Does the Mac Pro have to be in pairs?


macrumors 6502
Original poster
Jan 8, 2006
New Jersey
I'm looking to add another gig of ram into my Mac Pro. I've seen companies sell single sticks of DDR667 FB-DIMM RAM.

I've read that its good for performance to have it in pairs, but no one specifically says you can't install just one RAM module. Is it possible? 3 sticks on a riser? or 2 + 1?


macrumors 6502
Feb 6, 2004
I am not 100% sure because these new Macs use EFI instead of a traditional bios, EFI can be configured by Apple to lock out certain configurations.

If this was a traditional setup, or Apple was kind enough to setup EFI to take single stick and convert down to single channel, then you could purchase single dimms not in pairs for your machine.

I will be purchasing a Mac Pro for a small company next week and I also have found little to no documentation about its support for single DIMMS that are added without being in pairs.

Sun Baked

macrumors G5
May 19, 2002
Yes ... installation instructions here

Mac Pro Memory (FB-DIMM) DIY Replacement Instructions (pdf)

The machine is capable of Quad channel memory utilizing 2 pairs of matched memory (aka, 4 matched DIMMs) ... but 4 matched DIMMs aren't needed, just 1 pair at a time.

If you can understand the picture, there are two dual channel memory controllers on the machine. You should also see the need to alternate pair installations from Riser A to B, to make best use of the bandwidth -- because only two DIMMS can be accessed on the riser at a time.

Edit: That should confuse somebody. :D


macrumors member
Aug 31, 2006
Even if it did work, it would probably be like having 3 tires on a 4 wheel drive vehicle.


macrumors member
Sep 6, 2006
the diagram on the inside of the side panel of the MP's case shows to install 2 at a time.

Spaceman Spiff

macrumors regular
Oct 9, 2003
apfhex said:
"Does the Mac Pro have to be in pairs?"

No, but you sure would have a fast setup with two Mac Pros operating together. ;)
Ah, you beat me to it.

I think a nice 2 for 1 deal from Apple would be lovely, actually.

I'm still curious whether match pairs of pairs on the same riser make any difference.


macrumors 68040
May 9, 2006
I have a spinoff question on this - if I get a Mac Pro with the standard 1 meg of ram and want to add another more memory, does the next set of RAM modules need to be the same size to take advantage of the dual memory increase? Or could I add 2 1-meg modules to it (total of 3 meg) and still get the performance increase?