X-bit labs

First in DirectX 10.1: ATI Radeon HD 3850/3870 (page 3)

Category: Video

by Alexey Stepin , Yaroslav Lyssenko, Anton Shilov

[ 01/25/2008 | 10:31 AM ]

Architecture

Now we’ll discuss the specifications of the new Radeon and of the new graphics cards based on it.

Click to enlarge

Click to enlarge

As the table shows, AMD has gone further than Nvidia in exploring new tech processes: the new chip is the world’s first 55nm GPU, which helped achieve a frequency of 700MHz, at least with the senior model, Radeon HD 3800. The developer also reduced the complexity of the chip, probably by optimizing the internal architecture and simplifying the topology of the ring-bus memory controller because the RV670 has the same amount of functional subunits as the R600. This is the first case for a long time when a new GPU from ATI incorporates fewer transistors than a same-class GPU from Nvidia. ATI wins technical superiority again but how do the new products compare with the GeForce 8800 GT?

The ATI Radeon HD 3800 exceeds the Nvidia GeForce 8800 GT in core frequency but has fewer functional subunits, particularly TMUs. As we know, the advantage in the number of execution subunits is not very important, not only because the ALUs work at a higher frequency in Nvidia’s chips: the VLIW architecture implemented in the Radeon HD requires driver optimizations for effective paralleling of shader code. Otherwise, if not all of the ALUs are loaded with work, the efficiency of the Radeon HD’s superscalar shader processors plummets suddenly. In the worst case, only one out of five ALUs that are part of each of the 64 such processors is really working. Ironically, AMD/ATI has the most problems with software optimization as it doesn’t have access to the code of games developed within the framework of Nvidia’s The Way It’s Meant to Be Played program until their official release. This often has a negative effect on the performance of AMD/ATI’s graphics solutions in games.

If you view the flowchart of the RV670 only, you won’t find any difference from the R600. The difference can be found on a deeper level, namely support for DirectX 10.1 (Shader Model 4.1) and a full-featured hardware video-processor UVD similar to the one in the ATI Radeon HD 2600. The features and operation of the UVD were discussed in our review titled The Look of Perfect: Analysis of Graphics Accelerators Performance during Media Playback. The DirectX 10.1 support available in the new chip deserves a separate section in this article.