The CrossFire platform is similar to NVIDIA's SLI, but it's distinctive in some intriguing ways. CrossFire does allow for the teaming of a pair of PCI Express video cards to increase graphics rendering capacity, but it doesn't rely on a bridge between the two cards. Instead, CrossFire requires that one of the two cards in a system be a special, CrossFire-capable unit that has a custom composting chip onboard designed by ATI. This chip, onboard the master graphics card, takes the image from the local Radeon VPU and mixes it with the image from the slave graphics card, which it receives over a DVI connection. The CrossFire card then outputs the composite image via its own DVI output (or to VGA via a converter).



Link: The Tech Report