For an alphanumeric printer the printer-specific values that need to be
filled in printertag.asm are as follows:

   MaxColumns
      the maximum number of columns the printer can print across the page.

   NumCharSets
      the number of character sets which can be selected.

   8BitChars
      a pointer to an extended character table. If the field is null, the
      default table will be used.

   ConvFunc
      a pointer to a character conversion routine. If the field is null,no
      conversion routine will be used.