There are several actions that are generally used by handlers to allow messages returning from requested services (typically an Exec device) to look like incoming request packets. This allows the handler to request an asynchronous operation but be notified of the completion. For example, a handler sends the serial.device a request for a read, but instead of sending a plain IO request, it sends a DOS packet disguised as an IO request. The serial.device treats the packet like a normal IO request, returning it to the handler when it is finished. When the handler gets back its disguised DOS packet, it knows that the read has completed. ACTION_NIL 0 <internal> Although not specifically an action, many returns look like this value because the action field has not been filled in. ACTION_READ_RETURN 1001 <internal> Generally used to indicate the completion of an asynchronous read request. ACTION_WRITE_RETURN 1002 <internal> Generally used to indicate the completion of an asynchronous write request. ACTION_TIMER 30 <internal> Used to indicate the passage of a time interval. Many handlers have a steady stream of ACTION_TIMER packets so that they can schedule house keeping and flush buffers when no activity has occurred for a given time interval.
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