The final item in the stack frame is a pointer to the previous stack frame, often called the backpointer.
堆栈帧中的最后一个条目是一个指向前一堆栈帧的指针,通常被称为后向指针(back pointer)。
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Next, the current stack frame pointer is stored as the backpointer for the next stack frame, even though you haven't established the stack frame yet (this is done through negative offsets).