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ATI's CrossFire dual-graphics solution


Dually Radeons at last
by Scott Wasson — 8:00 AM on September 26, 2005

NVIDIA DID A VERY clever thing when it designed its GeForce 6 series graphics processors, building into them a compositing engine and provisions for a digital interconnect between graphics cards that could allow two (or more) graphics cards to work together in tandem. Once PCI Express became fairly common, NVIDIA pulled the curtains back on its SLI technology, scoring a coup on several fronts. The ability to team two graphics cards gave NVIDIA an indisputable edge in the otherwise tight struggle with ATI for graphics performance supremacy. The possibility of adding a second card gave enthusiasts an upgrade path they couldn't get elsewhere, and the exclusivity of SLI allowed NVIDIA to muscle would-be chipset competitors out of the high end of the motherboard market, selling a bundle of nForce4 SLI chipsets. The number of actual, fully configured SLI systems in the wild may not be huge, but NVIDIA has gotten tremendous mileage out of dangling the tantalizing prospect of dual-graphics goodness in front of PC enthusiasts.

Naturally, ATI wanted to match NVIDIA's dual-graphics capabilities stride for stride, but doing so wouldn't be easy. Yes, at its heart, the technology is fundamentally simple—the output from two graphics cards is combined to offer nearly double the pixel-pushing power of a single card. Yes, ATI graphics processors have had the ability to run in parallel configurations for some time—since the debut of the Radeon 9700—and high-end visualization systems like the Evans & Sutherland Renderbeast have put multiple ATI GPUs at work in teams as large as 64 chips. But the Radeon X800 family of GPUs wasn't built with provisions for a GPU-to-GPU communications and image compositing, and doing this work over a PCI Express link wouldn't be fast enough for real-time, high-resolution gaming.

In order to make a credible rival for SLI, ATI had to go a different route. ATI's consumer dual-graphics platform, known as CrossFire, would require a fair amount of ingenuity and some new, custom hardware. That hardware comes in the form of so-called "master" cards that incorporate additional chips to handle the communications and compositing needed to make CrossFire work. One of these master cards can hook up with a regular X800-series graphics card for multi-GPU bliss.


The Radeon X850 XT CrossFire Edition master card


Without its cooler, the master card's extra chips are easy to see


A close-up look at the five chips that make a master card special

CrossFire is a pretty slick scheme, really, given the limitations imposed by the original X800 hardware. ATI equipped its master cards with five new chips, pictured above. The second largest of the five chips there is a TMDS receiver made by Texas Instruments. To make up for the lack of a dedicated digital interconnect between GPUs, the master card can intercept and decode the DVI output of the slave card using this receiver. Next to it, the largest of the chips is a Spartan-series FPGA chip from Xilinx. FPGA stands for Field Programmable Gate Array, which is a fancy way of saying that this is a programmable logic chip. In this case, ATI has programmed the Xilinx FPGA to act as CrossFire's compositing engine, tasked with combining the images generated by the two Radeon GPUs into a single stream of video frames. The smaller chip just below the FPGA in the picture is a flash ROM; presumably, it holds the programming for the FPGA.

Once the images from the slave card have been decoded by the TMDS receiver and composited with the images from the master card's GPU by the FPGA, they have to be output to a display. That's where the last two chips come into the picture. The little, square chip above the FPGA is a RAMDAC chip made by Analog Devices. The RAMDAC converts digital video information for output to an analog display, such as a VGA monitor. Just above the TDMS receiver is a smaller, rectangular chip from Silicon Image. That's a TMDS transmitter capable of encoding images for output to a digital display via a DVI output.

All together, these five chips add the necessary functionality to ATI's master cards to allow a pair of graphics cards to run together in an SLI-like configuration with very little performance penalty for inter-chip communication or image compositing.