As designers have been buying more Intel-based Mac Pros, we are starting to see that this model is particularly sensitive to the quality of RAM installed.
The Mac Pro uses a new kind of RAM: Fully Buffered DIMMs, or FB-DIMMs. Each one has its own heat sink attached, which allows the Mac Pro to use fewer fans. With all third-party RAM, it’s best to purchase modules that specifically support your system. And in the case of the Mac Pro, that’s not just a recommendation — it’s a requirement.
Apple has such a tight thermal specification that just any RAM that fits in the slots isn’t always going to work. If RAM is installed that doesn’t meet Apple’s strict rules the machine will slow the bus speed down, and run the fans on higher speeds and in extreme situations just turn off the computer to keep from overheating.
This is all arguably good for the machine since it protects itself from getting damaged from excessive heat. However it does mean finding reliable sources of cheap RAM for a Mac Pro could be a problem for a while.
Note: While these details are specifically for the desktop Mac Pros, heat concerns and need for RAM meeting Apple’s specification above and beyond the normal RAM spec is applicable to the MacBook Pro laptops as well.
For more information, here are two additional sources:
Link: Inside the Mac Pro: RAM — MacWorld November 2006
Link: Thermal considerations for Mac Pro FB-DIMMs — Apple Developer Technote (pretty technical)
Source: We’d like to say a special thank you to Tom Lewellen, Professor of Radiology at the University of Washington’s Division of Nuclear Medicine for some of the research links we used while researching this issue. We’d also like to credit the November issue of Design Tools Monthly for pointing us to the excellent MacWorld article.