Commodore Amiga A2000 - COLLISION DETECTION

xyneg:
neg.w d3 ; y was negative, in one of [5,6]
cmp.w d3,d2 ; is y>x?
bmi xynygtx ; if so, octant is 6
moveq.l #OCTANT5+LINEMODE,d5 ; otherwise, octant is 5
bra lineagain
xynygtx:
exg d2,d3 ; X must be greater than Y
moveq.l #OCTANT6+LINEMODE,d5 ; we are in octant 6
lineagain:
mulu.w d9,d1 ; Calculate y1 * width
ror.l #4,d0 ; move upper four bits into hi word
add.w d0,d0 ; multiply by 2
add.l d1,a0 ; ptr += (xl >> 3)
add.w d0,a0 ; ptr += yl * width
swap d0 ; get the four bits of xl
or.w #$BFA,d0 ; or with VSEA, USEC, USED, F=A+C
lsl.w #2,d3 ; y = 4 * y
add.w d2,d2 ; X = 2 * X
move.w d2,d1 ; set up size word
lsl.w #5,d1 ; shift five left
add.w #$42,d1 ; and add 1 to height, 2 to width
btst #DMAB_BLTDONE-8,DMACONR(al) ; safety check
waitblit:
btst #DMAB_BLTDONE-8,DMACONR(a1) ; wait for blitter
bne waitblit
move.w d3,BLTBMOD(a1) ; B mod = 4 * Y
sub.w d2,d3
ext.l d3
move.l d3,BLTAPT(a1) ; A ptr = 4 * Y - 2 * X
bpl lineover ; if negative
or.w #SIGNFLAG,d5 ; set sign bit in conl
lineover:
move.w d0,BLTCON0(a1) ; write control registers
move.w d5,BLTCON1(a1)
move.w d4,BLTCMOD(a1) ; C mod = bitplane width
move.w d4,BLTDMOD(a1) ; D mod = bitplane width
sub.w d2,d3
move.w d3,BLTAMOD(a1) ; A mod = 4 * Y - 4 * X
move.w #$8000,BLTADAT(a1) ; A data = 0x8000
moveq.l #-1,d5 ; Set masks to all ones
move.l d5,BLTAFWM(a1) ; we can hit both masks at once
move.l a0,BLTCPT(a1) ; Pointer to first pixel to set
move.l a0,BLTDPT(a1)
move.w d1,BLTSIZE(a1) ; Start blit
rts ; and return, blit still in progress.
- 198 Blitter Hardware -