Printer Drivers

               Printer  drivers  exist in  memory  from  $7900-$7FFF.  This
          overlaps  part  of  screen 2.  Printer drivers  are  only  memory
          resident  when  they  are in use.  GEOS loads the  first  printer
          driver that it can find on the disk,  whenever it needs to  print
          something.  To make a particular printer driver always be the one
          found,  all  that  must be done is to place it ahead of  all  the
          other  printer  drivers in the directory.  There are  five  entry
          points  to a printer driver.  These form a jump table from $7900-
          $790E.

          $7900: InitForPrint
               The first entry point,  at $7900,  is the master reset. This
          routine must initialize any global variables that need to be set.
          This routine is called once when the driver is loaded.  Usually
          only the baud rate it set with this routine.

          $7903: StartPrint
               The   second  entry  point,   at  $7903,   is  the   printer
          initialization.  It is used to initialize the serial bus and open
          a logical file for the CBM KERNAL to address.  This  routine   is
          called just prior to printing an image. The X register must be set
          to an error code if the printer is not available.  This error code 
          is the C64's KERNAL status byte at location $90. This routine must
          initialize  any  temporary variables used by the driver.
 
          $7906: PrintBuffer
               The  third  entry point,  at $7906,  actually  performs  the
          printing of a line.  Locations $02-$03 R0  point to the bit image
          graphic data for the line (640 pixels,  8 bits high,  640 bytes).
          Locations $04-$05 R1 point to a similar buffer area free for use
          by  the driver if it needs it. This is mainly  to give  printers
          which only print  7 pixels at a time a  place to  accumulate  the
          extra  bits.  Locations $06-$07 R2 point to  the color   data for
          the  line.  This  is  only  for  the benefit  of  color printers.
          The printer driver must not modify any of these pointers.
               This routine works by calling SetDevice on device 4 and then
          calling InitForIO to access the CBM KERNAL.  DoneWithIO should be
          called before returning to the application.

          $7909: StopPrint
               The fourth entry point,  at $7909,  closes the printer. This
          routine  is called when the image is finished.  This allows 7 bit
          printers to print the contents of their buffers.  This is also to
          give  the  printer  driver  a  chance to  print  a  top  of  form
          character.  Like PrintBuffer, this routine requires $02-$03 R0 to
          point to any extra data to print, and $04-$05 to point to  a  640
          byte buffer for the driver's use.  It should send a form-feed for
          the end of page.  

          $790C: GetDimensions
               The  fifth  entry point,  at $790C,  returns the  number  of
          character  columns that the printer can print in the X  register.
          The  maximum  number  of  lines per page is  returned  in  the  Y
          register.  The  accumulator  is loaded with a  zero.  This  gives
          applications  the  ability  to compute  necessary  buffer  sizes.
          Typical  values  are 80 columns by 90 lines or 60 columns  by  90
          lines.

          $790F: PrintASCII
               The sixth entry point, at $790F, is used for printing lines
          of ASCII text in draft or nlq modes.  $02-$03 R0 should point to
          a  null-terminated  line  of ASCII  text for  printing.  $04-$05
          should point to a 640 byte buffer for print driver use.

          $7912: StartASCII
               The seventh entry point, at $7912, is the same as StartPrint
          except that it prepares the printer for draft or nlq printing. It
          should set up device 4 for the printer.

          $7915: SetNLQ
               This routine  should send whatever initialization string  to
          the printer which is necessary for it to print in its NLQ mode.