And this is only writable if we are in secure boot mode.

只有在我们处启动模式时才可写。

And it's not only that only this register is writable if that flag is set to one, but also the other one, this SPCSR0.

如果标志被设置为一，不寄存器是可写的，而且另一， SPCSR0 也是可写的。