Only 3.9GB useable out of 8GB (Windows 8 64bit)

I had the same problem.

I think that the answer to fixing this problem is to reduce the speed of the RAM manually by one speed level in the BIOS I reduced mine from 1600 Mhz to 1333Mhz. (The actual hardware speed of my Crucial chips are both 1600MHz. My RAM chips are both Crucial BLS4G3D1609DS1S00. They are not recommended by ASUS for the board. The nearest recommendation is Crucial BLT4G3D1608DT1TX0.16FM(XMP). Maybe the motherboard's BIOS is not stable yet. I loaded the latest BIOS version ie. Version 2101 dated Oct 2013 but this didn't fix the problem.

I then turned off the whole computer at the power socket in the wall. Afterwards, I switched the power on at the socket but made sure that I did not press the power button on the computer. (You must do it this way to get the MeMOK! button to work). I pressed the MeMOK! button on my Asus M5A97 R2.0 motherboard (It is near the RAM chips on the motherboard - see the manual of the motherboard). The MeMOK! light flashed a few times and made noises. The computer loaded up to the point on the BIOS where it asked me to press the F1 key and then save the changes that it made to the BIOS. I did all this and the system recognised my 8GB of RAM.

If this does not work I found that I got it to work by turning off the computer at the wall socket so that there is no power lights on the motherboard again. I then took out both the RAM chips and put them back into the same slots again (ie. the two blue sockets). I think that this seemed to tell the computer that the chips are new chips on the board. I turned on the power at the socket and then pressed the MeMOK! button again. The computer loaded up the BIOS to where it asked me to press the F1 key and then save the changes that it made to the BIOS (ie. save the changes when you exit the BIOS). I did all this and I found that I got the BIOS and Windows to recognise my 8GB RAM.

I found this out because I took out my RAM chips to find out their speed etc. After I took them out the system recognised my 8GB of RAM but said that only 3.9GB was useable again as before. Once you have got it to work leave your RAM chips in the sockets and there won't be any problems.

You might have to do all this a number of times to get it to work. I can't guarantee that it will work for you but I can say that it has definitely worked for me. I have since shut my computer down and reloaded it a number of times and it still recognises the 8GB without any problems.

(As you know to get into the BIOS on an ASUS you press the delete key or the F2 key when the message appears on the screen when you are loading up the computer (ie booting the computer). I write this to help others that don't understand how to get into the BIOS. Then you click on the "Advanced" button (F7 key or use the mouse if I remember correctly). Find the "Memory Speed" (I think it in the "Tweek" menu and change it. Then save your changes when you "exit" from the BIOS.)

I now have the full 8GB working properly without the 3.9GB useable message. I also notice that the Microsoft RAM speed moved from 5.9 to 7.4 when I did a new Speed check. It seemed to think that I changed my hardware and asked me to do the speed check again after I fixed the RAM to work properly. Everything works beautifully now.

This all looks very complicated but it isn't really. I have tried to includes as much detail as possible so that anyone can do it. Print this out to help you do it.