COPYING ARBITRARY REGIONS
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.
NOTE
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.
- Blitter Hardware 177 -
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