80 lines
1.2 KiB
ArmAsm
80 lines
1.2 KiB
ArmAsm
;==================
|
|
; scroll background
|
|
;==================
|
|
; background already loaded
|
|
; ANGLE 0-15 sets angle
|
|
; CV is Y position to display at
|
|
; 182/220... 220/16 = 13.75
|
|
; 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
|
|
; 0 11 22 34 45 56 68 79 91 102 113 125 136 147 159 170 182
|
|
|
|
scroll_offsets:
|
|
.byte 0,11,22,34,45,56,68,79,91,102,113,125,136,147,159,170
|
|
|
|
scroll_background:
|
|
ldy ANGLE ; use angle
|
|
lda scroll_offsets,Y ; lookup in table
|
|
sta OFFSET ; calculate offset
|
|
|
|
ldx #0
|
|
|
|
ldy CV ; lookup Y co-ord
|
|
lsr ; and set up self-modify code
|
|
|
|
lda gr_offsets,Y ; get position
|
|
sta bgsm1+1
|
|
lda gr_offsets+2,Y ; get position
|
|
sta bgsm2+1
|
|
lda gr_offsets+4,Y ; get position
|
|
sta bgsm3+1
|
|
lda gr_offsets+6,Y ; get position
|
|
sta bgsm4+1
|
|
|
|
|
|
iny
|
|
|
|
clc
|
|
|
|
lda gr_offsets,Y ; get position
|
|
adc DRAW_PAGE
|
|
sta bgsm1+2
|
|
lda gr_offsets+2,Y ; get position
|
|
adc DRAW_PAGE
|
|
sta bgsm2+2
|
|
lda gr_offsets+4,Y ; get position
|
|
adc DRAW_PAGE
|
|
sta bgsm3+2
|
|
lda gr_offsets+6,Y ; get position
|
|
adc DRAW_PAGE
|
|
sta bgsm4+2
|
|
|
|
ldy OFFSET
|
|
|
|
|
|
bgdraw_loop:
|
|
|
|
lda scroll_row1,Y
|
|
bgsm1:
|
|
sta $400,X
|
|
|
|
lda scroll_row2,Y
|
|
bgsm2:
|
|
sta $480,X
|
|
|
|
lda scroll_row3,Y
|
|
bgsm3:
|
|
sta $500,X
|
|
|
|
lda scroll_row4,Y
|
|
bgsm4:
|
|
sta $580,X
|
|
|
|
iny
|
|
inx
|
|
cpx #40
|
|
bne bgdraw_loop
|
|
|
|
rts
|
|
|
|
|