A new memory controller
Along with the rework pixel shader engine, ATI has designed a brand-new memory controller for the R520. Past ATI graphics chips used a centralized memory controller located in the center of the chip, and this placement caused problems. Lots of wires had to come into the center of the chip, and high wire density caused hotspots. The R520's new memory controller adopts a more distributed form of organization: a ring topology.
Communication is handled by means of a pair of 256-bit rings running in counter directions around the periphery of the chip. That makes for 512 bits of internal bandwidth, but external bandwidth to memory remains 256 bits, as in ATI's past high-end GPUs. ATI expects this new design to be capable of much higher memory clock speeds. The new memory controller also offers more access granularity by dividing its external memory interface up into eight 32-bit channels rather than four 64-bit channels. This memory controller's "smart" arbitration logic is programmable, and ATI expects to tune in on a per application basis in the future using the Catalyst A.I. facility of its graphics drivers.
The R520 is tweaked in a number of other ways to help improve memory performance, as well. The texture, color, Z, and stencil caches are now fully associative, making them more effective than in the past. The chip's Z compression has been revamped to achieve higher compression and maintain it longer. And the GPU's hidden surface removal technique for its hierarchical Z buffer has now uses floating-point math for more precise operation.
A grab-bag of other enhancements
There's much more to R520 and its derivatives, of course, but time is running short, so I'm going to bust out the machine gun and spray you with bullet points to cover the rest of the highlights. Many of them are quick but notable. Among them:
- HDR support with AA R520 and the gang can do filtering and blending of 16-bit per color channel floating-point texture formats, allowing for easier use of high-dynamic-range lighting effectsjust like the GeForce 6 and 7 series. Unlike NVIDIA's GPUs, the R500 series can also do multisampled antialiasing at the same time, complete with gamma-correct blends. ATI's new chips also support a 10:10:10:2 integer buffer format that offers increased color precision at the expense of alpha transparency precision. This is a smaller 32-bit format, but it should still be suitable for high-dynamic-range rendering.
Interestingly enough, ATI claims the Avivo display engine in the R500 series can do tone mapping, converting HDR images to display color depths with no performance penalty. Unfortunately, DirectX 9 doesn't currently offer a way to expose this capability to developers, and it's not in the drivers yet, but apparently the hardware can do it.
- Better edge and texture antialiasing ATI's new "Adaptive AA" feature mirrors NVIDIA's supersampled transparency AA, taking care of cases where alpha transparency creates jaggies that aren't on polygon edges. This one is enabled via a simple checkbox in the driver.
Another driver checkbox addresses a long-standing complaint of mine: angle-dependent anisotropic filtering. The R500 series includes a new, higher quality anisotropic filtering method that's not angle dependent. Most newer GPUs haven't included the ability to turn off angle-dependent aniso, and I'm pleased to see that ATI has made it happen.
- Avivo video and display engine Avivo is a branding concept that encompasses the whole of the "video pipeline," from the capture stage using ATI's Theater products to the decoding, playback, and display output stages that happen on a GPU. Avivo still uses pixel shaders rather than a dedicated video processing unit, but ATI is bringing over algorithms and capabilities from its Xilleon line of consumer electronics products in order to improve the Radeon's video and display features. Avivo features end-to-end 10-bit integer color precsion per channel for both analog and digital displays, and ATI proudly declares that Avivo will have the best H.264 acceleration around. We will have to cover Avivo in more depth in a future face-off against NVIDIA's PureVideo.