Atari XL - Page 130

120
The Aieri XL Handbook
The program listing turns into garbage because the
ANTIC chip is taking data for characters from an
area of RAM which contains system variables, not
character data. Press RESET to return to normal
viewing!
To make use of user defined characters you must
first copy all the characters in a set into the RAM,
and tell the ANTIC chip where to find them. You
can then alter some (or all) of the characters to your
own designs.
The first step in redefining characters then is to
copy
the entire character set in to RAM, in order
that we can modify some or all of the data. The
following program does this for you.
10 REM COpy CHARACTER SET
20 POKE 106,PEEK(106)-4
30 GRAPHICS 0
40 FOR Z=O TO 124
50 PRINT CHR${Z)i
60 IF Z=26 THEN Z=31
70 NEXT Z
75 FOR DELAY = 0 TO 500:NEXT
DELAY
80 CSET=PEEK(756)*256
90 RAMSET = 256*PEEK(106)
100 POKE 756,PEEK(106)
110 FOR Z=O TO 1023
120 POKE RAMSET+Z,PEEK{CSET+Z)
130 NEXT Z
140 PRINT:PRINT "COPYING COMPLETE"
Line 20 reserves space in RAM for the character
set; location 106 contains the page number of the
top of memory, i.e.
(256*PEEK( 106))-1