
\begin{tikzpicture}

\node (msp) at (0,0) {};
\draw [fill=black!10!white] (msp) rectangle (2,-3.5);
\node (msp) [anchor=north west] at (msp) {MSP430};

\node (rom1) at (1.5+2,0) {};
\draw [fill=black!5!white] (rom1) rectangle (1.5+4,-3.5/2+0.1);
\node [anchor=north west] at (rom1) {ROM};

\node (ram1) at (1.5+2,-3.5/2-0.1) {};
\draw [fill=black!5!white] (ram1) rectangle (1.5+4,-3.5);
\node [anchor=north west] at (ram1) {RAM};

\draw [line width=0.75, ->] (2.05,-0.7) -- (4-.55,-0.7); 
\draw [line width=2, <-] (2.05,-1.1) -- (4-.55,-1.1);

\draw [line width=0.75, ->] (2.05,-2.7+0.3) -- (4-.55,-2.7+0.3); 
\draw [line width=2, <->] (2.05,-3.1+0.3) -- (4-.55,-3.1+0.3);

\end{tikzpicture}