Evolution of the I/O Function (see Fig. 6)

Figure 6: Evolution of the I/O Function

• Processor directly controls a peripheral device. Example: CPU controls a flip-flop to implement a serial line
• Controller or I/O module is added
  • Processor uses programmed I/O without interrupts
  • Processor does not need to handle details of external devices
  • Example: A Universal Asynchronous Receiver Transmitter
    • CPU simply reads and writes bytes to I/O controller
    • I/O controller responsible for managing the signalling
• Controller or I/O module with interrupts. Processor does not spend time waiting for an I/O operation to be performed
• Direct Memory Access
  • Blocks of data are moved into memory without involving the processor
  • Processor involved at beginning and end only
• I/O module has a separate processor. Example: SCSI controller, controller CPU executes SCSI program code out of main memory
• I/O processor
  • I/O module has its own local memory, internal bus, etc.
  • It is a computer in its own right.
  • Example: Myrinet Multi-gigabit Network Controller