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I have a MacPro5,1 with 8 cores (2 quad-core processors) running OS X 10.10.2. From what I have read as per EveryMac.com's information about this unit it should be possible to have far beyond the normal maximum noted by Apple:

Actual Maximum RAM

...

Systems with dual processors, which EveryMac.com notes as the "Eight Core" and "Twelve Core" models, likewise officially support 32 GB of RAM, but again, OWC found they actually can support more. OWC first confirmed an actual maximum of 64 GB of RAM running Mac OS X 10.5 "Leopard" and later increased this maximum to 96 GB of RAM running Mac OS X 10.6 "Snow Leopard" or higher. More recently, OWC yet again discovered these dual processor models can support up to 128 GB, but only when running a 64-bit version of Windows XP or later or Linux. Mac OS X only can support 96 GB of RAM.

I initially had an arrangement of 8 4GB DIMMs. These brought my system up to 32GB.

I then purchased 2 16GB chips in hopes to raise the amount of RAM, however my system would simply not start up at all. Gave a fast power LED blink. I subsequently returned this RAM and acquired 4 8GB DIMMs.

Upon installing these, my system still shows 32GB of RAM, but the 7 & 8 DIMM slots are showing empty now at this point. Two of the DIMMS are being recognized while the others are not, even though there are 4 GB DIMMs in place and 4 4GB DIMMS:

current memory configuration

What kind of issue is going on here? Obviously we're getting mixed messages with the info above about being able to accept much larger memory configurations, but in practice things get quite whacky.

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  • Run /System/Library/CoreServices/Memory Slot Utility. What does it recommend? Also, based on the manual, the 4 GB DIMMs should be in slots 1-4 and the 8 GB DIMMs should be in slots 5-8.
    – tubedogg
    Feb 5 '15 at 20:45
  • This is interesting, I am doing that presently. Since it was not giving me proper information at first, ie missing DIMMs, I was not getting the complete picture from the Memory Slot Utility. After moving the DIMMs about, I'm getting some different results now and it appears that I'm close to a "proper" configuration. I'm curious, are you seeing anything about 16GB DIMMs anywhere?
    – ylluminate
    Feb 5 '15 at 20:48
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That machine will take 16's - my 3,1 does, to an unofficial maximum of 64GB [or more, according to Everymac, though I've not tested that myself].

The arrangement is not what you would initially think logical.

  • If you install equal-size DIMMs (for example, all 1 or 2 GB DIMMs) in your Mac Pro, Apple recommends that you fill the slots in the order listed in the table.

Note: If you install different-size DIMMs in your Mac Pro, follow the order in the table. If the DIMM configuration you install doesn't provide optimized performance, the Memory Slot Utility appears onscreen and recommends an improved configuration.

enter image description here

Source: Mac Pro (Mid 2012 and earlier): How to remove or install memory … & previous similar confusion when I upgraded mine ;)

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  • Well, let's say this scenario: Say I have 8 DIMMs; two are 16GB DIMMs and the rest are 4GB. What would the order need to be in order for that to properly work? I believe that I did as you note above, but it failed and so I'd like to get more specific.
    – ylluminate
    Feb 6 '15 at 9:04
  • the layout in my 3,1 is different, so it's not an exact match, it just primed me that it's 'not obvious'. I'd go 16, 16, then all the 4's. The layout does look more logical than on the 2008. Have a look further down that link page to see how odd that is.
    – Tetsujin
    Feb 6 '15 at 9:10
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Okay, so ultimately there are some specific requirements that the Mac Pro has in terms of where you stick the 8GB DIMMs. I am curious as to whether or not that could have had bearing on the 16GB DIMM issue wherein the system would simply not start up, but I wanted to at least share the ultimate configuration that worked after a couple of iterations (since the RAM simply was not appearing initially a few attempts had to be made) from from the suggestions of /System/Library/CoreServices/Memory Slot Utility.app:

Working Config

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  • Your solution here is right on the money. The system has two banks of memory and it's important to populate the two banks as mirror images DIMMs 1~4 are Bank 0 and DIMMs 5~8 are Bank 1.
    – Trane Francks
    Feb 5 '15 at 22:17
  • Thanks for the verification. Do you know anything regarding the usage of 16GB DIMMs on this unit? Sure would be nice to max this booger out.
    – ylluminate
    Feb 5 '15 at 23:32
  • Unofficially, you can fully populate the box with 16GB DIMMs for a total of 128GB of RAM. You'll need Mavericks or Yosemite to make full use of it; earlier OS X versions could only utilize up to 96GB RAM with the same DIMM configuration.
    – Trane Francks
    Feb 7 '15 at 0:25

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