Memory Not used

Hello Tom's Hardware people ...once again i seek your wisdom in the arts of computing and tech that i don't understand  

i just finished (with the help of Tom's Hardware) my new build ...here are my basic specifications :

ASUS sabertooth z77 Motherboard
Intel Core i7 3770K running at stock clock @3.50GHz
8GB of corsair vengeance ram
XFX HD7950 Double Dissipation BLACK (FX-795A-TDBC) with 3072 MB buffer
128GB Samsung MZ-7PC128B/WW SSD
1TB Western Digital 7500RPM hard drive

now the question is when i go to my computers properties it says : Memory 8GB(7.70GB is useable) ...

here is a photo :

http://postimage.org/image/pboh9g3id/

what is this and why is it like this ??not all memory is usable even though my motherboard supports up to 32GB memory ???

here is another photo of the task manager Ram status :

http://postimage.org/image/3vj8t5ykh/

the system is running fine and smoothly its just that i have to learn and i also ask out of curiosity

thanks in advance for any help..