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2018. szeptember 22., szombat

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Behind the scenes with ATI - Eric Demers interview

We are proud to present our latest interview with Eric Demers, one of the top hardware engineers from ATI. Learn the innards of R520, the Ring Bus, CrossFire and more.

R5xx

Eric Demers, 3D Architect and Hardware Design Manager for ATI was kind enough to answer some of our questions regarding some innards of the R520 architecture. Fasten your seat belts, because this isn't the ordinary lot-of-talk but not-to-much-info type of interview!


Eric Demers - source: www.driverheaven.com

PH!: Why do the top ATI GPU codenames always end with “20” instead of “00”, as in R420, R520? Is the “00” ending reserved fore something special?

ED: You know, I've been confused by our part numbers and have ceased to really try to understand the numbering scheme. Yes, the first number indicates architecture, but even that can be partially wrong. It's a number that we sometimes try to make mean something (i.e. engineering wise), sometimes meant to mean something else (i.e. marketing wise). I would not attach too much to those numbers. Even within engineer ing, we use codenames, since numbers change and aren't always meaningful. And yes, sometimes they will end in '20' and sometimes in '00' but it's more random than most things – I don't remember why we picked R520, for example. Perhaps it's done to confuse the enemy J We need cheat sheets to remember them all :)

PH!: We suspect that the architecture first shown with the R520 has lots of reserves. The former “big one”, the R300 has doubled in pipeline count and clock speed throughout its lifespan. We assume the R520 will do similar… will we ever see a 32 pipeline (or rather say, 8 quads) R520 running around 1 GHz or will the unified shader architecture wash it away beforehand? Does it make sense to speak about unified shaders at all when we have Ultra-Threading?

ED: Well, I won't comment on unannounced products, but there's a lot invested into a new generation. About 110 man years for the R5xx generation. So, trying to maximize the number of parts we can get from it is important, to justify all the investment. The R5xx series was designed to be more flexible than previous architectures, since the metrics of yesterday have become less meaningful. 2 years ago, it mattered more how many “pipelines” you have, perhaps with some notion of the number of Z's or textures per pipe, but the basic metric was that. Today, we have moved away from that paradigm. Today, applications don't use fix function pipelines anymore, but create powerful shader programs to execute on the HW. It's not “how many pixels can you pop out per second?”, it's “what is the throughput of your shader”?. Our R5xx architecture has moved away from simply scaling of pipelines, to now scaling in terms of ALU operations, texture operations, flow control, Z operations as well as more traditional raster operations (all of this bathing in a design that can maximize the work done by each part). So will there be a 1 GHz 32 pipeline R5xx part? Well, we've ceased to measure things that way, so it won't be so easy to describe. But, yes, we will have more parts from this generation :)

PH!: What top clock speeds are you expecting for the X1800 XT with air cooling and with extreme cooling? How far can the X1800 go without a die shrink?

ED: Well, with air cooling in the 90GT, we've seen core graphics clocks hit up to 800 MHz. With more extreme cooling, we've surpassed 1 GHz!! So there's a lot of headroom in these chips. On the memory side, I think we've gone well above 800 MHz, but the X1800XT now ships with 800 MHz memory, so, often, the limiting factor ends up being the dram speed. Board design also contributes to memory clock limitation. A few changes on the current board with faster memory could do significant improvements to memory. Perhaps a future product J


Radeon X1800 XT extreme cooled - source: www.muropaketti.com

But one thing to note is that our memory architecture was designed for GDDR4 up to 1.5GHz in speed, so it's got lots of headroom. I could see MCLK even hitting 1.6GHz with more aggressive cooling – Assuming the memory is there and the board is designed to work at such speeds!

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