Atari XL

158
The Atari XL Handbook
been possible. -One of the things we can do is to
scroll the display in all directions.
SCROLLING
We have met the term scrolling in Chapter 3 as the
name given to the way in which characters on the
screen move upwards and disappear off the top - as
happens when a large program is listed.
However, scrolling is not restricted to this one
direction. We can scroll the display both vertically
and horizon tally from wi thin a program, by
changing the address which ANTIC uses as the
start of display memory.
As mentioned earlier, the address of the start of
display memory is held in the fifth and sixth bytes
of the display list. To make the display scroll
horizontally we must add to (to scroll to the left) or
subtract from (to scroll to the right) the data stored
in these bytes.
The following program demonstrates horizontal
scrolling; use the + key to scroll left and the * key
to scroll right.
5 REM LEFT AND RIGHT SCROLLING
10 GRAPHICS 0
20 DIM MESSAGE$(40):MESSAGE$=
"LEFT AND RIGHT SCROLLING"
30 DISPLIST = PEEK(560)+256*
PEEK(561) _
40 DISPMEM = PEEK(DISPLIST+4)+
256*PEEK(DISPLIST+5)
50 POSITION O,ll:PRINT MESSAGE$
60 OPEN #: 1 , 4 , 0 , "K : "
70 GET #:l,X
80 IF X=43 THEN DISPMEM=DISPMEM+
l:GOSUB 100