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Ring-Bus Memory Controller Officially Dead

As for the memory controller, AMD’s graphics department has finally given up the ring-bus topology in the RV870.

Instead, they tried to achieve higher memory frequencies by introducing Error Detection Codes (EDC) and temperature compensation of the frequency of GDDR5. As a result, the memory is stable at frequencies up to 1250 (5000) MHz. This approach has helped increase the memory subsystem performance while keeping the PCB design simple, which has a positive effect on the manufacturing cost of the Radeon HD 5800.

ATI claims that the crossbar memory controller they use again will work just as efficient as the ring-bus one that was employed in the last three generations and was first introduced in ATI R520/Radeon X1800. Keeping in mind that the developer has practically doubled (or maybe increases even more) the size of all on-chip caches and used higher memory clock speed, we shouldn’t expect the RV870 efficiency to be any lower because of the memory controller topology.

Nevertheless, despite the doubling of the peak computational power together with doubling of the rasterization and texturing speeds, the memory bandwidth got only 30% higher. Therefore, we are a little concerned that the graphics processor won’t show its maximum performance in “extreme” conditions.

RBEs, Tessellation and Other Features

The amount of rasterization processors (AMD/ATI calls them render back-ends) has also been increased twofold, from 16 to 32. The performance of the rasterization subsystem has grown up accordingly:

The RBE architecture has been improved, too, but we don’t have any reliable info on this point. We only know that the GPU’s performance at full-screen antialiasing has been increased and it is now more efficient working with the Z-buffer/stencil buffer. The RV870’s RBEs support both classic antialiasing modes and programmable CFAA modes among which the Edge Detect mode is the most interesting. It delivers an ideal quality of antialiasing without any fuzziness typical of the CFAA Wide/Narrow Tent modes.

Before the arrival of the eighth generation of ATI Radeon GPUs, the hardware tessellation unit was perhaps the most questionable spot in the entire ATI Radeon HD series. Notwithstanding its support for both vertex and geometric shaders, it was never popular among game developers. The RV870 has got wider tessellation capabilities within the DirectX 11 framework. Two new shader types are support: hull and domain shaders. Tessellation itself can be performed in different ways including N patches, Bezier patches, displacement maps, etc.

There are some other interesting DirectX 11 features, like true multithreading, Order Independent Transparency which improves the quality of overlapping transparent objects by using DirectCompute 11, and the improved post-effects such as depth of field, etc. Besides, Nvidia is no more the exclusive provider of Ambient Occlusion technology. The new-generation Radeon HD supports High Definition Ambient Occlusion with improved shadowing quality.

As opposed to DirectX 10.1, DirectX 11 is going to have a warm welcome from game developers. A few projects will get support for the new API in Q4 of this year, with more such projects to come in 2010. This should give some thinking matter to Nvidia, the main and only opponent to AMD on the 3D graphics market.

Video Controller and PowerPlay

The display controller has not been revised for the RV870. Instead, it has been developed from scratch.

The new core offers very flexible display connection opportunities. Theoretically, it can output up to six video streams with 10-bit-per-component precision. Besides delivering them as independent content, the GPU can output a single picture at a huge resolution up to 5760x2160. It is supposed that you can actually play a game at that resolution, but we doubt the Radeon HD 5870, for all its impressive hardware resources, will be able to deliver a playable frame rate in such an extreme display mode. And we guess even building a CrossFireX subsystem out of four Radeon HD 5870 cards won’t help achieve a playable speed in Crysis Warhead at 5760x2160. Anyway, you will have to wait for a special version of the card, called Radeon HD 5870 Eyefinity6 Edition, that is equipped with six DisplayPort connectors in order to check this out in practice. The ordinary Radeon HD 5870 supports up to three monitors simultaneously which might be useful in some games like flight or racing simulators.

AMD’s PowerPlay technology has been improved, too. The peak power draw is declared to be only 188 watts. The new cards’ power consumption in idle mode is a mere 27W. The numbers look fantastic considering the tremendous complexity of the RV870 chip and need to be checked out like any other marketing claim. So, let’s get to practical tests now. First, let’s take a closer look at the graphics card itself.

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