CrossFire X - AMD's three- and four-way multi-GPU tech

By Koushik Saha on 14.3.08

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GPUs, it seems, are everywhere, breeding like rabbits. We see the introduction of a new GPU seemingly every month, and multi-GPU schemes like SLI and CrossFire are omnipresent. We now have multiple GPUs on a single graphics card, hybrid multi-GPU implementations involving integrated graphics, and more-than-two-way incarnations of both SLI and CrossFire.
The most intriguing bit of multi-GPU madness we've seen recently may be AMD's CrossFire X, simply because in this generation, AMD opted to chain together three or four mid-range GPUs in place of creating a separate high-end graphics processor. That's a bold move, fraught with peril, because multi-GPU schemes can be rather fragile, with iffy compatibility and less-than-ideal performance scaling. Then again, AMD's decision to rely on CrossFire X to round out the high end of its product lineup has surely helped to concentrate its attention on making the scheme work well. So who knows?

We've taken a quick look at AMD's first drivers for CrossFire X, and we have some interesting things to report. Read on to see what we learned.
Extending CrossFire to X
CrossFire X is, quite simply, an extension of the CrossFire dual-GPU feature to three and four GPUs. The hardware to make such a thing possible has been on the market for some time now, and last week's release of the Catalyst 8.3 driver revision finally enabled this feature in software, as well. The basic building block of CrossFire X is AMD's RV670 GPU, which is present in all of the various incarnations of the Radeon HD 3800 series of graphics cards. Getting to three or four GPUs can be achieved using a dizzying number of potential card combinations, which AMD has summarized in this helpful matrix:The caveat here is that CrossFire X will settle on the lowest core GPU clock, memory clock, and video RAM size to determine the operative clock speeds and effective memory size. As a result, a Radeon HD 3870 X2 paired with a Radeon HD 3850 256MB would perform like a trio of Radeon HD 3850 256MB cards. And, of course, that means the effective memory size for the entire GPU phalanx would effectively be 256MB, not 768MB, because memory isn't shared between GPUs in CrossFire (or in SLI, for that matter).

Like its dual-GPU predecessor, CrossFire X works on a fairly broad range of motherboards, including those based on AMD 480, 580, and 7-series chipsets, as well as boards based on many of Intel's more recent chipsets—among them: the 955, 965, 975, P35, G35, X38, and X48.
CrossFire X's performance and feature set will be more or less optimal depending on the chipset's topology and the motherboard's allocation of PCIe lanes. AMD cites its own 790FX chipset as the most optimal possible config, where the motherboard could dedicate eight lanes of PCIe 2.0 bandwidth to each of four PCIe x16 slots. On the other hand, Intel's P35 chipset would be less than ideal, since it has 16 lanes of PCIe 1.1 connectivity feeding a single PCIe x16 slot off of the north bridge chip, while the second PCIe x16 slot hangs off of the south bridge and has only four lanes connected. The P35's lower bandwidth will impose some limitations on CrossFire X: image compositing must be done in hardware (so you'll definitely need to have those CrossFire bridge connectors attached) and OpenGL support won't be possible.

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