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Resolved Is Apple installing the stock RAM incorrectly? Answer NO

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seanm9

macrumors regular
Original poster
Dec 29, 2007
143
0
Cape Cod, MA
The stock RAM in my iMac is in slots 1 and 3 and the open slots are 2 and 4...

When you look at it in About This Mac More Info it shows up side by side.
When you look at in System Information it has one listed as BANK 0/DIMM0 and the other as BANK 1/DIMM0.

All of the screen shots people have posted here show the same step up... and when we install a pair of 8 GB SODIMM's in the open slots you end up with a 4GB stick and an 8 GB stick in BANK 0, and a 4GB stick and an 8 GB stick in BANK 1...

everything I read says that RAM should be installed in matched pairs in the SAME BANK... SO SLOTS 1 and 2 are BANK 0 and 3 and 4 are BANK 1...

What am I missing? Is interleaving that important? This is a new RAM expansion design for Apple and possibly for Foxconn, is it possible that they need to color code the slots like PC motherboards do?
 
Last edited:
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WilliamG

macrumors G3
Mar 29, 2008
9,922
3,800
Seattle
1,3, and 2,4 are the correct pairings, from what I gather. That's how it comes from the factory in all new iMacs, so I'd keep it that way.
 
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Nuke61

macrumors 6502
Jan 18, 2013
325
1
Columbia, SC
seanm9 said:
The stock RAM in my iMac is in slots 1 and 3 and the open slots are 2 and 4...
Click to expand...
Two things to note:
1) This matches the pairing designated by Intel and the last PC that I assembled for dual-channel mode. The Intel description is here.
2) The difference in speed between single a dual channel isn't worth worrying about anymore because of DDR3 memory speeds. Will you see a difference in a benchmark? Yes. Will you see a difference in everyday use? Not unless you're using a stopwatch, which effectively makes it a test and not normal useage. http://www.behardware.com/art/imprimer/814/
 
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seanm9

macrumors regular
Original poster
Dec 29, 2007
143
0
Cape Cod, MA
Nermal said:
Hmm, Apple's instructions and specs don't mention banks at all. Unfortunately I don't have a "known good" system to check against so I'm not sure whether it's important or not :(
Click to expand...

Well I just moved my 8GB sticks to Bank 0 and the 4 GB sticks to Bank 1 (or that is what System information says anyhow)
it works... this probably also is only an issue for people with 2 different sized sets of RAM sticks (and even then the speed difference is probably minimal)

hfg said:
Isn't there an installation diagram on the inside surface of the removable RAM cover?
Click to expand...

there is but i think it was more how to move the tray out and not what slots are what bank but I didn't study it that much...

Now if I am correct then Stock 8GB machines can be faster (slightly) by putting the ram in the top 2 or bottom 2 slots...

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Nuke61 said:
Two things to note:
1) This matches the pairing designated by Intel and the last PC that I assembled for dual-channel mode. The Intel description is here.
2) The difference in speed between single a dual channel isn't worth worrying about anymore because of DDR3 memory speeds. Will you see a difference in a benchmark? Yes. Will you see a difference in everyday use? Not unless you're using a stopwatch, which effectively makes it a test and not normal useage. http://www.behardware.com/art/imprimer/814/
Click to expand...

Wish I has seen this before:eek:... the intel link is what i have been looking for... looks like i'll be in flex mode for the night (or until I go and by 2 more 8GB sticks)
 
D

drambuie

macrumors 6502a
Feb 16, 2010
751
1
If you pair them as 4 + 8 and 4 + 8 in each channel, the worst that happens is that the first 8GB will run in dual channel mode and the higher 16GB will run in single channel mode. The result is that you will be running in dual channel mode, except for the situations where you need more than 8GB, and then, only the upper 16GB will be in single channel mode.

In the MBP which has a single dual channel pair, the DIMMs are labelled as BANK0/DIMM0 and BANK1/DIMM0, which verifies the iMac's default pairings.
 
Giuly

Giuly

macrumors 68040
Oct 16, 2009
3,348
4
That depends whether you ask for timezone, state o
Nuke61 said:
Two things to note:
1) This matches the pairing designated by Intel and the last PC that I assembled for dual-channel mode. The Intel description is here.
2) The difference in speed between single a dual channel isn't worth worrying about anymore because of DDR3 memory speeds. Will you see a difference in a benchmark? Yes. Will you see a difference in everyday use? Not unless you're using a stopwatch, which effectively makes it a test and not normal useage. http://www.behardware.com/art/imprimer/814/
Click to expand...

Obviously, 1/2 the bandwidth is nothing you need a stopwatch for. It takes twice as long. Also, why would you even install single DIMMs when they're more expensive than the dual-channel kits for the same size?
 
D

drambuie

macrumors 6502a
Feb 16, 2010
751
1
Giuly said:
Obviously, 1/2 the bandwidth is nothing you need a stopwatch for. It takes twice as long. Also, why would you even install single DIMMs when they're more expensive than the dual-channel kits for the same size?
Click to expand...

The bandwidth of single channel memory is about 10% - 15% less than that of dual channel, as measured in a bandwith benchmark test where the memory is addressed contiguously. In actual use the advantage of dual channel is less because there's a lot of random addressing.

In single channel memory a RAS, (Row Address Select), action has to be performed for every address access. In dual channel, two addresses can be accessed by a single RAS, provided they are accessed in sequence. That's where the dual channel performance gain comes from.
 
N

Nuke61

macrumors 6502
Jan 18, 2013
325
1
Columbia, SC
Giuly said:
Obviously, 1/2 the bandwidth is nothing you need a stopwatch for. It takes twice as long. Also, why would you even install single DIMMs when they're more expensive than the dual-channel kits for the same size?
Click to expand...
It's not as simple as saying single channel is 1/2 the bandwidth of dual channel. If you read the test I referenced, there was roughly a 10% gain by going from single to dual channel when doing actual work and not just measuring bandwidth. You simply aren't going to notice a 10% difference unless you're timing it.
 
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