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\usepackage{../style}
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\begin{document}

%https://github.com/Storyyeller/Krakatau
%https://docs.oracle.com/javase/specs/jvms/se7/html/

\section*{Coursework 4 (Strand 1)}

\noindent This coursework is worth 6\% and is due on 7
December at 16:00. You are asked to implement a compiler for
the WHILE language that targets the assembler language
provided by Jasmin or Krakatau (both have very similar
syntax). 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 a mark of 0\% will
be awarded. You should use the lexer and parser from the
previous courseworks. 

\subsection*{Disclaimer}

It should be understood that the work you submit represents
your own effort. You have not copied from anyone else. An
exception is the Scala code I showed during the lectures,
which you can use. You can also use your own code from the
CW~1, CW~2 and CW~3.


\subsection*{Jasmin Assembler}

The Jasmin 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 into 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}
\small\url{http://www.ceng.metu.edu.tr/courses/ceng444/link/f3jasmintutorial.html}
\end{center}

\noindent and

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

\subsection*{Krakatau Assembler}

The Krakatau assembler is available from

\begin{center}
\url{https://github.com/Storyyeller/Krakatau}
\end{center}

\noindent This assembler requires Python and a package called
\pcode{ply} available from

\begin{center}
\url{https://pypi.python.org/pypi/ply}
\end{center}

\noindent This assembler is largely compatible with the Jasmin
syntax---that means for the files we are concerned with here,
it understands the same input syntax (no changes to your
compiler need to be made; ok maybe some small syntactic
adjustments are needed). You can generate Java Byte Code by
using 

\begin{center}
\texttt{python Krakatau-master/assemble.py loops.j}
\end{center}

\noindent where you may have to adapt the directory where
Krakatau is installed (I just downloaded the zip file from
Github and \pcode{Krakatau-master} was the directory where it
was installed). Again the resulting class-file you can run with
\texttt{java}.


%\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}

You need to lex and parse WHILE programs, and then generate
Java Byte Code instructions for the Jasmin assembler (or
Krakatau 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
be calculated. The Fibonacci program is given in
Figure~\ref{fibs}. You can write your own program for
calculating factorials. Submit your assembler code as
a file that can be run, not as PDF-text.

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

\subsection*{Question 2}

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

\begin{center}
\lstset{language=While}
\code{for} \;\textit{Id} \texttt{:=} \textit{AExp}\; \code{upto} 
\;\textit{AExp}\; \code{do} \textit{Block} 
\end{center}

\noindent The intended meaning is to first assign the variable
\textit{Id} the value of the first arithmetic expression, test
wether this value is less or equal than the value of the
second arithmetic expression. If yes, go through the loop, and
at the end increase the value of the loop variable by 1 and
start again with the test. If no, leave the loop. For example
the following instance of a \code{for}-loop is supposed to
print out the numbers \pcode{2}, \pcode{3}, \pcode{4}.


\begin{center}
\begin{minipage}{12cm}
\begin{lstlisting}[language=While, 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 \code{for}-loops; the other is to
translate the abstract syntax tree of \code{for}-loops into
an abstract syntax tree using existing language constructs.
For example the loop above could be translated to the
following \code{while}-loop:

\begin{center}
\begin{minipage}{12cm}
\begin{lstlisting}[language=While, numbers=none]
i := 2;
while (i <= 4) do {
    write i;
    i := i + 1;
}
\end{lstlisting}
\end{minipage}
\end{center}

\subsection*{Question 3 (marked with 1\%)}

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

\begin{center}
\begin{minipage}{12cm}
\begin{lstlisting}[language=While, numbers=none]
for i := 1 upto 10 do {
  for i := 1 upto 10 do {
    write i
  }
} 
\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?
Explain your decision and indicate what this program would
print out.

\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 or Krakatau---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}{12cm}
\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}
\begin{minipage}{12cm}
\begin{lstlisting}[language={},numbers=none]
javap -v HelloWorld
\end{lstlisting}
\end{minipage}
\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}{12cm}
\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 1 
    .limit stack 2  
    getstatic java/lang/System/out Ljava/io/PrintStream; 
    iload 0
    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}{12cm}
\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 1
   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}{12cm}
\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}{12cm}
\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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