interrupts as

课 Progress

0% Complete

Exam code:9618

Interrupt handling

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

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

Type	Definition	Example
Hardware Interrupts	Generated by external devices	Keyboard input, mouse movements, disk I/O requests
Software Interrupts	Triggered by software or the operating system	Application requests to open a file, division by zero errors
Trap Interrupts	Intentionally triggered by a program	Software debugging, handling unexpected error cases

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
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
Interrupt Service Routine (ISR) lookup
- The processor fetches the corresponding Interrupt Service Routine (ISR) associated with the interrupt type
ISR execution
- The processor transfers control to the ISR and executes the routine to handle the specific interrupt
Interrupt exit
- After the ISR completes, the processor restores the content of the registers from step 2
- The fetch-decode-execute cycle is resumed

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 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

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

Answer

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]