In a motherboard the CPU’s gateway to the world is the front-side bus connecting it to the northbridge. Whenever the CPU needs to read or write memory it does so via this bus. It uses some pins to transmit the physical memory address it wants to write or read, while other pins send the value to be written or receive the value being read. An Intel Core 2 QX6600 has 33 pins to transmit the physical memory address (so there are 2^33^ choices of memory locations) and 64 pins to send or receive data (so data is transmitted in a 64-bit data path, or 8-byte chunks). This allows the CPU to physically address 64 gigabytes of memory (2^33^ locations * 8 bytes) although most chipsets only handle up to 8 gigs of RAM.
Now comes the rub. We’re used to thinking of memory only in terms of RAM, the stuff programs read from and write to all the time. And indeed most of the memory requests from the processor are routed to RAM modules by the northbridge. But not all of them. Physical memory addresses are also used for communication with assorted devices on the motherboard (this communication is called memory-mapped I/O). These devices include video cards, most PCI cards (say, a scanner or SCSI card), and also the flash memory that stores the BIOS.
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. A bigger hole arises when memory addresses are reserved for video cards and PCI devices. This is why 32-bit OSes have problems using 4 gigs of RAM. In Linux the file /proc/iomem neatly lists these address range mappings. The diagram below shows a typical memory map for the first 4 gigs of physical memory addresses in an Intel PC: