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I have a mid-2011 iMac which came with 4GB RAM (2x2GB). I just bought 8 GB (2x4GB) and added it to get a total of 12GB. The current configuration is as follows:

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Given that the RAM is a different size, do I have the optimal locations for the different RAM chips? At the moment what I did was to simply add the 2 new sticks of RAM in the two empty slots.

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Related: apple.stackexchange.com/questions/15315/… –  Josh Jan 6 '12 at 15:10
    
@Josh: Yep I read that question. It addresses different sizes but not the seating configuration. –  Mike Kwan Jan 6 '12 at 15:13
    
Yeah, I wasn't saying it was a duplicate, just related, so others finding your question can see that one as well. –  Josh Jan 6 '12 at 15:27
    
The answers here are surprising, I was always taught that RAM should be ordered largest to smallest, but always identical sizes (and always identical everything else, brand, CL speed, etc.). –  Jason Salaz Jan 6 '12 at 17:56
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3 Answers 3

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That configuration should be perfectly fine. You can consult Apple's support documentation to be sure, but as long as your top two slots are filled using the original factory RAM and you installed the new RAM into the bottom slots (and you're using the right kind of RAM, of course) then your configuration should be OK.

Don't forget to run memtest!

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Thanks. This gives me the confidence it is in the right place. –  Mike Kwan Jan 6 '12 at 15:17
    
By the way I found a more convenient memory tool built into Windows 7 called Windows Memory Diagnostic which doesn't require downloading or installing. –  Mike Kwan Jan 6 '12 at 15:28
    
Memtest86+ doesn't require installing and in my experience is much more thorough than any other memory testing tool around. I have personally seen cases where it caught memory errors that other tools (including the Apple Hardware Test) did not. –  Josh Jan 6 '12 at 15:43
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I'm going to take WAGuess* and suggest that any gains (realized or not realized) due to memory-size matching will pale to insignificance beside the gains due to tripling your memory. My rule of thumb for processor speed is that our perceptions tend to be more logarithmic than linear and that we don't really notice a speed increase until it is 50% - 100% greater than what we had before.

If you're in a situation where a few - consistently acheived - percentage points matter, such as frequently or perpetually running long, compute-bound jobs, and and where finishing a job a few minutes earlier might have a significant benefit for someone, then there may be an advantage to optimizing your memory placements. Otherwise enjoy the speed of you newly augmented machine! You've already got 99++% of the benefits to be had.

'*' That guess isn't completely WA'd: I've already looked into using mis-matched memory sizes on two of my older Macs, and the above is what I concluded from at the time.

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He matched the RAM up properly because he installed them both in the user accessible slot in the bottom. –  Josh Jan 6 '12 at 15:08
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If a specific model of a Mac has preference to installing RAM in certain slots before others, or gets a speed boost by matching pairs, the installation manual (and sometimes printed on the mac itself) will have a section explaining the order preference.

All the newest iMacs that have empty slots put the memory in pairs matching left / right so as long as you added your pair left / right but either deeper or shallower from the front glass, you should be set for optimal speeds. I've tried installing it the other way and couldn't measure any differences in speed with several benchmarks, but why nor mirror the way Apple installs RAM just in case some models or some scenarios benefit.

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