Atari 400 Technical Reference Manual page 46

FILDAT [02FD] contains the fill data, and ROWCRS and COLCRS
contain the cursor coordinates of the line endpoint. The value
in ATACHR [02FB] will be used to draw the line; ATACHR always
contains the last data read or written, so if the steps above
are followed exactly, ATACHR will rtot have to be modified.
The value for the command byte for FILL is $12.
User-Alterable Data Base Variables
Certain functions of the Display Handler re~uire you to
examine and/or alter variables in the
as
database. The following
describes some of the more commonly used handler variables. (see
Appendix L, Bl-55, for additional descriptions).
Cursor Position
Two variables maintain the cursor position for the graphics
screen or mode 0 text screen. ROWCRS [0054] maintains the display
row number; and COLCRS [0055] maintains the display column
number. Both numbers range from 0 to the maximum number of
rows/columns, - 1. The cursor can be set outside of the defined
text margins with no ill effect. You can read and write this
region. The home position (0,0) for both text and graphics is the
upper left corner of the screen.
ROWCRS is a single byte. COLeRS is maintained at 2-bytes, with
the least significant byte being at the lower address.
When you alter these variables, the screen representation
of the cursor will not move until the next I/O operation
involving the display is performed.
Inhibit/Enable Visible Cursor Display
You can inhibit the display of the text cursor on the screen
by setting the variable CRSINH [02FO] to any nonzero value.
Subsequent I/O will not generate a visible cursor.
You can enable the display of the text cursor by setting
CRSINH to zero. Subsequent I/O will then generate a visible
cursor.
Text Margins
The text screen has user-alterable left and right margins. The OS
sets these margins to 2 and 39. The variable LMARGN [0052]
defines the left margin, and the variable RMARGN (0053J defines
the right margin. The leftmost margin value is 0 and the
OPERATING SYSTEM C016555 -- Section 5
61