Sidebar Sidebar
Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.

27" RAM Upgrade - why not 24GB?

I

Illydth

macrumors newbie
Aug 16, 2012
3
0
lixuelai said:
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.
Click to expand...

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?
 
Last edited:
Confuzzzed

Confuzzzed

macrumors 68000
Aug 7, 2011
1,630
0
Liverpool, UK
Ok, I will live up to my tag and declare myself well and truly confused! So, given the lack of consensus in the matter I suspect buying two further 8GB sticks and replacing Apple's stock 4GB will be the only safe way to optimise performance even though I do not need more than 24GB in reality
 
L

lixuelai

macrumors 6502a
Oct 29, 2008
958
327
Confuzzzed said:
Ok, I will live up to my tag and declare myself well and truly confused! So, given the lack of consensus in the matter I suspect buying two further 8GB sticks and replacing Apple's stock 4GB will be the only safe way to optimise performance even though I do not need more than 24GB in reality
Click to expand...

1) What you need to do to get dual channel is pretty simple... Buy your new RAM and put it into the empty slots in the iMac. Voila you are done. You can also do what Crucial suggests which is swap the Apple and new memory first then reinstall the Apple RAM into the empty slots but it doesn't really matter.

2) Even assuming you listened to some of the wrong advice in this thread (and I explained in my previous post why they are wrong) you will never notice it unless you run benchmarks for a living.
 
Confuzzzed

Confuzzzed

macrumors 68000
Aug 7, 2011
1,630
0
Liverpool, UK
I did what Crucial suggested!! It seems to work fine but then again most things I do don't really need more than 16GB so wouldn't really notice, but would be nice to have the most optimal set up just for personal satisfaction if that makes any sense
 
G

gaddster

macrumors member
Jan 26, 2013
85
0
liverpool, england
Guys, I did the opposite, for the reason that some forum members said the ram downgraded to 1300 mhz

So, I replaced the 2 x 4gb apple sticks with the crucial 8gb stick and the re-inserted the 2 x 4gb apple stick in the spare slots.. checked and I have 24gb at 1600mhz... woosh :)
 
C

Chippy99

macrumors 6502a
Apr 28, 2012
989
35
I have a Mac Mini. It only has 2 slots. This is useful to understand how apple label the memory channels. I have 16GB.

System Information says:

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 8GB DDR3 1600 MHz OK

This is properly functioning Dual Channel memory, with only 2 slots. Therefore we can see very clearly that BANK means CHANNEL.

So you must have the same configuration in each BANK, like this:

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 8GB DDR3 1600 MHz OK
BANK 0 /DIMM1 4GB DDR3 1600 MHz OK
BANK 1 /DIMM1 4GB DDR3 1600 MHz OK

Each BANK has the same amount of memory and the controller will access it in Dual Channel mode.

Not

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 4GB DDR3 1600 MHz OK
BANK 0 /DIMM1 8GB DDR3 1600 MHz OK
BANK 1 /DIMM1 4GB DDR3 1600 MHz OK

You can see the above has 16GB in BANK (i.e. channel) 0 and only 8GB in BANK 1.

This will run ALL the memory in single channel mode! It will not run some of it dual channel and some single.
 
I

iamthedudeman

macrumors 65816
Jul 7, 2007
1,385
246
nosnhojm said:
According to Intel "Dual-channel (interleaved) mode:
This mode offers higher memory throughput and is enabled when the memory capacities of both DIMM channels are equal. When using different speed DIMMs, the slowest memory timing will be used."

Apple preinstalls the 2x4gb dimms into bank0/dimm0 and bank1/dimm0 (channel A), similar to the following "Intel" diagram:
dualwith2.jpg


When you install additional 2x8gb dimms, you want to put them into the emty slots, similar to the following "Intel" diagram:
dualwith4.jpg


In this way, channel A (bank0/dimm0 + bank1/dimm0) = matching 4gb dimms, and channel B (bank0/dimm1 + bank1/dimm1) = matching 8gb dimms. Per the previous description, the capacities of both dimms per channel are equal.

Summary: Don't move around the preinstalled dimms, just install your new dimms into the slots

----------

[/COLOR]
Chippy99 said:
I have a Mac Mini. It only has 2 slots. This is useful to understand how apple label the memory channels. I have 16GB.

System Information says:

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 8GB DDR3 1600 MHz OK

This is properly functioning Dual Channel memory, with only 2 slots. Therefore we can see very clearly that BANK means CHANNEL.

So you must have the same configuration in each BANK, like this:

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 8GB DDR3 1600 MHz OK
BANK 0 /DIMM1 4GB DDR3 1600 MHz OK
BANK 1 /DIMM1 4GB DDR3 1600 MHz OK

Each BANK has the same amount of memory and the controller will access it in Dual Channel mode.

Not

BANK 0 /DIMM0 8GB DDR3 1600 MHz OK
BANK 1 /DIMM0 4GB DDR3 1600 MHz OK
BANK 0 /DIMM1 8GB DDR3 1600 MHz OK
BANK 1 /DIMM1 4GB DDR3 1600 MHz OK

You can see the above has 16GB in BANK (i.e. channel) 0 and only 8GB in BANK 1.

This will run ALL the memory in single channel mode! It will not run some of it dual channel and some single.
Click to expand...

Ding ding ding give that man a biscuit we have a winner. :D
Click to expand...
 
Last edited:
C

cleantone

macrumors newbie
May 11, 2013
1
0
New England, USA
Apologies on bumping this older thread but I just got my 5k Retina iMac. I bought it with 8GB and have 16GB of Crucial to add. I was surprised when I saw the way the RAM was installed. I was aimed to this thread for information to confirm what configuration I want to go with to add up to 24GB. Should I assume this information is still valid for the 5k model? Anyone know for sure? not much info out there. Thanks!
 
C

Chippy99

macrumors 6502a
Apr 28, 2012
989
35
cleantone said:
Apologies on bumping this older thread but I just got my 5k Retina iMac. I bought it with 8GB and have 16GB of Crucial to add. I was surprised when I saw the way the RAM was installed. I was aimed to this thread for information to confirm what configuration I want to go with to add up to 24GB. Should I assume this information is still valid for the 5k model? Anyone know for sure? not much info out there. Thanks!
Click to expand...

Just install it in the 2 vacant slots and you will be fine. (I am assuming no-one has messed with the already installed ram, which will have been installed by Apple into bank 0 and bank 1 already, to give you dual channel memory access.

That being the case the 2 empty slots are also bank 0 and bank 1, so you will end up with 12GB in bank 0 and 12GB in bank 1, which is what you want.

Once installed, you can check by looking at the memory config under System Information. You should be able to see 12GB in total in each bank 0 and 1.
 
You must log in or register to reply here.
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.
Top Bottom