A Few Reminders Unless working with an assem bler, you will not be able to write the commands you just learned as words. Instead, you'll have to
POKE the programs in place. But, even if you don't have an assembler, take heart. Keep reading, and in the end, I'll show you how to convert the
word commands to POKE statements. Just read about how to make pro grams, and instead of typing them in, just write them on paper. When you
get to the end, convert them to POKE statements and you're on your way. But, I do suggest that you buy the VIC 20 Programmer's Refer
ence Guide. Also, in the VIC 20 Programmer's
Color Test for the Commodore 64
by Ross Cherednik
gives all the locations in memory, and tells what they do.
How To Build Programs Machine language programs are
built logically. For example, to store a
to continue. Surprising combina tions turn out to work quite well with each other and can create exciting
visual effects. To run only border and background pairs and omit the print, simply de lete lines 40, 70, 80, 90, and 100.
Reference Guide, a memory map is on pages 170-177. A "memory map"
Typing CONT allows the program
A little often goes a long way, as is the case with the program at hand.
Explanation of Program
The twelve-line program displays on
Line 10 clears the screen.
the screen al! 4,096 possible combi
Lines 20-40 prepare the 16 colors
nations of border, background and
for use.
print colors. With a running time just
Line 50 is the border color.
under two hours, each combination
Line 60 is the background color.
OF in 1E00 (the first location in screen
is on the screen about a second and
Lines 70 and 80 spell "COLOR
memory, 7680 in decimal), you first
a half. This gives enough time to
TEST".
look over the commands. So you must do something like LDX #$0F,
then STX $1EOO. LDX loads the X register, STX stores the value of X
quickly assess the clarity and aes
Line 90 colors the print.
thetic effects of a combination.
Line 100 changes print color.
Pressing RUN/STOP will freeze a
Line 110 changes background color.
particular combination on the screen.
Line 120 changes border color,
somewhere. Now, let's try to subtract 50 from
80. Look over the commands. Write a short program to do the subtraction. Do it on your assembler.
Here is mine—1C00 LDA #580
1CO2SBC#S5O 1C04 BRK Run it from 1C00. The answer is in the accumulator now. Here is how it worked, LDA puts a number in A
5 REM ** PROGRRM BV ROSS CHEREDNIK *# 10 PRINT11:]" 20 FOR fl=0 TO 15 30 FOR B=8 TO 15
40 FOR O0 TO 15 50 POKE 53230, Fl 60 POKE 53231, B 70 POKE 1394,3:POKE POKE
SBC subtracts a number from A. BRK stops the program. Try doing the same program using the command
ADC instead of SBC. Experiment Type in short pro grams like the one above, but use different commands and see what
Next time, we will build more complex programs. Between now and then, practice. Experiment. Dis cover. Try new things Don't worry, you can't huit the computer from the keyboard (short of using a hammer to type)!
80 POKE
C
John Karcher, ago 15, is a home-schooled student who lives in Coimai, Pennsylvania
£■ ■ P|~|L'"C"
1396,12=
1398 1 IS
1400,20:POKE 1401 1 5:POKE 1402,13: POKE 1493,20 90 FOR J=0 TO 9:POKE <55666+J>,C: NEXT J 100 FOR X-l TO 1000: NEXT X :NEXT C 110 FOR X=l TO 1000: NEXT X :NEXT B 120
happens. It's really fun!
1397,15:POKE
1395, 1
FOR X=l
TO
1000: NEXT X =NEXT R
c