You are confused because Apple displays the memory config rather badly in System Profiler. It is true that Intel has had the same memory config since P4. So the easiest way I can convince you is to pull up a motherboard manual.
http://vfiles.cizgi.com.tr/cizgi/pms_dosya/153/p5wdhdeluxe_en.pdf
Page 2-14.
This is just one I know on top of my head is very clear about memory installation. You can check other motherboard manuals as well and they will be the same.
Now what Apple did was rearrange the order the slots are displayed in System Profiler. To give you an example 16GB of RAM in a 4 slot system will be seen like this:
bank 0/dimm0 - 8GB
bank 1/dimm0 - 8GB
bank 0/dimm1 - empty
bank 1/dimm1 - empty
In reality on a 2012 iMac the slots are 8/_/8/_ if you are to look at how the memory is installed. That is where the confusion lies.
Well I'll be...
So I just went through the process tonight doing EXACTLY what we're all talking about.
The gentelman here is correct...despite what I thought I knew about Dual Channel Setups.
Let me explain with layout to hopefully identify the issue.
When I opened the case on the 27" there were four slots:
----------------- Slot 4
----------------- Slot 3
----------------- Slot 2
----------------- Slot 1
And when I looked the factory installed RAM was configured as follows:
----------------- Slot 4
============= Slot 3
----------------- Slot 2
============= Slot 1
(where ==== is an installed RAM chip amd ----- is an empty slot)
Taking the advice from this thread I then replaced the factory RAM with my Crucial RAM in Slot 1 and Slot 3, and put the factory RAM back in slots 2 and 4.
============== Slot 4 (4G DIMM from Apple)
============== Slot 3 (8G DIMM from Crucial)
============== Slot 2 (4G DIMM from Apple)
============== Slot 1 (8G DIMM from Crucial)
According to the advice of Crucial and Apple (as posted on this thread) this is the EXACT configuration that is recommended.
I then booted the system and noted that it showed the full 24G of RAM.
HOWEVER when I go into About this Mac.., and then "More Info..." and then click on the "Memory" tab, I get exactly the picture posted by surripere a few posts below.
AND when I go into system profiler (Back up, Overview, System Report) and click Memory, I also get the second picture as surripere...Bank 0 and 1 of DIMM0 are the same chip (8G) and Bank 0/1 of DIMM1 are the same chip (4G).
If you ASSUME that Apple did not mis-install EVERYONE's RAM chips (i.e. all 27" iMacs are being shipped in single channel mode and not dual channel mode) and that all by default installations are in Dual Channel mode, then the 8/8/4/4 configuration in System Profiler IS what you will get when you install:
======4G======= Slot 4
======8G======= Slot 3
======4G======= Slot 2
======8G======= Slot 1
(This ASCII art is drawn as if you are looking at it with the back open and the RAM tray pulled down.) and that this IS the proper Dual Channel mode setup.
Further, if you look at lixu's Motherboard Manual link it becomes clear that in fact he's right (despite what I thought I knew), you actually are installing 8+4 in one channel and 8+4 in another channel. The important part however is that it must be 8+4 / 8+4 and not 8+4 / 4+8 or 8+8 / 4+4.
Sir: While I am having a hard time believing it, I will, at least for the moment, stand corrected on my prior post...you are in fact, right.
Now for the even more confusing question: Gogol's post seems to contridict this as the dual channel setup. But that is only possible if Apple is installing all RAM chips in Single Channel mode from the manufacturing plant, which I have a VERY hard time believing.
Is there a condition under which Single Channel mode will provide a better access time than Dual Channel?
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