Commodore Plus 4 - Page 223

Graphics Programming in BASIC 211
more of the left column is revealed, and the second column from the right is
concealed. When the scrolling position is 7, all but the left pixel of the left column
is visible, and all but the right pixel of the second column from the right is
invisible.
If the information in each column is moved right, and a new column is added
on the left while the scrolling position is simultaneously cleared to 0, a smooth
right scroll is obtained. This is best done in machine code. A left scroll is obtained
by reversing the procedure.
Smooth Scrolling Example Program
10 SOTCLR:FORI=0TO24:GOSUB120:NEXT
20 SR=65286
30 FORT=1TO2O:NEXT
40 P0KESR,(PEEK(SR)AND240)0R7
50 G0SUB120
60 F0RI=6T00STEP-l
70 P0RT=lT060:]SrEXT
80 POKESR,(PEEK(SR)A1TD24O)ORI
90 1OJXT
100 GETK$:IFK$=""THEITG0T030
110 P0KESR,(PEEK(SR)A]SrD240)0Rll:E]SrD
120 PRI1TTCHR$(13) "HIT A KEY TO STOP SCROLLING";:RETURN
Line-by-Line Explanation
10 Clear the screen and fill it with the message.
20 Set SR to scroll register address.
30 Wait loop.
40 Set to 24 lines and set the scroll register value to 7.
50 Put the message on the hidden line.
60 Count down for scroll register value.
70 Wait loop.
80 Set scroll register value.
90 Go back for next value.
100 See if a key has been pressed. If not, return to line 30.
110 Set to 25 lines and the default scroll register value (3).
120 Subroutine to print carriage return followed by message.