Atari XL - Page 129

Characters
119
Notice that the top and bottom lines of the
character are left blank. This is to leave a space
between lines of text on the screen.
REDEFINING CHARACTERS
You may be wondering how it is possible to create
new characters when the standard character
definitions are held in ROM - it's difficult to write
new numbers into read-only memory! The answer
is that we have to tell the XL to look somewhere
else for the character shapes, and define the new
characters in RAM. We have used this technique
before to swap between Standard and European
character sets. By POKEing the high byte of the
start address of the character set into location 756
we changed the memory from which the character
defini tions were taken.
Location 756 is the place where the XL stores the
high byte of the start address of character data (ie
to
find the address of the start of character data you
must multiply the contents of location 756 by 256).
This information is used by the chip which controls
all the display of the XL, known as the ANTIC
chip. Location 756 is a copy of one of the registers
on this chip, known as a shadow register. The
contents of the shadow register are copied to the
chip every sixtieth of a second and used to dictate
whereabouts in ROM the character set is to be
found.
This pointer can just as easily be made to point to .
an area of RAM - try this:
LIST the program to show some characters on the
screen, then type:
POKE 756,2