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question about iMac memory "pairing"

J

John Paul

macrumors member
Original poster
May 10, 2010
97
0
Washington DC
Just last night ordered one of the new 21.5" iMacs, which comes with 4 GB RAM standard (2 x 2GB). I know the iMac has 4 slots so am planning on upgrading the RAM once the computer arrives.

My question is, is there any reason (other than the extra cost) why I wouldn't want to add (2 x 4GB) more, giving me 12 GB, rather than adding (2 x 2 GB), for a total of 8 GB?

In other words, is there any "advantage" to having:

2 GB
2 GB
2 GB
2 GB

Over having

2 GB
2 GB
4 GB
4 GB

??? Does my question even make sense? I guess it's a way of saying, if "paired" memory is good, is "quadded" (not a real word) memory better? Or as long as it comes in pairs, will more always be better than less?
 
The iMac's motherboard supports dual channel memory. It is possible to get 2,2,2,2 in dual channel and possible to have 2,2,4,4 in dual channel as well. Just make sure each channel gets 2,4-- you want same amount of memory per channel.
 
2x2 + 2x4 is fine so long as you populate the banks correctly. Both memory stick in the same BANK should match in order to get dual channel support. The BANKs are arranged horizontally.

Install it as:
--2gb-- --2gb--
--4gb-- --4gb--

Do NOT install it as:
--2gb-- --4gb--
--2gb-- --4gb--
 
err404 said:
2x2 + 2x4 is fine so long as you populate the banks correctly. Both memory stick in the same BANK should match in order to get dual channel support. The BANKs are arranged horizontally.

Install it as:
--2gb-- --2gb--
--4gb-- --4gb--

Do NOT install it as:
--2gb-- --4gb--
--2gb-- --4gb--
Click to expand...

This

I wound up only adding another 4GB to my new iMac to save a bit of money but adding 8GB will work just as well. I just plan on upgrading mine to the full 16GB when it becomes cheaper.
 
What if I have only 3 stick of 4gb ram and left 1 stick of original 2gb ram. will my iMac run properly with 14gb ram with this configuration?

thanks
 
It'll have 14gb of ram, but only 8gb will be in dual channel bandwidth.
 
shakyhand said:
What if I have only 3 stick of 4gb ram and left 1 stick of original 2gb ram. will my iMac run properly with 14gb ram with this configuration?

thanks
Click to expand...

The answer is "maybe". Combining RAM from different vendors is always a bit of a lottery. It may run, it may crash. Combining two different RAM blocks in paired slots is even more risky though, I suggest you avoid it.
 
yusukeaoki said:
Or install
4GB x 4
and get it 16GB!
Click to expand...

Yeah.. that's what I've been thinking.. My intention was to buy 2 stick of 4GB RAM yesterday. However, it was so damn cheap and I ended up with 3. However, I'm 300km away from my iMac now, cannot test the RAM until I get home tonight... if the RAM works, no doubt I'll get another one to make it 16GB..
 
shakyhand said:
Yeah.. that's what I've been thinking.. My intention was to buy 2 stick of 4GB RAM yesterday. However, it was so damn cheap and I ended up with 3. However, I'm 300km away from my iMac now, cannot test the RAM until I get home tonight... if the RAM works, no doubt I'll get another one to make it 16GB..
Click to expand...

2x4GB are really cheap only like 30 bucks off newegg.
Just make it 60 bucks and you get 16GB!
 
yusukeaoki said:
2x4GB are really cheap only like 30 bucks off newegg.
Just make it 60 bucks and you get 16GB!
Click to expand...

Sure, but the question is; does 16GB result in any tangible improvement over 12GB. If not, your throwing away $30 on bragging rights.
 
err404 said:
Sure, but the question is; does 16GB result in any tangible improvement over 12GB. If not, your throwing away $30 on bragging rights.
Click to expand...

well look at your activity monitor if you have a lot of page outs with 12gb rm then 16gb ram will speed you up if you have a hdd. if you run your osx off an ssd the ram is less important. every page out means you used your hdd instead of ram. some users can actually put in 32gb ram and be short ram of course they are few and far in-between. I need around 9gb or

10 gb to avoid paging out but ram was cheap and i have 16gb. as my iphoto just keeps growing i will need it one day.
 
I will be doing a lot of video and photo editing (FCP X and photoshop CS5). Got a lot of 8mm tapes stuck in a drawer for the last 16 years and a new AVCHD video. Have been into photography since 2006 with a Canon 7D and a handful of red ring canon lenses..(another money wasting hobby).. Hopefully, the 12GB is enough..
 
shakyhand said:
I will be doing a lot of video and photo editing (FCP X and photoshop CS5). Got a lot of 8mm tapes stuck in a drawer for the last 16 years and a new AVCHD video. Have been into photography since 2006 with a Canon 7D and a handful of red ring canon lenses..(another money wasting hobby).. Hopefully, the 12GB is enough..
Click to expand...

I can run both PS CS5 and FCP X without any trouble and I only have 8GB of RAM. 12 Should work fine. The full 16 would run better, mind you, but 12 should do it.
 
First, thanks to all who replied so far.

But I am confused about "banks" because if I am reading this right, the 4 GB of RAM that came pre-installed by apple on the iMac in my office (21.5", bought new this January) is not properly paired (!?!)

Directly from System Profiler:

BANK 0/DIMM0:

Size: Empty
Type: Empty
Speed: Empty
Status: Empty
Manufacturer: Empty
Part Number: Empty
Serial Number: Empty

BANK 1/DIMM0:

Size: Empty
Type: Empty
Speed: Empty
Status: Empty
Manufacturer: Empty
Part Number: Empty
Serial Number: Empty

BANK 0/DIMM1:

Size: 2 GB
Type: DDR3
Speed: 1333 MHz
Status: OK
Manufacturer: 0x80CE
Part Number: 0x4D34373142353637334648302D4348392020
Serial Number: 0x8125BBB5

BANK 1/DIMM1:

Size: 2 GB
Type: DDR3
Speed: 1333 MHz
Status: OK
Manufacturer: 0x80CE
Part Number: 0x4D34373142353637334648302D4348392020
Serial Number: 0x8125BA91

So what they heck is that about? If I am reading this corrctly, then rather than pairing those two RAM modules in the same bank, they put one in one bank and one in the other. Which would mean if I were to add 2 more modules to this computer (let's say 2 x 4 GB just for discussion), it wouldn't simply be a matter of adding them to the two slots currently free?

Thought I understood things, but now am more confused than I was before.
 
If you opened the RAM panel you'd see what they mean, trust me. Essentially though there are 4 slots in the iMac for RAM but two banks. A bank is a pair of horizontal slots, one stick of RAM in each. What comes pre-installed is a 2GB stick on both top slots, thus both in the top bank. System profiler just lists is oddly. There will be two open slots for new RAM in the bottom bank. You can either add two sticks in there or remove the ones pre-installed and add 4 new sticks. In theory you could remove the apple RAM and pop in new ones in bank 1 but it would serve no purpose.
 
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