Command         Operation
-------         ----------
cmd_flush       remove all queued requests for the printer device.  does
                not affect active requests.

cmd_reset       reset the printer device to its initialized state.  all
                active and queued I/O requests will be aborted.

cmd_start       restart all paused i/o requests

cmd_stop        pause all active and queued i/o requests.

cmd_write       write out a stream of characters to the printer device.
                The number of characters can be specified or a
                NULL-terminated string can be sent.

prd_dumprport   dump the specified rastport to a graphics printer.

prd_prtcommand  send a command to the printer.

prd_query       return the status of the printer port's lines and
                registers.

prd_rawwrite    send unprocessed output to the the printer.


Exec Functions as Used in This Chapter
--------------------------------------
abortio()       abort a command to the printer device.

closedevice()   relinquish use of the printer device.  all requests must
                be complete before closing.

doio()          start a command and wait for completion (synchronous
                request).

opendevice()    obtain use of the printer device.

sendio()        start 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
                the printer message port.


Exec Support Functions as Used in This Chapter
----------------------------------------------
createport()    create a signal message port for reply messages from the
                audio device.  Exec will signal a task when a message
                arrives at the reply port.

createextio()   create an i/o request structure of type printerio.  this
                structure will be used to send commands to the printer
                device.

deleteport()    delete the message port created by createport().

deleteextio()   delete an i/o request structure created by createextio().