numeric.c:1432
static VALUE
fix_divmod(x, y)
    VALUE x, y;
{
    if (FIXNUM_P(y)) {
	long div, mod;

	fixdivmod(FIX2LONG(x), FIX2LONG(y), &div, &mod);

	return rb_assoc_new(LONG2NUM(div), LONG2NUM(mod));
    }
    return rb_num_coerce_bin(x, y);
}
