NMP can't run mismatched Ram?

[calaverasgrande]

I ordered my new Mac Pro "short loaded" with 16gb of ram. Expecting 1866 ram to come down in price over time, and even at today's prices it is cheaper to buy elsewhere.
What I've noticed at Crucial and OWC sites is that they state you cannot mix different sized ram chips.
"Installation of 16GB modules requires all previous lower density modules to be removed. You cannot mix Registered and Unbuffered memory in the same system."
I find this statement confusing. I've mixed different sizes of ECC before on other computers. Is this a limitation of Intel's memory controller? An Apple imposed limitation for stability reasons, or just ram merchant snake oil?

 

[Dranix]

The 16GB dimms have to be FB modules. 4 and 8GB is unbuffered. i guess you can mix 4 and 8 but not with 16.

 

[Umbongo]

I ordered my new Mac Pro "short loaded" with 16gb of ram. Expecting 1866 ram to come down in price over time, and even at today's prices it is cheaper to buy elsewhere.
What I've noticed at Crucial and OWC sites is that they state you cannot mix different sized ram chips.
"Installation of 16GB modules requires all previous lower density modules to be removed. You cannot mix Registered and Unbuffered memory in the same system."
I find this statement confusing. I've mixed different sizes of ECC before on other computers. Is this a limitation of Intel's memory controller? An Apple imposed limitation for stability reasons, or just ram merchant snake oil?

Has nothing to do with sizes and everything to do with the line below. The 4GB and 8GB DIMMs are ECC unbuffered, or Crucial are assuming they are. 16GB DIMMs only come as Registered.

"Installation of 16GB modules requires all previous lower density modules to be removed. You cannot mix Registered and Unbuffered memory in the same system."

 

[amack]

NMP can't run mismatched Ram?

Thanks, I didn't know that. Do the nMP that ship from apple not have ECC ram as standard anyway?

 

[Umbongo]

Thanks, I didn't know that. Do the nMP that ship from apple not have ECC ram as standard anyway?

Yeah they have ECC unbuffered RAM, but 16GB DIMMS are Registered ECC. Normal DIMMs are non-ECC unbuffered.

 

[calaverasgrande]

Ok, but they do make registered ECC 4 and 8gb chips. I have some in a tray right here for our HP workstations (I know blecch!).
That is why I quoted the full phrase. The last bit seems to imply that 4gb and 8gb chips aren't registered but it is hardly a rule. Heck we didn't even have 16gb ram chips not too long ago! So woudl that mean we havn't had registered ECC ram until now? Of course not.

I just kind of find it strange that back in the DDR400, DDR667 days we had to worry about matched sets of ram. Then more recently, with DDR2 and higher speeds coming out we no longer had to match both channels in order to take advantage of dual channel DDR speeds. Not that the speed changed anything in that regard, but the memory controllers became more sophisticated and less picky.

 

[Umbongo]

Ok, but they do make registered ECC 4 and 8gb chips. I have some in a tray right here for our HP workstations (I know blecch!).

Yes they do, but Apple don't use them for 4GB and 8GB capacities. It makes no sense to as the performance is fractionally lower and they would be more expensive for Apple to procure.

There are no 16GB unbuffered DIMMs, so if you want 16GB DIMMs you have to just use those on their own (or with other RDIMMs, but no one is considering you would go buy 4GB or 8GB 1866MHz RDIMMs to use).

http://support.apple.com/kb/HT6064