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Did you know that the Intel Nehalem Xeon 55xx series CPU uses 40-bits for memory addressing, even though it’s a 64-bit CPU? This means it can address a maximum of half-a-terabyte of memory. Consider the table below and see that 40-bit can address up to 512GB – the (current) configuration of a UCS blade therefore needs to fit its memory into this size, and 384GB does that.

Gabrie had a point when he asked “So why doesn’t a UCS blade use the full 512GB?”  This is purely down to combination of memory channels, sub-channels/slots and DIMMs with the Cisco Extended Memory Technology – here’s a great Cisco page on the whole shebang.  Basically, the maximum configuration is:

To get more memory would mean increasing one or more of channels, slots, or DIMM sizes and none of these is a trivial change and would make the available memory > 40 bits addressable:

• To increase the number of CPU memory channels = CPU architecture change
• To increase the number of memory slots = decrease in memory speed which severe impact on performance
• To increase the size of DIMMs = significant $cost and pushes well over the 512GB addressable

Thanks to Gabrie for making me add this extra piece!  You can get to Gabrie’s blog here: http://www.gabesvirtualworld.com/

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