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Graphic Operations
MOTOROLA
MC68322 USER’S MANUAL
12-9
When rendering to banded bit maps of 0
° pages or to frame and unbanded bit maps, the
displacement is added to the current destination address before the scanline is drawn. The
scanline run is then completed in a left to right direction. When rendering to banded bit maps
of 180
° pages, the run length is first subtracted from the current destination address and that
address is saved. Then the scanline run is carried out in a left to right direction and the
displacement is subtracted from the saved destination address.
12.6.3 Executing During Banded Applications
When executing a scanline table for a banded bitmap, the MC68322 performs boundary
checking to detect a band fault before executing each bit string specifier. A band fault occurs
when the destination bitmap is a banded bitmap and the scanline frame extends below the
end of the band. A band fault will cause the scanline transfer to terminate prematurely. The
remainder of the display list is processed in the event of a band fault, but the execution of
the scanline table resumes when the display list is rerun to render the next band of the page
image.
The MC68322 does not, however, check for a band fault while processing the scanline run
lengths. During banding applications, there are two ways to use bit string specifiers that can
cause unwanted destruction of data. The destruction occurs when the bit string specifier
causes processing to occur outside of the banded bitmap. One way is when a bit string
specifier contains a signed offset that references a previous band (see Figure 12-5). The
MC68322 checks only the lower boundary of the banded bitmap, so the violation goes
undetected. Normally, the MC68322 would process the bit string specifier’s run length at a
memory location that is not contained within the target bitmap.
Figure 12-5. Illegal Bit String Specifier Use
Another way is when a bit string specifier contains a run length that wraps from one scanline
to another and the new scanline is beyond the end of the current band. The MC68322 does
not check horizontal and vertical boundaries while processing the bit string specifier’s run.
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