Earlier this week hardware web site Anandtech.com reported that Apple designed its own CPU for the ne iPhone 5 rather than licensing a Cortex-A9 or next-generation Cortex-A15 from ARM. Among other reasons, 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 found in the previous Apple A5 processor.
Gwennap believes the custom A6 CPU is similar in complexity and performance to Cortex-A15 as well as to the Krait CPU that appears in Qualcomm's newest processors. To reach Apple's claim of a 2x performance gain over the iPhone 4S (which uses the Apple A5 processor), Gwennap expects the A6 contains two CPU cores clocking at roughly 1.2GHz. However, this speed is lower than what competing A15-class CPUs achieve, presumably to save power.
During the iPhone 5 presentation, Apple stated that the A6 processor is 22% smaller than the A5, putting its die size at about 96mm2. Gwennap believes that Samsung is manufacturing the A6 in its 32nm process.
"Simply taking the Apple A5X processor (used in the new iPad), which includes the quad-core GPU needed to deliver the iPhone 5's claimed 2x gain in GPU performance, and shrinking it from 45nm to 32nm would result in a die size of 82mm2," Gwennap says. "Because the dual Cortex-A9 CPUs in the A5X measure only 7mm2, the extra 14mm2 provides plenty of room for a more powerful custom CPU design as well as the chip?s new image-signal processor (ISP)."
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," Gwennap said.
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. However, the group reportedly faced difficulties in the development of the new architecture.
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.
According to Gwennap, Apple's team had completed the A6 microarchitecture designand started the physical-design phase in early 2010. 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.
Gwennap claims that Apple will continue to develop CPUs.
"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," Gwennap said.
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 rival Samsung does.