Complete Blitter Example - Commodore Amiga Hardware Reference Manual

Complete Blitter Example
The following example sets up the blitter to clear a block of memory. This program
assumes you have the required include files to get correct magic numbers.
This code is meant to be only an example. Programmers who wish to use the blitter
directly and who want their code to perform with the rest of the Amiga software must
do the appropriate OwnBlitterO, DisownBlitterO, and WaitBlitO calls. See the
Amiga ROM Kernel Manual
for information about using these calls.
include 'exec/types.i'
include 'hardware/custom.i'
include 'hardware/blit.i'
xref _custom
,
jBusy-wait for the previous blit to complete
WAITBLIT:
,
BTST
BNE.S
RTS
#DMAB_BLTDQNE-8,DMACONR(Al)
WAITBLIT
j
This routine uses a side affect in the blitter. lVhen each of the blits is
jfinished, the pointer in the blitter is pointing to the next word to be blitted.
jAO
=
pointer to first word to clear
jO
=
number of even bytes to clear
CLEARMEM:
LEA _CUSTOM,Al
jGet pointer to chip registers
MOVE.L AO,BLTPTD(Al)
jSet up destination to clear
CLR.W BLTMDD(Al) jSet modulo to no-skip
ASR.L #l,DO
jConvert to number of words
CLR.W BLTCONl(Al)
jNo special modes
MOVE.W #DEST,BLTCONO(Al)
jMinterms
=
0, enable only destination
j
This routine splits the blit into several parts to feed the blitter.
j
Firs t, the leftovers.
MOVEQ #$3F,Dl
AND.W DO,Dl
BEQ.S LABELl
SUB.L
OR
MOVE.W
Dl,DO
#$40,Dl
Dl,BL TSIZE(Al)
jExtract non-64-words-at-a-time part
jEven up the blit with a small one first
jMake it one row X leftover words
j
Trigger the blit
Blitter Hardware 193