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The RV670 GPU — continued


A block diagram of the RV670 GPU. Source: AMD.

The RV670's 3D graphics portions aren't totally cloned from the R600. AMD has added some new capabilities and has even convinced Microsoft to update DirectX 10 in order to expose them to programmers. The upcoming DirectX 10.1 and Shader Model 4.1 add support for some RV670-specific features and for some capabilities of the R600 that weren't available in DX10. These enhancements include, most notably, cube map arrays that allow multiple cube maps to be read and written in a single rendering pass, a capability AMD touts as essential for speeding up global illumination algorithms. DX10.1 also exposes much more direct control over GPU antialiasing capabilities to application developers, allowing them to create the sort of custom filters AMD provides with its Radeon HD drivers. Microsoft and AMD have worked to tighten up some requirements for mathematical precision in various stages of the rendering pipeline in DX10.1, as well, in addition to various other minor tweaks.

Because being DX10.1 compliant is an all-or-nothing affair, the RV670 is the world's only DX10.1-capable GPU, at least for the time being. I wouldn't get hung up on the differences between DX10 and 10.1 when selecting a video card, though. I'm pleased to see AMD moving the ball forward, especially on antialiasing, but these aren't major changes to the spec. Besides, only now are we starting to see differences in game support between DX9's Shader Models 2.0 and 3.0, and many of those differences are being skipped over in favor of moving wholesale to DX10. (I would be more optimistic about DX10.1 support becoming a boon for Radeon HD 3800-series owners at some point down the line were it not for the fact that nearly every major game I've installed in the past two months has come up with an Nvidia logo at startup. That says something about who's investing in the sort of developer-relations programs that win support for unique GPU features.)

Speaking of arcane capabilities changes, here's an interesting one for you: RV670 adds the ability to process double-precision floating-point datatypes. This sort of precision isn't typically needed for real-time graphics, but it can be very useful for non-graphics "stream computing" applications. AMD says the RV670 handles double-precision math at between a quarter and a half the speed of single-precision math, which isn't bad, considering the application. In fact, they've already announced the RV670-based FireStream 9170 card.

So the RV670 includes mojo for many markets. One of those markets is mobile computing, and this time around, AMD is pulling in a mobile-oriented feature to make its desktop chips more power-efficient. The marketing name for this particular mojo is "PowerPlay," which wraps up a number of power-saving measures under one banner. PowerPlay is to GPUs like Intel's SpeedStep is to CPUs. At the heart of the mechanism is a microcontroller that monitors the state of the GPU's command buffer in order to determine GPU utilization. With this info, the controller can direct the chip to enter one of several power states. At low utilization, the GPU remains in a relatively low-power state, without all of the 3D bits up and running. At high utilization, obviously, the GPU fires on all cylinders. The RV670 also has an intermediate state that AMD calls "light gaming" where some of portions of the graphics compute engine are active, while the rest are disabled in order to save power. PowerPlay can also scale core and memory clock speeds and voltages in response to load. These things are handled automatically by the chip, and niftily, AMD has included a GPU utilization readout in its driver control panel.


The Overdrive section of AMD's control panel now includes a GPU utilization readout

We will, of course, test the RV670's power consumption shortly.

The RV670 improves on the R600 in a couple of other areas. One of those is its support for HD video playback. AMD's UVD video decoder logic is fully present in the RV670, unlike the R600, so the RV670 can do most of the heavy lifting required for playback of high-definition video encoded with H.264 and VC-1 codecs. We've tested UVD's performance with Radeon HD 2400 and 2600 series cards and found that those cards couldn't scale video to resolutions beyond native 1080p (1920x1080). AMD claims the RV670 has sufficient bandwidth and shader power to scale movies up to 2560x1600 resolution, if needed.

To help make that possible, the RV670 includes support for HDCP over dual-link DVI connections and, like the R600, has a built-in digital audio controller it can use to pass sound over an HDMI connection. AMD offers a DVI-to-HDMI converter, as well.

The last bit of newness in the RV670 is the addition of PCI Express 2.0 connectivity. PCIe 2.0 effectively doubles the throughput of PCIe connections, with very little drama. PCIe 2.0 devices like the RV670 remain backward-compatible with older motherboards.

We've heard very little in the way of hype for PCIe 2.0, but AMD expects the faster interconnect to become quite useful when it enables "CrossFire X" via new video drivers slated for this coming January. When combined with the new RD790 chipset, RV670-based video cards will be able to run in two, three, or four-way configurations, and the four-way config would involve four expansion slots fed by eight PCIe 2.0 lanes each. In order to make such madness feasible, the RV670's CrossFire interconnect has been boosted to double the pixel rate per connector, so that only a single physical connector is needed for dual-card CrossFire configs. The second connector on each card could be used for some daisy-chaining action in three- and four-way setups.

AMD is betting in a big way on CrossFire, and plans to address some long-standing complaints with the tech in upcoming drivers. One planned change is the ability to support multiple displays "seamlessly," a capability one might have expected from the beginning out of multi-GPU acceleration. AMD's Overdrive utility now allows the overclocking of multiple GPUs, too. The biggest change, though, will be this one: rather than producing another high-end chip to replace the Radeon HD 2900 XT at the top of its lineup, AMD plans to introduce the Radeon HD 3870 X2 this winter, a dual-GPU-on-a-stick card reminiscent of the GeForce 7950 GX2. Given the challenges to multi-GPU performance scaling we've seen lately, even with only two GPUs, I'm not sure what to think of this new emphasis on CrossFire. The history of the 7950 GX2, after all, is not a happy one. Time will tell, I suppose.