gcdD_C

    gcdD_C:
    ;gcd(D,C)->HL
    ;binary GCD algorithm
        ld a,d \ or a \ ret z
        ld a,c \ or a \ ret z
        cp d \ ret z
        ld b,1
        or d \ rra \ jr c,+_
        inc b \ rr d \ rr c
        ld a,d \ or c \ rra \ jr nc,$-8
    _:
        srl d \ jr nc,$-2
        rl d
        ld a,c
    _:
        rra \ jr nc,$-1
        rla
        sub d
        jr z,+_
        jr nc,-_ \ neg
        ld c,d
        ld d,a
        ld a,c
        jp -_
    _:
        ld a,d
        add a,a
        djnz $-1
        ret