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22971473 UNPDF
The Java Virtual Machine Specification
Release 1.0 Beta
DRAFT
August 21, 1995
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Copyright Information
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Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Chapter 1: Java Virtual Machine Architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Chapter 2: Class File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Constant Pool. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.6 Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Chapter 3: The Virtual Machine Instruction Set . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.1 Format for the Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 Pushing Constants onto the Stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Loading Local Variables Onto the Stack . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Storing Stack Values into Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.5 Wider index for Loading, Storing and Incrementing . . . . . . . . . . . . . . . 35
3.6 Managing Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 Stack Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.8 Arithmetic Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.9 Logical Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.10 Conversion Operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.11 Control Transfer Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.12 Function Return. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.13 Table Jumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.14 Manipulating Object Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.15 Method Invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.16 Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.17 Miscellaneous Object Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.18 Monitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Appendix A: An Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
A.1 Constant Pool Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
A.2 Pushing Constants onto the Stack (_quick variants). . . . . . . . . . . . . . . . 74
A.3 Managing Arrays (_quick variants). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
A.4 Manipulating Object Fields (_quick variants) . . . . . . . . . . . . . . . . . . . . . 76
A.5 Method Invocation (_quick variants) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
A.6 Miscellaneous Object Operations (_quick variants) . . . . . . . . . . . . . . . . 80
Index of Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
March 4, 1996
Java Virtual Machine Specification
3
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4
Java Virtual Machine Specification
March 4, 1996
 
Preface
This document describes version 1.0 of the Java Virtual Machine and its instruction set. We have written this
document to act as a specification for both compiler writers, who wish to target the machine, and as a
specification for others who may wish to implement a compliant Java Virtual Machine.
The Java Virtual Machine is an imaginary machine that is implemented by emulating it in software on a real
machine. Code for the Java Virtual Machine is stored in .class files, each of which contains the code for at
most one public class.
Simple and efficient emulations of the Java Virtual Machine are possible because the machine’s format is
compact and efficient bytecodes. Implementations whose native code speed approximates that of compiled C
are also possible, by translating the bytecodes to machine code, although Sun has not released such
implementations at this time.
The rest of this document is structured as follows:
• Chapter 1 describes the architecture of the Java Virtual Machine.
• Chapter 2 describes the .class file format.
• Chapter 3 describes the bytecodes.
• Appendix A contains some instructions generated internally by Sun’s implementation of the Java
Virtual Machine. While not strictly part of the specification we describe these here so that this
specification can serve as a reference for our implementation. As more implementations of the Java
Virtual Machine become available, we may remove Appendix A from future releases.
Sun will license the Java Virtual Machine trademark and logo for use with compliant implementations of this
specification. If you are considering constructing your own implementation of the Java Virtual Machine please
contact us, at the email address below, so that we can work together to insure 100% compatiblity of your
implementation.
Send comments on this specification or questions about implementing the Java Virtual Machine to our
electronic mail address: java@java.sun.com .
August 22, 1995
Java Virtual Machine Specification
5
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