ATI’s Crossfire technology, like the Nvidia SLI technology, allows the use of two graphics cards in a single system with the benefit of boosting the system’s graphics capability. Similar to SLI, Crossfire also uses the PCI-Express 16x architecture to connect graphics cards.
The first Crossfire system made it’s appearance in 2005, but unlike Nvidia’s SLI offering, which required technologies to be build into their graphics processing units in order to transfer data, the first Crossfire systems utilised a special master card on which chips were installed to link the two cards together. On top of this, to use Crossfire, it was necessary to use a motherboard that was compatible with Crossfire, as well as a Crossfire master card and a second, regular ATI graphics card.
Initially, there were only two ATI master cards released: The X800 Crossfire Edition and the similarly named X850. To use Crossfire, the second card had to belong to the same family of card as the master, so you could use any variant of the X800 with the X800 Crossfire master card. In addition, this meant that the master card could auto-configure itself in line with the technologies on the slave card, meaning Crossfire wasn’t tied into running at a slower clock-speed as was found with Nvidia’s SLI.
Whereas the Nvidia SLI cards were required to be connected together inside the PC, early generations of Crossfire-enabled cards connected externally via a pass-through connector, which connected the slave card to the master card via a DVI output. The Crossfire master card had a decoder chip which could convert the signal received from the slave card so that it could be combined with the data produced by the master card and rendered. Later generations of Crossfire adopted the Nvidia approach, and Crossfire graphics cards are now connected internally using a bridge cable.
Crossfire-enabled motherboards are enabled from the BIOS, rather than by an invertible card, as in SLI.
Crossfire provides several ways to divide 3D rendering work between the graphics cards. The first mode used by Crossfire is ‘alternate frame rendering’ or AFR which first appeared with ATI’s Rage Fury MAXX graphics card, which used two Rage 128 graphics chips to render alternate frames. Crossfire AFR works in a similar fashion; each card takes on the job of rendering alternate frames, with one card rendering odd-numbered frames and the other rendering even-numbered frames.
The second mode of Crossfire is known as ‘scissoring’ which is similar to the split frame rendering used by Nvidia’s SLI mode. In ‘scissoring’ mode, the screen is divided into two areas so that each half requires a roughly equal amount of work to produce. Each card creates one part of the image, and the slave sends it’s work to the primary card for merging and display.
The third mode of Crossfire is known as ‘supertiling’. This mode involved dividing the screen into what would resemble a chessboard, with each graphics card processing alternate squares of the board. However, ‘supertiling’ mode is only found in games that use the Direct X Direct3D API and only graphics cards that both have 16 pixel pipelines each can properly utilise ‘supertiling’.
A fourth mode of Crossfire is also available, known as ‘super AA’. This involves the two graphics cards performing anti-aliasing processes to produce a smoother image and increase image quality. Each of the two cards perform anti-aliasing on each frame using different sample patterns which are them combined to give a cumulative level of anti-aliasing than would be possible on a single card without suffering a loss of frame rate.
In the early days of Crossfire, it proved very difficult for anyone looking to build a Crossfire enabled PC due to severe shortages of master cards. As a result, with the next generation of Crossfire, ATI released their Xpress 3200 motherboard chipset. This allowed Crossfire systems to fully utilise the full speed of the PCI Express x16 slot across 2 installed cards, allowing them both to run with all 16 lanes functional. When it was used in conjunction with two X1300, X1600 and X1800 graphics cards, it allowed users to run a Crossfire system without the need to have a dedicated Crossfire master card. Furthermore, because neither card in these setups have the input connector used on Crossfire master cards, the image data is shuttled to the display card via the PCI Express bus rather than using a connector cable inside the PC. However, more powerful cards such as the X1900 range require a connector cable to be used as well as a master card.
ATI announced in late-2006 that the Crossfire system would be further expanded to allow the format to perform physics calculations by linking cards in Crossfire mode with another, third card which carried out the physics calculations. However, such a setup would require three PCI Express x16 motherboards and would likely be accompanied by a hefty pricetag.