Interrupt handler

In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a callback function in microcontroller firmware, an operating system, or a device driver whose execution is triggered by the reception of an interrupt. In general, interrupts and their handlers are used to handle high-priority conditions that require the interruption of the current code the processor is executing.

Interrupt handlers have a multitude of functions, which vary based on what triggered the interrupt and the speed at which the interrupt handler completes its task. For example, pressing a key on a computer keyboard, or moving the mouse, triggers interrupts that call interrupt handlers which read the key, or the mouse's position, and copy the associated information into the computer's memory.

An interrupt handler is a low-level counterpart of event handlers. Interrupt handlers are initiated by either hardware interrupts or software interrupt instructions and are used for servicing hardware devices and transitions between protected modes of operation, such as system calls.

In several operating systems‍—‌Linux, Unix, macOS, Microsoft Windows, z/OS, DESQview and some other operating systems used in the past‍—‌interrupt handlers are divided into two parts: the First-Level Interrupt Handler (FLIH) and the Second-Level Interrupt Handlers (SLIH). FLIHs are also known as hard interrupt handlers or fast interrupt handlers, and SLIHs are also known as slow/soft interrupt handlers, or Deferred Procedure Calls in Windows.

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