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AMD's Radeon HD 6990 graphics card

Uber without the umlaut

Dual-GPU graphics cards have always been kind of strange. Technically, one of them is probably the fastest video card on the planet at any given time. For over a year now, for instance, the Radeon HD 5970 has held that title. Yet dual-GPU cards don't garner loads of attention, for a host of reasons: they're subject to the same performance and compatibility pitfalls as any multi-GPU configuration, they tend to have rather extreme power and cooling needs, and they're usually really freaking expensive, to name a few.

Nevertheless, these odd creations are here to stay. Much of the credit for that fact goes to AMD. The company has been carefully refining its dual-GPU products over the past few years, ever since it decided to stop producing as-big-as-you-can-make-them GPUs. Instead, recent flagship Radeons have been based on pairs of mid-sized chips.

The Radeon HD 5970 was odd in that it wasn't the absolute pinnacle of extremeness that one would expect out of this class of product. The card's sheer length and price tag were both plenty extreme, but its 725MHz default clock speed gave it performance closer to a pair of Radeon HD 5850s than to a pair of higher-end 5870s. The limiting factor there was power draw. AMD had to tune the card conservatively to ensure that it didn't exceed its 300W—its rated power draw and the max capacity provided by its 6- and 8-pin auxiliary power connectors—even in absolute peak cases. To skirt this limitation somewhat, AMD practically encouraged 5970 owners to venture into 400W territory by overclocking their cards, even going so far as to screen the chips to ensure they would reach clock speeds similar to the Radeon HD 5870's. It was innovation, of a sort, born of necessity.

Into Antilles

Now the 5970's successor has arrived in the form of a product code-named Antilles: the Radeon HD 6990, an all-new card based on a pair of the "Cayman" GPUs that power the Radeon HD 6970. Cayman is something of an incremental improvement over the Cypress chip that powered the Radeon HD 5970, so one might expect the 6990 to be an incremental step up from the 5970, as well. That's not quite the case, for a couple of reasons.

First, AMD has endowed the 6990 with a pair of 8-pin auxiliary power connectors and raised the card's max power rating to 375W. That gives the card quite a bit more headroom. Second, and more critically, AMD built a power-capping feature into the Cayman known as PowerTune that allows the GPU to monitor its own power draw and ramp back clock speeds if needed to stay within its prescribed power envelope. Although PowerTune doesn't often limit performance dramatically in typical gaming workloads, we've found that it will kick in when synthetic tests push the GPU past its normal bounds. That ability to prevent problems in worst-case scenarios has freed AMD to push for higher default clock speeds without fear of creating problems.

As a result of these and other changes, AMD has set the Radeon HD 6990's clock speed at 830MHz while leaving all of Cayman's execution units enabled. Each GPU on the card also has 2GB of GDDR5 memory clocked at 1250MHz, for an effective transfer rate of 5 GT/s. Those numbers put the 6990's theoretical peak performance right in between what one would expect from a couple of Radeon HD 6950s and a couple of Radeon HD 6970s—not too shabby, to say the least.

AMD apparently wasn't satisfied with that achievement, though. As you may know, all Radeon HD 6900-series cards have a dual-position switch on the top of the card near the CrossFire connector, ostensibly to enable one to switch to a recovery firmware in the event of a failed video BIOS flash attempt. On the 6990, however, moving that switch from its default position (2) to the other one (1) enables access to a hopped-up BIOS. AMD calls it the "Antilles Unlocking Switch for Uber Mode" or—yes, this is happening—AUSUM. Several things happen when your 6990 cards goes into the umlaut-impaired uber mode. The base GPU clock rises to 880MHz, same as the 6970, and the core GPU voltage rises from 1.12V to 1.175V. Also, the board's PowerTune limit is raised to 450W. You're essentially overclocking your card when you switch it into uber mode; AMD doesn't guarantee proper operation for everyone in every system. However, going AUSUM worked just fine with our 6990 sample on our Intel X58-based test system, much like the 5970 did for us at its "suggested" overclocked speed.

If that's not enough AUSUM-ness for you, AMD has given 6990 users more than enough leeway to get into real trouble. The Overdrive controls in the AMD control panel will allow GPU overclocks as high as 1200MHz, with memory overclocking as high as 1500MHz (or 6 GT/s).

  Peak pixel
fill rate
Peak bilinear
integer texel
filtering rate
Peak bilinear
FP16 texel
filtering rate
Peak shader
GeForce GTX 560 Ti 26.3 52.6 52.6 1263 1644 128
GeForce GTX 570 29.3 43.9 43.9 1405 2928 152
GeForce GTX 580 37.1 49.4 49.4 1581 3088 192
Radeon HD 6850 24.8 37.2 18.6 1488 775 128
Radeon HD 6870 28.8 50.4 25.2 2016 900 134
Radeon HD 6950 25.6 70.4 35.2 2253 1600 160
Radeon HD 6970 28.2 84.5 42.2 2703 1760 176
Radeon HD 5970 46.4 116.0 58.0 4640 1450 256
Radeon HD 6990 53.1 159.4 79.7 5100 3320 320
Radeon HD 6990 AUSUM 56.3 169.0 84.5 5407 3520 320

With or without the AUSUM switch enabled, the 6990's specifications are downright staggering. On paper, at least, it's far and away the fastest consumer graphics card ever. Of course, we're just adding up the capacities of its two individual GPUs in the table above and assuming the best—perfect scaling—will happen. That's not always how things work out in the real world, of course, but the 6990 has more than enough extra oomph to overcome less-than-ideal outcomes.