Why is smartphone RAM always layered with the CPU?

I've been poking around at the iFixIt teardowns and I've noticed that RAM is almost always layered on top of the CPU. Why is that?

What I've been looking at:

Nexus 5X:
"Samsung K3QF3F30BM-QGCF 2 GB LPDDR3 RAM, with the Qualcomm Snapdragon 808 layered beneath"

The iPhone 6:
"Apple A8 APL1011 SoC + SK Hynix RAM as denoted by the markings H9CKNNN8KTMRWR-NTH" (Talking about the CPU/RAM section)

And the LG G4:
"The Hexa-Core, 1.8 GHz Qualcomm Snapdragon 808 SOC is layered beneath the RAM."

Sorry if this is an amateur question, I'm just curious.

My main guess, Space, and power.

wiring things separately takes more space, mainly for the component, but also for additional power and data traces.


Imo it's just so they can sell them more expensive. Mostly the Iphones that everything is built in and you can't even add an SD card. That's lame. I switched to HTC One M8 Android.

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It's actually cheaper to produce ASIC CPUs with integrated RAM. The absence of SD cards in phones are completely up to the manufacturer and has nothing to do with how much RAM the ASIC CPU has. Phone doesn't have an SD card? Probably the manufacturer wants to create planned obsolescence to make you have to upgrade to a more expensive phone with more on-board Flash storage.

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Which they almost all do now. HTC seems to be the last holdout, but they made the non-removable battery mainstream, so they're no champions of user freedom.

LG still has removable batteries and SD cards.

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That's my point with the Iphone

Tad bit off-topic, but there are worse things to consider with the Iphone. Like the walled off OS that doesn't let you do anything and the shit prepackaged application store you are stuck with and cannot remove.

Yeah, but to be fair, a lot of android phones have the same thing. I am looking and you, Samsung.

Most of that is carrier based bloatware on Android. And its un-removable garbage that takes up space in your flash memory. Verizon and AT&T are notorious for this.

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Also. would the GPU not benefit from the proximity with speed? Gotta keep the UI snappy. But I am just guessing.

Complains about price
buys a <$700 phone

4chan is that way.

Not if you root. Of course, casual users aren't going to. It is scary the first time you do it.

Google is making it harder to root phones with the newest installment of Android, without a wipe and install, Android 6.0 cannot be rooted by the usual means as of right now.

more space for other components is my guess. And keeping the ram closer to the cpu would mean less latency and power draw?

That is correct. Shorter interconnects are critical for timing, particularly with ever faster ram. In fact keeping the interconnects between high bandwidth, low latency devices on a circuit board is crucial. It also has the benefit of saving materials, space and power. It also serves to reduce the footprint of the chip (surface area) since the ram interconnects can go out the top direct to the ram chip and the other inteconnects out the bottom.

The only area where it can sometimes be a disadvantage is with heat output, however with most embedded devices this is rarely a problem due to their inherent low power design.

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I meant the price for the Iphone versus the price for the same technology you'll pay less. Plus I didn't pay 700$ straight out of my pocket. I have an open contract that adds 10$ to my actual plan that cost 35$. So I pay around 60$ taxes included per month.

Space and power are certainly factors. I also imagine it has to do with speed. Putting RAM off SoC would inherently make it slower and make the cost to access it higher (slower speed) versus on SoC.

Plus it wouldn't be a "System on a Chip" if it didn't have ram on board.

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That's straight out of your pocket, every month. $10 *24 = $2400

24 is just the standard amount for a contract, so I'm assuming that amount. Anything less is still too much, though.

I don't have a contract. If I want to cancel the deal I have with them i'd only have to pay the balance of 240 maximum since they remove a few dollars each time I pay my bills. When i changed my cellphone, my balance was 20$ positive so it actually cost me 220$ for 24 months. I can cancel my contract and pay the rest right away though. When I check on the internet, I see that HTC one M8 cost around 500$. I consider it's not much paid. If I wanted to get an Iphone 6 for example, I'd still have to pay like 270$ right now and have the rest on a balance for 21$/month for 24 months. THIS. is alot.