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Quad Channel Memory Question

Discussion in 'Mac Pro' started by Chad Paulson, Nov 10, 2007.

  1. macrumors newbie

    Chad Paulson

    I currently have a Mac Pro with 2GB of RAM (2 x 512MB on both riser cards). I am thinking about purchasing 2 x 2G of RAM (it has gotten so cheap). Would I lose quad channel memory if I added the 2 x 2GB sticks to my current configuration?

    Thanks in advance.
  2. macrumors 6502

    Yes you would. You would need to purchase 4 2GB DIMMs and install them in A1, A2, B1 and B2 if you wanted to maintain quad channel.
  3. macrumors 6502a


    I believe you could also purchase (4) 1 x 1GB sticks if finances are a little tight.
  4. macrumors newbie

    Chad Paulson

    Thanks for the responses. I think I'm going to wait until after the holidays and upgrade with 4 x 2GB. I really want to keep quad channel memory. Here's hoping we continue to see a decline in memory prices.
  5. macrumors 601

    Anonymous Freak

    The Intel 5000X Chipset (the one used by the Mac Pro,) has two memory 'branches', with each branch having two channels.

    The chipset is capable of operating with only a single module, in what is basically just a diagnostic mode. Normally, you need to install modules in pairs, filling one slot of each channel in a branch. (Other than just one module, you cannot have an odd number of modules.)

    If you have a valid configuration with two memory modules in each branch, it will operate in quad-channel mode, even if the memory in each branch is mismatched.

    This means that as long as you have two modules on each memory riser of the Mac Pro, it will operate in quad-channel mode. Because if you are using both memory risers, it has to be using quad-channel mode. (And, similarly, as long as you have two valid modules in a riser, that riser is operating in dual-channel mode.)

    For more information than you can possibly digest, check out Intel's 5000X Datasheet, the memory section starts on page 305.

    I would suggest putting your existing 512 MB modules all on one riser, and the new modules on the other riser. Then, if you later get more 2 GB modules, divide up so you have two 2 GB modules and two 512 MB modules on each riser.

    The only requirement for matching DIMMs is that each pair has to be matched. You can have mismatched pairs on a single riser, as long as each pair matches itself, and you can have mismatched pairs on separate risers, as well as mismatched riser totals. As long as each pair of modules matches, it will operate just fine. (There will be a SLIGHT performance hit for having mismatched capacity risers, or having mismatched pairs on a single riser, but the increase from 2 GB to 6 GB total memory will effectively negate any such hit. Also, having more than four modules introduces some extra memory delay, but again, moving from 2 GB to 6 GB memory will be enough of an improvement to effectively negate the performance hit.)

    edit: Oh, and if you read the Intel datasheet, you will see that the 5000X chipset is just insane in its memory capabilities. It can actually put memory in RAID-1 mode, so that the two branches are mirrored, to make your memory ridiculously error-proof. As if ECC wasn't good enough, I guess.
  6. macrumors 603


    That wouldn't make it error proof as any errors would just be mirrored to the other set of RAM (the same as happens with RAID-1 hard drive arrays). What it does do is protect against failed RAM modules.
  7. macrumors newbie

    Chad Paulson

    Thanks so much ehurtley! Looks like I might place my order this week after all.

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