Atari XL

Advanced Graphics
157
Following this instruction there are 23 t2's. This is
the ANTIC instruction for a mode 0 display block.
When the XL interpreted the GRAElHICS 0
command in the program, it allocated sufficient
memory for the display, and created the display list
accordingly. The 23 (2's indicate that the display
should comprise 24 display blocks of 8 lines each.
There is a discrepancy here, since there are only 23
code 2s but mode 0 is a 24 line display. There is
another code 2 at the beginning of the display list
but it isn't obvious. We said earlier that the 66
code indicated that the next two bytes in the list
are to be read as a 16 bit address. In fact the code
for this is 64 and this has been added to the code 2
specifying the first mode 0 display block of the
screen (the top line of the display), in much the
same way as you addd 16 to a GRAPHICS
instruction in BASIC to modify its effect. The 64
added to the (2' instruction is called a modifier and
is discussed more fully later in this chapter.
Only the first of the last three bytes of the display
list is an instruction. The code 65 tells ANTIC to
jump back to the location specified in the following
two bytes - the start of the display list.
The display list we have examined is fairly short
and simple, being for a mode 0 display. For mode 8
it would be much longer, containing 192
instructions specifying one line display blocks, and
some other codes
A list of the display list instructions is given in
Appendix 9.
Armed with this information we can now make
changes to the display list which will allow us to
create more varied displays than have previously