I have a MacPro5,1 with 8 cores (2 quad-core processors) running OS X 10.10.2. From what I have read as per EveryMac.com's information about this unit it should be possible to have far beyond the normal maximum noted by Apple:
Actual Maximum RAM
Systems with dual processors, which EveryMac.com notes as the "Eight Core" and "Twelve Core" models, likewise officially support 32 GB of RAM, but again, OWC found they actually can support more. OWC first confirmed an actual maximum of 64 GB of RAM 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 higher. More recently, OWC yet again discovered these dual processor 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.
I initially had an arrangement of 8 4GB DIMMs. These brought my system up to 32GB.
I then purchased 2 16GB chips in hopes to raise the amount of RAM, however my system would simply not start up at all. Gave a fast power LED blink. I subsequently returned this RAM and acquired 4 8GB DIMMs.
Upon installing these, my system still shows 32GB of RAM, but the 7 & 8 DIMM slots are showing empty now at this point. Two of the DIMMS are being recognized while the others are not, even though there are 4 GB DIMMs in place and 4 4GB DIMMS:
What kind of issue is going on here? Obviously we're getting mixed messages with the info above about being able to accept much larger memory configurations, but in practice things get quite whacky.