Ascending And Descending Addressing - Commodore Amiga Hardware Reference Manual

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