Atari XL - Page 131

Characters
121
is the address of the top of free RAM. By POKEing
a value of four less into location 106, we protect the
top four pages of memory (1024 bytes) from use by
BASIC. It is here that we will store our character
set. Line 70 sets mode 0, and in doing so forces the
computer to check location 106 to find out how
much memory is available. Lines 40 to 70 put the
characters set on the screen, omitting the control
characters like screen clear and cursor characters.
After a short delay the character set pointer is
moved to our reserved area of RAM by line 80, and
the character data copied from ROM to RAM. As it
is copied, you will see the garbage which used to be
the character set return to normal. The computer
will behave as usual after copying, the difference
being that character data is now being taken from
RAM. Now that the character set is in RAM we can
change it to suit ouselves. As an example, if you
add these lines to the last program and then GOTO
1000, all the characters will be turned upside down
- you have redefined them!
200
1000
1010
1020
1030
1040
END
REM INVERT CHARACTERS
FOR Z=O TO 1023 STEP 8
FOR Y =
0 TO 7
POKE RAMSET+Z+Y,PEEK(CSET+
Z+(7-Y»
NEXT Y
Although this is a good example to illustrate
redefined character sets, if you ever do want to
invert the character set, there is a way of doing so'
which requires much less programming effort:
POKE 755,6
This will invert the character set, and