How to use all RAM in Windows 10?

Question

Windows

4 GB of RAM is reserved by hardware, identical (almost) memory sticks are used, all with the same frequency, speed and volume - 8 GB.
The paging file is enabled (turning it off does nothing)
Manipulating the checkbox (Msconfig -> Boot -> Advanced Options -> Maximum Memory) and the settings used by the memory in the system do not help from the word "no way".
Rearranging the planks - once every other time it helps, but I would like to know the answer to this question without dancing with a tambourine - I have been fighting with this for several years and I just can’t solve this problem once or twice.
Please help me figure this out once and for all
UPD
Linux (Manjaro XFCE) is the same
MemTotal: 12299952 kB
MemFree: 6804220 kB
MemAvailable: 9298224 kB

Answer 1

Memory is usually reserved in hardware by the graphics controller built into the processor. In most cases, you can adjust the amount through bios.

Answer 2

You have problems with memory sticks. Change frequency and/or timings.

Answer 3

Most likely a broken memory stick. Try slats 1 at a time. If on some one that bar does not start and is broken. Well, or a meme test from dosa.
Also, there may still be problems with the connector for the RAM, or important data not coming to the connector. Both because of faulty radio elements and because of the processor socket. Because of the memory controller, too, can be. Well, the most bonal, the motherboard no longer supports.