A Technical Introduction to CrossFire

A Technical Introduction to CrossFire
By , Last updated on: 12/3/2014

If you haven’t yet, start by reading the article, “A Technical Introduction to SLI,” to familiarize yourself with Nvidia’s similar technology, SLI. CrossFire, primarily ATI’s response to pressure from competing Nvidia’s SLI, is a multi-GPU system that can provide the much needed boost in computer graphics. Since we’ve knocked out the basics of multi-GPU in our other article, we’re going to jump right into the details for CrossFire.

Unlike SLI, CrossFire has 3 different modes, as opposed to the 2 offered form SLI. SLI contains AFR and SFR, whereas Crossfire contains AFR, Supertile, and Scissor. Ok I lied, there’s actually a 4th mode called SuperAA, but we’ll leave that for another article because it requires a lot more background information to understand. AFR is the same in both systems, where each card handles an alternating frame to increase graphics output. AFR requires more data to be transferred between cards, and thus is slower and less efficient than the other methods. CrossFire’s Scissor mode is comparable to Nvidia’s SFR mode. The only difference is that while Nvidia’s SFR mode splits the frame horizontally, Scissor splits the screen either horizontally or vertically, whichever provides a more equalized rendering. Thus, the Scissor mode might be slightly more advantageous than the SFR mode, but not significantly enough to matter.

Now, on to CrossFire’s new method: Supertile. To go over why Supertile does what it does, let us first talk about split screen rendering. Because most graphics are not always equal on the top and bottom halves of the screen, or even the left and right halves for that matter, it takes some time (we’re talking fractions of second, but the time does add up) to calculate where exactly the screen should be split. Because the calculating takes some time, it makes the entire processes slower. To eliminate this problem, Supertile splits the screen into a checker board fashion, dividing the screen into small squares with areas of 1024 pixels (32 pixels x 32 pixels). Then, picturing it like a checker board, all of the white squares go to one card, and all of the black squares go to another card. This eliminates the need for calculating the line, and just assigns smaller areas to each card, hoping to equalize the processing of each card.

So with this novel convention from ATI’s CrossFire, it seems that as long as the theory behind Supertile works, it should be faster than Nvidia’s SLI. Another aspect that could put CrossFire above its competition is the SuperAA mode, which we will go into in another, much more extensive, article or set of articles. Also, look for the follow up article pitting SLI against CrossFire in a head-to-head comparison.

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