ATi Radeon256 Preview

Hyper Z

Hyper Z is something that ATi created to alleviate the growing memory bandwidth issue as resolutions rise and color depths increase the bandwidth needed. The Hyper Z is a special Z-buffer cache that helps cut down on bandwidth and memory used when manipulating Z data. ATi is claiming to see somewhere around a 20% performance gain but the true test will be done once we have the board up and running on our test platform proving it.

Fill Rate

Fill-rate is something that we've discussed for a while now when comparing various cards "theoretical" abilities to each other but it hasn't always made sense in the real world. The architecture, clock speed and software situation will all effect how a given card will perform. Architectures can affect the fill-rate depending on how many pipelines they have, how many texturing unit per pipeline, and how efficiently they can do various filtering methods at the same time. If a given architecture needs more than one pass for a given multi-texturing situation then it is typically bad news on the performance side of things as it means wasting another pass to complete the task. The Radeon256 will take full advantage of these situations as both of its pipelines may do a triple textured and filtered pixel in one cycle. If we take a look at the pixel fill-rate, the Radeon256 might not appear to be very impressive.

First, we've given a range for the Radeon256, as the core speed wasn't final at the time we spoke with ATi. The numbers were based on a 200 MHz-core clock speed. You'll notice that the numbers it boasts aren't all that great actually but don't let this fool you. Let's take a look at the Texel Fill-Rate.

You can quickly see how the Radeon256 can quickly gain some ground as scenes become covered with tons of textures. The Radeon256 is just fine up until you have three textures per pixel but after that you're talking about needing to do an additional pass unlike the Viper II that can do a single textured pixel in a single pass (although only one pixel). There is much more to this as factors like total number of textures used per pixel, filtering modes and architecture efficiencies (like Hyper Z) alter real world performance. Once again, the only true proof can come from testing these capabilities by running various real world game applications.

Memory Bandwidth

Memory Bandwidth is yet another interesting topic to throw around because its such an expensive commodity these days. Higher fill-rates and leading edge features are just sucking away all available bandwidth and can kill performance. Companies are given a few choices to combat this issue. Increase memory bus width from the standard 128-bit to 256 or greater, increase memory speed (i.e. higher clock speeds or better memory - DDR) or provide features that are more memory efficient. ATi took the route of using fast DDR memory as well as implementing features like Hyper Z that help cache various data to decrease the amount of wasted rendering passes. This allows for more efficient use of the already available high-speed memory and essentially gives them a boost over the competition that uses the same type of memory.

We had to use a variable number once again for Memory Bandwidth as the DDR memory to be used was stated to be 166 MHz (effectively 333 MHz since it is DDR memory) and possibly up to 200 MHz later on. Factor in the various efficiencies that ATi has enhanced the Radeon256 with and we're talking about a very powerful card all of a sudden.