AMD Radeon HD 6870 & 6850


AMD just launched its third-generation 40nm GPU, but there is very little new in terms of architecture compared to the 5000 Series. To add to the confusion, the new cards are actually closer in performance to the Radeon HD 5830 and 5770 than the Cypress cards with which these cards share a suffix. You read right: The Radeon HD 5850 and 5870 will outperform the wet-behind-the-ears Radeon HD 6870 and 6850, respectively. Read on to find out how AMD managed to crater the price of performance graphics cards
without a die shrink or new architecture. The GPU inside the 6870 and 6850 consists of 1.7 billion transistors, which is close to half a billion fewer than the 5870 GPU. Die size has also slimmed from
Cypress’ 334mm2 to 255mm2, which enables AMD to crank out more 6870 GPUs per wafer. AMD’s secret to maintaining near 5870 performance with 25% fewer ICs is core clock. The Radeon HD 6870 and 6850 have 900MHz and 775MHz cores, respectively. Both GPUs have a 256-bit memory bus, but AMD set
the clocks at 1,050MHz for the 6870 and 1,000MHz for the 6850. Load/idle board power for the 6870 and 6850 are 151W/ 19W and 127W/19W, respectively.Architecturally, AMD’s engineers effectively took 25% of the horses dedicated to Cypress’ compute/shader/texture performance and hitched them instead to a rasterization/tessellation/ROP wagon, yieldingbetter real-world gaming performance. The SIMD engines found in the flagship Radeon HD 5870 return practically unchanged in the Barts XT and Pro chips,which have 14 and 12 SIMD engines respectively, for 1,120 and 960 stream processors each. The only real variation,however, is that each of the new SPs is no longer capable of FP64 calculations, which never come into play with 3D workloads, so gamers won’t notice the omission.
The more significant changes came to the tessellation unit, which is now capable of roughly twice the performance of the Cypress-based cards. The graphics engine also has a second rasterizer, which keeps the GPU’s horses fed and watered. These cards also support AMD’s new Morphological Anti-Aliasing mode, which performs fullscene anti-aliasing with less frame rate penalty compared to super-sampling.
We tested the Radeon HD 6870 and 6850 against Evga’s GeForce GTX 460 SuperClocked Edition, which got a $20 price cut on the eve of AMD’s launch. In the DirectX 9 Left 4 Dead 2, AMD’s new cards surpassed the mildly overclocked GTX 460 by more than 20fps (for the 6870) and 9fps (for the 6850).

The DX11 performance was significantly closer, with the 460 besting the 6850 inS.T.A.L.K.E.R.: Call
of Pripyat. The more tessellation-intensive Aliens vs. Predator demo seems to favor  AMD’s hardware, which speaks directly to theadvancements made in the Barts GPU.

AMD has a winner on its hands, but Nvidia’s price-slashed GTX 460 and 470 are more attractive than ever. The price/ performance battle is close, but AMD gets the nod here. But even if AMD’s parade
was moistened by Nvidia’s price cut, gamers in the market for a new graphics card come out the ultimate winners.

by Andrew Leibman


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