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Why does RAM always double?

marc weber

Joined: Aug 31, 2004
Posts: 11343

Warning: This post reveals my general ignorance of basic hardware/OS functionality. (Posting in Mac forum rather than General Computing since answer might be OS related.)

RAM options on a new iMac are...

4GB 1333MHz DDR3 SDRAM - 2x2GB
8GB 1333MHz DDR3 SDRAM - 2x4GB [Add $200.00]
16GB 1333MHz DDR3 SDRAM - 4x4GB [Add $600.00]

I'm going with the 8GB in the form of 2x4GB, which leaves two open slots. My question is, would there be a issue if I later added a third card of 4GB, for a total of 12GB? Or is there some reason I would need to jump from 8GB to 16GB?

"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer
Steve Fahlbusch

Joined: Sep 18, 2000
Posts: 601

Greetings Marc,

Now let me pile on my total misunderstanding of hardware.

Many years ago i was told that the DMA controller on microprocessors would scale down the DMA transfer from memory (RAM) to processor (either register or cache) to the slowest of all of the memory that way there could be no 'in transit' issues.

Now while i have never experenced this type of issue (to my knowledge) i still follow this advise -- no mattter how poor it be.

Bear Bibeault
Author and ninkuma

Joined: Jan 10, 2002
Posts: 63854

Dim recollections tell me (maybe erroneously) that there are two memory slots which, because of interleaving or some such, must contain the same size memory module.

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Henry Wong

Joined: Sep 28, 2004
Posts: 20525

Agreed. Many (actually most) CPU/memory architectures uses dual channel memory -- meaning dimms must be installed in pairs. For the options provided in your example, this looks like what you have. As an aside, my older i7 uses triple channel memory, which will get annoying if I want to add memory.

Anyway, back to your example, this mean that you can't go to 12gb by installing a 4gb dimm. To get to 12gb, you must add 2x2gb dimms.


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
marc weber

Joined: Aug 31, 2004
Posts: 11343

Henry Wong wrote:... Many (actually most) CPU/memory architectures uses dual channel memory...

Ah, "dual-channel" is the term I needed. I did some reading and it makes sense now. (My previous searches only turned up statements like, "RAM always doubles," or "must be installed in pairs," without any explanation.)

Thanks to everyone!

Edit: Here's a summary: Wikipedia - Dual-channel architecture.
Pat Farrell

Joined: Aug 11, 2007
Posts: 4659

When first released, many of the fastest of Intel's Core i7 chips were tripple channel for memory. This brought a lot of motherboards to market that had 3 and/or 6 DIMM slots. This lead lots of system builders to install 6GB of RAM.

I find it interesting that the recent Sandbridge Core i7 chips have dropped back to using dual-channel memory, following the older tradition.

The latest and greatest MacBook Pro that I got last month has 8GB of 1333 mHz DDR3.

I agree. Here's the link:
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