GPUs
For ATI and NVIDIA, DirectX 10 is a major evolution and they are both very enthusiast because of the new possibilities that will appear. We called both manufacturers and their point of views is identical except for one point. They are happy about internal optimisations of the API that reduces the CPU power consumption. For the same CPU load, the complexity of the scene will increase additionally to the evolution of the graphic pipeline that has been unchanged for a while. Geometry shaders and the stream output open new possibilities. The divergence point is the GPU architecture unification for calculation units. According to ATI it is the best solution, whereas for NVIDIA it isn´t yet. Here is a basic representation of architectures that expect to find for ATI and NVIDIA:
left, the G8x supposed implementation of Direct 3D 10, right, the supposed R6xx implementation of Direct 3D 10.Even if GPU manufacturers like to talk about completely new architectures developed from scratch etc., they are always evolutions. Developing a GPU is complex and obtaining good rendering results requires a lot of small optimisations to polish up the details and ensure that the data and operation flow aren´t interrupted. It requires a very important expertise that ATI and NVIDIA have acquired with time and that lacked to a lot of potential competitors like XGI, S3 and Matrox that haven´t obtained high enough results from their GPUs. For ATI and NVIDIA it is of course out of the question to completely leave this expertise and move in a direction that would be completely the opposite of previous architectures.
For ATI, changing for a unified architecture of execution units (= shaders processing) is a logical evolution that is a continuation of previous and current architectures. A unified architecture requires an advanced management of the different threads (tasks). The Radeon X1000 already do that even if it is only restricted to pixels shaders. With a unified architecture it is possible to attribute execution units to tasks that need them and to avoid to have unused units. ATI already has an experience of unified architecture because the Xbox 360 GPU has one.
For NVIDIA, the situation is much different because the efficiency of pixels calculations comes from a very long pipeline in which texture access is masked. Moving directly from such architecture to a unified architecture would represent a big risk that could seriously affect rendering. Also, NVIDIA´s current architecture is rather economical for the chip size whereas ATI´s is very costly. Not unifying the architecture with the G8x will make possible for NVIDIA to ensure a high level of performances and not increase too much the number of transistors. We expect that the G8x and the G80 (previously NV50) might not be unified or at least not completely.
Even if Direct 3D 10 defines a similar set of operations for all type of shaders, very slight differences will remain (only the geometry shader can create element for example) and each type of shader will have to process tasks that have a different profile. Optimising the same execution unit for a vertex shader or a pixel shader leads to different directions. These directions aren´t however that different if it is about geometry shaders or vertex shaders. This makes us think that Nvidia could develop a partial unification of the architecture for processing units that would be shared. Pixel shaders would keep improved dedicated units for the specificities of DirectX 10. With this compromise, NVIDIA will integrate the DirectX 10 support while still keeping a high yield per mm².
What would be the best solution ?
It is a little early to give our thoughts of course. NVIDIA will release the G80 before that DirectX 10 will be usable. The support won´t probably be directly activated in the drivers. For ATI, the R6xx will probably come with the DirectX 10 support because we estimate that it will be very easy to show the benefits of a unified architecture in different theoretical cases. ATI will use this possibility and they are right to do that. Now the thing is that theoretical cases maybe won´t represent the practice or at least not before a while.
With DirectX 9 and the first DirectX 10 games (that Microsoft announced at the same date), the G80 will behave like a GeForce 7900 with more calculation units and the R6xx like a more efficient Radeon X1900 (ATI speaks of 25% gain for some games with the same amount of calculation units between an unified and standard architecture). This doesn´t teach us anything except that it is too early to know which one will be the fastest. Frequencies will once more be of importance and NVIDIA won´t leave aside a double card like the GeForce 7950 GX2.
ATI and NVIDIA architectures will be valid for the entire GPU line and it isn´t because one architecture will be more efficient for high end products that it will also be the case for middle line and entry level products.
Speaking of yield per mm² for NVIDIA, we have to point out that this is in fact a very big simplification. Everyone admits it because the gap is enormous between ATI and NVIDIA´s chips. In practice, the size of the chip is only one of the elements that define the yield per mm² (of wafer). The frequency used is another factor. If a manufacturer has to use very high frequencies to remain in the competition, it reduces the number of chips that will be usable and the yield per mm² of wafer. One architecture might give more possibilities than another to integrate redundancy. This reduces the number of unusable chips and increases the yield per wafer. A bigger chip might have in the end a higher yield than a smaller one.
You probably have understood it, we are expecting NVIDIA to speak of the size of their chips (they will also mention the power consumption) and the ATI will defend the benefit of a unified architecture. As usual, these will be marketing simplifications and it will be important to step back a little to have a more pertinent opinion. To finish, developers too will have to give their opinion because the technological choices that they will make will push one or the other architecture.
