Lecture 19: 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 12 November 2002 CS 201J Fall 2002 2 Computer Architecture Processor does computation Memory stores bits Input Devices (mouse, keyboard) get input from user Output Devices (display, speakers) present output to user 12 November 2002 CS 201J Fall 2002 3 Central Processing Unit (CPU) 12 November 2002 CS 201J Fall 2002 4 Intel 4004 • First general purpose microprocessor, 1971 • 4-bit data • 46 instructions – 8-bit instructions! 12 November 2002 CS 201J Fall 2002 5 PC Motherboard Memory CPU From http://www.cyberiapc.com/hardwarebeg.htm 12 November 2002 CS 201J Fall 2002 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 12 November 2002 CS 201J Fall 2002 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 12 November 2002 CS 201J Fall 2002 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] 0040102B cmp eax,dword ptr [ebp+0Ch] int max (int a, int b) { 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; } 12 November 2002 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 2002 9 Java Virtual Machine 12 November 2002 CS 201J Fall 2002 10 Java Ring (1998) 12 November 2002 CS 201J Fall 2002 11 Java Card 12 November 2002 CS 201J Fall 2002 12 Java Virtual Machine • Small and simple to implement • All VMs will run all programs the same way • Secure 12 November 2002 CS 201J Fall 2002 13 Java Byte Codes • Stack-based virtual machine • Small instruction set: 202 instructions – 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 12 November 2002 CS 201J Fall 2002 14 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 12 November 2002 CS 201J Fall 2002 15 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 … 12 November 2002 CS 201J Fall 2002 16 Some Java Instructions Opcode 18 Mnemonic ldc <value> Description Push a one-word 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! 12 November 2002 CS 201J Fall 2002 17 Arithmetic Opcode 96 Mnemonic iadd Description Pops two integers from the stack and pushes their sum iconst_2 iconst_3 iadd 12 November 2002 CS 201J Fall 2002 18 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 12 November 2002 CS 201J Fall 2002 19 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? 12 November 2002 CS 201J Fall 2002 20 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) 12 November 2002 CS 201J Fall 2002 21 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. 12 November 2002 CS 201J Fall 2002 22 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)> 12 November 2002 CS 201J Fall 2002 23 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. 12 November 2002 CS 201J Fall 2002 24 Example public class Sample1 { static public void main (String args[]) { System.err.println ("Hello!"); System.exit (1); } } 12 November 2002 CS 201J Fall 2002 25 > 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 12 November 2002 CS 201J Fall 2002 26 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> 12 November 2002 CS 201J Fall 2002 27 Referencing Example Method void main(java.lang.String[]) 0 iconst_2 public class Locals1 { 1 istore_1 static public void main (String args[]) { 2 iconst_3 int a = 2; 3 istore_2 int b = 3; 4 iload_1 int c = a + b; 5 iload_2 6 iadd System.err.println ("c: " + c); } } 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.l 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 12 November 2002 CS 201J Fall 2002 28 public class Locals { static public void main (String args[]) { int a1, a2, a3, a4, a5, a6, a7, a8, a9; int a10, a11, a12, a13, a14, a15, a16, a17, a18, a19; int a20, a21, a22, a23, a24, a25, a26, a27, a28, a29; int a30, a31, a32, a33, a34, a35, a36, a37, a38, a39; int a40, a41, a42, a43, a44, a45, a46, a47, a48, a49; int a50, a51, a52, a53, a54, a55, a56, a57, a58, a59; int a60, a61, a62, a63, a64, a65, a66, a67, a68, a69; int a70, a71, a72, a73, a74, a75, a76, a77, a78, a79; int a80, a81, a82, a83, a84, a85, a86, a87, a88, a89; int a90, a91, a92, a93, a94, a95, a96, a97, a98, a99; int a100, a101, a102, a103, a104, a105, a106, a107, a108, a109; int a110, a111, a112, a113, a114, a115, a116, a117, a118, a119; int a120, a121, a122, a123, a124, a125, a126, a127, a128, a129; int a130, a131, a132, a133, a134, a135, a136, a137, a138, a139; int a140, a141, a142, a143, a144, a145, a146, a147, a148, a149; int a150, a151, a152, a153, a154, a155, a156, a157, a158, a159; int a160, a161, a162, a163, a164, a165, a166, a167, a168, a169; int a170, a171, a172, a173, a174, a175, a176, a177, a178, a179; int a180, a181, a182, a183, a184, a185, a186, a187, a188, a189; int a190, a191, a192, a193, a194, a195, a196, a197, a198, a199; int a200, a201, a202, a203, a204, a205, a206, a207, a208, a209; int a210, a211, a212, a213, a214, a215, a216, a217, a218, a219; int a220, a221, a222, a223, a224, a225, a226, a227, a228, a229; int a230, a231, a232, a233, a234, a235, a236, a237, a238, a239; int a240, a241, a242, a243, a244, a245, a246, a247, a248, a249; } bipush <n> Push a one-byte signed integer Why not just use lcd? Method void main(java.lang.String[]) 0 bipush 11 int a250, a251, a252, a253, a254, a255, a256; 2 istore 255 4 bipush 12 a255 = 11; 6 istore_w 256 a256 = 12; 10 return } 12 November 2002 CS 201J Fall 2002 29 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 pushIfExample edi public class { 00401019 lea edi,[ebp-40h] publicmov int max (intecx,10h a, int b) { 0040101C 00401021 if (a mov> b) { eax,0CCCCCCCCh 00401026 rep stos return a; dword ptr [edi] 7: if (a > b) { } else { 00401028 mov eax,dword ptr [ebp+8] return b; 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 MaxWhy is the compiled C code so much longer and more complicated? 12 November 2002 CS 201J Fall 2002 Method int max(int, int) 0 iload_1 1 iload_2 2 if_icmple 7 5 iload_1 6 ireturn 7 iload_2 8 ireturn 30 Creating Objects • new <class> – Pushes an object reference to new object of type <class> on the stack. (Note: doesn’t call a constructor!) 12 November 2002 CS 201J Fall 2002 31 Creating Objects public class NewExample { public StringBuffer makeBigBuffer () { StringBuffer res; res = new StringBuffer ("Hello!"); return res; } } Method java.lang.StringBuffer makeBigBuffer() 0 new #2 <Class java.lang.StringBuffer> Duplicates the reference, not the object! 3 dup 4 ldc #3 <String "Hello!"> 6 invokespecial #4 <Method java.lang.StringBuffer(java.lang.String)> 9 astore_1 10 aload_1 11 areturn 12 November 2002 CS 201J Fall 2002 32 Using Object Fields • getfield <field-spec> – Pops an object reference from the stack, and pushes the value of the field on the stack • putfield <field-spec> – Pops a value and an object reference from the stack, and stores the value in the in the object’s field 12 November 2002 CS 201J Fall 2002 33 public class Species { private String name; private Genome genome; public Species (String n, Genome g) { name = n; genome = g; } 0 1 4 5 6 9 10 11 14 aload_0 invokespecial #1 <Method java.lang.Object()> aload_0 aload_1 putfield #2 <Field java.lang.String name> aload_0 aload_2 putfield #3 <Field Genome genome> return 12 November 2002 CS 201J Fall 2002 34 Charge • Use javap –c <classname> to look at what the javac compiler produces for your code • How do we know bad things won’t happen? Method void main(java.lang.String[]) 0 iconst_2 1 iadd 2 return • If you want a review next class, send me questions by 3pm tomorrow! 12 November 2002 CS 201J Fall 2002 35