Anandtech now reveals, however, that the A6 is a custom Apple design:
The A6 is the first Apple SoC to use its own ARMv7 based processor design. The CPU core(s) aren't based on a vanilla A9 or A15 design from ARM IP, but instead are something of Apple's own creation.
Anandtech explains that Apple is one of a few ARM architecture licensees, which allows them to create their own custom ARM processor designs.
While Anadtech goes into the finer details, the ultimate benefit for Apple is the ability to tune their chips towards their own specific goals. In particular, Apple's design goals prioritize both power and performance while the generally licensable Cortex A15 design was reportedly targeted at server configurations.
Rumor has it that the original design goal for ARM's Cortex A15 was servers, and it's only through big.LITTLE (or other clever techniques) that the A15 would be suitable for smartphones. Given Apple's intense focus on power consumption, skipping the A15 would make sense but performance still had to improve.
Originally Posted by mark
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