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ATI announces CrossFire multi-VPU platform

Geoff Gasior
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WE’VE WATCHED OVER the course of the past six months as NVIDIA has reaped enormous publicity benefits by resurrecting the SLI multi-GPU graphics capability that it inherited from 3dfx, and we’ve wondered when ATI would join the fray. The folks at ATI have been talking about their own multi-card graphics capability for what seems like ages, and today, they’re finally taking the wraps off of their solution: ATI CrossFire.

The CrossFire platform is similar to NVIDIA’s SLI, but it’s distinctive in some intriguing ways. CrossFire does allow for the teaming of a pair of PCI Express video cards to increase graphics rendering capacity, but it doesn’t rely on a bridge between the two cards. Instead, CrossFire requires that one of the two cards in a system be a special, CrossFire-capable unit that has a custom composting chip onboard designed by ATI. This chip, onboard the master graphics card, takes the image from the local Radeon VPU and mixes it with the image from the slave graphics card, which it receives over a DVI connection. The CrossFire card then outputs the composite image via its own DVI output (or to VGA via a converter).


ATI’s CrossFire platform
Source: ATI

ATI’s Radeon VPUs have had the ability to combine images since the debut of the R300 chip, but this will be the first consumer-level implementation of the technology. CrossFire graphics cards will be able to work together several ways in order to produce a final image with higher performance than a single card. Those rendering modes are:

  • Supertiling — This is the tiling mode built into the original R300 VPU and used by ATI partner Evans & Sutherland in its high-end visualization systems. The screen is split up into 32×32 tiles and the workload is distributed according to a checkerboard-style pattern, with one card taking what would be the black squares and the other the squares that would be red. Splitting up the workload in such relatively small tiles should result in a distribution of the load that’s very close to an even split, and it should allow two cards to produce markedly higher fill rates and pixel shader throughputs than a single card. The vertex processing load will be duplicated on each card, however, so that a CrossFire system with supertiling won’t likely achieve any higher geometry throughput than a single card. Supertiling will be the default CrossFire mode for all Direct3D applications, but will not work in OpenGL.

  • Scissor mode — This mode will be familiar from SLI, where NVIDIA calls it split-frame rendering. Scissor mode divides the screen horizontally, giving the top portion of the screen to card A and the bottom portion to card B. The exact proportion of the split is adjusted on the fly as workloads change. This will be CrossFire’s default mode for OpenGL and will also work in Direct3D.

  • Alternate frame rendering — This one, ATI reminds us, is the mode used by its previous multi-chip solutions, like the Rage Fury MAXX. Alternate frame rendering involves buffering a few frames ahead of what’s being displayed onscreen, with the cards alternating drawing entire frames. AFR can be enabled on a CrossFire system via ATI’s control panel. This mode distributes both fill rate and geometry loads evenly between the cards, allowing for better scaling. ATI says AFR should be an option for both OpenGL and Direct3D applications.

  • Super antialiasing — Uniquely, CrossFire rigs may provide image quality benefits even in games where fill rate and geometry throughput isn’t normally at a premium via its Super AA capability. Super AA comes courtesy of the CrossFire compositing chip, which can combine images with different sample patterns produced by the two cards. CrossFire AA offers several new antialiasing modes from 8X up to 14X. 8X and 12X AA modes double up on 4X and 6X multisampling, respectively. 10X and 14X AA modes, on the other hand, combine 2X supersampling with 8X and 12X multisampling, respectively.

After talking to ATI’s Catalyst driver team, we learned that although CrossFire’s Direct3D and OpenGL defaults allow it to accelerate any application, the drivers will also ship with application-specific profiles. These profiles generally use Alternate Frame Rendering, which apparently offers better performance than superTiling or scissor modes. However, AFR apparently can’t be blindly enabled without the danger of causing display corruption or stability problems, so scissor and supertiling modes provide good fallback positions for games that lack AFR profiles.

CrossFire cards
ATI will initially sell CrossFire cards in three flavors, the most expensive of which is the $549 Radeon X850 CrossFire Edition 256MB card. This card will have 16 pixel pipelines, clock speeds to match the Radeon X850 XT, and support pairing with the Radeon X850 Pro, XT, and XT Platinum Edition. A CrossFire Edition of the Radeon X800 will also be available with 16 pixel pipelines and clock speeds to match the Radeon X800 XL. The X800 CrossFire Edition will come in 128MB and 256MB flavors that sell for $249 and $299, respectively, and be capable of teaming up with Radeon X800, Pro, XL, XT, and XT Platinum Edition. Here’s how the new CrossFire Edition cards fit into our handy graphics chip comparison chart:

Core clock (MHz) Pixel pipelines Peak fill rate (Mpixels/s) Texture units per pixel pipeline Peak fill rate (Mtexels/s) Memory clock (MHz) Memory bus width (bits) Peak memory bandwidth (GB/s)
Radeon X800 400 12 4800 1 4800 700 256 22.4
Radeon X800 Pro 475 12 5700 1 5700 900 256 28.8
Radeon X850 Pro 520 12 6240 1 6240 1120 256 35.8
Radeon X800 XL 400 16 6400 1 6400 980 256 31.4
Radeon X800 CrossFire Edition 400 16 6400 1 6400 980 256 31.4
Radeon X800 XT 500 16 8000 1 8000 1000 256 32.0
Radeon X800 XT Platinum Edition 520 16 8320 1 8320 1120 256 35.8
Radeon X850 XT 520 16 8320 1 8320 1120 256 35.8
Radeon X850 CrossFire Edition 520 16 8320 1 8320 1120 256 35.8
Radeon X850 XT Platinum Edition 540 16 8640 1 8640 1180 256 37.8

Breaking out the chip chart might seem a little indulgent, but it nicely illustrates the potential for CrossFire mismatches, particularly when 12-pipe cards are involved. Since both AFR and supertiling split the rendering load evenly across both cards, the system will be somewhat limited by the speed of the slowest card. To keep the fill rate disparity between mismatched cards to a minimum, CrossFire cards will actually throttle back to 12 pipes when they’re paired with 12-pipe partners. That should keep things pretty even, but it’s unfortunate that mismatches may cause surplus pixel pushing power to go unused. ATI may be able to leverage scissor mode’s dynamic screen segmentation to give extra work to the more powerful card in a CrossFire pair, but workloads are locked in at a 50/50 split for AFR and Supertiling.

CrossFire chipsets
In addition to requiring a CrossFire Edition graphics card, ATI’s new multi-VPU rendering scheme also requires a CrossFire-compatible chipset. ATI is offering such a chipset for both AMD and Intel platforms with its Radeon Xpress 200 CrossFire Edition. When two graphics cards are in-use, the Radeon Xpress 200 CrossFire Edition will provide each of a motherboard’s two PCI Express x16 slots with eight lanes of bandwidth. Single-card configurations will get a full 16 lanes of bandwidth from one of the x16 slots, and users won’t have to worry about installing a redirector card or swapping jumpers to switch between CrossFire and single-card modes.

Although their CrossFire capabilities are similar, only the Intel-compatible chipset will include ATI’s Radeon Xpress 200 integrated graphics processor. The IGP’s presence will allow the chipset to power up to five monitors thanks to ATI’s SurroundView technology, but it won’t lend a hand when it comes to 3D rendering. Not that the IGP has much horsepower to contribute, anyway.

Apart from their new graphics capabilities, the CrossFire Edition chipsets will support AMD and Intel’s respective dual-core processors. ATI is also claiming that south bridge Serial ATA and USB performance has been improved, and that it now equals or exceed that of the competition.

Interestingly, you won’t be locked into an ATI chipset if you want to run CrossFire. ATI will allow CrossFire to work with Intel chipsets, and is also in talks with NVIDIA to get CrossFire to cooperate with the nForce platform. Compatibility with VIA chipsets isn’t a priority for ATI, although it may be more likely than a cross-compatibility agreement with NVIDIA.

Conclusions
CrossFire’s ability to fall back on scissor and supertiling modes when it encounters a game that it doesn’t have an Alternate Frame Rendering profile for is unique. This gives CrossFire the potential to accelerate any Direct3D or OpenGL game, although fine-tuned AFR profiles may ultimately offer superior performance. NVIDIA continues to add new SLI game profiles to its drivers, but it can’t match CrossFire’s automatic compatibility.

Game compatibility aside, CrossFire’s most appealing attributes may be its flexibility. Despite the fact that a CrossFire combo may be limited by the speed of the slower card, it’s nice that mismatched cards will work at all. This should allow for relatively easy CrossFire upgrades, at least when compared to SLI, which requires that cards be installed in matched pairs.

CrossFire cards should start sampling in late June and arrive on the market this summer, with Abit, Asus, Gigabyte, Sapphire, MSI, VisionTek, and others slated to offer cards. ATI will offer a line of “Built by ATI” CrossFire cards, as well. Radeon Xpress 200 CrossFire Edition motherboards will be left to ATI’s motherboard partners, who include Asus, DFI, ECS, Gigabyte, MSI, Sapphire, and Tul.