Product: AMD Radeon HD 6870 & HD 6850 1GB Video Cards
Company: AMD
Authour: James Prior
Editor: Charles Oliver
Date: October 21st, 2010
Barts Architecture

AMD
AMD
The HD 6000 series is codenamed 'Northern Islands' and all the parts are named after islands in the northern hemisphere. The Caribbean island of Saint Barthélemy is used as 'Barts', and is being launched in two flavors, 'Pro' and 'XT', a tip o' the hat to the Radeons of yore that used 'XT' as the top performance designator and 'Pro' as the mainstream level. These internal GPU codenames translate to the product names used for the AMD Radeon HD 6800 series - 6850 is Barts Pro and 6870 is Barts XT.


The Barts core consists of 1.7Bn transistors and measures approximately 255mm2, using a combination of process engineering enhancements and design tweaking to get higher clock speeds and lower power than the HD 5800 series. The reduced transistor count is because of the reduced SIMD count compared to the Cypress Pro ASIC, and is the key to reducing die size, power consumption and heat.

Northern Islands - Barts
Northern Islands - Barts

One of the rumors circulating was there would be a change in Thread Processor design by changing the arrangement of the Stream Processors inside all the Northern Islands GPUs. This is not true, and shows in the architecture of the Barts ASIC - it keeps the VLIW5, or Vec5, design of RV770 and Evergreen, with four stream cores and one special function stream core, plus a branch unit and general purpose register. The dual rasterizer design of the Cypress single graphics engine is retained, but the tessellation stage is improved, now in 7th generation iteration. While the diagram indicates a change in dispatch processors, this is the same as Cypress but more accurately depicted.

VLIW5 Thread Processor
VLIW5 Thread Processor

Thread processors are still grouped in sets of sixteen with a 32Kb local data cache to make a SIMD Engine unit, with four texture units (TU) attached with 8Kb L1 texture cache. While this leaves a lower total TU count for Barts XT than Cypress Pro, the Barts XT core clocks are higher, giving arithmetic logic unit (ALU) and Texture capabilities on par with Cypress Pro. All the SIMD's share a 64Kb global data share. Bart's has a 256-bit memory interface, using four 64-bit memory controllers. Each memory controller has 128KB L2 cache, 16 Z/Stencil ROP's with Z/Stencil cache, and 4 color ROP's with color cache, all unchanged from Cypress. This gives a total of 128 Z/Stencil and 32 Color ROP's, like Cypress, however Barts lacks double precision floating point capability.

Cypress Pro Vs. Barts XT
Cypress Pro Vs. Barts XT

So far what we've seen is tweaked Evergreen, but there's more to the puzzle - once again, AMD has looked at image quality and worked to improve it. A refined algorithm for anisotropic filtering addresses problems with very 'noisy' textures. Contrary to popular belief, the AMD anisotropic filter doesn't under-sample, but actually oversamples. Problems with mipmaps transitions when very 'noisy' textures are implemented can be seen in the HD 5000 series where the filtering isn't quite keeping up with the changing kernels. The new algorithm addresses some of those concerns, and angle independent filtering is maintained, to further increase visual fidelity.

The abandonment of the 32nm process doesn't seem to have harmed AMD's refresh too much, providing them with an opportunity to do 'Cypress done right'.

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