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\layout Subject


\emph on 
Diplomarbeit
\layout Title

JustIce
\newline 

\size small 
A Free Class File Verifier for Java
\latex latex 

\backslash 
texttrademark\SpecialChar ~
  
\layout Author

Enver Haase
\newline 

\size tiny 
<ehaase@inf.fu-berlin.de>
\layout Date

September 2001
\layout Publishers

Freie Universität Berlin 
\newline 
Institut für Informatik
\newline 
Takustraße 9
\newline 
D-14195 Berlin
\layout Lowertitleback


\series bold 
\size scriptsize 
Revision
\series default 
 
\series bold 
\shape smallcaps 
$Id: JustIce.lyx 371539 2006-01-23 14:08:00Z tcurdt $
\layout Minisec

Erklärung
\begin_float footnote 
\layout Standard

I declare that I wrote this 
\emph on 
Diplomarbeit
\emph default 
 completely on my own and without the help of persons not listed.
 All sources of information are listed in the Bibliography section.
\end_float 
\layout Standard

Hiermit versichere ich, die vorliegende Diplomarbeit selbständig und ohne
 fremde Hilfe verfaßt zu haben.
 Es wurden nur die in der Bibliographie angegebenen Quellen benutzt.
\layout Minisec

Danksagung
\begin_float footnote 
\layout Standard

The creation of this 
\emph on 
Diplomarbeit
\emph default 
 paper was supported and supervised by Prof.
 Dr.
 Elfriede Fehr and Dipl.-Inform.
 Markus Dahm.
 Keith Seymour suggested a lot of language-related improvements.
 Thank you.
\end_float 
\layout Standard

Während der Anfertigung dieser Diplomarbeit wurde ich von Prof.
 Dr.
 Elfriede Fehr und Dipl.-Inform.
 Markus Dahm betreut, wofür ich mich an dieser Stelle herzlich bedanke.
\layout Standard

Desweiteren bedanke ich mich bei Keith Seymour, der mir eine Reihe sprachspezifi
scher Verbesserungsvorschläge sandte.
\layout Minisec

Autor
\begin_float footnote 
\layout Standard

Author
\end_float 
\layout Standard

Enver Haase
\newline 
Gubener Straße 18
\newline 
D-10243 Berlin
\newline 

\layout Standard


\begin_inset LatexCommand \tableofcontents{}

\end_inset 


\layout Addchap

Abstract
\layout Standard

When Sun Microsystems developed their 
\emph on 
Java Platform
\emph default 
 in the early 1990s, it was originally designed for use in networked and
 embedded consumer-electronics applications.
 But when they introduced it around 1995, it quickly became used in World
 Wide Web browser software.
 This was a way to bring interactive content to demanding World Wide Web
 users.
 Sun took great care for the robustness of the platform: they planned to
 connect embedded devices and let them share data and code over a network.
 Defective devices transmitting bad data or unreliable network connections
 should not cause other devices to crash.
 This property made Java a good choice for the code-executing engine in
 World Wide Web browsers: defective server software or transmission errors
 would not cause the 
\emph on 
Java Platform
\emph default 
 to crash; this is also true for purposely malicious code hidden on the
 Web.
 The code-executing part of the 
\emph on 
Java Platform
\emph default 
 is called 
\emph on 
The Java Virtual Machine
\emph default 
 (the 
\emph on 
JVM
\emph default 
, for short).
 This execution engine has to assure that the code to be executed is well-behave
d; it has to 
\emph on 
verify
\emph default 
 the code.
 Therefore, the 
\emph on 
verifier
\emph default 
 is an integral part of every JVM, but JustIce implements a verifier that
 is not integrated in a JVM.
 It was implemented using a software library called the 
\emph on 
Byte Code Engineering Library
\emph default 
 (the 
\emph on 
BCEL
\emph default 
, for short) by Markus Dahm 
\begin_inset LatexCommand \cite{BCEL98,BCEL-WWW}

\end_inset 

.
\layout Standard

The BCEL is intended to give users a convenient mechanism to analyze, create
 and manipulate (binary) Java class files.
 It offers an object-oriented view of otherwise raw data, including program
 code.
 This library is, therefore, well-respected especially in the compiler-writer
 community whenever the JVM is chosen as the target machine of the compiler.
 Compiler back-ends use the BCEL to produce code for the JVM; and as new
 compilers may be faulty, they may produce bad code.
 Testing these compilers often is a difficult task.
 The generated code should not only be semantically correct, but it also
 has to pass the verifiers of all existing JVM implementations.
 Normally, a lot of human interaction is required to run test cases.
 If the code is rejected by a verifier, one often does not know why.
 Most verifiers emit error messages which do not identify the offending
 instruction.
\layout Standard

JustIce presents an Application Programming Interface (API) that may be
 used to automate the procedure sketched above.
 The constraints imposed on class files are designed to be strict, therefore
 eleminating the need to run several verifiers on the generated code.
 If code passes the JustIce verifier, it should pass all other verifiers.
 JustIce was also designed to output human-understandable messages if the
 verification of some code fails.
\layout Standard

The application range of JustIce is not limited to compiler back-ends, in
 the same sense as the BCEL is not only useful in this area.
 Transformations of existing code and even generation of hand-crafted code
 fall into its scope, too.
 As a side effect, JustIce exports some data structures such as a control
 flow graph; so its API may also be used for applications targeting other
 problem areas such as static analyses of program code.
\layout Chapter

Introduction
\layout Section

Low Level Security as a Part of a Many-Tiered Strategy
\layout Standard

The Java programming language is well-known for its inherent security facilities
 such as the lack of pointer arithmetic or the need for memory allocation
 and deallocation.
 Lesser known is that this is only the top of an iceberg; the 
\emph on 
Java Platform
\emph default 
 implements a many-tiered security strategy 
\begin_inset LatexCommand \cite{Yellin-WWW}

\end_inset 

.
 It was designed to run even untrusted code -- code that possibly was not
 produced by a compiler for the Java programming language, code that may
 be corrupt or code that may have malicious intent (such as stealing credit
 card number information from a hard disk drive).
 Three considerations were made:
\layout Itemize

Untrusted code could damage hardware, software, or information on the host
 machine.
\layout Itemize

It could pass unauthorized information to anyone.
\layout Itemize

It could cause the host machine to become unusable through resource depletion.
\layout Standard

While some security features such as type-safety or the already-mentioned
 lack of pointer arithmetic of the Java programming language are a convenient
 help for programmers, they can only help to reduce programming errors.
 Of course these features do not help targeting the above problems.
 At a lower level, however, the 
\emph on 
Java Plat\SpecialChar \-
form
\emph default 
 implements a so-called sandbox: an area where code can be executed but
 that has well-defined boundaries shielding the rest of the system.
 This is achieved by means of a 
\emph on 
Java Virtual Machine
\emph default 
 (JVM) emulation; the host platform does not directly run untrusted code,
 but a 
\emph on 
run-time system
\emph default 
 which in turn runs the code, restricting its access to system resources.
\layout Standard

A run-time system cannot safely assume that untrusted code is well-behaved.
 Code could cause stack overflows, stack underruns, or otherwise erroneous
 behaviour that may bring the run-time system into an undefined state --
 possibly allowing access to protected memory areas.
 One could protect the run-time system by letting it predict the effects
 of every single instruction just in time while actually executing it --
 but that would be too time-consuming to be applicable in practice.
\layout Standard

Therefore, good behaviour of program code has to be enforced 
\emph on 
before
\emph default 
 it is actually executed -- at least as far as this is possible.
 This is the lowest level of Java security; there has to be an integral
 component in every JVM implementation doing so (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 420).
 This part of the JVM is called the 
\emph on 
class file verifier
\emph default 
, yet better known as the 
\emph on 
bytecode verifier.
 
\emph default 
Technically speaking, bytecode verification is only a part of class file
 verification so 
\emph on 
class file verifier
\emph default 
 is a more embracing term.
 JustIce implements a whole class file verifier.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 396
file chap1.eps
width 3 100
flags 9

\end_inset 


\layout Caption

Concept of Class File Verification
\end_float 
\layout Section

Why Another Verifier?
\layout Standard

As said before, every JVM implementation must contain a class file verifier,
 so it is reasonable to ask for the motivation behind creating just another
 class file verifier -- especially one that is 
\emph on 
not
\emph default 
 part of a JVM implementation.
\layout Subsection

Bytecode Engineers Need JustIce
\layout Standard

Shortly after the 
\emph on 
Java Platform
\emph default 
 was introduced, it was adopted with pleasure because of its inherent independen
ce from operating systems and concrete hardware.
 Industry and educational institutions with heterogenous networked computers
 could now run the same software program on different host machines.
 Soon, many efforts were put into research and development of compilers
 for programming languages other than the Java programming language that
 use the JVM bytecode as target.
\layout Standard

Nowadays, many other programming languages do have the JVM as its target
 platform; e.g.
 Fortran 
\begin_inset LatexCommand \cite{f2j}

\end_inset 

, Ada 
\begin_inset LatexCommand \cite{AppMag-WWW}

\end_inset 

, Scheme 
\begin_inset LatexCommand \cite{KAWA-WWW}

\end_inset 

 or modified Java language versions 
\begin_inset LatexCommand \cite{GJ-WWW,PMG-WWW}

\end_inset 

.
 A vast collection of programming languages targeting the JVM can be found
 on the World Wide Web 
\begin_inset LatexCommand \cite{PL4JVM}

\end_inset 

.
\layout Standard

All these compilers emit code for the JVM -- and so all these compilers
 have to pass the JVM's verifier.
 Implementors of such compilers have to consider the security related constraint
s the JVM poses on the generated code.
 It is difficult to test if the emitted code works on all JVM implementations,
 passing all JVM verifier implementations.
 This is especially problematic if not all of the project's class files
 are loaded into the JVM during a test run, because then they will not be
 verified.
\layout Standard

Having an opportunity to verify the transitive hull of referenced class
 files (starting with some main class file) would be of help; JustIce offers
 it.
\layout Standard

The Bytecode Engineering Library by Markus Dahm is often used as a compiler
 back-end to emit code, but it is also used to hand-craft code or to implement
 bytecode transformations.
 Because JustIce works closely together with the BCEL, users of the BCEL
 do not even have to leave their development environment to run the JustIce
 verifier.
\layout Standard

To our knowledge, JustIce is the only implementation of a Java class file
 verifier that was written in the Java programming language 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

 itself
\begin_float footnote 
\layout Standard

In a personal communication, Robert Stärk told the author that there was
 a Java implementation of the verifier discussed in 
\begin_inset LatexCommand \cite{JBook}

\end_inset 

, written by Joachim Schmid using the BCEL.
 However, it is not released for public use yet.
\end_float 
.
 Because of its 
\emph on 
Verification API
\emph default 
, it can be included in other software projects written in Java with more
 ease than any other verifier implementation in a different programming
 language could provide.
\layout Subsection

JustIce is Verbose
\layout Standard

Usually, when classes pass the verifier, it is mute.
 JustIce, in contrast, distinguishes between verification results and messages.
 Messages are often warnings, but the reason for emitting such a warning
 instead of a negative verification result is because the class file does
 not pose a threat to the integrity of the JVM and thus does not have to
 be rejected.
\layout Standard

When a verification error occurs and the class file is rejected, even the
 built-in verifiers usually produce some output saying so.
 As an example, consider the following verifier run:
\newline 

\newline 

\family typewriter 
ehaase@haneman:/home/ehaase > java Cc 
\newline 
Exception in thread "main" java.lang.VerifyError: 
\newline 
(class: Cc, method: ttt signature: ()V)
\newline 
Recursive call to jsr entry
\family default 

\newline 

\latex latex 

\newline 

\layout Standard

One might ask 
\emph on 
which
\emph default 
 
\begin_inset Quotes eld
\end_inset 

jsr entry
\begin_inset Quotes erd
\end_inset 

 (a branch target of a
\latex latex 
 
\backslash 
texttt{jsr}
\latex default 
 or a 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction) is called recursively and which instructions may be responsible
 for this.
 Compare this to JustIce's output:
\newline 

\newline 
[...]
\layout Standard


\family typewriter 
Pass 3b, method number 0 ['public static void ttt()']:
\layout Standard


\family typewriter 
VERIFIED_REJECTED
\layout Standard


\family typewriter 
Constraint violated in method 'public static void ttt()':
\layout Standard


\family typewriter 
Subroutine with local variable '1', JSRs '[ 36: jsr[168](3) -> astore_1,
 8: jsr[168](3) -> astore_1, 30: jsr[168](3) -> astore_1, 23: jsr[168](3)
 -> astore_1]', RET ' 62: ret[169](2) 1' is called by a subroutine which
 uses the same local variable index as itself; maybe even a recursive call?
 JustIce's clean definition of a subroutine forbids both.
\newline 

\family default 
[...]
\layout Standard


\family typewriter 
Warnings:
\layout Standard


\family typewriter 
Pass 2: Attribute 'LineNumber(0, 4), LineNumber(0, 5), LineNumber(15, 8),
 LineNumber(39, 11), LineNumber(47, 12), LineNumber(57, 13), LineNumber(64,
 15)' as an attribute of Code attribute '<CODE>' (method 'public static
 void ttt()') will effectively be ignored and is only useful for debuggers
 and such.
\layout Standard


\family typewriter 
Pass 2: Attribute 'LineNumber(0, 1), LineNumber(4, 1)' as an attribute of
 Code attribute '<CODE>' (method 'public void <init>()') will effectively
 be ignored and is only useful for debuggers and such.
\layout Standard


\family typewriter 
Pass 3a: LineNumberTable attribute 'LineNumber(0, 4), LineNumber(0, 5),
 LineNumber(15, 8), LineNumber(39, 11), LineNumber(47, 12), LineNumber(57,
 13), LineNumber(64, 15)' refers to the same code offset ('0') more than
 once which is violating the semantics [but is sometimes produced by IBM's
 'jikes' compiler].
\newline 

\layout Standard

This output obviously has an answer to the above question; it shows the
 only 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instructions possibly responsible for a recursive call (which is not allowed
 by the specification of the JVM).
 For the special --but clean-- definition of subroutines JustIce uses, please
 see section 
\begin_inset LatexCommand \ref{Subroutines_Def}

\end_inset 

.
\layout Standard

Note also the warning messages.
 Class files that were not generated by Sun's 
\emph on 
javac
\emph default 
 compiler have a tendency to look a little different in some corner cases.
 IBM's 
\emph on 
jikes
\emph default 
 compiler, for instance, produces LineNumberTable attributes (see 
\begin_inset LatexCommand \ref{LineNumberTableAttribute}

\end_inset 

) which look different from those created by 
\emph on 
javac
\emph default 
.
 Detecting such differences is desirable because future JVMs will have stricter
 verification checks
\begin_float footnote 
\layout Standard

The Solaris port of Sun's JVM, version 1.3.0_01, already has (some of) the
 stricter checks built in.
 You may enable them using the command-line option '-Xfuture'.
 Nothing about this issue is mentioned in the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
\end_float 
 (which most old 
\emph on 
javac
\emph default 
-compiled class files will probably still pass).
 JustIce guides bytecode engineers to create class files that are indistinguisha
ble from those created by 
\emph on 
javac
\emph default 
 to retain compatibility with Sun's future JVM implementations.
 Figure 
\begin_inset LatexCommand \ref{FigVenn}

\end_inset 

 graphically shows the relationship between class files and the verifier
\begin_float footnote 
\layout Standard

This is a simplicistic figure; unfortunately, there are class files produced
 by the 
\emph on 
javac
\emph default 
 compiler that do not pass the verifier.
 Please see section 
\begin_inset LatexCommand \ref{javacRejected}

\end_inset 

 for more details.
\end_float 
.
\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 378
file VennDiag.eps
width 3 100
height 3 45
flags 9

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{FigVenn}

\end_inset 

Venn diagram showing the operating domain of the Java verifier.
\end_float 
\layout Subsection

JustIce is Free
\layout Standard

Currently, there is no other free and complete open source verifier available
 known to the author.
 You may have a look at the JVM's source code by Sun Microsystems but you
 are not allowed to use the knowledge from that inspection for your own
 projects or even use their code.
 JustIce is a clean-room implementation: the author wrote JustIce by only
 reading the Java
\latex latex 

\backslash 
texttrademark
\latex default 
\SpecialChar ~
 Virtual Machine Specification, Second Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

 and comparing the behaviour of JustIce with the behaviour of commercial
 implementations of Sun Microsystems and IBM Corporation.
\layout Standard

The open source JVM implementation 
\emph on 
Kaffe
\emph default 
 
\begin_inset LatexCommand \cite{Kaffe-WWW}

\end_inset 

, for example
\emph on 
,
\emph default 
 does not have a 
\emph on 
complete
\emph default 
 verifier built in (although mandated by the JVM specification).
\layout Standard


\emph on 
Kissme
\emph default 
 
\begin_inset LatexCommand \cite{kissme-WWW}

\end_inset 

, another open source JVM implementation, currently does not include any
 verifier at all.
 
\layout Standard

The JVM implementations
\emph on 
 SableVM
\emph default 
 
\begin_inset LatexCommand \cite{SableVM-WWW}

\end_inset 

 and Intel Corporation's 
\emph on 
Open Runtime Platform
\emph default 
 
\begin_inset LatexCommand \cite{ORP-WWW}

\end_inset 

 are platforms to experiment with performance-enhancements.
 They are not intended to work as general-purpose JVMs so they do not need
 to implement verifiers.
\layout Standard

Other open source projects that could make use of a free verifier include
 the Java compiler 
\emph on 
gcj
\emph default 
 which is part of the GNU compiler collection 
\begin_inset LatexCommand \cite{GCC-WWW}

\end_inset 

.
\layout Standard

JustIce is covered by the well-known and respected software license 
\emph on 
GNU General Public License
\emph default 
 (GPL); see section 
\begin_inset LatexCommand \ref{GPL}

\end_inset 

.
 The author hopes other free software will benefit from it; from the JustIce
 software 
\begin_inset LatexCommand \cite{JustIce}

\end_inset 

 as well as from this paper describing some of the inner workings of JustIce.
\layout Chapter

The Java Virtual Machine
\layout Standard

The Java Virtual Machine (JVM) is an abstract machine specified in 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 It has no knowledge about the Java programming language; but only of a
 certain binary file format: the class file format.
 A class file contains machine instructions for the JVM (called 
\emph on 
bytecodes
\emph default 
), a symbol table (called 
\emph on 
constant pool
\emph default 
) and some other ancillary information.
\layout Standard

On method invocation, a local stack frame is set up called the 
\emph on 
execution frame
\emph default 
.
 It consists of an 
\emph on 
operand stack
\emph default 
 and 
\emph on 
local variables
\emph default 
 (which may be compared to registers of traditional machines).
\layout Standard

The instructions in the code arrays of class files are interpreted by the
 JVM.
 There are 212 legal instructions; they have read-access to the class file's
 constant pool and they can modify the operand stack and the local variables
 in their execution frame.
 An invoked method reads its arguments from the local variables.
 Certain instructions pass a return value to the invoking method.
\layout Section


\begin_inset LatexCommand \label{Classfile Structure}

\end_inset 

The ClassFile Structure
\layout Standard

Traditionally, the JVM loads its programs from files stored on file systems
 of host machines; these files have names that end with 
\emph on 

\begin_inset Quotes eld
\end_inset 

.class
\begin_inset Quotes erd
\end_inset 


\emph default 
.
 It is possible to store the files in various other ways; a so-called 
\emph on 
class loader
\emph default 
 is then used to transform the files internally to the desired, basic class
 file format.
 Therefore, it suffices to explain the structure of traditional class files.
 Every class file consists of a single 
\family typewriter 
ClassFile
\family default 
 structure as defined below.
 It defines a single class as known from the Java Programming Language 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

.
 The terms 
\emph on 
class
\emph default 
 and 
\emph on 
class file
\emph default 
 may therefore be used interchangeably.
\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 526
file classfile.eps
width 3 100
flags 9

\end_inset 


\layout Standard

A class file consists of constants, fields, methods, attributes and some
 ancillary information.
 This figure was taken from 
\begin_inset LatexCommand \cite{BCEL98}

\end_inset 

, used with permission of the author.
\layout Caption

A Class File
\end_float 
\layout Standard

As we will see, the 
\family typewriter 
ClassFile
\family default 
 structure and its sub-structures are defined for upwards compatibility,
 i.e., new structure definitions can be added to the specification easily
 at a later time.
\newline 

\newline 

\family typewriter 
ClassFile { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 magic;
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 minor_version; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 major_version; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 constant_pool_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
cp_info constant_pool[constant_pool_count-1]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 access_flags; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 this_class; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 super_class; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 interfaces_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 interfaces[interfaces_count]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 fields_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
field_info fields[fields_count]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 methods_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
method_info methods[methods_count]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attributes_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
attribute_info attributes[attributes_count]; 
\newline 
}
\newline 

\newline 

\family default 
You may read an '
\family typewriter 
u
\family default 
' as 'byte times'; e.g., '
\family typewriter 
u2
\family default 
' means 'two bytes in size'.
 We will not delve into too much detail here; the exact specification of
 the entries are published by Sun 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 But one should note that besides some other information, a class file basically
 defines 
\emph on 
attributes
\emph default 
,
\emph on 
 constants
\emph default 
, 
\emph on 
fields
\emph default 
 and 
\emph on 
methods
\emph default 
.
 Also, there are strong structural constraints imposed on class files.
 It is a verifier's task to validate them.
\layout Subsection

Attributes
\layout Standard

The general format of an attribute is defined below.
\newline 

\newline 

\family typewriter 
attribute_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attribute_name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 attribute_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 info[attribute_length]; 
\newline 
}
\family default 

\newline 

\newline 
An attribute is basically a typed data container; its type is determined
 by its name.
 Every JVM is required to be silent about attributes of types it does not
 know.
 On the other hand, newly defined attributes are required not to impose
 a semantical change on the class file.
 These attributes should be uniquely named; in fact, the pair (<attribute
 name>, <attribute length>) is required to be unique.
 This is guaranteed because attributes not defined by Sun Microsystems have
 to be named according to the package naming scheme of the Java Programming
 Language 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

.
 Certain basic attributes are predefined.
 They are used in the 
\family typewriter 
ClassFile
\family default 
 (see section 
\begin_inset LatexCommand \ref{Classfile Structure}

\end_inset 

), 
\family typewriter 
field_info
\family default 
 (see section 
\begin_inset LatexCommand \ref{Fields}

\end_inset 

) and 
\family typewriter 
method_info
\family default 
 (see section 
\begin_inset LatexCommand \ref{Methods}

\end_inset 

).
 Also, attributes may be nested: the 
\family typewriter 
Code
\family default 
 attribute references other attributes.
\layout Standard

Some examples for predefined attributes are listed below.
\layout Subsubsection


\begin_inset LatexCommand \label{ConstantValueAttribute}

\end_inset 

The ConstantValue attribute
\layout Standard

The ConstantValue attribute has the following format:
\newline 

\newline 

\family typewriter 
ConstantValue_attribute { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attribute_name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 attribute_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 constantvalue_index; 
\newline 
}
\family default 

\newline 

\newline 
The 
\family typewriter 
ConstantValue
\family default 
 attribute represents the value of a constant field.
 It has a fixed length: it contains only a two-byte reference into the constant
 pool.
 Only 
\family typewriter 
field_info
\family default 
 structures (see section 
\begin_inset LatexCommand \ref{Fields}

\end_inset 

) contain this type of attribute.
\layout Subsubsection


\begin_inset LatexCommand \label{CodeAttribute}

\end_inset 

The Code Attribute
\layout Standard

The 
\family typewriter 
Code
\family default 
 attribute is used in the 
\family typewriter 
method_info
\family default 
 (see section 
\begin_inset LatexCommand \ref{Methods}

\end_inset 

) structure.
 It represents the program code of a method and it is defined as follows:
\newline 

\newline 

\family typewriter 
Code_attribute { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attribute_name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 attribute_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 max_stack; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 max_locals; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 code_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 code[code_length]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 exception_table_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
{ 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 start_pc; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 end_pc; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 handler_pc; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 catch_type; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
} exception_table[exception_table_length]; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attributes_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
attribute_info attributes[attributes_count]; 
\newline 
}
\family default 

\newline 

\newline 
This is the most complex of all predefined attributes.
 Every method that has code (i.e., every non-native, non-abstract method)
 must have such an attribute.
 Note that the maximum stack depth and the number of local variables for
 a method invocation are defined here.
 This is important for the JVM when it creates an 
\emph on 
execution frame
\emph default 
 (see section 
\begin_inset LatexCommand \ref{LV_and_OpStack}

\end_inset 

) at the time the method is invoked.
\layout Standard

Also, the exception handlers are defined here.
 Exception handlers prevent an executing method from an abrupt completion
 if an exceptional situation occurs.
 Code areas are said to be protected against a class of exceptional situations
 by an exception handler
\begin_float footnote 
\layout Standard

The JVM closely reflects the 
\emph on 
exception
\emph default 
 mechanism of the Java programming language 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

.
 In the Java programming language, exceptions can be 
\emph on 
thrown
\emph default 
, and they can be 
\emph on 
caught
\emph default 
 explicitly.
 If an internal JVM error occurs, the JVM also --implicitly-- throws an
 exception.
\end_float 
.
 Algorithm 
\begin_inset LatexCommand \ref{ExcHdAlgo}

\end_inset 

 shows an example for the use of exception handlers.
 The exact meaning of the instruction opcodes is not important here, the
 most common instructions are explained later in this paper.
\layout Standard

\begin_float alg 
\layout Standard

[Let 
\family typewriter 
start_pc
\family default 
 and 
\family typewriter 
end_pc
\family default 
 protect the area A to B, inclusive.
 Let the 
\family typewriter 
catch_type
\family default 
 be 
\begin_inset Quotes eld
\end_inset 


\family typewriter 
java.lang.NullPointerException
\family default 

\begin_inset Quotes erd
\end_inset 

.
 Let the 
\family typewriter 
handler_pc
\family default 
 point to C.]
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
aconst_null\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; push a NULL onto the operand stack.
\layout Standard


\family typewriter 
A:\SpecialChar ~
nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; do nothing
\layout Standard


\family typewriter 
B:\SpecialChar ~
getfield Foo::bar\SpecialChar ~
\SpecialChar ~
; dereference NULL, cause NullPointerExc.
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
return\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
;\SpecialChar ~
never executed
\layout Standard


\family typewriter 
C:\SpecialChar ~
nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
;\SpecialChar ~
this is executed: we could handle
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
;\SpecialChar ~
the NullPointerException
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
return\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
;\SpecialChar ~
leave method (complete normally)
\layout Caption


\begin_inset LatexCommand \label{ExcHdAlgo}

\end_inset 

Use of Exception Handlers
\end_float 
\layout Standard

The most important item, however, is the 
\family typewriter 
code
\family default 
 item.
 It defines the bytecode of this method; i.e., the JVM machine instructions.
\layout Subsubsection


\begin_inset LatexCommand \label{LineNumberTableAttribute}

\end_inset 

The LineNumberTable Attribute
\layout Standard

The 
\family typewriter 
LineNumberTable
\family default 
 attribute is defined as follows:
\newline 

\newline 

\family typewriter 
LineNumberTable_attribute { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attribute_name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u4 attribute_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 line_number_table_length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
{ 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 start_pc; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 line_number; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
} line_number_table[line_number_table_length]; 
\newline 
}
\newline 

\family default 

\newline 
This attribute describes the relation between source code line numbers and
 JVM instruction offsets in the 
\family typewriter 
code
\family default 
 array of the 
\family typewriter 
Code_attribute
\family default 
; it can be used by debuggers to show the source code of currently executing
 JVM machine instructions.
 This attribute is usually a sub-attribute of a 
\family typewriter 
Code_attribute
\family default 
.
 Multiple 
\family typewriter 
LineNumberTable
\family default 
 attributes may together represent a given line of a source code file.
 
\layout Subsection

Constants
\layout Standard

All the constants together form the 
\emph on 
constant pool
\emph default 
.
 The general 
\family typewriter 
cp_info
\family default 
 structure is straightforward.
\newline 

\newline 

\family typewriter 
cp_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 tag; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 info[]; 
\newline 
}
\family default 

\newline 

\newline 
The 'tag' defines what 'info' follows it.
 Constants define either constant values or constant symbolic references,
 such as references to other classes.
 Currently, eleven constant types are defined: 
\family typewriter 
Class
\family default 
, 
\family typewriter 
Field\SpecialChar \-
ref
\family default 
, 
\family typewriter 
Method\SpecialChar \-
ref
\family default 
, 
\family typewriter 
In\SpecialChar \-
ter\SpecialChar \-
face\SpecialChar \-
Method\SpecialChar \-
ref
\family default 
, 
\family typewriter 
String
\family default 
, 
\family typewriter 
In\SpecialChar \-
teger
\family default 
, 
\family typewriter 
Float
\family default 
, 
\family typewriter 
Long
\family default 
, 
\family typewriter 
Double
\family default 
, 
\family typewriter 
Name\SpecialChar \-
And\SpecialChar \-
Type
\family default 
 and 
\family typewriter 
Utf8
\family default 
.
\layout Standard

Most of the names are self-explanatory; the interested reader will find
 more information in the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 Constants can be nested; this is done by referring to the constant pool
 index of the enclosed constant.
\layout Standard

See the following examples.
\newline 

\newline 

\family typewriter 
CONSTANT_Utf8_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 tag; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 length; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 bytes[length]; 
\newline 
}
\newline 

\newline 

\family default 
A CONSTANT_Utf8 represents a constant string.
 Such a string is e.g.
 used to describe names of methods, names of fields, names of attributes,
 types of methods or types of fields.
 This string is encoded in UTF-8 format, a variant of the unicode character
 set 
\begin_inset LatexCommand \cite{Unicode}

\end_inset 

.

\family typewriter 
 
\family default 
The tag for this type of constant is simply the number 1, as defined in
 the Java Virtual Machine Specification, Second Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
\family typewriter 

\newline 

\newline 
CONSTANT_NameAndType_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 tag; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 descriptor_index; 
\newline 
}
\family default 

\newline 

\newline 
A Constant_NameAndType represents a name and a signature of a method, the
 tag is the number 12.

\family typewriter 
 
\family default 
Both 
\family typewriter 
class_index 
\family default 
and
\family typewriter 
 descriptor_index 
\family default 
refer to a
\family typewriter 
 CONSTANT_Utf8
\family default 
.
\family typewriter 

\newline 

\newline 
CONSTANT_InterfaceMethodref_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u1 tag; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 class_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 name_and_type_index; 
\newline 
}
\family default 

\newline 

\newline 
A 
\family typewriter 
CONSTANT_InterfaceMethodref
\family default 
 describes a reference to a method defined in an interface class (see section
 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

 for an explanation of interfaces), the tag is number 11.
 The interface class is referenced via a two-byte index into the constant
 pool.
 A 
\family typewriter 
Constant_Class
\family default 
 is expected there describing a reference to some class file.
 Every method has a name, zero or more argument types and a return type;
 this is described in the 
\family typewriter 
CONSTANT_NameAndType
\family default 
 that is also referenced via a two-byte constant pool index.
\layout Standard

Note that there are implicit constraints on the integrity of a class file:
 for example, there must not be a 
\family typewriter 
CONSTANT_Integer
\family default 
 where a 
\family typewriter 
CONSTANT_Utf8
\family default 
 is expected for a certain entity.
 As another example, the names and the types of methods are encoded as strings
 in UTF-8 format 
\begin_inset LatexCommand \cite{Unicode}

\end_inset 

.
 They have to be well-formed (according to the specification) to be valid.
\layout Subsection


\begin_inset LatexCommand \label{Fields}

\end_inset 

Fields
\layout Standard

Each field is described by a field_info structure as defined below.
\newline 

\newline 

\family typewriter 
field_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 access_flags; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 descriptor_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attributes_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
attribute_info attributes[attributes_count]; 
\family default 

\newline 
}
\newline 

\newline 
A field has to be unique in a class file with respect to its name and descriptor
\begin_float footnote 
\layout Standard

The descriptor of a field describes its type.
 E.g., a descriptor of 
\begin_inset Quotes eld
\end_inset 

[I
\begin_inset Quotes erd
\end_inset 

 means 
\begin_inset Quotes eld
\end_inset 

one-dimensional array of 
\family typewriter 
int
\family default 

\begin_inset Quotes erd
\end_inset 

.
\end_float 
.
 We see that fields reference constants in the constant pool via their constant
 pool indices (such as a 
\family typewriter 
CONSTANT_Utf8
\family default 
 describing a field's name).
 An important attribute used by fields is the ConstantValue attribute (see
 section 
\begin_inset LatexCommand \ref{ConstantValueAttribute}

\end_inset 

).
\layout Standard

The 
\family typewriter 
access_flags
\family default 
 entry is a bit vector that specifies the accessibility and other properties
\begin_float footnote 
\layout Standard

Often called 
\emph on 
visibility
\emph default 
.
\end_float 
 of the field.
 E.g., a field with the 
\family typewriter 
ACC_PRIVATE
\begin_float footnote 
\layout Standard

Bit number 1.
\end_float 
 bit set is not accessible to other classes.
 A field with the 
\family typewriter 
ACC_PUBLIC
\begin_float footnote 
\layout Standard

Bit number 0.
\end_float 
 bit set is accessible to any other class.
 Any combination with both the 
\family typewriter 
ACC_PRIVATE
\family default 
 and the 
\family typewriter 
ACC_PUBLIC
\family default 
 bit set is not valid.
\layout Standard

The 
\family typewriter 
descriptor_index
\family default 
 refers to a 
\family typewriter 
CONSTANT_Utf8
\family default 
 that symbolically encodes the type of the field.
\layout Subsection


\begin_inset LatexCommand \label{Methods}

\end_inset 

Methods
\layout Standard

Each method is described by a method_info structure as defined below.
\newline 

\newline 

\family typewriter 
method_info { 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 access_flags; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 name_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 descriptor_index; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
u2 attributes_count; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
attribute_info attributes[attributes_count]; 
\newline 
}
\family default 

\newline 

\newline 
As we can easily see, this is exactly the same structure we already know
 as 
\family typewriter 
field_info
\family default 
 (see section 
\begin_inset LatexCommand \ref{Fields}

\end_inset 

).
 The difference lies in the meaning of the enlisted entities.
 For example, an access flag saying a field was volatile (non-cacheable)
 would not make any sense if set in a 
\family typewriter 
method_info
\family default 
 structure.
 Vice versa, an access flag saying the floating point instructions should
 work in 
\begin_inset Quotes eld
\end_inset 

FP-strict
\begin_inset Quotes erd
\end_inset 

 mode would be of no use if set in a 
\family typewriter 
field_info
\family default 
 structure.
\layout Standard

Methods use a different set of attributes than fields; for example, the
 
\family typewriter 
Constant\SpecialChar \-
Value
\family default 
 attribute (see section 
\begin_inset LatexCommand \ref{ConstantValueAttribute}

\end_inset 

) is of no use here.
 The 
\family typewriter 
Code
\family default 
 and 
\family typewriter 
Exceptions
\family default 
 attributes frequently used by methods are of no use for fields on the other
 hand.
\layout Section

The Execution Engine
\layout Standard

Before a piece of code (the code of a 
\begin_inset Quotes eld
\end_inset 

method
\begin_inset Quotes erd
\end_inset 

) is executed, an 
\emph on 
execution frame
\emph default 
 is set up.
 It consists of a program counter (as known from traditional CPUs), a set
 of local variables (similar to registers known from traditional CPUs),
 and an operand stack.
 For each new invocation instance of a method, a new execution frame is
 set up; it is destroyed on method termination.
\layout Standard

Because a method may invoke other methods or itself recursively, there is
 a global method invocation stack.
\layout Standard

There also is a garbage-collected heap shared among the execution frames.
 This heap is used for object allocation (see section 
\begin_inset LatexCommand \ref{Instructions}

\end_inset 

).
\layout Standard

The number of local variables is not fixed.
 Every method defines how many local variables are used for its code (up
 to 65536).
\layout Standard

Also note that there is no equivalent of a 
\emph on 
Processor Status Word
\emph default 
 (PSW) in the JVM.
 Traditionally, a PSW has flags that are set implicitly during execution
 of the instructions (such as an overflow or is-zero flag).
 This is often used for conditional branching.
 The JVM, however, uses the operand stack to store the result of a comparison
 instruction explicitly.
 This result is often read from the stack by the JVM's conditional branching
 instructions.
\layout Standard

Should exceptional situations occur (such as an out-of-memory situation),
 the JVM does not lock up.
 Instead, an 
\begin_inset Quotes eld
\end_inset 

exception is thrown
\begin_inset Quotes erd
\end_inset 

; the currently executing program is signalled.
 These signals can be processed (
\begin_inset Quotes eld
\end_inset 

exceptions can be caught
\begin_inset Quotes erd
\end_inset 

).
 If such a signal is not handled by the currently executing method, the
 JVM will search a handler through the invocation hierarchy and stop execution
 only if none was found.
\layout Standard

There is a thread mechanism in the JVM.
 Basically every thread creates an own method invocation stack (so there
 may be more than one active execution frame at a time), but this feature
 is not important for the rest of this text.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 379
file exframe.eps
width 3 100
flags 9

\end_inset 


\layout Standard

This figure shows a method invocation stack.
 Method 
\family typewriter 
main
\family default 
 was invoked by the system, 
\family typewriter 
main
\family default 
 invoked 
\family typewriter 
foo
\family default 
, 
\family typewriter 
foo
\family default 
 invoked 
\family typewriter 
bar
\family default 
, and 
\family typewriter 
bar
\family default 
 invoked 
\family typewriter 
foo
\family default 
 recursively.
 This figure assumes 
\family typewriter 
main
\family default 
 allocates one local variable and one operand stack slot, 
\family typewriter 
foo
\family default 
 allocates three local variables and two operand stack slots and 
\family typewriter 
bar
\family default 
 allocates one local variable and two operand stack slots.
\layout Caption

Method Invocation Stack
\end_float 
\layout Subsection


\begin_inset LatexCommand \label{LV_and_OpStack}

\end_inset 

Local Variables and the Operand Stack
\layout Standard

The method information in a class file defines how many local variables
 are used on this method's invocation.
 It also defines the maximum operand stack size.
 Together, the local variables array and the operand stack are called the
 
\emph on 
execution frame
\emph default 
.
\layout Standard

A single stack slot has a width of 32 bits, which is also the width of a
 local variable.
 Therefore, values of types that occupy 64 bits (
\emph on 
double
\emph default 
 and 
\emph on 
long
\emph default 
) must be stored in two consecutive stack slots or local variables.
\layout Standard

The verifier takes care that the stack cannot overflow and that it cannot
 underflow.
 Also, it takes care that instructions may only access local variables if
 they contain a value of a known, correct type (see section 
\begin_inset LatexCommand \ref{Pass3Spec}

\end_inset 

).
 
\layout Subsection


\begin_inset LatexCommand \label{Instructions}

\end_inset 

Introduction to JVM Instructions
\layout Standard

This section is derived from section 2.2 of 
\begin_inset LatexCommand \cite{BCEL98}

\end_inset 

, used with permission of the author.
\layout Standard

The JVM's instruction set currently consists of 212 instructions, 44 opcodes
 are marked as reserved and may be used for future extensions or intermediate
 optimizations within the Virtual Machine.
 The instruction set can be roughly grouped as follows:
\layout Description

Stack\SpecialChar ~
operations: Constants can be pushed onto the stack either by loading
 them from the constant pool with the 
\latex latex 

\backslash 
texttt{ldc}
\latex default 
 instruction or with special ``short-cut'' instructions where the operand
 is encoded into the instructions, e.g., 
\latex latex 

\backslash 
texttt{iconst
\backslash 
_0}
\latex default 
 or 
\latex latex 

\backslash 
texttt{bipush}
\latex default 
 (push byte value).
\layout Description

Arithmetic\SpecialChar ~
operations: The instruction set of the JVM distinguishes its operand
 types using different instructions to operate on values of specific type.
 Arithmetic operations starting with 
\latex latex 

\backslash 
texttt{i}
\latex default 
, for example, denote an integer operation.
 E.g.,
\latex latex 
 
\backslash 
texttt{iadd}
\latex default 
 that adds two integers and pushes the result back on the operand stack.
 The Java types 
\latex latex 

\backslash 
texttt{boolean}
\latex default 
, 
\latex latex 

\backslash 
texttt{byte}
\latex default 
, 
\latex latex 

\backslash 
texttt{short}
\latex default 
, and 
\latex latex 

\backslash 
texttt{char}
\latex default 
 are handled as integers by the JVM.
\layout Description


\begin_inset LatexCommand \label{RetDesc}

\end_inset 

Control\SpecialChar ~
flow: There are branch instructions like 
\latex latex 

\backslash 
texttt{goto} 
\latex default 
and 
\latex latex 

\backslash 
texttt{if
\backslash 
_icmpeq}
\latex default 
, which compares two integers for equality.
 There is also a 
\latex latex 

\backslash 
texttt{jsr}
\begin_float footnote 
\layout Standard

There is a 
\begin_inset Quotes eld
\end_inset 

wide
\begin_inset Quotes erd
\end_inset 

 version of 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 called 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
.
 The instructions 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 and 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 play in important role in chapter 
\begin_inset LatexCommand \ref{Pass3Spec}

\end_inset 

.
\end_float 
 (jump into subroutine) and 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 (return from subroutine) pair of instructions.
 Exceptions may be thrown with the 
\latex latex 

\backslash 
texttt{athrow}
\latex default 
 instruction.
 Branch targets are coded as offsets from the current byte code position,
 i.e., they are coded with an integer number.
\layout Description

Load\SpecialChar ~
and\SpecialChar ~
store\SpecialChar ~
operations for local variables like 
\latex latex 

\backslash 
texttt{iload}
\latex default 
 and 
\latex latex 

\backslash 
texttt{istore}
\latex default 
.
 There are also array operations like 
\latex latex 

\backslash 
texttt{iastore}
\latex default 
 which stores an integer value into an array.
\layout Description

Field\SpecialChar ~
access: The value of an instance field may be retrieved with 
\latex latex 

\backslash 
texttt{getfield}
\latex default 
 and written with 
\latex latex 

\backslash 
texttt{putfield}
\latex default 
.
 For static fields, there are 
\latex latex 

\backslash 
texttt{getstatic}
\latex default 
 and 
\latex latex 

\backslash 
texttt{putstatic}
\latex default 
 counterparts.
\layout Description

Method\SpecialChar ~
invocation: Methods may either be called via static references with
 
\latex latex 

\backslash 
texttt{invokestatic}
\latex default 
 or be bound virtually with the 
\latex latex 

\backslash 
texttt{invokevirtual}
\latex default 
 instruction.
 Super class methods and private methods are invoked with 
\latex latex 

\backslash 
texttt{invokespecial}
\latex default 
.
\layout Description

Object\SpecialChar ~
allocation: Class instances are allocated with the 
\latex latex 

\backslash 
texttt{new}
\latex default 
 instruction, arrays of basic type like 
\latex latex 

\backslash 
texttt{int[]}
\latex default 
 with 
\latex latex 

\backslash 
texttt{newarray}
\latex default 
, arrays of references like 
\latex latex 

\backslash 
texttt{String[][]}
\latex default 
 with 
\latex latex 

\backslash 
texttt{anewarray}
\latex default 
 or 
\latex latex 

\backslash 
texttt{multianewarray}
\latex default 
.
\layout Description

Conversion\SpecialChar ~
and\SpecialChar ~
type\SpecialChar ~
checking: For stack operands of basic type there exist
 casting operations like 
\latex latex 

\backslash 
texttt{f2i}
\latex default 
 which converts a float value into an integer.
 The validity of a type cast may be checked with 
\latex latex 

\backslash 
texttt{checkcast}
\latex default 
 and the 
\latex latex 

\backslash 
texttt{instanceof}
\latex default 
 operator can be directly mapped to the equally named instruction.
\layout Standard

Most instructions have a fixed length, but there are also some variable-length
 instructions: In particular, the 
\latex latex 

\backslash 
texttt{lookupswitch}
\latex default 
 and 
\latex latex 

\backslash 
texttt{tableswitch}
\latex default 
 instructions, which are often used by compilers to implement the Java language
 
\latex latex 

\backslash 
texttt{switch()}
\latex default 
 statements.
 Since the number of 
\latex latex 

\backslash 
texttt{case}
\latex default 
 clauses may vary, these instructions contain a variable number of statements.
\layout Standard

In a class file, the 
\family typewriter 
code
\family default 
 item in the 
\family typewriter 
Code
\family default 
 attributes (which in turn are attributes of 
\family typewriter 
method_info
\family default 
 structures), is a byte array in which binary representations of JVM instruction
s are stored sequentially.
 This is also called 
\emph on 
bytecode
\emph default 
.
\layout Standard

The JVM is a stack-based machine.
 There are local variables which may be compared to registers, but most
 instructions work on the operand stack.
 E.g., the 
\latex latex 

\backslash 
texttt{iadd}
\latex default 
 instruction pops two integers from the operand stack and pushes the result
 of the add operation on top of the stack.
\layout Standard

We will not list all of the instructions here, since these are explained
 in detail in the JVM specification.
 However, you will find the most common instructions in table 
\begin_inset LatexCommand \ref{typeprefixes}

\end_inset 

, cited with slight corrections and modifications from chapter 4 of 
\begin_inset LatexCommand \cite{JNS}

\end_inset 

.
\layout Standard

\begin_float tab 
\layout Caption


\begin_inset LatexCommand \label{typeprefixes}

\end_inset 

Type Prefixes and the Most Common JVM Instructions
\layout Standard
\align center 

\begin_inset  Tabular
<lyxtabular version="2" rows="9" columns="2">
<features rotate="false" islongtable="false" endhead="0" endfirsthead="0" endfoot="0" endlastfoot="0">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="true" width="" special="">
<row topline="true" bottomline="true" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Prefix
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Bytecode type
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

i
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Integer
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

f
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Floating point
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

l
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Long
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

d
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Double precision floating point
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

b
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Byte
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

s
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Short
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

c
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Character
\end_inset 
</cell>
</row>
<row topline="true" bottomline="true" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

a
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Object reference
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 


\end_float 
\layout Standard


\begin_inset  Tabular
<lyxtabular version="2" rows="29" columns="10">
<features rotate="false" islongtable="true" endhead="1" endfirsthead="0" endfoot="0" endlastfoot="0">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="left" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="center" valignment="top" leftline="true" rightline="false" width="" special="">
<column alignment="left" valignment="top" leftline="true" rightline="true" width="4cm" special="">
<row topline="true" bottomline="true" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
Instruction
\end_inset 
</cell>
<cell multicolumn="0" alignment="left" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
int
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
long
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
float
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
double
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
byte
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
char
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
short
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
object ref.
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard


\size scriptsize 
Function
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2c
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="left" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to character
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2d
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to double
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2i
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to integer
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2f
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to float
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2l
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to long
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?2s
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Convert value of type <?> to short
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?add
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Add two values of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?aload
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Push an element of type <?> from an array onto the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?and
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform logical AND on two values of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?astore
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Pop an element of type <?> from the stack and store it in an array of type
 <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?cmp
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Compare two long values.
 If they are equal push 0, if the first is greater push 1, else push -1
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?cmpg
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Compare two IEEE values of type <?> from the stack.
 If they are equal push 0, if the first is greater push 1, if the second
 is greater push -1.
 If either is NaN (not a number) push 1
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?cmpl
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Compare two IEEE values of type <?> from the stack.
 If they are equal push 0, if the first is greater push 1, if the second
 is greater push -1.
 If either is NaN (not a number) push -1
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?const
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Push a constant value of type <?> onto the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?div
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform a division using two values of type <?> and push the quotient onto
 the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?inc
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Increment the top of the stack (possibly by a negative value)
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?ipush
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Push a sign extended byte or short value onto the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?load
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Push a value of type <?> from a local variable onto the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?mul
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform multiplication of two values of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?neg
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Negate a value of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?newarray
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Create a new array of object references
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?or
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform logical OR on two values of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?rem
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform a division using two values of type <?> and push the remainder onto
 the stack
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?return
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Return a value of type <?> to the invoking method
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?shl
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform arithmetic shift left on a value of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?shr
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform arithmetic shift right on a value of type <?>
\end_inset 
</cell>
</row>
<row topline="true" bottomline="false" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?store
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Pop a value of type <?> and store it into a local variable
\end_inset 
</cell>
</row>
<row topline="true" bottomline="true" newpage="false">
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

?sub
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

X
\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="false" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell multicolumn="0" alignment="center" valignment="top" topline="true" bottomline="false" leftline="true" rightline="true" rotate="false" usebox="none" width="" special="">
\begin_inset Text

\layout Standard

Perform a subtraction using two values of type <?>
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 


\layout Standard

The opcode names are mostly self-explanatory.
 In this paper, all bytecode is commented to support the intuitive understanding.
 Algorithms 
\begin_inset LatexCommand \ref{facjavapl}

\end_inset 

 and 
\begin_inset LatexCommand \ref{facjavabytecode}

\end_inset 

 show an example bytecode taken from 
\begin_inset LatexCommand \cite{BCEL98}

\end_inset 

.
 It implements the well-known faculty function.
 To understand this example, it is important to know that method arguments
 are stored into the local variables of a newly created execution frame
 upon method invocation.
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{facjavapl}

\end_inset 

Methed 
\emph on 
fac
\emph default 
 in a class 
\emph on 
Faculty
\emph default 
, Java programming language version
\layout Standard


\family typewriter 
public static final int fac(int n){
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
return (n==0)?1:n*fac(n-1);
\layout Standard


\family typewriter 
}
\end_float 
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{facjavabytecode}

\end_inset 

Method 
\emph on 
fac
\emph default 
 in a class 
\emph on 
Faculty
\emph default 
, Java bytecode version
\layout Standard


\family typewriter 
\size footnotesize 
Faculty.fac (I)I
\layout Standard


\family typewriter 
\size footnotesize 
0:\SpecialChar ~
\SpecialChar ~
iload_0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; load argument onto stack
\layout Standard


\family typewriter 
\size footnotesize 
1:\SpecialChar ~
\SpecialChar ~
ifne #8\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; non-zero? Then branch to 8.
\layout Standard


\family typewriter 
\size footnotesize 
4:\SpecialChar ~
\SpecialChar ~
iconst_1\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; push constant 1 onto stack
\layout Standard


\family typewriter 
\size footnotesize 
5:\SpecialChar ~
\SpecialChar ~
goto #16\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; jump to 16
\layout Standard


\family typewriter 
\size footnotesize 
8:\SpecialChar ~
\SpecialChar ~
iload_0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; load argument onto stack
\layout Standard


\family typewriter 
\size footnotesize 
9:\SpecialChar ~
\SpecialChar ~
iload_0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; load argument onto stack
\layout Standard


\family typewriter 
\size footnotesize 
10:\SpecialChar ~
iconst_1\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; push constant 1 onto stack
\layout Standard


\family typewriter 
\size footnotesize 
11:\SpecialChar ~
isub\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; subtract the stack top from
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; the stack next-to-top which becomes
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; the new stack top
\layout Standard


\family typewriter 
\size footnotesize 
12:\SpecialChar ~
invokestatic Faculty.fac (I)I\SpecialChar ~
\SpecialChar ~
; call method fac recursively,
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; the new invocation
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; instance's argument is the stack top
\layout Standard


\family typewriter 
\size footnotesize 
15:\SpecialChar ~
imul\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; multiply the return value with the
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; argument given to the current
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; invocation instance
\layout Standard


\family typewriter 
\size footnotesize 
16:\SpecialChar ~
ireturn\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; return value on top of the
\layout Standard


\family typewriter 
\size footnotesize 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; stack to the invoking method
\end_float 
\layout Chapter


\begin_inset LatexCommand \label{SpecPasses}

\end_inset 

Specification of the Verification Passes
\layout Standard

Sun describes a four-pass class file verifier in The Java Virtual Machine
 Specification, Second Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 It is not necessary to implement the verification algorithms literally;
 and it is not possible anyway (see section 
\begin_inset LatexCommand \ref{SpecSubroutines}

\end_inset 

).
 However, implementing a verifier with a multiple-pass architecture makes
 sense.
 It is a good thing to stay close to the specification because it is well-known
 throughout the bytecode engineering community.
 Also, the boundaries between the passes are not arbitrary.
 They are drawn to improve the performance of the verifiers built into JVMs.
 For example, classes are not verified (completely) before they are actually
 used but they are loaded as soon as they are referenced in a certain way.
 Most verifiers use the traditional multiple-pass architecture, including
 Kimera 
\begin_inset LatexCommand \cite{Kimera-WWW}

\end_inset 

.
 Work in other directions (for instance, the one-pass-architecture proposed
 by Fong 
\begin_inset LatexCommand \cite{Fong-WWW}

\end_inset 

) did not yield lasting results.
\layout Standard

Pass one is basically about loading a class file into the JVM in a sane
 way and pass two verifies that the loaded class file information is consistent.
 Pass three verifies that the program code is well-behaved; pass four verifies
 things that conceptually belong to pass three but are delayed to the run-time
 for performance reasons.
\layout Standard

Sometimes implementation details are discussed in this chapter.
 Whenever the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

 was ambigous about some issue, the behaviour of Sun's JVM implementations
 was observed.
 The discussed details are part of the specification of the JustIce verifier.
\layout Section


\begin_inset LatexCommand \label{PassOneSpec}

\end_inset 

Pass One
\layout Standard

The first pass of the verifier is only vaguely specified.
 It is there to assure a class file 
\begin_inset Quotes eld
\end_inset 


\series bold 
has the basic format of a class file.
 The first four bytes must contain the right magic number.
 All recognized attributes must be of the proper length.
 The class file must not be truncated or have any extra bytes at the end.
 The constant pool must not contain any superficially unrecognizable information
\series default 

\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 141).
\layout Standard

The right magic number is 0xCAFEBABE (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 94), which is easy to assure.
\layout Standard

It is not clear what 
\begin_inset Quotes eld
\end_inset 

superficially unrecognizable information
\begin_inset Quotes erd
\end_inset 

 exactly is, however.
 If an attribute is not known to the JVM (or verifier) implementation, it
 has to be ignored -- so this does not seem to be 
\begin_inset Quotes eld
\end_inset 

superficially unrecognizable information
\begin_inset Quotes erd
\end_inset 

.
 Attributes that are not used cannot be detected in pass one.
 One would have to look at the bytecodes to decide whether an attribute
 is used or not (which is not the domain of pass one, but of pass three).
\layout Standard

Observations show that most existing JVM verifiers
\begin_float footnote 
\layout Standard

An example of a verifier with this behaviour is the one implemented in Sun's
 Solaris port of the JVM, version 1.3.0_01.
\end_float 
 ignore 
\begin_inset Quotes eld
\end_inset 

extra bytes at the end
\begin_inset Quotes erd
\end_inset 

 instead of rejecting class files bearing them.
\layout Standard

The other two statements specify verification of the class file structure
 (and the structure of the attributes therein).
 But this is also the domain of pass two! Only by inspecting the way the
 JVM 
\emph on 
loads
\emph default 
, 
\emph on 
resolves
\emph default 
 and 
\emph on 
prepares
\emph default 
 classes one will understand the precise boundary between verification passes
 one and two 
\begin_inset LatexCommand \cite{Fong-WWW}

\end_inset 

.
\layout Standard

'Being careful when loading a class file' is a good definition for pass
 one: the structure of the file to load is untrusted.
 Every implicit statement such as 
\begin_inset Quotes eld
\end_inset 

this attribute has a length of 1234 bytes in total
\begin_inset Quotes erd
\end_inset 

 is validated.
\layout Standard


\emph on 
Resolution
\emph default 
 is the transformation of a symbolic reference to an actual reference --
 i.e., as long as there is only a symbolic reference to an entity, this entity
 cannot be verified at all because it has not been loaded yet.
 Passes two and three are performed during the 
\emph on 
resolution
\emph default 
 of a class file; while loading of the class file --pass one-- must have
 been performed before.
 
\emph on 
Resolution
\emph default 
 as such is meaningless to JustIce; the term is only used to draw the borders
 between the verification passes.
\layout Section


\begin_inset LatexCommand \label{SpecPassTwo}

\end_inset 

Pass Two
\layout Standard

The checks performed in pass two enforce that the following constraints
 are satisfied.
\layout Itemize

Ensuring that final classes are not subclassed and that final methods are
 not overridden.
\layout Itemize

Checking that every class (except 
\family typewriter 
java.lang.Object
\family default 
) has a direct superclass.
\layout Itemize

Ensuring that the constant pool satisfies the documented static constraints:
 for example, that each 
\family typewriter 
CONSTANT_Class_info
\family default 
 structure in the constant pool contains in its 
\family typewriter 
name_index
\family default 
 item a valid constant pool index for a 
\family typewriter 
CONSTANT_Utf8_info
\family default 
 structure.
\layout Itemize

Checking that all field references and method references in the constant
 pool have valid names, valid classes, and a valid type descriptor.
\layout Standard

As Frank Yellin puts it 
\begin_inset LatexCommand \cite{Yellin-WWW}

\end_inset 

: pass two 
\begin_inset Quotes eld
\end_inset 

performs all verification that can be performed without looking at the bytecodes
\begin_inset Quotes erd
\end_inset 

.
 Also, 
\begin_inset Quotes eld
\end_inset 

this pass does not actually check to make sure that the given field or method
 really exists in the given class; nor does it check that the type signatures
 given refer to real classes.
\begin_inset Quotes erd
\end_inset 

 Note that again 
\emph on 
resolution
\emph default 
 plays an important role to create the boundary between two passes; here
 it is the boundary between pass two and pass three.
 Because linking-time verification enhances the performance of the JVM,
 checks that basically belong to pass two are delayed to pass three.
 This leads to the obvious contradiction in the sentences cited above.
\layout Standard

This performance enhancement has an ugly side effect.
 Consider a reference to a method m contained in a class file C that does
 not exist.
 As long as this reference is not 
\emph on 
used
\emph default 
, i.e., 
\emph on 
resolved
\emph default 
, the absence of C cannot be detected.
 Such a reference should in the author's opinion regarded as 
\begin_inset Quotes eld
\end_inset 

superficially unrecognizable information
\begin_inset Quotes erd
\end_inset 

 (see section 
\begin_inset LatexCommand \ref{PassOneSpec}

\end_inset 

) and therefore be detected.
\layout Standard

This pass has to verify the integrity of the clas file's data structures
 as explained in section 
\begin_inset LatexCommand \ref{Classfile Structure}

\end_inset 

.
 As an example, consider the Line\SpecialChar \-
Number\SpecialChar \-
Table atribute.
 Sun did not specify there has to be exactly one 
\family typewriter 
Line\SpecialChar \-
Number\SpecialChar \-
Table
\family default 
 attribute (or none at all) per method, so possibly there is more than one
 attribute of that kind.
 This lax specification is not necessary due to the fact that you can put
 all information in a single 
\family typewriter 
Line\SpecialChar \-
Number\SpecialChar \-
Table_attri\SpecialChar \-
bute
\begin_float footnote 
\layout Standard

Any number of 
\family typewriter 
line_number_table 
\family default 
array entries fits nicely in a single 
\family typewriter 
LineNumberTable_attribute
\family default 
 attribute.
\end_float 
, but Sun did specify it this way (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 129).
\layout Standard

Verifiers are requested to reject class files with inconsistent information
 in their attributes.
 However, here it may be that only by looking at all 
\family typewriter 
Line\SpecialChar \-
Number\SpecialChar \-
Table_attribute
\family default 
s of a method, an inconsistency can be detected.
 JustIce does so and rejects class files with inconsistent 
\family typewriter 
Line\SpecialChar \-
Number\SpecialChar \-
Table
\family default 
 information.
\layout Standard

Furthermore, it issues warnings if such an attribute is detected at all
 to discourage its use (see section 
\begin_inset LatexCommand \ref{Pass2Impl}

\end_inset 

).
 This is done because of possible different interpretations of the specification.
\layout Standard

It should be noted that the use of attributes raises a few more problems
 to class file verification.
 A simple case is the presence of an unknown attribute that may safely be
 ignored.
 It is explicitly stated that such a class file must not be rejected.
 On the other hand, how should a verifier react if --for example-- a 
\family typewriter 
field_info
\family default 
 (see section 
\begin_inset LatexCommand \ref{Fields}

\end_inset 

) structure encloses a 
\family typewriter 
Code_attribute
\family default 
? JustIce will issue a warning but not reject the class file.
\layout Section


\begin_inset LatexCommand \label{Pass3Spec}

\end_inset 

Pass Three
\layout Standard

Performing pass three basically means 
\emph on 
verifying the bytecode
\emph default 
.
 There are so-called 
\begin_inset Quotes eld
\end_inset 

static constraints
\begin_inset Quotes erd
\end_inset 

 on both the instructions in the code array and their operands.
 There are also so-called 
\begin_inset Quotes eld
\end_inset 

structural constraints
\begin_inset Quotes erd
\end_inset 

.
 The structural constraints specify constraints on relationships between
 JVM instructions, so some people (including the author) regard 
\begin_inset Quotes eld
\end_inset 

structural constraints
\begin_inset Quotes erd
\end_inset 

 as a misnomer; they should be called 
\begin_inset Quotes eld
\end_inset 

dynamic constraints
\begin_inset Quotes erd
\end_inset 

.
\layout Standard

Static constraints are easily enforced using very simple checks.
 Here is an example for such a check: let there be a 
\family typewriter 
Code
\family default 
 (see section 
\begin_inset LatexCommand \ref{CodeAttribute}

\end_inset 

) attribute with a 
\family typewriter 
max_locals
\family default 
 value of 2.
 Only local variables number 0 and 1 may be accessed by the bytecode in
 this 
\family typewriter 
Code
\family default 
 attribute.
 For all instructions accessing local variables, make sure they do not access
 any other local variable.
\layout Standard

Structural constraints are enforced using an algorithm sketched by Sun;
 it implements a symbolic execution of a method's code, by means of data
 flow analysis including type inference (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, pages 143-151).
 This algorithm is called the 
\emph on 
data flow analyzer.

\emph default 
 It is intuitively easy to understand, but it is hard to prove its correctness.
 The reason for that is the very weak specification of its subtleties; especiall
y 
\emph on 
subroutines
\emph default 
, 
\emph on 
wide date types
\emph default 
 and 
\emph on 
object initialization
\emph default 
 (see below).
 The general approach, however, is sound 
\begin_inset LatexCommand \cite{BCV-Soundness}

\end_inset 

.
 Here is an example for a structural constraint enforced by this algorithm:
 during program execution, at any given point in the program the operand
 stack is always of the same height, no matter which code path was taken
 to reach that point.
 
\layout Standard

Pass three is the core of the verifier.
 Note that we will split this pass up into two passes, namely a pass verifying
 the static constraints and a pass verifying the structural constraints
 of a method's code.
 We will call these passes 
\begin_inset Quotes eld
\end_inset 

pass 3a
\begin_inset Quotes erd
\end_inset 

 and 
\begin_inset Quotes eld
\end_inset 

pass 3b
\begin_inset Quotes erd
\end_inset 

.
 In a way, they resemble pass one and pass two: the former pass carefully
 parses an entity, while the latter pass performs additional verification.
 
\layout Standard

By defining pass four, the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

 implicitly excludes 
\begin_inset Quotes eld
\end_inset 

certain tests that could in principle be performed in Pass 3
\begin_inset Quotes erd
\end_inset 

, because they are 
\begin_inset Quotes eld
\end_inset 

delayed until the first time the code for the method is actually invoked
\begin_inset Quotes erd
\end_inset 

.
 On the other hand, verifiers are allowed to perform pass four partially
 or completely as a part of pass three.
 JustIce performs the pass four checks in pass 3a.
\layout Subsection

Static Constraints: Pass 3a
\layout Standard

Sun gives examples of what the verifier does before starting the data flow
 analyzer (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, pages 143-144):
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
Branches must be within the bounds of the code array for the method.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
The targets of all control-flow instructions are each the start of an instructio
n.
 In the case of a 
\latex latex 

\backslash 
texttt{wide}
\latex default 
 instruction the 
\latex latex 

\backslash 
texttt{wide} 
\latex default 
opcode is considered the start of the instruction, and the opcode giving
 the operation modified by that 
\latex latex 

\backslash 
texttt{wide}
\latex default 
 instruction is not considered to start an instruction.
 Branches into the middle of an instruction are disallowed.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
No instruction can access or modify a local variable at an index greater
 than or equal to the number of local variables that its method indicates
 it allocates.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
All references to the constant pool must be an entry of the appropriate
 type.
 For example: the instruction 
\latex latex 

\backslash 
texttt{ldc}
\latex default 
 can be used only for data of type int or float or for instances of class
 String; the instruction 
\latex latex 

\backslash 
texttt{getfield}
\latex default 
 must reference a field.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
The code does not end in the middle of an instruction.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
Execution cannot fall off the end of the code.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
For each exception handler, the starting and ending point of the code protected
 by the handler must be at the beginning of an instruction or, in the case
 of the ending point, immediately past the end of the code.
 The starting point must be before the ending point.
 The exception handler code must start at a valid instruction, and it may
 not start at an opcode being modified by the 
\latex latex 

\backslash 
texttt{wide}
\latex default 
 instruction.
\layout Standard

Most of these constraints are either static constraints on instructions
 or on their operands.
 A full list of constraints can be found in the Java Virtual Machine Specificati
on, Second Edition (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, pages 133-137).
\layout Standard

The check for execution falling off the end of the code is an exception:
 this is a structural constraint and should therefore be performed in pass
 3b.
 Sun's verifiers, however, reject code that has an unreachable 
\latex latex 

\backslash 
texttt{nop} 
\latex default 
at the end of the code array.
 Obviously, they reject the code before performing data flow analysis.
 For the sake of compatibility, JustIce performs this check in pass 3a.
\layout Standard

Note that the JVM's instructions differ in length.
 Some instructions occupy only one byte (such as 
\family typewriter 
nop
\family default 
), others occupy three bytes (such as 
\family typewriter 
goto
\family default 
).
 Branch instructions could therefore target operands of instructions.
 For example, line 1 of algorithm 
\begin_inset LatexCommand \ref{facjavabytecode}

\end_inset 

 reads 
\begin_inset Quotes eld
\end_inset 


\family typewriter 
1: ifne #8
\family default 

\begin_inset Quotes erd
\end_inset 

.
 If it would read 
\begin_inset Quotes eld
\end_inset 


\family typewriter 
1: ifne #7
\family default 

\begin_inset Quotes erd
\end_inset 

, this code was malformed.
 A special case is the instruction 
\family typewriter 
wide
\family default 
.
 This instruction takes another instruction 
\emph on 
as its operand
\emph default 
, so one could be misguided into thinking this embedded instruction was
 a valid target for branches.
 It is not.
\layout Standard

The checks Sun delays until pass four are performed in pass 3a by JustIce.
 These are checks to ensure allowed and possible access to a referenced
 type, listed below.
 
\layout Itemize

Is the type (class or interface) currently under examination allowed to
 reference the type
\begin_float footnote 
\layout Standard

Interfaces may contain code, this is normally used for static initialization
 of 
\family typewriter 
final
\family default 
 variables.
\end_float 
?
\layout Itemize

Does the referenced method or field exist in the given class?
\layout Itemize

Does the referenced method or field have the indicated descriptor (signature)?
\layout Itemize

Does the method currently under examination have access to the referenced
 method or field?
\layout Subsection

Structural Constraints: Pass 3b
\layout Standard

The structural constraints of JVM instructions are enforced by a data flow
 analyzer.
 This algorithm ensures the following constraints (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 142).
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
The operand stack is always the same size and contains the same types of
 values.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
No local variable is accessed unless it is known to contain a value of an
 appropriate type.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
Methods are invoked with the appropriate arguments.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
Fields are assigned only using values of appropriate types.
\layout Itemize
\pextra_type 1 \pextra_width 10mm


\series bold 
All opcodes have appropriate type arguments on the operand stack and in
 the local variable array.
\layout Standard

A full list of structural constraints can be found in The Java Virtual Machine
 Specification, Second Edition (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, pages 137-139).
\layout Subsubsection


\begin_inset LatexCommand \label{SunCoreAlgo}

\end_inset 

Sun's Verification Algorithm
\layout Standard

Sun specifies the data flow analyzer by giving an informal algorithm (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, pages 144-146).
 This algorithm it cited here completely because it is the very core of
 the verifier.
 According to this algorithm, every bytecode instruction has a 
\begin_inset Quotes eld
\end_inset 

changed
\begin_inset Quotes erd
\end_inset 

 bit.
 Initially, only the 
\begin_inset Quotes eld
\end_inset 

changed
\begin_inset Quotes erd
\end_inset 

 bit of the first instruction is set.
\layout Enumerate
\pextra_type 1 \pextra_width 10mm


\series bold 
Select a virtual machine instruction whose "changed" bit is set.
 If no instruction remains whose "changed" bit is set, the method has successful
ly been verified.
 Otherwise, turn off the "changed" bit of the selected instruction.
\layout Enumerate
\pextra_type 1 \pextra_width 10mm


\series bold 
Model the effect of the instruction on the operand stack and local variable
 array by doing the following:
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If the instruction uses values from the operand stack, ensure that there
 are a sufficient number of values on the stack and that the top values
 on the stack are of an appropriate type.
 Otherwise, verification fails.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If the instruction uses a local variable, ensure that the specified local
 variable contains a value of the appropriate type.
 Otherwise, verification fails.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If the instruction pushes values onto the operand stack, ensure that there
 is sufficient room on the operand stack for the new values.
 Add the indicated types to the top of the modeled operand stack.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If the instruction modifies a local variable, record that the local variable
 now contains the new type.
\layout Enumerate
\pextra_type 1 \pextra_width 10mm


\series bold 
Determine the instructions that can follow the current instruction.
 Successor instructions can be one of the following:
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
The next instruction, if the current instruction is not an unconditional
 control transfer instruction (for instance goto, return, or athrow).
 Verification fails if it is possible to "fall off" the last instruction
 of the method.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
The target(s) of a conditional or unconditional branch or switch.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
Any exception handlers for this instruction.
 
\layout Enumerate
\pextra_type 1 \pextra_width 10mm


\series bold 
Merge the state of the operand stack and local variable array at the end
 of the execution of the current instruction into each of the successor
 instructions.
 In the special case of control transfer to an exception handler, the operand
 stack is set to contain a single object of the exception type indicated
 by the exception handler information.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If this is the first time the successor instruction has been visited, record
 that the operand stack and local variable values calculated in steps 2
 and 3 are the state of the operand stack and local variable array prior
 to executing the successor instruction.
 Set the "changed" bit for the successor instruction.
\newline 

\latex latex 

\backslash 
textbullet\SpecialChar ~

\latex default 
If the successor instruction has been seen before, merge the operand stack
 and local variable values calculated in steps 2 and 3 into the values already
 there.
 Set the "changed" bit if there is any modification to the values.
\layout Enumerate
\pextra_type 1 \pextra_width 10mm


\series bold 
Continue at step 1.
 
\layout Standard
\pextra_type 1 \pextra_width 10mm


\series bold 
To merge two operand stacks, the number of values on each stack must be
 identical.
 The types of values on the stacks must also be identical, except that different
ly typed reference values may appear at corresponding places on the two
 stacks.
 In this case, the merged operand stack contains a reference to an instance
 of the first common superclass of the two types.
 Such a reference type always exists because the type Object is a superclass
 of all class and interface types.
 If the operand stacks cannot be merged, verification of the method fails.
\layout Standard
\pextra_type 1 \pextra_width 10mm


\series bold 
To merge two local variable array states, corresponding pairs of local variables
 are compared.
 If the two types are not identical, then unless both contain reference
 values, the verifier records that the local variable contains an unusable
 value.
 If both of the pair of local variables contain reference values, the merged
 state contains a reference to an instance of the first common superclass
 of the two types.
\layout Standard

Certain instructions and data types complicate the data flow analyzer, most
 notably the instruction 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 (see section 
\begin_inset LatexCommand \ref{RetDesc}

\end_inset 

).
 The algorithm above even uses a special definition of 
\emph on 
merging
\emph default 
 for the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction (see 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 151).
 The 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction is parameterized with a value of type 
\family typewriter 
returnaddress
\family default 
 which is read from a local variable and used as a branching target.
 The 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction is there to implement a (control flow) return from a 
\emph on 
subroutine
\emph default 
.
\layout Subsubsection

Reachability of Instructions
\layout Standard

For the data flow analysis algorithm, you need to know all the possible
 control flow successors of every instruction, i.e., you need to build a 
\emph on 
control flow graph
\emph default 
 (see below).
 Without the instructions
\latex latex 
 
\backslash 
texttt{jsr}
\begin_float footnote 
\layout Standard

Remember, a 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction is an unconditional branch instruction that jumps into a 
\emph on 
subroutine
\emph default 
.
 Usually a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction leaves the 
\emph on 
subroutine
\emph default 
.
\end_float 
,
\latex latex 
 
\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 and 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 this calculation would be easy.
 But to calculate successors of a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction, you need a complete control flow graph: you need to find out
 which 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or
\latex latex 
 
\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 and
\latex latex 
 
\backslash 
texttt{ret}
\latex default 
 pairs belong together.
 Therefore, a cycle of self-dependency is created that has to be broken
 somewhere.
 This is explained in detail below.
\layout Standard

This was also an issue that led to the definition of the term
\emph on 
 subroutine
\emph default 
 that JustIce uses.
 This definition allows the prediction of a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction's target without performing control flow analysis.
\layout Subsubsection


\begin_inset LatexCommand \label{SpecSubroutines}

\end_inset 

Subroutines
\layout Standard

Subroutines make the verification algorithm extremely difficult.
 They are harshly underspecified.
 Although 
\begin_inset Quotes eld
\end_inset 

the Java virtual machine has no guarantee that any file it is asked to load
 was generated by that compiler
\begin_inset Quotes erd
\end_inset 

, the subroutine specification explains how 
\emph on 
javac 
\emph default 
transforms 
\begin_inset Quotes eld
\end_inset 


\latex latex 

\backslash 
texttt{try}
\latex default 
/
\latex latex 

\backslash 
texttt{catch}
\latex default 
/
\latex latex 

\backslash 
texttt{finally}
\latex default 

\begin_inset Quotes erd
\end_inset 

 clauses into subroutines 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 Intuitively, one gets the idea that a subroutine starts with some jump
 target of a 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction and ends with a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction.
 But the specification fails to correctly specify what subroutines exactly
 are at machine instruction level.
 Consider algorithm 
\begin_inset LatexCommand \ref{jsrpopalgo}

\end_inset 

.
\layout Standard

\begin_float alg 
\layout Standard


\family typewriter 
00 jsr\SpecialChar ~
03\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Jump to 
\begin_inset Quotes eld
\end_inset 

subroutine
\begin_inset Quotes erd
\end_inset 

 at offset 03; push return
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; address 03 onto stack.
\layout Standard


\family typewriter 
03 pop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Pop the return address off the stack.
\layout Standard


\family typewriter 
04 nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; No operation.
\layout Caption


\begin_inset LatexCommand \label{jsrpopalgo}

\end_inset 

Is This a Subroutine?
\end_float 
\layout Standard

What is this? Is the 
\emph on 
NOP
\emph default 
 instruction part of a subroutine or not? Algorithm 
\begin_inset LatexCommand \ref{OneOrTwoSubroutinesAlgo}

\end_inset 

 shows another example.
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{OneOrTwoSubroutinesAlgo}

\end_inset 

One or Two Subroutines?
\layout Standard


\family typewriter 
00 iload_0\SpecialChar ~
\SpecialChar ~
; Load a numerical 0 onto the stack.
\layout Standard


\family typewriter 
01 jsr\SpecialChar ~
05\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Jump to "subroutine" at offset 05; push return
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; address 04 onto stack.
\layout Standard


\family typewriter 
04 return\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Leave the method.
\layout Standard


\family typewriter 
05 dup\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Duplicate the stack's top.
\layout Standard


\family typewriter 
06 astore\SpecialChar ~
0\SpecialChar ~
; Store the return address from the stack into
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; local variable 0.
\layout Standard


\family typewriter 
07 astore\SpecialChar ~
1\SpecialChar ~
; Store the return address from the stack into
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; local variable 1.
\layout Standard


\family typewriter 
08 ifeq\SpecialChar ~
12\SpecialChar ~
\SpecialChar ~
; If there is a 0 on top of the stack, jump to
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; offset 12.
\layout Standard


\family typewriter 
11 ret\SpecialChar ~
0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Return to offset 4 (because this is in local
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; variable 0 here).
\layout Standard


\family typewriter 
12 nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; No operation.
\layout Standard


\family typewriter 
13 ret\SpecialChar ~
1\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Return to offset 4 (because this is in local
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; variable 1 here).
\end_float 
\layout Standard

Do we deal with one subroutine (which is the case if you define subroutines
 to start with a
\latex latex 
 
\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
's target) or are these two subroutines (which is the case if you count
 the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instructions and believe that there must be exactly one 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 per subroutine)?
\layout Standard

Recursive calls to subroutines are forbidden by the specification; however,
 Sun's verifier implementations are not consequently deciding which recursive
 calls to reject
\begin_float footnote 
\layout Standard

This was experimentally found by the author and also published in 
\begin_inset LatexCommand \cite{JBook}

\end_inset 

.
\end_float 
.
 This is a failure due to a missing definition of the term 
\emph on 
subroutine
\emph default 
.
\layout Standard

While the first example passes Sun's verifier, the second example is rejected.
 The exact definition of the term 
\emph on 
subroutine
\emph default 
 cannot be deducted from ther behaviour of Sun's verifier.
\layout Standard

A new, clean specification had to be defined.
 Such a specification can of course not be compatible with the behaviour
 of Sun's verifier in all corner cases.
\layout Subsubsection


\begin_inset LatexCommand \label{Subroutines_Def}

\end_inset 

A Precise Definition of the Term 
\emph on 
Subroutine
\layout Standard

Because Sun --inappropriately-- describes how 
\emph on 
javac
\emph default 
 creates subroutines, the definition presented here is based on the observation
 of 
\emph on 
javac
\emph default 
's behaviour.
 This makes the definition compatible with a lot of existing code, but without
 violating the validity of far-reaching conclusions earned by exploiting
 a clean definition
\begin_float footnote 
\layout Standard

Unfortunately, in some rare cases, 
\emph on 
javac
\emph default 
 produces code that is incompatible with the constraints related to our
 definition of 
\emph on 
subroutine
\emph default 
.
 However, 
\emph on 
javac
\emph default 
 also produces code which is incompatible with Sun's verifier (see section
 
\begin_inset LatexCommand \ref{StaerkJreject}

\end_inset 

).
\end_float 
.
 
\layout Itemize

Every instruction of a method is part of exactly one subroutine (or the
 top-level).
\layout Itemize

The first instruction of a subroutine is an 
\latex latex 

\backslash 
texttt{astore N}
\latex default 
 instruction that stores the return address in local variable number 
\emph on 
N
\emph default 
.
\layout Itemize

There must be exactly one 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction per subroutine.
 This instruction must work on the local variable 
\emph on 
N
\emph default 
; i.e., it is a 
\latex latex 

\backslash 
texttt{ret N}
\latex default 
 instruction.
\layout Itemize

Subroutines are not protected by exception handlers.
\layout Itemize

No instruction that is part of a subroutine is the target of an exception
 handler.
\layout Itemize

Subroutines of a subroutine do not access local variable 
\emph on 
N
\emph default 
.
 A subsubroutine of a subroutine is also considered a subroutine here, in
 a recursive sense.
\layout Standard

As we can see, a subroutine can be characterized by its set of instructions,
 the most important instruction being the target of some 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or
\latex latex 
 
\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction that is not part of the subroutine itself.
 Another important property is the local variable 
\emph on 
N
\emph default 
 the
\latex latex 
 
\backslash 
texttt{ret}
\latex default 
 instruction is working on.
\layout Standard

This way, we can make sure subroutines are properly nested, so that JustIce
 would reject both the example bytecodes in algorithms 
\begin_inset LatexCommand \ref{jsrpopalgo}

\end_inset 

 and 
\begin_inset LatexCommand \ref{OneOrTwoSubroutinesAlgo}

\end_inset 

.
\layout Standard

The 
\latex latex 

\backslash 
texttt{astore}
\latex default 
 instruction mentioned above is so important because there is no JVM instruction
 that can read values of a 
\latex latex 

\backslash 
texttt{returnaddress}
\latex default 
 type from local variables.
 After entering a subroutine, the 
\latex latex 

\backslash 
texttt{astore}
\latex default 
 instruction pops the return address off the operand stack and writes it
 into local variable number 
\emph on 
N
\emph default 
.
 Therefore we can be sure it will not be duplicated or deleted as in algorithms
 
\begin_inset LatexCommand \ref{jsrpopalgo}

\end_inset 

 and 
\begin_inset LatexCommand \ref{OneOrTwoSubroutinesAlgo}

\end_inset 

.
\layout Standard

The constraints concerning exception handlers are defined to make sure that
 we can observe the control flow statically.
 If an exception is thrown from within a subroutine, the method simply 
\begin_inset Quotes eld
\end_inset 


\emph on 
completes abruptly
\emph default 

\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 74).
 If we would allow subroutine instructions to be protected by exception
 handlers, it would not be clear if the handling instructions are part of
 the subroutine or not.
\layout Standard

We can also derive subsubroutines of subroutines recursively by exploiting
 the properly-nested property explained above.
\layout Subsubsection

The Control Flow Graph
\layout Standard

A control flow graph is a directed graph with edges that represent possible
 branches of control flow.
 Similarly, the nodes describe groups of physically adjacent instructions
 that have to be executed one after another -- without any possible control
 flow branch to another instruction but the physical successor
\begin_float footnote 
\layout Standard

More information about control flow graphs can be found in 
\begin_inset LatexCommand \cite{DragonBook}

\end_inset 

.
\end_float 
.
 Figure 
\begin_inset LatexCommand \ref{convcfg}

\end_inset 

 shows such a control flow graph for algorithm 
\begin_inset LatexCommand \ref{facjavabytecode}

\end_inset 

, the implementation of the faculty function discussed earlier.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 368
file conventcfg.eps
width 3 100
flags 9

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{convcfg}

\end_inset 

A Conventional Control Flow Graph
\end_float 
\layout Standard

The JVM defines a sort of control flow orthogonal to the common execution
 of instructions, namely, the exception mechanism.
 Because every instruction could possibly throw an exception (say, a 
\family typewriter 
java.lang.VirtualMachineError
\family default 
) during its execution, the control flow graph calculated by JustIce always
 uses only one instruction per node.
 This also reflects the original verification algorithm given by Sun Microsystem
s.
 Figure 
\begin_inset LatexCommand \ref{justicecfg}

\end_inset 

 shows an example for such a control flow graph.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 473
file justicecfg.eps
width 3 100
flags 9

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{justicecfg}

\end_inset 

A Control Flow Graph as Used by JustIce
\end_float 
\layout Standard

Instruction nodes are augmented with a data structure that represents the
 simulated operand stack and the simulated local variables array.
 When running the core verification algorithm, these nodes are put into
 a queue which is equivalent to tagging them with a 
\emph on 
changed
\emph default 
 bit as Sun describes
\begin_float footnote 
\layout Standard

As explained later, JustIce uses a queue that allows duplicates: this is
 a slight semantical change.
\end_float 
.
\layout Subsubsection

Subroutines Revisited: Interplay With the Data Flow Analyzer
\layout Standard

There is another problem concerning subroutines.
 Normally, when merging the type information of two simulated local variables,
 the common type is recorded as 
\emph on 
unusable
\emph default 
 if the types differ.
 This 
\emph on 
unusable
\emph default 
 value is then propagated to subsequent instructions to prevent read access.
\layout Standard

This is not the case with the successors of the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction.
 These successors are physical successors of some 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instructions.
\layout Standard

Subroutines are said to be 
\emph on 
polymorphic
\emph default 
 with respect to their local variables arrays.
 As an example, consider algorithm 
\begin_inset LatexCommand \ref{lvpolymorphalgo}

\end_inset 

.
 This algorithm shows legal JVM code.
 In line 11, local variable 0 may contain a value of the 
\family typewriter 
integer
\family default 
 or the 
\family typewriter 
float
\family default 
 type; depending on the 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 instruction that entered the subroutine.
 Normally, this would cause the verifier to mark local variable 0 as 
\emph on 
unusable 
\emph default 
and propagate this information.
 The successors of the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction are the instructions in lines 5 and 10.
 However, a correct verifier does 
\emph on 
not
\emph default 
 mark local variable 0 as 
\emph on 
unusable
\emph default 
 for them, because the local variable 0 was not accessed or modified in
 the subroutine.
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{lvpolymorphalgo}

\end_inset 

Local Variables are Polymorphic in Subroutines
\layout Standard


\family typewriter 
0 : iconst_0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; load integer constant 0 onto stack
\layout Standard


\family typewriter 
1 : istore 0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; move it into local variable 0
\layout Standard


\family typewriter 
2 : jsr 11\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; enter subroutine
\layout Standard


\family typewriter 
5 : fconst 0.0\SpecialChar ~
; load float constant 0.0 onto stack
\layout Standard


\family typewriter 
6 : fstore 0\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; move it into local variable 0
\layout Standard


\family typewriter 
7 : jsr 11\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; enter subroutine again
\layout Standard


\family typewriter 
10: return\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; complete method
\layout Standard


\family typewriter 
11: astore 1\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Subroutine entry: move return address
\layout Standard


\family typewriter 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; into local variable 1
\layout Standard


\family typewriter 
12: nop\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; do nothing
\layout Standard


\family typewriter 
13: ret 1\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; return from subroutine
\end_float 
\layout Standard

Basically, only the local variables accessed in the called subroutine (and
 the subroutines called from there, recursively) are merged with the correspondi
ng successor of a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction.
 This means that in this special case, three sources are used to construct
 the merged array of local variables type information (instead of only two):
 the 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction, the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction and the "old" type information of the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction's target (which is the physical successor of the 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w} 
\latex default 
instruction).
\layout Standard

One possibility to deal with this situation is 
\emph on 
inlining
\emph default 
.
 For instance, the verifier of the ElectricalFire JVM 
\begin_inset LatexCommand \cite{EF}

\end_inset 

 uses this approach: instruction nodes of subroutines are duplicated for
 every calling 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
 or 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction.
 This approach is equivalent to the one sketched by Sun (see 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 151).
 
\layout Standard

JustIce uses a variant of this approach: instruction nodes are augmented
 with sets of local variables arrays.
 The local variables array used for merging a 
\latex latex 

\backslash 
texttt{ret}
\latex default 
's type information with the physical successor of some 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction is keyed by that 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction itself.
 This still implies a special merging mechanism for the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instruction: only the physical successor of one 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instruction can be merged with the 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 at a time, because other 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 instructions have possibly not been symbolically executed yet and thus
 bear no type information at the time of merging.
 In this scenario, an instruction in a subroutine plays multiple roles;
 one for each occurence of a 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 that is calling the subroutine.
 The queue holding the instructions to symbolically execute is therefore
 required to allow duplicates.
\layout Subsubsection

Wide Data Types
\layout Standard

The types 
\family typewriter 
long
\family default 
 and 
\family typewriter 
double 
\family default 
use two consecutive local variables if written to or read from a local variables
 array.
 Similarly, they use two operand stack slots.
 This makes type verification a bit more difficult because of subtle special
 cases.
 For example, when a method uses three local variables at maximum (local
 variables 0, 1 and 2), the code is not allowed to store a 
\family typewriter 
double
\family default 
 value in local variable 2 (because local variable 3 would have to be occupied,
 too).
\layout Subsubsection

Instance Initialization and Newly Created Objects
\layout Standard

It would be difficult to verify that a newly created instance is initialized
 exactly once, given all possible paths of execution flow in a method.
 Fortunately (from a verifier implementor's view), Sun puts constraints
 on object initialization that match the behaviour of the verifier --- instead
 of putting sane constraints on object initialization and actually verifying
 them.
\layout Standard


\begin_inset Quotes eld
\end_inset 

A valid instruction sequence must not have an uninitialized object on the
 operand stack or in a local variable during a backwards branch [\SpecialChar \ldots{}
].
 Otherwise, a devious piece of code might fool the verifier into thinking
 it had initialized a class instance when it had, in fact, initialized a
 class instance created in a previous pass through a loop
\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 148).
\layout Section


\begin_inset LatexCommand \label{Pass4Spec}

\end_inset 

Pass Four
\layout Standard

Pass four performs 
\begin_inset Quotes eld
\end_inset 

certain tests that could in principle be performed in Pass 3
\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 142).
 These tests are usually delayed by JVM implementations until run-time,
 because they possibly trigger the loading of referenced class file definitions.
 This is a performance enhancement.
 However, 
\begin_inset Quotes eld
\end_inset 

A Java virtual machine implementation is allowed to perform any or all of
 the Pass 4 steps as part of Pass 3
\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 143).
 The tests
\layout Itemize

ensure that the referenced method or field exists in the given class
\layout Itemize

check that the referenced method or field has the indicated descriptor (signatur
e)
\layout Itemize

check that the currently executing method has access to the referenced method
 or field.
\layout Standard

JustIce has no run-time system and so the tests of pass four are performed
 in pass 3a.
\layout Standard

There are tests that have to be performed at run-time: for example, if an
 object referenced by an object reference on top of the operand stack implements
 a certain interface or not 
\begin_inset LatexCommand \cite{Fong2-WWW}

\end_inset 

.
 These are not considered part of the pass four verification.
\layout Chapter

Implementation of the Verification Passes
\layout Standard

Occasionally, the behaviour of other verifier implementations was explained
 in section 
\begin_inset LatexCommand \ref{SpecPasses}

\end_inset 


\emph on 
.
 
\emph default 
This is not a mistake; the Java Virtual Machine Specification, Second Edition
 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

 is unfortunately not detailed enough to make a clean-room implementation
 of the JVM verifier possible.
 Having a close look at the behaviour of existing verifier implementations
 is sometimes necessary to interpret the specification correctly.
 For that reason, the behaviour of these implementations is part of the
 specification of JustIce whereever appropriate.
 Still, there are some minor differences in behaviour between JustIce and
 the traditional JVM built-in verifiers.
 These differences were observed by using the traditional verifiers, not
 by inspecting their source code.
\layout Standard

JustIce is implemented in the Java programming language 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

 using the Byte Code Engineering Library 
\begin_inset LatexCommand \cite{BCEL-WWW,BCEL98}

\end_inset 

.
\layout Section

Pass One
\layout Standard

The Byte Code Engineering Library (BCEL) presents an object oriented view
 of the class file structure.
 Therefore, an integral part of that library is parsing class files.
 JustIce uses the BCEL, so there was nothing left to do to load a class
 file in.
 Only minor changes were made to the BCEL to make it more verbose when exception
al situations occur; i.e., when a garbled class file is loaded in.
 The BCEL uses Java's exception mechanism to signal these situations; JustIce
 transforms this behaviour into the behaviour expected by users of the Verificat
ion API (see section 
\begin_inset LatexCommand \ref{Verification API}

\end_inset 

).
\layout Subsubsection

Comparison to Sun's Implementation
\layout Standard

There does not seem to be any difference in behaviour between JustIce and
 the traditional verifiers.
 Still, this conviction is a result of black box tests so it might not be
 true in corner cases.
\layout Standard

Unknown attributes are ignored (though JustIce records a warning message,
 where the traditional verifiers don't).
\layout Standard

Trailing bytes at the end of the class file are ignored in both versions,
 contradicting the specification.
 This was necessary because some Java run-time environments are broken concernin
g the handling of .JAR archive files.
 The mechanism of loading class files from these archives files using the
 Java Platform's API is used by BCEL and probably by Sun's JVM, too.
 It is possible that this is the reason why Sun's verifier itself does not
 enforce this constraint.
 However, it does not really pose a threat to the integrity of any JVM known
 to the author.
 There is no entry in the 
\family typewriter 
ClassFile
\family default 
 structure (see section 
\begin_inset LatexCommand \ref{Classfile Structure}

\end_inset 

) stating how long the class file is in its entirety, so a JVM implementor
 cannot possibly base a wrong decision on that.
 
\layout Section


\begin_inset LatexCommand \label{Pass2Impl}

\end_inset 

Pass Two
\layout Standard

JustIce does perform 
\begin_inset Quotes eld
\end_inset 

all verification that can be performed without looking at the bytecodes
\begin_inset Quotes erd
\end_inset 

 in pass two.
 For some reasons (like determining a valid ancestor hierarchy of a class),
 pass two of JustIce has to load referenced classes.
 Of course, this is done in a careful way: by pass-one-verifying them.
 If loading of a referenced class should fail (i.e., verification pass one
 fails on this class), the referencing class is rejected by JustIce's pass
 two.
 Pass two of JustIce does not pass-two-verify any referenced classes.
\layout Standard

Also, JustIce's pass two emits a wealth of (warning) messages.
 Their target is to guide a bytecode engineer to create class files that
 are indistinguishable from those created by Sun's 
\emph on 
javac
\emph default 
 compiler with no debugging output.
 For example, the use of 
\family typewriter 
LineNumberTable
\family default 
 attributes (see section 
\begin_inset LatexCommand \ref{LineNumberTableAttribute}

\end_inset 

) is discouraged, because these atributes are only useful for debugging
 purposes.
 Still, they can be the reason for a class file to be rejected -- to be
 on the safe side, finished applications for the JVM should not be shipped
 with this debug information.
\layout Standard

Most of the checks of pass two were implemented using the Visitor programming
 pattern 
\begin_inset LatexCommand \cite{DesignPatterns}

\end_inset 

 provided by the BCEL's 
\emph on 
de.fub.byte\SpecialChar \-
code.class\SpecialChar \-
file
\emph default 
 API.
 This made it possible to have all the verification split into several methods
 without having to define artificial boundaries.
 For instance, a 
\family typewriter 
ConstantValue
\family default 
 attribute is verified in a method called 
\emph on 
visitConstantValue(ConstantValue)
\emph default 
.
 This is a use of the object oriented view of class files the BCEL offers.
\layout Subsubsection

Comparison to Sun's Implementation
\layout Standard

JustIce does not distinguish between run-time or link-time because it was
 not intended to implement a JVM.
 Therefore, the notion of 
\emph on 
resolving 
\emph default 
(see section 
\begin_inset LatexCommand \ref{SpecPassTwo}

\end_inset 

) is useless for JustIce.
 The author believes that the specification of pass two given by Sun closely
 reflects their implementation (or the other way around)
\begin_float footnote 
\layout Standard

The Java Virtual Machine Specification, Second Edition, began as an internal
 project documentation (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page xiv).
 Unfortunately, this can still be felt sometimes.
\end_float 
.
\layout Standard

Sometimes, there are ambiguities in the specification.
 For instance, it is said that 
\begin_inset Quotes eld
\end_inset 

If the constant pool of a class or interface refers to any class or interface
 that is not a member of a package, its 
\family typewriter 
ClassFile
\family default 
 structure must have exactly one 
\family typewriter 
InnerClasses
\family default 
 attribute in its 
\family typewriter 
attributes
\family default 
 table
\begin_inset Quotes erd
\end_inset 

.
 A class or interface that is 
\begin_inset Quotes eld
\end_inset 

not member of a package
\begin_inset Quotes erd
\end_inset 

 is better known as a 
\emph on 
nested class
\emph default 
 or 
\emph on 
inner class
\emph default 
 
\begin_inset LatexCommand \cite{InnerSpec}

\end_inset 

, but this is something specific to the Java language.
 The 
\emph on 
javac
\emph default 
 compiler creates multiple, often funny-named
\begin_float footnote 
\layout Standard

For anonymous classes defined in a class 
\emph on 
X
\emph default 
 the names are 
\emph on 
X$1
\emph default 
, 
\emph on 
X$2
\emph default 
 and so on.
 For a named inner class 
\emph on 
I
\emph default 
 defined in class 
\emph on 
C
\emph default 
 the name is 
\emph on 
C$I
\emph default 
.
 There is, however, no guarantee for that: this is only observed behaviour
 of javac.
 Please see section 
\begin_inset LatexCommand \ref{InnerBug}

\end_inset 

 for an example how this behaviour can lead to unexpected problems.
\end_float 
 class files that are otherwise indistinguishable from normal class files.
\layout Standard

Therefore, it is generally not possible to decide if such an attribute is
 missing; therefore Sun's implementation does not check this constraint.
 JustIce, in contrast, uses its warning mechanism if the name of a referenced
 class or interface could be a name of an inner class created by the 
\emph on 
javac
\emph default 
 compiler and the 
\family typewriter 
InnerClass
\family default 
 attribute is missing.
\layout Standard

The sets of accepted or rejected class files concerning pass two are equal
 using both Sun's implementation and JustIce, as exhaustive tests show.
 This can, however, not be proven because one would need to analyze Sun's
 source code for that (which is not intended: as already mentioned, JustIce
 is a clean-room implementation).
\layout Section

Pass Three
\layout Subsection

Pass 3a
\layout Standard

One feature of the BCEL's 
\emph on 
de.fub.bytecode.generic
\emph default 
 package is parsing code attributes of methods and transforming them into
 so-called 
\family typewriter 
Instruction\SpecialChar \-
List
\family default 
 objects.
 Consequently, this feature is used to implement pass 3a; a few additional
 checks have been implemented where BCEL is too 
\begin_inset Quotes eld
\end_inset 

trustful
\begin_inset Quotes erd
\end_inset 

 when parsing, i.e., where BCEL relies on the correctness of the class file.
\layout Standard

Pass 3a consists of the checking of static constraints on instructions and
 static constraints on operands of these instructions.
 The successful creation an an 
\family typewriter 
Instruction\SpecialChar \-
List
\family default 
 object already implies that the static constraints on instructions are
 satisfied.
 Similar to pass one, JustIce transforms the behaviour of BCEL's exception
 mechanism into the behaviour expected by users of the Verification API
 (see section 
\begin_inset LatexCommand \ref{Verification API}

\end_inset 

).
\layout Standard

The 
\emph on 
de.fub.byte\SpecialChar \-
code.ge\SpecialChar \-
ne\SpecialChar \-
ric 
\emph default 
API provided by BCEL offers a Visitor design pattern similar to the one
 of the 
\emph on 
de.fub.byte\SpecialChar \-
code.class\SpecialChar \-
file
\emph default 
 API.
 The tests for the static constraints on operands of instructions are implemente
d by using it.
 For example, the constraints put on the operands of any 
\latex latex 

\backslash 
texttt{iload}
\latex default 
 instruction are verified using a 
\emph on 
visitILOAD(ILOAD)
\emph default 
 method defined in a Visitor class.
 This Visitor class implements all the checks for integrity of all instruction's
 operands.
 Algorithm 
\begin_inset LatexCommand \ref{visitILOADstaticoperands}

\end_inset 

 shows the impementation of the 
\emph on 
visitILOAD(ILOAD)
\emph default 
 method.
\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{visitILOADstaticoperands}

\end_inset 

visitILOAD, Visitor ensuring static constraints on operands of instructions
\layout Standard


\family typewriter 
\SpecialChar \-
\SpecialChar ~
/** Checks if the constraints of operands of the said instruction(s) are
 satisfied.
 */
\newline 
\SpecialChar \-
public void visitILOAD(ILOAD o){
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
int idx = o.getIndex();
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
if (idx < 0){
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
constraintViolated(o, "Index '"+idx+"' must be non-negative.");
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
else{
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
int maxminus1 = max_locals()-1;
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (idx > maxminus1){
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
constraintViolated(o, "Index '"+idx+"' must not be greater than max_locals-1
 '"+maxminus1+"'.");
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
} 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\newline 
}
\end_float 
\layout Standard

JustIce does not provide any run-time, so the tests of pass four (see section
 
\begin_inset LatexCommand \ref{Pass4Spec}

\end_inset 

) are not delayed until run-time, but performed here.
\layout Subsubsection

Comparison to Sun's Implementation
\layout Standard

Sun does not distinguish pass 3a and pass 3b.
 However, Sun's verifiers also have to ensure that the static constraints
 on instructions are satisfied before starting data flow analysis.
\layout Standard

This is obvious because a data structure has to be built before the data
 flow analyzer can be run; and this data structure has to be built carefully
\begin_float footnote 
\layout Standard

This actually means verifying the structural integrity of the bytecodes.
\end_float 
 because passes one and two did not look at the bytecodes before.
\layout Standard

JustIce does implement pass four checks in pass 3a which Sun's verifiers
 do not.
 Because JustIce provides no run-time, the outcome of a verification failure
 is reported instantly.
 Traditional JVMs are required to silently delay the actions triggered by
 that knowledge until run-time.
\layout Subsection

Pass 3b
\layout Standard

JustIce aims at implementing Sun's data flow analyzing algorithm as closely
 as possible.
 First, a control flow graph is built --- which implies analyzing a method's
 subroutine calling structure first.
\layout Standard

After that an implementation of the core algorithm sketched by Sun Microsystems
 is started.
 Verification failure is internally signalled by the Java exception handling
 mechanism which is then transformed to match the Verification API (see
 section 
\begin_inset LatexCommand \ref{Verification API}

\end_inset 

).
\layout Subsubsection


\begin_inset LatexCommand \label{SubroutineImpl}

\end_inset 

Subroutines
\layout Standard

Subroutines are modeled as instances of the 
\family typewriter 
Subroutine
\family default 
 interface
\emph on 
.

\emph default 
 They provide the following methods (note that an 
\family typewriter 
InstructionHandle
\family default 
 is the BCEL's programming handle to instruction objects and that 
\emph on 
X[]
\emph default 
 is the common Java notation for 
\emph on 
array of
\emph default 
 
\emph on 
X
\emph default 
):
\layout Itemize


\emph on 
boolean contains(InstructionHandle)
\emph default 

\newline 
Returns true if and only if the given 
\family typewriter 
InstructionHandle
\family default 
 refers to an instruction that is part of this subroutine,
\layout Itemize


\emph on 
InstructionHandle[] getInstructions()
\emph default 
 
\newline 
Returns all instructions that together form this subroutine,
\layout Itemize


\emph on 
int[] getAccessedLocalsIndices()
\emph default 

\newline 
Returns an array containing the indices of the local variable slots accessed
 by this subroutine (read-accessed, write-accessed or both); local variables
 referenced by subroutines of this subroutine are not included,
\layout Itemize


\emph on 
int[] getRecursivelyAccessedLocalsIndices()
\emph default 
 
\emph on 

\newline 

\emph default 
Returns an array containing the indices of the local variable slots accessed
 by this subroutine (read-accessed, write-accessed or both); local variables
 referenced by subroutines of this subroutine are included,
\layout Itemize


\emph on 
Subroutine[] subSubs()
\emph default 
 
\emph on 

\newline 

\emph default 
Returns the subroutines that are directly called from this subroutine,
\layout Itemize


\emph on 
InstructionHandle[] getEnteringJsrInstructions() 
\emph default 

\newline 
Returns all the JsrInstructions that have the first instruction of this
 subroutine as their target,
\layout Itemize


\emph on 
InstructionHandle getLeavingRET() 
\emph default 

\newline 
Returns the one and only RET that leaves the subroutine.
\layout Standard

Together with information from a simple analysis of the possible control
 flow transfer of all the other instructions but 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 (see section 
\begin_inset LatexCommand \ref{Pass3Spec}

\end_inset 

), a control flow graph is built.
\layout Subsubsection

The Control Flow Graph
\layout Standard

The control flow graph is a single instance with respect to a given method
 to verify.
 It is defined by providing access to a set of contexts of instructions.
 These are modeled as instances of the
\emph on 
 
\family typewriter 
\emph default 
In\SpecialChar \-
struc\SpecialChar \-
tion\SpecialChar \-
Con\SpecialChar \-
text
\family default 
 interface.
\layout Standard

These instances enclose 
\family typewriter 
InstructionHandle
\family default 
 objects (which represent an instruction in the bytecode), but they augment
 these objects with type information (a set of 
\family typewriter 
Frame
\family default 
s, see below) as needed by the data flow analysis algorithm.
 Also, a method called 
\emph on 
getSuccessors() 
\emph default 
is provided that calculates the possible control flow successors of a given
 
\family typewriter 
In\SpecialChar \-
struc\SpecialChar \-
tion\SpecialChar \-
Con\SpecialChar \-
text
\family default 
 instance.
\layout Standard

The most notable method defined in the 
\family typewriter 
In\SpecialChar \-
struc\SpecialChar \-
tion\SpecialChar \-
Con\SpecialChar \-
text
\family default 
\emph on 
 
\emph default 
interface is, however, the 
\emph on 
execute(Frame, ArrayList, InstConstraintVisitor, ExecutionVisitor)
\emph default 
 method.
 This method is used to symbolically execute a given instruction.
\layout Standard

The 
\family typewriter 
ArrayList
\family default 
\emph on 
 
\emph default 
argument is there to record the subroutine calling chain.
 The properly-nested property of JustIce subroutines is exploited here:
 one can simply count 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
/
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 and 
\latex latex 

\backslash 
texttt{ret}
\latex default 
 instructions, similar to counting opened and closed braces in mathematical
 expressions.
\layout Standard

A 
\family typewriter 
Frame
\family default 
 is JustIce's model of an 
\emph on 
execution frame
\emph default 
: a local variables array model together with an operand stack model.
 Every 
\emph on 
InstructionContext
\emph default 
 instance is augmented with such a frame (to be precise, a set of such frames
 as discussed in the specification of subroutines, see section 
\begin_inset LatexCommand \ref{Pass3Spec}

\end_inset 

).
\layout Standard

When frames are merged, the 
\emph on 
execute(Frame, ArrayList, InstConstraintVisitor, ExecutionVisitor) 
\emph default 
method of some successor 
\family typewriter 
InstructionContext
\family default 
 is called.
 The 
\family typewriter 
Frame
\family default 
 argument represents is the current type information of the predecessing
 
\family typewriter 
InstructionContext.
\layout Subsubsection

Visitors
\layout Standard

As in pass 3a, the Visitor pattern of the BCEL 
\emph on 
de.fub.byte\SpecialChar \-
code.ge\SpecialChar \-
ne\SpecialChar \-
ric
\emph default 
 API is also used in pass 3b.
 While it was used to verify the static constraints of pass three in pass
 3a, it is now used to verify the structural constraints.
\layout Standard

Before an instruction 
\family typewriter 
X
\family default 
 is symbolically executed, the corresponding 
\emph on 
visitX(X)
\emph default 
 method is invoked on an 
\family typewriter 
InstConstraintVisitor
\family default 
 instance.
 This instance is there to verify all the preconditions are met to safely
 execute the instruction 
\family typewriter 
X
\family default 
.
 The 
\family typewriter 
InstConstraintVisitor
\family default 
 class therefore holds information about the preconditions of all 212 valid
 Java bytecode instructions.
 A simplified version of this Visitor's 
\emph on 
visitILOAD(ILOAD)
\emph default 
 method is listed in algorithm 
\begin_inset LatexCommand \ref{visitILOADInstConstraints}

\end_inset 

.
\layout Standard

Similarly, the
\emph on 
 
\family typewriter 
\emph default 
ExecutionVisitor
\family default 
 class contains information about the behaviour of every bytecode instruction.
 An instance of this class is used to model the effect of the bytecode instructi
ons on a 
\emph on 
Frame
\emph default 
 instance.
 Algorithm 
\begin_inset LatexCommand \ref{visitILOADExecution}

\end_inset 

 shows the 
\emph on 
visitILOAD(ILOAD)
\emph default 
 method of this Visitor.
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{visitILOADInstConstraints}

\end_inset 

visitILOAD, Visitor ensuring the structural (dynamic) constraints of instruction
s
\layout Standard


\family typewriter 
public void visitILOAD(ILOAD o){
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
int produce = o.produceStack(cpg);
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if ( produce + stack().slotsUsed() > stack().maxStack() ){ 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
constraintViolated(o, "Cannot produce "+produce+" stack slots: only "+(stack().ma
xStack()-stack().slotsUsed())+" free stack slot(s) left.
\backslash 
nStack:
\backslash 
n"+stack()); 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
[\SpecialChar \ldots{}
]
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\end_float 
\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{visitILOADExecution}

\end_inset 

visitILOAD, Visitor symbolically executing instructions
\layout Standard


\family typewriter 
/** Symbolically executes the corresponding Java Virtual Machine instruction.
 */ 
\newline 
\SpecialChar \-
public void visitILOAD(ILOAD o){ 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
stack().push(Type.INT);
\newline 
\SpecialChar \-
}
\end_float 
\begin_float alg 
\layout Caption

Simplified Core Verification Algorithm of Pass 3b
\layout Standard


\series bold 
\size small 
public VerificationResult do_verify(Method m)
\series default 
{
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
ControlFlowGraph cfg;
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
if (m.hasCode()) 
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
cfg = new ControlFlowGraph(m)
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
else
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
return Good_VerificationResult;
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
Frame f = new Frame(); 
\shape slanted 
// local variables and operand stack
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
f.localVariables().initialize(m.signature()); 
\shape slanted 
// put formal param types into loc.
 vars
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
InstConstraintVisitor icv = new InstConstraintVisitor();
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
ExecutionVisitor ev = new ExecutionVisitor();
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
try{
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
circulationPump(cfg, f, icv, ev);
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
catch(VerificationFailure){
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
return Bad_VerificationResult;
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
return Good_VerificationResult;
\layout Standard


\size small 
} 
\newline 

\layout Standard


\series bold 
\size small 
public void circulationPump(ControlflowGraph cfg, Frame startFrame, InstConstrai
ntVisitor icv, ExecutionVisitor ev) throws VerificationFailure
\series default 
{
\layout Standard


\size small 
Instruction start = cfg.getFirstInstruction();
\layout Standard


\shape slanted 
\size small 
/*
\layout Standard


\shape slanted 
\size small 
Now merge the first frame (type info) into the first instruction.
\layout Standard


\shape slanted 
\size small 
Empty list -> no instructions have been executed before.
\layout Standard


\shape slanted 
\size small 
*/
\layout Standard


\size small 
start.execute(startFrame, EmptyInstructionList, icv, ev); 
\layout Standard


\shape slanted 
\size small 
/*
\layout Standard


\shape slanted 
\size small 
Q is a Queue of pairs (Instruction, InstructionList).
\layout Standard


\shape slanted 
\size small 
*/
\layout Standard


\size small 
Queue Q = EmptyQueue;
\layout Standard


\shape slanted 
\size small 
/*
\layout Standard


\shape slanted 
\size small 
Put the first instruction into the queue.
 This is similar to initializing a breadth first search.
\layout Standard


\shape slanted 
\size small 
*/
\layout Standard


\size small 
Q.add (start, EmptyInstructionList); 
\layout Standard


\shape slanted 
\size small 
/*
\layout Standard


\shape slanted 
\size small 
The main loop
\layout Standard


\shape slanted 
\size small 
*/
\layout Standard


\size small 
while (Q.isNotEmpty()){
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
Instruction u = fst(Q.head());
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
InstructionList ec = snd(Q.head());
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
Q.removeHead();
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
InstructionList oldchain = ec;
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
InstructionList newchain = ec++[u];
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
for (all successors v of u){
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~

\shape slanted 
/*
\layout Standard


\shape slanted 
\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
execute returns true if type info has changed.
 It may throw VerificationFailures.
\layout Standard


\shape slanted 
\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
*/
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (v.execute(u.getOutFrame(oldchain), newchain,icv,ev))
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
Q.add((v, newchain));
\layout Standard


\size small 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
}
\layout Standard


\size small 
}
\end_float 
\layout Subsubsection


\begin_inset LatexCommand \label{ComparisonSubroutines}

\end_inset 

Comparison to Sun's Implementation
\layout Standard

JustIce was originally aimed to be as compatible to Sun's implementation
 as possible.
 However, the unclear specification prevents clean room implementations
 (i.e., implementations whose programmers did not look into Sun's code) from
 perfect compatibility.
\layout Standard

Fortunately, it JustIce closely matches Sun's implementation in its behaviour.
 As a test case, the author verified the transitive hull of the referenced
 class files starting with the 
\emph on 
de.fub.bytecode.verifier.Verifier
\emph default 
 class.
 This set includes most of the classes of the Java 2 API supplied by Sun
 Microsystems, i.e., a few hundreds of apparently correct classes.
 A very small number of class files was rejected by JustIce because of its
 different specification of subroutine constraints.
 No other rejects were encountered.
\layout Standard

Most class files that are found to be rejected by Sun's verifier implementations
 are rejected by JustIce, too.
\layout Standard

However, there are class file rejected by Sun's verifier implementations
 but not by JustIce.
 This should not occur, but JustIce does not mimic the programming errors
 of Sun's verifiers so far.
 Please see section 
\begin_inset LatexCommand \ref{javacRejected}

\end_inset 

 for a discussion on a selected incompatibility issue.
\layout Standard

An automated testing suite could solidify the trust in JustIce's implementation
 which is not implemented yet.
 Please see section 
\begin_inset LatexCommand \ref{VerifierValidationSuite}

\end_inset 

 for a discussion on that topic.
\layout Section

Pass Four
\layout Standard

The tests Sun's verifiers perform during run-time but which in principle
 could be performed in pass three 
\emph on 
are
\emph default 
 performed in pass 3a by JustIce.
\layout Subsubsection

Comparison to Sun's Implementation
\layout Standard

It sems natural that Sun's verifier implements the specification by Sun.
 Obviously, JustIce has no run-time so JustIce has no pass four.
 The checks Sun performs in pass four
\begin_float footnote 
\layout Standard

Some JVMs expose implementation mistakes concerning pass four verification.
 See section 
\begin_inset LatexCommand \ref{PassFourBug}

\end_inset 

.
\end_float 
 are performed in pass 3a by JustIce.
\layout Chapter


\begin_inset LatexCommand \label{Verification API}

\end_inset 

The Verification API
\layout Section

Introduction
\layout Standard

The Application Programming Interface (API) of JustIce uses object oriented
 design patterns 
\begin_inset LatexCommand \cite{DesignPatterns}

\end_inset 

.
 Readers not familiar with design patterns are encouraged to read at least
 about the 
\emph on 
Visitor
\emph default 
, 
\emph on 
Singleton
\emph default 
, 
\emph on 
Observer
\emph default 
 and 
\emph on 
Factory
\emph default 
 patterns.
\layout Standard

JustIce currently consists of four packages: 
\emph on 
de.fub.byte\SpecialChar \-
code.veri\SpecialChar \-
fier
\emph default 
, 
\emph on 
de.fub.
 byte\SpecialChar \-
code.veri\SpecialChar \-
fier.exc
\emph default 
, 
\emph on 
de.fub.byte\SpecialChar \-
code.veri\SpecialChar \-
fier.statics
\emph default 
 and 
\emph on 
de.fub.byte\SpecialChar \-
code.veri\SpecialChar \-
fier.
 struc\SpecialChar \-
tu\SpecialChar \-
rals
\emph default 
.
 (We shall from now on omit the preceding 
\emph on 
de.fub.byte\SpecialChar \-
code
\emph default 
.) The most important of them is the 
\emph on 
verifier
\emph default 
 package.
 The class 
\family typewriter 
VerifierFactory
\family default 
 can be found here; this is the place where all verification starts.
 The 
\family typewriter 
Veri\SpecialChar \-
fier\SpecialChar \-
Fac\SpecialChar \-
tory
\family default 
 creates 
\family typewriter 
Verifier
\family default 
 instances; only the 
\family typewriter 
VerifierFactory
\family default 
 can create these instances.
 A 
\family typewriter 
Verifier
\family default 
 instance, in turn, has a one-to-one relationship with a class file to verify,
 
\begin_inset Quotes eld
\end_inset 

its class
\begin_inset Quotes erd
\end_inset 

.
 You can instruct a 
\family typewriter 
Verifier
\family default 
 instance to run a verification pass on its class yielding a 
\family typewriter 
VerificationResult
\family default 
.
\layout Standard

All class files are fetched from the BCEL's class file repository, i.e., the
 class 
\family typewriter 
Re\SpecialChar \-
po\SpecialChar \-
si\SpecialChar \-
to\SpecialChar \-
ry
\family default 
.
 The class files stored there are either put there by the user or they are
 read from the file system.
 For a bytecode engineer who uses the BCEL this is convenient, because one
 does not have to save the dynamically created class file first in order
 to load it into JustIce.
\layout Standard

Pass 1 and pass 2 are related to the 
\family typewriter 
ClassFile
\family default 
 structure as such; passes 3a and 3b verify the bytecode of a method.
 If a class file was created using the BCEL, the BCEL user already knows
 how the 
\family typewriter 
JavaClass
\family default 
 object looks like
\begin_float footnote 
\layout Standard

A 
\family typewriter 
JavaClass
\family default 
 object represents a class file in the BCEL.
\end_float 
.
 The number of methods is known and the order of the methods in the class
 file is known.
\layout Standard

However, if this is not the case, one usually does not know the number of
 methods in a class file or the order of these methods.
 To carefully extract this information from an untrusted class file, one
 should first let a pass-2-verification run on this file.
 Afterwards, the information can be read from the 
\family typewriter 
JavaClass
\family default 
 object the BCEL offers.
\layout Standard

Finally, one is able to supply the 
\begin_inset Quotes eld
\end_inset 

method index
\begin_inset Quotes erd
\end_inset 

 needed by verification passes 3a and 3b.
\layout Standard

Basically, after pass 2 has been run successfully on a class file, one can
 safely use the methods in the BCEL's
\emph on 
 classfile 
\emph default 
package
\emph on 
 
\emph default 
on that class file.
 After pass 3a has been run successfully on a method, one can safely work
 on that method using the BCEL's 
\emph on 
generic
\emph default 
 package.
 After pass 3b has been run successfully on all methods in a class file,
 this class file will not be rejected by other verifiers.
\layout Standard

Often, the run of a verification pass implies recursively verifying other
 class files as well (because they are somehow referenced).
 Therefore, 
\emph on 
Verifier
\emph default 
 instances for these referenced classes are created transparently.
 To be notified when such an event occurs, one can implement the 
\emph on 
VerifierFactoryObserver 
\emph default 
interface and let the 
\emph on 
VerifierFactory
\emph default 
 register your implementation.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 863
file VerificationAPI.eps
width 3 100
angle 90
flags 1

\end_inset 


\layout Caption

UML class diagram of the Verification API
\end_float 
\layout Standard

A Verifier creates instances of PassVerifiers.
 A PassVerifier instance in charge of performing some later verification
 pass transparently creates PassVerifier instances for the preceding passes.
 Therefore, users of the Verification API do not have to care about the
 order of verification passes; i.e., earlier passes are run always before
 later passes.
 All verification results are cached; this way an unsual order of calls
 to the 
\emph on 
doPassX()
\emph default 
 methods of the 
\emph on 
Verifier
\emph default 
 class does not even waste computing time.
\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 631
file V_API_SD.eps
width 3 100
height 3 75
flags 9

\end_inset 


\layout Caption

Informal UML sequence diagram showing the dependency of verification pass
 two on verification pass one.
\end_float 
\layout Section

Some Example Code
\layout Standard

The code below shows an example of how to use the API provided by JustIce.
 It will verify the transitive hull of all referenced class files.
 Normally, while verifying a class, referenced classes are recursively verified
 performing 
\emph on 
earlier
\emph default 
 passes.
 Verifiers that are using pass 1 on their class will not load in any other
 classes (see section 
\begin_inset LatexCommand \ref{SpecPasses}

\end_inset 

).
 Therefore, normally the transitive hull is 
\emph on 
not
\emph default 
 verified completely (it usually does not make sense to verify it, though
 -- it's done here only to give an example of what can be done).
\family typewriter 
\size small 

\newline 

\newline 
01\SpecialChar ~
package de.fub.bytecode.verifier;
\newline 
02\SpecialChar ~
import de.fub.bytecode.verifier.*; 
\newline 
03\SpecialChar ~
import de.fub.bytecode.classfile.*; 
\newline 
04\SpecialChar ~
import de.fub.bytecode.*;
\newline 
05\SpecialChar ~
/** 
\newline 
06\SpecialChar ~
\SpecialChar ~
* This class has a main method implementing a demonstration program
\newline 
07\SpecialChar ~
\SpecialChar ~
* of how to use the VerifierFactoryObserver.
 It transitively verifies
\newline 
08\SpecialChar ~
\SpecialChar ~
* all class files encountered; this may take up a lot of time and,
\newline 
09\SpecialChar ~
\SpecialChar ~
* more notably, memory.
 
\newline 
10\SpecialChar ~
\SpecialChar ~
* 
\newline 
11\SpecialChar ~
\SpecialChar ~
* @author Enver Haase
\newline 
12\SpecialChar ~
\SpecialChar ~
*/ 
\newline 
13\SpecialChar ~
public class TransitiveHull implements VerifierFactoryObserver{
\newline 
14\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
/** Used for indentation.
 */
\newline 
15\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
private int indent = 0;
\newline 
16\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
/** Not publicly instantiable.
 */ 
\newline 
17\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
private TransitiveHull(){ } 
\newline 
18
\newline 
19\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
/* Implementing VerifierFactoryObserver.
 */ 
\newline 
20\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
public void update(String classname){
\newline 
21\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
for (int i=0; i<indent; i++) { 
\newline 
22\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.print(" "); 
\newline 
23\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
} 
\newline 
24\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println(classname); 
\newline 
25\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
indent += 1;
\newline 
26\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
Verifier v = VerifierFactory.getVerifier(classname); 
\newline 
27\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
VerificationResult vr; 
\newline 
28\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
vr = v.doPass1(); 
\newline 
29\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (vr != VerificationResult.VR_OK) 
\newline 
30\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println("Pass 1:
\backslash 
n"+vr);
\newline 
31\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
vr = v.doPass2(); 
\newline 
32\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (vr != VerificationResult.VR_OK)
\newline 
33\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println("Pass 2:
\backslash 
n"+vr);
\newline 
34\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (vr == VerificationResult.VR_OK){
\newline 
35\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
JavaClass jc = Repository.lookupClass(v.getClassName()); 
\newline 
36\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
for (int i=0; i<jc.getMethods().length; i++){ 
\newline 
37\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
vr = v.doPass3a(i); 
\newline 
38\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (vr != VerificationResult.VR_OK)
\newline 
39\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println(v.getClassName()+", Pass 3a, method "+
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
i+" ['"+jc.getMethods()[i]+"']:
\backslash 
n"+vr);
\newline 
40\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
vr = v.doPass3b(i);
\newline 
41\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (vr != VerificationResult.VR_OK)
\newline 
42\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println(v.getClassName()+", Pass 3b, method "+
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
i+" ['"+jc.getMethods()[i]+"']:
\backslash 
n"+vr); 
\newline 
43\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
} 
\newline 
44\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
45\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
indent -= 1;
\newline 
46\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
47
\newline 
48\SpecialChar ~
\SpecialChar ~
/**
\newline 
49\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
* This method implements a demonstration program
\newline 
50\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
* of how to use the VerifierFactoryObserver.
 It transitively 
\newline 
51\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
* verifies all class files encountered; this may take up a
\newline 
52\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
* lot of time and, more notably, memory.
 
\newline 
53\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
*/ 
\newline 
54\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
public static void main(String[] args){ 
\newline 
55\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (args.length != 1){
\newline 
56\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.out.println("Need exactly one argument: The root class 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
to verify."); 
\newline 
57\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
System.exit(1);
\newline 
58\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
59\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
int dotclasspos = args[0].lastIndexOf(".class"); 
\newline 
60\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (dotclasspos != -1)
\newline 
61\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
args[0] = args[0].substring(0,dotclasspos); args[0] = 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
args[0].replace('/', '.'); 
\newline 
62\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
TransitiveHull th = new TransitiveHull(); 
\newline 
63\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
VerifierFactory.attach(th); 
\newline 
64\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
VerifierFactory.getVerifier(args[0]); // the observer is called 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
back and does the actual trick.
\newline 
65\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
VerifierFactory.detach(th); 
\newline 
66\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
67\SpecialChar ~
}
\newline 

\layout Standard


\size small 
First, an instance of the 
\emph on 
TransitiveHull
\emph default 
 class is created in line 62.
 Note that this class implements the 
\emph on 
VerifierFactoryObserver
\emph default 
 interface.
\layout Standard


\size small 
A reference to the newly created instance is then passed to the 
\emph on 
VerifierFactory
\emph default 
 in line 63 by invoking its 
\emph on 
attach(VerifierFactoryObserver)
\emph default 
 method.
 After registering the new observer, the 
\emph on 
VerifierFactory
\emph default 
 will call the instance's
\emph on 
 update(String)
\emph default 
 method (defined in lines 20-46) whenever a new 
\emph on 
Verifier
\emph default 
 instance is created.
\layout Standard


\size small 
To trigger the verification, a first 
\emph on 
Verifier
\emph default 
 instance is fetched from the 
\emph on 
VerifierFactory
\emph default 
.
 Because it is the very first 
\emph on 
Verifier
\emph default 
 instance that is fetched, we know that it has to be newly created.
 This is done in line 64.
 This instance is not used in the 
\emph on 
main(String[])
\emph default 
 method; but its creation leads to a invocation of the 
\emph on 
update(String)
\emph default 
 method which is defined in lines 20-46.
\layout Standard

There, the name of the class to verify is printed (lines 21-25, line 45)
 and the four verification passes provided by JustIce are run.
 Note that one has to be careful not to try to verify a method that does
 not exist.
 JustIce would in this case throw an 
\emph on 
InvalidMethodException
\emph default 
.
 Therefore, after successfully verifying that the structure of the class
 file to verify is well-formed (verification up to and including pass two,
 lines 26-31), the number of methods is fetched from the corresponding JavaClass
 object.
 (It is necessary to perform verification pass two on a class file to safely
 find out how many methods are defined in this class file.)
\layout Standard

After determining the number of methods, these methods are verified performing
 passes 3a and 3b on them (lines 32-44).
\layout Standard

By applying all verification passes on some class file 
\emph on 
C
\emph default 
, all class files referenced by 
\emph on 
C
\emph default 
 are found.
 Therefore, new 
\emph on 
Verifier
\emph default 
 instances are created which are responsible for them.
 Because of that, the 
\emph on 
update(String)
\emph default 
 method described above is called for every referenced class.
 This is a recursive loop; the program terminates when there is no referenced
 class left to be verified.
\layout Standard

The example above is simple yet powerful.
 Admittedly, it is of limited use to verify classes provided by the JVM
 vendor; therefore one would not normally verify all the transitive hull
 of referenced class files.
 However, a common use is verifying all classes of a project.
 Inserting a new line between line 20 and 21 like
\layout Standard


\family typewriter 
if (!(classname.startsWith(
\begin_inset Quotes eld
\end_inset 

de.fub.bytecode.verifier
\begin_inset Quotes erd
\end_inset 

)) return;
\newline 

\family default 
would easily accomplish this goal if JustIce itself is the project to verify
 and all the project's class files are referenced by another class file
 in the project.
\layout Section


\begin_inset LatexCommand \label{GUI_APP}

\end_inset 

An Application Prototype
\layout Standard

The API of JustIce is used to offer bytecode engineers an opportunity to
 create their own application programs.
 However, this dimension of configurability is often not needed.
\layout Standard

JustIce comes with an application prototype which provides an easy-to-use
 user interface.
 Figures 
\begin_inset LatexCommand \ref{GUI1fig}

\end_inset 

 and 
\begin_inset LatexCommand \ref{GUI2fig}

\end_inset 

 show screen shots of this prototype built on the JustIce verifier.
 The boxes to the right contain verification information.
 From the top to the bottom the boxes represent the verification passes
 one, two, 3a and 3b and the warning messages, respectively.
\layout Standard

\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 757
file GUI1.eps
width 3 100
height 3 90
angle 90
flags 9

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{GUI1fig}

\end_inset 

Verification of the Mini.MiniParser class file.
 Verification is passed, but JustIce suggests to remove unnecessary (debug
 information) attributes.
\end_float 
\begin_float fig 
\layout Standard
\align center 

\begin_inset Figure size 595 757
file GUI2.eps
width 3 100
height 3 90
angle 90
flags 9

\end_inset 


\layout Caption


\begin_inset LatexCommand \label{GUI2fig}

\end_inset 

Verification of the java.io.ObjectInputStream class file.
 Verification is not passed because of an unsatisfied constraint related
 to subroutines.
\end_float 
\layout Chapter

Conclusion
\layout Section

What Was Achieved
\layout Standard

About a third of the development time of JustIce was spent examining the
 various issues in connection with subroutines, i.e., issues concerning the
 bytecode instructions 
\latex latex 

\backslash 
texttt{jsr}
\latex default 
, 
\latex latex 

\backslash 
texttt{jsr
\backslash 
_w}
\latex default 
 and 
\latex latex 

\backslash 
texttt{ret}
\latex default 
.
 This led to a new definition of the term 
\emph on 
subroutine
\emph default 
 (section 
\begin_inset LatexCommand \ref{SpecSubroutines}

\end_inset 

)
\begin_float footnote 
\layout Standard

A request for clarification of the subroutine issue, sent to the electronic
 mail address 
\family typewriter 
jvm@java.sun.com
\family default 
 was not answered.
\end_float 
, a new implementation of this verification area (section 
\begin_inset LatexCommand \ref{SubroutineImpl}

\end_inset 

) and a discussion on the arising incompatibilities (sections 
\begin_inset LatexCommand \ref{ComparisonSubroutines}

\end_inset 

 and 
\begin_inset LatexCommand \ref{StaerkJreject}

\end_inset 

).
\layout Standard

Only a few different verifier implementations exist at all, and most of
 them are incomplete.
 JustIce is a complete class file verifier implementation including a bytecode
 verifier.
\layout Standard

The development of JustIce also led to improvements of the Byte Code Engineering
 Library 
\begin_inset LatexCommand \cite{BCEL-WWW,BCEL98}

\end_inset 

.
 For instance, the 
\family typewriter 
returnaddress
\family default 
 data type was introduced there.
 It was modeled as a parameterized type.
 Also, a programming error was repaired that led to inconsistent treatment
 of exception handlers in the BCEL.
\layout Standard

The control flow graph used by JustIce can also be used in other projects;
 the Verification API provides access to this data structure
\begin_float footnote 
\layout Standard

A 
\family typewriter 
Control\SpecialChar \-
Flow\SpecialChar \-
Graph
\family default 
 instance can be created by invoking the 
\emph on 
Control\SpecialChar \-
Flow\SpecialChar \-
Graph(Method\SpecialChar \-
Gen)
\emph default 
 constructor.
 A 
\family typewriter 
Method\SpecialChar \-
Gen
\family default 
 is the BCEL's representation of a method.
\end_float 
.
 Only because of the clarification of the subroutine issues could such a
 data structure be defined statically.
\layout Standard

As an Open Source project, JustIce provides algorithms which may be re-used
 in own projects.
 For example, every compiler targeting the JVM has to calculate the maximum
 amount of stack memory used by a method.
 This is also done by JustIce.
\layout Standard

Finally, the need for a discussion on the meaning of 
\emph on 
Java security
\emph default 
 was identified (see section 
\begin_inset LatexCommand \ref{LinePrincipleInfoHidingAndSecurity}

\end_inset 

).
\layout Section

What Could Not Be Achieved
\layout Subsection

A Constraint Database
\layout Standard

Efforts have been made to make JustIce verifier highly configurable.
 Unfortunately, this could not be accomplished by the author.
 For instance, it was planned to build a constraint database which would
 make it possible to turn on or off single checks during verification.
\layout Standard

While this might be possible in some cases, in general the constraints of
 the class file verifier are highly intertwined.
 For instance, without a well-formed constant pool one could not run the
 data flow analyzer in a sane way.
 As another example, if a user preferred not to care about stack underflow
 the verification algorithm would require complicated user interaction;
 i.e., the user would have to decide what type to put onto the simulated operand
 stack just before it is read.
\layout Standard

One could model the interdependencies of the various constraints and allow
 only groups of checks to be turned on or off together.
 However, the author doubts this could be done in a way that is not prone
 to errors and that can be validated easily.
\layout Standard

This is also the reason why only one error is reported if verification fails.
 Trying to continue verification and find more constraint violations leads
 only to consequential verification errors.
\layout Standard

JustIce implements caching of verification results.
 If a bytecode engineer works on a class file and needs to run JustIce several
 times against it, JustIce will cache the verification results of the recursivel
y referenced class files.
 Because of this, JustIce will be fast every subsequent time it is used
 to verify the class.
 This minimizes the impact of the above shortcomings.
\layout Subsection

A Perfect Verifier
\layout Standard

JustIce does not implement a perfect verifier.
 Some class files with code that is safe to execute are rejected.
 Unfortunately, there has to be some degree of uncertainty concerning which
 class files to reject.
\layout Standard

The JVM performs 
\emph on 
initialization
\emph default 
 of class files after loading and verifying them without error.
 This includes running the code in the special class initialization method
 called 
\emph on 
<clinit>
\emph default 
 if it exists (see 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 53).
 For the correct operation of the JVM it is important that this method does
 not contain an infinite loop.
 Verifying if this constraint is true is similar to the Halting Problem
 and therefore not generally computable 
\begin_inset LatexCommand \cite{Unknowable}

\end_inset 

.
 A verifier has to omit the check and pass potentially unsafe class files.
\layout Standard

For another example, consider algorithm 
\begin_inset LatexCommand \ref{StackOverflowAlgo}

\end_inset 

 below.
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{StackOverflowAlgo}

\end_inset 

Rejected class
\layout Standard


\family typewriter 
public static int always_true()
\layout Standard


\family typewriter 
Code(max_stack = 1, max_locals = 1, code_length = 2)
\layout Standard


\family typewriter 
0: iconst_1\SpecialChar ~
\SpecialChar ~
; push constant 1 onto stack
\layout Standard


\family typewriter 
1: ireturn\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; return constant 1 (
\begin_inset Quotes eld
\end_inset 

true
\begin_inset Quotes erd
\end_inset 

)
\newline 

\layout Standard


\family typewriter 
public static void good_method()
\layout Standard


\family typewriter 
0: invokestatic NewClass0.always_true ()I (18)
\layout Standard


\family typewriter 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Push 
\begin_inset Quotes eld
\end_inset 

true
\begin_inset Quotes erd
\end_inset 

 on stack
\layout Standard


\family typewriter 
3: ifne #10\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; If 
\begin_inset Quotes eld
\end_inset 

true
\begin_inset Quotes erd
\end_inset 

 is on stack jump to 10
\layout Standard


\family typewriter 
6: pop \SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; Pop a value off the stack
\layout Standard


\family typewriter 
7: goto #6 \SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; jump to 6
\layout Standard


\family typewriter 
10:return\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
; complete method
\end_float 
This code is harmless, because lines 6 and 7 can never be executed (it would
 underflow the operand stack in an infinite loop).
 A class file with this code is rejected by JustIce and other verifiers,
 because the endless loop seems to be a malicious threat to the integrity
 of the JVM.
\layout Standard

We conclude that there cannot be a perfect verifier.
 All that could be done is reduce the degree of uncertainty.
 For practical purposes, i.e., to be compatible with Sun's implementation,
 one should not even do that.
\layout Standard

There is also a simple proof showing a perfect verifier does not exist in
 
\begin_inset LatexCommand \cite{JNS}

\end_inset 

, chapter 6.
 It uses a diagonalization argument.
\layout Section

Future Work
\layout Standard

Class file verification is an integral component of Java security; and applicati
on programs running on the Java Virtual Machine are often used in security
 critical areas.
 Several security holes and flaws have been found both in implementations
 and the specification of the Java class file verifier since it was introduced.
\layout Standard

Recently, the area has experienced a leap as a theoretically founded, sound
 and complete Java environment was defined in 
\begin_inset LatexCommand \cite{JBook}

\end_inset 

.
 Possibly Sun's engineers will use this work to improve Java and the Java
 verifier.
 JustIce will have to change to always keep close to the industry standard.
 
\layout Standard

But JustIce itself can also be improved concerning practicability, and new
 software can be developed on top of the Verification API.
\layout Subsection

Improvements to JustIce
\layout Subsubsection

Introduction of Unique Identifers for Verification Results and Warning Messages
\layout Standard

Currently, warning messages and verification results are conceptually text-based.
 Only 
\emph on 
VerificationResult
\emph default 
 objects include a numeric value which programs can use to decide if some
 class verification failed or not.
 A program like the prototype introduced in section 
\begin_inset LatexCommand \ref{GUI_APP}

\end_inset 

 can currently not hide specific messages from the user without parsing
 text.
 This limitation should be removed in the future by using unique message
 numbers.
 This would also make translation of the messages into other languages easier.
\layout Subsubsection


\begin_inset LatexCommand \label{NewVerificationStrategy}

\end_inset 

A New Verification Strategy
\layout Standard

The core verification algorithm cited in section 
\begin_inset LatexCommand \ref{SunCoreAlgo}

\end_inset 

 works by generalizing the knowledge about an object type along the inheritance
 hierarchy.
\layout Standard

For instance, let there be an object of type 
\family typewriter 
java.util.Ab\SpecialChar \-
stract\SpecialChar \-
List
\family default 
 on the simulated stack of some modeled instruction.
 Let there be a loop so that the algorithm has to visit that same instruction
 again, this time with an object of type 
\family typewriter 
java.util.Ab\SpecialChar \-
stract\SpecialChar \-
Set
\family default 
 in that same stack slot.
 The verifier will compute the meet of the two types and record that there
 is some object of type 
\family typewriter 
java.util.Ab\SpecialChar \-
stract\SpecialChar \-
Collection
\family default 
 in that stack slot.
\layout Standard

Remember that the instruction will be marked with a 
\emph on 
changed
\emph default 
 bit until no such re-typing change occurs any more (JustIce will actually
 put it into a queue).
\layout Standard

This approach does not work very well when it comes to interface types instead
 of class files.
 For example, the meet of a 
\family typewriter 
java.lang.In\SpecialChar \-
teger
\family default 
 and a 
\family typewriter 
java.lang.Doub\SpecialChar \-
le
\family default 
 is a 
\family typewriter 
java.lang.Num\SpecialChar \-
ber
\family default 
 because 
\family typewriter 
java.lang.Num\SpecialChar \-
ber
\family default 
\emph on 
 
\emph default 
is the first common super class.
 Both classes also implement the 
\family typewriter 
java.lang.Com\SpecialChar \-
parable
\family default 
 interface, but 
\family typewriter 
java.lang.Num\SpecialChar \-
ber
\family default 
 does not.
 This information is lost when replacing the type information.
 However, current verifiers do not reject the class files but make additional
 run-time checks necessary.
\layout Standard

Fong noticed that this could be the reason for the 
\latex latex 

\backslash 
texttt{invoke\SpecialChar \-
interface}
\latex default 
 opcode to be underspecified 
\begin_inset LatexCommand \cite{Fong2-WWW}

\end_inset 

 (also see section 
\begin_inset LatexCommand \ref{InvokeInterfaceDescFONG}

\end_inset 

).
\layout Standard

Stärk et al.
 suggest the use of 
\emph on 
sets
\emph default 
 of reference types instead (
\begin_inset LatexCommand \cite{JBook}

\end_inset 

, pages 229-231).
 This could also be implemented in JustIce.
\layout Subsubsection

Keeping up with Specification Clarifications
\layout Standard

As a clean-room implementation, JustIce depends on the clearness of the
 specification.
 Ambiguities could lead to programming errors.
\layout Standard

Here we give one example: methods can be inherited in Java (for example,
 the method 
\emph on 
clone() 
\emph default 
is declared in the 
\family typewriter 
java.lang.Ob\SpecialChar \-
ject
\family default 
 class and therefore inherited by every other class).
\layout Standard

Let a class 
\family typewriter 
A
\family default 
 be a subclass of 
\family typewriter 
java.lang.Ob\SpecialChar \-
ject
\family default 
 and let class 
\family typewriter 
B
\family default 
 be a subclass of 
\family typewriter 
A
\family default 
.
 Also, let class 
\family typewriter 
B
\family default 
 override the definition of 
\emph on 
clone()
\emph default 
 with an own implementation.
\layout Standard

If 
\emph on 
javac
\emph default 
 compiles a Java program that invokes this method, it is either referenced
 as 
\emph on 
java.lang.Ob\SpecialChar \-
ject::clone()
\emph default 
 or as 
\emph on 
B::clone()
\emph default 
.
 However, because 
\family typewriter 
A
\family default 
 inherits this method, the reference 
\emph on 
A::clone()
\emph default 
 is legal, too.
\layout Standard

In The Java Virtual Machine Specification, Second Edition (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 291) it is said that the reference must be a 
\begin_inset Quotes eld
\end_inset 

symbolic reference to the class in which the method is to be found
\begin_inset Quotes erd
\end_inset 

.
 Statically, the method 
\emph on 
clone()
\emph default 
 can of course not be found in class 
\family typewriter 
A
\family default 
.
 One could therefore think the reference 
\emph on 
A::clone()
\emph default 
 was not legal.
\layout Standard

In the meanwhile, Sun's engineer Gilad Bracha clarified this issue: 
\begin_inset Quotes eld
\end_inset 

Of course.
 This is discussed in JVMS 5.4.3.4, which describes interface method resolution.
 I don't see the text on page 280 as contradicting that.
 The symbolic reference does give an interface in which the required method
 can be found, albeit as an inherited member.
 We could try and reword it in a more precise way, to eliminate any misunderstan
dings.
\begin_inset Quotes erd
\end_inset 


\layout Standard

Keeping up with clarifications like this is an inevitable and on-going part
 of the development of JustIce.
\layout Subsubsection

Keeping up with Java Extensions
\layout Standard

Recently, Sun Microsystems introduced a new attribute: the 
\family typewriter 
StackMap
\family default 
 attribute which is an attribute local to the 
\family typewriter 
Code
\family default 
 attribute (see section
\emph on 
 
\begin_inset LatexCommand \ref{CodeAttribute}

\end_inset 


\emph default 
).
 It was specified in 
\begin_inset LatexCommand \cite{J2ME-CLDCS}

\end_inset 

.
\layout Standard

It is there to provide 
\begin_inset Quotes eld
\end_inset 

limited devices
\begin_inset Quotes erd
\end_inset 

 that perform a one-pass verification with type information that would normally
 have to be inferred by the verifier.
\layout Standard

It is not used by the verification algorithm of JustIce now: it's currently
 an 
\emph on 
unknown attribute
\emph default 
 to JustIce.
\layout Subsubsection

Detecting Local Variable Accesses out of Scope
\layout Standard

The 
\family typewriter 
LocalVariableTable
\family default 
 attribute is a debug information attribute.
 Basically, it gives debuggers information about the original (source code)
 name and type of a given local variable.
\layout Standard

JustIce builds data structures to warn if it detects contradicting and overlappi
ng areas; e.g., if some local variable is anounced to carry an 
\family typewriter 
int
\family default 
 value and a 
\family typewriter 
float
\family default 
 value at the same time.
\layout Standard

It could also be interesting to warn if a local variable is accessed for
 which no debug information exists.
 This is currently not implemented.
\layout Subsubsection

Extending the Verification API
\layout Standard

JustIce can easily be extended to run certain analyses related to symbolic
 bytecode execution.
\layout Standard

This includes the computation of the maximum number of used operand stack
 slots in a method or the computation of unused local variables in a method.
\layout Standard

These analyses are normally costly to implement
\begin_float footnote 
\layout Standard

Often, heuristics are used such as the method MethodGen.getMaxStack() in
 the BCEL 
\begin_inset LatexCommand \cite{BCEL-WWW,BCEL98}

\end_inset 

.
\end_float 
, but they are a waste product of the verifier's core algorithm.
\layout Subsubsection


\begin_inset LatexCommand \label{VerifierValidationSuite}

\end_inset 

A Verifier Validation Suite
\layout Standard

The Kimera project 
\begin_inset LatexCommand \cite{Kimera-WWW}

\end_inset 

 was the first known project to implement a stand-alone Java verifier.
 The people behind the project had to test the behaviour of their verifier
 against the behaviour of the previous implementations.
 Tests have been run in order to validate the Kimera verifier.
 These tests range from simply introducing random one-byte errors into class
 files and automatically running Kimera against other verifiers to elaborate
 research work 
\begin_inset LatexCommand \cite{Kimera-ProdGram,Kimera-TestingJVM}

\end_inset 

.
\layout Standard

Currently, JustIce comes only with a very limited possibility of running
 test cases against the native verifier of the host machine's JVM.
 The pioneering work of the Kimera project could be used to implement a
 validation suite for JustIce.
\layout Subsection


\begin_inset LatexCommand \label{Firewall}

\end_inset 

A Verifier Protecting an Intranet
\layout Standard

Often, Java Virtual Machines are built into software used to browse the
 World Wide Web such as the KDE project's 
\emph on 
Konqueror 
\begin_inset LatexCommand \cite{KDE}

\end_inset 


\emph default 
 or Mozilla.org's 
\emph on 
Mozilla
\emph default 
 
\begin_inset LatexCommand \cite{Mozilla}

\end_inset 

 products.
 Such Internet technology is also often used in corporate networks.
 Corporate networks based on internet technology are called 
\emph on 
intranets
\emph default 
; these networks are normally protected from the Internet by a so-called
 
\emph on 
firewall
\emph default 
 computer.
 
\layout Standard

This computer's task is to provide access to the internet only to privileged
 employees and --even more important-- it blocks access from unauthorized
 persons outside the intranet.
 The firewall machine is a single, bi-directional point of access.
\layout Standard

However, normally web-browsing is considered harmless, so that the employees
 can unrestrictedly gather information, possibly visiting Java-enabled web
 sites.
 The JVMs built into the browser software run software downloaded from the
 World Wide Web; while the the built-in verifiers make sure that no dangerous
 code can be executed.
\layout Standard

Let us assume someone discovered a security hole in the verifier implementation
 or implementations that are used on the corporate network's workstations;
 let us also assume a patch exists that would fix the problem.
 
\layout Standard

A system administrator would have to spent a lot of time to repair every
 single verifier.
 A cheaper solution would be a verifier built into the firewall machine;
 such a verifier can easily be implemented using JustIce and its Verification
 API.
\layout Subsection

A Java Virtual Machine Implementation Using JustIce
\layout Standard

The Java verifier is originally a part of the Java Virtual Machine.
 JustIce could also be part of a Java Virtual Machine.
 JustIce's class files (the program code JustIce consists of) could simply
 be integrated into the core Java class files.
 The execution engine would then run JustIce without actually verifying
 JustIce's class files themselves.
 
\layout Standard

For scientific purposes one could also implement a JVM in the Java programming
 language.
 Such an implementation could, for example, serve as a debugger.
\layout Subsection


\begin_inset LatexCommand \label{LinePrincipleInfoHidingAndSecurity}

\end_inset 

Drawing a Clear Line Between the Principle of Information Hiding and Security
\layout Standard

The principle of information hiding has been (and still is!) a practice
 of experienced programmers for many years.
 It is there to reduce programming errors.
\layout Standard

In the Modula-2 programming language 
\begin_inset LatexCommand \cite{M2}

\end_inset 

 this is achieved by explicitely dividing the program code in definition
 modules and implementation modules.
 In older programming languages, such as in the C programming language 
\begin_inset LatexCommand \cite{C}

\end_inset 

, this principle is implicitely used, too.
 Basically this is achieved by defining interfaces that only describe what
 the code of a program module does.
 These interface 
\begin_inset Quotes eld
\end_inset 

headers
\begin_inset Quotes erd
\end_inset 

 are included into user code instead of simply including the code itself.
 
\layout Standard

In object-oriented programming languages such as in Delphi 
\begin_inset LatexCommand \cite{D3}

\end_inset 

, C++ 
\begin_inset LatexCommand \cite{CPP-D,CPP-E}

\end_inset 

 or Java 
\begin_inset LatexCommand \cite{langspec2}

\end_inset 

, this principle is refined to what is called object encapsulation.
 When a class is defined, certain key words such as 
\family typewriter 
private
\family default 
, 
\family typewriter 
protected
\family default 
, 
\family typewriter 
friend
\family default 
, 
\family typewriter 
public
\family default 
, 
\family typewriter 
published
\family default 
 set the access rules for the members
\begin_float footnote 
\layout Standard

The members of a class are its components: methods (program code) and fields
 (also called attributes or variables).
\end_float 
 of an object of the given class.
\layout Standard

Still, this refined technique does not have anything to do with security.
 It is only there to aid programmers create a reasonable design.
 If every piece of code could manipulate every data structure, one would
 not know where to look for a programming error in the program source code.
 On the other hand, if some field is private in C++, one could (with some
 knowledge about the compiler used) still reference and modify this field
 by pointer manipulation.
 In addition to that, a second program like a debugger could watch even
 the data of private fields.
\layout Standard

However, when a Java program is compiled into the language of the JVM, the
 information about the access rights of the fields and methods is included.
 This is where the principle of information hiding is exploited to provide
 security.
 For example, the verifier of the JVM has to make sure private fields are
 never accessed from a foreign piece of code.
 But there are many implementations of the JVM which have security flaws
 such as not honouring the access rights.
 There are debuggers for JVM bytecodes, too.
\layout Standard

When one thinks about security, one has to think of some enemy who could
 try to harm the computer or information stored on that computer.
 From a JVM user's point of view, the JVM is relatively secure.
 Even running untrusted code cannot do much harm.
 Because the security flaws in different JVM implementations differ, they
 are probably not exploited most times.
\layout Standard

From a Java programmer's point of view, the JVM is not secure.
 Untrusted users can do much harm.
 For example, an online banking application storing important data in Java
 fields (such as access information to the bank's database management system)
 is a threat to both the bank and its customers.
 This information could easily be extracted by a malicious user.
\layout Standard

Another problem for Java programmers is the amount of symbolical information
 stored in class files.
 Today, it is easy to de-compile a Java class file back to Java language
 source code 
\begin_inset LatexCommand \cite{JODE-WWW}

\end_inset 

.
 This source code can then be read and analyzed by the user.
 Facing this problem, the 
\begin_inset Quotes eld
\end_inset 

only safe course of action is to assume that ALL Java code will at some
 point be decompiled
\begin_inset Quotes erd
\end_inset 

 (
\begin_inset LatexCommand \cite{JNS}

\end_inset 

, page 68).
\layout Standard

We conclude that the principle of information hiding is not enough to provide
 a degree of security that both --users and programmers-- could accept.
 Programmers should not believe a good design makes a program 
\emph on 
secure
\emph default 
.
 
\layout Chapter

Appendix
\layout Section

History of JustIce
\layout Standard

The author of JustIce once started to implement a class file decompiler
 like Jode 
\begin_inset LatexCommand \cite{JODE-WWW}

\end_inset 

.
 It soon became clear that to successfully implement it, one should exploit
 the 
\begin_inset Quotes eld
\end_inset 

well-behaved
\begin_inset Quotes erd
\end_inset 

 property of class files (which essentially means that they pass a verifier,
 especially pass three) 
\begin_inset LatexCommand \cite{Krakatoa-WWW}

\end_inset 

.
 
\layout Standard

JustIce was then developed to understand the 
\begin_inset Quotes eld
\end_inset 

well-behaved
\begin_inset Quotes erd
\end_inset 

 property of usual class files.
 It took much longer to complete than estimated because of the many inherent
 bugs and ambiguities in The Java Virtual Machine Specification, Second
 Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
\layout Standard

Its name starts with a 
\emph on 
J
\emph default 
 like Java does, referring to the tradition of giving Java-related software
 such names.
 The second part of the name, 
\emph on 
ICE
\emph default 
, was inspired by a novel by William Gibson 
\begin_inset LatexCommand \cite{Neuromancer}

\end_inset 

.
 It is an acronym for 
\emph on 
Intrusion Countermeasures Electronics
\emph default 
, something that is very much like today's firewall systems (see section
 
\begin_inset LatexCommand \ref{Firewall}

\end_inset 

).
 He credits the invention of 
\emph on 
ICE
\emph default 
 to Tom Maddox.
 The missing three letters were inserted to create a word that makes sense;
 in fact, choosing the three-letter combination 
\emph on 
ust 
\emph default 
resulted in the creation of a word with a double sense via bi-capitalization.
\layout Standard

JustIce was written using and extending the excellent Byte Code Engineering
 Library 
\begin_inset LatexCommand \cite{BCEL-WWW,BCEL98}

\end_inset 

 by Markus Dahm.
 It really helped a lot and sped up development time.
\layout Standard

It was also --last but not least-- written to earn its author a German
\emph on 
 Dipl.-Inform.

\emph default 
 degree which one may compare to a 
\emph on 
master
\emph default 
 degree.
\layout Section

Flaws and Ambiguities Encountered
\layout Standard

While designing, implementing and testing JustIce, a lot of interesting
 flaws and ambiguities were found in the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, the Java compiler 
\emph on 
javac
\emph default 
 and the JVM 
\emph on 
java
\emph default 
.
\layout Subsection

Flaws in the Java Virtual Machine Specification
\layout Standard

The Java Virtual Machine Specification, Second Edition was derived from
 an in-house document describing the as-is implementation of Sun's genuine
 Java Virtual Machine (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page xiv).
 This sometimes leads to problems as there are still a few points left where
 Sun's engineers forgot to describe specification details to the public,
 in error assuming they would be implementation details.
 Another source of mistakes are ambiguities, inherent to natural languages
 auch as English.
\layout Subsubsection

A Code Length Maximum of 65535 Bytes per Method
\layout Standard

On page 152, The Java Virtual Machine Specification, Second Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

 says that code arrays may at most have a length of 65536 bytes because
 certain indices that point into the code are only 16 bits of width.
 Page 134 states the code must have 
\begin_inset Quotes gld
\end_inset 

less than
\begin_inset Quotes grd
\end_inset 

 65536 bytes.
 Therefore, the limitation stated on page 152 is not helpful, but only confusing.
\layout Subsubsection

Subroutines
\layout Standard

The implementation of a provably correct verifier is not possible because
 of the ambiguities in the specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 To reach this goal, various efforts have been made to describe the verifier
 and the JVM formally 
\begin_inset LatexCommand \cite{Qian,StataAbadi,FreundMitchell,JBook,JPaper}

\end_inset 

.
 By restricting the code 
\emph on 
javac
\emph default 
 produces or by redefining the verifier's behaviour, however, they are never
 one-to-one with the behaviour of the existing JVMs.
\layout Standard

Sun's specification does not define the term 
\emph on 
subroutine
\emph default 
 although it is used.
 Instead, it is explained what bytecode the Java 
\emph on 
compiler
\emph default 
 generates when a 
\family typewriter 
finally
\family default 
 clause appears in the Java 
\emph on 
language
\emph default 
 source code -- this definitely does not belong there, because a verifier
 must never assume the code it verifies was created by Sun's 
\emph on 
javac
\emph default 
 compiler.
\layout Standard

Clarifying this issue could lead to an 
\emph on 
official
\emph default 
 formal specification.
\layout Subsubsection

The Specification Sometimes Satisfies the Verifier
\layout Standard


\begin_inset LatexCommand \label{InvokeInterfaceDescFONG}

\end_inset 

Fong 
\begin_inset LatexCommand \cite{Fong2-WWW}

\end_inset 

 found in 1997 that the 
\family typewriter 
invokeinterface
\family default 
 opcode was underspecified in the first edition of the Java Virtual Machine
 Specification.
 He managed to create a class file that did not implement a specific interface
 but nevertheless used 
\family typewriter 
invokeinterface
\family default 
 to invoke a method.
 This class file passed the verifier (up to pass three), but the JVM found
 the problem during run-time (pass four).
 Fong concluded that the omission in the specification was done on purpose
 because the implementation of the data flow analyzer does not allow to
 check this constraint (please see section 
\begin_inset LatexCommand \ref{NewVerificationStrategy}

\end_inset 

 for a description of how this limitation could be overcome).
 However, in The Java Virtual Machine Specification, Second Edition 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, the specification of 
\family typewriter 
invokeinterface
\family default 
 is corrected.
\layout Standard

Still, there is another case where one would suspect the specification describes
 the behaviour of the verifier: on pages 147 and 148 of the specification
 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, verification of instance initialization methods and newly created objects
 is explained.
 
\begin_inset Quotes eld
\end_inset 

A valid instruction sequence must not have an uninitialized object on the
 operand stack or in a local variable during a backwards branch, or in a
 local variable in code protected by an exception handler or a 
\family typewriter 
finally
\family default 
 clause
\begin_inset Quotes erd
\end_inset 

.
 Note that the Java language keyword 
\family typewriter 
finally
\family default 
 does not really belong here (Sun should speak of 
\emph on 
subroutines
\emph default 
), but more important is that this specification is made to satisfy the
 verification algorithm: 
\begin_inset Quotes eld
\end_inset 

Otherwise, a devious piece of code might fool the verifier
\begin_inset Quotes erd
\end_inset 

.
 
\layout Subsubsection


\begin_inset LatexCommand \label{InnerBug}

\end_inset 

The '$' Character as a Valid Part of a Java Name
\layout Standard

Because the 
\emph on 
javac
\emph default 
 compiler may create class files with a '$' character in their names as
 a result of Java source files defining inner classes, this character should
 no longer be a valid part of a Java name to avoid problems.
 I.e., the method invocation 
\emph on 
ja\SpecialChar \-
va.lang.Cha\SpecialChar \-
rac\SpecialChar \-
ter.is\SpecialChar \-
Ja\SpecialChar \-
va\SpecialChar \-
Iden\SpecialChar \-
tifier\SpecialChar \-
Part('$');
\emph default 
 should return the value 
\family typewriter 
false
\family default 
.
\layout Subsection

Flaws in the Implementation of the 
\emph on 
Java Platform
\layout Subsubsection


\begin_inset LatexCommand \label{javacRejected}

\end_inset 

Sun's Verifier Rejects Code Produced by Sun's Compiler
\layout Standard

Surprisingly, there are a number of examples in which such a thing happens.
\layout Paragraph


\begin_inset LatexCommand \label{StaerkJreject}

\end_inset 

Another Problem With Subroutines
\layout Standard

In 
\begin_inset LatexCommand \cite{JPaper}

\end_inset 

, Stärk and Schmid give a few code examples which are compiled correctly
 by the 
\emph on 
javac
\emph default 
 compiler but the resulting code is rejected by the traditional verifiers.
 Algorithms 
\begin_inset LatexCommand \ref{StaerkJLang}

\end_inset 

 and 
\begin_inset LatexCommand \ref{StaerkJByteCode}

\end_inset 

 show one of their examples given in the Java programming language and the
 resulting output of the 
\emph on 
javac
\emph default 
 compiler.
\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{StaerkJLang}

\end_inset 

Stärk and Schmid's Rejected Class, Java Language Version
\layout Standard


\family typewriter 
class Test1{
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
int test(boolean b){ 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
int i; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
try{
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (b) return 1; 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
i=2;
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
finally {
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
if (b) i = 3;
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
return i;
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\newline 
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
}
\end_float 
\layout Standard

\begin_float alg 
\layout Caption


\begin_inset LatexCommand \label{StaerkJByteCode}

\end_inset 

Stärk and Schmid's Rejected Class, JVM Bytecode Version
\layout Standard


\family typewriter 
int test(boolean arg1)
\layout Standard


\family typewriter 
Code(max_stack = 1, max_locals = 6, code_length = 39)
\layout Standard


\family typewriter 
0: iload_1 
\layout Standard


\family typewriter 
1: ifeq #11 
\layout Standard


\family typewriter 
4: iconst_1 
\layout Standard


\family typewriter 
5: istore_3 
\layout Standard


\family typewriter 
6: jsr #27 
\layout Standard


\family typewriter 
9: iload_3 
\layout Standard


\family typewriter 
10: ireturn 
\layout Standard


\family typewriter 
11: iconst_2 
\layout Standard


\family typewriter 
12: istore_2 
\layout Standard


\family typewriter 
13: jsr #27 
\layout Standard


\family typewriter 
16: goto #37 
\layout Standard


\family typewriter 
19: astore %4 
\layout Standard


\family typewriter 
21: jsr #27 
\layout Standard


\family typewriter 
24: aload %4 
\layout Standard


\family typewriter 
26: athrow 
\layout Standard


\family typewriter 
27: astore %5 
\layout Standard


\family typewriter 
29: iload_1 
\layout Standard


\family typewriter 
30: ifeq #35 
\layout Standard


\family typewriter 
33: iconst_3
\layout Standard


\family typewriter 
34: istore_2 
\layout Standard


\family typewriter 
35: ret %5 
\layout Standard


\family typewriter 
37: iload_2 
\layout Standard


\family typewriter 
38: ireturn
\end_float 
If one tries to run this bytecode using a JVM by IBM Corporation, the code
 is rejected
\begin_float footnote 
\layout Standard

It is also rejected by Sun's JVMs and the Kimera verifier 
\begin_inset LatexCommand \cite{Kimera-WWW}

\end_inset 

.
\end_float 
:
\newline 

\family typewriter 
ehaase@haneman:/home/ehaase > java Test1
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
Exception in thread "main" java.lang.VerifyError:
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
(class: Test1, method: test signature: (Z)I) 
\newline 
\SpecialChar \-
\SpecialChar ~
\SpecialChar ~
Localvariable 2 contains wrong type
\newline 

\newline 

\family default 
In his lectures, Stärk explains that the problem lies in the polymorphic
 nature of JVM subroutines 
\begin_inset LatexCommand \cite{JLectures}

\end_inset 

.
 Consider algorithm 
\begin_inset LatexCommand \ref{StaerkJByteCode}

\end_inset 

.
 In line 12, an 
\family typewriter 
int
\family default 
 is put into local variable number 2.
 The subroutine starting at line 27 is then called from line number 13.
 Note that this subroutine accesses the local variable number 2.
 Finally, line 16 transfers control to line 37 where the verification problem
 occurs.
 An 
\family typewriter 
int
\family default 
 should be read from local variable number 2, but this is marked 
\family typewriter 
unusable
\family default 
, because it was accessed in the subroutine.
\layout Standard

However, the specification (
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

, page 151) states:
\layout Itemize

For any local variable that [\SpecialChar \ldots{}
] has been accessed or modified by the subroutine,
 use the type of the local variable at the time of the 
\family typewriter 
ret
\family default 
.
\layout Itemize

For any other local variables, use the type of the local variable before
 the 
\family typewriter 
jsr
\family default 
 instruction.
\layout Standard

As one can see, in the above example local variable number 2 holds an 
\family typewriter 
int
\family default 
 data type in both cases; there is no need to mark it 
\family typewriter 
unusable
\family default 
.
 This is the reason why JustIce does not reject the above bytecode, thus
 being slightly incompatible with the behaviour of other verifiers.
\layout Paragraph

The Maximum Method Length May Be Exceeded
\layout Standard

The 
\emph on 
javac
\emph default 
 compiler Sun included in the Java Development Kit version 1.3.0_01 does not
 check for the maximum method length of the 
\family typewriter 
code
\family default 
 array in a 
\family typewriter 
Code
\family default 
 attribute (see section 
\begin_inset LatexCommand \ref{CodeAttribute}

\end_inset 

).
 A test file containing 65000 lines like 
\begin_inset Quotes eld
\end_inset 


\family typewriter 
Sys\SpecialChar \-
tem.out.println(
\begin_inset Quotes eld
\end_inset 

Test
\begin_inset Quotes erd
\end_inset 

);
\family default 

\begin_inset Quotes erd
\end_inset 

 was compiled, but the resulting class file was rejected by the verifier.
\layout Standard

IBM Corporation's 
\emph on 
jikes
\emph default 
 compiler does not even generate code, but it locks up while compiling the
 test file.
\layout Subsubsection

A Compiler Issue Related to Inner Classes
\layout Standard

The 
\emph on 
javac
\emph default 
 compiler has to name class files, even those of so-called anonymous classes
 
\begin_inset LatexCommand \cite{InnerSpec}

\end_inset 

.
\layout Standard

This can cause problems: an inner class 
\emph on 
I
\emph default 
 defined in a class 
\emph on 
A
\emph default 
 will be compiled into a class file called 
\emph on 
A$I.class
\emph default 
.
 A Java class named 
\emph on 
A$I
\emph default 
 will also be compiled into a class file named 
\emph on 
A$I.class
\emph default 
 overwriting the former class file.
 Because Sun did not forbid the '
\emph on 
$
\emph default 
' character as a legal part of a Java identifier, the 
\emph on 
javac
\emph default 
 compiler should use a more sophisticated naming scheme.
\layout Subsubsection


\begin_inset LatexCommand \label{PassFourBug}

\end_inset 

Pass Four is Only Partially Implemented
\layout Standard

Pass four defines run-time tests for constraints that could also be verified
 in pass three; it is only for performance reasons that these tests are
 delayed.
 Instead of having all the tests in one place, they are unnecessarily spread
 
\begin_inset Quotes eld
\end_inset 

making the validation of the verification algorithm itself extremely difficult
\begin_inset Quotes erd
\end_inset 

 
\begin_inset LatexCommand \cite{Fong-WWW}

\end_inset 

.
 Risking security for better performance is often regarded as a bad decision.
 For instance, in the 
\layout Standard


\family typewriter 
java version "1.3.0_01"
\layout Standard


\family typewriter 
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0_01) 
\layout Standard


\family typewriter 
Java HotSpot(TM) Client VM (build 1.3.0_01, mixed mode)
\layout Standard

Java Virtual Machine, the pass four check for access rights was unintentionally
 omitted.
 Sadly, other vendors license Sun's code and base their own implementations
 on that code.
 Therefore, mistakes are often inherited throughout the JVM vendors.
 The
\layout Standard


\family typewriter 
java version "1.3.0"
\layout Standard


\family typewriter 
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0)
\layout Standard


\family typewriter 
Classic VM (build 1.3.0, J2RE 1.3.0 IBM build cx130-20010626 (JIT enabled: jitc))
\layout Standard

Java Virtual Machine by IBM Corporation, for example, exposes the same mistake.
\layout Section

Related Work
\layout Subsection

The Kimera Project
\layout Standard

It is a misfortune that the Kimera 
\begin_inset LatexCommand \cite{Kimera-WWW}

\end_inset 

 project closed the World Wide Web presence and that the source code of
 the Kimera verifier was never released -- it would have been quite interesting
 to see how that respected verifier implementation deals with the problems
 arising concerning subroutine verification.
\layout Standard

However, Kimera is the single other stand-alone verifier besides JustIce
 the author knows of.
 The people behind the project found important security breaches in JVM
 implementations of various World Wide Web browsers.
\layout Standard

Also, they validated their verifier implementation and published several
 papers on JVM implementation verification 
\begin_inset LatexCommand \cite{Kimera-ProdGram,Kimera-TestingJVM}

\end_inset 

.
\layout Subsection

The Verifier by Stärk, Schmid and Börger
\layout Standard

In 
\begin_inset LatexCommand \cite{JBook}

\end_inset 

, the authors define the Java programming language and the Java virtual
 machine formally using 
\emph on 
Abstract State Machines
\emph default 
 (ASM).
 This also includes the verifier; its specifications have also been implemented
 in the functional programming language AsmGofer 
\begin_inset LatexCommand \cite{AsmGofer}

\end_inset 

.
 This implementation is included on the CD-ROM that accompanies the book.
\layout Standard

The 
\begin_inset Quotes eld
\end_inset 


\emph on 
JBook verifier
\emph default 

\begin_inset Quotes erd
\end_inset 

 does not implement a complete class file verifier.
 It currently only implements the bytecode verification.
 Its input files are not class files itself, but a textual representation
 of class files in so-called Jasmin format 
\begin_inset LatexCommand \cite{JVM}

\end_inset 

.
 Therefore, this implementation is merely of theoretical interest.
\layout Standard

It does, however, implement a bytecode verifier that is founded on a 
\emph on 
solid
\emph default 
 theory.
 This theory could become the standard for the interpretation of the JVM
 specification 
\begin_inset LatexCommand \cite{vmspec2}

\end_inset 

.
 It could even change the specification to remove its ambiguities.
\layout Standard

There is also an unreleased version of this verifier implemented in the
 Java programming language using the BCEL.
 This implementation, if it should ever be released, promises a lot as it
 could combine usability and a solid theory.
\layout Section


\begin_inset LatexCommand \label{GPL}

\end_inset 

The GNU General Public License
\layout Standard


\emph on 
GNU GENERAL PUBLIC LICENSE
\layout Standard

Version 2, June 1991
\layout Standard

Copyright (C) 1989, 1991 Free Software Foundation, Inc.
\layout Standard

59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
\layout Standard

Everyone is permitted to copy and distribute verbatim copies of this license
 document, but changing it is not allowed.
\layout Standard


\emph on 
Preamble
\layout Standard

The licenses for most software are designed to take away your freedom to
 share and change it.
 By contrast, the GNU General Public License is intended to guarantee your
 freedom to share and change free software--to make sure the software is
 free for all its users.
 This General Public License applies to most of the Free Software Foundation's
 software and to any other program whose authors commit to using it.
 (Some other Free Software Foundation software is covered by the GNU Library
 General Public License instead.) You can apply it to your programs, too.When
 we speak of free software, we are referring to freedom, not price.
 Our General Public Licenses are designed to make sure that you have the
 freedom to distribute copies of free software (and charge for this service
 if you wish), that you receive source code or can get it if you want it,
 that you can change the software or use pieces of it in new free programs;
 and that you know you can do these things.
\layout Standard

To protect your rights, we need to make restrictions that forbid anyone
 to deny you these rights or to ask you to surrender the rights.
\layout Standard

These restrictions translate to certain responsibilities for you if you
 distribute copies of the software, or if you modify it.
 For example, if you distribute copies of such a program, whether gratis
 or for a fee, you must give the recipients all the rights that you have.
 You must make sure that they, too, receive or can get the source code.
 And you must show them these terms so they know their rights.
\layout Standard

We protect your rights with two steps:
\layout Standard

(1) copyright the software, and
\layout Standard

(2) offer you this license which gives you legal permission to copy, distribute
 and/or modify the software.
\layout Standard

Also, for each author's protection and ours, we want to make certain that
 everyone understands that there is no warranty for this free software.
 If the software is modified by someone else and passed on, we want its
 recipients to know that what they have is not the original, so that any
 problems introduced by others will not reflect on the original authors'
 reputations.
\layout Standard

Finally, any free program is threatened constantly by software patents.
 We wish to avoid the danger that redistributors of a free program will
 individually obtain patent licenses, in effect making the program proprietary.
 To prevent this, we have made it clear that any patent must be licensed
 for everyone's free use or not licensed at all.
\layout Standard

The precise terms and conditions for copying, distribution and modification
 follow.
\layout Standard


\emph on 
GNU GENERAL PUBLIC LICENSE
\layout Standard


\emph on 
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
\layout Standard

0.
 This License applies to any program or other work which contains a notice
 placed by the copyright holder saying it may be distributed under the terms
 of this General Public License.
 The "Program", below, refers to any such program or work, and a "work based
 on the Program" means either the Program or any derivative work under copyright
 law: that is to say, a work containing the Program or a portion of it,
 either verbatim or with modifications and/or translated into another language.
 (Hereinafter, translation is included without limitation in the term "modificat
ion".) Each licensee is addressed as "you".
 Activities other than copying, distribution and modification are not covered
 by this License; they are outside its scope.
 The act of running the Program is not restricted, and the output from the
 Program is covered only if its contents constitute a work based on the
 Program (independent of having been made by running the Program).
 Whether that is true depends on what the Program does.
\layout Standard

1.
 You may copy and distribute verbatim copies of the Program's source code
 as you receive it, in any medium, provided that you conspicuously and appropria
tely publish on each copy an appropriate copyright notice and disclaimer
 of warranty; keep intact all the notices that refer to this License and
 to the absence of any warranty; and give any other recipients of the Program
 a copy of this License along with the Program.
 You may charge a fee for the physical act of transferring a copy, and you
 may at your option offer warranty protection in exchange for a fee.
\layout Standard

2.
 You may modify your copy or copies of the Program or any portion of it,
 thus forming a work based on the Program, and copy and distribute such
 modifications or work under the terms of Section 1 above, provided that
 you also meet all of these conditions:
\layout Standard

a) You must cause the modified files to carry prominent notices stating
 that you changed the files and the date of any change.
\layout Standard

b) You must cause any work that you distribute or publish, that in whole
 or in part contains or is derived from the Program or any part thereof,
 to be licensed as a whole at no charge to all third parties under the terms
 of this License.
\layout Standard

c) If the modified program normally reads commands interactively when run,
 you must cause it, when started running for such interactive use in the
 most ordinary way, to print or display an announcement including an appropriate
 copyright notice and a notice that there is no warranty (or else, saying
 that you provide a warranty) and that users may redistribute the program
 under these conditions, and telling the user how to view a copy of this
 License.
 (Exception: if the Program itself is interactive but does not normally
 print such an announcement, your work based on the Program is not required
 to print an announcement.) These requirements apply to the modified work
 as a whole.
 If identifiable sections of that work are not derived from the Program,
 and can be reasonably considered independent and separate works in themselves,
 then this License, and its terms, do not apply to those sections when you
 distribute them as separate works.
 But when you distribute the same sections as part of a whole which is a
 work based on the Program, the distribution of the whole must be on the
 terms of this License, whose permissions for other licensees extend to
 the entire whole, and thus to each and every part regardless of who wrote
 it.
 Thus, it is not the intent of this section to claim rights or contest your
 rights to work written entirely by you; rather, the intent is to exercise
 the right to control the distribution of derivative or collective works
 based on the Program.
 In addition, mere aggregation of another work not based on the Program
 with the Program (or with a work based on the Program) on a volume of a
 storage or distribution medium does not bring the other work under the
 scope of this License.
\layout Standard

3.
 You may copy and distribute the Program (or a work based on it, under Section
 2) in object code or executable form under the terms of Sections 1 and
 2 above provided that you also do one of the following:
\layout Standard

a) Accompany it with the complete corresponding machine-readable source
 code, which must be distributed under the terms of Sections 1 and 2 above
 on a medium customarily used for software interchange; or,
\layout Standard

b) Accompany it with a written offer, valid for at least three years, to
 give any third party, for a charge no more than your cost of physically
 performing source distribution, a complete machine-readable copy of the
 corresponding source code, to be distributed under the terms of Sections
 1 and 2 above on a medium customarily used for software interchange; or,
\layout Standard

c) Accompany it with the information you received as to the offer to distribute
 corresponding source code.
 (This alternative is allowed only for noncommercial distribution and only
 if you received the program in object code or executable form with such
 an offer, in accord with Subsection b above.) The source code for a work
 means the preferred form of the work for making modifications to it.
 For an executable work, complete source code means all the source code
 for all modules it contains, plus any associated interface definition files,
 plus the scripts used to control compilation and installation of the executable.
 However, as a special exception, the source code distributed need not include
 anything that is normally distributed (in either source or binary form)
 with the major components (compiler, kernel, and so on) of the operating
 system on which the executable runs, unless that component itself accompanies
 the executable.
 If distribution of executable or object code is made by offering access
 to copy from a designated place, then offering equivalent access to copy
 the source code from the same place counts as distribution of the source
 code, even though third parties are not compelled to copy the source along
 with the object code.
\layout Standard

4.
 You may not copy, modify, sublicense, or distribute the Program except
 as expressly provided under this License.
 Any attempt otherwise to copy, modify, sublicense or distribute the Program
 is void, and will automatically terminate your rights under this License.
 However, parties who have received copies, or rights, from you under this
 License will not have their licenses terminated so long as such parties
 remain in full compliance.
\layout Standard

5.
 You are not required to accept this License, since you have not signed
 it.
 However, nothing else grants you permission to modify or distribute the
 Program or its derivative works.
 These actions are prohibited by law if you do not accept this License.
 Therefore, by modifying or distributing the Program (or any work based
 on the Program), you indicate your acceptance of this License to do so,
 and all its terms and conditions for copying, distributing or modifying
 the Program or works based on it.
\layout Standard

6.
 Each time you redistribute the Program (or any work based on the Program),
 the recipient automatically receives a license from the original licensor
 to copy, distribute or modify the Program subject to these terms and conditions.
 You may not impose any further restrictions on the recipients' exercise
 of the rights granted herein.
 You are not responsible for enforcing compliance by third parties to this
 License.
\layout Standard

7.
 If, as a consequence of a court judgment or allegation of patent infringement
 or for any other reason (not limited to patent issues), conditions are
 imposed on you (whether by court order, agreement or otherwise) that contradict
 the conditions of this License, they do not excuse you from the conditions
 of this License.
 If you cannot distribute so as to satisfy simultaneously your obligations
 under this License and any other pertinent obligations, then as a consequence
 you may not distribute the Program at all.
 For example, if a patent license would not permit royalty-free redistribution
 of the Program by all those who receive copies directly or indirectly through
 you, then the only way you could satisfy both it and this License would
 be to refrain entirely from distribution of the Program.
 If any portion of this section is held invalid or unenforceable under any
 particular circumstance, the balance of the section is intended to apply
 and the section as a whole is intended to apply in other circumstances.
 It is not the purpose of this section to induce you to infringe any patents
 or other property right claims or to contest validity of any such claims;
 this section has the sole purpose of protecting the integrity of the free
 software distribution system, which is implemented by public license practices.
 Many people have made generous contributions to the wide range of software
 distributed through that system in reliance on consistent application of
 that system; it is up to the author/donor to decide if he or she is willing
 to distribute software through any other system and a licensee cannot impose
 that choice.
 This section is intended to make thoroughly clear what is believed to be
 a consequence of the rest of this License.
\layout Standard

8.
 If the distribution and/or use of the Program is restricted in certain
 countries either by patents or by copyrighted interfaces, the original
 copyright holder who places the Program under this License may add an explicit
 geographical distribution limitation excluding those countries, so that
 distribution is permitted only in or among countries not thus excluded.
 In such case, this License incorporates the limitation as if written in
 the body of this License.
\layout Standard

9.
 The Free Software Foundation may publish revised and/or new versions of
 the General Public License from time to time.
 Such new versions will be similar in spirit to the present version, but
 may differ in detail to address new problems or concerns.
 Each version is given a distinguishing version number.
 If the Program specifies a version number of this License which applies
 to it and "any later version", you have the option of following the terms
 and conditions either of that version or of any later version published
 by the Free Software Foundation.
 If the Program does not specify a version number of this License, you may
 choose any version ever published by the Free Software Foundation.
\layout Standard

10.
 If you wish to incorporate parts of the Program into other free programs
 whose distribution conditions are different, write to the author to ask
 for permission.
 For software which is copyrighted by the Free Software Foundation, write
 to the Free Software Foundation; we sometimes make exceptions for this.
 Our decision will be guided by the two goals of preserving the free status
 of all derivatives of our free software and of promoting the sharing and
 reuse of software generally.
\layout Standard


\emph on 
NO WARRANTY
\layout Standard

11.
 BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR
 THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.
 EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
 PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
 EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH
 YOU.
 SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY
 SERVICING, REPAIR OR CORRECTION.
\layout Standard

12.
 IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL
 ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE
 THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING
 ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF
 THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS
 OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR
 THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
 EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY
 OF SUCH DAMAGES.
\layout Standard


\emph on 
END OF TERMS AND CONDITIONS
\layout Addchap

Glossary
\layout Description

Access\SpecialChar ~
modifiers In the Java programming language, the use of the keywords
 
\family typewriter 
private
\family default 
, 
\family typewriter 
protected
\family default 
, 
\family typewriter 
public
\family default 
 (or the use of no keyword) defines the access rights for data or program
 code (also called visibility).
 This information is also used by the JVM: it is part of the class files.
 The most important modifier is 
\family typewriter 
private
\family default 
 which is used to globally deny access to a field or method.
\layout Description

Access\SpecialChar ~
rights Access rights are granted or denied by the use of 
\latex latex 

\backslash 
(
\backslash 
triangleright
\backslash 
)
\latex default 
access modifiers.
\layout Description

API Applications Programming Interface.
 Such an interface is used to include functionality of foreign program modules
 (often
\latex latex 
 
\latex default 
Java
\latex latex 
 
\backslash 
(
\backslash 
triangleright
\backslash 
)
\latex default 
packages) into own programs.
\layout Description

Debugger A program used to investigate the behaviour of another program.
 Often used to find and remove programming errors, so-called bugs.
\layout Description

Descriptor A symbolic description of type information.
 In the JVM's class files, strings in UTF-8 format 
\begin_inset LatexCommand \cite{Unicode}

\end_inset 

 are used to describe type information.
\layout Description

Field A member of a Java object or class, also called variable or attribute.
\layout Description

Method A member of a Java object or class.
 Methods include program code or they are abstract representatives for program
 code.
 A method can be compared to a 
\emph on 
function 
\emph default 
in programming languages like C or Pascal.
\layout Description

Opcode Operation Code.
 This denotes an instruction in an assembly-like computer language; to some
 people it means its binary representation.
\layout Description

Package A package is an entity used in both the Java programming language
 and the Java Virtual Machine definition.
 It is used to group classes that in the eyes of the programmer belong together.
 Package definitions have impact on 
\latex latex 

\backslash 
(
\backslash 
triangleright
\backslash 
)
\latex default 
access rights granted to other classes.
\layout Description

Signature A method has a (possibly empty) set of arguments it expects, and
 it has a return type (possibly the 
\family typewriter 
void
\family default 
 type).
 The type information of the arguments and the return type together is called
 signature.
 A signature can be expressed in terms of a 
\latex latex 

\backslash 
(
\backslash 
triangleright
\backslash 
)
\latex default 
descriptor.
\layout Description

Type A field or a method argument has a type such as 
\family typewriter 
int
\family default 
 or 
\family typewriter 
String
\family default 
.
 In the JVM's context, all values are typed.
 Types can be expressed in terms of a 
\latex latex 

\backslash 
(
\backslash 
triangleright
\backslash 
)
\latex default 
descriptor.
\layout Standard


\begin_inset LatexCommand \listoffigures{}

\end_inset 


\layout Standard


\latex latex 

\backslash 
addcontentsline{toc}{chapter}{List Of Figures}
\layout Standard


\begin_inset LatexCommand \listofalgorithms{}

\end_inset 


\layout Standard


\latex latex 

\backslash 
addcontentsline{toc}{chapter}{List Of Algorithms}
\layout Bibliography
\bibitem [AppMag-WWW]{AppMag-WWW}


\latex latex 

\backslash 
addcontentsline{toc}{chapter}{Bibliography}
\latex default 
AverStar's AppletMagic(tm): Ada for the Java Virtual Machine.
\newline 

\emph on 
http://www.appletmagic.com
\layout Bibliography
\bibitem [AsmGofer]{AsmGofer}

Joachim Schmid: AsmGofer.
\newline 

\emph on 
http://www.tydo.org
\layout Bibliography
\bibitem [BCEL98]{BCEL98}

Markus Dahm: Byte Code Engineering with the BCEL API.
 Freie Universität Berlin, Institut für Informatik.
 Technical Report B-17-98.
\layout Bibliography
\bibitem [BCEL-WWW]{BCEL-WWW}

Markus Dahm: Byte Code Engineering Library.
\emph on 

\newline 
http://bcel.sourceforge.net
\layout Bibliography
\bibitem [BCV-Soundness]{BCV-Soundness}

Cornelia Pusch: Proving the Soundness of a Java Bytecode Verifier Specification
 in Isabelle/HOL.
 Technische Universität München, Institut für Informatik.
 
\newline 

\emph on 
http://www.in.tum.de/~pusch/
\layout Bibliography
\bibitem [C]{C}

Brian W.
 Kerninghan, Dennis M.
 Ritchie: The C Programming Language, Second Edition, ANSI C.
 Prentice-Hall 1998, ISBN 0131103628.
\layout Bibliography
\bibitem [CPP-D]{CPP-D}

Bjarne Stroustrup: Die C++ Programmiersprache.
 Addison-Wesly-Longman, 1998, ISBN 3-8273-1296-5.
\layout Bibliography
\bibitem [CPP-E]{CPP-E}

Bjarne Stroustrup: The C++-Programming Language, Third Edition.
 Addison-Wesley 1997, ISBN 0-201-88954-4.
\layout Bibliography
\bibitem [D3]{D3}

Guido Lang, Andreas Bohne: Delphi 3.0 lernen.
 Addison-Wesley-Longman 1997, ISBN 3-8273-1190-x.
\layout Bibliography
\bibitem [DesignPatterns]{DesignPatterns}

Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: Design Patterns
 Elements of Reusable Object-Oriented Software.
 Addison-Wesley 1995, ISBN: 0201633612.
\layout Bibliography
\bibitem [DragonBook]{DragonBook}

Alfred V.
 Aho, Ravi Sethi, Jeffrey D.
 Ullman: Compilers: Principles, Techniques, and Tools.
 Addison-Wesley 1985, ISBN: 0201100886.
\layout Bibliography
\bibitem [EF]{EF}

ElectricalFire.
\emph on 

\newline 
http://www.mozilla.org/projects/ef/
\layout Bibliography
\bibitem [f2j]{f2j}

Keith Seymour: f2j - Fortran-to-Java Compiler.
\newline 

\emph on 
http://cs.utk.edu/f2j/
\layout Bibliography
\bibitem [Fong-WWW]{Fong-WWW}

Philip W.
 L.
 Fong: The mysterious Pass One, first draft, September 2, 1997.
 
\newline 

\emph on 
http://www.cs.sfu.ca/people/GradStudents/pwfong/personal/ JVM/pass1/
\layout Bibliography
\bibitem [Fong2-WWW]{Fong2-WWW}

Philip W.
 L.
 Fong: A Flaw with the Specification of the Invokeinterface Opcode.
 
\newline 

\emph on 
http://www.cs.sfu.ca/people/GradStudents/pwfong/personal/ JVM/invokeinterface/
\layout Bibliography
\bibitem [FreundMitchell]{FreundMitchell}

Stephen N.
 Freund, John Mitchell: A Formal Framework for the Java Bytecode Language
 and Verifier.
 Department of Computer Science, Stanford University.
 Stanford, CA 94305-9045.
 Appeared in OOPSLA '99.
\layout Bibliography
\bibitem [GCC-WWW]{GCC-WWW}

GCC, The GNU compiler collection.
\emph on 

\newline 
http://gcc.gnu.org
\layout Bibliography
\bibitem [GJ-WWW]{GJ-WWW}

GJ.
 A Generic Java Language Extension.
\newline 

\emph on 
http://www.cis.unisa.edu.au/~pizza/gj/
\layout Bibliography
\bibitem [InnerSpec]{InnerSpec}

Sun Microsystems: Inner Classes Specification.
\newline 

\emph on 
http://java.sun.com/products/jdk/1.1/docs/guide/
\newline 
innerclasses/spec/innerclasses.doc.html
\layout Bibliography
\bibitem [J2ME-CLDCS]{J2ME-CLDCS}

Sun Microsystems: J2ME
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 Connected Limited Device Configuration Specification.
\newline 

\emph on 
http://jcp.org/aboutJava/communityprocess/final/jsr030/
\layout Bibliography
\bibitem [JBook]{JBook}

Robert Stärk, Joachim Schmid, Egon Börger: Java
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\latex default 
 and the Java
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\latex default 
 Virtual Machine.
 Springer-Verlag 2001, ISBN 3-540-42088-6.
\newline 

\emph on 
http://www.inf.ethz.ch/~jbook/
\layout Bibliography
\bibitem [JPaper]{JPaper}

Robert F.
 Stärk, Joachim Schmid: Java bytecode verification is not possible.
 ETH Zürich, Department of Computer Science 2000.
\emph on 

\newline 
http://www.inf.ethz.ch/~staerk/pdf/jbv00.pdf
\layout Bibliography
\bibitem [JLectures]{JLectures}

Robert F.
 Stärk: Java and the JVM: Definition and Verification (37-474).
\newline 

\emph on 
http://www.inf.ethz.ch/~jbook/eth37474/
\newline 
http://www.inf.ethz.ch/~jbook/eth37474/javaBV.pdf
\layout Bibliography
\bibitem [JNS]{JNS}

Robert Macgregor, Dave Durbin, John Owlett, Andrew Yeomans: JAVA
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 Network Security.
 Prentice Hall 1998, ISBN 0137615299.
\layout Bibliography
\bibitem [JODE-WWW]{JODE-WWW}

JODE is a java package containing a decompiler and an optimizer for java.
\newline 

\emph on 
http://jode.sourceforge.net
\layout Bibliography
\bibitem [JustIce]{JustIce}

Enver Haase: JustIce.
 A Free Class File Verifier for Java
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.Freie Universität Berlin, Takustraße 9, D-14195 Berlin; September 2001.
\newline 

\emph on 
http://bcel.sourceforge.net/
\newline 
http://bcel.sourceforge.net/justice
\layout Bibliography
\bibitem [JVM]{JVM}

Jon Meyer, Troy Downing: JAVA Virtual Machine.
 O'Reilly 1997, ISBN 1-56592-194-1.
\layout Bibliography
\bibitem [Kaffe-WWW]{Kaffe-WWW}

Kaffe.
 Kaffe is a cleanroom, open source implementation of a Java virtual machine
 and class libraries.
\emph on 

\newline 
http://www.kaffe.org
\layout Bibliography
\bibitem [KAWA-WWW]{KAWA-WWW}

Kawa, the Java-based Scheme system.
\emph on 

\newline 
http://http://www.gnu.org/software/kawa/
\layout Bibliography
\bibitem [KDE]{KDE}

KDE, the K desktop environment.
\newline 

\emph on 
http://www.kde.org
\layout Bibliography
\bibitem [Kimera-WWW]{Kimera-WWW}

The Kimera Verifier.

\emph on 
 
\emph default 

\newline 
Currently off-line because of a World Wide Web presentation rework.
\emph on 

\newline 
http://kimera.cs.washington.edu/verifier.html
\newline 
http://www-kimera.cs.washington.edu
\layout Bibliography
\bibitem [Kimera-TestingJVM]{Kimera-TestingJVM}

Emin Gün Sirer: Testing Java Virtual Machines.
 An Experience Report on Automatically Testing Java Virtual Machines.
 University of Washington, Dept.
 of Computer Science and Engineering.
\newline 

\emph on 
http://kimera.cs.washington.edu
\layout Bibliography
\bibitem [Kimera-ProdGram]{Kimera-ProdGram}

Emin Gün Sirer, Brian N.
 Bershad: Using Production Grammars in Software Testing.
 University of Washington, Department of Computer Science.
\newline 

\emph on 
http://kimera.cs.washington.edu
\layout Bibliography
\bibitem [kissme-WWW]{kissme-WWW}

kissme.
 A free Java Virtual Machine.
\emph on 

\newline 
http://kissme.sourceforge.net
\layout Bibliography
\bibitem [Krakatoa-WWW]{Krakatoa-WWW}

Todd A.
 Proebsting, Scott A.
 Watterson: Krakatoa: Decompilation in Java (Does Bytecode Reveal Source?).
 The University of Arizona, Department of Computer Science.
\newline 

\emph on 
http://www.cs.arizona.edu/people/saw/papers/Krakatoa-COOTS97.ps.Z
\layout Bibliography
\bibitem [langspec2]{langspec2}

James Gosling, Bill Joy, Guy Steele, Gilad Bracha: The Java Language Specificati
on, Second Edition.
 Addison-Wesley 2000, ISBN 0201310082.
\layout Bibliography
\bibitem [M2]{M2}

Niklaus Wirth: Programming in Modula-2, Fourth Edition.
 Springer-Verlag 1988, ISBN 3-540-50150-9.
\layout Bibliography
\bibitem [Mozilla]{Mozilla}

Mozilla.org (The Mozilla Origanization): Mozilla.
\newline 

\emph on 
http://www.mozilla.org
\layout Bibliography
\bibitem [Neuromancer]{Neuromancer}

William Gibson: Neuromancer.
 Ace Books 1994, ISBN 0441000681.
\layout Bibliography
\bibitem [ORP-WWW]{ORP-WWW}

Open Runtime Platform.
 A Platform For Bytecode System Research.
\newline 

\emph on 
http://www.intel.com/research/mrl/orp/index.htm
\layout Bibliography
\bibitem [PL4JVM]{PL4JVM}

Robert Tolksdorf: Programming Languages for the Java Virtual Machine.
\newline 

\emph on 
http://www.robert-tolksdorf.de/vmlanguages.html
\layout Bibliography
\bibitem [PMG-WWW]{PMG-WWW}

PMG.
 Poor Man's Genericity for Java.
 
\newline 

\emph on 

\layout Bibliography
\bibitem [Qian]{Qian}

Zhenyu Qian: A Formal Specification of Java
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 Virtual Machine Instructions for Objects, Methods and Subroutines.
 Bremen Institute for Safe Systems (BISS), FB3 Informatik, Universität Bremen,
 D-28334 Bremen, Germany.
\layout Bibliography
\bibitem [SableVM-WWW]{SableVM-WWW}

SableVM.
 A Bytecode Interpreter.
\emph on 

\newline 
http://www.sablevm.org
\layout Bibliography
\bibitem [StataAbadi]{StataAbadi}

Raymie Stata and Martin Abadi: A Type System for Java Bytecode Subroutines.
 In: ACM Transactions on Programming Languages and Systems, Vol.
 21, No.
 1, January 1999, Pages 90-137.
\layout Bibliography
\bibitem [Unknowable]{Unknowable}

G.J.
 Chaitin: The Unknowable.
 Springer-Verlag 1999, ISBN 981-4021-72-5.
\newline 

\emph on 
http://www.umcs.maine.edu/~chaitin/unknowable/
\layout Bibliography
\bibitem [Unicode]{Unicode}

The Unicode Consortium: The Unicode Standard, Version 2.0.
 Niso Press 1996, ISBN 0-201-48345-9.
\newline 

\emph on 
http://www.unicode.org
\layout Bibliography
\bibitem [Yellin-WWW]{Yellin-WWW}

Frank Yellin: Low Level Security in Java.
\emph on 

\newline 
http://java.sun.com/sfaq/verifier.html
\layout Bibliography
\bibitem [VMSPEC2]{vmspec2}

Tim Lindholm, Frank Yellin: The Java
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\latex default 
 Virtual Machine Specification, Second Edition.
 Addison-Wesley 1999, ISBN 0-201-43294-4.
\the_end