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Computex 2005: ATI CrossFire Technology May 30, 2005 Brandon Sandman Bell |
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Summary: For months NVIDIAs SLI technology has been the only game in town for users looking to combine two cards together for more performance. But soon that will change, as today ATI is unveiling their SLI equivalent: CrossFire! ATI is taking image quality to the next level with CrossFire, supporting AA levels up to 14x and a new rendering mode used in advanced simulation systems dubbed supertiling. Read all the details in this article!
 Introduction | Page:: ( 1 / 5 ) |
Ever since 3dfx first uttered the words S-L-I in 1998, gamers everywhere have been entranced by the idea of combining multiple cards for additional performance. Unlike a lot of technologies in the graphics market, the SLI concept can appeal to both the hardcore hardware enthusiast as well as the gamer on a budget.
For the high-end crowd the concept is simple: you get the performance of tomorrow’s high-end cards today. By dropping in two cards at the same time you can not only enjoy new levels of performance at the screen resolutions you’ve used previously, SLI also opened up new screen resolutions. In the case of Voodoo2 for instance, 1024x768 became available.
Meanwhile, gamers on a budget upgraded incrementally, in many cases purchasing the first card at full MSRP and then taking advantage of price cuts to pick up the second card 6-12 months later at a much lower price.
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As a result, 3dfx’s Voodoo2 and their SLI “Power of Two” campaign was an enormous success, it has been estimated that 30% of Voodoo2 card owners were SLI users. This is an enormously high percentage for a product that was derided by many as a niche product with no future.
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Knowing this, NVIDIA was quick to cash in on SLI’s previous success when they were ready to launch their multi-GPU technology last year, even though the rendering technique used was completely different. Since then NVIDIA’s SLI solution has gone on to enjoy tremendous success with rapidly falling prices on motherboards and video cards spurring demand for products based on the technology. The one key downside to NVIDIA SLI however has been application support. NVIDIA’s driver team has been busy literally optimizing individual games for SLI. In our original SLI Performance Preview from November, we noted three games that didn’t take advantage of SLI: Lock On: Modern Air Combat, Unreal Tournament 2004, and the original Splinter Cell (as well as its follow-up Pandora Tomorrow). Since then, NVIDIA has only reworked their SLI driver to take advantage of one of these three games, LOMAC.
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Now ATI is poised to release their equivalent to NVIDIA’s SLI technology, only they boast one key feature – their solution works with all games! ATI’s dubbed their multi-VPU product Crossfire.
| Crossfire implementation | Page:: ( 2 / 5 ) |
Differences between Crossfire and SLI
While ATI and NVIDIA’s multiple graphics card technologies share some key concepts, there are also a number of differences between ATI’s and NVIDIA’s implementations.
For starters, the most basic difference between ATI Crossfire and NVIDIA SLI is that ATI has a dedicated CrossFire card designed just for linking to your existing ATI X800/X850 series graphics card. For now, ATI provides two different CrossFire boards, a RADEON X800 Crossfire Edition, and a RADEON X850 Crossfire Edition.
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As their names imply, the RADEON X800 Crossfire Edition supports the X800 series of R420 and R430 VPUs, this includes the RADEON X800, X800 XL, X800 PRO, and X800 XT/X800 XT Platinum Edition. ATI lists two SKUs for the X800 CrossFire board, one with 128MB of memory and a second with 256MB, but we think the 256MB board will probably be the predominant solution. ATI’s RADEON X850 Crossfire Edition ships with 256MB of memory and supports cards based on the R480 VPU, this includes the RADEON X850 PRO, X850 XT, and X850 XT Platinum Edition.
Since the X800 Crossfire supports R420 PCI Express boards launched nearly a year ago, ATI claims that Crossfire already supports nearly 1 million people that are “Crossfire-ready.”
NVIDIA, on the other hand builds their SLI technology into the graphics core of every GeForce 6600 GT and 6800 series card, the user merely needs to purchase a second PCI-E NVIDIA GPU in order to take advantage of SLI. Purchasing two X850 boards wouldn’t give you Crossfire, you’d merely have two distinct X850 cards which can’t be linked together.
| ATI CrossFire Pricing | |
| CrossFire Edition Card | Estimated Price |
| RADEON X850 CrossFire Edition (256MB only) | $549 |
| RADEON X800 CrossFire Edition 256MB | $299 |
| RADEON X800 CrossFire Edition 128MB | $249 |
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While ATI has given us pricing for CrossFire X800 and X850, at Computex this week they’ve been adamant that the pricing data they’ve given above is merely preliminary. In other words, the prices you see above may, or may not change by the time the first CrossFire cards are available in July. While we weren’t told this by ATI, we believe ATI’s final pricing strategy for Crossfire will ultimately depend on NVIDIA’s upcoming next-generation GPU, codenamed G70. Quite simply, if NVIDIA is able to deliver G70 on time and with good performance, ATI will likely feel the need to lower prices on CrossFire boards substantially.
| Hardware implementation (cont’d) | Page:: ( 3 / 5 ) |
So what separates a CrossFire board from a regular RADEON card? Simple, CrossFire’s unique compositing engine chip. The compositing engine chip is an external chip located near the graphics core and is responsible for handling all the communication between the two cards, as well as handling blending and arithmetic operations.
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By going with an external solution in the compositing engine, CrossFire is able to support a wider variety of cards. As we just mentioned previously, CrossFire supports R420-based X800 PRO/XT cards that were first launched last year. The compositing engine chip can actually communicate with a wide variety of cards, and isn’t picky about the revision of the board or the card’s BIOS. You can also mix and match manufacturers.
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Basically, just plug your RADEON card in and the compositing engine onboard your CrossFire card should be able to communicate with it perfectly. NVIDIA’s SLI technology on the other hand requires that the boards come from the same manufacturer and use the same BIOS in order to support SLI (although NVIDIA is working on resolving this).
ATI also feels that their compositing engine boasts lower latency than NVIDIA’s solution, although by press time we weren’t given specific figures or other details such as transistor count.
CrossFire’s software implementation
Like NVIDIA’s SLI, CrossFire supports multiple rendering modes, including alternate frame rendering, scissors, and a new mode which is unique to ATI, supertiling. You’re probably familiar with alternate frame rendering and scissor mode, so we’ll briefly go over those first, before describing supertiling.
With alternate frame rendering (AFR), each card handles alternating frames. Graphics core one handles everything in frame 1, while graphics core two then handles frame number 2. Each chip renders every other frame instead of alternating lines in the same frame, as 3dfx had done with scan-line interleave.
Scissor mode is the equivalent of NVIDIA SLI’s single frame rendering mode, or SFR. In this mode, the workload is split horizontally across the screen. One card takes the upper portion while the second card takes the lower segment. The frame buffer data is then combined and sent to the monitor.
The final rendering mode ATI CrossFire supports is supertiling. In supertiling, the scene is split into 32x32 squares known as “tiles”. Each card then handles alternating tiles which are then output to the display.
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The key benefit supertiling brings is performance. Unlike other modes, with supertiling CPU overhead is nearly nonexistent. Because of this, supertiling is the preferred mode for CrossFire and is used by default in all Direct3D applications. For OpenGL, AFR is used for all DOOM 3/Quake 3 engine games, while scissor mode is used for older OpenGL apps.
| SuperAA/Platforms | Page:: ( 4 / 5 ) |
One new feature ATI has implemented into CrossFire is what they call “Super anti-aliasing”. This refers to two abilities which are unique to Crossfire. The first is CrossFire’s ability to mix sampling patterns from both cards, providing sharper visuals, and the second is CrossFire’s support for higher AA modes than previous ATI offerings. As a result of these features, Super AA provides even more optimal sampling patterns and more samples per pixel than previous AA implementations. We’ll start with the enhanced sample patterns first.
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In order to improve AA quality, each CrossFire board uses slightly different rotated-grid sampling patterns, proving excellent load balancing. ATI’s compositing engine chip then takes the different sample patterns from the two boards and blends them together to produce the final image. This results in an even better looking final image.
Higher AA modes
In addition to the standard 2, 4, and 6 sample AA modes ATI has provided previously, for CrossFire, ATI also adds 8x, 10x, 12x, and 14x modes for even better-looking visuals.
In the case of CrossFire’s 8xAA mode, both cards are running in 4xAA mode, for an effective equivalent of 8xAA (at 12xAA, both cards are running in 6x mode). The 10x and 14x modes mix multisampling with supersampling for even better image quality. At the 10x setting for example, the cards mix 8x multisampling with 2x supersampling, while 14x combines 12x multisampling with 2x supersampling.
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Chipsets
On the chipset side, ATI is providing CrossFire solutions for both Intel and AMD platforms. Both chipsets build largely on ATI’s XPRESS 200 chipset, which was launched by ATI several months ago, the key additions are support for newer technologies such as DDR2-667 on the Intel platform as well as 1066MHz FSB, while the AMD platform supports newer storage technologies natively such as 300MB/sec SATA with NCQ support (provided externally), SMP, and CAS latencies as low as 1.5 on the memory.
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Both chipsets are built on a 0.13-micron manufacturing process, so they require no active cooling for operation. ATI even integrates 4-phase power circuitry on their reference AMD board for even greater levels of overclocking, while the Intel platform is offered solely with integrated graphics (ATI has no immediate plans for an IGP CrossFire AMD-based board). ATI is also quick to point out that their CrossFire solution doesn’t require a selector card or jumpers, unlike NVIDIA SLI.
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| Conclusion | Page:: ( 5 / 5 ) |
After months of waiting, ATI’s multi-VPU solution is almost here, and on paper it definitely looks pretty promising. We’re eager to test ATI’s claims of CrossFire support with all games. As many of you know, NVIDIA’s SLI currently supports approximately 70 titles natively; ATI claims their figure for CrossFire is in the thousands.
When we quizzed them about this, they indicated that they can boast support for so many titles because of SuperAA. In other words, they’re not claiming you’ll see a performance improvement in every game you play thanks to CrossFire, instead their solution allows you to crank up the AA to levels never seen before previously. Technically this allows them to claim CrossFire support, but it isn’t quite the same as the performance proposition NVIDIA pitches for their SLI solution. Although ATI feels they’ll be pretty strong in this regard too, claiming performance superior to NVIDIA in games such as Splinter Cell: Chaos Theory and Need for Speed Underground 2.
ATI’s unique compositing engine also allows them to support cards released around this time last year, such as the PCI-E X800 PRO and X800 XT. We bet many of you never dreamed you’d be able to pair up your card with a CrossFire for even more performance. X800 XL owners are probably drooling right now too.
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On the motherboard side, ATI has signed up manufacturers that they haven’t worked with previously such as DFI and ECS. Long-time partners such as Sapphire and TUL are also onboard to, as are the heavy-hitters such as ASUS, Gigabyte, and MSI. The first motherboards should arrive by the end of the month.
On the graphics side, ATI is going to handle early CrossFire production themselves, with their board partners to follow shortly thereafter. ATI expects the first CrossFire boards will be available in July, although as we stated earlier final pricing hasn’t been determined.
This is where NVIDIA’s G70 throws a wrench in ATI’s plans. If NVIDIA is able to launch G70 on time and it delivers the performance many expect it to, G70 could make life tough for CrossFire. After all, to start from scratch with CrossFire you’d have to purchase a new $200 motherboard, and as it stands right now a $300-$550 CrossFire card, in addition to a second RADEON card. That currently equates to over $1000 for a high-end setup, and about $800 for an X800 XL config. This is quite a bit more than NVIDIA is likely to charge for G70.
If G70 is as fast as many expect it to be, many enthusiasts may just pick up a G70. With ATI’s R520 launch rumored to be delayed until this Fall, ATI will have to quickly adjust pricing on their RADEON and CrossFire cards to hold them over until R520 is ready.
Right now it’s just a waiting game for both G70 and CrossFire though, so for right now, this is all just speculation. But it certainly looks like ATI has designed CrossFire to be more flexible than SLI, and that will likely win them points with many existing ATI card users who already own high-end ATI cards.
ATI stresses that they could also integrate CrossFire into their X700 series and even lower-end boards such as the X300, but they don’t feel these solutions make as much sense from a performance perspective. After all, by the time you’ve purchased a new Crossfire motherboard and two X700 cards, you could have bought a single RADEON X800 XL card that would outperform it.