"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."
