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 (ISR)

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

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	Division by zero errors
Trap Interrupts	Intentionally triggered by a program	Software debugging, handling unexpected error cases

The interrupt process

1. Saving Current State:
   • The processor pushes the values in the registers into a memory structure called the stack.
   • These values are stored in a stack frame to preserve the program’s current state.
2. Executing the ISR:
   • The Program Counter (PC) is updated to point to the instructions of the ISR.
   • The ISR executes its FDE cycle to handle the interrupt.
3. Restoring State:
   • After the ISR completes, the processor retrieves the saved values by popping the stack frame, restoring the original program’s state.
   • The PC is updated to point back to the interrupted program, and the processor resumes the interrupted program.
4. Handling Priorities:
   • If a higher-priority interrupt occurs during an ISR, the higher-priority ISR takes precedence, and its state is also saved in the stack.

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