Model A1419 / EMC 2806 & EMC 2834/ Late 2014 or Early 2015 or late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 Processor / Retina 5K display

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How to couple RAM Modules in iMac 27?

Hi, I have just upgraded my iMac by adding 16GB (2x8GB) to the pristine 8GB (2x4GB). Since I know, but I may be wrong, that modules should be coupled, how do I have to do it?

By now I have made a try, putting physically close, in the 4 available trays, similar modules (4GB with 4GB, 8GB with 8GB).

This is the configuration I got from system info (RAM modules coupled in the same bank)

BANK0/DIMM0 8GB

BANK1/DIMM0 4GB

BANK0/DIMM1 8GB

BANK1/DIMM1 4GB

Is this correct, or should I insert them physically alternated, so they I have the same modules coupled in the same DIMM?

Thanks for your help

Francesco

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4 Answers

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Rule of thumb is to populate the bank with the same module. Which it appears you did here.

Bank0 8GB (times 2)

Bank1 4GB (times 2)

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Thanks, I had doubts because the pristine configuration involved two modules in two different banks. However it all works great now, so following the rule of thumbs gave good results!

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Unless RAM layout changed between the 2015 and the 2017 iMac, I'm pretty sure this is wrong.

I just upgraded the memory in my 2017 iMac and tests with Geekbench show that the modules should indeed be paired by DIMM slot, not by bank.

The difference is significant too. With the same modules in different banks I get a memory bandwidth of 30 GB/s, with the modules in the same bank (but different DIMM slots) memory bandwidth drops to 16 GB/s.

(Which pair of modules goes in which DIMM slot does not seem to make any appreciable difference. I ran the test just once each because numbers were so close. )

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Correct! You want the SAME sized modules within the BANK.

Sadly, you have different sized modules within the BANK:

  • BANK0/DIMM0 has a 4GB
  • BANK0/DIMM1 has a 8GB

Where you want:

  • BANK0/DIMM0 will have a 4GB
  • BANK0/DIMM1 will have a 4GB

The other bank would be setup:

  • BANK1/DIMM0 will have a 8GB
  • BANK1/DIMM1 will have a 8GB

Or you could go the other way were Bank 0 has the larger modules.

Note: The number after the word BANK thats the bank number NOT the DIMM number.

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Thanks a lot. I was very unsettled. This morning I talked to apple and they told me this way is the right one (slot 1+3 each 4GB, slot 2+4 each 8GB)

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Sorry, I may be wrong, but I was told to use the same amount of RAM/memory in each channel/bank, 12 in each. the pristine configuration (4GB Slot 1 and 4GB Slot 3 - iMac Late 2015) would validate this (regarding to dual channel).

BANK 0/DIMM0:

Größe: 4 GB

Typ: DDR3

Geschwindigkeit: 1867 MHz

Status: OK

BANK 0/DIMM1:

Größe: 8 GB

Typ: DDR3

Geschwindigkeit: 1867 MHz

Status: OK

BANK 1/DIMM0:

Größe: 4 GB

Typ: DDR3

Geschwindigkeit: 1867 MHz

Status: OK

BANK 1/DIMM1:

Größe: 8 GB

Typ: DDR3

Geschwindigkeit: 1867 MHz

Status: OK

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The way you have your RAM installed you have lost interleaf services. While it will work this way it is not the most efficient.

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Is possible that same RAM sizes should be in DIMMs and not in BANKs? Maybe apple just call them differently. Because here is screenshot how looks 12GB from factory in iMac.

Block Image

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No, the word Bank is a JEDEC Standards term, and is using it correctly!

Your iMac uses a dual channel architecture to gain the most performance by Interleaving the same sized RAM modules. So when you have two of the SAME modules within a Bank you gain the best performance!

Yes, Mixing your RAM willy nilly will also work!

But, now you are forcing the computer to treat the RAM as a Single Channel AND running at the slowest common RAS & CAS speed of all of the RAM modules you have installed!

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So I would reseat the modules as I outlined BANK 0 have the same sized modules (x2 2 GB) and BANK 1 the same sized modules (x2 4 GB).

Clearly the person who upgraded your system (it was not Apple) didn't know what they where doing ;-} Apple never shipped a 12 GB system. The dealer you bought your system from added the RAM.

Now one last point! Don't mix different makes or models of DIMM modules that are the same size! You want what we call a Matched Pair Here you can learn more soon that and RAS & CAS Memory DIMM's

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Dear Dan,

I fully understand what is Dual channel and what advantages it has. What I don't understand why Apple is not using dual channel. For example I just upgraded old 2009 late iMac (RAMs from bottom), and it has 2 RAM modules "horizontally", and 2 slots above (again horizontally) was empty. If you look at any video on youtube, this is original position from factory, which is in OS represented as Bank0/Dimm1 and Bank1/Dimm1 - so different Banks. Thoughts?

https://youtu.be/Z4UqhXIcEQ4?t=2m5s

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@peterk2 - Take a look here for more details: Guide to RAM Memory Channels as Fast As Possible

Sadly, there is a lot of contradicting information on how the DIMM's need to be installed. So I understand your confusion.

As to the layout of what your 2009 iMac had I don't have one to reference so I can't tell you if the physical layout of the DIMM's (using two 2 GB DIMMS) where or were not installed correctly. The marking of the slot should tell you the layout if a Bank was across or one on top of the other. Its possible the person assembling the iMac messed up.

As I mustily maxed out our iMac or Mac Mini's with the same DIMM's I never needed to worry about it as I was always putting in four matching 4 GB DIMM's.

Why don't you prove to your self what is the best setup. Since you have a 2009 system with only 4 GB run a timed RAM test with the DIMMS across the banks and within the Bank to see which runs faster.

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I had a problem with my iMac 27 2014.

I have 2 DIMMs 4GB and 2 DIMMs 8GB.

I put (by myself) the modules with the same size on the same banks.

After having this issues (my iMac freezed every once in a while) I send my iMac to Apple Store and after several days they found the memory was not in the correct position and they changed it so that the same size are on the same DIMM.

I never had a problem since then

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