Screen Control Functions for C02

This is a prototype definition for a library providing functions
to manipulate the screen and move the cursor. Since this code is
highly system dependent, a separate library will need to be 
created for each system type.

The following functions should be defined:

  clrscn();           Clears the screen and returns the cursor to the 
                      home position.
                      
                      Note: May  call a system routine, or may print
                      "clear screen" character sequence.
                      
  crsrhm();           Moves cursor to home position (upper left corner
                      of the screen).    

                      Note: May call mvcrsr(0,0); or may print "cursor
                      home" character sequence.
                      
  crsrlf();           Moves cursor one column to the left. May or may not
                      screen wrap, depending on system.

                      Note: May call mvcrsr; or may print "cursor left"
                      character sequence.

  crsrrt();           Moves cursor one column to the right. May or may not
                      screen wrap, depending on system.
                      
                      Note: May call mvcrsr; or may print "cursor right"
                      character sequence.

  crsrup();           Moves cursor one row up.
                      
                      Note: May call mvcrsr; or may print "cursor up"
                      character sequence.

  crsrdn();           Moves cursor one row down.
                      
                      Note: May call mvcrsr; or may print "cursor down"
                      character sequence.

  mvcrsr(r, c);       Moves cursor to row r, column c.

                      Note: May call system routine, or set system variables.
  
  h = scnhgt();       Returns screen height in rows.
  
                      Note: May call system routine, or read system variable.
                                            
  w = scnwid();       Returns screen width in columns.
  
                      Note: May call system routine, or read system variable.
                                            
  c = scncol();       Returns current screen column.
  
                      Note: May call system routine, or read system variable.
                                            
  r = scnrow();       Returns current screen row.
  
                      Note: May call system routine, or read system variable.
                                            
  txtclr(c);          Sets text color to system color c. 
            
                      Note: May call system routine or may print color change
                      sequence.

  bkgclr(c);          Sets background color to system color c.
                      
                      Note: May call system routine or may print color change
                      sequence.


Assembly Constants (Defined in System Header file)

  BLACK             Code for color Black
  BLUE              Code for color Blue
  GREEN             Code for color Green
  CYAN              Code for color Cyan
  RED               Code for color Red
  MAGNTA            Code for color Magenta
  YELLOW            Code for color Yellow
  WHITE             Code for color White
   
  BREAK             ASCII code for Break/Stop Key
  BCKSPC            ASCII code for Backspace key
  CLEAR             ASCII code for Clear Key
  DELETE            ASCII code for Delete key
  DOWN              ASCII code for Cursor Down Key
  ENTER             ASCII code for Return/Enter key (usually CR)
  ESCAPE            ASCII code for Escape/Abort key (usually ESC)
  FNx               ASCII code for Function Key x
  HOME              ASCII code for Home Key
  INSERT            ASCII code for Insert Key
  LEFT              ASCII code for Cursor Left Key
  RIGHT             ASCII code for Cursor Left Key
  TAB               ASCII code for Tab Key  
  UP                ASCII code for Cursor Up Key

  BTMLFT            ASCII code for Box Drawing Bottom Left Corner
  BTMRGT            ASCII code for Box Drawing Bottom Right Corner
  BTMTEE            ASCII code for Box Drawing Bottom to Cetter Tee
  CTRCRS            ASCII code for Box Drawing Center Cross
  HRZLIN            ASCII code for Box Drawing Horizontal Line
  LFTTEE            ASCII code for Box Drawing Left To Center T
  RGHTEE            ASCII code for Box Drawing Right To Center T
  TOPLFT            ASCII code for Box Drawing Top Left Corner
  TOPRGT            ASCII code for Box Drawing Top Right Corner
  TOPTEE            ASCII code for Box Drawing Top to Center T
  VRTLIN            ASCII code for Box Drawing Verical Line