1. Lab 9 Java Bytecode & The Jasmin Assembler

2. 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.

3. 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

4. Jasmin
Jasmin is a Java Assembler that can translate Bytecode instructions to a binary class
You can download jasmin from
$ sudo mv jasmin.jar /usr/local/lib
$ export CLASSPATH="/usr/local/lib/jasmin.jar:$CLASSPATH“

5. Using Jasmin Write your program using the bytecode instructions
Assemble the code using jasmin to generate class files
Execute the class files using java

6. Jasmin Code Structure
Comment Area
Header Area
Field Area
Method Area

7. Jasmin Code Structure Comment Area Header Area ; Classfile version:
; Major: 49
; Minor: 0
.source MyApp.java
.class public MyApp
.super java/lang/Object

8. Jasmin Code Structure 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

9. 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; }

10. Example Create the file simple.j and write the instructions.
.class public simple
.super java/lang/Object
.method public <init>()V
aload_0
invokespecial java/lang/Object/<init>()V
return
.end method

11. 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
istore 50 ; storage for a dummy integer for ; reading it by read

12. 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
istore 50 ; storage for a dummy integer for ; reading it by read

13. Example 1 2 .. 50 51 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 2 .. 50 51 99
Stack
Locals

14. Example 49 1 2 .. 50 51 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 49 1 2 .. 50 51 99
Stack
Locals

15. Example 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 2 .. 50 51 49 99
Stack
Locals

16. Example 49 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 49 1 2 .. 50 51 49 99
Stack
Locals

17. Example Ascii for LF 10 49 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1
Ascii for LF 10 49 1 2 .. 50 51 49 99
Stack
Locals

18. Example 39 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 39 1 2 .. 50 51 49 99
Stack
Locals

19. Example 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 2 .. 50 51 49 99
Stack
Locals

20. Example 49 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 49 1 2 .. 50 51 49 99
Stack
Locals

21. Example 32 49 1 2 .. 50 51 49 99 Ascii for Space Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 32 49 1 2 .. 50 51 49 99
Ascii for Space
Stack
Locals

22. Example 17 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 17 1 2 .. 50 51 49 99
Stack
Locals

23. Example 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 2 .. 50 51 49 99
Stack
Locals

24. Example 49 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 49 1 2 .. 50 51 49 99
Stack
Locals

25. Example 48 49 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 48 49 1 2 .. 50 51 49 99
Stack
Locals

26. Example 1 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 1 2 .. 50 51 49 99
Stack
Locals

27. Example 10 1 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 10 1 1 2 .. 50 51 49 99
Stack
Locals

28. Example 10 1 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 10 1 1 2 .. 50 51 49 99
Stack
Locals

29. Example 1 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 1 2 .. 50 51 49 99
Stack
Locals

30. Example 1 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 1 2 .. 50 51 49 99
Stack
Locals

31. Example 1 2 .. 50 51 49 99 Stack Locals Label1:
getstatic java/lang/System/in Ljava/io/InputStream;
invokevirtual java/io/InputStream/read()I
istore 51
iload 51
ldc 10
isub
ifeq Label2
ldc 32
ldc 48
iload 50
imul
iadd
istore 50
goto Label1 1 2 .. 50 51 49 99
Stack
Locals

32. Example Label2: ; now our dummy integer contains the
; integer read from the keyboard
iload ; read function ends here
istore ; store this value in x
iload 1
ldc 3
iadd
istore ; x=x+3
getstatic java/lang/System/out Ljava/io/PrintStream;
swap
invokevirtual java/io/PrintStream/println(I)V ; print x
return ; return from main
.end method

33. 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

34. 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; }