APPENDIX EIGHT



So let's analyze the DL for GRAPHICS 2 that we printed above:

112     These three instructions print
112     24 blank scan lines at the top
112     of the screen
71      GR.2 with LMS instruction added
112     Address of the first line of screen data
158     158 * 256 + 112 = 40560
7       Display the rest of the data in
7       GR.2, so we have a total of
7       ten GR.2 lines, or 10 * 16 =
7       160 scan lines used.
7
7
7
7
7
66      GR.0 with LMS instruction added
96      Address of the text window at bottom
159     159 * 256 + 96 = 40800
2       GR.0 for text window, so we have
2       a total of four lines
2
65      Jump and wait for vertical blank
88      Address of display list itself
158     158 * 256 + 88 = 40536
        (return to the top of this list)

Now examine the list for GR.2 + 16. You can see that it adds two 7's
to replace the GR.0 lines at the bottom of the screen. A little math
shows us that the screen in both cases has a total of 192 scan lines.
That's an important number; if you want your screen to come out
properly, you must insure that you get as close to this figure as
possible; otherwise you'll end up with blank lines at the bottom of your
screen, or worse -- in the display itself.

You will find the value 112 in every Atari display list. The three of them
are used to bring the display to a readable location on your set. Try
replacing one or more of them with a zero to see what happens without
them. The jump instructions are also used to skip across a 1K
boundary, since the DL itself cannot cross a 1K boundary without such
a jump. Also, DL data cannot cross a 4K boundary, so you must use an
LMS instruction before crossing one.

The critical factor in designing your own display list is to make sure
that the data and the scan lines match. This may require you to
manipulate your data so that you have the proper number of bytes per
line so that the display appears correctly on the screen. Here are the