Information about the printer in use can be obtained by reading the
printerdata and printerextendeddata structures.  the values found in these
structures are determined by the printer driver selected through
Preferences.  The data structures are defined in devices/prtbase.h.

Printer specific data is returned in printerio when the printer device is
opened.  To read the structures, you must first set the printerdata
structure to point to iodrp.io_Device of the printerio used to open the
device and then set printerextendeddata to point to the extended data
portion of printerdata.

     printer_data.c