• I/O devices allow us to communicate with the computer system. A computer communicates with the outside world through its input/output (I/O) subsystem.
• I/O is the transfer of data between primary memory and various I/O peripherals.
• I/O devices are not connected directly to the CPU. I/O devices connect to the CPU through various interfaces.
• The CPU communicates to these external devices via input/output registers.
This exchange of data is performed in two ways:
o In memory-mapped I/O, the registers in the interface appear in the computer’s memory and there is no real difference accessing memory and accessing an I/O device. It uses up memory space in the system.
o With instruction-based I/O, the CPU has specialized instructions that input and output. Although this does not use memory space, it requires specific I/O instructions.
• Interrupts play a very important part in I/O, because they are an efficient way to notify CPU that input or output is available for use.