Unit 7

• I/O requirements

  ...

  • Means for addressing different peripheral devices
  • A way for peripheral devices to initiate communication with the CPU
  • Means for handling devices with different control requirements
  • Buses that connect high-speed I/O devices with the computer must support high data transfer rates
• Why are I/O interfaces necessary?

  Because...

  • Different formats required by the devices
  • Bursts of data v.s streaming data
  • Device control requirements that would tie up too much CPU time
  • Incompatibilities in speed between the devices and the CPU make synchronization difficult
• Interrupts

  Signals that cause the CPU to alter its normal flow of instruction execution. (Frees CPU from waiting for events, provides control for external I/O initiation)

  Examples:

  • Unexpected input
  • Abnormal situations
  • Illegal instructions
  • Multitasking/processing
• Use of interrupts

  ...

  • Notify that an external event has occurred
  • Signal completion
  • Allocate CPU time
  • Indicate abnormal event
  • Software interrupts
• What is direct memory access (DMA)?


  A computer feature that allows hardware devices to transfer data to and from the main memory without involving the CPU

  
  

  Required conditions for DMA:

  • I/O interface and memory must be connected
  • I/O module must be capable of reading and writing to memory
  • Conflicts between the CPU and I/O module must be avoided
  • Interrupt required for completion
• I/O module functions

  ...

  • Recognizes messages from devices addressed to it and accepts commands from the CPU
  • Communicates with CPU
  • Copies data from its buffer to the device/from the CPU to its buffer
  • Physically controls the device
  • Provides the necessary registers and controls to perform a direct memory transfer