Gigabyte GA-7DX User Manual page 59

Memory
The category is display-only which is determined by POST (Power On Self Test) of the
BIOS.
Base Memory Size
The POST of the BIOS will determine the amount of base (or conventional)
memory installed in the system.
640 K for systems with 640 K or more memory installed on the motherboard.
Extended Memory Size
The BIOS determines how much extended memory is present during the POST.
This is the amount of memory located above 1 MB in the CPU's memory
address map.
52
BIOS Setup