Mac Pro (Mid 2010): Registered DIMM support
Learn more about the types of memory you can use in quad core, 6-core, 8-core, and 12-core Mac Pro (Mid 2010) computers.
The Mac Pro (Mid 2010) supports 8GB registered DIMMs (R-DIMMs) for up to 64GB of memory in 8-Core and 12-Core computers, and up to 32GB of memory in Quad-Core and 6-Core systems.
Do not mix R-DIMMs with unregistered DIMMs. If you install both types of DIMMs in the same Mac Pro, it will not start up.