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%https://github.com/Storyyeller/Krakatau

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

\section*{Coursework 4 (Strand 1)}

\noindent This coursework is worth 10\% and is due on 11

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 look are concerned with,

it understands the same input syntax (no changes to your

compiler need to be made). 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).

%\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 (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.

\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 4\%)}

Extend the syntax of your language so that it contains also

\texttt{for}-loops, like

\begin{center}

\end{center}

\noindent The intended meaning is to first assign the variable

\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

an abstract syntax tree using existing language constructs.

For example the loop above could be translated to the

\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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