AMD Radeon HD 6990 Review - PAGE 2

The Cayman architecture uses a newly redesigned core architecture that has dual graphics engines, new computing features and a modified VLIW4 core architecture. The Cayman core now features 24 SIMD engines, which give it a maximum of 96 texture units. In addition, the render-ends have been upgraded for better Anti-Aliasing performance, and it has a 256-bit GDDR5 memory interface. This allows it to support memory speeds of up to 5.5Gbps.

One of the more dramatic changes to the Cayman architecture is the switch from the VLIW5 to VLIW4 design. Previous generation AMD graphics cards utilized a VLIW5 engine that had 4 simple and 1 complex processing units. This design worked well, but its utilization wasn't necessarily very efficient.  The new VLIW4 engine now features 4 units that have equal capabilities and a "T" unit which sees its functions distributed throughout. This improves the ALU and Raster utilization, which further increases the overall efficiency of the design over the previous generations' architecture. The new design allows more SIMD engines to be placed in the same space, which improves the overall efficiency per millimeter by about 10%.

The Cayman architecture uses a similar geometry engine to that of the previous generation graphics cards, but with the HD 6900 series AMD has effectively doubled it. This gives both the HD 6970 and HD 6950 dual graphics engines, allowing them to process two primitives per clock, giving it twice the transform, backface cull and tessellation sub-division rates. This improves the DX11 tessellation performance, which is highlighted in the next slide.

With the redesigned architecture of the Cayman GPU, the HD 6900 series has up to three times the tessellation performance in comparison to Cypress based graphics cards.

PowerTune is a new power management system that is being introducing with the AMD 6900 series, and will continue to be used throughout future AMD graphics products. In essence PowerTune is a means to set a predefined TDP by adjusting the clock speeds in real time. The way in which PowerTune is utilized is very different than the on-board regulation chips on NVIDIA’s 500 series. NVIDIA’s power management system monitors the power coming from the rails, while AMD’s technology instead relies on performance counters that are embedded throughout the GPU.

These performance counters have an internal algorithm that can dynamically calculate how much power is being used, and adjust according. This allows PowerTune to maintain the power draw at the predefined level, effectively eliminating huge surges in power from occurring. Since games operate at a lower peak power rate than benchmarking utilities such as Kombuster, in-game performance will not be negativity affected.

The main technology that allows AMD to create a dual GPU solution such as the HD 6990 is their PowerTune power management system. This allowed them to build a board running at up to 375W and not be limited by the power requirements of synthetic benchmarks such as Furmark or OCCT. Without the limitation of having to design a board for maximum synthetic benchmarks, AMD was able to optimize the clock speeds for games and not have to worry about not exceeding the maximum TDP. This increases the overall performance for the graphics card and allows it to use every bit of power available.

EQAA or Enhanced Quality Anti-Aliasing is new as of the 6900 series, and is an AA option that improves in-game image quality by effectively doubling the amount of coverage samples. According to AMD, the performance loss while it's enabled is minimal.

With the release of the Cayman architecture AMD has continued support for their new Morphological-AA, which is essentially implemented in the same manner as it was with the 6800 series. Morphological Anti-Aliasing is an AA method that uses a post process filtering technique with DirectCompute. This approach can deliver full screen Anti-Aliasing, faster super sampling and is compatible with DX9 through DX11 games. In addition, Morphological-AA can be used alongside MSAA to ensure the best image quality without losing any in-game detail.

Another feature is AMD's HD3D technology. Stereoscopic 3D support is not new to the market, and in fact Nvidia has employed 3D support in products for some time now. What is new however is AMD's approach to the technology. Where Nvidia uses a closed-sourced method to drive stereoscopic 3D, AMD is using an open-source method that allows many manufactures to create their own means of support. This means that there will not be a specific standard between compatible 3D displays, glasses and software, but rather multiple options available to the consumer. Still, this is in the early stages and it could be some time before AMD's HD3D technology is readily implemented.