previous：前一个Webflow拥有这个对象；当那前一个Webflow被从栈中删除时，它将被删除。
Previous: the previous Webflow owns the object; it is removed when that previous Webflow is removed from the stack.
对每个异常，CPU控制单元也生成一个硬件错误码，并在开始执行处理函数前将其压入内核空间栈中。
For some exceptions, the CPU control unit also generates a hardware error code and pushes it on the Kernel Mode stack before starting the exception handler.
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