I think I'm right in saying (and someone please correct me if I have the wrong end of the stick here?) Apple's rated clock speeds are the maximum sustainable in the iPads (i.e. the A12Z can run up to 2.5GHz pretty much indefinitely, that's it's top sustained clock and it doesn't go over it) unlike Intel who quote their base clock speed and then turbo above it for heavy loads (which is when active cooing comes into play).
You are entirely correct. I think this is one if the most interesting differences between Intel and Apple chips.
Intel introduces turbo boost as a way to have both high performance and good energy efficiency. By scaling clock on demands they can adapt to many different scenarios. The range of turbo boost in current Intel chips can be quite ridiculous, from under 2ghz for sustained multi-core scenarios (to hit the advertised 15W limit) to up to 5ghz for burst workloads, on the same chip. The overall behavior of the CPUs is unintuitive to most users, e.g. a 15W CPU can draw much more power for short periods of time. Basically, turbo boost as we have it now renders TDP almost meaningless.
Apple on the other hand has a much lower turbo boost range, I think it’s something like 2.5-2.7 for the A13. But that also seems to be the upper limit for the CPU, as the power consumption spikes like crazy afterwards. To put it differently, Intel architecture has its peak efficiency clock and it’s peak performance clock fairly far from each other, while in Apple designs they are very close together. In practical terms, what this means that if Apple manages to move the performance just a bit higher with the new A14, their peak efficiency will likely match Intel’s peak performance.
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