Atari XL

Advanced Graphics
163
compensation for the disappearing scan lines of
line 11 when scrolling starts. This prevents the
whole of the rest of the screen from moving up with
line 11.
Lines 300 to 330 and 400 to 440 increment and
decrement the vertical scrolling register, with a
delay, to gradually increase or decrease the vertical
offset of lines 12 and 13 on the screen and produce
the effect of smooth scrolling.
This technique can be applied to any area of the
screen, and also works for modes 1 and 2.
GRAPHICS MODE
It is possible to modify the entire display list to
create a display mode which the BASIC command
GRAPHICS cannot give. If you look at the table
of ANTIC instruction codes in Appendix 9, you'll
notice that code 3 isn't used. This is used to select
another text mode which is very similar to mode 0,
but is composed of display blocks which are 10 scan
lines tall. The only way to use this mode is to
modify the display list, and the following program
will do just that:
5 REM MODE 0.5
10 DL=PEEK(560)+256*PEEK(561)
20 POKE DL+3,67
30 FOR Z=6 TO 23:POKE DL+Z,3
40 NEXT Z
50 FOR Z=24 TO 26:POKE DL+Z,
PEEK(DL+Z+5)
60 NEXT Z
List the program, then try running it. You will see
the screen 'stretch', leaving more space between
the lines of the listing then usual. This is because
each display block is now 10 scan lines high instead