; hence the classical graphics card came with a higher number of pixel shader units than vertex shader units. The ATI Radeon X1950XTX for example had a pixel shader processor/vertex shader processor ratio of 48:8, the GeForce 7900 series had a ratio of 24:1. There are situations where the application needs more vertices than pixels, and the graphics card is limited to the vertex speed. Take for example a GeForce 7800GTX with 24 Pixel Shader units and 8 Vertex Shader Units. There are instances where the card will only go as fast as the vertices are fed.

The Unified Shader architecture on the G80 can allocate the units in a different way. If the application calls for it, it can allocate 24 Vertex Shader units to vertices and 8 Pixel Shader units for pixel shading assuming the same number of units as the G70 or geometry shading or physics shading which is new with Microsoft's DirectX 10 coming with Microsoft's Vista Operating System. The exact number of shader units applied to each application can be dynamically changed, allowing efficient use of the resources of the card compared to a 48:8 (R580) or 24:8 (G70) ratio of earlier video cards.