gcdHL_DE:
;gcd(HL,DE)->HL
;binary GCD algorithm
    ld a,h \ or l \ ret z
    ex de,hl
    ld a,h \ or l \ ret z
    sbc hl,de
    jr z,step4
    add hl,de
    ld b,1
    ld a,e \ or l \ rra \ jr c,+_
    inc b
    rr h \ rr l
    rr d \ rr e
    ld a,e \ or l \ rra \ jr nc,$-12
_:
    srl h \ rr l \ jr nc,$-4 \ adc hl,hl
    ex de,hl
_:
    srl h \ rr l \ jr nc,$-4 \ adc hl,hl
    xor a \ sbc hl,de
    jr z,step4
    jr nc,-_ \ sub l \ ld l,a \ sbc a,a \ sub h \ ld h,a
    jp -_-1
step4:
    ex de,hl
    dec b
    ret z
    add hl,hl
    djnz $-1
    ret