Unrais use only haf of the memory installed

Hi

I have a supermicro motherboard with two Xeon cpu's and 16GB ram, when i look under

Memory:8 GB (max. installable capacity 64 GB)

DIMM x 2 = 2048 MB, 667 MT/s

DIMM x 2 = 2048 MB, 667 MT/s

DIMM x 2 = 2048 MB, 667 MT/s

DIMM x 2 = 2048 MB, 667 MT/s

But when you look under memory tab it shows that i have 8 sticks of 2GB

Memory Summary

Number Of Devices:8

even on the dashboard it shows i have 16GB but only 8GB usable

Memory size total usable 16.046 GB

installed 8 GB

How can i allocate the rest of 8 GB.

Thanks