Only 3.2GB of 4GB RAM detected on 64-bit Debian
Solution 1:
You need to turn Memory Remapping ON in the BIOS. Otherwise, it won't remap memory above the 4GB boundary and some memory will be covered up by hardware mappings.
Solution 2:
PCI also uses some of the address space. Both PCI cards and PCI devices build into the motherboard (such as the chipset).
Usually this meant that out of 4GB only 3¼ to 3½ GB was available, even if you did not use a modern GPU with a lot of onboard RAM.