Interrupts (Cambridge International AS Computer Science)

Interrupt handling

What are interrupts?

• An interrupt is a **signal **to the processor that stops its current task and performs a different task temporarily
• Interrupts can be** hardware events or time-sensitive tasks**
• When an interrupt occurs, the processor suspends the current program execution and transfers control to an interrupt service routine

Purpose and role of interrupts

• Real-time Event Handling: hardware errors and signals from input devices e.g. hard disk failure
• Device Communication: alerts from external devices e.g. printer jams and network errors
• Multitasking: suspending processing in one application so that the user can switch to another

Types of interrupts

The interrupt process

1. Interrupt raised

2. A hardware device or software sends an interrupt and an interrupt flag is set in the interrupt register

3. Interrupt detected

4. At the end of the current F-E cycle, the processor checks the interrupt register

5. The priority of the interrupt is compared with the current process

6. If it is lower priority, the processor carries on with the next F-E cycle and the interrupt waits

7. Current state saved

8. If it is higher priority, the current contents of the processor registers are saved on the stack

9. ISR called and executed

10. The processor calls the appropriate Interrupt Service handling Routine (ISR) for that interrupt

11. The ISR runs to handle the interrupt

12. Return

13. When the ISR finishes, the processor checks for any further higher-priority interrupts

14. Otherwise, the register contents are restored from the stack and the interrupted process continues with the next F-E cycle

The interrupt process

What is an ISR?

• An ISR is a special function that handles a particular interrupt type

• Each type of interrupt has a corresponding routine

  • E.g. printer jam, hard disk failure, file download error, network connection error all have routines to be followed when they happen

• ISRs should be concise, efficient, and carefully designed to minimise the time taken to execute, as they often need to handle time-sensitive events

Interrupt priority and nesting

• Interrupt prioritisation means the processor can acknowledge and switch to resolving a higher-priority interrupt
• Prioritising interrupts is vital because many things can go wrong at the same time
• Lower-priority ISRs may be temporarily suspended until the higher-priority ISR completes the execution
• Nesting of interrupts refers to the ability of the processor to handle interrupts within interrupts
• Proper management of nested interrupts avoids potential conflicts and ensures system stability

Interrupt priority handling

System handling of interrupt priority

Worked Example Explain how the computer handles an interrupt when a key is pressed on the keyboard. [5]

Final answer: Answer

1 mark per bullet (max 5):

• An interrupt flag is raised in the interrupt register [1 mark]
• The system finishes its current fetch-execute cycle [1 mark]
• The interrupt register is checked for a higher priority interrupt [1 mark]
• If a higher priority interrupt is found, the current register contents are pushed to the stack [1 mark]
• The appropriate Interrupt Service Routine (ISR) is executed [1 mark]
• The ISR processes the key press and completes [1 mark]
• The register contents are restored from the stack [1 mark]
• Control is returned to the original process [1 mark]