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!
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