adding frames to example figures

book.tex

@@ -2426,13 +2426,14 @@ errors.
   as the exit code.}
 
 \begin{figure}[tbp]
-\begin{lstlisting}
+\begin{lstlisting}[frame=single]
 	.globl main
 main:
 	movq	$10, %rax
 	addq	$32, %rax
 	retq
 \end{lstlisting}
+
 \caption{An x86 program that computes
   \racket{\code{(+ 10 32)}}\python{10 + 32}.}
 \label{fig:p0-x86}