Screen Control Functions for C02

This module contains functions for screen and cursor manipulation.

The accompanying assembly code vary by platform, so when invoking the
cross-compilers, the command line option -s must be used to ensure that
the correct header files are included.

At the beginning of the program use the directives

  #include <screen.h02>

The following functions should be defined:

  clrscn();         Clears the screen and returns the cursor to the home
                    position.

                    Returns with no action if the platform does not
                    support the clear screen action.

  crsrhm();         Moves cursor to home position (upper left corner
                    of the screen).

                    Returns with no action if the platform does not
                    support cursor positioning.

  setpos(c,r);      Moves cursor to column c on row r. The topmost row
                    and leftmost column are numbered 0.

                    Returns with no action if the platform does not
                    support cursor positioning.

  c,r = getpos();   Returns the current row and column of the cursor. The
                    topmost row and leftmost column are numbered 0.

                    Note: Returns 255 for both the width and height if
                    the platform does not support cursor positioning.

  w,h = getsiz ();  Returns the current screen or window width and height
                    in rows and columns.

                    Note: Returns 255 for width and/or height if either or
                    both are cannot be determined via software. Since all
                    supported platforms have less than 128 screen rows and
                    columns, the :- test can be used on these values.
