The memory is already mapped by the BIOS/EFI firmware, before the kernel takes control.
By default, whenever the memory modules used in all different channels have the same size, e.g. two 8 GB modules, the firmware maps the modules with interleaved addresses, to ensure a double throughput for 2 channels, or triple/quadruple/etc. for workstation/server motherboards with more memory channels.
By default, whenever the memory modules used in all different channels have the same size, e.g. two 8 GB modules, the firmware maps the modules with interleaved addresses, to ensure a double throughput for 2 channels, or triple/quadruple/etc. for workstation/server motherboards with more memory channels.