Lecture 18 0xCAFEBABE Java Byte Codes CS201j Engineering Software University of Virginia Computer Science David Evans http www cs virginia edu evans Menu Running Programs Crash Course in Architecture CS333 Crash Course in Compilers CS571 Java Virtual Machine Byte Codes 4 November 2003 CS 201J Fall 2003 2 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 4 November 2003 CS 201J Fall 2003 3 Central Processing Unit CPU 4 November 2003 CS 201J Fall 2003 4 Intel 4004 First general purpose microprocessor 1971 4 bit data 46 instructions 8 bit instructions 4 November 2003 CS 201J Fall 2003 5 PC Motherboard Memory CPU From http www cyberiapc com hardwarebeg htm 4 November 2003 CS 201J Fall 2003 6 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 4 November 2003 CS 201J Fall 2003 7 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 4 November 2003 CS 201J Fall 2003 8 6 int max int a int b push ebp push instruction is 1 byte 00401010 00401011 mov ebp esp mov instruction is 2 bytes 00401013 Dealing with sub esp 40h function call 00401016 push ebx updating 00401017 push esi stack 00401018 push edi moving 00401019 lea edi ebp 40h In Visual C see arguments 0040101C mov ecx 10h assembly 00401021 mov eax 0CCCCCCCCh code by running Debug 00401026 rep stos dword ptr edi then 7 if a b Window Disassembly 00401028 mov eax dword ptr ebp 8 cmp eax dword ptr ebp 0Ch int max int a int b 0040102B 0040102E jle max 25h 00401035 if a b 8 return b return b 00401030 mov eax dword ptr ebp 0Ch else 00401033 jmp max 28h 00401038 return a 9 else 10 return a 4 November 2003 00401035 00401038 00401039 0040103A 0040103B 0040103D 0040103E mov pop pop pop mov pop ret eax dword ptr ebp 8 edi esi ebx esp ebp ebp CS 201J Fall 2003 Cleanup and return 9 Java Virtual Machine 4 November 2003 CS 201J Fall 2003 10 Java Ring 1998 4 November 2003 CS 201J Fall 2003 11 Java Card 4 November 2003 CS 201J Fall 2003 12 Java Virtual Machine Small and simple to implement All VMs will run all programs the same way Secure 4 November 2003 CS 201J Fall 2003 13 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 4 November 2003 CS 201J Fall 2003 14 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 4 November 2003 CS 201J Fall 2003 15 Stack Based Computation push put something on the top of the stack pop get and remove the top of the stack Stack push 2 push 3 add 2 5 3 Does 2 pops pushes sum 4 November 2003 CS 201J Fall 2003 16 Some Java Instructions Opcode Mnemonic Description 0 nop Does nothing 1 aconst null Push null on the stack 3 iconst 0 Push int 0 on the stack 4 iconst 1 Push int 1 on the stack 4 November 2003 CS 201J Fall 2003 17 Some Java Instructions Opcode 18 Mnemonic ldc value Description Push a one word 4 bytes constant onto the stack 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 4 November 2003 CS 201J Fall 2003 18 Arithmetic Opcode 96 Mnemonic iadd Description Pops two integers from the stack and pushes their sum iconst 2 iconst 3 iadd 4 November 2003 CS 201J Fall 2003 19 Arithmetic Opcode Mnemonic Description 96 iadd Pops two integers from the stack and pushes their sum 97 ladd Pops two long integers from the stack and pushes their sum 106 fmul Pops two floats from the stack and pushes their product 119 dneg Pops a double from the stack and pushes its negation 4 November 2003 CS 201J Fall 2003 20 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 4 November 2003 CS 201J Fall 2003 21 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 4 November 2003 CS 201J Fall 2003 22 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 4 November 2003 CS 201J Fall 2003 23 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 4 November 2003 CS 201J Fall 2003 24 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 4 November 2003 CS 201J Fall 2003 25 Example public class Sample1 static public void main String args System err println Hello System exit 1 4 November 2003 CS 201J Fall 2003 26 javap c Sample1 public class Sample1 static public void main String args System err println Hello System exit 1 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 4 November 2003 CS 201J Fall 2003 27 Referencing Memory iload varnum Pushes the int in local variable varnum 1 bytes on the stack istore varnum Pops the