Buying more RAM for my Quad Xeon

Discussion in 'Mac Pro' started by AaronICT, May 9, 2007.

  1. AaronICT macrumors member

    Feb 8, 2007
    • 8 Total Slots

    That I understand.

    • Modules must be installed in Matched Pairs


    • DDR2 667MHz ECC PC5300 240 Pin
    • Fully-Buffered DIMM
    • Mac Pro Specific Heatsink on Module


    • 72 Bit (64 Bit + 8 bit ECC)

    I understand that

    • Install 4 DIMMs or More for Best Performance

    ?? Is this so each bank on each riser has RAM on it? Like if I want to add 4GB to the 1GB I have now, do I need to get 4x1GB, so that I have a total of 6 DIMMS installed?

    • FB DIMM technology takes advantage of multiple channel access. Apple recommends the use of 4 or more modules to take advantage of this 256-Bit Wide memory architecture. Modules must be installed in pairs and each module is 64-Bit Wide + 8 Bits for Error Correction (ECC).

    If someone could please explain that to me, I'd love them forever.

    Also, is it okay to add 4x1GB to my machine, keeping the 2x512MB in there, or should I just sell those back to OWC or on eBay and keep the machine at 4GB of totally matched DIMMs?

  2. tribe3 macrumors 6502

    Nov 1, 2005
    Vienna, VA - USA
    Hi, I'm not an expert so don't ask me "why", but I recently installed memory on my MP and did some research.

    It is important that you fill slot 1 and 2 (the ones closer to the motherboard) on each Riser. So You should install 2 x 1GB on slots 1/2 of Riser A; the same on riser B, and you can add the 2 x 512 on slots 3 and 4 of Raiser A ( Some will say these 2 X 512 should go on Riser B but the insude of the lid of your computer shows a sequence like the one I just described.

    Good Luck
  3. ^squirrel^ macrumors 6502a


    Apr 4, 2006
    Are you aure about that?

    I thought you fill one riser first then the other..... I've just done this on 3 machines today and they work fine.
  4. CanadaRAM macrumors G5


    Oct 11, 2004
    On the Left Coast - Victoria BC Canada
    256 bit access is why the "4 modules" thing. It provides a tiny real-world improvement, I wouldn't sweat it unduly.

    The way it works (and the order you should install your RAM) - Start with your largest capacity modules.

    Riser A slot 1 & 2
    Riser B Solt 1 & 2 = 256 bit access if these 4 are matching
    Riser A slot 3 & 4
    Riser B slot 3 & 4

    The RAM in slots 3 and 4 are slower to access (extra latency) than 1 and 2, so put your smallest or odd pair RAM in 3 and 4 after populating 1 and 2 on both risers.

  5. ^squirrel^ macrumors 6502a


    Apr 4, 2006
  6. thomasp macrumors 6502a

    Sep 18, 2004
    So, if you bought a stock Mac Pro (with 1Gb RAM in 2x 512's), and then bought four 1Gb chips from, say Crucial, would it be best to install two 1Gb's in Riser A, slots 1 & 2, then the other 2x 1Gb's in Riser B, slots 1 & 2, and park the remaining 2x 512's in Riser A, slots 3 & 4?
  7. tribe3 macrumors 6502

    Nov 1, 2005
    Vienna, VA - USA
    Yes, I'm sure because I've asked all over the web.

    According to the gurus you should fill slots 1 and 2 of both risers first for best performance

    Here is a guide:

    Also, on the inside of the tower cover there is a graphic sequence that shows more or less the same.

  8. CanadaRAM macrumors G5


    Oct 11, 2004
    On the Left Coast - Victoria BC Canada
    Yup, that's it in one.
  9. dkoralek macrumors 6502


    Sep 12, 2006
    In response to your last question, you are likely going to see better performance for having more RAM than you are going to lose by not having all the RAM in quad channel.

  10. patrick0brien macrumors 68040


    Oct 24, 2002
    The West Loop

    Rule of thumb: You should make the risers as identical as possible in RAM DIMM arrangements, the larger capacities closest to the Logic Board Connector.
  11. iSavant macrumors regular

    Mar 31, 2007

    I think the location of the dimms issue is pretty well covered here... on another aspect of your post:

    I picked up a new Mac Pro (1GB) on Saturday
    Ordered NetList from OWC (4 x 1 GB) on Sunday
    Rec'd ram today and installed 1 GB sticks at a1 & a2 & b1 & b2
    Moved my original 512 MB sticks to a3 & a4

    I'm keeping my original RAM rather than sell it to OWC, of course YMMV.

    Dang! That additional RAM makes a huge and immediately noticable difference! :D
  12. AaronICT thread starter macrumors member

    Feb 8, 2007
    That's good to hear, because that's the exact route I'm going. How much of a difference did it make? Because I am very dissatisfied with my Mac Pro with just 1GB.
  13. lssmit02 macrumors 6502

    Mar 25, 2004
    Rule of thumb for memory

    One question to your post: it has been said that there is a "rule of thumb" that you need 1 gig of ram minimum per processor for a Mac Pro to run well under OS X. Any truth to that, or is that just so much "horse hockey" commonly posted on these boards?
  14. Lycanthrope macrumors 6502a


    Nov 1, 2005
    Brussels, Belgium, Europe
    Gawd, so many options for something so simple...

    I have here, clutched in my sweaty paws 2x2GB from OWC - I would have assumed without any reading that this should be placed 2GB and the stock 512MB in slot 1 & 2 on each riser.

    But on looking at the diagram above it seems more correct that I have 2GB x 2 in riser 1 and 2x512MB in the other.

    Reading this seems to concur with that also as does the Apple Memory Replacement Instructions.

    I think, to avoid confusion I will aim to fill it up to 16GB in the coming year :)
  15. GlowingApple macrumors newbie


    Jul 25, 2007
    Hi, I was researching RAM upgrades for my brand-new Mac Pro (Quad Xeons) and saw this post. After looking through some manuals, web sites, and eventually calling Apple's tech support and talking to a Mac Pro specialist, I've come to the following consensus.

    1. RAM must be in matched pairs
    2. Combinations of 4 in equal sizes offers the best speed advantage (so 4 x 1 GB would be better than 2 x 2 GB)
    3. Even if the sizes are not matched, 4 sticks offers a minute speed improvement over 6 sticks.
    4. More RAM sticks means more power consumption than less sticks (even with the same size), though not by much.
    5. The prior statement also holds true for heat.
    6. In the end, more RAM is still better, even if multiples of 4 aren't matched, so long as multiples of 2 are. i.e. 4 or 8 are better than 2 or 6, but more RAM is still best; 6 GB is still better than 4 GB.
    So in my case, Apple told me that having 2 x 1 GB sticks in the Pro already, buying 2 x 2 GB sticks is slightly faster than 4 x 1 GB sticks. They said the speed improvement is very minor though, so if the 1 GB sticks are cheaper they'd recommend that rather than paying any more for the 2 GB. Since the 2 GB sticks ended up being cheaper this was good news.

    Hopefully this helps to clear up any remaining confusion for some. If you're looking for places to buy, has the RAM, matching all of Apple's specs (including thermal), for about $400 cheaper than Kingston or Crucial (for 4 GB). It's not name brand, but I've bought from OWC before and always gotten good hardware.

    P. S. Thanks to the previous posters for all the info. Special thanks to Lycanthrope for the link to That page was very helpful.

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