I had 16gb usable but now i have 7.95gb

Aug 11, 2018
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Hello.I have a MSI B350 Gaming Carbon Pro, AMD Ryzen 1800,EVGA GTX 1080, 2monitors 1 is 144Ghz second is 60Ghz, 16GB of Trident Z RBG and I can only use 8gb from it.It says I have 16gb but I can only use 7.95gb.When i built my pc I had 16 gb usable but now I have only 7.95gb and the pc is maybe 1 month old.All drivers are updated. Any idea what can it be?