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 for interrupt handler
From The Free On-line Dictionary of Computing (30 December 2018) :

  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.

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