爲什麼linux將中斷向量拷貝到0xffff0000？

ARM supports two exception address: 0x0000_0000(normal address) and 0xffff_0000(high vector address), up to CP setting.
    After MMU is enabled, 0x0000_0000 locates in user sapce, so we should set it to 0xffff_0000 which is in kernel space