Introduction - do read this.
CrossFireX previewA bit of history, if you will.
Released in September 2005, ATI brought its own multi-GPU technology - where two graphics cards are leveraged in tandem to increase performance - to the desktop, to compete with NVIDIA's SLI.
Dubbed CrossFire, it allowed the user to purchase a master card and regular slave card, based on the same underlying GPU, and team them together to produce up to 2x the base card's performance. The entire point of CrossFire was to promote additional performance, which could either equate to higher frame-rates at a particular resolution or higher image-quality settings, for the same resolution, than a single card's.
Support for the nascent CrossFire technology necessitated the provision of a motherboard capable of running the two PCI-Express cards - be it ATI or Intel chipset-based - albeit with split-lane PCIe arrangements. The two cards were connected via an external cable and the maximum supported resolution was 1,600x1,200 - limited by the specifications of the compositing chip
Higher performance was achieved by a variety of rendering modes, and the actual scalability of a gaming title was dependent on just how well the engine queued up frames and how well-tuned the CATALYST driver was.
ATI created profiles for popular games but the unwieldy nature of the setup, together with the need for a master card, ensured that first-generation (two-card) CrossFire was strictly an enthusiast-only adventure.
ATI's next CrossFire evolution abandoned the inelegant master/slave arrangement and provided inter-card communication via the PCIe bus.
Limited in pure bandwidth stakes when compared to NVIDIA's bridge-attaching SLI solution, CrossFire finally became a viable solution with the introduction of the Radeon X1950 Pro GPU, released in October 2006, reviewed over here, and integrated the compositing engine - which takes the frame data produced by one card and composites it with the other, to produce the final output - for internal CrossFire, meaning that the previous master/slave relationship became moot.
Subsequent series of GPUs carried the internal compositing engine and, thinking of higher-end cards, were attached to one another via an NVIDIA-like bridge fashion, with the interconnect providing data output right up to a screen-busting 2,560x2,048.
ATI, now part of AMD, upgraded CrossFire support on the recently-released Radeon HD 3800-series of GPUs by providing higher inter-card bandwidth and the provision for two connectors, paving the way for three- and four-card support.
NVIDIA, meanwhile, had beaten ATI to the performance three-way punch with its three-way SLI, reserved for its high-end GeForce 8800 GTX and Ultra GPUs.
ATI, though, formally launched its highest-performing desktop product, Radeon HD 3870 X2, in January 2008, which leveraged two Radeon HD 3870 GPUs - albeit with slightly different clockspeeds - on to one card: internal CrossFire on a single PCB. One could hook a couple of these together to form a four-GPU subsystem, but doing so required a specific driver that was still a work in progress at that time.
This brief history lesson brings us nicely to CrossFireX - the latest incarnation of ATI's multi-GPU technology that hardware-launched with the Spider platform in November 2007.