There are two shift controls. Bits
15
through
12
of BLTCONO select the shift value for
source
A.
Bits 15 through 12 of BLTCONI select the shift value for source
B.
Both
values are normally
set
the same.
The
shift controls are used in a special way during
line drawing. See "Line Drawing" below.
Masking
If
an object
is
not an even multiple of
16
bits in width, the blitter can mask
off
either
the left
or
the right edge or both in order to work with only the actual bit-boundary rec-
tangle enclosing the object. First- and last-word masking
is
particularly useful when you
need
to
store the images of a text font in a packed edge-to-edge organization.
For
example, assume a packed font
that
contains both an
"H"
and an
"I"
as shown
in
figure 6-4.
111
1111111
11
11 11
11
11
11
111111111
11
111111111
11
11 11
11
11 11 11
111
1111111
Figure
6-4:
A Packed
Font
To
isolate the
"I"
character, the first
11
bits along the left edge of the enclosing rectan-
gle
must be masked.
The
blitter includes this capability, called the first-word mask, and
applies it
to
the leftmost word on each horizontal line. Only when there
is
a 1 bit
in
the
first-word mask will
that
bit of source A actually appear
in
the logic operation.
For
example, if the first-word mask (BLTAFWM)
is
0000000000001111, the
data
the
blitter will see, using the input for source A shown above,
is
shown
in
figure 6-5.
178
BIitter Hardware
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