.INCLUDE "zeropage.inc"

.EXPORT _fiba

.CODE

_fiba:
    cmp #0      ;set up loop
    beq end_z
    tay
    lda #1      ;ptr1 = 0 ptr2 = 1 ptr3 = 0
    ldx #0
    stx ptr1
    stx ptr1 + 1
    sta ptr2
    stx ptr2 + 1
    stx ptr3
    stx ptr3 + 1
loop:
    clc
    lda ptr2    ;ptr1 = ptr2 + ptr3
    adc ptr3
    sta ptr1
    lda ptr2 + 1
    adc ptr3 + 1
    sta ptr1 + 1
    lda ptr2    ;ptr3 = ptr2
    sta ptr3
    lda ptr2 + 1
    sta ptr3 + 1
    lda ptr1    ;ptr2 = ptr1
    sta ptr2
    lda ptr1 + 1
    sta ptr2 + 1
    dey
    beq end
    jmp loop
end:
    lda ptr1
    ldx ptr1 + 1
    rts
end_z:
    lda #0
    ldx #0
    rts