KB-60601 Mac Pro 5,1 RAM Configurations


The Mac Pro has 4 RAM slots per processor. This article covers dual processor boards, however everything would probably work with a single CPU board, keeping in mind to only use the slots available on that board.

Compatible RAM Types

The Mac Pro 2009 to 2012 uses DDR3 DIMMs. Conventionally, 1066MHz or 1333MHz speed RAM is installed – this corresponds to PC3-8500 or PC3-10600 designated RAM. Either will work, and RAM of each type can be mixed, however will run at the slowest speed of those installed. Faster RAM can also be installed, but will run at the speed the Mac Pro is capable of.

Since the Mac Pro uses Xeon processors, the system is also able to operate error-correcting RAM, or ECC memory. These are designated with the suffix R (that is, PC3-8500R or PC3-10600R). When using ECC memory, all installed modules should be ECC (do not install ECC memory together with non-ECC memory).

Some DDR3 RAM is low-voltage compatible. These can also be installed with no issue. These can also be installed alongside RAM that isn’t low voltage compatible. The Mac Pro doesn’t run the RAM at low voltage, so all RAM will be run at standard voltage.

The Mac Pro is not able to use load-reduced (sometimes known as fully buffered) RAM. These are usually denoted with the suffix L.

All RAM module capacities can be installed – RAM modules up to 16GB per module can be installed without any further configuration. Commonly, modules up to 8GB can be found either ECC or non ECC, while 16GB modules are usually ECC, but check the label or the part number to confirm. 32GB DDR3 modules are also available, however these require OpenCore to be installed and the individual DIMMs to be added to the configuration file. Details can be found here

64GB DDR3 DIMMS exist, however no vendor produced 64GB DIMMs that weren’t load reduced, therefore these aren’t compatible.

RAM Configuration

The Mac Pro Xeon processors are capable of up to triple-channel RAM operation. Multi-channel operation allows for the processors to access the RAM at a faster rate. Theoretically, dual-channel RAM can offer double the bandwidth, but in normal use performance will increase less so.

To use multi-channel RAM, install identically sized RAM modules in the first slots of each processor. For dual channel, populate slots 1 and 2 (and 5 and 6). For triple channel populate 1, 2 and 3 (and 5, 6 and 7). When using all 8 slots, the system can only operate in dual channel mode. The fastest RAM performance is available in triple channel mode, whereas the largest RAM capacity is only available in dual channel mode.

Theoretically, with PC3-10600R offering 10GB/s of memory bandwidth:

  • Dual channel operation can offer 20GB/s per CPU (40GB/s overall)
  • Triple channel operation can offer 30GB/s per CPU (60GB/s overall)

Overall memory throughput will be limited by the bus interface between the processors (Intel Quick Path Interconnect) and however the system decides to use the RAM (whether software can utilise dual processors, if data in memory is required to be sent to processing). Not all work requires high bandwidth, and not all work required large RAM capacity.

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