Command         Operation
-------         ---------
CMD_FLUSH       Purge all active and queued requests for the narrator
                device.

CMD_READ        Read mouth shapes associated with an active write from the
                narrator device.

CMD_RESET       Reset the narrator port to its initialized state. All
                active and queued I/O requests will be aborted.  Restarts
                the device if it has been stopped.

CMD_START       Restart the currently active speech (if any) and resume
                queued I/O requests.

CMD_STOP        Stop any currently active speech and prevent queued I/O
                requests from starting.

CMD_WRITE       Write a stream of characters to the narrator device and
                generate mouth movement data for reads.


Exec Functions as Used in This Chapter
--------------------------------------
AbortIO()       Abort a command to the narrator device. If the command is
                in progress, it is stopped immediately.  If it is queued,
                it is removed from the queue.

BeginIO()       Initiate a command and return immediately (asynchronous
                request).  This is used to minimize the amount of system
                overhead.

CloseDevice()   Relinquish use of the narrator device.  All requests must
                be complete.

CheckIO()       Return the status of an I/O request.

CloseLibrary()  Relinquish use of a previously opened library.

DoIO()          Initiate a command and wait for completion (synchronous
                request). Should be used with care because it will not
                return control if the request does not complete.

OpenDevice()    Obtain use of the narrator device.

OpenLibrary()   Obtain use of a library.

SendIO()        Initiate a command and return immediately (asynchronous
                request).

WaitIO()        Wait for the completion of an asynchronous request. When
                the request is complete the message will be removed from
                reply port.


Exec Support Functions as Used in This Chapter
----------------------------------------------
CreateExtIO()   Create an extended I/O request structure of type
                narrator_rb.  This structure will be used to communicate
                commands to the narrator device.

CreatePort()    Create a signal message port for reply messages from the
                narrator device.  Exec will signal a task when a message
                arrives at the port.

DeleteExtIO()   Delete an extended I/O request structure created by
                CreateExtIO().

DeletePort()    Delete the message port created by CreatePort().