Radeon X1800 XT 512MB
$549 MSRP [Check US Prices]
Radeon X1800 XL 256MB
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R520: The Never Ending Story?

2005 has hardly been a banner year for ATI. Following on from the success of the R300 generation of products, and obviously moving fast from the removal of the initial R400 project from the roadmap, which was almost certainly partly due to the addition of XBOX 360's "Xenos" graphics processor to the roadmap, ATI moved into a series of products based on the R420 architecture. The architecture itself bore much similarity to R300's, with some changes to the shader capabilities, but with the high end products primarily leaning on the performance of the 130n low-k process and a whole bunch more parallelism.

With ATI's engineers suggesting to management that to really see the benefits of Shader Model 3.0 the number of transistors required to support their implementation the 90nm process node would be needed, R420's primary goal appears to have been to keep the architectural changes to a minimum in order to hit the transition to PCI Express. ATI's strategy was to present the market with a full range of PCI Express products to pick up a number of OEM positions and, in the meantime, they could work on a larger architectural transition for another year. Initially things appeared to have worked out as ATI had expected, with their PCI Express products capturing numerous OEM slots; however it started to come unstuck with the realisation that the adoption of PCI Express outside of the OEM's was nowhere near as fast (probably partially due to the AMD market being reliant virtually solely on third party chipset vendors, who aren't marching to Intel's beat) and with their RIALTO AGP bridge chip late to the market ATI were reliant on previous generation AGP products much longer than initially expected. The strategy was almost certainly also reliant on a timely introduction to R520, which would indeed use a 90nm process, in order to give them an architecture and feature boost; something else that clearly didn't go to plan.

R520's commercial availability was scheduled for introduction roughly one year after R420, i.e. late spring to summer 2005. Initially R520 was on target for this type of timescale, and just three days after the release of the Radeon X850 series, based on R480, the refresh to R420/R423, ATI received their first packaged R520 chips, ready for the bring-up process. Things appeared to be going as planned, with sightings of the product in operation at E3; however, following on from that nothing appeared, even with NVIDIA's release of their next generation G70 product ATI didn't announce any products. Soon stories of leaking issues appeared and eventually Dave Orton, ATI's CEO, announced that the scheduled release had indeed been pushed back due to "a variety of design issues".

According to public reports ATI noticed that as late as July, issues occurred that prevented the R520 core being clocked close to its target speeds, which is consistent with leakage issues. Curiously, the issue did not occur across all their 90nm products - ATI had already delivered Xenos to Microsoft using the same 90nm process R520 does, and other derivatives of the R520 line suffered the same issue (RV530) but others did not (RV515) - the fact R520 and RV530 share the same memory bus, while RV515 and Xenos have different memory busses is not likely to be coincidental in this case. ATI were open about talking about the issue they faced bringing up R520, sometimes describing the issue in such detail that only Electronic Engineers are likely to understand, however their primary issue when trying to track it down was that it wasn't a consistent failure - it was almost random in its appearance, causing boards to fail in different cases at different times, the only consistent element being that it occurs at high clockspeeds. Although, publicly, ATI representatives wouldn't lay blame on exactly were the issue existed, quietly some will point out that when the issue was eventually traced it had occurred not in any of ATI's logic cells, but instead in a piece of "off-the-shelf" third party IP whose 90nm library was not correct. Once the issue was actually traced, after nearly 6 months of attacking numerous points where they felt the problems could have occurred, it took them less than an hour to resolve in the design, requiring only a contact and metal change, and once back from the fab with the fix in place stable, yield-able clockspeeds jumped in the order of 160MHz.

Now, though, nearly a year after it first came back, ATI are ready to announce and introduce Radeon X1800, based on R520.

Pipeline Overview

The R520 architecture has been designed along the same principles as the R420 and R300 architectures preceding it and, although it it is another four quad (16 pipeline) design, there have been greater changes to the architecture than there certainly was from R300 to R420, not least because R520's design supports the Shader Model 3.0 (SM3.0) DirectX capabilities.

Click for a bigger version

R520 Pipeline Overview

From the overview diagram, the four rendering quad engines are fed by a unit ATI are calling the "Ultra Threaded Dispatch Processor" which is one of the important architectural changes introduced in R520. Before taking a closer look at the Radeon X1800 board we'll first take a more detailed look at the architecture and capabilities R520 brings with it.