Using blitter for filling areas:

The blitter offers a crude area-fill option that works in tandem with the data-copy operation described above. Fill mode is activated by BLTCON1's IFE (Inclusive Fill Enable) bit or EFE (Exclusive Fill Enable) bit prior to turning on the blitter. After the blitter reads data from memory and manipulates it according to the miniterm settings, the specified fill operation is performed on the data before it is written to destination memory. Fill operations work IN DESCENDING MODE ONLY! The figure below illustrates the effect of each type of fill operation has on data:
After: Before FCI = 0 FCI = 1
00011000 00011000 11111111 00100100 00111100 11100111 01000010 01111110 11000011 Fill 01000010 01111110 11000011 00100100 00111100 11100111 00011000 00011000 11111111
00011000 00001000 11110111 00100100 00011100 11100011 Exclusive 01000010 00111110 11000001 Fill 01000010 00111110 11000001 00100100 00011100 11100011 00011000 00001000 11110111
In all fills, the blitter evaluates data one bit at a time, starting at the rightmost edge and moving to the left. Before the fill begins, however, the blitter takes note of the FCI bit (2) of the BLTCON1 register. The value of this bit becomes the initial fill value of the blitter's fill bit. For the sake of clarity, let's assume FCI starts at zero.
Here's how inclusive fill works: As the blitter moves left, it changes all zero bits to zero, because zero is the current value of the fill bit. When the blitter encounters a 1 bit, it toggles the fill bit, changing it to a one. Now the blitter changes the following 0 bits to 1, because that is the current value of the fill bit. Every time the blitter encounters a 1 bit in the data, the fill bit changes value. The figure above illustrates the difference between fills that start with FCI = 0 or FCI = 1.
Exclusive fills take one extra step when they encounter a 1 bot. In addition to toggling the fill bit, the blitter replaces that 1 bit with the new fill-bit value.
Both types of fills expect borders to be defined by a single 1 bit - one bit to tell the blitter that it's inside an area, and the other to tell it that it's outside. When the blitter encounters two one bits in a row, it assumes that there is no space to be filled since there is no zero bit between pixels. If the blitter encounters three 1 bits in a row, the blitter assumes that it has gone in, out, then back in to an area to be filled. (The fill bit has been toggled three times.) This happens when the blitter runs across an odd number of adjacent bits.
To execute a fill operation, simply set up the blitter registers for a standard copy operation, set either the IFE or EFE bit, set the FCI bit to the desired value, set the DESC to put the blitter in descending mode, then start the blitter by writing the area size to the BLTSIZE register. By setting the blitter's source address equal to the destination address, you can fill an area without having to copy data from one separate location to another.
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