Question POST shows not the whole installed memory as seen in BIOS

[JohnLockeG]

I use the ASUS Sabertooth 990 FX with an AMD FX-8350 and this memory modules:

1. Corsair XMS3 DIMM Kit 16GB, DDR3-1333, CL9-9-9-24 (CMX16GX3M2A1333C9)
2. Corsair XMS3 DIMM 4GB, DDR3-1600, CL9-9-9-24 (CMX4GX3M1A1600C9)

I know about the problems in this case with different volt, speed etc. That's ok for me.

As the manual says, that should not be a problem:

You may install varying memory sizes in Channel A and Channel B. The system maps the total size of the lower-sized channel for the dual-channel configuration. Any excess memory from the higher-sized channel is then mapped for single-channel operation.

But when booting my computer in POST he only shows 12 GB of RAM.
In BIOS all 20 GB of RAM is shown.

Why he did not show the whole 20 GB in POST?
How much memory could now be used in Windows 10 64bit?

I cannot upload the Pictures, so you will find them here.

Thank your for helping me.

 

[Fix_that_Glitch]

In task manager under performance tab what does it show there for memory? Pic possible?

 

[helpstar]

reset the BIOS by jumper RTC
try the memok! button

check if all modules are ok by inserting just a single one and by testing with memtest86+ booted from an USB flash drive