Back in early 2018 I wrote one of my first articles as a writer for Low End Mac, covering how to get 64 GB of RAM working in your Mac Pro 1,1 (2006) or Mac Pro 2,1 (2007). At the time, the project felt like a massive risk considering the cost of the RAM, and spotty reporting of whether or not this even worked.. but it worked. Consistently.

In the years since the initial project + article, I’ve learned some things along the way as well as starting on another project more recently. In this article, I will share my updated findings, anecdotes, and recommendations.

Inside Your Mac

Officially, the RAM memory ceiling for the 2006 and 2007 Mac Pro is 16 GB of 667 MHz ECC PC2-5300f DDR2, configured as eight 2 GB DIMMS. Many users found this wasn’t the case, as reports surfaced over time about 32 GB working. But was 32 GB the actual limit?

• The logic board inside your Mac uses an Intel 5000X memory controller, which has a memory ceiling of 64 GB in non-mirrored mode, or 32 GB in a mirrored configuration.
  • See: Intel 5000X Chipset overview (Page 20)
  • See also: Intel 5000X Chipset product brief
    .
• According to Apple’s 2006 Mac Pro press-release, the 2006-7 Mac Pro has a 256-Bit wide memory architecture.
  .
  • “With 667 MHz DDR2 fully-buffered memory, the Mac Pro also boasts a 256-bit wide memory architecture for amazing bandwidth.”
    .
  • Each DDR2 channel has 64-bit data bus, while ECC RAM has an additional 8 bits for a 72-bit memory channel.
    .
  • Unlike later Intel CPUs (Nehalem and later), these Clovertown and Woodcrest CPUs don’t have an integrated memory controller – instead they talk to the Intel 5000X. This is also why it’s possible to register 64 GB RAM even with 2x Dual-Core x51xx CPUs.

FB-DIMM channels are organized into 2 branches for support of RAID1 (mirroring), with a memory read bandwidth of 5.325 GB/s per-channel, totaling four memory channels alotgether. Although the Intel chipset can support up to 16 physical RAM modules, your Mac Pro has a total of 8. This means each RAM channel may populate with two RAM sticks each.

• Total theoretical RAM bandwidth = 21 GB/s

What kind of RAM should I use?

Your best bet will be either Micron or Kingston 2Rx4 8 GB PC2-5300f DDR2 FBDIMMs. Samsung modules may or may not work as I haven’t tested those, but they are out there.

Do NOT buy 4Rx4 modules, they will not work and will result in an unbootable Mac. I have mostly seen these on Dell-Micron branded modules, which I accidentally bought in 2017 since they were cheaper at the time versus 2Rx4 RAM. After trying these modules in multiple Macs (including a 3,1), I came to the conclusion they just won’t work in any Mac.

• It is best to have all the RAM in matched pairs. If you have more of one type of module, it’s better to fill the remaining slots with ones that match.
  .
• The correct RAM modules will look like some version of “8GB 2Rx4 PC2-5300f-555-1x-xx”.
  .

What you need to know

• You’ll find random LED errors from time to time; rest assured, it’s normal. Just keep swapping around the sticks until something works.
  .
• RAM with thinner heatsinks have less surface area to dissipate heat – it’s strongly suggested to have fans ramped up to a higher default (~1000 rpm or more), to prevent premature failure.
  .
• If you plan to swap the heftier Apple or SK Hynix heatsinks onto these RAM modules, keep in mind not all are created equal. SK Hynix modules tend to have 4 fins, whereas some other modules including Apple ones have 7 fins, which means more surface area = better cooling.
  .
  • If you plan to swap heatsinks, be very careful and make sure all the old thermal paste is off the old RAM. You will want the thermal pads on the better heatsinks (if there are any) to sit comfortably on the RAM.
    .
• 64 GB RAM has only been tested with at least OS X 10.7.5 + and at least both CPU sockets populated with Xeons onboard.

The End Result