Inside the SoC The Xbox One SoC appears to be implemented like an enormous variant of the Llano/Piledriver architecture we described for the PS4. One of our theories was that the chip would use the same “Onion” and “Garlic” buses. That appears to be exactly what Microsoft did. The interesting ESRAM cache First, there’s the fact that while we’ve been calling this a 32MB ESRAM cache, Microsoft is representing it as a series of four 8MB caches. Bandwidth to this cache is apparently 260GB/s. The math on this is… odd. It’s not clear if the ESRAM cache is actually a group of 4x8MB caches that can be split into chunks for different purposes or how its purposed. The implication is that the cache is a total of 1024 bits wide, running at the GPU’s clock speed of ~850MHz for 209GB/s in uni-directional mode. The other major mystery of the ESRAM cache is the single arrow running from the CPU cache linkage down to the GPU-ESRAM bus. It implies that there’s a way for the CPU to snoop the contents of ESRAM, but there’s no mention of why that capability isn’t already provided for on the Onion/Garlic buses and it’s not clear why they’d represent this option with a tiny black arrow rather than a fat bandwidth pipe. Even with this new information, the use and capabilities of the ESRAM remain mysterious. It’s not clear what Microsoft expects it to be used for — if it’s for caching GPU data, why break it into 8MB chunks, and why does the CPU have a connection to it?
Posted on: Sat, 19 Oct 2013 20:47:05 +0000
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