Commodore VIC-20 Programmer's Reference Manual page 106

point lo RAM, however, we will be displaying memory that we can
change any way we want.
Add the following line to the program:
5 POKE 36869,255
(This will give us 64 programmable characters, which we can set
up as an 8-by-a character matrix, which will give us a 64-dot by
64-dot HI-RES screen—just what we were looking for.)
RUN the program now. Garbage appeared on the screen,
right? Just like the regular screen, we have to clear the
HI-RES screen before we use it. Printing a key won't work in
this case. Instead we have to clear out the section of memory
used for our programmable characters. Add the following
line to your program to clear the HI-RES screen;
6 FOR I- 7168 TO 7679 : POKE I, 0 : NEXT
Now RUN the program again, You should see nothing but black
on the screen—your program is a success. What we want to add
now is the means to turn dots on and off on the Hl-RES screen.
To SET a dot (turn the dot on) or UNSET a dot you must know
how to find the correct bit in the character memory that you must set
to one. In other words, you have to find the character you need to
change, the row of the character, and which bit of the row that you
have to change. We need a formula to calculate this.
We will use X and Y to stand for the horizontal and vertical
position of the dot. The dot where X = 0 and Y - 0 is at the upper-left
of the display. Dots to the right have higher X values, and the dots
toward the bottom have higher Y values.
The dots where 0< = X< = 7 and 0< - Y< = 7 are in character
number 0, which we placed at the upper-left corner of the screen.
Each character contains 64 dots. 8 rows of 8 dots each.
These are the simple calculations to decide which dot of which
character is being altered:
The character number is. . .
CHAR - INT{X/8) *3 + INT(Y6}
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