1

Java Virtual Machine: Instruction Set

Cheng-Chia Chen

22

Java Program

class SumI {

public static void main (String[] args) {

int count=10;

int sum =0;

for (int index=1;index<count;index++)

sum=sum+index;

} // method main

} // class SumI

33

Java ByteCode

Method void main(java.lang.String[]) 0 bipush 10// byte push 10 to stack (0x10) 2 istore_1 // load 10 (top of stack) to count 3 iconst_0 // push 0 to stack 4 istore_2 // load 0 (top of stack to sum 5 iconst_1 // push 1 to stack 6 istore_3 // load 1 (top of stack) to index 7 goto 17 // go to 17

43

Java ByteCode

10 iload_2 // load sum to stack 11 iload_3 // load index to stack 12 iadd // add 13 istore_2 // store “top of stack” to sum 14 iinc 3 1 // index ++ 17 iload_3 // load index to stack 18 iload_1 // load count to stack 19 if_icmplt 10 // if index < count goto 10 22 return

5

import java.io.*;class ReadFile { public static void main (String[] args) { try { FileInputStream in = new FileInputStream(args[0]); int c; while((c = in.read()) != -1) System.out.println(c); in.close(); } catch(Exception e) { e.printStackTrace(); } } // method main} // class ReadFile

6

JVM Instruction Sets Stack and Local Variable Operations Type Conversion Integer Arithmetic Logic and Bit manipulation Floating Point Arithmetic Objects and Arrays Control Flow Exceptions Finally Clauses Method Invocation and Return Thread Synchronization

7

Menmonics for instruction type

i for an int operation, l for long, s for short, b for byte, c for char, f for float, d for double, and a for reference/array. EX: iload, lload, fload, dload, aload,…

8

Load and Store Instructions

transfer values between the local variables and the operand stack

load: localVar Stack store: Stack localVar [i | l | f | d | a]load u8,// or wide load u16 [i | l | f | d | a]load_<n>, // n=0..3 [i | l | f | d | a]store u8, // or wide load u16 [i | l | f | d | a]store_<n>, // n=0..3

9

Push Constants onto stack

iconst_x » x=m1..5; push int x onto stack.

fconst_x» x=0..2; push float x onto stack.

lconst_x ; dconst_x :» x=0 or 1; push long (or double) x onto stack

aconst_null: push null onto stack. bipush s8; sipush s16,

» push (int) s8 [or (int) s16] onto stack

10

Push Constants onto stack

ldc u8; ldc_w u16, ldc2_w u16,» push single word (or double worlds for ldc2

) from constant_pool entry u8 (or u16) » ldc/ldc_w can be used to push byte/short/c

har/int/float/String constants.

11

Generic Stack Operations nop // do nothing pop, pop2, // pop top (two) words dup, dup2, // duplicate top (two) words swap. // swap top two words dup_x1, dup_x2

» duplicate top words and put 1 ( 2 ) down» w3, w2, w1 => w3,w1,w2,w1 or (w1,w3,w2,w1)

dup2_x1, dup2_x2, » w4, w3, w2, w1 =>w4, w2,w1, w3,w2,w1 or (w2,w1,w4,

w3,w2,w1)

12

Integer and floating pointArithmetic Instructions

Add: iadd, ladd, fadd, dadd. Subtract: isub, lsub, fsub, dsub. Multiply: imul, lmul, fmul, dmul. Divide: idiv, ldiv, fdiv, ddiv. Remainder: irem, lrem, frem, drem. Negate: ineg, lneg, fneg, dneg. Local variable increment:

» iinc u8 s8 // add s8 to lcalvar[u8].» wide iinc u16 s16 // add s16 to local var at u16

13

Shift and Bitwise operations

Shift: ishl, ishr, iushr, lshl, lshr, lushr.» ishl : … int2, int1 => … , int2<<(int1 & 0x001f)» lushr: … long2, int1=> … , long2 >>> (int1 & 0x

003f) Bitwise OR: ior, lor. Bitwise AND: iand, land. Bitwise exclusive OR: ixor, lxor.

14

Comparison operations

dcmpg, dcmpl,» …d1,d2=>… int; (d1 < d2 =>-1; d2=d1=> 0; d

1>d2 =>1 ) or NaN=>(g =>1; l=>-1) fcmpg, fcmpl, // like above lcmp.

» … long1, long2 => … int k where» k = -1, 0 or 1 depending on long1 <, = or > l

ong2.

15

Type Conversion Instructions

widening conversions:» int long float double :» i2l, i2f, i2d, l2f, l2d, f2d. » // sign extension+round-to-nearest mode

Narrowing conversions:» double float long int :» d2f, d2l, d2i, f2l, f2i, l2i // truncate+ sign extension

+ rtn mode» int short, byte, char:» i2s, i2c, i2b // truncate+ sign extension

Note: byte, short, char int are done automaticaly.

16

Object Creation and Field Access

new » Create a new class instance on the heap,» and push a reference:

Field access:» getfield fieldRefIndex; putfield fieldRefIndex,» … objRef … value; …objRef, value …» getstatic fieldRefIndex putstatic fieldRefIndex» … value; …value …

Type Checking: » instanceof class_index : … objRef … RltVal» checkcast class_index: …obj1 … obj1 if obj1 ca

n be casted to class type, o/w throw CastException

17

Array Creation newarray aType:

» pop length, allocate new primitive array of type given by atype, push the reference.

» aType : (z,c,f,d,b,s,i,l) ->(4,5,6,7,8,9,10,11)» … u16 -> arrayRef

anewarray Class_index, » pop length, allocate new array of type given by class_i

ndex, push the reference multianewarray type_index dim:u8.

» pop dim number of array lengths, allocate multi-dimensional array of type class_index, pop reference.

» new int[10][20] ==> p10, p20, multianewarray [[i 2.

18

Array Component Access array component stack :

» baload, caload, saload, iaload,» … arrayRef, index:s32 … (int) array[index]» laload, faload, daload, aaload.

stack array component» bastore, castore, sastore, iastore, » … arrayRef, index:s32, val … » lastore, fastore, dastore, aastore.

Get the length of array: arraylength» … arrayRef … (int)array.length

19

Control flow Instructions1. Conditional Branches

Conditional branch:» unary comparsions

– ifeq, iflt, ifle, ifne, ifgt, ifge, // =0?, <0?,…– ifnull, ifnonnull, // null or not null

» binary comparisons– if_icmpeq, if_icmpne, if_icmplt, if_icmpgt, if_icmple, if_icmpge, – if_acmpeq, if_acmpne. // compare reference

For comparison of long, double and float» use dcmpg, dcmpl, fcmpg, fcmpl, lcmp and» unary comparisons on int.

20

Control flow Instructions :2.Compound Conditional

Branches lookupswitch <0-3bytepads for word alignment> d

efaultOffset #pairs {case offset}#pairs:» all (blue) fields are int32 values» case must be in ascending order» e.g.: lookupswitch lab0 3 2 lab1 10 lab2 13 lab3

tableswitch <0-3pads> defOffset lowCase highCase offset high-low+1. » goto offset[top-low+1] iff low ≤ top ≤ high» else goto defOffset.» e.g.: tableswitch lab0 11 14 lab11 lab12 lab13 lab14.

21

Control flow Instructions: Unconditional branches

goto s16; goto_w s32» pc = pc + s16 (or s32).

// for implementing finally clauses

// intraMethod subroutine call jsr s16, jsr_w s32,

» push return addr and jumpToSubroutine pc+s16 for a finally block

» … … returnAddr

ret u8; widen ret u16,» return to addr given by localVar u8 or u16 .

22

Method Invocation and Return Instructions

invokevirtual : for normal methods invokeInterface:

» for interface methods // search of implementation is needed

invokespecial: » for private method, superclass method and instance i

nitialization (constructors) methods invokestatic : for static methods format: opcode u16 // methodRefIndex return :

» ireturn, lreturn, freturn, dreturn, areturn» return : for void, instance, class (or interface) initializat

ion methods

23

Exceptions related Instructions

Throw Exception:» athrow» can also be thrown by other instructions if abnorm

al condition detected.» … ExcObj …

Implementing finally jsr s16, jsr_w s32,

» jumpToSubroutine pc+s16 for a finally block

ret u8; widen ret u16,» return to addr given by localVar u8 or u16 .

24

Synchronization via monitor construct:

» monitorenter, monitorexit monitorenter

» pop objectref, acquire the lock associated with the object.

» … ObjRef … monitorexit

» pop objectref, release the lock associate with objectref

» … ObjRef …

25

Classes needing special support of JVM

Reflection:» java.lang.reflect.* and java.lang.Class.

Loading and creation of a class or interface.» ClassLoader.

Linking and initialization of a class or interface. Security:

» java.security.** and other classes such as SecurityManager.

Multithreading, such as the class Thread. Weak references: java.lang.ref.* …