What’s relevant to the discussion of the 6800GT SLI is the ability of the motherboard to use 2 PCI Express slots in tandem for graphics tasks like games. The Gigabyte GA-K8N SLI has 2 PCI Express X16 slots. The end user can insert 2 PCI Express graphics cards of the same type and model from the same manufacturer. The concept was first introduced by a company called 3dfx. 3dfx is sadly no longer with us (having gone bankrupt in December 2000), but most of the assets were purchased by NVIDIA. One of the concepts in that deal was called SLI (Scalable Link Interface). SLI was the codename for 3dfx’s dual card solution for graphics cards. The Voodoo 2 used 2 PCI cards to double the performance in games.

When one PCI Express card is installed on the GA-K8N SLI, the slot operates at full X16 speed. With a bandwidth of 250 MB/second each way, this gives a maximum memory bandwidth of 4 GB and 8 GB when bi-directional bandwidth is considered. This nearly quadruples the bandwidth from the PCI Express slot from the previous AGP 8x slot from 2.1 to 8 GB. With 2 cards installed, each slot operates in X8 mode. As games hardly use the full 2.1 GB bandwidth of AGP 8x, this is not currently a limitation.

There are three modes of SLI operation AFR, SFR and Compatibility mode. AFR stands for Alternate Frame Rendering. AFR has each graphics card render a separate frame. In other words graphics card #1 will render all even frames, and graphics card #2 will render all odd frames. SFR stands for Single Frame Rendering. In this mode the screen is split between the two graphics cards with the driver detecting the split done by each graphics card. In theory, AFR is superior, as there will be times when the prediction is incorrectly done, forcing one card to wait for the other to finish the frame.

NVIDIA’s driver engineers have profiled many of the top games on the market today, to determine whether AFR or SFR should be used in the game. The third mode is used when a game isn’t profiled to use AFR or SFR called Compatibility mode. Compatibility mode has the game rendered by one of the graphics cards, making it exactly as if only one card was used in the system.