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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  • metafor - Tuesday, November 01, 2011 - link

    Fair enough. But that really doesn't take away from the fact that the A5 is a relatively large chip and from the UV-scans of it, looks to use quite a bit of that die area for the GPU.

    I don't know if a similar scan has been done of Exynos but one can't safely say both chips are far bigger than SoC's traditionally used in this space.

    Though that trend appears to be moving forward with MSM8960 and Tegra 3.....
    Reply
  • PeteH - Tuesday, November 01, 2011 - link

    That leads to an interesting question: will Apple always have the largest SoCs, and thus (most likely) the highest performance in the mobile space?

    The reason I could see this happening is that Apple doesn't have to sell their SoC's at a profit, so they're paying closer to cost for the chips (excluding the fab mark up). Other manufacturers (like NVIDIA) need to make a profit on their chips.
    Reply
  • name99 - Thursday, November 03, 2011 - link

    "I'm not entirely sure why they had to use such a powerful GPU, though. "

    And you know EXACTLY how Apple use the GPU do you?
    Does Siri run some of its workload on the GPU? Does the faster camera stuff (eg fast HDR) run on the GPU? Does Apple already have OpenCL running (for internal use) on iOS?
    Reply
  • doobydoo - Friday, December 02, 2011 - link

    He must be an Android fan.

    Androids new marketing campaign will offer a revolutionary 'new' feature - the ability to have a slower GPU than other phones!!!

    Magical.
    Reply
  • InternetGeek - Monday, October 31, 2011 - link

    They might give AMD and nVidia a run for their money if they ever tried creating desktop products... Reply
  • sprockkets - Monday, October 31, 2011 - link

    Kyro 2 was a good chip, but obviously went to focus on the desktop market. Reply
  • tipoo - Tuesday, November 01, 2011 - link

    Maybe, but there's a reason such crossovers usually take so long. Look at Intel trying to get into this space, I don't doubt they will be good at it but it takes years of development. Imagination specializes in low power, it would take lots of development effort to get into the high power desktop game. Reply
  • _tangent - Tuesday, November 01, 2011 - link

    I think this might be intentionally ironic given they got out of that game a long time ago :P

    On point though, anyone would given AMD and nVidia a run for their money with the right up front cash and expertise. I imagine the barrier to entry into that market is truly colossal though. Point is, the SGX543 MP2 is no evidence one way or the other.
    Reply
  • lurker22 - Monday, October 31, 2011 - link

    Before buying many people who got a 4s on AT&T told me how much better it was than their prior AT&T iphones.

    Anand, thanks for confirming and explaining the reasons.
    Reply
  • LordSojar - Monday, October 31, 2011 - link

    Can't we have reviews as detailed as this for the really big name Android phones? They are always far less detailed and lack a lot of the testing put into this.... thing....

    Apple makes a few adjustments, tweaks a few things, adds in the same processor that's in the iPad 2, and we have a highly detailed, scientific review that covers every single aspect, even if said aspects are the same. Samsung releases a new phone that has overall better features, faster CPU, faster NAND, a different and arguably better (or at least equal) screen, and mums the word?

    The bias is getting a bit out of hand at this point... We get that you're big time Apple fans, but cmon... At least do a major review of this caliber for the Droid RAZR and the Samsung Galaxy Nexus and the Galaxy S2 Skyrocket (LTE on AT&T!). Even if you combine them into one review, just make it THIS detailed for once instead of giving Apple the huge, super detailed ultra review!
    Reply

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