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Bytecode basics
A first look at the bytecodes of the Java virtual machine
Page 3 of 10
| Opcode | Operand(s) | Description |
|---|---|---|
iconst_m1 |
(none) | pushes int -1 onto the stack |
iconst_0 |
(none) | pushes int 0 onto the stack |
iconst_1 |
(none) | pushes int 1 onto the stack |
iconst_2 |
(none) | pushes int 2 onto the stack |
iconst_3 |
(none) | pushes int 3 onto the stack |
iconst_4 |
(none) | pushes int 4 onto the stack |
iconst_5 |
(none) | pushes int 5 onto the stack |
fconst_0 |
(none) | pushes float 0 onto the stack |
fconst_1 |
(none) | pushes float 1 onto the stack |
fconst_2 |
(none) | pushes float 2 onto the stack |
The opcodes shown in the previous table push ints and floats, which are 32-bit values. Each slot on the Java stack is 32 bits wide. Therefore each time an int or float is pushed onto the stack, it occupies one slot.
The opcodes shown in the next table push longs and doubles. Long and double values occupy 64 bits. Each time a long or double is pushed onto the stack, its value occupies two slots on the stack. Opcodes that indicate a specific long or double value to push are shown in the following table:
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- "The Java Virtual Machine Specification" represents the official word from Sun.
http://java.sun.com/1.0alpha3/doc/vmspec/vmspec_1.html - When it comes out, the book The Java Virtual Machine Specification, by Tim Lindholm and Frank Yellin (ISBN 0-201-63452-X), will be the definitive JVM reference. This book is part of The Java Series, from Addison-Wesley.
http://www.aw.com/cp/lindholm-yellin.html
http://www.aw.com/cp/javaseries.html
- Previous Under The Hood articles:
- The lean, mean virtual machine -- Gives an introduction to the Java virtual machine. Look here to see how the garbage collected heap fits in with the other parts of the JVM.
- The Java class file lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.
- Java's garbage-collected heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the JVM in particular.
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