call_stack 与 call_frame

call_stack由一个或多个call_frame组成。

Each call_frame corresponds to a call to a subroutine which has not yet terminated with a return. For example, if a subroutine named DrawLine is currently running, having been called by a subroutine DrawSquare, the top part of the call stack might be laid out like this:

 上图就是一个call_stack, 它由两个call_frame组成。

参考：http://en.wikipedia.org/wiki/Call_stack