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.
Overview
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.
A FLIH implements at minimum platform-specific interrupt handling similar to interrupt routines. In response to an interrupt, there is a context switch, and the code for the interrupt is loaded and executed. The job of a FLIH is to quickly service the interrupt, or to record platform-specific critical information which is only available at the time of the interrupt, and schedule the execution of a SLIH for further long-lived interrupt handling.
FLIHs cause jitter in process execution. FLIHs also mask interrupts. Reducing the jitter is most important for real-time operating systems, since they must maintain a guarantee that execution of specific code will complete within an agreed amount of time. To reduce jitter and to reduce the potential for losing data from masked interrupts, programmers attempt to minimize the execution time of a FLIH, moving as much as possible to the SLIH. With the speed of modern computers, FLIHs may implement all device and platform-dependent handling, and use a SLIH for further platform-independent long-lived handling.
FLIHs which service hardware typically mask their associated interrupt (or keep it masked as the case may be) until they complete their execution. An (unusual) FLIH which unmasks its associated interrupt before it completes is called a reentrant interrupt handler. Reentrant interrupt handlers might cause a stack overflow from multiple preemptions by the same interrupt vector, and so they are usually avoided. In a priority interrupt system, the FLIH also (briefly) masks other interrupts of equal or lesser priority.
A SLIH completes long interrupt processing tasks similarly to a process. SLIHs either have a dedicated kernel thread for each handler, or are executed by a pool of kernel worker threads. These threads sit on a run queue in the operating system until processor time is available for them to perform processing for the interrupt. SLIHs may have a long-lived execution time, and thus are typically scheduled similarly to threads and processes.
In Linux, FLIHs are called upper half, and SLIHs are called lower half or bottom half. This is different from naming used in other Unix-like systems, where both are a part of bottom half.