the width of the large image and the width of the smaller window.
The blitter has a separate modulo register for each of the three possible source images
and one for the destination image (four
in
all). This allows the larger bit map image of
each source and the destination to
be
a different size, even though the smaller window
for each
is
identical.
Note
that
although the hardware deals
in
words for pointers and modulos, the values
loaded into these hardware registers from the 68000 are treated as byte counts. For
example, a jump value of 4 for a modulo would actually
be
an 8 when written from the
68000.
Ascending
and
Descending
Addressing
It
is
important
to
be
able to control the direction of the address increment or decrement
when the source and destination areas overlap. Ascending or descending
is
specified for
overlapping
data
moves either
to
move a block of
data
or
to
fill
a region with a particu-
lar value.
If you wish
to
move
data
toward a higher address in memory with an overlap between
source and destination areas, you should use the descending (address decrement) mode
for the
data
move.
If
you wish
to
move
data
toward a lower address in memory with an
overlap between the source and destination areas, you should
use
the ascending (address
increment) mode for the
data
move.
The
descending mode
is
selected with bit 1 of
BLTCONl.
If
the source and destination
data
areas overlap in a blitter operation, there
is
a possibil-
ity of writing
to
a particular location as the destination before
it
was read as the source.
To
prevent this kind of
data
destruction, you must take care
to
correctly choose ascend-
ing or descending mode. Also, you may need to offset the source or destination.
Using table
6-4
at
the end of the chapter, you can observe the order of operations and
determine the required offset or mode. Pay careful attention to the notes.
It
helps to
draw pictures.
170 Blitter Hardware
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