\documentclass[12pt]{article}
\usepackage{hyperlatex}
\usepackage{graphicx}

\begin{document}

\begin{center}
\begin{image}{box1}
\fbox{\parbox{120pt}{
\fbox{\parbox{100pt}{\tt  main \\ a}} \\[15pt]
\fbox{\parbox{100pt}{\tt  print\_message \\ a}}
}}
\end{image}
\end{center}

\end{document}