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Linley on Mobile

How Apple Designed Own CPU For A6

September 15, 2012

Author: Linley Gwennap

At the iPhone 5 announcement, Apple did not disclose what CPU it used in its new A6 processor. Recent information, however, indicates that Apple designed its own CPU rather than licensing a Cortex-A9 or next-generation Cortex-A15 from ARM. The fact that Apple requires iPhone 5 apps to be recompiled to a new architecture variant called ARMv7s indicates that the A6 does not use the same Cortex-A9 CPUs that are in the previous Apple A5 processor. The website anandtech.com is now reporting that the A6 uses an internally designed CPU.

Although Apple has been licensing cores from ARM for its previous processors, the company’s interest in CPU design dates back to its $278 million acquisition of PA Semi in April 2008. That acquisition included a CPU design team that had developed a high-performance PowerPC processor under the leadership of Jim Keller and Pete Bannon. More important, some of the team members had previously worked on low-power StrongArm processors under PA Semi CEO Dan Dobberpuhl at Digital Equipment (DEC) in the 1990s. Within a month of that deal, Apple secretly signed an architecture license with ARM that allowed the company to develop its own ARM-compatible CPUs, becoming one of the few companies in the world with that right.

While one group of PA Semi employees set to work on the Apple A4 processor using an ARM CPU core, another group began defining the microarchitecture for the new CPU. According to one source at Apple, Steve Jobs initially set an “insanely great” bar for the performance of the new CPU, but he eventually realized that his CPU team was limited by the same laws of physics that apply to everyone else. For whatever reason, the project took a long time to get through the initial definition and design phase.

During this period, some of PA Semi’s top people—including CEO Dobberpuhl, COO Leo Joseph, and VP of System Architecture Mark Hayter—left Apple, causing reports that the CPU design team was dissolving. But at PA Semi, Dobberpuhl and Joseph were involved mainly on the business side, and Hayter worked at the SoC level and not on the CPU, so these departures were not as significant as they appeared. Keller and Bannon continued to lead Apple’s chip development, and the company expanded its team through vigorous hiring. In February 2010, the company hired Gerard Williams, an ARM Fellow who was the technical lead for the Cortex-A8 and Cortex-A15 CPUs; Williams became Apple’s chief CPU architect. Keller recently left Apple to move to AMD.

In early 2010, the team had completed the A6 microarchitecture design and started the physical-design phase. To bolster its physical-design capabilities, Apple spent an estimated $120 million in April 2010 to acquire Intrinsity. This deal brought in an experienced team of chip designers that specialized in high-speed physical design, having just finished boosting the speed of Samsung’s Hummingbird CPU (which Apple used in its A4 processor). The A6 taped out about a year later, and Apple received the first samples last summer. To support the iPhone 5 launch, the new processor must have been cleared for production around June.

Now that it has completed its first CPU design, Apple is not likely to stop there. To keep pace with competitors using ARM’s own cores, the company will have to crank out a new CPU design every couple of years. We believe Apple is already working on a next-generation CPU, which will likely implement the 64-bit ARMv8 instruction set. This new CPU probably won’t debut until 2014, so for its 2013 products, Apple will have to rely on the same CPU design, perhaps in a quad-core configuration and with a higher-performance GPU.

At this point, Apple has spent about $400 million to buy PA Semi and Intrinsity, tens of millions for a license to design its own ARM CPUs, and probably north of $100 million to support its CPU design efforts over the past four years. It appears that the end result will be that Apple ships a Cortex-A15-class CPU about three months before arch-enemy Samsung does. These three months happen to come during the big holiday buying season, during which the iPhone 5 could generate $25 billion in revenue. So that half billion dollars could be money well spent.

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