% In context of the Sphinx TikZ extension we can not use the specific
% drawstack environment here and have to encapsulate it into a standard
% tikzpicture environment --> write the full declaration with \stacktop{}
% at the beginning and \stackbottom{} on the end.
\begin{tikzpicture}[x=0.7cm,y=0.7cm]
\stacktop{} % do not remove
%%  now we on \begin{drawstack}
%%  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    \small

    \startframe
    % \cellcom writes something on the right-hand side of a cell.
    \cell{loc2} \cellcom{-8(\%ebp)}
    \cell{loc1} \cellcom{-4(\%ebp)}
    % \esp and \ebp are stack pointer and base pointer in Pentium.
    % These macros are simple shortcuts for \cellptr{...}
    \cell{Backup \%ebp} \esp \ebp
    \cell{@ return} \cellcom{4(\%ebp)}
    \finishframe{function\\ {\tt op()}}
    \startframe
    \cell{} \cellcom{8(\%ebp)}
    \cell{}
    \finishframe{function\\ {\tt main()}}

%%  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%  now we into \end{drawstack}
\stackbottom{} % do not remove
\end{tikzpicture}

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