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\section*{Coursework 4 (Strand 1)}

\noindent This coursework is worth 10\% and is due on 12th
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
where you might need to give the correct path to the class file. 
For example:

\begin{center}
\texttt{java -cp . loops/loops}
\end{center}

\noindent 
There are also other resources about Jasmin on the Internet, for example

\begin{center}
\url{http://www.ceng.metu.edu.tr/courses/ceng444/link/f3jasmintutorial.html}

and

\url{http://www.csc.villanova.edu/~tway/courses/csc8505/s2011/handouts/JVM%20and%20Jasmin.pdf}
\end{center}

\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 5\%)}

You need to lex and parse WHILE programs, and then generate Java Byte
Code instructions for the Jasmin assembler. As solution you need to
submit the assembler instructions for the Fibonacci and Factorial
programs. Both should be so modified that a user can input on the
console which Fibonacci number and which Factorial should
calculated. The Fibonacci program is given in Figure~\ref{fibs}. You
can write your own program for calculating factorials.

\begin{figure}[t]
\lstinputlisting[language=while]{../progs/fib.while}
\caption{The Fibonacci program in the WHILE language.\label{fibs}}
\end{figure}

\subsection*{Question 2 (marked with 5\%)}

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
program

\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}

\noindent 
Note that in this program the variable \pcode{i} is used
twice. You need to make a decision how it should be compiled?

\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
following example: Compile using the usual Java compiler the simple Hello World 
program below:

\begin{center}
\begin{minipage}{10cm}
\begin{lstlisting}[language=Java,numbers=none]
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 to see the assembler instructions of the Java byte
code that has been generated for this 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,numbers=none]
object HelloWorld {
   def main(args: Array[String]) {
      println("Hello World!")
  }
}
\end{lstlisting}
\end{minipage}
\end{center}


\subsection*{Library Functions}

You need to generate code for the commands \texttt{write} and
\texttt{read}. This will require the addition of some
``library'' functions to your generated code. The first
command even needs two versions, because you need to write out
an integer and string. The Java byte code will need two
separate functions for this. For writing out an integer, you
can use the assembler code

\begin{center}
\begin{minipage}{12cm}
\begin{lstlisting}[language=JVMIS, 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}[language=JVMIS, 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).

Writing out a string is similar. The corresponding library function uses strings 
instead of integers:

\begin{center}
\begin{minipage}{12cm}
\begin{lstlisting}[language=JVMIS, 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
The code that needs to be generated for \code{write "some_string"} commands 
is

\begin{center}
\begin{minipage}{8cm}
\begin{lstlisting}[language=JVMIS,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.

The code for \texttt{read} is more complicated. The reason is that inputting a string
will need to be transformed into an integer. The code in Figure~\ref{read} does this.
It can be called with

\begin{center}
\begin{minipage}{8cm}
\begin{lstlisting}[language=JVMIS,numbers=none]
invokestatic XXX/XXX/read()I 
istore n
\end{lstlisting}
\end{minipage}
\end{center}

\noindent 
where \texttt{n} is the index of the variable that requires an input. If you
use Windows you need to take into account that a ``return'' is not just a newline,
\code{'\\10'}, but \code{'\\13\\10'}. This means you need to change line~12 in 
Figure~\ref{read} to \pcode{ldc 13}.  


\begin{figure}[t]\small
\begin{lstlisting}[language=JVMIS]
.method public static read()I 
      .limit locals 10 
      .limit stack 10

      ldc 0 
      istore 1  ; this will hold our final integer 
Label1: 
      getstatic java/lang/System/in Ljava/io/InputStream; 
      invokevirtual java/io/InputStream/read()I 
      istore 2 
      iload 2 
      ldc 10   ; the newline delimiter 
      isub 
      ifeq Label2 
      iload 2 
      ldc 32   ; the space delimiter 
      isub 
      ifeq Label2
      iload 2 
      ldc 48   ; we have our digit in ASCII, have to subtract it from 48 
      isub 
      ldc 10 
      iload 1 
      imul 
      iadd 
      istore 1 
      goto Label1 
Label2: 
      ;when we come here we have our integer computed in Local Variable 1 
      iload 1 
      ireturn 
.end method
\end{lstlisting}\normalsize
\caption{Assembler code for reading an integer from the console.\label{read}}
\end{figure}

\end{document}

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