Understanding the iPhone 3GS
by Anand Lal Shimpi on July 7, 2009 12:00 AM EST- Posted in
- Smartphones
- Mobile
More Detail on ARM11 vs. Cortex A8
We’ve gone through the basic architectural details of the ARM11 and Cortex A8 cores, and across the board the A8 is far ahead. It gets even better for the new design once we drill a little deeper.
The L1 cache in the A8 gets a significant improvement. The ARM11 core had a 2 cycle L1 cache, while the A8 has a single cycle L1. In-order cores depend heavily on fast memory access, so an even faster L1 will have a dramatic impact on performance.
ARM11 actually supported a L2 cache but it was rarely used; the Cortex A8 is designed with a tightly coupled L2 cache varying in size. Vendors can choose from cache sizes as small as 128KB all the way up to 1MB, with a minimal access latency of 8 cycles. The L2 access time is programmable, with slower access more desirable to save power.
The caches also include way prediction to minimize the number of cache ways active when doing a cache access, this sort of cache level power management was also used by Intel back on the first Pentium M processors and is still used today in modern x86 processors.
The ARM11 core supported a 64-bit bus that connected it to the rest of the SoC; Cortex A8 allows for either a 64-bit or 128-bit bus. It’s unclear what vendors like Samsung and T.I. have implemented on their A8 based SoCs.
The S is for speed. Powered by the ARM Cortex A8.
With a deeper pipeline, the Cortex A8 also has a much more sophisticated branch prediction unit. While the ARM11 core had a 88% accurate branch predictor, the Cortex A8 can correctly predict branches over 95% of the time. If you care about stats, the A8 has a 512 entry branch target buffer and a 4K entry global history buffer. The accuracy of the branch predictor in the Cortex A8 is actually as high as what AMD claimed with its first Athlon processor, and this is an in-order core in a smartphone. With a 13-stage pipe however, a very accurate predictor was necessary.
While ARM11 supported some rudimentary SIMDfp instructions, Cortex A8 adds a full SIMDfp instruction set with NEON. ARM expects a greater than 2x improvement on media processing applications thanks to the A8’s NEON instructions - of course you’ll need to compile directly for NEON in order to see those gains. If you’re looking for a modern day relation, NEON is like the A8’s SSE whereas ARM11 basically had a sophisticated MMX equivalent. Both are very important.
The Cortex A8 is a more power hungry core than the ARM11, but the design also has much more extensive clock gating (turning off the clock to idle parts of the chip) than the ARM11. Since the A8 is newer it’s also going to be manufactured on a smaller manufacturing process. The bulk of ARM11 based SoCs used 90nm transistors, while A8 based SoCs are shipping at 65nm. ARM11 has started to transition down to 65nm, while A8 will move down to 45nm.
At the same clock speed and with the same L2 cache sizes, ARM shows the Cortex A8 as being able to execute 40% more instructions per second than the ARM11. That’s a generational performance improvement, something that can’t be delivered by clock speed alone, but the comparison is conservative. Cortex A8 designs won’t ship at the same clock speed and cache configurations as ARM11 chips; as far as I can tell, none of the major ARM11 based smartphones even had a L2 cache while Cortex A8 designs are expected to have one.
Furthermore, the ARM11 based smartphones were much lower in the frequency curve than the early A8 platforms. While a 40% improvement in instruction throughput is reasonable at the same specs, I would expect far larger real world performance improvements from a Cortex A8 based SoC compared to a ARM11 SoC.
Overall the Cortex A8 is much more like a modern day microprocessor. It’s still an in-order core, but it adds superscalar execution, a deeper pipeline, larger caches and a broader instruction set among other things. For any current high end smartphone there doesn’t seem to be a reason to choose the ARM11 over it, companies that insist on using ARM11 based designs even in 2009 are either not agile enough to implement a better chip in a quick manner or have no concern for performance and are more focused on cost savings. Neither option is a particularly good one and it is telling that the two manufacturers who seem to have gotten how to properly design a smartphone, Apple and Palm, have both opted to go with a Cortex A8 before most of the more established players.
A Call to Action
This leads me to a further point: we need more transparency in specs from smartphone manufacturers. The mobile phone market is all too shielded from the performance metrics and accountability that we’ve had in the PC space. When Intel was shipping Pentium 4s that performed slower than the Pentium IIIs they were replacing, we called them out on it. To this day, Apple refuses to talk about the processor in the iPhone 3GS. We get to hear all about what’s in the Nehalem Mac Pro, but the hardware behind the 3GS is off limits - despite the fact that it’s very good. This policy of not delivering specifics and a general unwillingness to talk about specs is absurd at best. It doesn’t take much more than a teardown and some homebrew code to figure out what CPU at what frequency is in any modern day smartphone; manufacturers should show pride in their hardware, or refrain from putting something inside a phone that’s they can’t be proud of.
What we need are cache sizes, clock speeds, full architecture disclosures. They don’t have to be on the phone’s marketing materials but make them accessible and at least some of the focus. These SoCs are so incredibly cool, they pack more power than the desktops of 10 years ago into a single chip smaller than my thumbnail - boast about them! Palm had a tremendous leg up on the competition with its OMAP 3430 processor, yet there was hardly any attention paid to it by Palm. I get that the vast majority of consumers don’t get, but those who do, would help tremendously if given access to this information. It’s something to get excited about.
And if the manufacturers won’t devote time and energy to this stuff, then I will.
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shank2001 - Wednesday, July 8, 2009 - link
Wow, talk about fanboyism. Let me get this straight, just because it is an article on the iPhone you feel there is no knowledge to be gleaned from an in depth article like this one? You would rather that no one wasted YOUR time with more articles about the iPhone, is that right?Well I, for one, learned a lot about smartphones in general, and not just the iPhone, but I especially loved this article getting into the nitty gritty of the iPhone.
The iPhone happens to be the best smartphone out right now... it might not be that way forever.... probably not. But I really like the direction Apple has been taking the smartphone market since they introduced the iPhone.
After years of using so called "smartphones" running windows mobile, and Pocket PC OS, etc. the iPhone is a breath of fresh air, and is the best smartphone I have ever owned.
It took Apple to force the market into making a true smartphone. I am glad that Palm has woken up from their stupor and come out with the Pre, I hope it is enough to turn around their fortunes.
Although, for me, it does not come close to the iPhone, but then again, most of the things that people seem to dislike, I actually LOVE... like the touchscreen typing for example. I am way faster typing on my iPhone than I ever was on my old smartphones with the chicklet keyboards. You just have to trust the autocorrection... once you learn to trust it, it is amazingly speedy. I no longer think twice about typing lengthy comments, such as this one, on my mobile phone any more! I just do it.
I have not yet heard a single viable reason to hate the iPhone, I think I will coin the term "Hateboy" to describe these boys, they certainly act like little kids filled with illogical hate. No matter if you love the iPhone or hate it though, you have to admit that the iPhone DID revolutionize how smartphones will work from now on. Kudos to Palm for recognizing this fact, hopefully others will as well. The consumer is who wins. And good for Apple for finally getting some recognition for their amazing products that have led the industry from the very beginning in so many ways.
Myrandex - Wednesday, July 8, 2009 - link
I will agree that the iPhone revolutionized the smartphone market, and it has done some pretty amazing things pretty darn well good, but I still have some key complaints that prevent me from EVER getting one, unless I see a fundamental change in Apple, which never happens.I will not buy a phone without a Standard connection interface. Just beause the iPhone is so popular doesn't mean that its proprietary apple dock connector is standard. Use standard mini USB or micro USB, or I won't touch it.
The touchscreen keyboard is good, and it does have a nice autocorrection, however I still prefer tactile feedback. As good as that keyboard is, there is no way that I can type without looking at the keyboard like I can on my current HTC smartphone. The keyboard on my phone is very comfortable, and I'd imagine that I am faster on that than some people on their computer keyboards. I've written 4 page emails from my phone with minimal effort. Some words I do not want autocorrected either. Acronyms many times are autocorrected by a phone, or names too, but many times I type these accurate and want them to stay that way, which on a software keyboard it slows you down because of verifying every key. Passwords also are like this, much slower when I'm using an iPhone compared to my smartphone (my fiancee has had both the 1st Gen iPhone and the iPhone 3G).
iTunes. I hate it. If I have to use it to use a phone, I will not use the phone. I want USB Mass Storage device access, I want to copy and paste music to my phone with my organization that I have already decided upon (whether or not tags are filled out correctly), and I don't want to have to sync it, just copy and paste what I want ONTO it and OFF OF it. I want to be able to copy music from my desktop onto my phone, plug my phone into my laptop, and copy it off of there. No profiles, no syncing, just pure file access. Thanks Microsoft, no thanks Apple.
Apple controlling application. This is corrected with jailbreaking, however I choose to vote with my wallet. I do not want to support a company that enforces this. Any application that I find that I want written for my phone, I will install on my phone. If it offends someone at Apple, then more power to me to want to install it on my phone. They will not make a dollar off of me for their stongarmed tactics.
There are probably other small things as well, but those are the major ones.
Jaosn
christinme7890 - Thursday, July 9, 2009 - link
if you are using your keyboard without looking you are most likely driving...a large portion of the accidents in cars happen because idiots are using their phones while driving. If you are doing this please stop or you could kill someone...no joke. I had a friend whose mom died because some idiot was text while driving.Second the itunes store is great for most people. Sure it doesn't have what you are looking for which is essentially a hdd but if it did then installing apps would suck. Why do you think the APP store is doing so well. Because it is a one stop shop. If I want a app, i go to the APP store and search for it and then click buy/install then sync and finished.
If I were using a WinMo device I would have to first find a list of all the devs that offer the app i am looking for and visit each and every webpage and sit through all the trash that they claim their software does. Pay attention to the finding devs that produce the app I am looking for...this takes longer than you think, especially for a n00b consumer. I also have to pay attention to which WinMo OS the app supports. Many times you need the newest and greatest OS in order for it to work and when I had a PPC, verizon didn't let me have the latest upgrade to the OS. I had to hack the PPC to allow me to use the updated OS. Then once I find the right dev, the software is usually a lot more expensive because there is no immediate competition. So i end up paying a ton of money. Sure you can find free software that does similar but it is not backed by a good support system...merely a live forum. Then once I find the software I have to give them my credit card information and email address. I will more than likely end up getting a email daily from the company about their new crappy software. Now that I have spent 20 minutes entering in my underwear size and preferred deodorant brand I download my app. After downloading it I have to go through their custom install procedure. Then I have to hope that it installed correctly. Then if I want it to sync with my desktop I have to buy another piece of software and install it and then figure out how to get it to sync. So much hassle and running around and time wasted. I can install 1 APP store app in less than a minute and it could be a game that is 100mb installed over wifi.
And about the music. Easy, you create what we call a playlist and drag and drop your music to the playlist and then sync your phone. You then play your playlist. Playlists are the same as folders. Not sure what is so difficult about that. Yes it would be nice to be able to connect my iphone to multiple computers to copy music but then you get what APPLE, along with every other business wants to avoid, and that is illegal sharing...duh. Just because people don't let you do anything you want doesn't mean you have to spew hate.
"If it offends someone at Apple, then more power to me to want to install it on my phone." This attitude is selfish and you are probably one of those people that loves to steal and pirate software all the time. You care little for the developer and only about yourself.
shank2001 - Wednesday, July 8, 2009 - link
That was a good rebuttal comment. I agree with some of what you say, especially the having to look at the keyboard. It is a definite necessity. I do miss that about an actual physical keyboard.iwodo - Wednesday, July 8, 2009 - link
This is no way fanaticism. But the general enthusiasm in terms of great technology improvement. It was an GREAT article from Anand. ( Yes i read it all, give me lollipops :D ) May be you are too young to figure this out.What drive you to see more SSD stuff? I want to see more SSD review too. Why? Simply because HDD it is the SINGLE bottleneck living inside out current Computer. Be it PC or Mac.
Upgrading from an Core2Duo to Core2Quad or even Core i7, Double Channel to Tri Channel, DDR2 to DDR3, 2GB to 4GB Memory, Geforce 9500 to GTX 290.... If you are not a gamer, any of these upgrades, or even if you do ALL of these upgrades, wont even land you a 10% overall performance increase in your 90+% day to day usage of computer. And even if they do show more then 10% in benchmark. There is a very small chance these are even human / user perceivable.
You will properly feel your system being faster if you reinstall Windows rather then upgrading your Hardware.
That is why SSD is so important and many people want one. It actually brings Significant perceivable speed improvement that is not seen FOR MANY YEARS.
The last time we seen any improvement was in the Pre Pentium 4 days...
iPhone, or Internet Mobile Devices, are in exactly the same period technology growth when PC were in the 486 and Pentium Era. 700% increase in Graphics, 100% increase in CPU speed? When was the last time you seen any of these in PC.
The next technological advance are in the Mobile / Phone space. They are the new PC. Just like how X86 manage to utilize its Desktop strength to gain market share in server space. May be ARM could finally dethrone x86. ( At least i hope so )
iGo - Tuesday, July 7, 2009 - link
Absolutely agree with cdrsft, never mind that guy. He probably read just the first and last page.Not always you come across the article which provides lot of information, on and off topic... and not always you find more than required information in an article which is actually useful and help you learn. Not to mention, all this written in absolutely enjoyable manner. :)
Thank you, Mr. Shimpi for such wonderful article... and many more before this.
cdrsft - Tuesday, July 7, 2009 - link
never mind that guy - your article was great and very helpful for those of us who want to understand more....... thank you!bowtech - Monday, November 8, 2010 - link
can u explain why cortex a8 did not beat arm 11 in almost any of these tests then.http://www.pengutronix.de/development/kernel/arm-b...MassiveTurboLag - Tuesday, April 30, 2013 - link
By the look of that video screenshot Anand drives a Porsche Cayenne. I hope he didn't see Jeremy Clarkson's video on it.sirk - Friday, June 11, 2021 - link
nice read