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Why does a motherboard like a SuperMicro MBD-X8DTH-6F-O support 24gb of unbuffered memory but also support 192gb of registered memory?

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up vote 5 down vote accepted

You've kind of answered this yourself, by the use of buffering - by using buffers memory operations don't have to be precisely synchronised allowing for more memory per channel. It's why registered memory is more expensive, it allows more memory per channel.

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Buffered/Registered memory doesn't use the data bus to power the logic gates at every memory chip on the module; it only powers the buffer IC, thus putting significantly less demand on the bus and allowing more modules. –  Chris S May 11 '11 at 16:41
    
Nicely put Chris. –  Chopper3 May 11 '11 at 16:43
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Chopper3 noted that registered memory allows one to install more DIMMs per memory channel. The other half of the equation is the amount of memory per DIMM. Supermicro (the mobo vendor) tested 16 GB registered DIMMs, and 2 GB unbuffered DIMMs. That isn't to say that larger unbuffered DIMMs won't work, only that supermicro didn't bother testing larger ones.

But, the final aspect is that one shouldn't discount documentation changes. Per the supermicro website (http://www.supermicro.com/products/motherboard/QPI/5500/X8DTH-6F.cfm), the UDIMM max is 48 GB (4GB DIMMs).

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