SLI vs. Crossfire: What they are and how they work

SLI and Crossfire can serve an important purpose in your system, allowing you to drastically increase the power you’ll get from your graphics cards — even though there’s a price.

There are a few things to keep in mind for both SLI and Crossfire. For example, to run either of them you’re going to need a compatible motherboard, two compatible graphics cards, and a so-called “bridge.”

But how do the two systems work? Sometimes figuring out which one is right for you comes with a deeper understanding of what makes them tick. Here’s a guide to help you learn.

NVIDIA SLI

Image courtesy of Coaster J

SLI was developed by NVIDIA and it basically acts as a link between GPUs for transferring information like synchronization and pixel data. SLI works through a product called the SLI Bridge — which is able to handle two graphics cards of the same model. That’s important — you can’t use two different graphics cards with SLI, however the two cards can be from different manufacturers, as long as they’re based on the same design.

SLI basically works in one of two different ways, giving different information to the two graphics cards. SLI always uses a slave card and a master card — with the master card being the first processor and the slave being the second. As the names suggest, the slave card sends all of its information to the master card through the SLI bridge, and the master card consolidates all the information, including the information that it processed, and sends it all to your display.

The first way that SLI works is called Split frame rendering, and it basically means that each frame is split in half horizontally, and one half is sent to each of the cards. It’s important to note that frames aren’t split based on pixels — they’re split based on workload. So, if there’s almost nothing to render at the top of the frame, but a lot that needs rendering towards the bottom, there might be more of the actual frame sent to one card, but only 50 percent of the work load.

Alternate frame rendering, on the other hand, essentially means that the two graphics cards are given alternate frames to render. For example, card 1 might be given frames 1, 3, and 5, while card 2 is given frames 2, 4, and 6. Alternate frame rendering is the most common example of how both SLI and Crossfire work.

AMD Crossfire

Image courtesy of D-Kuru/Wikimedia Commons

Crossfire is essentially AMD’s answer to SLI, and it works slightly differently. While Crossfire has historically needed both a master card and a slave card, the most recent versions eliminate the need for this. The most recent version, called Crossfire XDMA, doesn’t even need a bridging port — instead it uses XDMA to open a direct channel between the two GPUs in a Crossfire system, which all works through PCI Express 3.0.

Unlike SLI, Crossfire allows for you to use different graphics card models, although you should really use very similar models if you can at all help it. The two graphics cards you use do, however, have to be built by AMD, and they have to be of the same generation.

Like SLI, Crossfire can either use split frame rendering or alternate frame rendering, however one disadvantage is that Crossfire only works in full-screen mode — not in windowed mode. Still, Crossfire is compatible with more motherboards, and it’s generally available on cheaper motherboards — which helps if you’re on a budget.

Conclusions

So what’s the main difference? Well, in the end SLI is a little more consistent and powerful, however Crossfire is more flexible, allowing for different setups. If you can afford to go for SLI, you’ll probably get better results, but if not Crossfire is a great option too.