GenBlit

         General guidelines on using blitter at hardware level:


         When programming the blitter at the hardware level
         with multitasking turned on, you must be sure to
         grab the blitter for your own exclusive use so other
         programs don't try to use it.  Using the library
         function call OwnBlitter() you can reserve the
         blitter for your own personal use.  The function
         call DisownBlitter() frees the blitter so other
         programs can use it.


         Before writing to any of the blitter registers, you
         must be sure the blitter isn't currently in
         operation, even after a call to OwnBlitter().  To
         ensure the blitter's dormancy, you can use the
         function call WaitBlit(), or you can read the status
         of the BBUSY bit found in the DMACONR register at
         $dff002.


         Under normal operation conditions, the Amiga's
         microprocessor has priority over the blitter when it
         comes to accessing chip RAM.  Because of the way
         memory cycles are allocated, memory conflicts rarely
         occur between the blitter and the 68000.  However,
         if time is a critical factor, you can give the
         blitter a higher priority than the 68000 by setting
         the BLTPRI bit in the DMACON register ($dff096).


         The blitter can perform so many special operations
         on a word of data that it's important to know the
         order in which these operations take place.  Masking
         via the BLTAFWM and BLTALWM register takes effect
         first (assuming the data was obtained from A and is
         either the first or last word of a horizontal
         line.); next the data is shifter, if a shift was
         specified my the source's shift bits (ASHx or BSHx);
         the miniterm operations are performed next; and
         finally, the data is filled according to the status
         of BLTCON1's EFE, IFE, and FCI bits.  Only after all
         this does the data reach its destination.


         As a general rule, you should always write zeroes to
         any unused bits in a blitter register.  On future
         versions of the Amiga blitter, these bits may be
         used to perform new functions - functions that could
         freak your program if mistakingly activated!