SLI and Crossfire Explained

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Could it be you are still puzzled over SLI and Crossfire. Which one is the best? No doubt you have seen the logos branded across many adverts and websites but just what exactly is this technology and why would you want it?

Nvidia SLI and ATI Crossfire Technology Explained

A Detailed Explanation of Multiple GPU Graphics Hardware

Scaleable Link Technology or SLI isn't a new technology. The idea of using two graphics cards to improve performance was first developed by a company called 3DFX way back in 1998. 3DFX Interactive specialised in cutting edge 3D graphics cards and dominated the industry throught the 1990's until it went bankrupt in 2000. The Voodoo2 graphics card was the first example of connecting two graphics cards together with each card drawing half the scanlines of the screen. For the price of a second card, enthusiasts could double the 3D gaming performance and bump resolutions upto an exciting 1024 x 768. After the company went into liquidation, the technology disappeared for awhile until 2004 when NVIDIA introduced a new technology called SLI (Scalable Link Interface).  It didn't take long for ATI came up with their own such solution, which it called CrossFire.  Both SLI and CrossFire have the same basic purpose: to allow multiple graphics cards to operate in parallel, processing more graphics data in the same amount of time.  The benefits are obvious: frame rates in a game would go up, since more frames could be generated every second.  Alternatively, each frame could be a better image, but without slowing it down for the graphcs card to keep up.  You could also run at higher resolutions without a loss in frame rate.

How Does SLI and Crossfire Work?

SLI and CrossFire each achieve their purpose in different ways.  In an SLI system, the two graphics cards are identical and connected by a "bridge," which is just a high-bandwidth connection between the two cards so they can communicate and keep the overlap in processing to a minimum.  You plug your monitor into either video card, and the software takes care of the rest.  Effectively what you have in an SLI system is the equivalent of one video card with almost twice the performance, twice the memory, and twice the memory bandwidth.  The result: your graphics are rendered almost twice as well, whether it's double the frame rate or a higher level of anti-aliasing or anisotropic filtering (which each improve picture quality in different ways).

Very recently NVIDIA released drivers for quad SLI, which allows for four graphics cards in an SLI setup.  It works exactly the same way, though the bridge is a little more complicated.  For a while, quad SLI was only available through professional builders, and for good reason.  Four graphics cards, each using a lot of power, taking up a lot of space, and producing a lot of heat all in one confined space is a hard setup to implement in a system.

CrossFire, unlike SLI, does not utilize the same bridge technique, pointing to the fact that the two graphics cards don't communicate with one another as much in a CrossFire setup.  The two cards are instead joined together on the outside, with a special cable that connects the monitor to one output on each video card, rather than just one of them.  This way, each card can send whatever it's working on to the monitor, without worrying about the other card at all.  They can alternate frames very easily this way, for example.  In a CrossFire system the two graphics cards are different, with one of them being a slightly more expensive "CrossFire Edition" card of the same model, with the other being a standard card.  The CrossFire Edition Card has a separate chip that handles all the CrossFire functions.  But despite the differences the end result is the same; again, you get almost twice the graphics performance (it's almost twice because two graphics cards can never really be 100% independent when working on the same task).


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