Atari XL

162
The Atari XL Handbook
The following program illustrates this technique
by scrolling two lines of text in the middle of the
screen.
5 REM SMOOTH SCROLLING
10 VERTSCROLL=54277
20
DISPLIST=PEEK(560)+256*
PEEK(561)
30 GRAPHICS 0
40 FOR
Z=l TO 10
50 PRINT "NON-SCROLLING LINE "jZ
60 NEXT Z
70 PRINT "HIDDEN LINE
11"
80 PRINT "S-M-O-O-T-H S-C-R-O-L-
L-I-N-G-!"
90 PRINT "HIDDEN LINE 13"
100 FOR Z=14 TO 23
110 PRINT "NON-SCROLLING LINE "jZ
120 NEXT Z
200 POKE DISPLIST+15,2+32:POKE
DISPLIST+16,2+32
300 FOR Y=O TO 7
310 POKE VERTSCROLL,Y
320 FOR DELAY=Q TO 100:NEXT DELAY
330 NEXT Y
400 FOR Y=7 TO 0 STEP-l
410 POKE VERTSCROLL,Y
420 FOR DELAY=O TO 100:NEXT DELAY
430 NEXT Y
440 GOTO 300
The program works like this:
Lines 10 to 120 create a display to illustrate the
scrolling mechanism. .
Line 200 adds a modifier of 32 to the display list
codes for lines 11 and 12 of the graphics 0 screen to
indicate vertical scrolling. (The code for a graphics
o display block is 2, as described earlier). Line 13,
which has no modifier, is initially hidden to allow