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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

Contestado! View the answer Yo también tengo este problema

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6 Respuestas

Solución Elegida

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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Andreas is correct! Thanks to his comment, I changed out the RAM order per his suggestion, and my brand new iMac runs geekbench MUCH faster now. (so to reiterate, RAM pairs should NOT be side by side, they should be staggered, ie slot 1 and 3 for pair 1, slot 2 and 4 for pair 2.

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@danj It might be helpful to others if you delete this post, as it is incorrect and with your reputation on this site, it looks like people are using your incorrect advice instead of Apple's. Please see my other comment that according to JEDEC standards a bank is not a collection of memory modules; a memory module (physical or logical) is made up of banks. So with 2 physical sticks of ram making up one logical DIMM, the first stick gets the bank 0 (BA0) address and the second stick the BA1 address. Memory_bank and Interleaved_memory might help explain better.

TLDR: it makes sense to group sticks of identical ram so they have the same DIMM numbers (and one will be assigned bank 0 and the other bank 1).

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@Adam Dunn - You are correct this is a mess! But I have it right, Apple confused us with odd terms!

A Bank must hold the same RAM chips. Now let's look at the time point of this post back in 2015 where at the time SO-DIMM's where still setup as discreet units Not seen in the context of a channel. To add to this Apple was clear as mud! So we didn't have any guidance from them! At least Apple learned, look at the new Mac Pro memory covers all neatly laid out!

So If you look as the older SIP/DIM chipped logic boards you'll find the markings In the context of a collection of same RAM within the bank. So BANK0 would be Set-A and BANK1 would have Set-B of the discreet ram chips. Even the original Macintosh 128 which used DIMM's was also marked in the same manner.

Now we know the word DIMM here should have been CHANNEL (Starting with the 2011 iMac models) So all RAM with in the given DIMM(number) needs to be the same!

The BANK is within the given CHANNEL, so reading left to right the proper labeling should have been:

CHANNEL0 (DIMM0) - BANK0 & BANK1

CHANNEL1 (DIMM1) - BANK0 & BANK1

Thanks Apple for not using the correct terms and needing to comprehend your mud from right to left!

- de

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Respuesta Más Útil

After reading contradictory answers, I decided to make some tests to determine what the correct way to add (2 x 8 GB) RAM to existing (2 x 4 GB) RAM. And this are the results I found.

Tested on iMac (27-inch Mid 2011) Intel Core i7 - 3.4 Ghz with High Sierra:

  • With the following configuration from System Information / Memory (by BANK): ***

BANK 0/DIMM0: 4 GB - DDR3 - 1333 Mhz

BANK 1/DIMM0: 8 GB - DDR3 - 1333 Mhz

BANK 0/DIMM1: 4 GB - DDR3 - 1333 Mhz

BANK 1/DIMM1: 8 GB - DDR3 - 1333 Mhz

Geekbench 4 shows and memory bandwitdh at 8.76 GB/sec for Single-Core and 8.61 GB/sec for Multi-Core

(Full results at : https://browser.geekbench.com/v4/cpu/101...)

  • With the following configuration System Information / Memory (by DIMM): ***

BANK 0/DIMM0: 4 GB - DDR3 - 1333 Mhz

BANK 1/DIMM0: 4 GB - DDR3 - 1333 Mhz

BANK 0/DIMM1: 8 GB - DDR3 - 1333 Mhz

BANK 1/DIMM1: 8 GB - DDR3 - 1333 Mhz

Geekbench 4 shows and memory bandwitdh at 15.4 GB/sec for Single-Core and 16.6 GB/sec for Multi-Core

(Full results at : https://browser.geekbench.com/v4/cpu/101...)

The good configuration to exploit the Dual Channel to get the best performance is “by DIMM” meaning to have same RAM sizes in same DIMM number.

Daniel

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Comentarios:

Daniel (MQA) is correct. The identical matching ram should be paired by DIMM number and NOT Bank number. I ran the Geekbench test in both memory configurations, confirming similar findings. Thanks Daniel.

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This is the correct answer. I did the same testing with my new 2x16GB modules in my iMac 27" (2017)

Here's the memory bandwidth result for Geekbench 4

BANK 0/DIMM0: 16GB

BANK 0/DIMM1: Empty

BANK 1/DIMM0: 16GB

BANK 1/DIMM1: Empty

Single-core: 28.0 GB/sec

Multi-core: 29.9GB/sec

-------------------------------

BANK 0/DIMM0: 16GB

BANK 0/DIMM1: 16GB

BANK 1/DIMM0: Empty

BANK 1/DIMM1: Empty

Single-core: 16.6 GB/sec

Multi-core: 16.0 GB/sec

-------------------------------

With this I can confirm that the RAM modules should be installed pair-wise per DIMM and NOT per BANK on iMac 2017.

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On the 2019 iMac, the slots are labeled differently:

BANK 0 / Channel A - DIMM 0

BANK 1 / Channel A - DIMM 1

BANK 2 / Channel B - DIMM 0

BANK 3 / Channel B - DIMM 1

Best performance is when BANK 0 matches BANK 2, and BANK 1 matches BANK 3.

- de

Don't you just love Apple!

First misusing the word Bank! And now finally using a better word Channel! But still making it confusing!

Populate same sized SO-DIMM's across the different channels.

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@danj It might be helpful to others if you delete this post, as it is incorrect and with your reputation on this site, it looks like people are using your incorrect advice instead of Apple's. Please see my other comment above that according to JEDEC standards a bank is not a collection of memory modules; a memory module (physical or logical) is made up of banks. So with 2 physical sticks of ram making up one logical DIMM, the first stick gets the bank 0 (BA0) address and the second stick the BA1 address. https://en.wikipedia.org/wiki/Memory_geo... and https://en.wikipedia.org/wiki/Interleave... might help explain better.

TLDR: it makes sense to group sticks of identical ram so they have the same DIMM numbers (and one will be assigned bank 0 and the other bank 1).

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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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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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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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On my 2014 iMac retina 27 Apple told me to couple the modules by dimm. I tried the other way but I had a lot of issues!

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I found it really easy to upgrade the RAM on my iMac.

First, you can assume the factory installed ram is installed correctly so if you are not intending to swap those then do not remove them. They should both be in DIMM 0.

If you are filling two empty slots on a 4 slot iMac, just shove in a new matching pair and reboot. It should show the original pair occupying DIMM 0 and the new pair occupying DIMM 1 which is how you want it. You can disregard what banks they are in.

If you also want to replace the original memory, then remove the original SO-DIMMS and replace them with a matching pair. So, if you are looking at the 4 slots you have a top row and a bottom row (regardless whether your imac is lying face-up or face-down). You need to have a matching pair in the top slots and/or a matching pair in the bottom slots. You do not need to match a top slot with a bottom slot.

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I tried the same size = same DIMM configuration (as above comments suggested it would be quicker) and my machine wouldn’t boot. Turns on but screen stays blank. Had to revert to installing by BANK. Any suggestions as to why?

iMac 27’ retina late 2015

2x stock 4GB DDR3 1867Mhz

2x OWC 16GB DDR3 1867Mhz

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Did you tried to inverse the DIMM ? I have already experienced solving the problem by inversing the DIMM.

For example if you have :

DIMM 0 - 4GB

DIMM 1 - 16GB

Then try

DIMM 0 - 16GB

DIMM 1 - 4GB

or vice-versa.

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Ha! Well I thought I had but obviously not. Just tried it again and it’s worked this time. Thank you! :)

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Memory bandwidth at previous config was 12.2 single core and 11.8 multi core. It’s now up at 22.6 single and 22.4 multi. So a significant improvement.

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