3.
Run the blitter
in
the descending direction. The
fill
operation operates
correctly only in the descending mode (right
to
left).
(BLTCONl, Bit 1
=
1)
4.
Use
the control bit called "FCI" (for fill-carry-in)
to
define
how
the
fill
opera-
tion should
be
performed.
(BLTCONl, Bit 2
=
0)
This defines the
fill
start
state as a
O.
5.
Define the horizontal and vertical size of a rectangle of words
that
will enclose
the lines around the area
to
be filled. This value must be written
to
the size
control (BLTSIZE) register
tostart
the
fill.
The
blitter uses the FCI bit as the starting
fin
state, beginning
at
the rightmost edge of
each line. For each
"1"
bit in the source area, the blitter "flips" the
fill
state, either
filling or not filling the space with
l's.
This continues for each line until the left edge of
the blit is reached.
At
that
point, the filling stops. For another example, examine the
figure below. Only the 1 bits are shown
in
figure
6-7.
The 0 bits are blank. The figure
is not drawn
to
scale.
Before
After
1
1 1 1
11111
11111
1 1 1 1
11111
11111
1 1 1 1
1111
1111
1 1 1 1
111
111
11
11
11
11
1 1
1 1
111 111
1 1 1 1
1111
1111
1 1
1
1
11111
11111
Figure
6-7:
Use of the FCI Bit - Bit Is a 0
If
the FCI bit
is
a 1 instead of a
0,
the area outside the lines
is
filled with
Is
and the
area inside the lines
is
left with
Os
in between.
Blitter Hardware
181
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