Atari XL

Advanced Graphics 161
To cause an area of the screen to scroll smoothly,
we must modify the display list for the area of the
screen we are interested in. To do this, a modifier
instruction must be added to the display list opcode
for
each display block which is to be smooth
scrolled.
The modifiers are:
HORIZONTAL SCROLL add 16
VERTICAL SCROLL
add 32
The modifier indicates to ANTIC that a display
block is to be treated differently from normal.
When such a modifier is encountered, ANTIC
checks the appropriate scroll register and uses the
number it finds to determine how many (if any)
scan lines of that display block to display.
An example will make this clear.
Suppose we wanted to vertically smooth scroll just
one line of a mode 0 display - the following steps
are required:
Add 32 to the display list opcode for that display
block, to indicate to ANTIC that this block is to be
vertically scrolled.
Write a number between 0 and 7, corresponding to .
the number of scan lines of that block to be
displayed initially, into the vertical scroll register.
A value of7 would display the entire block whereas
o would result in none of the block being displayed.
Increment or decrement the scroll register to
smooth scroll that display block up or down.