Of course, weвЂ™re just adding up the capacities of its two individual GPUs in the table above and assuming the best-perfect scaling-will happen. On paper, at least, itвЂ™s far and away the fastest consumer graphics card ever. With or without the AUSUM switch enabled, the 6990вЂ™s specifications are downright staggering. 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). If thatвЂ™s not enough AUSUM-ness for you, AMD has given 6990 users more than enough leeway to get into real trouble. 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 вЂњsuggestedвЂќ overclocked speed. YouвЂ™re essentially overclocking your card when you switch it into uber mode AMD doesnвЂ™t guarantee proper operation for everyone in every system. Also, the boardвЂ™s PowerTune limit is raised to 450W. The base GPU clock rises to 880MHz, same as the 6970, and the core GPU voltage rises from 1.12V to 1.175V. Several things happen when your 6990 cards goes into the umlaut-impaired uber mode. AMD calls it the вЂњAntilles Unlocking Switch for Uber ModeвЂќ or-yes, this is happening-AUSUM. On the 6990, however, moving that switch from its default position (2) to the other one (1) enables access to a hopped-up BIOS.

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. Those numbers put the 6990вЂ™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.ДЄMD apparently wasnвЂ™t satisfied with that achievement, though. Each GPU on the card also has 2GB of GDDR5 memory clocked at 1250MHz, for an effective transfer rate of 5 GT/s. That ability to prevent problems in worst-case scenarios has freed AMD to push for higher default clock speeds without fear of creating problems.ДЄs a result of these and other changes, AMD has set the Radeon HD 6990вЂ™s clock speed at 830MHz while leaving all of CaymanвЂ™s execution units enabled. Although PowerTune doesnвЂ™t often limit performance dramatically in typical gaming workloads, weвЂ™ve found that it will kick in when synthetic tests push the GPU past its normal bounds. 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. That gives the card quite a bit more headroom. It was innovation, of a sort, born of necessity.ДЇirst, AMD has endowed the 6990 with a pair of 8-pin auxiliary power connectors and raised the cardвЂ™s max power rating to 375W. 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 5870вЂ™s. AMD had to tune the card conservatively to ensure that it didnвЂ™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. The limiting factor there was power draw.

The cardвЂ™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 Radeon HD 5970 was odd in that it wasnвЂ™t the absolute pinnacle of extremeness that one would expect out of this class of product. Instead, recent flagship Radeons have been based on pairs of mid-sized chips. 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. Much of the credit for that fact goes to AMD.

Nevertheless, these odd creations are here to stay. Yet dual-GPU cards donвЂ™t garner loads of attention, for a host of reasons: theyвЂ™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 theyвЂ™re usually really freaking expensive, to name a few. For over a year now, for instance, the Radeon HD 5970 has held that title. Technically, one of them is probably the fastest video card on the planet at any given time. Dual-GPU graphics cards have always been kind of strange.

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