interrupt handler

software A routine which is executed when an interrupt

occurs. Interrupt handlers typically deal with low-level

events in the hardware of a computer system such as a

character arriving at a serial port or a tick of a

real-time clock. Special care is required when writing an

interrupt handler to ensure that either the interrupt which

triggered the handler's execution is masked out (inhibitted)

until the handler exits, or the handler is re-entrant so

that multiple concurrent invocations will not interfere with

each other.

If interrupts are masked then the handler must execute as

quickly as possible so that important events are not missed.

This is often arranged by splitting the processing associated

with the event into "upper" and "lower" halves. The lower

part is the interrupt handler which masks out further

interrupts as required, checks that the appropriate event has

occurred (this may be necessary if several events share the

same interrupt), services the interrupt, e.g. by reading a

character from a UART and writing it to a queue, and

re-enabling interrupts.

The upper half executes as part of a user process. It waits

until the interrupt handler has run. Normally the operatingsystem is responsible for reactivating a process which is

waiting for some low-level event. It detects this by a shared

flag or by inspecting a shared queue or by some other

synchronisation mechanism. It is important that the upper and

lower halves do not interfere if an interrupt occurs during

the execution of upper half code. This is usually ensured by

disabling interrupts during critical sections of code such

as removing a character from a queue.

(2002-07-24)