AMD PowerTune is a new technology that opens up a new direction for maximum performance versus TDP. Pretty much AMD can now limit the maximum TDP applied to a card.
AMD can regulate the TDP with the help of active monitoring. Basically you can lower TDP, have it at normal, or increase the thermal headroom of the graphics cards though the Catalyst drivers. The new feature allows the GPU to be designed with higher engine clock speeds which can be applied on the broad set of applications that have thermal headroom.
So at default it will try and keep your power consumption and TDP at a pre-defined baseline, say 200 Watt. But you'll also have a margin to increase that TDP to say +20%, or vice versa, when you want to limit your power draw you can lower the power usage by -20% (or anything in-between).
AMD PowerTune can enable higher performance that is optimized to the thermal limits of the GPU by dynamically adjusting the engine clock during runtime based on an internally calculated GPU power assessment.
AMD PowerTune technology also deals with applications that would otherwise exceed the GPUs TDP like OCCT, Furmark or 3DMark's perlin noise tests. It does so by dynamically managing the engine clock speeds based on calculations which determine the proximity of the GPU to its TDP limit.
AMD PowerTune allows for the GPU to run within its TDP budget at higher nominal clock speeds than otherwise possible.
So a monitoring function on the graphics card can be used to downclock your card when needed. And the flipside of the coin is that it can be directly adjusted by the user using the AMD Catalyst Control Center and used for tweaking and overclocking as you can allow for a more aggressive power containment (and therefore more aggressively limit power and heat) or be used by enthusiasts to relax the enforcement of factory thermal constraints on their AMD Radeon HD 6900 Series Radeon graphics card and squeeze every last bit of performance - at the cost of a much higher TDP of course.
Mind you that if you use ATI Overdrive for tweaking the R6990, you'll need to tweak and apply the setting for each GPU independently.
What's That New Switch?
If you look at the photo below you'll notice a tiny micro-switch next to the Crossfire connector. The R6990 cards have one firmware flashable BIOS and one (non flashable) default BIOS, with the switch you can select BIOS 1 or 2.
AMD initially implemented the feature likely to prevent the RMA rate. They know very well that the enthusiast community often re-flashes their cards, often unsuccessfully after which they enter a very expensive RMA procedure at AMD's cost.
With the R6990 AMD decided to expand that function a little as you'll get control over clock-frequencies and TDP. So the dual-BIOS feature now became an Unlocking Switch. The switch toggles between the factory-supported Performance BIOS of 375W TDP (830 MHz - BIOS1), and an Extreme Performance BIOS (880 MHz - BIOS2) which unlocks higher clock speeds and up to 450W TDP of performance. it also applies a slightly higher voltage.
Position 1 450W Extreme Performance BIOS (BIOS2).
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