Friday, 9 July 2010

Tech Report: A Look At The EDRAM On Valhalla

We brought you our inside the 360 slim feature just over a week ago, revealing the internal layout of the console, and the all important details surrounding the chip and die shrinks contained within. However, one piece of the puzzle was missing – both the size and exact whereabouts of the EDRAM in the CPU/GPU package. This is something which is of particular interest seeing as this piece of silicon has always been behind in terms of process node changes, and has yet to be intergrated into the same die as the GPU.

This still seems to be the case - as you will see in the image of the console’s motherboard below – that the EDRAM is a separate entity from both the CPU/GPU combo, which now appears to be housed on a single die.

So, both the GPU and CPU are firmly on a single die, and the EDRAM is once again separate, all of which is housed on a single package, codenamed Valhalla. The CPU/GPU is now on a 45nm process node, although the EDRAM looks to be noticeably larger. The overall die size is some 34% smaller than the Jasper chipset’s CPU and GPU combined, inc EDRAM. And is 53% smaller than the ones used in the first 360 chipsets. For reference the Jasper chipset featured both the CPU and GPU at 65nm, whilst the original 360 featured 90nm versions of both chips.

From the above image it doesn’t look like the EDRAM has been fabbed at 45nm – it’s far too large in size for that. Instead all signs point to it being around 55nm or 65nm as speculated in our original article from a couple of months back. Interestingly, this is opposite to what the latest pieces of scattered information were saying in our later article – in which a 45nm EDRAM was hotly expected to be included.

Why the difference in die size compared to the other chips then? Well, the EDRAM itself has always been behind on process reductions, with complications in shrinking the chip being the main reason behind such slow comparative progress being made. Cost is also another factor. It could be that it is simply too expensive to economically reduce the EDRAM in size whilst obtaining optimal yields during production (a certain number of chips produced are unusable). Plus, the slower progress of reliable die shrinks to the EDRAM could have also held back development of the Valhalla CPU/GPU combination, with the complex integration of the EDRAM at a different size posing unnecessary problems and expenditure.

Integrating all three chips (CPU, GPU and EDRAM) would take the cooperation of ATI, NEC and IBM in order to make it happen given the increased complexity required for such a design. The cost of which would have been larger than simply producing three separate chips, or in this case two on one package. Another issue is getting the design and final production grade silicon ready on time, and given the increased complexity it is unlikely that this was ever going to happen. Effectively, all things considered, the cost/benefit ratio to having not only die shrinks, but also complete integration of all three chips on one die was perhaps too poor for consideration.

That’s not to say that later on down the line the EDRAM won’t be included in another internal revision of the 360, because it is very likely that Microsoft are working to do exactly that at some point in the future. It is possible for this to happen now, just not quite being the cost saving measure they are after at this point in time. Maybe with the next, and possibly last revision to the hardware, we will find that all three chips will have been completely integrated into a single die, rather than a single package. At that point, we expect that the EDRAM along with the now combined CPU/GPU will all be produced on the same process node.

With the current Xbox 360 S (slim) we have here what is clearly the best price/performance ratio available at this point in time with regards to heat reduction, and overall saving in silicon. Whatever changes we (and many others) might speculate, there is no doubt that Microsoft have done their very best in producing the most efficient, and cost effective version of the 360 to date. It is also the quietest and most solidly built too, which is not something you can really say about the console before the Slim’s arrival.

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