PC Processors

[vanster]

one CPU is an FX 8320-E, the other is a Ryzen 7-2700.

So one uses DDR3, and the other DDR4 RAM

Both systems have 16 GB RAM installed, across 4 slots (24 GB in 2x8 and 2x4 matched pairs), but can use only 12 GB.  Even though according to the BIOS all 4 slots are filled with the correct memory. I have other DDR3 and DDR4 Ram to swap out, but nothing changed.  Board diagnosis says the B1 and B2 slots are fine, but whatever ram is in those slots is not detected by the CPU. 

 Is this common to AMD CPUS, and is there some way to test the CPU alone to verify that it is the problem?

[sebakke]

It sounds to me like you didn't pair your 2 sets of DIMMs properly.  Did you put both 8GB sticks in the A slots and then both 4GB sticks into the B slots?   You should have the 8GB in A1 and B1 and then the 4GB sticks in A2 and B2 or they won't interleave properly.

[vanster]

I have tried installing them both ways:  in A1/B1, A2/B2  and in A1/A2 and B1/B2.  that is Both of the 1 sockets, Both of the 2 Sockets, Both of the channel A sockets and both of the channel B sockets. 

Anything that goes into the A2/B2 sockets is fully utilized by the system.

anything that goes into the A1/B1 sockets  is seen by the BIOS,  but the OS cannot see it, whether Windows or Linux.

I have matched pairs from Crucial, Kingston, HP. 

I have the same issues on an AM4 socket system (DDR4) and an AM3+ socket system (DDR3)

the AM4 system is using a Ryzen7 2700  and the AM3+  is using a FX8320-E