Commodore Plus 4 - Page 222

210 Programming Graphics
bottom of the screen can then be placed invisibly on the line and scrolled onto the
screen. To shrink the screen, bit 3 of the scroll register must be cleared with
POKE 65286,PEEK(65286)A1TD247
To restore the screen later, use
POKE 65286,PEEK(65286)0R8
The low three bits of the scroll register determine the scrolling position; normally
it is 3. To set the scrolling position, use
POKE 65286,(PEEK(65286)AKTD248)ORscroJiin^position
When the scrolling position is set to 7, the bottom line is completely invisible and
the top line is completely visible. Then, as the scrolling position is decreased to 0,
more and more of the bottom line is revealed, and the top line is concealed. When
the scrolling position is 0, all but the bottom pixel of the bottom line is visible, and
all but the bottom pixel of the top line is invisible.
If the existing lines are each moved up one, and a new line is placed at the
bottom while the scrolling position is simultaneously reset to 7, a smooth upward
scroll is achieved. To accomplish this, the operation should be done in machine
language and timed with the raster beam (see Chapter 5). But upward scrolling
can be done fairly well in BASIC. The example program shows an upward scroll.
Downward scrolling is accomplished by reversing the operations.
Horizontal scrolling is handled by memory location 65287 ($FF07). The first
step is normally to shrink the screen to 38 columns. The effect of this is to expand
the border to cover two character columns. Then, the information to be added at
the left or right of the screen can be placed invisibly on the line and scrolled onto
the screen. To shrink the screen, bit 3 of the scroll register must be cleared with
POKE 65287,PEEK(65287)A1TO247
To restore the screen later, use
POKE 65287,PEEK(65287)0R8
The low three bits of the scroll register determine the scrolling position; normally
it is 0. To set the scrolling position, use
POKE 65287,(PEEK(65287)A]SrD248)0RsicroJJ2ii^position
When the scrolling position is set to 0, both the left and right columns are
completely invisible. Then, as the scrolling position is increased to 7, more and