Of all the things announced during the iPad“reveal”, I was most intrigued by the device’s processor, the “Apple A4“.

More surprising to me was how this announcement was but a single bullet-point in the presentation, and hasn’t drawn much discussion on the web following the announcement.

I spent some time gathering what little information there is out there on the chip and have collected my findings here.

This is probably the best single resource available on the web at the moment.  It describes what is (publicly) known about the A4 as well as discussing the chip’s “origin story” and enough background about Apple’s processor choices to understand where things may head from here.

This is an article from 2008 discussing Apple’s acquisition of PA Semi, the engineering behind the A4.

A few things come to mind reading through these articles.  The first is that Apple has switched CPU’s (more interestingly, CPU instruction sets) several times and each time the change happens rather quickly and without much (if any) end-user impact.  These changes have been driven by a pragmatic need to improve some aspect of performance (processing power, efficiency, etc.).  Contrast this with Microsoft and the x86 architecture where an instruction-set-level change is so taboo that it hasn’t been done even when the cost of maintaining this “compatibility” has been crippling.

Secondly Apple didn’t leave the POWER architecture for performance or other reasons, but for the fact that future (expected) chips in the series could not achieve the thermal efficiency Apple needed for mobile devices (laptops in particular).  In other words, the decision to switch to Intel chips was driven by Watts not FLOPS.  As early as 2007 PA Semi delivered a PowerPC G5 chip (IBM’s POWER4 architecture) capable of 2.0Ghz speeds and consuming only 13 watts (25 peak) and with two cores to boot (I’d like to compare this to the POWER4 chip used in desktop Macs at the time but I can’t find definitive information on this at the moment).

To me the moral of the story here is that Apple’s move to use their own silicon in the iPad is a natural progression of their interest in controlling everything about their computers, and of their lack of fear in venturing into the realm of processor design.  Some have shot down the idea of Apple using these chips outside of their mobile device realm based on the complexity of moving Mac OS to a new chip but history shows Apple has no reservation in making a move like this, when it makes sense to do so.

There may be other sound reasons that Apple will continue using Intel chips in the Mac line and keep the A* chips use limited to mobile devices but their history and behavioral patterns indicate otherwise.