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📁 Jvm 规范说明。The Java Virtual Machine was designed to support the Java programming language. Some concep

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<html><head><title>VM Spec  </title></head><body BGCOLOR=#eeeeff text=#000000 LINK=#0000ff VLINK=#000077 ALINK=#ff0000> <a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc94.html">Prev</a> | <a href="Instructions2.doc96.html">Next</a> | <a href="Mnemonics.doc.html">Index</a><hr><br> <a name="ifnull"></a><h2>ifnull</h2><a name="ifnull.Operation"></a><b>Operation</b><br><a name="68607"></a>Branch if <code>reference</code> is <code>null</code><p><Table Border="3"><tr><td><a name="68602"></a> <i>ifnull</i><td><a name="87568"></a> <tr><td><a name="68604"></a> <i>branchbyte1</i><td><a name="87568"></a> <tr><td><a name="68606"></a> <i>branchbyte2</i><td><a name="87568"></a> </Table><Table><tr><td></Table><p><a name="ifnull.Forms"></a><b>Forms</b><br><a name="68608"></a><i>ifnull</i> = 198 (0xc6)<p><a name="ifnull.Stack"></a><b>Stack</b><br><a name="68609"></a>..., <i>value</i> <img src="chars/arrwdbrt.gif"> ...<p><a name="ifnull.Description"></a><b>Description</b><br><a name="68610"></a>The <i>value</i> must of type <code>reference</code>. It is popped from the operand stack. If <i>value</i> is <code>null</code>, <em>the unsigned</em> <i>branchbyte1</i> and <i>branchbyte2</i> are used to construct a signed 16-bit offset, where the offset is calculated to be (<i>branchbyte1</i> << 8) | <i>branchbyte2</i>. Execution then proceeds at that offset from the address of the opcode of this <i>ifnull</i> instruction. The target address must be that of an opcode of an instruction within the method that contains this <i>ifnull</i> instruction.<p><a name="68611"></a>Otherwise, execution proceeds at the address of the instruction following this <i>ifnull</i> instruction.<p><hr><!-- This inserts footnotes--><p><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc94.html">Prev</a> | <a href="Instructions2.doc96.html">Next</a> | <a href="Mnemonics.doc.html">Index</a><p><font size=-1>Java Virtual Machine Specification (HTML generated by dkramer on February 25, 1997)<br><i><a href="Copyright.doc.html">Copyright &#169 1996 Sun Microsystems, Inc.</a>All rights reserved</i><br>Please send any comments or corrections to <a href="mailto:doug.kramer@sun.com">doug.kramer@sun.com</a></font></body></html>

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