o
If
the big picture
in
memory
is
larger than the display window, you must
respecify the modulos. The modulo must be some value other than
O.
o You must allocate more memory for the larger memory picture.
Specifying
the
Modulo
For
a memory picture wider than the display window, you need to respecify the modulo
so
that
the correct
data
words are fetched for each line of the display. As an example,
assume the display window
is
the standard 320 pixels wide, so
40
bytes are to be
displayed on each line. The big picture
in
memory, however, is exactly twice as wide as
the display window, or
80
bytes wide. Also, assume
that
you wish
to
display the left
half of the big picture. Figure 3-14 shows the relationship between the big picture and
the picture to be displayed.
START
I
Width
of
the Bit-Plane Defined in RAM
Width
of
defined
•
screen
on which
bit-plane data
is
to
appear
START+78
Figure 3-14: Memory Picture Larger
than
the Display
Because 40 bytes are to be fetched for each line, the
data
fetch for line 1 is as shown
in
figure 3-15.
66
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