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The UEVENT statements enable, disable, or suspend a user-defined event.
 
UEVENT ON
UEVENT OFF
UEVENT STOP
 
Usage Notes
    ■ UEVENT ON enables trapping of a user-defined event. A user-defined
      event is usually a hardware interrupt (although it can be a software
      interrupt). If a user-defined event occurs after a UEVENT ON
      statement, the routine specified in the ON UEVENT statement
      is executed.
    ■ UEVENT OFF disables trapping of a user-defined event. No trapping
      takes place until another UEVENT ON statement is executed. Events
      occurring while trapping is off are ignored.
    ■ UEVENT STOP suspends trapping of a user-defined event. No trapping
      takes place until a UEVENT ON statement is executed. Events
      occurring while trapping is suspended are remembered and processed
      when the next UEVENT ON statement is executed. However, remembered
      events are lost if a UEVENT OFF statement is executed.
    ■ When a user-defined trap occurs (that is, the GOSUB is
      performed), an automatic UEVENT STOP is executed so that recursive
      traps cannot take place. The RETURN from the trapping routine
      automatically executes a UEVENT ON statement unless an explicit
      UEVENT OFF was performed inside the routine.
    ■ If your program contains event-handling statements and you are
      compiling from the BC command line, use the BC /W or /V option.
      (The /W option checks for events at every label or line number; the
      /V option checks at every statement.) If you do not use these
      options and your program contains event traps, BASIC generates the
      error message, "ON event without /V or /W on command line."
    ■ For more information, see Chapter 9, "Event Handling" in the
      Programmer's Guide.