Although nVidia launched the first PC based graphics solution to incorporate a unified shader model in the shape of the GeForce 8800 GTX, it’s worth remembering that ATI had a unified shader model part over a year earlier inside the Xbox 360. The Xenos graphics chip inside the X360 was the first unified shader model part to hit the market, and is still the most advance graphics processor ever seen inside a games console – the PlayStation 3 is basically using a variant of the GeForce 7800, so it’s a way behind its main rival when it comes to graphics architecture, despite being launched a year later!
To understand the benefits of a unified shader model, you need to understand how traditional graphics architecture worked. Previous generations of graphics hardware incorporated a number of dedicated vertex shaders and pixel shaders. In principle there’s nothing wrong with this system, with each array of dedicated shaders springing to life when needed and hammering through their instructions. Obviously the more shaders you had, the faster you could process those instructions, and with most games requiring more pixel processing that vertex processing, graphics cards tended to have the shader ratio weighted in favour of pixels.
The problem with the dedicated shader model is that it’s not always that efficient. You see there are times when there will be a significant amount of complex geometry processing, which can leave the pixel shaders twiddling their thumbs. Likewise, when there’s masses of pixel processing to do, your vertex shaders are sitting around doing nothing but wasting clock cycles.
A unified shader model solves the problem of dormant hardware and wasted clock cycles. In essence, the unified shader architecture does away with dedicated shaders for vertex and pixel processing, replacing them with shaders that are capable of executing both vertex and pixel operations. This means that if there is masses of complex geometry to deal with, all the shaders can become vertex processors, and when there’s loads of pixel processing to do, you’ll have a whole host of pixel shaders.
The significant difference between the AMD’s HD 2900 XT and nVidia’s GeForce 8800 is that this card represents AMD’s second generation unified shader part, with the Xenos chip merrily processing away in Xbox 360s for the past 18 months. AMD is adamant that it has learned a lot about unified shader architecture since designing the Xenos, and is confident that this latest incarnation will make the most of Microsoft’s forthcoming DirectX 10 platform.