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I recently bought an used MacBook (mid 2007). I have 1 GB (two 512 MB) 667 MHz PC2-5300 and I would like to upgrade the system. I have read that the limit from my machine is 3Gb.

At this point I have two solutions:

  1. upgrade to 2gb (2x1 Gb)
  2. upgrade to 3gb (1x1 Gb + 1x2 Gb)

What is the best one? I know that with twins ram it is possible to exploit the Dual Channel, isn't it?

Thank you!!

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3GB seems to be a weird number. I would've expected it to be either 2GB or 4GB (in general, powers of 2). –  user76528 May 15 '11 at 17:00
    
I've never heard of the 3 GB limit either. –  boehj May 15 '11 at 17:07
    
its oddly common, my thinkpad r61 of the same era has the same limit. –  Journeyman Geek May 17 '11 at 5:59

2 Answers 2

Even with the 1Gb + 2Gb configuration you will get full dual channel memory up till the 1+1Gb limit, and single channel performance after that, so 1+2 is better than 1+1.

See this link for what Intel calls "Flex mode" DRAM.

Quoting the above link:

The figure below shows a flex mode configuration using two DIMMs. The operation is as follows:

The 512 MB DIMM in the Channel A, DIMM 0 socket and the lower 512 MB of the DIMM in the Channel B, DIMM 0 socket operate together in dual channel mode. The remaining (upper) 512 MB of the DIMM in Channel B operates in single channel mode.

So for 512MB + 1GB configuration, the bottom 512MB of both DIMMs will be in dual channel mode and the top 512MB of the 1GB DIMM will be in single channel mode.

I'm not familiar with AMD motherboards, but they almost certainly have the same behavior.

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From what I can tell, the MacBooks with the 3GB system limit don't support flex mode. Some sources say a 2GB+1GB configuration runs in single channel mode, while others say it runs in asymmetric dual-channel mode. A 2GB + 2GB configuration will then result in dual-channel mode for at least the first GB in each memory module, as Scott mentions, or (if those that say MacBooks interleave addressing are correct) dual channel mode for the entire addressable region. –  outis Dec 1 '11 at 3:40

You could always opt for installing 4Gb (2x2Gb). I did this in my MacBook (also a mid-2007 model). OS X will only see and use 3Gb of memory, but the System Profiler will correctly report the two 2Gb sticks.

I'm not sure how the dual channel gets portioned out. I read the article referenced by DasBoot, so in the best case you'd get the lower 1Gb of each stick in dual channel, and the remaining 1Gb in one stick in single channel.

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