I know it's not my thread but I'll chime in anyway.
At least you've got 16GB Of my 64GB of RAM I get a whopping... 1.8GB of usable RAM.
From what I read in other topics, 'half the RAM' being available can be attributed to using DELL RAID cards (e.g. the H310 I use), which can be mitigated by blocking the SMBus pins. That's something I'll try once parity sync is through.
However, I use 4 sticks á 16GB and I get 1/8th of a single stick available, so something very strange is going on. 4 Sticks being dead is very unlikely, but I'll test them also after parity. I've also added my diagnostics for good measure, where it mentions This.
41 --- WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 63456MB of RAM.
bentobox-diagnostics-20230622-1239.zip
Things I'll try but currently haven't:
- update Asus Prime Z390-A BIOS (it's somewhat old)
- block SMBus pins on IT'd H310
- check for bad slots / dimms