SpringWater
Member
Why does my win 8 pro 64-bit pc show only 6gb of ram in dxdiag, I upgraded to win 8 pro 64-bit to have 8gb of ram and it gives me only 6gb... 

Do you have a graphics card with shared video memory? If so that could account for the missing 2GB.
Whether it is consumed or not, if you right click on computer and go properties the computer information that comes up should show the correct amount.
You most likely have a problem. See #7.
What does your bios show?
Sometimes it will tell you "hardware reserved" for me if I don't manually set the clock speed and latency settings manually in my motherboard for the RAM. What it defaults too is always wrong and I have to set it myself. Try that. Also try reseating all the memory sticks or trying different slots. Also +1 to what Nev said.
Have you made sure memory remapping is off in the BIOS?
If you have six slots then put 3 matching sticks in alternating slots. Will look like this
3-1-3-1-3-1
or this
1-3-1-3-1-3