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Building a better CrossFire
CrossFire configs with Radeon X1800 cards retain many of the same limitations that Radeon X800 and X850 versions have, including the need for a specialized "master" or CrossFire Edition card, an external dongle or Y-cable, and driver-based game profiles in order to improve performance. Radeon X1800 CrossFire still defaults to SuperTiling mode for load balancing if no application-specific profile specifying a different mode is available in ATI's drivers, and based on our experience so far, SuperTiling remains a net performance loss versus a single card more often than not.

The X1800 version, though, isn't limited to 1600x1200 resolution at 60Hz. It can run at up to 2560x1600 thanks to a more powerful compositing engine and the use of a dual-link DVI connection from the slave card to the master. (Err, card A to card B, if you're intent on being politically correct.)


A bevy o' chips on the X1800 CrossFire card

Even the new Radeon X1000 series GPUs weren't designed with CrossFire capabilities expressly built in, so the "master" cards need some additional chips in order to handle the image compositing work for multi-GPU bliss. The Radeon X1800 CrossFire card really packs 'em in, too, with a total of at least seven different chips onboard to help.

The large chip on the left in the picture above is the heart of the CrossFire engine, a Xilinx Spartan XC3S400 FPGA, or programmable logic chip, that has been programmed to do the image compositing for CrossFire. This is a more powerful FPGA than the one on the Radeon X850 master card. ATI has taught the FPGA a new trick, too: how to do the blending required for CrossFire's Super Antialiasing mode. Doing the blend in hardware here should alleviate some of the performance penalty associated with Super AA. ATI claims this new compositing engine is more flexible than the old one and could be reprogrammed to do new and different things via a flash ROM update, although they don't seem to have any specific new capabilities in mind for it just yet. The flash ROM itself is the small chip just above the FPGA in the photo.

The two relatively large chips in the top right of the picture are Silicon Image SiI 163B TMDS receivers. These chips receive data from the slave card via the dual DVI links passed through the CrossFire dongle. Those links run at 165MHz, and each link can pass an image of up to 1600x1200 resolution at 60Hz. Combined, they give the Radeon X1800 CrossFire Edition the ability to pass through resolutions as high as 2560x1600. The two smaller Silicon Image chips are matching TDMS transmitters, used to drive any displays connected to the master card via the CrossFire dongle's dual-link DVI output.

To the left of the TMDS transmitters is the same Analog Devices RAMDAC chip used on the Radeon X850 CrossFire. This little guy converts digital frame buffer info into an analog video signal suitable for CRT displays.

Obviously, adding all of this logic to the CrossFire card isn't the ideal way to do things, and indeed, FPGA chips are often used to prototype logic that is later converted into a custom ASIC. I wouldn't be shocked to see ATI incorporate CrossFire compositing logic directly into future GPUs, as NVIDIA has done with SLI.


The CrossFire connector is now a funky bifurcated job

ATI's Master plan
ATI says Radeon X1800 CrossFire Edition cards will be available today at online retailers. Given ATI's recent track record on that front, I'd say we can expect the cards to be generally available before Christmas, but I wouldn't bet the bank on immediate gratification. These cards will ship at a princely list price of $599, and ATI expects to see the things actually selling at that price—or close to it. Hopefully, prices won't rocket into the stratosphere the way those of the GeForce 7800 GTX 512 have, but that will depend on ATI's supply and the markets demands, I suppose.

As for Radeon X1600 CrossFire cards, well, there's been an apparent change in plans. ATI initially said it would be providing a CrossFire Edition of the Radeon X1600, but the new plan is to begin by offering "connectorless" CrossFire for this card. Simply plug a pair of these into the system and go—no master card or connector dongle is needed. The X1600 cards will have to pass data to one another via PCI Express, and doing so could hamper frame rates. However, rumor has it that ATI is planning to introduce a new chipset with dual 16-lane PCI Express slots, much like NVIDIA's nForce SLI X16. This chipset might provide enough PCI Express bandwidth to sustain solid CrossFire performance for a pair of Radeon X1600s. ATI may yet decide to produce an X1600 CrossFire Edition master card, too, but I would be surprised to see it happen.