Crazy idea. 32GB RAM modules in a cMP. Anyone tried this?

[tsialex]

- Do get RAM with a designation ending in R ("Registered") i.e. PC3-10600R. Such RAM is able to run reliably at higher densities (e.g. in 32GB DIMMs). You shouldn't mix Registered with regular U ("Unbuffered") RAM, though. The chipset will then just treat all RAM as unbuffered, negating the benefit of Registered.

You can't mix registered and unbuffered sticks, Mac Pro won't even POST and the power LED will blink SOS.

 

[DefaultModeNetworx]

What it comes down to is you can't mix UDIMM with RDIMM.
Both RDIMMs and UDIMMs have ECC, the difference is in the bank switching logic and the latency. UDIMMs have lower timings but can support less memory per stick. RDIMMs allow to pack more memory per stick, but rely on a buffer which will slow things down somewhat.

And definitely not L
I learned that the hard way

And thank you Tsialex for the correction