Commodore Amiga A1000, Hardware Reference Manual

Page: 204 / 380

BLITTER KEY POINTS
This is a list of some key points that should be remembered when programming the
blitter.
o Write BLTSIZE last; writing this register starts the blit.
o Modulos and pointers are in bytes; width is in words and height is in pixels. The least
significant bit of all pointers and modules is ignored.
o The order of operations in the blitter is masking, shifting, logical combination of sources,
area fill, and zero nag setting.
o In ascending mode, the blitter increments the pointers, adds the modules, and shifts to
the right.
o In descending mode, the blitter decrements the pointers, subtracts the modules, and
shifts to the left.
o Area fill only works correctly in descending mode.
o Check BLTDONE before writing blitter registers or using the results of a blit.
o Shifts are done on immediate data as soon as it is loaded.
EXAMPLE: ClearMem
;
; Blitter example - memory clear
;
include 'exec/types.i'
include 'hardware/custom.i'
include 'hardware/dmabits.i'
include 'hardware/blit.i'
include 'hardware/hw examples.i"
xref _custom
;
; Wait for previous blit to complete.
;
waitblit:
btst.b #DMAB_BLTDONE-8,DMACONR(a1)
waitblit2:
btst.b #DMAB_BLTDONE-8,DMACONR(a1)
bne waitblit2
rts
;
; This routine uses a side effect in the blitter. When each
- Blitter Hardware 195 -