FillMode

         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.