mirror of
https://github.com/deater/dos33fsprogs.git
synced 2024-11-16 08:05:31 +00:00
80 lines
1.2 KiB
ArmAsm
80 lines
1.2 KiB
ArmAsm
;==================
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; scroll background
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;==================
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; background already loaded
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; ANGLE 0-15 sets angle
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; CV is Y position to display at
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; 182/220... 220/16 = 13.75
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; 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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; 0 11 22 34 45 56 68 79 91 102 113 125 136 147 159 170 182
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scroll_offsets:
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.byte 0,11,22,34,45,56,68,79,91,102,113,125,136,147,159,170
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scroll_background:
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ldy ANGLE ; use angle
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lda scroll_offsets,Y ; lookup in table
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sta OFFSET ; calculate offset
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ldx #0
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ldy CV ; lookup Y co-ord
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lsr ; and set up self-modify code
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lda gr_offsets,Y ; get position
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sta bgsm1+1
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lda gr_offsets+2,Y ; get position
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sta bgsm2+1
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lda gr_offsets+4,Y ; get position
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sta bgsm3+1
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lda gr_offsets+6,Y ; get position
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sta bgsm4+1
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iny
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clc
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lda gr_offsets,Y ; get position
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adc DRAW_PAGE
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sta bgsm1+2
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lda gr_offsets+2,Y ; get position
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adc DRAW_PAGE
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sta bgsm2+2
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lda gr_offsets+4,Y ; get position
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adc DRAW_PAGE
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sta bgsm3+2
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lda gr_offsets+6,Y ; get position
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adc DRAW_PAGE
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sta bgsm4+2
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ldy OFFSET
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bgdraw_loop:
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lda scroll_row1,Y
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bgsm1:
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sta $400,X
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lda scroll_row2,Y
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bgsm2:
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sta $480,X
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lda scroll_row3,Y
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bgsm3:
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sta $500,X
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lda scroll_row4,Y
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bgsm4:
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sta $580,X
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iny
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inx
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cpx #40
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bne bgdraw_loop
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rts
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