ATI CrossFire: Imperfect Implementation

The problem with the original CrossFire was that ATI had only out-dated RADEON X850/X800 series processors then which didn’t support modern graphics technologies and were far inferior to the GeForce 7 series in performance. ATI’s approach to creating a multi-GPU technology was not perfect, either. Contrary to NVIDIA, ATI put an emphasis on the hardware part of the technology and tried to make CrossFire as independent from software and drivers as it could possibly be. To implement this concept in silicon, a special mechanism for sewing together the parts of the frame rendered by the different GPUs was necessary. They created it basing on the programmable Xilinx Spartan-3 field programmable gate array (FPGA) that consisted of 400 thousand gates.

Unfortunately, there was a flaw in the Compositing Engine (as the “image-sewing” unit was called) which almost ruined the whole concept. How? A Master card with a Compositing Engine on board was connected to a Slave via an external cable and the data were transferred through a DVI interface, thus requiring a TMDS receiver to be converted into the format the Compositing Engine could understand. The Engine actually included a single-channel 165MHz TMDS receiver, but these characteristics of the receiver limited the maximum display mode to 1600x1200 at 60Hz refresh rate. Moreover, even if CrossFire-compatible Master cards of the first generation had carried a receiver with a higher operating frequency or two ordinary receivers coupled in dual-link mode, the mentioned limitation would have still persisted as ordinary RADEON X850 and X800 graphics cards were equipped with single-channel transmitters that supported single-link mode only.

Thus, the first version of CrossFire was in fact limited to 1280x1024 resolution, but high performance in high resolutions is among the main features of any multi-GPU technology, isn’t it? Large TFT monitors capable of working at 1600x1200 were and are rare and expensive, while using 1600x1200@60Hz mode on a CRT display is prohibited for ergonomic reasons – you’ll strain your eyes in no time at such a low refresh rate.

So the first implementation of the multi-GPU technology from ATI was technically a failure, especially since there were some software-related problems besides the defective design of the Compositing Engine. Despite ATI’s claiming CrossFire’s independence from software, 7 out of 21 games we used in our tests (see this article for details) had a zero or even negative performance gain. With the above-mentioned hardware deficiencies and after the arrival of NVIDIA’s new-generation and faster graphics cards, CrossFire had no real market perspectives in its original implementation. On the other hand, we then said that the next version of ATI’s multi-GPU technology might come out on new-generation graphics processors, free of the original flaws and competitive against NVIDIA’s SLI. So today we are going to see if our expectations have come true because ATI has introduced its RADEON X1800 CrossFire Edition graphics card to the public!