IO Interrupts

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I/O Interrupts

• Computer Architecture 215

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Interrupts

• I/O interrupt
• It interrupts program when I/O is ready, return
  when done with data transfer
• I/O interrupt is asynchronous
• Not associated with any instruction
• Doesnt need to be handled immediately

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Fig. 8.12 Simplified Interrupt Circuit for an I/O
interface

• ireq interrupt request signal
• iack interrupt acknowledge signal
• The data bus is used to return interrupt
  information and vector bits.

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One and only one requesting device must receive
an acknowledge signal solution Fig. 8.13
Daisy Chain

• Notice that ireq signal is a single, continuous
  bus wire to all devices, iack is logically
  different at every device, with each device
  producing iack for the next one in a linear chain.

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What is a software interrupt handler? It is
a special procedure of the process, devoted to
completing the I/O request after an interrupt
occurs.The general functions of an interrupt
handler

• 1. Save the state of the interrupted program
• 2. Do programmed I/O operations to satisfy the
  interrupt request
• 3. Restart or turn off the interrupting device
• 4. Restore the state and return to the
  interrupted program

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• Interrupt Priority and Nested Interrupts
• Response deadline maximum time that a handler
  can take between when a request is issued and
  when the device must be serviced.
• Nested interrupts A system of interrupts that
  allows an interrupt handler to be interrupted.
• Critical section a code sequence in the handler
  during which a second interrupt could interfere.

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The general structure of a nested interrupt
handler

• 1. Save the state changed by interrupt
• 2. Disable lower priority interrupts
• 3. Re-enable exception processing
• 4. Service interrupting device
• 5. Disable exception processing
• 6. Re-enable lower priority interrupts
• 7. Restore saved interrupt state
• 8. Return to interrupted program and re-enable
  exceptions.

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• Interrupt mask a bit vector use to
  enable/disable the individual interrupts.
• Priority mask use to set the interrupts
  priority of different devices.

• higher-priority devices come first and
  lower-priority devices come last.

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References

• Computer Systems Design and Architecture
• http//cs.wcsu.edu/joelw/arch/09_IO.ppt
• http//en.wikipedia.org/wiki/Interrupt
• http//dme.uma.pt/jcardoso/Teaching/AC/Lectures/5
  B65D20Entrada20e20Saida.pdf