The late 2013 Apple Mac Pro supports up to 128GB of 1866MHz DDR3 unregistered ECC-DIMM and registered DIMM, using a four-channel memory controller.
Please note:
1.Registered DIMMs cannot be mixed with unregistered ECC-DIMMs.
2.Due to Intel's Xeon CPU limitation, the system will run at 1066Mhz when installed a quad rank memory module.
3.Due to the mechanical design, the late 2013 Mac Pro cannot use the memory modules with heat sinks and VLP (Very Low Profile) type.
Please see our video:
https://www.youtube.com/watch?v=Zhg18fbRDDw&feature=player_embedded