Commodore VIC-20 User Manual page 217

340 REM PROTECT MEMORY
350
POKE
52,24
POKE
56,24
~:55
REM
FORGET VARIABLES
360
CLR
365
REM START THE FUN
370 LOAD "MAIN"
Chapter 6 Graphics
203
The VIC will load and run the program called "MAIN." The CLR is
needed to erase any variables defined in the character builder. BASIC stores
variables in memory immediately after the program text. Because your main
program will probably be larger than the character builder, part of it will be
stored in the area occupied by the character builder's variables. If the CLR
were not there and you had a variable in the main program with the same
name as one in the character builder, BASIC would store values assigned to
that variable in the program's area. The results of damaging a program this
way are unpredictable, but you can avoid them easily: Always include the
CLR statement before the LOAD, to tell BASIC to "forget" the old
variables.
ACCESSING THE BUILT·IN
CHARACTER SET
Your program may also need to work around the loss of the standard
character set. If your program displays messages, you may still be able to
make use of the built-in character set. If you are not redefining all 256
characters, simply start your custom character set at location 7168; the
built-in character set is made available by printing reversed characters. This
technique exploits a side effect of the VIC's hardware design. The section
"VIC Chip's Window into Memory," later in this chapter, explains why it
works. Remember that the reverse control is reset at the end of every line;
your program should start each message with a
R VS ON
character.
100
REM DEMONSTRATE MESSAOES WITH CUSTOM CHARACTERS
110
REM START CUSTOM
CHAR~CTERS
AT
7168
120
FOKE
36969,255
.
400 REM DISPLAY FROO
CHA~ACTER