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ARM Cortex A15: a deeper look

ARM Cortex A15: a deeper look
The ARM Cortex A15 is the next generation processor that will end up on a huge number of devices in 2013. But the first gadgets with it are already out - the new $249 Chromebook and the Nexus 10 are both powered by Cortex A15-based SoCs.

Cortex A15 is the successor to Cortex A9, and it comes at just about the right time. Companies like Qualcomm and now Apple are customizing the existing A9 design to build their own processing cores like the Krait processor in the Snapdragon S4 and the Apple Swift core in the A6 chip on the iPhone 5. While based on A9, these customized solutions are much more agile and virtually obliterate the A9. That is exactly why the timing for Cortex A15 seems about right to fight off the newcomers.

To understand the Cortex A15, we have to take a look back at ARM designs and make the inevitable comparison with Intel’s Atom chips. First thing that makes the ARM chips different is the fact that they prioritize power efficiency from their earliest versions and are now making huge gains in performance. For Intel, the way with Atom is the complete opposite. The company has perfected its architecture to a great performance level and over the last few years is working on getting power usage down.

The Cortex A15 builds on that legacy of low-power devices, but for the first time can truly match Intel Atom’s performance and aims to go at more than just smartphones and tablets. This time, ARM wants a chunk of the server space, an area previously reserved for Intel and AMD.

Cortex A15 vs Cortex A9
So the Cortex A15 is a chip that is truly designed to become the first step towards an increasing presence of ARM in servers. This generation of chips will be the segway to ARM’s server entry and will mostly end up being used in smartphones and tablets. Still, that makes it quite a multipurpose design, and that’s something we need to take into account.

The A15 is in many ways a logical evolution over the A9. It is a 32-bit chip with several new instructions added to the underlying ARMv7 ISA architecture. It bumps up the instruction decoder from two-wide in A9 to three-wide (just like in Krait and Switf), and can issue double the number of micro operations per clock cycle, 8. It is also communicating faster with lowest-level cache (L1) via a 128-bit channel, up from 64-bit one in the A9.

ARM Cortex A15: a deeper look
The Cortex A15 comes with a deeper, 15-level pipeline, up from an 8-stage pipeline in the A9. Such a boost to the pipeline depth means less gets done in one clock cycle, so the processor can run at higher clocks, up to 2.5GHz. However, mispredicted branches take longer to complete and can punish such a decision by slowing down the performance. To address that, ARM brings an improved branch predictor that can execute more out-of-order instructions and also can process 128-bit NEON instructions in one cycle. NEON is a technology that speeds up multimedia processing like encoding and decoding of video, audio and gaming.

Pipeline depth:
Cortex A8...............13
Cortex A9...............8
Cortex A15..............15

How will this affect the actual performance? We have already seen the Cortex A15 smoke Intel’s Atom N570 chip in Chromebooks, and as more devices come in we will get a better understanding of how powerful ARM’s new processor is.

Connecting it all
However, at this point of performance, power efficiency starts to become an issue for ARM as well and it addresses it with the so called big.LITTLE setup. In it, a Cortex A15 processor can be paired with the less powerful, but much more efficient A7 so that the A7 is used for trivial tasks, and the A15 kicks in when games, the browser or other more requiring applications are fired up. That is the kind of setup that will end up in many servers as well, and that big.LITTLE scheme is the way to go for ARM in the future as well when the next generation of processors, the Cortex A50 and A53 arrive.

To make this connection between a power-efficient and performance-oriented core possible, ARM is introducing the CoreLink CCI-400 (Cache Coherent Interconnect). That is the exact component that makes pairing all components like the various CPUs, graphics and others possible.

Each of those component bind to the CCI-400 via a 128-bit connector

The CCI-400 basically makes it clear that for ARM, the focus of big.LITTLE is on tablets and servers, and less on phones. We are still likely to see A15-based phones, especially in the high-end, but that boost in productivity might mean a lower battery life or at least a lower than the maximum supported clock speed that is said to be around 2.5GHz.

Devices with it
When it comes to actual chips built using the A15 processor, we already have devices out on the market with the Samsung Exynos 5250 (aka Exynos 5 Dual) on Nexus 10 and ChromeBook. We do expect to be swept by A15-based smartphones and tablets in 2013, and rumor has already started floating that an eventual Samsung Galaxy S IV would use a quad-core A15 setup.

Nvidia is also coming up strong with its Tegra 4 chip next year that is also based on the A15 processor, and here are the other competing companies.

Future chips based on Cortex A15
Nvidia Tegra 4 Wayne.............28nm........Q1-Q3 2013
ST-Ericsson Nova A9600........28nm........2013
TI OMAP 5.............................28nm........Q2 2013

In the very near future of mid-2013, the Cortex A15 will go head-to-head with Intel’s upcoming new Atom Z2580 chips that are expected to bump up performance hugely for the first time in the five years since the introduction of the series in 2008, and finally deliver a boost in graphics with a built-in PowerVR SGX 544MP2 GPU.

We don’t know how those will look, but for the time being, it seems that ARM continues being ahead in the mobile chip game.

Images courtesy of AnandTech, ITProPortal.

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