ldx #0                  ;top = 0
                                            ;we use Xreg as top
loop_fractal_line   cpx #0
                    bpl @+                  ;if x<0 go to drawing
                    jmp exit_fractal_line
@                   lda sx2,x               ;dx = abs(sx2-sx1)
                    sec
                    sbc sx1,x
                    bcs @+
                    eor #$FF
                    adc #1
@                   cmp #2                  ;if dx<2 skip to plot point
                    bcs subdiv_fractal_line
                    ldy sx1,x
                    lda sy1,x
                    sta heights,y
                    dex
                    bne loop_fractal_line
                    beq loop_fractal_line
subdiv_fractal_line lsr                     ;ed = dx/4
                    lsr
                    sta edge_displacement
                    
                    lda sx2,x               ;sx2 = int((sx1+sx2)/2)
                    sta sx2+1,x             ;use it to copy to top+1
                    clc
                    adc sx1,x
                    lsr
                    sta sx2,x
                    sta sx1+1,x             ;copy to top+1 also
                    
                    lda sy2,x               ;sy2 = int((sy1+sy2)/2)
                    sta sy2+1,x             ;use it to copy to top+1
                    clc
                    adc sy1,x
                    lsr
                    sta sy2,x
                    
                    lda st2,x               ;st2 = st1+st2
                    sta st2+1,x             ;use it to copy to top+1
                    clc
                    adc st1,x
                    sta st2,x
                    sta st1+1,x             ;copy to top+1 also
                    
                    bvs @+                  ;if v<>0 skip displacement
                    bmi n_equal_1
                    lda sy2,x               ;n == 0
                    clc                     ;sy2 += ed
                    adc edge_displacement
                    sta sy2,x
                    jmp @+
n_equal_1           lda sy2,x               ;n == 1
                    sec                     ;sy2 -= ed
                    sbc edge_displacement
                    sta sy2,x

@                                           ;Copy new Y to top+1
                    lda sy2,x
                    sta sy1+1,x
                    
                    inx                     ;top += 1
                    jmp loop_fractal_line
exit_fractal_line