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One of the most common uses of the blitter is to move arbitrary rectangles
of data from one bitplane to another, or to different positions within a
bitplane. These rectangles are usually on arbitrary bit coordinates, so
 shifting  and  masking  are necessary. There are further complications.
It may take several readings and some experimentation before everything in
this section can be understood.

A source image that spans only two words may, when copied with certain
 shifts , span three words.  Our 23 pixel wide rectangle above, for
instance, when  shifted  12 bits, will span three words. Alternatively, an
image spanning three words may fit in two for certain  shifts .  Under all
such circumstances, the blit size should be set to the larger of the two
values, such that both source and destination will fit within the blit
size.  Proper  masking  should be applied to mask out unwanted data.

Some general guidelines for copying an arbitrary region are as follows.

1. Use the A  DMA channel , disabled, preloaded with all ones and the
   appropriate  mask  and  shift values , to mask the  cookie-cut  function.
   Use the B channel to fetch the source data, the C channel to fetch
   the destination data, and the D channel to write the destination
   data.  Use the  cookie-cut  function $CA.

2. If  shifting , always use ascending mode if bit shifting to the right,
   and use  descending mode  if bit shifting to the left.

   These shifts are the shifts of the bit position of the leftmost edge
   within a word, rather than  absolute shifts , as explained previously.

3. If the source and destination overlap, use ascending mode if the
   destination has a lower memory address (is higher on the display) and
    descending mode  otherwise.

4. If the source spans more words than the destination, use the same
    shift  value for the A channel as for the source B channel and set the
   first and last word  masks  as if they were masking the B source data.

5. If the destination spans more words than the source, use a  shift 
   value of zero for the A channel and set the first and last word  masks 
   as if they were masking the destination D data.

6. If the source and destination span the same number of words, use the
   A channel to  mask  either the source, as in 4, or the destination, as
   in 5.

   Conditions 2 and 3 can be contradictory if, for instance, you are
   trying to move an image one pixel down and to the right.  In this
   case, we would want to use  descending mode  so our destination does
   not overwrite our source before we use the source, but we would want
   to use ascending mode for the right  shift .  In some situations, it is
   possible to get around general guideline 2 above with clever  masking .
   But occasionally just  masking  the first or last word may not be
   sufficient; it may be necessary to mask more than 16 bits on one or
   the other end. In such a case, a mask can be built in memory for a
   single raster row, and the A  DMA channel  enabled to explicitly fetch
   this mask. By setting the A modulo value to the negative of the width
   of the mask, the mask will be repeatedly fetched for each row.

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