The Memory Interface

Most SoCs deployed in smartphone designs implement a package-on-package (PoP) stack of DRAM on top of the SoC package. As its name implies, PoP refers to the physical stacking of multiple packages and not layering of raw die. The SoC is typically the lowest level with its memory bus routed to pads on the top of the package. A DRAM package is then stacked on top of the SoC. Avoiding having to route high-speed DRAM lines on the PCB itself not only saves space but it further reduces memory latency.


An example of a PoP stack

The iPhone has always used a PoP configuration for its SoCs and Apple has always been kind enough to silkscreen the part number of the DRAM on the outer package of the SoC. In the past we've seen part numbers from both Samsung and Elpida on Apple SoCs. As both companies can provide similarly spec'd DRAM it makes sense for Apple to source from two suppliers in the event that one is unable to meet demand for a given period.


iPhone 4 mainboard, courtesy iFixit

If we look at iFixit's teardown of the iPhone 4 we see the following DRAM part number: K4X4G643G8-1GC8. Most DRAM vendors do a pretty bad job of providing public data about their part numbers used in chip stacks, so we have to do a little bit of inferring to figure out exactly what Apple used last generation.

The first three characters tell us a bit about the type of DRAM. The K means it's memory, the 4 tells us that it's DRAM and the X tells us that it's mobile DDR (aka LPDDR). The next two characters tell us the density of the DRAM, in this case 4G is translated literally to 4Gbit or 512MB. Characters 6 and 7 are also of importance - they tell us the DRAM organization. Samsung's public documentation only tells us that 16 refers to a 16-bit interface and 32 here would mean a 32-bit interface. Based on that we can safely assume that the 4Gbit DRAM on the A4 is 64-bits wide. In the mobile world a 32-bit interface typically refers to a single channel, which confirms the A4's DRAM interface is two 32-bit channels wide.

The last two characters in the part number, C8, tell us the source clock frequency of the DRAM. Samsung's datasheets tell us that C8 corresponds to a 5ns cycle time with a CAS latency of 3 clocks. Taking the inverse of that gives us 200MHz (frequency = 1 / clock period). Remember, we're talking about double data rate (DDR) SDRAM so data is transferred at both the rising and falling edges of the clock, making the effective data rate 400MHz.

All of this tells us that the iPhone 4's A4 SoC has a 64-bit wide LPDDR1 memory interface with a 400MHz data rate. Multiply all of that out and you get peak theoretical bandwidth of 3.2GB/s. DDR memory interfaces are generally 80% efficient at best so you're looking at a limit of around 2.5GB/s. To put this in perspective, the A4 has as much memory bandwidth as the original AMD Athlon 64 released in 2003.

iPhone 4S mainboard, courtesy iFixit

With the A5 Apple definitely stepped up the memory interface. Once again we turn to iFixit's teardown of the iPhone 4S to lift that oh-so-precious part number: K3PE4E400B-XGC1.

The K once again tells us we're dealing with Samsung memory, while the 3P reveals there are two mobile DDR2 with 4n prefetch (aka LPDDR2-S4) DRAM die on the package. Why not a 4 this time? Technically the 4 refers to a discrete DRAM while the 3 implies a DRAM stack, obviously both are stacked DRAM so I'm not entirely sure why there's a difference here. Each of the next two E4s tell us the density of the two DRAM die. Samsung's public documentation only goes up to E3 which corresponds to a 1Gbit x32 device. Given that we know the A5 has 512MB on-package, E4 likely means 2Gbit x32 (256MB 32-bit). There are two E4 die on package which makes up the 512MB 64-bit DRAM stack.

Once again the final two characters reveal the cycle time of the DRAM: 2.5ns. The inverse of 2.5ns gives us a 400MHz clock frequency, or an 800MHz data rate (source clock frequency is actually 200MHz, but with a 4n prefetch we can transfer at effectively 800MHz). Peak bandwidth to the A5 is roughly double that of the A4: 6.4GB/s. This is as much memory bandwidth as AMD's Athlon 64 platform offered in late 2004, just 7 years later and in a much smaller form factor.

The doubling of memory bandwidth requires a sufficiently large workload to really show it. We see this in Geekbench's memory bandwidth results where the A5 doesn't appear to offer any more bandwidth than the A4 in all but one of the tests:

Memory Bandwidth Comparison - Geekbench 2
  Apple iPhone 4 Apple iPhone 4S
Overall Memory Score 593 700
Read Sequential 318.7 MB/s 302.3 MB/s
Write Sequential 704.9 MB/s 809.2 MB/s
Stdlib Allocate 1.55 Mallocs/sec 1.55 Mallocs/sec
Stdlib Write 1.25 GB/s 2.54 GB/s
Stdlib Copy 724.5 MB/s 490.1 MB/s
Overall Stream Score 280 281
Stream Copy 413.5 MB/s 396.4 MB/s
Stream Scale 313.3 MB/s 317.4 MB/s
Stream Add 518.0 MB/s 527.1 MB/s
Stream Triad 363.6 MB/s 373.9 MB/s

Memory bandwidth tests are extremely sensitive to architecture optimizations, particularly for single threaded tests like these so I wouldn't read too much into the cases where you see no gains or a drop.

The increase in raw memory bandwidth makes a lot of sense. Apple doubled the number of CPU cores on the A5, with each one even more bandwidth hungry than the single A4 core. The 4x increase in GPU compute combined with an increase in clock speeds give the A5 another big consumer of bandwidth. Add things like 1080p video capture and the memory bandwidth increase seems justified.

Looking back at the evolution of the iPhone's memory interface gives us an idea of just how quickly this industry has been evolving. Back in 2007 the original iPhone debuted with a 16-bit wide LPDDR-266 memory interface connected to a meager 128MB of DRAM. The 3GS delivered a huge increase in memory bandwidth by doubling the interface width and increasing the data rate to 400MHz. Scaling since then has been even more dramatic:

Memory capacity on the other hand has seen more of a step-function growth:

By using a mobile optimized OS Apple has been able to get around large memory requirements. The growth pattern in memory size partially illustrates the lag between introducing faster hardware and developers building truly demanding applications that require that sort of performance. Apple was able to leave the iPhone 4S at 512MB of RAM because the target for many iOS apps is still the iPhone 3GS generation. Don't be surprised to see a move to 1GB in the next iPhone release (we won't see 768MB due to the dual-channel memory requirement) as the app developer target moves to 512MB.

The A5 Architecture & CPU Performance GPU Performance Using Unreal Engine 3
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  • medi01 - Tuesday, November 08, 2011 - link

    To care about "improved vibration" I have first to be somehow concerned with current "unimproved" vibration. Which I don't. And I don't know any person that is. Reply
  • doobydoo - Friday, December 02, 2011 - link

    You don't have to be concerned with something to care about getting an improved version.

    For example, I may be quite happy to work for a salary of £1,000,000 a year, but that doesn't mean I wouldn't care if my salary was doubled.
    Reply
  • Grandpa - Wednesday, November 09, 2011 - link

    Anyone heard of staticgate yet? How come nobody gives a review about the static issue on the iphone? Reply
  • mymomentummedia - Thursday, November 10, 2011 - link

    Here is a funny iphone 4s review... iphone 4s plus a 50.cal Barret sniper rifle. what do you get?

    http://www.youtube.com/watch?v=RsVEbRt6g94
    Reply
  • chillstatus - Monday, November 14, 2011 - link

    I made sure to switch my dataplan over to the "Unlimited Data Plan for iPhone 4S" and when I run the field test, it still says it's using wap.cingular. I live in Los Angeles, and am wondering how I can get it to use the faster network. Anyone else have this issue? Reply
  • rhch - Sunday, February 05, 2012 - link

    I just found out that my 4S wifi range is not as good as my old iPhone 4 or iPad 2.
    4S gets much earlier "no connection" with Safari, Appstore etc when i take distance to my hotspot.
    Can someone confirm this?
    Reply
  • AlexD - Monday, February 06, 2012 - link

    With this commend I ask for help to find a developer for an iPhone-App that enables to use the 4S for the catching of light below 380nm (UV) and above 780 nm (NIR). The purpose is to identify light emission of materials (inks, plastics) outside the visible range and to visualize it with the smartphone. Even a slight range (e.g. >320nm or <850nm would help me.
    I would also be very greatful if somebody could explain what range of wavelength the 4S camera sensor is catching, if there are filters which can be removed or exchanged or the like to enable the above function. Thanks in advance for your support.
    Reply
  • meace - Sunday, February 12, 2012 - link

    Is the POP memory attachment to the A4 permanent?
    As in... could that pop ddr sdram of 128 *2 be replaced with 256 *2 ?
    I'm guessing there's no way that it's coming apart without damage
    (maybe even in a clean room) but I'll ask.
    Reply
  • Rizi - Tuesday, July 17, 2012 - link

    At first glance <a href="http://cellocean.com/iphone-4s-specifications-2210... phone</a> seems identical to its predecessor, except for the distinctive top-mounted incisions which signifies that a new design of the antenna is lying in wait inside. However, when you switch on the phone you will find that things are different. The 4S does not require activation via the computer except that you are restoring the data from a pre-existing iTunes backup. The phone can be set up as a new phone or restored from an iCloud account. In the event that you do not currently have an Apple ID, there is the option of creating one. A number of iPhone 4 users will ask themselves why they would need the iPhone 4S when the iOS 5 upgrade, which is free of charge, will equip their existing phone with the majority of the new features that are in excess of 200. However, it is important to note that the iPhone 4S is the best phone that Apple has built to date. The A5 dual-core processor, which makes things obviously nippier, is new and it is a feature that is also incorporated in the iPad 2. Web pages load quicker, apps launch faster, multi-tasking is a great deal more fluid and Pages and other resource-hungry apps now permit you to edit documents with no lag. Another great feature of the A5 processor is that it allows you to mirror the content of the iPhone 4S over AirPlay. When operated wirelessly, the iPhone 4S can push out up to 720p to the Apple TV. When connected to HDMI, the number will increase to 1080p. The ease of pushing content of the small screen to the big screen of the fly is an unexpected delight, particularly when you begin to rotate and zoom. In addition, it is something to brag about to owners of iPhone 4; the earlier model just cannot handle with the demands of this technological wizardry. The two antennas that run across the top of the iPhone 4S can both receive and transmit data, enabling quicker 3G connections. Apple has asserted that highest speed of download by way of HSDPA is 14.4Mbps, which is two times as quick as the iPhone 4. A major overhaul has been done to the camera, it now has the capacity to record 1080p video and take eight-megapixel stills. In addition, face detection is an upgrade and it works really well. Reply
  • vitaprimo - Saturday, August 18, 2012 - link

    I don't think it's the phone's fault that much. I have two iPhone 4s (not 4Ss) on different carriers (Telcel and Iusacell) and the never lose signal, no matter how tightly I grip them and I don't use cases. Now, one of the carrier is relatively new in my state and out of the major cities I've had a few dropped calls but they always connect back. I use to have iPhone 4Ss and I noticed no difference; I lost a bag with the phones in it so I'm using my old 4s until the sixth comes out--hopefully in less than two months.

    This whole antenna issue seems to just be affecting carriers in the US--that being AT&T.
    Reply

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