What Are SLI and CrossFire?

A couple of years ago graphics manufacturer NVIDIA introduced a new technology that it called SLI (Scalable Link Interface), a technology that provided a computer with the ability to utilize two graphics cards to produce one higher quality video output. Eventually 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 Each System 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).

What You Need for an SLI or CrossFire System
First of all, SLI and CrossFire are only supported on PCI-Express x16 slots (which I will hereby refer to simply as PCI-Express slots), and to put in two of them you need two slots. (The only current exception is NVIDIA’s GeForce 7950GX2, which is basically two 7900GTX cards that fit into one slot.) SLI is supported on motherboards using select nForce4 and nForce 500 chipsets. In general, if it has an NVIDIA chipset and has two PCI-Express slots, it’ll work with SLI. CrossFire is supported on select ATI chipsets, including the CrossFire Xpress 1600 and 3200 and some Radeon Xpress 200. ATI drivers also support CrossFire on Intel chipsets with two PCI-Express slots, which currently means only the 975X chipset. NVIDIA technically could support Intel chipsets if they wanted to, but they currently do not.

Next you’ll need two video cards. With an SLI setup the two cards have to have the same GPU and the same memory size. They can be different brands and different clock speeds. For a CrossFire setup you’ll need one of the two to be a CrossFire Edition card.

Your power supply is also worth taking a look at if you want to use more than one video card. Video cards today take more and more power, and depending on your video card you could need a lot of power on the +12V rails. Your driver will warn you if your video card is not receiving enough power, and it’ll bring down your cards to a lower power level until the issue is corrected. If you see such a warning, it’s time for a power supply upgrade. Many power supplies feature an SLI certification provided by NVIDIA; ATI has recently also begun to do the same, but an SLI certification is fine for a CrossFire system of equivalent performance.

SLI/CrossFire and Physics
One of the newer issues manufaturers are trying to address today is the calculation of physics in games. Up until recently, the only physics calculations in a game were the ones that were absolutely essential, and they were done by the CPU. But physics can also make a game look better. Imagine watching a car drive by, and for once the wheels are rolling along the pavement, instead of sinking into it while gliding across the screen. Or two cars crashing and being deformed, like they would in real life. In games today the cars are just treated as boxes, and when they hit each other they bounce off each other the way boxes would; they don’t have the shape, strength, or momentum of the actual car.

That’s where multiple video cards might be able to help. With multiple graphics cards, one of them could be used to perform physics calculations, while the other one(s) could render the game as usual. Both NVIDIA and ATI are currently working on incorporating physics into SLI and CrossFire systems.