I recently upgraded from an old Intel CPU to a new Ryzen 2700 with an ASUS Prime X470-Pro motherboard. After installing the new CPU/Mobo and keeping all the old parts the same, I booted up to find that only half of my 4x4GB kit was usable in Windows (16 GB seen, 8 GB usable). After not having any luck updating the BIOS version, I thought maybe the RAM was bad so bought a newer 2x8GB kit and replaced the old memory with that, but no luck. It also defaults the memory speed to 2100MHz when it's a 3200MHz rated pair, and using the D.O.C.P profile causes the computer to not boot and force me back to the BIOS screen to reset to defaults. Trying to manually tweak it and raise it in small increments, even to 2200 MHz causes the same issue. The memory sticks are listed on the ASUS QVL list.
I then got an RMA for the motherboard thinking it was a faulty slot (I made sure the memory was properly seated and in the correct channels per manual), however the replacement (new, not refurbished) did not fix the issue. While waiting for the motherboard to come back, I put back the old Intel combo with the new sticks and got the full 16GB usable. Also setting to the default XMP profile gives me the rated 3200MHz stable.
So I then got a replacement for the CPU which just came in today, but again same problem. I'm at a loss right now as the only components that have changed are the Motherboard and CPU, and when returning both of them the same issue persists. It also shows in the BIOS that it recognizes both 8 GB sticks, but only shows 8GB so it's not an OS problem. I reformatted my computer multiple times due to the hardware changes and no luck. I also read that the cooler can be on too tight, so tried loosening it (aftermarket Noctua) and when that didn't work, applied the stock Spire cooler that came with the CPU. My old Intel CPU uses more power than the new AMD one, so it does not appear to be power related. Are there any known issues with this?
So to recap:
https://imgur.com/a/TnIOn1g shows what I see in the BIOS and OS (this was taken with the 4x4GB kit but the 2x8GB kit is the same)
This is typically a ram compatibility issue
1) you are really sure your ram is on the mobo compatibility list
2) boot up with all bios settings on auto/default
3) remove half the ram and boot
... now ram should show up complete then update the bios to 4011
Ryzens CPU are pretty sensitive to the type of RAM being used. One User had the same problem. Turned out that his RAM was not compatible with the Ryzen CPU/APU. He was also using 4 gbytes RAM Memory modules.
Here is your Asus Motherboard's QVL list for the 2nd generation Ryzen CPUs list : http://dlcdnet.asus.com/pub/ASUS/mb/SocketAM4/PRIME_X470-PRO/PRIME-X470-PRO-memory-QVL-for-Ryzen2nd-... .
If your memory is not listed, it is a good indication why you are only seeing 50% of your memory.
I'm using CMK16GX4M2B3000C15 which is listed on the QVL (checked beforehand). I've also tried with two other sets all with the same results. I've booted up with Defaults multiple times and tried with one stick, which shows the proper 8GB.
Under Task Manager - Performance - Memory, What is it showing for the "Reserved Memory"? 8 gbytes of Reserved Memory?
Sometimes motherboards with integrated Graphics, Windows and BIOS allocates a certain amount of RAM Memory for the Integrated Graphics.
If it happens to be a problem with the Ryzen, install Ryzen Master and see if you can control the RAM memory usage from there. This a User guide for Overclocking the Ryzen using Ryzen Master but is useful and showing the different features of the program.: https://download.amd.com/documents/AMD-Ryzen-Processor-and-AMD-Ryzen-Master-Overclocking-Users-Guide...
There is a new BIOS for your motherboard (05/02/2018) version 4011. Is that the one you updated too?
This is the latest AMD CHIPSET for the Ryzen AM4 - x470 Motherboards from AMD Support: Chipset .
So it does show 8.1 GB system reserved, but it also shows 8GB under the BIOS even though it sees both RAM sticks (or all 4 if using those instead). I have heard about making sure the iGPU isn't using it, but I could never find that option in the BIOS. So I went through all the options in the manual and saw that it's under an NB section that I don't have, and that it isn't supported on Ryzen CPUs (http://dlcdnet.asus.com/pub/ASUS/mb/SocketAM4/PRIME_X470-PRO/E13883_PRIME__X470-PRO_UM_WEB.pdf section 3.6.2). It also explains why when I tried taking out my graphics card to just use the onboard, I could never get that to work. I also tried enabling Maximum Memory under msconfig Advanced options.
I have the Ryzen Master tool, but as soon as I start to tweak any of the settings past default (other than voltage), the computer won't POST and I have to set it back to defaults. In the BIOS I've also tried tweaking the way memory pairing works (channel, die, none, etc.) and no effect there either.
Unfortunately not an actual fix. I ended up returning the motherboard and got a different one. I wasn't having luck with the X470's for whatever reason, so got an X370 to try and it's worked without issue.