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SLI vs Crossfire
Nvidia vs ATI | Posted by jonchappell on Aug 09th 05 06:27 AM
SLI stands for Scalable Link Interface. It is a method of linking two graphics cards together in order to combine processing power and render a scene faster. Theoretically performance is doubled, but in reality performance actually increases by 50-80%.

SLI only works on newer PCI-Express motherboards and requires the board to have to x16 PCI-E slots side-by-side. It also requires two cards that support SLI which is Nvidia's GeForce 6 series and above. Both cards must be identical. The cards are physically connected by a small PCB connector.

The system works in one of two ways. Either both cards work on the frame 50/50 or one renders a frame and the other renders the next in advance.

ATI's Crossfire technology (currently unreleased) is identical in concept to Nvidia's offering but with some essential differences. Firstly, SLI requires game-specific driver optimisations which means that when a new game is released, it might take a while for a new driver to appear that supports the game. Crossfire supposedly supports all DirectX and OpenGL games out of the box.

Secondly, Crossfire does not require the cards to be identical or from the same manufacturer. It does, however, require that one card is a special Crossfire master card. Also, the cards are not physically connected by a circuit board but their output is combined by a monitor splitter cable.

Crossfire currently allows four rendering modes:
  • Super Tiling (DirectX only) splits the screen up into chessboard-like squares which each card alternately processes.
  • Scissor splits the screen horizontally in half and each card calculates one half. Load balancing can be used to make sure that the faster card renders a greater proportion of the screen in order to improve rendering speed
  • Alternate Frame Rendering (AFR) is identical to the mode offered by SLI
  • SuperAA mode is used to improve image quality, not necessarily frame rates. This mode can double anti-aliasing quality without affecting speed. For example, a 16x AA image could be achieved if each card was set to 8x AA.
Conclusion
ATI's Crossfire solution would appear to offer more flexibility than Nvidia's SLI as you can use non-identical cards. On the other hand, whilst there are lots of modes available, you cannot choose which specific mode to use (with the exception of SuperAA).

 
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