"When the northbridge receives a physical memory request it
decides where to route it: should it go to RAM? Video card maybe?
This routing is decided via the memory address map. For each region
of physical memory addresses, the memory map knows the device that
owns that region. The bulk of the addresses are mapped to RAM, but
when they aren’t the memory map tells the chipset which
device should service requests for those addresses. This mapping of
memory addresses away from RAM modules causes the classic hole in
PC memory between 640KB and 1MB."
