\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{pgf, tikz}
\usetikzlibrary{shapes, chains, calc, arrows, positioning}

\begin{document}

\begin{center}
\begin{tikzpicture}[%
	>=triangle 60,              
	start chain=going below,    
	node distance=6mm and 60mm, 
	every join/.style={norm},
]
\tikzset{
base/.style={draw, on chain, on grid, align=center, minimum height=4ex},
zbox/.style={base, rectangle, text width=8em},
ebox/.style={base, diamond, aspect=1.5, text width=10em},
term/.style={zbox, rounded corners},
norm/.style={->, draw},
coord/.style={coordinate, on chain, on grid, node distance=6mm and 25mm}}

\node [zbox] (z1) {\verb"R0 <- 29"};
\node [above=0mm of z1, text width=8em] {$Z_{1}$};
\node [zbox] (z2) {\verb"R1 <- R0 - 10"};
\node [above=0mm of z2, text width=8em] {$Z_{2}$};
\draw [->] (z1) -- (z2);

\node [zbox] (z3) {\verb"R2 <- R1 >> 9"};
\node [above=0mm of z3, text width=8em] {$Z_{3}$};
\draw [->] (z2) -- (z3);

\node [zbox] (z4) {\verb"R3 <- R2 and 30"};
\node [above=0mm of z4, text width=8em] {$Z_{4}$};
\draw [->] (z3) -- (z4);

\node [zbox] (z5) {\verb"R0 <- R3 <| R0"};
\node [above=0mm of z5, text width=8em] {$Z_{5}$};
\draw [->] (z4) -- (z5);

\node [zbox] (z6) {\verb"R1 <- R0 >> R0"};
\node [above=0mm of z6, text width=8em] {$Z_{6}$};
\draw [->] (z5) -- (z6);

\node [zbox] (z7) {\verb"R2 <- R1 >> R0"};
\node [above=0mm of z7, text width=8em] {$Z_{7}$};
\draw [->] (z6) -- (z7);

\end{tikzpicture}

\end{center}

\end{document}
