CrossFire is a brand name for ATI Technologies multi-GPU solution, which competes with its rival nVidia's Scalable Link Interface (SLI). The technology allows a pair of graphics cards to be used in a single computer to improve graphics performance. Although only recently announced for consumer level hardware, similar technology known as AMR has been used for some time in professional grade cards for flight simulators and similar applications available from Evans & Sutherland.

The system requires a CrossFire-compliant motherboard with a pair of PCI Express (PCIe) graphics cards, which can be enabled via either hardware or software. Radeon x800s, x850s, x1800s and x1900s come in a 'CrossFire Edition' that has 'master' capability built into the hardware. One must buy a Master card, and pair it with a normal card from the same series. Radeon x1300s and x1600s have no 'CrossFire Edition' but are enabled via software. ATI currently has not created the infrastructure to allow FireGL cards to be set up in a CrossFire configuration. Another point to note is that the 'slave' graphics card needs to be from the same family as the 'master', regardless of whether the 'master' is designated by the hardware or by software.

However, with ATI's release of the new Motherboard Chipset named "CrossFire Xpress 3200", the 'master' card is no longer required for every "CrossFire Ready" card (with the exception of the Radeon X1900 series). With the CrossFire Xpress 3200, two normal cards can be run in a Crossfire setup. This move from ATI is viewed as an overall improvement in market strategy due to the fact that Crossfire Master cards are expensive, in very high demand, and largely unavailable on the retail level.

Although the CrossFire Xpress 3200 chipset is indeed capable of CrossFire through the PCI-e bus for every Radeon series below the X1900s, the driver accommodations for this CrossFire method has not yet materialized for the X1800 series. ATI has said that future revisions of the Catalyst driver suite will contain what is required for X1800 dongleless CrossFire, but has not yet mentioned a specific date.

CrossFire was first made available to the public on September 27, 2005.
Contents


* 1 Rendering modes
* 2 Advantages over NVIDIA SLI
* 3 Disadvantages compared to SLI
* 4 See also
* 5 External links



Rendering modes

The CrossFire system supports four different rendering modes, each offering their own specific advantages and disadvantages.

* SuperTiling: CrossFire's standard dual-GPU rendering mode. It divides the screen up like a checkerboard, allocating adjacent squares ('quads') to alternate GPUs. (To continue the checkerboard analogy, one card would render the white squares, and the other the black). SuperTiling supports all Direct3D applications (but not OpenGL). However, it provides the least performance enhancement of the four modes, a rough estimate being 1.15 times the power of a single equivalent GPU. This is due to the fact that SuperTiling does not allow the geometry of a scene to be scaled between two cards. It is worth noting that SuperTiling only works on cards that have an even number of pixel quads so a setup with an X800 with 12 pixel pipelines paired with an X800 master card will not be able to render the SuperTiling mode.

* Scissor: Divides the screen into two rectangles, one above the other. This is the default operating mode for OpenGL-based applications. Unfortunately, the performance boost with Scissor mode is approximately equal to the SuperTiling mode. This render mode is more commonly known as Split Frame Rendering (SFR), which is how nVidia refers to it in SLI. In theory, SuperTiling should provide higher performance, because there is a better chance the work will be evenly divided between the two cards. Using Scissor mode means that the system has to carefully choose the "cutting point" in order to balance the load.

* Alternate Frame Rendering: The fastest mode, Alternate Frame Rendering (as the name suggests) sets one GPU to render odd frames, and one the even frames. While this produces a high performance boost, it is incompatible with games using render-to-texture functions because one card doesn't have direct access to the texture buffer of the other. Like nVidia, ATI uses game profiles for Alternate Frame Rendering, but nVidia allow you to create profiles to use AFR on any application.

* CrossFire Super AA: This mode is not designed for a large increase in frames per second; rather, it is intended to improve the quality of the frames rendered (hence 'Super AA' - super anti-aliasing). Super AA is able to double the anti-aliasing factor (eg. 4x, 8x and 12x) without any drop in framerate.



Advantages over NVIDIA SLI

* ATi has opened the Crossfire architecture to Intel, allowing CrossFire to be enabled on certain Intel chipsets which boast two 16x PCI-E slots. SLI, however, requires an nForce motherboard which is SLI certified (such as the nForce 590 SLI).



Disadvantages compared to SLI

* If an OpenGL game does not have a Crossfire profile, the Catalyst AI system will default the rendering mode to Scissor, with no way to change it to a more suitable or faster mode, such as AFR. However SLI allows the rendering mode to be set for each application manually, even for games which do not have an existing profile. It should be noted that setting Catalyst AI to 'Advanced' allows manual mode setting for Direct 3D games, but not OpenGL games, to AFR.

* Some CrossFire implementations use an external dongle to connect the two cards, so the interconnect between them is not as fast as the link chip used for SLI. However, low- to mid-range GPUs do not need a Master Card when used with the new Xpress 3200 Chipset, because there the two cards are linked through the motherboard. Unfortunately, Master Cards are still necessary with the X1800 or X1900 series Crossfire, as communicating over the PCI-E bus becomes far too much of a bottleneck for such high-end GPUs. It is rumoured that a later version of Catalyst will aleviate the requirement for an external dongle. (The upcoming Radeon R600 GPU will feature integrated compositing engines on every Crossfire-compatible GPU, overcoming almost all disadvantages of Master Cards and 'masterless' Crossfire. It is not known yet how two such cards will connect together; nVidia has patented the internal bridge connector, so ATi may have to continue using special dongles.)

* If one already had a card such as the X1900GT, and wished to use it in a Crossfire system, they would need to buy an expensive X1900XT Master card. The X1900GT is a 12(36) pipeline part with 256MB of memory, while the X1900XT is a 16(48) pipeline part with 512MB of memory. On top of this, the XT runs at higher speeds in MHz. To compensate for this large gap in speed, the Master card would disable four(16) of its pipes, ignore half of its memory, and clock its GPU/RAM speed down to match. Many see this as poor value, with an expensive Master card performing at the level of a card of half the price if paired with a slower card.

* The CrossFire implementation in the Radeon X850 XT Master Card uses a compositing chip from Silicon Image (SiI 163B TMDS) which limits a X850 CrossFire setup to a resolution of 1600x1200 @60 Hz or 1920x1440 @52 Hz and could be a problem for some CRT owners wishing to use CrossFire to play games at high resolutions. As many people find a 60 Hz refresh rate with a CRT to strain ones eyes, the practical limit becomes 1280x1024, which does not push crossfire enough to justify the cost. LCD screens capable of higher resolutions are also quite expensive. However, the implementation in the X1800XT / X1900XT Master Card uses two compositing chips working in parallel, removing the resolution limit and allowing for a resolution of 2560x1600 @60 Hz, which is only found in large LCD monitors.