How do you upgrade the RAM in the "Early 2009/Nehalem" Mac Pro? How much RAM of what type does it support?

Just as it is for the original and "Early 2008" Mac Pro models, upgrading the memory in the "Early 2009/Nehalem" Mac Pro is quite simple. Even though the memory used in the "Early 2009/Nehalem" Mac Pro models do not have the same enormous heatsinks as memory used by earlier Mac Pro models, it nevertheless is critical that the memory meet precise standards to function properly.

The "Early 2009/Nehalem" Mac Pro models use "1066 MHz DDR3 ECC SDRAM", but more specifically must conform to the following specifications:

• PC3-8500, 1066 MHz, DDR3 SDRAM UDIMMs
• Error-correcting code (ECC)
• 72-bit wide, 240-pin ECC modules
• 36 ICs maximum per ECC UDIMM

Please note that the low-end "Quad Core" models -- Mac Pro "Quad Core" 2.66 (2009), "Quad Core" 2.93 (2009) and "Quad Core" 3.33 (2009) -- have four memory slots on a single "slide out" processor tray. The high-end "Eight Core" models -- Mac Pro "Eight Core" 2.26 (2009), Mac Pro "Eight Core" 2.66 (2009) and Mac Pro "Eight Core" 2.93 (2009) -- have eight memory slots (four on each of two "processor trays").

Officially, starting December 4, 2009, Apple supports 16 GB of RAM in the "Quad Core" models and 32 GB of RAM in the "Eight Core" models using 4 GB memory modules.

However, site sponsor OWC first discovered that the "Quad Core" models actually were able to support 32 GB of RAM (in both 32-bit and 64-bit mode) and later increased this maximum to 48 GB of RAM.

Likewise, OWC first discovered the "Eight Core" models could support 64 GB of RAM when booting the system in 64-bit mode 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 10.7 "Lion." More recently, OWC yet again discovered these "Eight Core" 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.

Apple covers installing memory in the "Early 2009" Mac Pro models on pages 41-58 of the PDF User's Guide -- complete with drawings -- and this absolutely should be read before upgrading the memory.

However, OWC noticed the manual and the Memory Slot Utility software that is pre-installed on the "Early 2009" Mac Pro provide contradictory information about which slots should be populated to maximize performance. It's best to use the information provided by the application rather than the manual.

Perhaps even better than the official drawings, OWC provides a step-by-step video of the upgrade process:

From testing both systems, the always excellent BareFeats discovered that three memory modules in the Mac Pro "Quad Core" 2.66 (2009/Nehalem) and six memory modules in the Mac Pro "Eight Core" 2.26 (2009/Nehalem) are faster than the maximum of four and eight modules, respectively. This is because each Nehalem processor has three memory controllers, so it is slower for the processor to access the fourth one. However, BareFeats noted that the "vast majority of real world applications do not saturate the memory bandwidth" so it wouldn't necessarily be slower to maximize the memory.

In addition to providing helpful installation videos, Other World Computing also has quality memory that is guaranteed to work with the "Early 2009/Nehalem" Mac Pro at an affordable price.