An Update on Apple’s A7: It's Better Than I Thought

When I reviewed the iPhone 5s I didn’t have much time to go in and do the sort of in-depth investigation into Cyclone (Apple’s 64-bit custom ARMv8 core) as I did with Swift (Apple’s custom ARMv7 core from A6) the year before. I had heard rumors that Cyclone was substantially wider than its predecessor but I didn’t really have any proof other than hearsay so I left it out of the article. Instead I surmised in the 5s review that the A7 was likely an evolved Swift core rather than a brand new design, after all - what sense would it make to design a new CPU core and then do it all over again for the next one? It turns out I was quite wrong.

Armed with a bit of custom code and a bunch of low level tests I think I have a far better idea of what Apple’s A7 and Cyclone cores look like now than I did a month ago. I’m still toying with the idea of doing a much deeper investigation into A7, but I wanted to share some of my findings here.

The first task is to understand the width of the machine. With Swift I got lucky in that Apple had left a bunch of public LLVM documentation uncensored, referring to Swift’s 3-wide design. It turns out that although the design might be capable of decoding, issuing and retiring up to three instructions per clock, in most cases it behaved like a 2-wide machine. Mix FP and integer code and you’re looking at a machine that’s more like 1.5 instructions wide. Obviously Swift did very well in the market and its competitors at the time, including Qualcomm’s Krait 300, were similarly capable.

With Cyclone Apple is in a completely different league. As far as I can tell, peak issue width of Cyclone is 6 instructions. That’s at least 2x the width of Swift and Krait, and at best more than 3x the width depending on instruction mix. Limitations on co-issuing FP and integer math have also been lifted as you can run up to four integer adds and two FP adds in parallel. You can also perform up to two loads or stores per clock.

I don’t yet have a good understanding of the number of execution ports and how they’re mapped, but Cyclone appears to be the widest ARM architecture we’ve ever seen at this point. I’m talking wider than Qualcomm’s Krait 400 and even ARM’s Cortex A15.

I did have some low level analysis in the 5s review, where I pointed out the significantly reduced memory latency and increased bandwidth to the A7. It turns out that I was missing a big part of the story back then as well…

A Large System Wide Cache

In our iPhone 5s review I pointed out that the A7 now featured more computational GPU power than the 4th generation iPad. For a device running at 1/8 the resolution of the iPad, the A7’s GPU either meant that Apple had an application that needed tons of GPU performance or it planned on using the A7 in other, higher resolution devices. I speculated it would be the latter, and it turns out that’s indeed the case. For the first time since the iPad 2, Apple once again shares common silicon between the iPhone 5s, iPad Air and iPad mini with Retina Display.

As Brian found out in his investigation after the iPad event last week all three devices use the exact same silicon with the exact same internal model number: S5L8960X. There are no extra cores, no change in GPU configuration and the biggest one: no increase in memory bandwidth.

Previously both the A5X and A6X featured a 128-bit wide memory interface, with half of it seemingly reserved for GPU use exclusively. The non-X parts by comparison only had a 64-bit wide memory interface. The assumption was that a move to such a high resolution display demanded a substantial increase in memory bandwidth. With the A7, Apple takes a step back in memory interface width - so is it enough to hamper the performance of the iPad Air with its 2048 x 1536 display?

The numbers alone tell us the answer is no. In all available graphics benchmarks the iPad Air delivers better performance at its native resolution than the outgoing 4th generation iPad (as you'll soon see). Now many of these benchmarks are bound more by GPU compute rather than memory bandwidth, a side effect of the relative lack of memory bandwidth on modern day mobile platforms. Across the board though I couldn’t find a situation where anything was smoother on the iPad 4 than the iPad Air.

There’s another part of this story. Something I missed in my original A7 analysis. When Chipworks posted a shot of the A7 die many of you correctly identified what appeared to be a 4MB SRAM on the die itself. It's highlighted on the right in the floorplan diagram below:


A7 Floorplan, Courtesy Chipworks

While I originally assumed that this SRAM might be reserved for use by the ISP, it turns out that it can do a lot more than that. If we look at memory latency (from the perspective of a single CPU core) vs. transfer size on A7 we notice a very interesting phenomenon between 1MB and 4MB:

That SRAM is indeed some sort of a cache before you get to main memory. It’s not the fastest thing in the world, but it’s appreciably quicker than going all the way out to main memory. Available bandwidth is also pretty good:

We’re only looking at bandwidth seen by a single CPU core, but even then we’re talking about 10GB/s. Lookups in this third level cache don’t happen in parallel with main memory requests, so the impact on worst case memory latency is additive unfortunately (a tradeoff of speed vs. power).

I don’t yet have the tools needed to measure the impact of this on-die memory on GPU accesses, but in the worst case scenario it’ll help free up more of the memory interface for use by the GPU. It’s more likely that some graphics requests are cached here as well, with intelligent allocation of bandwidth depending on what type of application you’re running.

That’s the other aspect of what makes A7 so very interesting. This is the first Apple SoC that’s able to deliver good amounts of memory bandwidth to all consumers. A single CPU core can use up 8GB/s of bandwidth. I’m still vetting other SoCs, but so far I haven’t come across anyone in the ARM camp that can compete with what Apple has built here. Only Intel is competitive.

 

Introduction, Hardware & Cases CPU Changes, Performance & Power Consumption
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  • jelloboy - Saturday, November 02, 2013 - link

    I've been coming here for many years. Their breakdown of products are fantastic, I always checkout what they have to say about something before picking it up. Reply
  • beggerking@yahoo.com - Saturday, November 02, 2013 - link

    Where is the nexus 5 review? Why is it not up yet? Reply
  • golem - Sunday, November 03, 2013 - link

    Why are you expecting one so soon? Most major outlets only have previews or first impressions and Anandtech's reviews usually come late anyway. Reply
  • jelloboy - Saturday, November 02, 2013 - link

    I just got my Air today, I'm upgrading from an iPad 3 and it's actually quite impressive in your hand, much more so than I thought it would be. Very comfortable and light to hold but still feels like an Apple product (which is a good thing) - very fast and the biggest thing I was worried about isn't an issue. I was worried since they cut some width off the sides that you might run into an issue of not having anyplace to rest your thumbs - however this isn't an issue at all. Since the iPad Air is a good amount lighter you don't to use as much leverage - plus I guess the software has some sort of thumb reject stuff in it - regardless it works.

    Obviously the unit is much faster than the iPad 3 - just generally usage is much more enjoyable on the Air. The iPad 3 wasn't slow, but it certainly wasn't as fast as my iPhone 5 - the Air obviously doesn't have the issue.

    So I wasn't sure if this new iPad would really be all that different, if the size and weight changes would be as dramatic as what I've read about here and other places - but I'm happy to say it really is quite an improvement.

    Also Android fanboys before you start crying, so you know I have 4 Sony Google TVs (which I love), and Nvidia Shield (which is awesome) and I've owned 3 different Android tablets over the years (Transformer TF100, Nexus 7, Transformer TF300). For the record the Android tablets are awful, terrible - slow, crappy software, lots of bugs. Meanwhile the Shield, which thinks it's a phone, actually runs very well. Anyways my point is Android has a long long long long long long way to go to be competitive in the tablet market. A long way.
    Reply
  • Gadgetmanjohn - Sunday, November 03, 2013 - link

    I'm an Apple fan, I have an iPad 1 and 3 but skipped 4 as it came out too soon after I'd purchased the 3rd gen. However, having tried the iPad air in the shop I was completely put off by the sound vibrations through the chassis. You can feel it all the way from the bottom to the top, very off putting when watching video or playing music whilst holding it. I just can't believe more people aren't talking about it, let's hope the mini doesn't suffer the same problem. Reply
  • Origin64 - Monday, November 04, 2013 - link

    You guys still pretending this is anything new? Same rectangle, different day. Reply
  • Chrispy_ - Monday, November 04, 2013 - link

    No matter how good this device may be, I can't get over the fact that iOS7 is such a blatant copy for the visual style of Android. The default wallpaper on this reminds me of several of the 4.2 JellyBean wallpapers:

    http://www.smartkeitai.com/wp-content/uploads/2012...
    http://www.smartkeitai.com/wp-content/uploads/2012...
    Reply
  • Brakken - Thursday, November 14, 2013 - link

    I was able to skin my android in iOS7 five months before iOS7 was released. Who's copying who now, huh? Huh?! Reply
  • nasqb112 - Monday, November 04, 2013 - link

    Great review (as always) as this looks like an awesome device. I want to get an opinion from the Anandtech crowd:

    I currently have a MacBook Air (2011) and a Nexus 7 and want to consolidate.

    I was thinking Surface Pro 2 or iPad Air with keyboard case (similar to the Surface keyboard blades) as I need to be productive on the device. I'd also watch movies, surf the web, game while traveling, etc.

    Can anyone offer their thoughts on iWork vs Office (particularly for spreadsheets/data analysis and presentations?). I know what Office is supposedly coming to iPad, but I'll believe it when I see it.

    So what would you choose, Surface Pro 2 or iPad Air with keyboard and why? Thanks all!
    Reply
  • tripleverbosity - Wednesday, November 13, 2013 - link

    If your biggest concern is running office I don't think the iPad Air with keyboard would be a good fit for you. Who knows when Office actually shows up for iOS and what level of functionality Microsoft will expose. As for iWork, it definitely isn't as comprehensive of an offering as Office at this point. That said, there's no way I'd buy a Surface Pro 2. If you can fit in into your budget, spend a couple hundred more and upgrade to the latest Macbook Air. It seems like you are trying to replace the laptop experience with a tablet/keyboard combo. Why not just go with a great laptop like the Air? You'll get better performance, better software capabilities, a superior keyboard and trackpad, and the best battery life out in the segment. Reply

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