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CPU ram limitations.

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December 14, 2011 6:01:07 PM

I cannot seem to find a definitive answer.

On intel's site, it says the RAM limit is 32gb for a single CPU such as an E3-1245.
Conversely there are motherboards that claim to support as much as 64gb on a single socket motherboard. Dual sockets supporting 64gb then makes sense because each CPU would get 32gb.

However, as I google, I see people claiming to have as much as 256gb installed in a server system. Even with quad sockets, that should be limited to 128gb for intel systems right? (AMD's new Zambezi says it recognizes 64gb per CPU).

So the question is simply, are these CPU limitations real? If I were to put 48gb in a motherboard that claimed to support it, will it really only use 32gb if the chip says it only supports 32gb?

Confused about how to maximize my single CPU ram potential.

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January 13, 2012 3:32:03 PM

phate said:
I cannot seem to find a definitive answer.

On intel's site, it says the RAM limit is 32gb for a single CPU such as an E3-1245.
Conversely there are motherboards that claim to support as much as 64gb on a single socket motherboard. Dual sockets supporting 64gb then makes sense because each CPU would get 32gb.

However, as I google, I see people claiming to have as much as 256gb installed in a server system. Even with quad sockets, that should be limited to 128gb for intel systems right? (AMD's new Zambezi says it recognizes 64gb per CPU).

So the question is simply, are these CPU limitations real? If I were to put 48gb in a motherboard that claimed to support it, will it really only use 32gb if the chip says it only supports 32gb?

Confused about how to maximize my single CPU ram potential.


It basically boils down to how many memory ranks a certain CPU can support. Any dual-channel CPU that can't use registered memory can support four dual-ranked DIMMs. The exact quantity supported at any particular point in time depends on how large of memory ICs memory makers are putting on DIMMs at a given time. DDR3 is supposed to allow for 8 Gbit memory ICs, which means 8 GB per rank of 8 ICs, and 16 GB per dual-ranked module. Nobody makes 8 Gbit ICs yet- they only make 4 Gbit ones at best- so the maximum dual-ranked module size right now is 8 GB. So, both are correct- you can put no more than 32 GB (four 8 GB modules) in the system today, but in the future you can put up to 64 GB in there (four 16 GB modules.)

Dual-socket machines and servers play according to different rules because they can use registered memory. That allows for more memory modules per channel than systems like the E3-1245 that only use unbuffered memory. They can also have larger memory modules because registered memory allows for quad-rank DIMMs and also four-bit-wide memory ICs, so theoretically you can have up to 64 GB in a single registered DDR3 DIMM, whereas unbuffered tops out at 16 GB per DIMM. A dual Socket G34 Opteron setup can support 12 registered DIMMs per CPU and up to 64 GB per DIMM, giving a maximum capacity of 768 GB per socket. Currently they could have up to 384 GB per socket since 64 GB quad-ranked registered DIMMs don't exist yet, but 32 GB ones do.
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January 23, 2012 1:44:14 PM

Best answer selected by phate.
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January 23, 2012 3:21:10 PM

This topic has been closed by Mousemonkey
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