Atari XL

Advanced Graphics 159
90 IF X=42 THEN DISPMEM=DISPMEM-
l:GOSUB 100
95 GOTO 70
100 DISPHI = INT(DISPMEM/256)
110 DISPLO = DISPMEM-(DISPHI*256)
120 POKE DISPLIST+4,DISPLO
130 POKE DISPLIST+5,DISPHI
140 RETURN
This program makes the display scroll because it
alters the address from which the ANTIC chip
reads display data, but it does not alter the
addresses into which the BASIC routines write the
picture data. In normal use, the pointer to display
memory is set by the operating system to the
address at which BASIC begins storing display
data when the graphics mode is set, or the machine
is swi tched on.
When you run this program, you will notice that as
you scroll the display to the right, a series of
graphics and other characters appear at the top of
the screen. This is the lower end of the display list,
and appears because to scroll to the right we
subtracted one from the display memory pointers.
This tells ANTIC to take display data from the
address one lower than that where display memory
starts, and as the display list is always stored
immediately before display memory, it is treated as
screen data and displayed.
Vertical scrolling can be achieved in much the
same way, but the display memory pointers must.
be incremented by 40, as the following program
demonstrates.
5 REM UP AND DOWN SCROLLING
10 GRAPHICS a
20 DIM MESSAGE$(40): MESSAGE$=
"WHAT GOES UP MUST COME DOWN!"