Custom CPU core design in iPhone 5 marks an Apple first
More details of the iPhone 5 CPU emerged this week, confirming Apple’s claimed performance gains. But more importantly, they are the first indications of the impact of Apple’s custom chip design, rather than relying on standardized cores licensed from ARM.
With the iPhone 5’s A6 CPU, Apple has taken a big step: gaining complete control over the processor implementations for its iPhone and iPad product lines. That change may put Apple ahead of archrival Samsung’s processor development, also based on ARM chip technology. And it gives Apple the power to more fully control the complex tradeoffs of battery demand, CPU performance and graphics performance in the end user’s computing experience.
Teardowns, such as the one by iFixit.com and Chipworks, and by UBM TechInsights confirm that the A6 uses two ARM-based but Apple-designed cores for the CPU, along with three integrated Imagination Technologies’ PowerVR graphics cores. Apple seems to have systematically pieced together the elements to at least double the iPhone 5 performance over that of the iPhone 4S.
In a variety of benchmark tests, such as those by Anandtech.com, the CPU and graphics performance puts the iPhone at or near the top of the heap compared to high-end rivals such as the HTC One S or X, LG Optimus G and Samsung Galaxy S 3. The tests also show that the iPhone 5 compute and graphics performance is often comparable to, or better than, the new iPad, which runs the A5X chip.
The A5X and its earlier brethren are different from the A6 in one very important way. Until now, Apple licensed and used ARM’s microprocessor core designs, along with the relevant software, for its mobile processors. This is the same approach used by many other companies, including Broadcom, Nvidia and Texas Instruments. All of them combine these cores with a variety of other components — baseband chips, memory, graphics processors and so on — to create their own system-on-chip (SoC) which gets plugged into the final assembly for a phone or tablet. This approach allows for a lot of innovation by the chip designers, who can combine different numbers of cores, or run them at different clockspeeds, for example.
But Apple is one of a much smaller group of companies (Qualcomm is another) that also has licensed the underlying ARM instruction set architecture (ISA), in this case ARMv7 which is widely used in other mobile products. With this instruction set “recipe,” Apple’s own chip designers can craft their own cores, and tailor them specifically to the company’s mobile platform priorities. “You have absolute control over your [chip] roadmap and the features that are important to you,” says James Bruce, lead mobile strategist with ARM. “It’s not an exercise for the fainthearted. You probably need about 50 to 100 highly skilled engineers. And it’s a multi-year project.”
And Apple has such a team. In April 2008, Apple acquired semiconductor design firm PA Semi for $278 million. “That acquisition included a CPU design team that had developed a high-performance PowerPC processor under the leadership of Jim Keller and Pete Bannon,” notes a recent blog post by Linley Gwennap, principal analyst for The Linley Group, a technology analysis firm focused on semiconductors. “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.”
Shortly after that purchase, Apple secretly signed the architecture license with ARM. One team began working on the A4, using licensed ARM core designs, but another team “began defining the microarchitecture for the new CPU,” according to Linley. The design was finished by early 2010, and Apple then launched the physical design work. About the same time, Apple hired as its chief CPU architect Gerard Williams from ARM, where he was the technical lead for the Cortex-A8 and Cortex-A15 CPUs, according to Linley. And shortly after, Apple made its second silicon-related acquisition: paying $120 million in April 2010 for Intrinsity, with expertise in high-speed physical design.
This expertise now puts Apple on a sustainable evolution in processor design, tailored to its specific needs.
“Mobile processors have been using Cortex-A9 [core designs] for the past two-plus years,” Linley says in an email. “Cortex-A15 is the next step in ARM’s roadmap, and the first Cortex-A15 should appear in phones around the end of this year. It offers a large increase in performance, although at some cost to battery life.”
But it’s also designed to meet requirements for a very wide range of end products, from smartphones to big servers. Apple will forgo ARM’s A15 designs but create an A15-class CPU of its own specifically for mobile devices. “Having control of the CPU allows Apple to optimize the design to meet its own needs,” Linley says. “Apple is willing to spend a little more money — on a more expensive CPU — if it makes the end product, such as an iPhone, noticeably better.”
According to a battery of initial tests by Anandtech.com, Apple has done exactly that with its first custom CPU, running the iPhone 5.
“Overall, the performance of the A6 CPU cores seems to be very good,” writes Anand Lal Shimpi. “Apple claimed a 2x CPU performance advantage compared to the iPhone 4S during the launch event for the 5. How does that claim match up with our numbers? Pretty good actually. … This is hardly the most comprehensive list of CPU benchmarks, but on average we’re seeing the iPhone 5 deliver 2.13 times the scores of the iPhone 4S.”
Part of the gain is realized by moving to a smaller die process for the chip, to 32 nanometers from 45 nm. But by itself that’s not enough, according to Lal Shimpi.
Among his findings:
- “The memory interface on the A6 seems tangibly better than any previous ARM based design, and the advantage here even outpaces Intel’s own Medfield SoC.”
- “The A6 … features a three core PowerVR SGX 543MP3 [graphics processor], running at higher clock speeds to deliver a good balance of die size while still delivering on Apple’s 2x GPU performance claim.”
- “The result is compute performance that’s similar to the A5X in Apple’s 3rd generation iPad, but with a smaller overall die area.”
- “As we’ve seen in the past, these gains don’t typically translate into dramatically higher frame rates in games, but games with better visual quality instead.”
The iPhone 5 ranks at or near the top in these benchmarks, compared with an array of high-end rivals. Lal Shimpi noted that Qualcomm’s ARM-based Snapdragon S4 Pro CPU with Adreno 320 GPU puts LG’s Optimus G “hot on the heels of the new iPhone.”
Why not just run the Cortex-A9 cores found in the earlier Apple A5 SoC at a higher frequency?
“To push frequency you have to push voltage, which has an exponential impact on power consumption,” according to another post by Lal Shimpi. “Running your cores as close as possible to their minimum voltage is ideal for battery life. The right approach to scaling CPU performance is a combination of increasing architectural efficiency ([the number of] instructions executed per clock goes up), multithreading and conservative frequency scaling.”
With full control of its CPU design, Apple now can exploit these kinds of synergies.
And that’s one reason why Apple hasn’t rushed into quad-core CPU designs for the iPhone. A range of tech blogs and news sites argued last year and this year that quad-core CPUs were needed to “compete” with Android smartphones that were making use of these powerful processors, such as the LG Optimus 4X HD and HTC One X.
“The problem with quad-core today is that apps must be modified to use all four cores,” says Linley. “Few Android apps, for example, can do that, so Apple is not at a big disadvantage by having a dual core. By mid-2013, however, quad-core Android apps will be common, as will quad-core Android phones, so I think it would be a good time for Apple to follow suit.”
But the real change, likely in 2014 according to Linley, will be a shift to 64-bit chip architectures for mobile processors. Linley believes Apple is already at work on this, likely implementing the 64-bit ARMv8 instruction set.
“A 64-bit processor can more easily handle 4GB of DRAM and higher,” Linley says. “The iPhone 5 uses 1GB of DRAM [itself doubling the DRAM of iPhone 4S], and some competing phones use 2GB, so it is easy to imagine an iPhone in 2014 with 4GB of DRAM. Also, ARM has included many other innovations in its 64-bit ARMv8 design, so it’s kind of a package deal. If Apple wants to keep pace with the leading ARM processors, it has to go to 64 bits in 2014.”