Memory Mapped I/O

Memory I/O devices are mapped into the system memory map with ROM and RAM. To access a hardware  device, simply write or  read  to  those  'special'  addresses  by using  the  normal  memory  access instructions. The advantage to this method is that all instruction which may access memory may be utilized to manipulate an I/O device. The disadvantage to this method is that the complete address bus might be completely decoded for all devices.   For instance, a machine with a 32-bit address bus would need logic gates to resolve  the  state  of  all  32  address  lines to accurately decode the particular address of any device. This increases the cost of adding hardware to the machine.