Lab 9 Java Bytecode & The Jasmin Assembler. Java Bytecode javac compiles your java source code into...

Lab 9Java Bytecode & The Jasmin Assembler

Java Bytecode

• javac compiles your java source code into an intermediate set of instruction called the bytecode.

• JVM (Java Virtual Machine) is a machine that interfaces with the hardware to run the byte code instructions.

Java Bytecode

• JVM can be used as an interface to other programming languages that translates to the byte code instruction set

• But, it is hard to translate to a class file which is a binary file

Jasmin

• Jasmin is a Java Assembler that can translate Bytecode instructions to a binary class

• You can download jasmin from http://sourceforge.net/projects/jasmin/files/jasmin/jasmin-2.4/jasmin-2.4.zip/download.

$ sudo mv jasmin.jar /usr/local/lib$ export CLASSPATH="/usr/local/lib/jasmin.jar:$CLASSPATH“

http://sourceforge.net/projects/jasmin/files/jasmin/jasmin-2.4/jasmin-2.4.zip/download

http://sourceforge.net/projects/jasmin/files/jasmin/jasmin-2.4/jasmin-2.4.zip/download

http://www.antlr.org/download/antlr-4.4-complete.jar

Using Jasmin

• Write your program using the bytecode instructions

• Assemble the code using jasmin to generate class files

• Execute the class files using java

Jasmin Code Structure

Comment AreaHeader Area

Field AreaMethod Area


• Comment Area

• Header Area

; Classfile version: ; Major: 49 ; Minor: 0

.source MyApp.java

.class public MyApp

.super java/lang/Object


• Field Area

• Method Area

.field public static x I

.method public <init>()V aload_0 invokenonvirtual java/lang/Object/<init>()V return.end method.method public static main([Ljava/lang/String;)V .limit stack 10 .limit locals 10 return.end method

Example

• Our task is to write the following program in bytecode instructions,

public class simple {public static int main(String[] args){

int x = 0;x = System.in.read();x = x + 3;System.out.println(x);return 0;

} }

• Create the file simple.j and write the instructions.

Example

.class public simple

.super java/lang/Object

.method public <init>()V aload_0 invokespecial java/lang/Object/<init>()V return .end method

Example

.method public static main([Ljava/lang/String;)V .limit stack 5 .limit locals 100 ldc 0 istore 1 ; initialize x to zero and store it ; in local variable 1

; the read function starts at this point ldc 0 istore 50 ; storage for a dummy integer for ; reading it by read

ExampleLabel1: getstatic java/lang/System/in Ljava/io/InputStream; invokevirtual java/io/InputStream/read()I istore 51 iload 51 ldc 10 isub ifeq Label2 iload 51 ldc 32 isub ifeq Label2 iload 51 ldc 48 isub ldc 10 iload 50 imul iadd istore 50 goto Label1

0

1 0

2

..

..

50 0

51

..

99

Stack

Locals


49 0

1 0

2

..

..

50 0

51

..

99

Stack

Locals


0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


49 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


10

49

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals

Ascii for LF


39 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


49 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


32

49

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals

Ascii for Space


17 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


49 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


48

49

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


1 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


10

1

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


0

10

1

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


0

1

0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


1 0

1 0

2

..

..

50 0

51 49

..

99

Stack

Locals


0

1 0

2

..

..

50 1

51 49

..

99

Stack

Locals

ExampleLabel2: ; now our dummy integer contains the ; integer read from the keyboard iload 50 ; read function ends here istore 1 ; store this value in x iload 1 ldc 3 iadd istore 1 ; x=x+3 iload 1 getstatic java/lang/System/out Ljava/io/PrintStream; swap invokevirtual java/io/PrintStream/println(I)V ; print x return ; return from main.end method

Example

• Use jasmin to translate the simple.j file to the simple.class binary file

$ java -cp $CLASSPATH jasmin.Main simple.j

Or,$ java –jar jasmin.jar simple.j

• Execute the class file

$ java simple

Activity

• Write the equivalent bytecode instructions for the following class and use jasmin to generate the class file.

public class simple {public static int main(String[] args){

int x = 0;int y = 5;x = System.in.read();x = x * y - 2;System.out.println(x);return 0;

} }

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