Interrupt driven I/O

Interrupt driven I/O

1. Benefit

   • Overcomes CPU waiting time

2. How it works

   • Processor issue an I/O command to module and do others processing
     • I/O module will check the availbality
       • Interrupt the processor when I/O is ready to exchange data
         • The processor will stop the current processing task and executs when data finished transfer

3. Design Issue

   • Unable to determine which device issued the interrupt
   • Unable to determine the priority of the interrupt
   • Ways to overcome
     • Multiple interrupt line
     • Software poll
       • When interrupt happends the interrupt service program will ask which I/O module interrupted it
         • *This process is time consuming
     • Daisy Chain
       • How it works
         • uses a hardware poll
           • When processor got interrupt it sends out the interrupt acknowledgement signal
             • The signal goes to all the I/O module
               • The requesting I/O module will respond by placing its address on the data line
                 • So now the processor know which I/O module interrupted and can proceed with necessary actions
     • Bus arbitration
       • I/O must gain control of the bus before it can issue an interrupt
         • This way only allow one interrupt in given time

4. Disadvantages

   • The processor is tied up in managing an I/O transfer because a number of instrustion must be executed by the processor
   • The I/O transfer rate is limited by the speed of the processor