An interrupt handler is a low-level counterpart of event handlers. These handlers are initiated by either hardware interrupts or interrupt instructions in software, and are used for servicing hardware devices and transitions between protected modes of operation such as system calls.
A FLIH implements at minimum platform specific interrupt handling similarly 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. Otherwise, these interrupt handlers might cause a stack overflow from multiple preemptions by the same interrupt vector. 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.
It is worth noting that in many systems the FLIH and SLIH are referred to as upper halves and lower halves, hardware and software interrupts, or a derivation of those names.