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184 lines
2.1 KiB
ArmAsm
184 lines
2.1 KiB
ArmAsm
; Bresenham Lines
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; by Vince `deater` Weaver <vince@deater.net>
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; based on code from https://gist.github.com/petrihakkinen/
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.include "zp.inc"
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.include "hardware.inc"
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B_Y1 = $F0
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B_Y2 = $F1
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B_X1 = $F2
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B_X2 = $F3
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B_DY = $F4
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B_DX = $F5
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B_SY = $F6
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B_SX = $F7
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B_ERR = $F8
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lines:
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jsr SETGR ; set lo-res 40x40 mode
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; A=$D0 afterward
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lda #6
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jsr SETCOL
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lda #10
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sta B_X1
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sta B_Y1
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lda #30
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sta B_X2
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sta B_Y2
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jsr draw_line
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end:
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jmp end
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draw_line:
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jsr init_bresenham
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line_loop:
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ldy B_X1
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lda B_Y1
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jsr PLOT ; PLOT AT Y,A
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ldy B_X1
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cpy B_X2
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bne line_no_end
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lda B_Y1
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cmp B_Y2
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beq done_line
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line_no_end:
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jsr step_bresenham
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jmp draw_line
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done_line:
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rts
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;========================
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; step
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;========================
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step_bresenham:
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; err2 = err
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lda B_ERR
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pha ; push err2
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; if err2 > -dx:
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; err = err - dy
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; x = x + sx
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clc
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adc B_DX ; skip if err2 + dx <= 0
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bmi skip_x
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beq skip_x
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lda B_ERR
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sec
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sbc B_DY
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sta B_ERR
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lda B_X1
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clc
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adc B_SX
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sta B_X1
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skip_x:
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; if err2 < dy:
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; err = err + dx
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; y = y + sy
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pla ; pop err2
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cmp B_DY ; skip if err2 - dy >= 0
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bpl skip_y
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lda B_ERR
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clc
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adc B_DX
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sta B_ERR
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lda B_Y1
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clc
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adc B_SY
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sta B_Y1
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skip_y:
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rts
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;========================
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; init
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;========================
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init_bresenham:
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; dx = abs(x2 - x1)
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; dy = abs(y2 - y1)
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; sx = x1 < x2 ? 1 : -1
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; sy = y1 < y2 ? 1 : -1
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; err = dx > dy ? dx : -dy
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; dx = dx * 2
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; dy = dy * 2
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; if y1 < y2:
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; sy = 1
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; dy = y2 - y1
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; else:
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; sy = -1
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; dy = y1 - y2
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ldx #$ff ; X = -1
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lda B_Y1
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sec
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sbc B_Y2 ; A = y1 - y2
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bpl yskip
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ldx #1 ; X = 1
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jsr neg ; A = y2 - y1
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yskip:
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sta B_DY
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stx B_SY
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; if x1 < x2:
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; sx = 1
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; dx = x2 - x1
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; else:
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; sx = -1
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; dx = x1 - x2
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ldx #$ff ; X = -1
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lda B_X1
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sec
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sbc B_X2 ; A = x1 - x2
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bpl xskip
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ldx #1 ; X = 1
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jsr neg ; A = x2 - x1
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xskip:
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sta B_DX
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stx B_SX
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; err = dx > dy ? dx : -dy
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; lda B_DX
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cmp B_DY ; dx - dy > 0
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beq errneg
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bpl skiperr
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errneg:
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lda B_DY
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jsr neg
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skiperr:
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sta B_ERR
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; dx = dx * 2
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; dy = dy * 2
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asl B_DX
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asl B_DY
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rts
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neg:
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eor #$ff
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clc
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adc #1
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rts
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