Can i mix memory size ?

enny001

macrumors member
Original poster
Apr 30, 2013
77
0
Hi guys i have mac pro 2009 2.66 and have 12x3 gb owc with one free slot can add another 8 gb in to that free slot or it hsas to match i was told by own it has to match

Thanks
 

T5BRICK

macrumors G3
Aug 3, 2006
8,102
2,095
Oregon
*edit* disregard my answer. I accidentally clicked on the Mac Pro section, I was thinking MacBook Pro.

To answer the question in the subject line. Yes you can mix different sizes of memory.
 

enny001

macrumors member
Original poster
Apr 30, 2013
77
0
why was i told by owc that i cant they toled me that it has to be 4 gb all of them. Will i get any problems if i place another 8 gb with 3 sticks 4 gb i will have 20gb if i do this

Thanks
 

Studio K

macrumors 6502
Feb 17, 2013
361
7
United States
why was i told by owc that i cant they toled me that it has to be 4 gb all of them. Will i get any problems if i place another 8 gb with 3 sticks 4 gb i will have 20gb if i do this

Thanks
Try placing the largest memory module into Slot 1. 8GB in slot 1 and 4GB in slots 2,3, & 4. The worst thing that can happen is the Mac Pro won't boot. If it does start up, then OS X will notify you if the memory is installed in the correct confiuration.

I'm using 4GB modules in slots 1 & 2 with 2GB modules in slots 3 & 4.
 

Draeconis

macrumors 6502a
May 6, 2008
968
264
You can't do that, no. 8Gb DIMMs can't be mixed with smaller modules, the Mac Pro won't pass POST if you try.
 

Draeconis

macrumors 6502a
May 6, 2008
968
264
We can, I did that already. It's about the spec, but not the size (e.g. we can't mix RDIMM with UDIMM).
You can't, because I've tried exactly this and it doesn't work.

8Gb and 16Gb modules can't be mixed with 1, 2 and 4Gb.
 

h9826790

macrumors G5
Apr 3, 2014
14,107
6,732
Hong Kong
Done! Case closed.

I can mix 8G with 2G sticks, and ECC RAM with non ECC RAM, and 1333 with 1066 RAM, and in the non optimum order / slots.

The result is all RAM works at 1066 without ECC. That's it.

Screen Shot 2015-07-27 at 18.16.51.jpg

Screen Shot 2015-07-27 at 18.17.12.jpg

Screen Shot 2015-07-27 at 18.17.25.jpg
 
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IowaLynn

macrumors 68000
Feb 22, 2015
1,555
356
You can't do that, no. 8Gb DIMMs can't be mixed with smaller modules, the Mac Pro won't pass POST if you try.
In the early days of the 4,1 8GB DIMMs could not (and they were not available even to use or test in early 2009) - but that changed in the year afterwards, maybe by the time the Mid-2010 came out.
 

h9826790

macrumors G5
Apr 3, 2014
14,107
6,732
Hong Kong
Are they all UDIMM? The biggest restriction AFAIK is you can't mix RDIMM with UDIMM.

Anyway, there is no guarantee mix RAM can work flawlessly. All I can prove is just that's possible.
 
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