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Understanding Java Byte Codes | CS201J Fall 2003 Lecture Series

Learn about Java Byte Codes, the Java Virtual Machine, CPU architecture, compilers, and more in this informative lecture from the University of Virginia.

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Understanding Java Byte Codes | CS201J Fall 2003 Lecture Series

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  1. Lecture 18: 0xCAFEBABE (Java Byte Codes) David Evans http://www.cs.virginia.edu/evans CS201j: Engineering Software University of Virginia Computer Science

  2. Menu • Running Programs • Crash Course in Architecture (CS333) • Crash Course in Compilers (CS571) • Java Virtual Machine • Byte Codes CS 201J Fall 2003

  3. Computer Architecture Processor does computation Memory stores bits Input Devices (mouse, keyboard, accelerometer) get input from user and environment Output Devices (display, speakers) present output to user CS 201J Fall 2003

  4. Central Processing Unit (CPU) CS 201J Fall 2003

  5. Intel 4004 • First general purpose microprocessor, 1971 • 4-bit data • 46 instructions • 8-bit instructions! CS 201J Fall 2003

  6. PC Motherboard Memory CPU From http://www.cyberiapc.com/hardwarebeg.htm CS 201J Fall 2003

  7. Inside the CPU • Registers • Loads and decodes instructions from memory • ALU: Arithmetic Logic Unit • Does arithmetic • Can only operate on values in registers • Must load values from memory into registers before computing with them CS 201J Fall 2003

  8. Compiler • Translates a program in a high-level language into machine instructions • Calling convention • How are parameters passed to functions • How is the stack managed to return • Register allocation • Figure out how to use registers efficiently CS 201J Fall 2003

  9. 6: int max (int a, int b) { 00401010 push ebp 00401011 mov ebp,esp 00401013 sub esp,40h 00401016 push ebx 00401017 push esi 00401018 push edi 00401019 lea edi,[ebp-40h] 0040101C mov ecx,10h 00401021 mov eax,0CCCCCCCCh 00401026 rep stos dword ptr [edi] 7: if (a > b) { 00401028 mov eax,dword ptr [ebp+8] 0040102B cmp eax,dword ptr [ebp+0Ch] 0040102E jle max+25h (00401035) 8: return b; 00401030 mov eax,dword ptr [ebp+0Ch] 00401033 jmp max+28h (00401038) 9: } else { 10: return a; 00401035 mov eax,dword ptr [ebp+8] 00401038 pop edi 00401039 pop esi 0040103A pop ebx 0040103B mov esp,ebp 0040103D pop ebp 0040103E ret push instruction is 1 byte mov instruction is 2 bytes Dealing with function call: updating stack, moving arguments In Visual C++, see assembly code by running Debug, then Window | Disassembly int max (int a, int b) { if (a > b) { return b; } else { return a; } } Cleanup and return CS 201J Fall 2003

  10. Java Virtual Machine CS 201J Fall 2003

  11. Java Ring (1998) CS 201J Fall 2003

  12. Java Card CS 201J Fall 2003

  13. Java Virtual Machine • Small and simple to implement • All VMs will run all programs the same way • Secure CS 201J Fall 2003

  14. Implementing the JavaVM load class into memory set the instruction pointer to point to the beginning of main do { fetch the next instruction execute that instruction } while (there is more to do); Some other issues we will talk about Thursday and next week: Verification – need to check byte codes satisfy security policy Garbage collection – need to reclaim unused storage CS 201J Fall 2003

  15. Java Byte Codes • Stack-based virtual machine • Small instruction set: 202 instructions (all are 1 byte opcode + operands) • Intel x86: ~280 instructions (1 to 17 bytes long!) • Memory is typed • Every Java class file begins with magic number 3405691582 = 0xCAFEBABE in base 16 CS 201J Fall 2003

  16. Stack-Based Computation • push – put something on the top of the stack • pop – get and remove the top of the stack Stack push 2 5 2 push 3 3 add Does 2 pops, pushes sum CS 201J Fall 2003

  17. Some Java Instructions CS 201J Fall 2003

  18. Some Java Instructions Constant may be an int, float or String ldc “Hello” ldc 201 The String is really a reference to an entry in the string constant table! CS 201J Fall 2003

  19. Arithmetic iconst_2 iconst_3 iadd CS 201J Fall 2003

  20. Arithmetic CS 201J Fall 2003

  21. Java Byte Code Instructions • 0: nop • 1-20: putting constants on the stack • 96-119: arithmetic on ints, longs, floats, doubles • What other kinds of instructions do we need? CS 201J Fall 2003

  22. Other Instruction Classes • Control Flow (~20 instructions) • if, goto, return • Method Calls (4 instructions) • Loading and Storing Variables (65 instructions) • Creating objects (1 instruction) • Using object fields (4 instructions) • Arrays (3 instructions) CS 201J Fall 2003

  23. Control Flow • ifeq <label> Pop an int off the stack. If it is zero, jump to the label. Otherwise, continue normally. • if_icmple <label> Pop two ints off the stack. If the second one is <= the first one, jump to the label. Otherwise, continue normally. CS 201J Fall 2003

  24. Method Calls • invokevirtual <method> • Invokes the method <method> on the parameters and object on the top of the stack. • Finds the appropriate method at run-time based on the actual type of the this object. invokevirtual <Method void println(java.lang.String)> CS 201J Fall 2003

  25. Method Calls • invokestatic <method> • Invokes a static (class) method <method> on the parameters on the top of the stack. • Finds the appropriate method at run-time based on the actual type of the this object. CS 201J Fall 2003

  26. Example public class Sample1 { static public void main (String args[]) { System.err.println ("Hello!"); System.exit (1); } } CS 201J Fall 2003

  27. public class Sample1 { static public void main (String args[]) { System.err.println ("Hello!"); System.exit (1); } } > javap -c Sample1 Compiled from Sample1.java public class Sample1 extends java.lang.Object { public Sample1(); public static void main(java.lang.String[]); } Method Sample1() 0 aload_0 1 invokespecial #1 <Method java.lang.Object()> 4 return Method void main(java.lang.String[]) 0 getstatic #2 <Field java.io.PrintStream err> 3 ldc #3 <String "Hello!"> 5 invokevirtual #4 <Method void println(java.lang.String)> 8 iconst_1 9 invokestatic #5 <Method void exit(int)> 12 return CS 201J Fall 2003

  28. Referencing Memory • iload <varnum> • Pushes the int in local variable <varnum> (1 bytes) on the stack • istore <varnum> • Pops the int on the top of the stack and stores it in local variable <varnum> CS 201J Fall 2003

  29. Referencing Example Method void main(java.lang.String[]) 0 iconst_2 1 istore_1 2 iconst_3 3 istore_2 4 iload_1 5 iload_2 6 iadd 7 istore_3 8 getstatic #2 <Field java.io.PrintStream err> 11 new #3 <Class java.lang.StringBuffer> 14 dup 15 invokespecial #4 <Method java.lang.StringBuffer()> 18 ldc #5 <String "c: "> 20 invokevirtual #6 <Method java.lang.StringBuffer append(java.lang.String)> 23 iload_3 24 invokevirtual #7 <Method java.lang.StringBuffer append(int)> 27 invokevirtual #8 <Method java.lang.String toString()> 30 invokevirtual #9 <Method void println(java.lang.String)> 33 return public class Locals1 { static public void main (String args[]) { int a = 2; int b = 3; int c = a + b; System.err.println ("c: " + c); } } CS 201J Fall 2003

  30. Charge • PS6 will involve reading and writing Java byte codes • Use javap –c <classname> to look at what the javac compiler produces for your code • Thursday: what would this program do? Method void main(java.lang.String[]) 0 iconst_2 1 iadd 2 return CS 201J Fall 2003

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