What are interrupts and how interrupt handling is done in modern operating systems?

Interrupts are signals generated by hardware or software when a particular event requires immediate attention from the processor. These signals inform the CPU about high-priority, urgent information that demands interruption of the current working process to handle time-critical tasks.

When an interrupt occurs, the processor completes the current instruction execution and transfers control to a special routine called an Interrupt Service Routine (ISR) or interrupt handler. This mechanism ensures that critical events are processed promptly without losing system responsiveness.

Types of Interrupts

Interrupt Handling in Modern Operating Systems

Modern operating systems like Linux, macOS, and Windows implement a two-level interrupt handling mechanism to balance responsiveness and system stability −

First-Level Interrupt Handlers (FLIH)

Also known as hard interrupt handlers or upper half in Linux −

• Handle platform-specific, time-critical interrupt processing

• Execute with interrupts disabled (masked) to prevent nesting

• Perform minimal, essential work to maintain system timing

• May cause execution jitter due to immediate processing requirements

Second-Level Interrupt Handlers (SLIH)

Also known as soft interrupt handlers or bottom half in Linux −

• Complete lengthy interrupt processing tasks deferred from FLIH

• Run as dedicated threads with longer execution times

• Execute with interrupts enabled, allowing preemption

• Handle complex processing that doesn't require immediate attention

Interrupt Handling Mechanism

The operating system uses an interrupt vector table that maps interrupt numbers to their corresponding ISR addresses. When an interrupt occurs, the processor uses this table to quickly locate and execute the appropriate handler routine.

Key Features

• Prioritized handling − Higher-priority interrupts can preempt lower-priority ones

• Context preservation − CPU state is saved before and restored after interrupt handling

• Nested interrupts − Modern systems support interrupt nesting with proper priority management

• Deferred processing − Non-critical tasks are postponed to maintain system responsiveness

Conclusion

Interrupt handling is fundamental to modern operating systems, enabling efficient multitasking and responsive I/O operations. The two-level approach balances immediate response requirements with system stability, ensuring that critical events are handled promptly while maintaining overall system performance.