Java Virtual Machine Instruction Set Architecture

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Java Virtual MachineInstruction Set Architecture

• Justin Dzeja

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What is a Java Virtual Machine?

• JVM is an abstract computing machine
• Like an actual computing machine, it has an
  instruction set and manipulates various memory
  areas at run time
• A JVM enables a set of computer software programs
  and data structures to use a virtual machine
  model for the execution of other computer
  programs and scripts
• Not just Java, JVM now supports many languages
• Ada, C, LISP, Python

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Why a Virtual Machine?

• The Java platform was initially developed to
  address the problems of building software for
  networked consumer devices
• It was designed to support multiple host
  architectures and to allow secure delivery of
  software components
• To meet these requirements, compiled code had to
  survive transport across networks, operate on any
  client, and assure the client that it was safe to
  run
• "Write Once, Run Anywhere"

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Java Timeline

• 1991 James Gosling begins work on Java project
• Originally, the language is named Oak
• 1995 Sun releases first public implementation
  as Java 1.0
• 1998 - JDK 1.1 release downloads top 2 million
• 1999 - Java 2 is released by Sun
• 2005 - Approximately 4.5 million developers use
  Java technology
• 2007 Sun makes all of Javas core code
  available under open-source distribution terms

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Java Principles

• Sun set five primary goals in the creation of the
  Java language,
• It should be "simple, object oriented, and
  familiar".
• It should be "robust and secure".
• It should be "architecture neutral and portable".
• It should execute with "high performance".
• It should be "interpreted, threaded, and
  dynamic".

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JVM Instruction Set Architecture

• Instructions
• A Java virtual machine instruction consists of a
  one-byte opcode specifying the operation to be
  performed, followed by zero or more operands
  supplying arguments or data that are used by the
  operation
• Operands are not required, there are many
  instructions that consist of only the opcode
• One-byte instructions allow for up to 256
  instructions but only about 200 are used in class
  files, leaving room for more instructions to be
  added
• Each instruction has a mnemonic name which is
  mapped to the binary one-byte opcode

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JVM Instruction Set Architecture

• Instruction Format
• The mnemonic operation names often include the
  data type followed by the operation name
• iadd, ladd, fadd, dadd
• int, long, float, double
• JVM supports conversion operations that convert
  from one data type to another, these include both
  data types in the operation name
• i2l, i2f, i2d, l2f, l2d, f2d

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Operation Types

• The JVM ISA is a CISC architecture, thus having
  many instructions
• They can be classified into broad groups
• Load and store
• Arithmetic and logic
• Type conversion
• Object creation and manipulation
• Operand stack management
• Control transfer
• Method invocation and return

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Operation Types

• Load and store
• Used to move values from local variable array or
  heap to the operand stack
• iload, istore
• Arithmetic and logic
• JVM supports addition, subtraction, division,
  multiplication, remainder, negation, increment
• irem, idiv, iinc
• Type conversion
• Allows converting from one primitive data type to
  another
• i2l, i2f, i2d, l2f, l2d, f2d
• Object creation and manipulation
• Instantiating objects and manipulating fields
• new, putfield
• Operand stack management
• swap, dup2
• Control transfer
• ifeq, goto
• Method invocation and return
• invokespecial, areturn

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JVM Data Types

• The Java virtual machine operates on two kinds of
  types primitive types and reference types
• Integral Types
• Byte - 8-bit signed two's-complement integers
• Short - 16-bit signed two's-complement integers
• Int - 32-bit signed two's-complement integers
• Long - 64-bit signed two's-complement integers
• Char - 16-bit unsigned integers representing
  Unicode characters

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JVM Data Types
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JVM Data Types

• Floating Point Types
• Float - values are elements of the float value
  set (typically 32-bit single-precision but may
  vary with implementation)
• Double - values are elements of the double value
  set(64-bit double-precision)
• Boolean - values true and false
• JVM has very little support for the Boolean data
  type
• Boolean variables in a Java program are compiled
  to use values of the JVM int data type
• returnAddress - are pointers to the opcodes of
  Java virtual machine instructions

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JVM Data Types

• Three kinds of reference types
• Class types
• Array types
• Interface types
• These reference dynamically created classes,
  arrays, or interface implementations
• Can be set to null when not referencing anything
  and then cast to any type

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JVM Data Types

• The basic unit of size for data values in the
  Java virtual machine is the word
• a fixed size chosen by the designer of each Java
  virtual machine implementation
• The word size must be large enough to hold a
  value of type byte, short, int, char, float,
  returnAddress, or reference
• at least 32 bits

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JVM Runtime Data Areas

• Since JVM is a virtual machine it doesnt have
  any physical registers , instead it defines
  various runtime data areas that are used during
  execution of a program
• One of the areas defined is the program counter
  register
• Each thread of control has its own PC register
• The register is wide enough to contain a
  returnAddress or a native pointer on the specific
  platform

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JVM Runtime Data Areas

• JVM Stack
• Each thread gets its own JVM stack when it is
  created
• Stacks store frames which hold data and play a
  role in method invocation and return
• The actual memory for a JVM stack does not need
  to be contiguous
• The stack can be either of a fixed size or
  dynamically contracted and expanded as needed

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JVM Runtime Data Areas

• JVM Frames
• A frame is used to store data and partial
  results, as well as to perform dynamic linking ,
  return values for methods, and dispatch
  exceptions
• A new frame is created each time a method is
  invoked and destroyed when the method is
  completed
• Only one frame, for the executing method, is
  active at any point
• Each frame contains a local variable array
• Local variables can store primitive or reference
  data types
• Variables are addressed by index, starting from
  zero
• Data types long and double occupy two consecutive
  local variables
• Frames also contains an operand stack
• Last-in-first-out (LIFO)
• JVM loads values from local variables or fields
  onto the stack
• Then JVM instructions can take operands from the
  stack, operate on them, and the push the result
  back onto the stack
• The operand stack size is fixed at compile time
  based on method associated with the frame

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JVM Operand Stack

• Code
• iload_0 // push the int in local variable 0
• iload_1 // push the int in local variable 1
• iadd // pop two ints, add them, push result
• istore_2 // pop Int, store into local variable 2
  

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JVM Runtime Data Areas

• JVM Heap
• The heap is a data area shared by all JVM threads
• Memory from the heap is allocated for instances
  of classes and arrays
• Can be either of fixed size or dynamic
• Does not to be in contiguous memory space
• Maintained by an automatic storage management
  system or garbage collector

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JVM Runtime Data Areas

• Method Area
• The method area is also shared among all JVM
  threads
• It stores per-class structures
• such as the runtime constant pool, field and
  method data, code for methods and constructors,
  including the special methods used in class and
  instance initialization
• The method area is logically part of the heap,
  but depending on the implementation it may or may
  not be garbage collected or compacted

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JVM Runtime Data Areas

• Runtime Constant Pool
• The runtime constant pool is a per-class runtime
  representation of the constant pool table in a
  class file
• It contains numeric constants as well as method
  and field references that are resolved at runtime
• This is similar to a symbol table for a
  conventional programming language, although it
  stores a wider range of data

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JVM Addressing Modes

• JVM supports three addressing modes
• Immediate addressing mode
• Constant is part of instruction
• Indexed addressing mode
• Accessing variables from local variable array
• Stack addressing mode
• Retrieving values from operand stack using pop

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JVM Method Calls

• Sample Code
• Compiles to

int add12and13() return addTwo(12, 13)
Method int add12and13() 0 aload_0 // Push
local variable 0 (this) 1 bipush 12 // Push
int constant 12 3 bipush 13 // Push int
constant 13 5 invokevirtual 4 // Method
Example.addtwo(II)I 8 ireturn // Return int on
top of operand stack it //is the int result
of addTwo()
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Design Principles

• Simplicity favors regularity
• Examples of this principle can be found
  throughout the JVM specification
• Instructions are all a standard opcode that is
  one byte in size
• The naming conventions for opcode mnemonics are
  standard across different types of operations
• Smaller is faster
• Data areas such as the heap are dynamic in size
  resulting in memory space saved when not in use
• JVM has a large instruction set, which results in
  a smaller code size when converted to byte code

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Design Principles

• Make the common case fast
• JVM includes instructions to increment variables
  or to arithmetically shift values for fast
  execution of common operations
• Good design demands good compromises
• The JVM finds a good balance between high
  performance and being secure and robust

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JVM Advantages/Disadvantages

• A self-contained operating environment as if its
  a separate computer, this gives JVM two main
  advantages
• System Independence a Java application will run
  the same on any JVM, regardless of the underlying
  system
• Security Since a Java program operates in a
  self-contained environment there is less risk to
  files and other applications
• The disadvantage is that running the virtual
  machine is extra overhead on the system, which
  can impair performance in some situations

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Sources

• http//java.sun.com/docs/books/jvms/second_edition
  /html/VMSpecTOC.doc.html
• http//www.cis.cau.edu/121/lecture05-2.htm
• http//www.particle.kth.se/lindsey/JavaCourse/Boo
  k/Part1/Supplements/Chapter01/JVM.html