ARM Detail The Cortex-A35, A New High Efficiency Core

ARM Holdings are an interesting British company that design central processor units, or application cores. ARM license these designs to many other businesses such as Apple, MediaTek, HiSilicon, Samsung, NVIDIA and Qualcomm, who either use the stock reference design (such as the ARM Cortex-A7, Cortex-A53, Cortex-A72) or redesign the core for their own custom chip (the Apple A-range, Qualcomm's Krait and Kyro cores). This means that when ARM announce a new reference application core, there could be a wide ranging impact on the industry. And ARM have just announced a new low power application process, the Cortex-A35, which is designed for the ultra-high efficiency end of the current ARM core line up. To put this into context, ARM have three tiers of core design: high performance, high efficiency and ultra-high efficiency. The ARM Cortex-A57 (as used in the Snapdragon 810, Exynos 7420 chipsets) is a high performance core. The ARM Cortex-A53 (a very common application core used in a huge number of current chipsets) is a high efficiency core, and the Cortex-A35 slots in below the A53 and seemingly designed to replace ARM's older Cortex-A5 and Cortex-A7 cores. We rarely see these cores in modern smartphones, but many Android Wear devices are based around the 32-bit ARM Cortex-A7 core. The Cortex-A35 is going to be a very common application core.

In brief, the ARM Cortex-A35 is expected to be used at smaller processor node sizes, immediately in the 14nm or 16nm points, with clock speeds capable of exceeding 2.0 GHz. The application core uses the ARMv8 instruction set and is a full 64-bit core, unlike the A5 and A7, which are older ARMv7 32-bit units. It is capable of buddying up with the full range of other 64-bit cores, the A53, A57 and A72, so we can expect chipset designers to implement a big.LITTLE processor based around the A35 with either the A53 or A72 (the A57 is less likely to be used as it is an older and less efficient design than the A72). We are likely to see a future MediaTek triple-tier chip based around the A35, A53 and A72 cores.

Under the skin, ARM are using a refined architecture for the Cortex-A35 and it features improved power and performance efficiency compared with older A5 and A7. In particular, the A35 uses much of the higher performance memory technology as we find in the A53 and features a redesigned instruction fetch unit with improved branch prediction technology. The better the chip can second-guess the next instruction, the less power it uses. The A35 core is physically a smaller chip than the A53, which is going to be useful for squeezing components into a very slim smartphone design (and we would hope, bigger batteries). By Qualcomm's numbers, then, the A35 is reputed to use around 10% less power than the Cortex-A7 whilst giving a performance improvement of between 6% to 40%, depending on the application. Compared with the A53, the A35 promises to offer comparable performance for many applications (although notably not for web browsing, where Qualcomm's figures show the A35 is only 80% as quick as the A53), but using a core that is 75% the size of the A53 and using a little over two thirds of the power. Since the A35 is capable of exceeding 2.0 GHz, manufacturers could simply run the core at a higher clock speed than an A53 in order to make up the performance deficit and dispense with the A53 cores. This may not save power, but will result in a physically smaller chip. We are very likely to see big.LITTLE chipsets based around the A35 and A72 cores.

So far, so good: the new ARM Cortex-A35 is smaller and more power efficient than the older designs, and appears to be set up to compete with the A53 core. Unfortunately, there is bad news: ARM does not expect the first devices to ship with the Cortex-A35 until the end of 2016. We have a year to wait before we see everything from smartphones to wearables and tablets with one or more A35s inside.

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About the Author
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David Steele

Senior Staff Writer
I grew up with 8-bit computers and moved into PDAs in my professional life, using a number of devices from early Windows CE clamshells and later. Today, my main devices are a Nexus 5X, a Sony Xperia Z Tablet and a coffee cup.