Upgrading memory in a Mac Pro is unlike any other computer I’ve added memory to. The specific requirements and procedure on the upgrade have a lot to do with the architecture of the machine concerning the type of memory and the memory controller in the Mac Pro.

The experience I’ll share here has to do with the Early 2008 version of the Mac Pro. The previous version does vary slightly.

My MP came with 2gb of Ram, the stock configuration. The Mac Pro utilizes 2 memory riser cards each having 4 memory slots each giving the machine a total dimm capacity of 8 slots. To save a little money, I decided to just pick up 6 more 1gb sticks to fully populate the two cards. Having dealt with countless memory upgrades over the years, I didn’t give this plan a 2nd thought as to it being a challenge.

The MP comes with 2 1gb dimms, one on the top riser card and one in the bottom, slot 0 on each (slot closest to the motherboard when installed). The memory I purchased off of eBay were actually Mac Pro pulls for systems that had been upgraded and ended up being the exact brand and model that my MP came with. Sweet! 8 perfectly matched dimms. Well, not quite as I soon learned....

The physical install is quite easy, pull the risers, pop in the dimms, and put the risers back in. After doing this however, when I powered my machine back up I saw... 4 GB of RAM!

As it turns out, the MP has a specific way that memory must be installed as the dimms even within a specific brand/model are paired. Upon closer inspection of the dimms, there were slight variations in the model number, I assumed revision or perhaps firmware (though dimms don’t usually have firmware per se) version. As it turns out, when populating the cards, you first must move the original stock DIMM up next to it’s pair on the top card. Then, you begin adding the dimms in pairs first populating the top card and then the bottom card.

My dimms were Nanya brand... and had all identical numbers with the exception of the last 2 digits of one of the numbers on the dimm label. My stock dimm’s had a ‘30’ and each of the other 3 pairs had similarly matching numbers (one pair had 31, one had 29 and one had 42). I simply matched each of those pairs up and installed the rest of the memory as described below.

Here are a couple shots to illustrate this dimm shuffle:


Stock Setup:
memory1

Move one dimm to the other riser:

memory2


From here, you just fill in the same set of dimm slots on the now empty riser with the next pair of (perfectly matched) dimms. If a 3rd pair is being installed, move back to the top riser and fully populate it. Finally the last pair can fully populate the bottom riser, just make sure each of the pairs are 100% identical.

To help further with this rather picky memory install procedure, here is a link to a great video on OWC Computing (macsales.com) that should clear up any confusion:

Memory Install Video - Early 2008 Mac Pro