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\section*{Coursework 3}

\noindent

This coursework is worth 4\% and is due on 13 December at 16:00. You are asked to 

implement a compiler for the WHILE language that targets the

assembler language provided by Jasmin. This assembler 

is available from

\begin{center}

\url{http://jasmin.sourceforge.net}

\end{center}

\noindent

There is a user guide for Jasmin

\begin{center}

\url{http://jasmin.sourceforge.net/guide.html}

\end{center}

\noindent

and also a description of some of the instructions that the JVM understands

\begin{center}

\url{http://jasmin.sourceforge.net/instructions.html}

\end{center}

\noindent

If you generated a correct assembler file for Jasmin, for example

\texttt{loops.j}, you can use

\begin{center}

\texttt{java -jar jasmin-2.4/jasmin.jar loops.j}

\end{center}

\noindent

in order to translate it to Java byte code. The resulting class file can be

run with

\begin{center}

\texttt{java loops}

\end{center}

\noindent

\noindent

You need to submit a document containing the answers for the two questions 

below. You can do the implementation in any programming language you like, but you need 

to submit the source code with which you answered the questions. Otherwise

the submission will not be counted.  However, the coursework 

will \emph{only} be judged according to the answers. You can submit your answers

in a txt-file or as pdf.\bigskip

\subsection*{Question 1 (marked with 2\%)}

You need to lex and parse WHILE programs and submit the assembler 

instructions for the Fibonacci program and for the program you submitted

in Coursework 2 in Question 3. The latter should be so modified that 

a user can input the upper bound on the console (in the original question

it was fixed to 100).

\subsection*{Question 2 (marked with 2\%)}

Extend the syntax of you language so that it contains also \texttt{for}-loops, like

\begin{center}

\texttt{for} \;\textit{Id} \texttt{:=} \textit{AExp}\; \texttt{upto} \;\textit{AExp}\; \texttt{do} \textit{Block} 

\end{center}

\noindent

The intended meaning is to first assign the variable \textit{Id} the value of the first arithmetic 

expression, then go through the loop, at the end increase the value of the variable by 1, 

and finally test wether the value is not less or equal to the value of the second

arithmetic expression. For example the following instance of a \texttt{for}-loop 

is supposed to print out the numbers \texttt{2}, \texttt{3}, \texttt{4}.

\begin{center}

\begin{minipage}{6cm}

\begin{lstlisting}[language=While,basicstyle=\ttfamily, numbers=none]

for i := 2 upto 4 do {

    write i	

}

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

There are two ways how this can be implemented: one is to adapt the code generation 

part of the compiler and generate specific code for \texttt{for}-loops; the other is to

translate the abstract syntax tree of \texttt{for}-loops into an abstract syntax tree using

existing language constructs. For example the loop above could be translated

to the following \texttt{while}-loop:

\begin{center}

\begin{minipage}{6cm}

\begin{lstlisting}[language=While,basicstyle=\ttfamily, numbers=none]

i := 2;

while (i <= 4) do {

    write i;

    i := i + 1;

}

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

In this question you are supposed to give the assembler instructions for the

\begin{center}

\begin{minipage}{6cm}

\begin{lstlisting}[language=While,basicstyle=\ttfamily, numbers=none]

for i := 1 upto 10000 do {

  for i := 1 upto 10000 do {

  skip

  }

} 

\end{lstlisting}

\end{minipage}

\end{center}

\subsection*{Further Information}

The Java infrastructure unfortunately does not contain an assembler out-of-the-box

(therefore

you need to download the additional package Jasmin---see above). But it does contain a 

disassembler, called \texttt{javap}. A dissembler does the ``opposite'' of an assembler: it

generates readable assembler code from Java byte code. Have a look at the

program below:

\begin{center}

\begin{minipage}{10cm}

\begin{lstlisting}[language=Java,basicstyle=\ttfamily]

class HelloWorld {

    public static void main(String[] args) {

        System.out.println("Hello World!");

    }

}

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

You can use the command

\begin{center}

\texttt{javap -v HelloWorld}

\end{center}

\noindent

program. You can compare this with the code generated for the Scala

version of Hello World.

\begin{center}

\begin{minipage}{10cm}

\begin{lstlisting}[language=Scala,basicstyle=\ttfamily]

object HelloWorld {

   def main(args: Array[String]) {

      println("Hello World!")

  }

}

\end{lstlisting}

\end{minipage}

\end{center}

\subsection*{Library Functions}

command even needs two versions, because you might want to write out an

integer or a string. The Java byte code will need two separate functions for this.

\begin{center}

\begin{minipage}{12cm}

\begin{lstlisting}[basicstyle=\ttfamily, numbers=none]

.method public static write(I)V 

    .limit locals 5 

    .limit stack 5 

    iload 0 

    getstatic java/lang/System/out Ljava/io/PrintStream; 

    swap 

    invokevirtual java/io/PrintStream/println(I)V 

    return 

.end method

\end{lstlisting}

\end{minipage}

\end{center}

\noindent 

This function will invoke Java's \texttt{println} function for integers. Then if you need

to generate code for \texttt{write x} where \texttt{x} is an integer variable, you can generate

\begin{center}

\begin{minipage}{8cm}

\begin{lstlisting}[basicstyle=\ttfamily, numbers=none]

iload n 

invokestatic XXX/XXX/write(I)V

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

where \texttt{n} is the index where the value of the variable \texttt{x} is

stored. The \texttt{XXX/XXX} needs to be replaced with the class name 

which you use to generate the code (for example \texttt{fib/fib} in case

of the Fibonacci numbers).

\begin{center}

\begin{minipage}{12cm}

\begin{lstlisting}[basicstyle=\ttfamily, numbers=none]

.method public static writes(Ljava/lang/String;)V

   .limit stack 2

   .limit locals 2

   getstatic java/lang/System/out Ljava/io/PrintStream;

   aload 0

   invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V

   return

.end method

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

is

\begin{center}

\begin{minipage}{8cm}

\begin{lstlisting}[basicstyle=\ttfamily, numbers=none]

ldc "some_string"

invokestatic XXX/XXX/writes(Ljava/lang/String;)V

\end{lstlisting}

\end{minipage}

\end{center}

\noindent

Again you need to adjust the \texttt{XXX/XXX} part in each call.

\end{document}

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