ATI Crossfire Multi GPU for the Radeons

What is Crossfire?

Crossfire is ATI's solution to Nvidia's SLI (Scan Line Interleave). It allows you to have two ATI graphics cards in the same PC working together side by side in order to almost double the performance not only games but in all applications and day to day tasks as well. Crossfire allows you to play games with high resolutions as well as having Full Screen Anti-Aliasing and other image quality increasing features, without having a big performance hit.

The Main attraction that came out of the release of SLI from Nvidia was that consumers would buy a relatively cheap graphics card such as the Geforce 6600GT (which turned out to be the card of choice) and either ran it as a single card until they required a performance boost, or bought two of these cards straight away and had a top notch graphics machine for less than the cost of the flagship product of the time. Crossfire will offer similar benefits to those who are on a budget while still giving power hungry users the chance to double up on the top of the range cards.

What hardware is required?

In order to run a system capable of running the crossfire multi visual processing units you will need to make sure of a few hardware specifications. Firstly you will need a system with a crossfire capable motherboard. Searching hardware sites for crossfire in the motherboards sections will get you some results. ATI have their own chipset ready to take the multi graphics card system, the Radeon® Xpress 200 Crossfire chipset.

Obviously you will need 2 ATI graphics cards if you are running a dual graphics card system, but you need to be a little more specific to get a working crossfire system up and running. Firstly you need a crossfire ready graphics card any type of X800 or X850 is crossfire ready providing that it is a PCI-express based card and not any type of AGP version. ATI is likely to release a crossfire version of any later graphics cards released after this article is written. The second card has to a be a Crossfire edition card. This has the hardware to enable the Crossfire technology to work inside your PC.

You will also need a beefy PSU as taking two graphics cards onboard especially powerful ones will take up a few watts to say the least. Then Lastly you will need a case with a decent amount of space, two graphics cards will take up a lot of room and create a fair amount of heat, so a spacious case with some good cooling is definitely preferable.

Crossfire Edition and Crossfire ready

The difference between these two types of ATI Radeon graphics cards is very slight but incredibly important. The Crossfire ready cards are graphics card which can be placed in a crossfire system, however you cannot use two crossfire ready graphics cards together in this system. This is where the Crossfire edition cards come into play. These graphics cards contain an extra chip (a compositing engine) that is required to merge the partial images created by both graphics cards in order to create a complete frame. Be aware that you require one of each type to make this work and they must be from the same range i.e. both X800's. However unlike Nvidia's solution you can mix and match varieties of these cards. For example you can pair a X800 XT Crossfire edition with a standard X800 pro. You wont see as much benefit as having two X800 XT's but it is possible and could help some people with lower speed X800 / X850 cards.

How CrossFire Works

So we know that crossfire shares the workload of a sequence of frames in a game or the workload of rendering an image in a CAD program for example. But ATI and crossfire have 3 different ways of doing this, each with there own merits.

AFR - Alternate Frame Rendering.

Alternate frame rendering is also a technique that Nvidia's SLI can use. AFR is as simple as it sounds. One frame is fully rendered by one card while the second card is already working on the next frame before switching back again. This can almost double performance upto the point where the work on one frame becomes two much and then other modes become more useful. Also some games cannot use AFR as each frame can be dependent on the frame before it, therefore frame 2 cannot be rendered until frame 1 has been completed, defeating the object of AFR entirely. I a crossfire system I would recommend using Radeon cards capable of running the frame rate on there own adequately or this system becomes less efficient.

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