Solution

The correct answer is Option 4.

Key Points

• To understand the correct sequence of a service routine, it is essential to follow the logical order of operations when handling an interrupt:
  • Save contents of processor registers (A): This ensures that the current state of the processor is preserved, so it can be restored later.
  • Check which flag is set (D): Determine which device or event caused the interrupt.
  • Service the device whose flag is set (C): Perform the necessary actions to handle the interrupt.
  • Restore contents of processor registers (E): Restore the processor to its previous state.
  • Turn the interrupt facility on (B): Re-enable interrupts to allow further interrupt handling.

The correct sequence is: A, D, C, E, B.

Additional Information

• This sequence ensures that the system correctly handles the interrupt without losing any critical information or causing inconsistencies.
• Each step is crucial for maintaining the integrity of the processor state and ensuring that the system can continue to operate correctly after the interrupt is serviced.