Input / Output Subsystem

There are three basic forms of input and output systems –

• Programmed I/O      
                             
• Interrupt driven I/O
                     
• Direct Memory Access(DMA)
                     

With programmed I/O, the processor executes a program that gives its direct control of the I/O operation, including sensing device status, sending a read or write command, and transferring the data.

With interrupt driven I/O, the processor issues an I/O command, continues to execute other instructions, and is interrupted by the I/O module when the I/O module completes its work.

In Direct Memory Access (DMA), the I/O module and main memory exchange data directly without processor involvement.