Commodore VIC-20 User Manual page 200

186
The VIC 20 User Guide
giving the illusion that it is moving. For example, this program produces a
windshield with wipers that sweep back and forth.
113 DL' , I=1513
213 REM BUILD WINDSHIELD
313 PRINT
u:']..--.."
49 PRINT
"I I"
50 PRINT" '---'''
69 REM
S~JEEP
WIPERS TO R IOHT
79 FOR 1=1 TO 4
813 ON I C;OSLIB 1813,240,3013,360
99 FOR
J=l
TO DLY :
NE~T
190 NEXT
!
19 REM SWEEP
~JIPERS
TO LEFT
129 FOR 1=3 TO 2 STEP -1
130 ON I GOSUB 1813., 240, 300, 360
140 FOR J=l TO DLY : NEXT
159 NEXT
169 (lOTO 713
179 REM SHOW WIPERS AT 113 O'CLOCK
1813 POKE 7793,77
1913 POKE 7794,32
299 POKE 7795,77
2113 POKE 77136,32
2213 RETURN
230 REM
SHO~J
WIPERS AT 12 O'CLOCK
249 POKE 7793,32
250 POKE 7794,191
269 POKE 77135,32
279 POKE 7796,191
289 RETURN
299 REM SHOW WIPERS AT 2 O'CLOCK
399 POKE 7793,32
319 POKE 7794,78
329 POKE 7795,32
3313 POKE 77,136,78
349 RETURN
350 REM SHOW WIPERS AT 3 O'CLOCK
3613 POKE 7794,1913
379 POKE 7796,199
389 RETURN
This program has several major parts. We will look at each in detail.
Lines 30 through 50 clear the screen and display the windshield using
PRINT statements.
Lines 70 through 100 sweep the wipers from left to right. They are then
swept back to the left by lines 120 through 150. These loops change the