SLI

With the recent release of CrossFireX by AMD/ATI and 3-way SLI by nVidia we think it is a good time to make a technical comparison between all incarnations of these two technologies, which have the same goal: to allow video cards to be connected in parallel in order to increase gaming performance.

Let’s start first talking about SLI, since it was the first of the two technologies to be released. SLI was originally introduced by 3dfx in 1998 with their Voodoo 2 card. At that time SLI meant Scan Line Interleaving and worked by making each GPU to process one group of lines (one GPU processing odd lines and the other processing even lines). nVidia bought 3dfx on April 19^th 2001 and introduced a similar but updated concept for their video cards in June 2004, renaming SLI to Scalable Link Interface.

SLI can work under the following modes:

• SFR (Split Frame Rendering), where each frame is divided in two and each half is sent to a different GPU to be processed. This is the mode used by SLI configurations using two video cards. See Figure 1.
• AFR (Alternate Frame Rendering), where each GPU processes an entire frame, but while one GPU is processing the current frame, the other GPU is already rendering the next frame, so when the first frame is delivered, the second frame is already rendered (ideally) or almost rendering. This is the mode used by three-way SLI. See Figure 2.
• AFR of SFR, which is used on Quad SLI system. Here two GPUs process the first frame under SFR mode while the other two GPUs are processing the next frame also under SFR mode.
• SLI AA (Anti-Aliasing). This mode isn’t intended to improve gaming performance but increasing image quality. In fact under this mode the game usually runs slower but with a better image quality. While with a single video card you can typically only obtain up to 4x or 8x anti-aliasing modes, with SLI this number is elevated to 8x, 16x or 32x, depending on the SLI mode. So far this mode does not work under Windows Vista.

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Figure 1: Split Frame Rendering (SFR) mode.

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Figure 2: Alternate Frame Rendering (AFR) mode.

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Figure 3: AFR of SFR mode.

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