%%%%%%%%%%
\fbox{
 \begin{minipage}[t]{\textwidth}
  STARTUP \\
  PARAMCHECK
 \end{minipage}
}
\\
\fbox{ \begin{minipage}[t]{\textwidth}
  {\tt $\uparrow$}
  \begin{minipage}[c]{.6\textwidth}
      BASEGRID\\
      INITIAL, POSTINITIAL
  \end{minipage}
\end{minipage} }
\\
\fbox{ \begin{minipage}[t]{\textwidth}
  {\tt $\downarrow$}
  \begin{minipage}[c]{.6\textwidth}
      RESTRICT
  \end{minipage} 
\end{minipage} }
\\
\fbox{ \begin{minipage}[t]{\textwidth}
  {\tt $\uparrow$}
  \begin{minipage}[c]{.6\textwidth}
      ANALYSIS
  \end{minipage}
\end{minipage} }
\\
\fbox{ \begin{minipage}[t]{\textwidth}
  Main loop\\
  \fbox{ \begin{minipage}[c]{.8\textwidth}
      $\uparrow$
      \begin{minipage}[c]{.8\textwidth}
         REGRID
      \end{minipage}
  \end{minipage} }
  \\
  \fbox{ \begin{minipage}[c]{.8\textwidth}
      $\uparrow$
      \begin{minipage}[c]{.9\textwidth}
         Advance time\\
         EVOL (incl. MoL)
      \end{minipage}
  \end{minipage} }
  \\
  \fbox{ \begin{minipage}[c]{.8\textwidth}
      $\downarrow$
      \begin{minipage}[c]{.9\textwidth}
         RESTRICT
      \end{minipage}
  \end{minipage} }
  \\
  \fbox{ \begin{minipage}[c]{.8\textwidth}
      $\uparrow$
      \begin{minipage}[c]{.9\textwidth}
         CHECKPOINT\\
         ANALYSIS
      \end{minipage}
  \end{minipage} }
  \\
\end{minipage} }
\\