System Interrupts (OCR A Level Computer Science)

System Interrupts

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 Request (IRQ)

   • An external device or software generates an interrupt, signalling the processor to stop its current task

   • The interrupt controller passes this to the interrupt handler for assessment

2. Interrupt Acknowledge

   • The interrupt handler decides if the interrupt needs to be dealt with now or later

   • If yes, the current contents of the processor registers are saved in memory

3. Interrupt Service Routine (ISR) Lookup

   • The processor fetches the corresponding Interrupt Service Routine (ISR) associated with the interrupt type

4. ISR Execution

   • The processor transfers control to the ISR and executes the routine to handle the specific interrupt

5. Interrupt Exit

   • After the ISR completes, the processor restores the content of the registers from step 2

   • The fetch-decode-execute cycle is resumed

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

Nesting of interrupts

Interrupt priority handling

System handling of interrupt priority