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.

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
Krait.......................11
Swift........................?
Cortex A15..............15
Atom........................16

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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25 Comments

1. Captain_Doug

Posts: 1037; Member since: Feb 10, 2012

Cool cool cool.

8. PhenomFaz

Posts: 1236; Member since: Sep 26, 2012

The Note 3 with an overclocked a-15 quad would get me really really high :)

25. Nadr1212

Posts: 741; Member since: Sep 22, 2012

THIS IS THE LONGEST ARTICLE EVER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

10. MeoCao unregistered

It's very interesting how A15 compare to customized A9 from Qualcomm and Apple. It looks like existing benchmarks are not capable of measuring A15 performance.

2. eisenbricher

Posts: 973; Member since: Aug 09, 2012

Awesome article. I thoroughly enjoyed reading this. I am very positive about the big.LITTLE chips. I already use equivalent example in my laptop. It is the AMD switchable graphics tech. When I am not playing games / using graphics intensive apps the onboard intel sandy bridge handles the graphics duty, providing excellent battery life. Whereas the AMD chip takes duty whenever needed, providing excellent performance. Sounds simple, but works like magic. My friend's laptop without this capability keeps using the dedicated graphics all the time heating up and consuming power. Mine keeps it cool :)

12. Captain_Doug

Posts: 1037; Member since: Feb 10, 2012

I wanna point out that the Omap 5430/32 will use the big.LITTLE setup in a way. It'll use 2A15 cores and 2CortexM4 cores which is like an A8. Also, I know A15 is just arriving but now I'm super excited for the 2A57 + 2A53 setup. Probably won't arrive for another 2 years but I'm excited nonetheless. There's just such a huge jump up from the previous big.LITTLE setup. Phone tech is so cool. It is advancing so much faster than pretty much all other fields.

16. eisenbricher

Posts: 973; Member since: Aug 09, 2012

Nice to know that, bro!

26. VADroidTS555

Posts: 9; Member since: Nov 07, 2012

AMD Switchable graphics work awesome. Intel HD 3000 handle little graphics and when required AMD comes in role. AMD work like Turbocharger here to boost up the performance. Have you noticed that, even if AMD is in role, still Intel keeps working. For evidence see CPU meter while switching windows.

3. baghelvijay01

Posts: 8; Member since: Nov 09, 2012

A very first time I am seeing some technical article in comparison to consumer oriented ones

4. thelegend6657 unregistered

I would love to see a 1080p 5 incher with tegra 4 Wayne device coming in Q1 2013 I am having SPM examinations now and my uncle promise me rm1K for every A I have . Results coming in March , hopefully I get 7A and use about 2K for a new gadget

18. darkskoliro

Posts: 1092; Member since: May 07, 2012

tegra 4 wont come out till at least Q2.

5. shuaibhere

Posts: 1986; Member since: Jul 07, 2012

Exynos 5!!!! Sammy leading all the way!!!!

6. Shubham412302

Posts: 581; Member since: Nov 09, 2011

hope intel brings quadcore with hyperthreading it will slap arm cortex a15

7. Bozzor

Posts: 248; Member since: May 02, 2012

My hope is for a Cortex A15 quadcore Nexus with 5 inch IGZO 1080p 5 inch screen, that's bright in sunlight, 3 GB RAM, min 2500mAh battery, 32GB + micro sd LTE and a really loud speaker. Oh...and all that for less than $400... :-D

9. MeoCao unregistered

This sounds ridiculous now but who knows? Could we think Nexus 4 was possible at this time last year?

22. AWiseGuy

Posts: 68; Member since: Oct 30, 2012

Good observation. In fact, I'm thinking that an upgraded version of the Nexus 4 (maybe the Nexus 5, judging by the 5" screen size, haha!) would carry the specs Bozzor mentioned. The main problem is that the Nexus line, by requirement, cannot have sd card slots. Google doesn't allow them in the Nexus line, which ticks me off royally.

11. rickywinataa

Posts: 262; Member since: Apr 05, 2012

maybe the performance between the A15 & A9 right now is not that big but A15 is still a dual core after all. quad core A15 will be a real monster for mobile devices

14. cretinick

Posts: 148; Member since: Jan 25, 2011

The information I had previously was that the Krait had been developed from the licensing of the Cortex-A15. Not the A9.

15. Daniel.95

Posts: 89; Member since: Sep 20, 2012

Intel will destroy these with a 48-core phone or tablet CPU soon

17. flyboy21

Posts: 14; Member since: Aug 27, 2012

so now is the question buying the note two or waiting for the galaxy s4 with that processor

24. Twisted247

Posts: 282; Member since: Oct 12, 2012

Buy Both!

19. mail2ankit82

Posts: 3; Member since: Jun 12, 2012

"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. " This wrong info. Qualcomm Kraits not based on A9 or customized A9. They are on their own, they are comparable or better in performance than A9 and they have their own design which is not a customized A9. Get the facts right.

20. VebbX

Posts: 41; Member since: Feb 26, 2012

According to Asus' site the Padfone 2 has a Cortext 15 prosessor, don't know if it's right:http://www.asus.com/Mobile/PadFone2/#specifications Edited: Spelling error

21. VebbX

Posts: 41; Member since: Feb 26, 2012

Why isn't Snapdragon S4 mentioned? http://www.qualcomm.com/snapdragon/processors/s4

23. pikapowerize

Posts: 1869; Member since: May 03, 2012

i still dontt get all of these... in a paragraph please explain, is ARM making something that will work on x86 apps? or these cortex A15 is just for phones? which will be better? intel's or ARM?

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