Five months ago NVIDIA released the GeForce 6800 in a marketing blitz. Based on new architecture, the GeForce 6800 comes in three versions, the cheapest costing you around $300. Of course, we are not all prepared to invest that amount of money in a graphic card. NVIDIA knows that perfectly well and decided to launch a new graphic card based on the GeForce 6 architecture. This mid-level graphic card’s name is the GeForce 6600.
The GeForce 6600 GT
The main elements of the GeForce 6800 are in the GeForce 6600. All the GeForce 6800 units are in the GeForce 6600, and are almost identical but lesser in number. Indeed, the GeForce 6600 GT is made of 144 millions transistors with a 110 nm fabrication process instead of 222 million and 130 nm fabrication process for the GeForce 6800. To give you a better idea, a GeForce FX 5900 has 130 million transistors and a GeForce FX 5700 82 million.
Here is a reminder of the GeForce 6800 3D architecture:
There are 6 Vertex Shaders followed by 16 pixel pipelines. Each one of them has a large unit which handles both shaders and standard texturing, and a smaller unit dedicated to shaders. These pipelines are connected to a 16 ROP Engine (ROP for Raster Operation), which manages anti-aliasing, blending, Z-Buffer compression and double Z (possibility to calculate twice more Z data than the standard data / clock with the 6800). The standard GeForce 6800 has 5 Vertex Shader and 12 Pixel Pipelines. All this information is then transmitted to the 256 bit memory.
What about the GeForce 6600’s architecture?
As you can see this time, units were reduced. The number of pixel shaders was reduced from 6 to 3, the pixel pipelines from 16 to 8 and the ROP engine from 16 to 4. The memory drops to 128 bits. With this architecture, the GeForce 6600 has performance restrictions for simple pixels, for example, with a shader of one instruction or a simple texture with bilinear filtering. If the pixel pipelines are able to process 8 simple pixels per clock cycle, in practice the ROP will restrict the chip to 4 pixels per clock cycle.
Of course, with more complex pixels, the problem is different. With trilinear filtering, double texturing or a more complex shader, the pixel pipelines will only process 4 pixels per clock cycle. It seems obvious that this choice has been made regarding the size of the available bandwidth. With a 128 bits bus, 8 ROPs would have been significantly restricted by the bandwidth and performance gains couldn’t have been visible in practice (only the more complex pixels are affected). It is better to save some money on transistors, now that pixels displayed in games are more and more complex and can’t be calculated in only one clock cycle.
Here is a table with the main specifications of the 6800 and the 6600 GT.
The figures speak for themselves. In terms of sheer power the 6600 GT is close to the standard 6800. Only the bandwidth is smaller and will probably provide a lesser performance in practice for games in high resolution and with anti-aliasing / anisotropic filtering. We will check this assumption in the following pages. A standard GeForce 6600 is also expected out. The differences will be mainly on the frequency level with 300 MHz for the GPU and at least 275 MHz for the memory. Several manufacturers intend to go beyond these specifications.
The GeForce 6600GT functions are identical to the GeForce 6800’s. The GeForce 6600GT uses the Shader Model 3.0 enabling longer shaders and also introduces dynamic branching, that are however costly to performance. We are waiting to see how developers use this function to know if it’s really a plus for the current graphic card generation.
The GeForce6 is also the first chip to provide a full FP16 format support. It has a Buffer in floating point and is also able to process operations like blending or filtering. It makes it possible to manage the HDR with more efficiency and flexibility.
The 6800 video engine is also in the GeForce 6600. It’s able to decode in hardware the WMV9 and encode in hardware the MPEG1/2/4. It is, however, not yet used by the drivers. The manufacturer said they would talk to us soon about this issue. Is this good news?
