How to couple RAM Modules in iMac 27?

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!

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

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

@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.

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

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

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)

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!

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.

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.

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

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.

I have the 2020 27" iMac with two 4 GB RAM (2667MHz) from Apple installed in slots 1 (Channel A/DiMM1) and 3 (Channel B DiMM1). When I followed the instructions, and installed two OWC 16GB RAM chips (2667MHz) in the free slots (2 and 4), it told me that my Mac had 4 memory slots each of which accepts a 2133 MHz DDR4 memory module - which is clearly incorrect! When I installed the 2 x 4GB Apple memory in slots 1 & 2 and the 2 x 16GB OWC in memory slots 3 & 4 it (correctly) told me that my Mac had 4 memory slots each of which accepts a 2667 MHz DDR4 memory module. So now I am completely non-plussed! I dint use Geekbench or anything but if the Mac thinks it has 2133 MHz memory modules, surely this is a bad thing????

@peter: cleverest answer ever. Had the same problem: 2x4 stock in 2&4, 2667 mhz (of course, from factory). Adding identical dimm’s from a 2019 27” in 1&3, got 2133 mhz slowdown. Placed stock in 1&2 and remaining in 3&4 solved the problem. My final opinion is: why Apple’s KB about ram installation in (at least) the 27” is not clear???