Commodore VIC-20 - Page 258

244 The VIC 20 User Guide
90 FOR 1=9 TO 123:NEXT
109 FOR Da128 TO 133
119 POKE 36874, D
129 NEXT
125 READ A: IF A=~~~ THEN 298
126 POKE 36875,A
139 POKE 36874,9
149 FOR 1-9 TO 300:NEXT
159 GOTO 13
209 POKE 36878,0:RESTORE:GOTO 10
The most important thing to keep in mind when working with this kind
of program is that doing more things within your loops (such as adding more
READ statements) adds time to the loop. Test statements, such as the one
on line
15, also take some time. Be sure to account for every line. If you are
unsure
of your timing, run the program and listen for timing problems. By
adding to or subtracting from the timing loops, as on lines 50 or 140, you can
usually compensate for timing errors.
The VIC Electronic Organ
Here's a program that "GETs" notes from the keyboard and plays
them. We've used the note values from Table
7-2 and POKEd them into the
middle and high registers.
300 GET A': IF A .... "" THEN 399
310 IF A'-"Q" THEN A-195
329 IF A'-"2" THEN A-19~
330 IF A'="II/" THEN A=201
340 IF A'-"3" THEN A=203
350 IF A'-"E" THEN A"'207
360 IF A'-"R" THEN FI-209
370 IF A'-"5" THEN FI=212
380 IF A'a"T" THEN A=215
390 IF A'="6" THEN A=217
490 IF AS-/lY" THEN A-21S1
410 IF A.*"7" THEN A-221
420 IF A'-"U" THEN A-223
430 IF A''''''I'' THEN A=225
433 POKE 36875,A:POKE 36876,A
440 FOR J-13 TO " STEP -.73
430 POKE 36878,J:NEXT
460 Aae: OOTO 300
This program was written for only one octave. By increasing the
number
of keys read, you can add more octaves. To make the keys play