gcd proc
        mov eax, a
        mov ecx, b
        .while eax != 0 && ecx != 0
            xor edx, edx
            .if SDWORD PTR eax > ecx
                idiv ecx
                xchg eax, edx
            .else
                xchg eax, ecx
                idiv ecx
                xchg eax, edx
            .endif
        .endw
        add eax, ecx
        ret
    gcd endp