\documentclass{article}
\usepackage{listings,color}
\definecolor{verbgray}{gray}{0.9}
\lstnewenvironment{code}{\lstset{backgroundcolor=\color{verbgray},frame=single, framerule=0pt,basicstyle=\ttfamily,columns=fullflexible}}{}
\definecolor{shadecolor}{rgb}{.9, .9, .9}
\begin{document}
text
\begin{code}
for i:=maxint to 0 do begin
j:=square(root(i)); end;
\end{code}
\end{document}