;
; Simple starfield effect
;
stars_update:
    ld x, <stars
    ld a, >stars

stars_update_loop:
    ld y, [a : x]
    inc x
    bo
    inc a
    ld b, [a : x]
    ; is the end of list?
    cmp b, 0xff
    bz
    ret

    push a
    push x

    ; erase old
    ld x, 0
    ld [b : y], x
    ld x, [sp + 0]

    ; update address
    inc x
    bo
    inc a
    push x
    ld x, [a : x]
stars_update_addr:
    add y, 128
    bo
    inc b
    cmp b, 0xfe
    bnz
    jmp stars_no_wrap
    cmp y, 0x80
    bc
    ld b, 0xbf

    ; add some randomness
    push a
    push x
    push b
    call rnd
    pop b
    pop x
    pop a

stars_no_wrap:
    dec x
    bnz
    jmp stars_update_addr
    pop x

    ; draw new: color
    inc x
    bo
    inc a
    ld x, [a : x]
    ld [b : y], x

    pop x
    pop a

    ; save new address
    ld [a : x], b
    dec x
    bo
    dec a
    ld [a : x], y

    ; next star
    add x, 4
    bo
    inc a
    jmp stars_update_loop