Section 6 - System Interrupts

Computers provide a mechanism by which other modules may interrupt the normal processing sequence

• Some devices can be many orders of magnitude slower than the CPU
• Interrupts allow the CPU to work, pausing to service external devices only when they signal that they require attention
• The processor and operating system are responsible for suspending and resuming the user program appropriately

Common Classes of Interrupts

• Program
  • e.g. overflow, division by zero
• Timer
  • Generated by internal processor timer
• I/O
  • An I/O controller signals operation completion
• Hardware Failure
  • e.g. power failure, memory parity error

Flow Control

Consider a program which is intermittently required to write data

• This is done by a separate I/O program with its own preparation and completion operations
• Interrupts allow this to be done efficiently
• The main program may still be required to wait
  

Interrupt Cycle

As part of the instruction cycle the processor checks for an interrupt signal

• If there's no interrupt, fetch the next instruction
• If an interrupt is pending:
  • Suspend execution of the current program and save its "state",
  • Set the PC to the start address of the interrupt handler routine and process the interrupt,
  • Restore the previous state and continue the interrupted program

Augmented Instruction Cycle

Multiple Interrupts

Approach 1: Disabled interrupts ( sequential )

• The processor ignores further interrupts whilst processing one interrupt
• Ignored interrupts remain pending and are checked after the first interrupts has been processed
• Interrupts are handled in the sequence as they occur
  

Approach 2: Define Priorities ( nested )

• Low priority interrupts can be interrupted by higher priority interrupts
• When the higher priority interrupt has been processed the processor returns to the previous interrupt
  

Summary

• Software allows a computer to execute different instructions without reconfiguring the hardware
• The meaning of the contents of Machine Word of memory is defined by the context in which it is used
• It pays to set slow external processes to run concurrently but to keep interrupts to check their status