Commodore VIC-20 User Manual page 224

210
The VIC 20 User Guide
220 PRINT ";[:KS[#+3;"
230 PRINT "aIDLTU,4("
240 PRINT
"£MUJ~-5="
250 PRINT "ilFNVt&.6)"
260 PR
I
NT
":!J.30W~1 171"
Now the upper left corner of the screen has become a high-resolution work
area, where you can start to experiment.
Changing the
Dots
In the
High-Resolution Work Area
After running the Setup program, the portion of character memory
that defines the work area looks like this.
Memory
Memory
Memory
Location
Column Location
Column Location Column
Row
o-r'H"1vt.n-..o,.....
ooQ\S:=~~:! ~ .
-...0 r-.OO C'o 0-('1
('f")
V)
'-"\.11")11')\0\0\0\0
0 7168
0
7232
0
7616
0
7169
7233 7617
2 7170
7234
7618
3 7171
7235 7619
4
7172
7236 7620
60 7228
0
7692
0
7676
0
61
7229
7693
7677
62
7230
7695
7678
63
7231
7695
7679
To change a dot on the screen, your program must find the right
location to POKE and the value to put there. The 64 bits of the X dimension
are broken up into eight bytes of eight bits each. To find the correct column
of bytes, use the following formula:
450 COL=INT(X/8)
Since each column of bytes is 64 rows high, the column number must be
multiplied by 64. The calculation to find the right location to POKE is
460 PL=7168+Y+64*COL