Date:
Monday , May 30, 2005
Author:
Brent Justice
Editor:
Kyle Bennett
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ATI CrossFire Preview

Today ATI announces their tandem video card solution branded CrossFire. Inside we will preview this new technology and tell you what you need to know from a gamer’s perspective.

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Radeon Xpress 200 CrossFire Edition Chipset:

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We’ll start off with the motherboard chipset. Much as NVIDIA’s SLI requires the nForce4 SLI chipset with PCI-Express, ATI’s solution also requires a special motherboard chipset. ATI is using their Radeon Xpress 200 chipset to provide the necessary support for CrossFire.

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The Radeon Xpress 200 CrossFire Edition chipset is available on both an Intel and AMD platform, and both are loaded with all the latest features. Both the AMD and Intel versions support dual core CPUs. You can read about our experiences with the AMD platform Radeon Xpress 200 here where we also examine performance. From our experience, the Radeon Xpress 200 is a very fast and capable chipset. You can also read our introduction of this chipset on the Intel platform here. Both of those articles provide much more detail into the capabilities of the Radeon Xpress 200.

With the Radeon Xpress 200 CrossFire Edition, you will have two PCI-Express graphics slots. Unlike NVDIA, ATI does not require any kind of hardware selector card or jumper settings to enable CrossFire -- everything is handled via the BIOS. In CrossFire operation both the CrossFire Edition and regular X800 or X850 series video card will be running in x8 PCI-Express mode.

Radeon X850 and X800 CrossFire Edition:

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The next two components you need to make CrossFire work are the CrossFire Edition graphics card and a regular Radeon X800 or X850 series graphics card. Right now, everyone who owns an X800 or X850 series graphics card, or who goes out and buys one off the shelf, already supports ATI’s CrossFire multiple graphics card configuration. This is a great plus for gamers because those of you that already own an X800 or X850 series graphics card are only two steps away from being able to set up a multiple video card system with it. The only downside is that you will have to change your motherboard.

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So what is the Radeon CrossFire Edition graphics card you ask? In essence, this is a Radeon X800 series or X850 series-based video card with an extra hardware chip known as the compositing engine. This hardware chip is on the printed circuit board itself and not built into the actual core. This compositing engine contains FIFO buffers and logic of a programmable nature to carry out its task of blending the image from both video cards and outputting that blended image. ATI says that this compositing engine imposes no performance penalty.

In a CrossFire system, each GPU has its own dedicated PCI Express link to the North Bridge of the motherboard chipset, and is allocated its own command buffer and non-local storage space in system memory. Allocating separate command buffers allows each GPU to be assigned its own unique set of tasks.

Another portion of system memory is set aside for sharing of data between the GPUs. Shared data includes synchronization commands, textures, off-screen buffers, and other temporary data generated during the rendering process. This configuration takes advantage of the high speed bi-directional PCI Express links to ensure smooth, efficient co-operation between the GPUs.

When each GPU has completed its assigned tasks for a given frame, the resulting outputs are sent to the CrossFire Compositing Engine. This device combines the results from each GPU according to the selected operating mode, and sends the final frames out to the display device. It is capable of performing advanced blending operations without burdening either of the GPUs.

As you can see in the first slide in the paragraph above, a high-speed data transfer cable will connect both video cards. The Radeon CrossFire Edition video card has a special high density DMS port that you plug one end of the high-speed cable into; you then plug the other end of the cable into a DVI port on your regular Radeon X800 or X850 series graphics card. Texture and geometry data will be passed to each card over the PCI-Express bus. The high speed cable transmits raw pixel data which is fed to the compositing engine for final display out. You will not lose the use of that DVI port either since the high-speed data transfer cable is also a Y cable providing an extra DVI output connector.

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Here is how the products will lineup. There will be one Radeon X850 CrossFire Edition video card with a 256MB framebuffer and an MSRP of $549. This is simply a Radeon X850 XT 256MB video card with the compositing engine and DMS connector to use as the controller card. This card can be used with the Radeon X850 XT-PE, Radeon X850 XT, and Radeon X850 Pro.

Under that, there are two Radeon X800 CrossFire Edition video cards. One has a 256MB framebuffer and has an MSRP of $299, and the other is the 128MB model at $249. These cards work with the Radeon X800 XT-PE, Radeon X800 XT, Radeon X800 XL, Radeon X800 Pro, and X800.

There are only two criteria that must be met and be identical between the CrossFire Edition card and your regular Radeon X800 or X850 series video card to work properly. You must have the same amount of framebuffer memory between both cards, so if you have a 128MB video card get the X800 128MB CrossFire Edition, if you have a 256MB card get the 256MB X800 or X850 CrossFire Edition. You must also achieve parity with the number of pixel pipelines, so all the Radeon X 850 series have 16 pixel-pipelines, and as such, the Radeon X850 CrossFire Edition has 16 pixel-pipelines.

What does not need to be the same are the clock speeds between both video cards. Unlike what you might think, the controller video card will not downclock itself to match a slower secondary video card. The clock speeds of both video cards can be different as long as the pixel-pipeline count and framebuffer size are the same. With the buffers in the compositing engine, it is able to buffer up data if one card is still drawing while the other is waiting for the data before blending and still maintain a performance advantage.

You also do not need to have the same BIOS or brands of video cards paired up. Theoretically, for example, you could have a Radeon X850 CrossFire Edition video card and a Sapphire Radeon X850 XT video card work without any problems at all. All you need to make sure is that you buy the right edition of the CrossFire boards with the same size framebuffer.

You are probably wondering how much power this is going to take to make all this happen. Power Supplies are one component of a computer system that is often overlooked, but are an increasingly important part of the system. For this complete CrossFire experience, ATI is recommending a good 475 watt power supply. ATI told us they had a CrossFire system working great with an Enermax 475 watt power supply running an Xpress 200-based motherboard, hard drives in RAID, and an optical drive without problems.