Atari XL

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The Atari XL Handbook
DISPLIST = PEEK(560)+256*
PEEK(561)
DISPMEM = PEEK(DISPLIST+4)+256
* PEEK(DISPLIST+5)
DISPMAX = DISPMEM+880:DISPMIN
=DISPMEM
POSITION 0,23:PRINT MESSAGE$
REM SCROLL UP
DISPMEM = DISPMEM+40
IF DISPMEM > DISPMAX THEN 80
GOSUB 100:GOTO 60
REM SCROLL DOWN
DISPMEM = DISPMEM-40
IF DISPMEM < DISPMIN THEN 60
GOSUB lOO:GOTO 80
DISPHI = INT(DISPMEM/256)
DISPLO = DISPMEM-(DISPHI*256)
POKE DISPLIST+4,DISPLO
POKE DISPLIST+5,DISPHI
RETURN
This technique is simple to use but has the
disadvantage that the scrolling is (coarse', that is
the motion is not smooth because the display is
moved by a whole character height or width of
eight pixels at a time. With a bit more program-
ming, we can cause the display to scroll by a single
pixel at a time within a single character space. A
combination of both techniques will allow smooth
scrolling over the entire screen.
SMOOTH SCROLLING
Smooth scrolling is achieved with the use of two
GTIA
registers:
VERTICAL SCROLL
HORIZONTAL SCROLL