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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><html><head><title>VM Spec </title></head><body BGCOLOR=#eeeeff text=#000000 LINK=#0000ff VLINK=#000077 ALINK=#ff0000><table width=100%><tr><td><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc13.html">Prev</a> | <a href="Instructions2.doc15.html">Next</a> | <a href="VMSpecIX.fm.html">Index</a></td><td align=right><i><i>The Java<sup><font size=-2>TM</font></sup> Virtual Machine Specification</i></i></td></tr></table><hr><br> <a href="Instructions2.doc.html">A</a><a href="Instructions2.doc1.html">B</a><a href="Instructions2.doc2.html">C</a><a href="Instructions2.doc3.html">D</a><a href="Instructions2.doc4.html">F</a><a href="Instructions2.doc5.html">G</a><a href="Instructions2.doc6.html">I</a><a href="Instructions2.doc7.html">J</a><a href="Instructions2.doc8.html">L</a><a href="Instructions2.doc9.html">M</a><a href="Instructions2.doc10.html">N</a><a href="Instructions2.doc11.html">P</a><a href="Instructions2.doc12.html">R</a><a href="Instructions2.doc13.html">S</a><a href="Instructions2.doc14.html">T</a><a href="Instructions2.doc15.html">W</a><a name="tableswitch"></a><hr><h2>tableswitch</h2><a name="tableswitch.Operation"></a><p><b>Operation</b><br><blockquote><a name="70136"></a>Access jump table by index and jump<p><Table Border="1"></blockquote><p><b>Format</b><br><blockquote><tr><td><a name="70107"></a> <i>tableswitch</i><td><a name="87535"></a> <tr><td><a name="70109"></a> <0-3 byte pad\><td><a name="87537"></a> <tr><td><a name="70111"></a> <i>defaultbyte1</i><td><a name="87539"></a> <tr><td><a name="70113"></a> <i>defaultbyte2</i><td><a name="87541"></a> <tr><td><a name="70115"></a> <i>defaultbyte3</i><td><a name="87543"></a> <tr><td><a name="70117"></a> <i>defaultbyte4</i><td><a name="87545"></a> <tr><td><a name="70119"></a> <i>lowbyte1</i><td><a name="87547"></a> <tr><td><a name="70121"></a> <i>lowbyte2</i><td><a name="87549"></a> <tr><td><a name="70123"></a> <i>lowbyte3</i><td><a name="87551"></a> <tr><td><a name="70125"></a> <i>lowbyte4</i><td><a name="87553"></a> <tr><td><a name="70127"></a> <i>highbyte1</i><td><a name="87555"></a> <tr><td><a name="70129"></a> <i>highbyte2</i><td><a name="87557"></a> <tr><td><a name="70131"></a> <i>highbyte3</i><td><a name="87559"></a> <tr><td><a name="70133"></a> <i>highbyte4</i><td><a name="87561"></a> <tr><td><a name="70135"></a> <i>jump offsets</i>...<td><a name="87563"></a> </Table><br></blockquote><p><a name="tableswitch.Forms"></a><p><b>Forms</b><br><blockquote><a name="70137"></a><i>tableswitch</i> = 170 (0xaa)</blockquote><p><a name="tableswitch.Operand"></a><p><b>Operand Stack</b><br><blockquote><a name="70138"></a>..., <i>index</i><em> </em><img src="chars/arrwdbrt.gif"> ...</blockquote><p><a name="tableswitch.Description"></a><p><b>Description</b><br><blockquote><a name="499986"></a>A <i>tableswitch</i> is a variable-length instruction. Immediately after the <i>tableswitch</i> opcode, between 0 and 3 null bytes (zeroed bytes, not the null object) are inserted as padding. The number of null bytes is chosen so that the following byte begins at an address that is a multiple of 4 bytes from the start of the current method (the opcode of its first instruction). Immediately after the padding follow bytes constituting three signed 32-bit values: <i>default</i>, <i>low</i>, and <i>high</i>. Immediately following those bytes are bytes constituting a series of <i>high </i>- <i>low </i>+ 1 signed 32-bit offsets. The value <i>low</i> must be less than or equal  to <i>high</i>.  The <i>high </i>- <i>low </i>+ 1  signed 32-bit  offsets  are treated as a 0-based jump table. Each of these signed 32-bit values is constructed as (<i>byte1</i> << 24) | (<i>byte2</i> << 16) | (<i>byte3</i> << 8) | <i>byte4</i>.</blockquote><p><blockquote><a name="70140"></a>The <i>index</i> must be of type <code>int</code> and is popped from the operand stack. If <i>index</i> is less than <i>low</i> or <i>index</i> is greater than <i>high</i>, then a target address is calculated by adding <i>default</i> to the address of the opcode of this <i>tableswitch</i> instruction. Otherwise, the offset at position <i>index </i>- <i>low</i> of the jump table is extracted. The target address is calculated by adding that offset to the address of the opcode of this <i>tableswitch</i> instruction. Execution then continues at the target address.</blockquote><p><blockquote><a name="70141"></a>The target address that can be calculated from each jump table offset, as well as the ones that can be calculated from <i>default</i>, must be the address of an opcode of an instruction within the method that contains this <i>tableswitch</i> instruction.</blockquote><p><a name="tableswitch.Notes"></a><p><b>Notes</b><br><blockquote><a name="70142"></a>The alignment required of the 4-byte operands of the <i>tableswitch</i> instruction guarantees 4-byte alignment of those operands if and only if the method that contains the <i>tableswitch</i> starts on a 4-byte boundary.</blockquote><p><hr><!-- This inserts footnotes--><p><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc13.html">Prev</a> | <a href="Instructions2.doc15.html">Next</a> | <a href="VMSpecIX.fm.html">Index</a><p><font size="-1"><i>The Java</i><sup><font size=-2>TM</font></sup><i> Virtual Machine Specification </i><br><!-- HTML generated by Suzette Pelouch on March 30, 1999 --><i><a href="Copyright.doc.html">Copyright © 1999 Sun Microsystems, Inc.</a>All rights reserved</i><br>Please send any comments or corrections to <a href="mailto:jvm@java.sun.com">jvm@java.sun.com</a></font></body></html>⌨️ 快捷键说明
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