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<td><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc13.html">Prev</a> | <a href="Instructions2.doc15.html">Next</a> | <a href="Lindholm.INDEX.html">Index</a></td><td align=right><i><i>The Java<sup><font size=-2>TM</font></sup> Virtual Machine Specification</i></i></td>
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<a name="tableswitch">
</a><h2>tableswitch</h2>
<a name="tableswitch.Operation"></a>
<b>Operation</h3></b>
<a name="70136"></a>
Access jump table by index and jump<p><Table Border="1">
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<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><br><p>
<a name="tableswitch.Forms"></a>
<b>Forms</h3></b>
<a name="70137"></a>
<i>tableswitch</i> = 170 (0xaa)<p>
<a name="tableswitch.Stack"></a>
<b>Stack</h3></b>
<a name="70138"></a>
..., <i>index</i><em> </em><img src="chars/arrwdbrt.gif"> ...<p>
<a name="tableswitch.Description"></a>
<b>Description</h3></b>
<a name="159620"></a>
A <i>tableswitch</i> is a variable-length instruction. Immediately after the <i>tableswitch</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 following byte 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 the bytes constituting a series of signed 32-bit values: <i>default</i>, <i>low</i>, <i>high</i>, and then <i>high </i>- <i>low </i>+ 1 further 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>.<p>
<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.<p>
<a name="70141"></a>
The target address which 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.<p>
<a name="tableswitch.Notes"></a>
<b>Notes</h3></b>
<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.<p>
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<a href="VMSpecTOC.doc.html">Contents</a> | <a href="Instructions2.doc13.html">Prev</a> | <a href="Instructions2.doc15.html">Next</a> | <a href="Lindholm.INDEX.html">Index</a>
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<font size = -1>Java Virtual Machine Specification <br>
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