Zero Detection; Figure 6-5 Blitter Masking Example - Commodore Amiga Hardware Reference Manual

1111
11
11
11
11
11
11
11
1111
Figure 6-5: Blitter Masking Example
In a similar way, the blitter's last-word mask (BLTALWM) masks the rightmost word of
the source A data. Thus, it is possible to extract rectangular data from a source whose
right and left edges are between word boundaries.
If the window is only one word wide (as illustrated above), the first and last word masks
will overlap, and source-A bits will be passed only where both masks are true. This
example assumed the last word mask was loaded with all Is ($FFFF) as all masks should
be when they are not needed.
Zero Detection
A blitter zero flag is provided that can be tested to determine if the logic operation
selected has resulted in a null (empty
=
all zeros) logic operation result. The zero flag
(BZERO) in bit 13 of DMACONR will stay true if the result is all zeros.
This feature is usually used to assist collision detection by "and"ing two images together
to test for overlap. The operation D
=
AB is performed (D can actually be disabled),
and if images A and B do not overlap, the zero flag will stay true.
When the purpose of a blit is only to do zero detection and not to generate a D destina-
tion image, the USED bit (bit 8 of BLTCONO) can be turned off to save time and bus
cycles.
Blitter Hardware 179