Commodore VIC-20 - Page 100

253
254
255
5120
S144
7168
BASIC RAM
BASIC RAM
BASIC RAM
20
24
29
This table assumes that screen memory starts at 7680 (1EOO).
However, it can be moved to other locations. The number of
characters you have to work with at each location might change in
that case.
There are two problems involved in creating your own special
characters. First, it is an all or nothing process. Generally, if you use
your own character set by telling the VIC chip to get the character
information from the area you have prepared in RAM, the standard
VIC characters are unavailable to you. To solve this problem, you
must copy any letters, numbers, or standard VfC graphics you
intend to use into your own character memory in RAM, You can pick
and choose, take only the ones you want, and don't even have to
keep them in order!
The second problem with programmable characters is that your
character set takes memory space away from your BASIC
program. This is a trade off situation, since you only have a limited
amount oi RAM available. If you decide to create a character set for
a program, the program has to be smaller than a program which
uses the standard VIC characters.
There are two locations in the VIC to start your character set that
should not be used with BASIC—0 and 4096. The first should not
be used because BASIC stores important data on page 0. The
second cant be used because that is where your BASfC program
starts! (If you expand your VIC, or use machine language1 you can
start your characters at 4096 if you want. This limit only applies to
the unexpanded VIC.)
The best place to put your character set for use with BASJC while
experimenting is at 7168. This is done by POKEing location 36869
with 255, giving you 64 characters to work with. Try the POKE now,
like this;
POKE 36369,255
Immediately all the letters on the screen turn to garbage. This is
because there are no characters set up at location 7166 right now
. . . only random bytes. Set the VIC back to normal by using the
RUN/STOP and RESTORE keys.
Now let's begin creating graphics characters. To protect your
character set from BASIC, you should reduce the amount of
memory BASIC thinks that it has. The amount of memory fn your
computer stays the same . + . it's just that you've told BASIC not to
use some of ft.