Chipworks Confirms Xbox One SoC Has 14 CUs with 2 Disabled
by Anand Lal Shimpi on November 26, 2013 8:57 PM EST
In our mini-review of the Xbox One I speculated that the shipping version of Microsoft's console featured 14 AMD GCN CUs (Graphics Core Next Compute Units), with two disabled to improve yields. Microsoft publicly stated that Xbox One development kits featured 14 CUs and Sony similarly had 20 CUs with only 18 enabled with the PS4. With Xbox One hardware in the wild, Chipworks went to task delayering the SoC/APU and confirmed the speculation - the Xbox One does indeed feature 14 CUs (pictured above).
Microsoft claims it weighed the benefits of running 12 CUs (768 cores) at 853MHz vs. 14 CUs (896 cores) at 800MHz and decided on the former. Given that the Xbox One APU only features 16 ROPs and ROP performance scales with clock speed, Microsoft likely made the right decision. Thermal and yield limits likely kept Microsoft from doing both - enabling all CUs and running them at a higher frequency. Chances are that over time Microsoft will phase out the extra CUs, although it may take a while to get there. I'm not sure if we'll see either company move to 20nm, they may wait until 14/16nm in order to realize real area/cost savings which would mean at least another year of shipping 14/20 CU parts at 28nm.
Compared to the PS4's APU, we see a very similar layout. The on-die SRAM sits next to the GPU array, and far away from the CPU, which makes sense given that the latter isn't allowed direct access to the eSRAM. You can very clearly see the tradeoff Microsoft had to make in order to accommodate its eSRAM. The GPU area shrinks considerably.
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Mr Perfect - Wednesday, November 27, 2013 - link
Kings Landing? Do you mean Knights Landing?LemmingOverlord - Wednesday, November 27, 2013 - link
A geek wrapped in a GoT fan mindsetdylan522p - Wednesday, November 27, 2013 - link
Yup. I am reading the books currently and it accidentally popped up in my head and I typed it instead of Knights.tipoo - Tuesday, November 26, 2013 - link
So the larger pool of eSRAM can only be accessed by the CPU after copying to main memory, right? I wonder if the smaller pool right by the CPU part instead of the GPU part might have to do with the MOVE engines, or perhaps SHAPE, or if it's just a scratchpad between CPU and GPU...DanNeely - Tuesday, November 26, 2013 - link
IIRC the CPU cache is sram; maybe the block between the CPU clusters is a top level cache for them.Kevin G - Wednesday, November 27, 2013 - link
The CPU should be able to read/write to the 32 MB of eSRAM. That catch is that the bandwidth will be bottlenecked by the CPU-GPU link on die as well as take a small latency hit due to going through the GPU's memory controller. The bandwidth and latency should still be better than the DDR3 main memory.milli - Wednesday, November 27, 2013 - link
Better latency yes but I don't think more bandwidth. As far as I know, there's 'only' a 30GB/s link between the CPU cluster and the rest of the system. But I don't think anybody is going to use the eSRAM for the CPU.Alexvrb - Wednesday, November 27, 2013 - link
For the CPU, they won't be using the 32MB of eSRAM. The quad channel DDR3 provides way more bandwidth than the CPU needs, and it's fairly low latency to boot.Owls - Tuesday, November 26, 2013 - link
I have no dog in this fight as I will get both consoles anyway (already got the PS4) but it's been proven by everyone that the PS4 has the superior design. However, I still have some hope that the XBone will even out in the graphics department down the line.Morawka - Wednesday, November 27, 2013 - link
I think the general consensus on reviewers is that xbox one is the one to get if you can only get 1 console. The podcasters are going nuts over the xbox one's kinect and tv features.