Source : Free On-Line Dictionary of Computing

interrupt handler
     
         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 {operating
        system} 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)