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Can anyone speculate on how much of the design is from Apple and how much is from ARM? How easily will the other ARM licensees be able to be in the same ballpark as Apple?

This is an apples and oranges question. (Sorry.) There are some pretty nice features in the ARM64 design which allow chip designers to save a lot of power--or make the chip faster. Apple has done a very good job of balancing the two to get a nice solution.

But the big thing that Apple did was speed in getting both the chip and the software ready. I don't know how much of the software and firmware Apple is required to make open, or more important how much of the ARM64 OS code will be useful to others building Linux on other ARM64 chips.

But I do think that it is now pretty clear that ARM64 is a more efficient design than AMD64. Nothing against the AMD64 designers. Technology has improved a lot since then. It has both resulted in additional requirements for high-end chips (in particular security kernel support and guest OS support), and in different tradeoff parameters. So being able to turn off parts of the chip is now common, even in high-speed chips, and the ARM64 design maximizes the size of the areas that can be turned off.

Also renaming registers make increasing the number of integer registers inexpensive if not free. The live virtual registers need silicon, any inactive registers don't need silicon or power. The ARM64 ISA makes this a bit easier, but it also provides 30(+2) general purpose registers without instruction prefixes. Decoding AMD64 instructions at high speed is expensive in power and real-estate. Part of that cost is for folding in prefixes and eliminating instructions which do register moves. (Technically, if the software needs more than 16 registers, AMD64 code has to spill the registers to memory. Not all that expensive in time, because all high-end AMD64 chips will pull the value out of the write pipe if needed, or just recognize that it is still in a renaming register. But not having to do that for speed saves power.
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