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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.doc143.html">Prev</a> | <a href="Instructions2.doc145.html">Next</a> | <a href="Mnemonics.doc.html">Index</a><hr><br> <a name="lookupswitch"></a><h2>lookupswitch</h2><a name="lookupswitch.Operation"></a><b>Operation</b><br><a name="69543"></a>Access jump table by key match and jump<p><Table Border="3"><tr><td><a name="69522"></a> <i>lookupswitch</i><td><a name="87568"></a> <tr><td><a name="69524"></a> <<i>0-3 byte pad</i>><td><a name="87568"></a> <tr><td><a name="69526"></a> <i>defaultbyte1</i><td><a name="87568"></a> <tr><td><a name="69528"></a> <i>defaultbyte2</i><td><a name="87568"></a> <tr><td><a name="69530"></a> <i>defaultbyte3</i><td><a name="87568"></a> <tr><td><a name="69532"></a> <i>defaultbyte4</i><td><a name="87568"></a> <tr><td><a name="69534"></a> <i>npairs1</i><td><a name="87568"></a> <tr><td><a name="69536"></a> <i>npairs2</i><td><a name="87568"></a> <tr><td><a name="69538"></a> <i>npairs3</i><td><a name="87568"></a> <tr><td><a name="69540"></a> <i>npairs4</i><td><a name="87568"></a> <tr><td><a name="69542"></a> <i>match-offset pairs</i>...<td><a name="87568"></a> </Table><Table><tr><td></Table><p><a name="lookupswitch.Forms"></a><b>Forms</b><br><a name="69544"></a><i>lookupswitch</i> = 171 (0xab)<p><a name="lookupswitch.Stack"></a><b>Stack</b><br><a name="69545"></a>..., <i>key</i> <img src="chars/arrwdbrt.gif"> ...<p><a name="lookupswitch.Description"></a><b>Description</b><br><a name="69546"></a>A <i>lookupswitch</i> is a variable-length instruction. Immediately after the <i>lookupswitch</i> opcode, between zero and three null bytes (zeroed bytes, not the null object) are inserted as padding. The number of null bytes is chosen so that the <i>defaultbyte1</i> begins at an address that is a multiple of four bytes from the start of the current method (the opcode of its first instruction). Immediately after the padding follow a series of signed 32-bit values: <i>default</i>, <i>npairs</i>, and then <i>npairs</i> pairs of signed 32-bit values. The <i>npairs</i> must be greater than or equal to 0. Each of the <i>npairs</i> pairs consists of an <code>int</code> <i>match</i> and a signed 32-bit <i>offset</i>. Each of these signed 32-bit values is constructed from four unsigned bytes as (<i>byte1</i> << 24) | (<i>byte2</i> << 16) | (<i>byte3</i> << 8) | <i>byte4</i>.<p><hr><!-- This inserts footnotes--><p><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc143.html">Prev</a> | <a href="Instructions2.doc145.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 © 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>⌨️ 快捷键说明
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