[paulryanrogers]

But how far has Apple's chip diverged from reference ARM designs? And does ARM preserve backwards compatibility as aggressively as Intel?

Because I see compatibility as the biggest reason for the dominance of x86 and x64.

[saagarjha]

They present the same interface, and that's all that really matters. (AArch64 is incompatible with the 32-bit instruction set and thumb, FWIW.)