ATI Radeon HD 2900 XT: Calling a Spade a Spade
by Derek Wilson on May 14, 2007 12:04 PM EST- Posted in
- GPUs
Texturing, Caches, and Memory
Texturing
R600 features less texture hardware than we would expect to see, though AMD stands by the argument that compute power will come out on top when it matters. At the same time, we can't compute anything if we don't have any data to work with. So let's take a look at what AMD has done with their texture units.
There are four texture units in R600, one for each SIMD unit. These units don't share resources with the hardware in the SIMD units and are independently scheduled by AMD's dispatch processor. The dispatch processor is able to determine what data will be needed for threads about to execute and can handle setting up the texture units without waiting for the SIMD unit to request data and come up empty.
Texture units on the R600 are able to make both filtered and unfiltered texture requests no matter what shader is running. Unfiltered textures are useful with non-image-based texture data like vertex textures, normal maps, and generic blocks of data. Filtered requests will generally be for image data to be used in determining the color of a pixel. R600 can address one unfiltered texture per clock per texture unit and one filtered textures per clock per texture unit. Filtered units can be used to request unfiltered textures if necessary, providing an extra four unfiltered textures in place of one filtered texture.
The unfiltered texture requests will come back through four fp32 texture samplers (one per component), while the filtered requests will return 16 data points which will be run through the texture filtering hardware resulting in four filtered texture samples. The hardware can at best produce 32 single component fp16 unfiltered results per texture unit per clock. More practically, each texture unit can produce four bilinear filtered four component fp16 samples per clock alongside four unfiltered results. For textures with fp32 components, two clocks would be required to complete a bilinear filter process, as only half the data is loaded at a time to conserve bandwidth.
This is definitely a step up for R600, as R5xx hardware doesn't have texture filtering hardware for floating point textures. All told, with each of its four texture units working, R600 can consume up to 32 unfiltered textures or 16 unfiltered textures plus 16 filtered textures (as long as they're fp16 or fewer bits and we're only using bilinear filtering).
G80 is built with four texture address units and eight texture filters per block of 16 SPs. In total, this means NVIDIA's hardware can produce 32 filtered texture samples per clock (again these are fp16 and bilinear filtered). Of course, NVIDIA is operating on twice as many threads per clock, so it is conceivable that they would benefit more from having the extra filtered data.
We will have to wait and see if AMD's approach of providing unfiltered and filtered texture access in parallel pays off. For the general case on pixel shaders, we would want to see more filtered textures per clock, but with vertex and geometry shaders coming into the mix this could be a good way to save hardware space while offering more texturing power. On a final texturing note, AMD implemented "percentage closer" filter hardware for depth stencil textures. This will allow developers to implement fast soft shadows. The details of the implementation weren't indicated though.
Facebook
Twitter
RSS
86 Comments
View All Comments
yyrkoon - Tuesday, May 15, 2007 - link
See, the problem here is: guys like you are so bent on saving that little bit of money, by buying a lesser brand name, that you do not even take the time to research your hardware. USe newegg , and read the user reviews, and if that is not enough for you, go to the countless other resources all over the internet.yyrkoon - Tuesday, May 15, 2007 - link
Blame the crappy OEM you bought the card from, not nVIdia. Get an EVGA card, and embrace a completely different aspect on video card life.MSI may make some decent motherboards, but their other components have serious issues.
LoneWolf15 - Thursday, May 17, 2007 - link
Um, since 95% of nvidia-GPU cards on the market are the reference design, I'd say your argument here is shaky at best. EVGA and MSI both use the reference design, and it's even possible that cards with the same GPU came off the same production line at the same plant.DerekWilson - Thursday, May 17, 2007 - link
it is true that the majority of parts are based on reference designs, but that doesn't mean they all come from the same place. I'm sure some of them do, but to say that all of these guys just buy completed boards and put their name on them all the time is selling them a little short.at the same time, the whole argument of which manufacturer builds the better board on a board component level isn't something we can really answer.
what we would suggest is that its better to buy from OEMs who have good customer service and long extensive warranties. this way, even if things do go wrong, there is some recourse for customers who get bad boards or have bad experiences with drivers and software.
cmdrdredd - Monday, May 14, 2007 - link
you're wrong. 99% of people buying these high end cards are gaming. Those gamers demand and deserve the best possible performance. If a card that uses MORE power and costs MORE (x2900xt vs 8800gts) and performs generally the same or slower what is the point? Fact is...ATI's high end is in fact slower than mid range offerings from Nvidia and consumes alot more power. Regardless of what you think, people are buying these based on performance benchmarks in 99% of all cases.AnnonymousCoward - Tuesday, May 15, 2007 - link
No, you're wrong. Did you overlook the emphasis he put on "NOT ALWAYS"?You said 99% use for gaming--so there's 1%. Out of the gamers, many really want LCD scaling to work, so that games aren't stretched horribly on widescreen monitors. Some gamers would also like TVout to work.
So he was right: faster is NOT ALWAYS better.
erwos - Monday, May 14, 2007 - link
It'd be nice to get the scoop on the video decode acceleration present on these boards, and how it stocks up to the (excellent) PureVideo HD found in the 8600 series.imaheadcase - Tuesday, May 15, 2007 - link
I agree! They need to do a whole article on video acceleration on a range of cards and show the pluses and cons of each card in respective areas. A lot of people like myself like to watch videos and game on cards, but like the option open to use the advanced video features.Turnip - Monday, May 14, 2007 - link
"We certainly hope we won't see a repeat of the R600 launch when Barcelona and Agena take on Core 2 Duo/Quad in a few months...."Why, that's exactly what I had been thinking :)
Phew! I made it through the whole thing though, I even read all of those awfully big words and everything! :)
Thanks guys, another top review :)
Kougar - Monday, May 14, 2007 - link
First, great article! I will be going back to reread the very indepth analysis of the hardware and features, something that keeps me a avid Anandtech reader. :)Since it was mentioned that overclocking will be included in a future article, I would like to suggest that if possible watercooling be factored into it. So far one review site has already done a watercooled test with a low-end watercooling setup, and without mods acheived 930MHz on the Core, which indirectly means 930MHz shaders if I understand the hardware.
I'm sure I am not the only reader extremely interested to see if all R600 needs is a ~900-950MHz overclock to offer some solid GTX level performance... or if it would even help at all. Again thanks for the consideration, and the great article! Now off to find some Folding@Home numbers...