129 lines
5.9 KiB
HTML
129 lines
5.9 KiB
HTML
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<title>VAX-branch - Using as</title>
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<link rel="up" href="Vax_002dDependent.html#Vax_002dDependent" title="Vax-Dependent">
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<link rel="prev" href="VAX_002dopcodes.html#VAX_002dopcodes" title="VAX-opcodes">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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<body>
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<div class="node">
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<p>
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<a name="VAX_002dbranch"></a>Next: <a rel="next" accesskey="n" href="VAX_002doperands.html#VAX_002doperands">VAX-operands</a>,
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Previous: <a rel="previous" accesskey="p" href="VAX_002dopcodes.html#VAX_002dopcodes">VAX-opcodes</a>,
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Up: <a rel="up" accesskey="u" href="Vax_002dDependent.html#Vax_002dDependent">Vax-Dependent</a>
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<hr><br>
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</div>
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<h4 class="subsection">9.34.5 VAX Branch Improvement</h4>
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<p><a name="index-VAX-branch-improvement-1767"></a><a name="index-branch-improvement_002c-VAX-1768"></a><a name="index-pseudo_002dops-for-branch_002c-VAX-1769"></a>Certain pseudo opcodes are permitted. They are for branch
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instructions. They expand to the shortest branch instruction that
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reaches the target. Generally these mnemonics are made by
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substituting <span class="samp">j</span> for <span class="samp">b</span> at the start of a DEC mnemonic.
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This feature is included both for compatibility and to help
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compilers. If you do not need this feature, avoid these
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opcodes. Here are the mnemonics, and the code they can expand into.
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<dl>
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<dt><code>jbsb</code><dd><span class="samp">Jsb</span> is already an instruction mnemonic, so we chose <span class="samp">jbsb</span>.
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<dl>
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<dt>(byte displacement)<dd><kbd>bsbb ...</kbd>
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<br><dt>(word displacement)<dd><kbd>bsbw ...</kbd>
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<br><dt>(long displacement)<dd><kbd>jsb ...</kbd>
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</dl>
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<br><dt><code>jbr</code><dt><code>jr</code><dd>Unconditional branch.
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<dl>
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<dt>(byte displacement)<dd><kbd>brb ...</kbd>
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<br><dt>(word displacement)<dd><kbd>brw ...</kbd>
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<br><dt>(long displacement)<dd><kbd>jmp ...</kbd>
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</dl>
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<br><dt><code>j</code><var>COND</var><dd><var>COND</var> may be any one of the conditional branches
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<code>neq</code>, <code>nequ</code>, <code>eql</code>, <code>eqlu</code>, <code>gtr</code>,
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<code>geq</code>, <code>lss</code>, <code>gtru</code>, <code>lequ</code>, <code>vc</code>, <code>vs</code>,
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<code>gequ</code>, <code>cc</code>, <code>lssu</code>, <code>cs</code>.
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<var>COND</var> may also be one of the bit tests
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<code>bs</code>, <code>bc</code>, <code>bss</code>, <code>bcs</code>, <code>bsc</code>, <code>bcc</code>,
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<code>bssi</code>, <code>bcci</code>, <code>lbs</code>, <code>lbc</code>.
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<var>NOTCOND</var> is the opposite condition to <var>COND</var>.
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<dl>
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<dt>(byte displacement)<dd><kbd>b</kbd><var>COND</var><kbd> ...</kbd>
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<br><dt>(word displacement)<dd><kbd>b</kbd><var>NOTCOND</var><kbd> foo ; brw ... ; foo:</kbd>
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<br><dt>(long displacement)<dd><kbd>b</kbd><var>NOTCOND</var><kbd> foo ; jmp ... ; foo:</kbd>
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</dl>
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<br><dt><code>jacb</code><var>X</var><dd><var>X</var> may be one of <code>b d f g h l w</code>.
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<dl>
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<dt>(word displacement)<dd><var>OPCODE</var><kbd> ...</kbd>
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<br><dt>(long displacement)<dd>
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<pre class="example"> <var>OPCODE</var> ..., foo ;
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brb bar ;
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foo: jmp ... ;
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bar:
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</pre>
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</dl>
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<br><dt><code>jaob</code><var>YYY</var><dd><var>YYY</var> may be one of <code>lss leq</code>.
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<br><dt><code>jsob</code><var>ZZZ</var><dd><var>ZZZ</var> may be one of <code>geq gtr</code>.
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<dl>
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<dt>(byte displacement)<dd><var>OPCODE</var><kbd> ...</kbd>
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<br><dt>(word displacement)<dd>
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<pre class="example"> <var>OPCODE</var> ..., foo ;
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brb bar ;
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foo: brw <var>destination</var> ;
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bar:
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</pre>
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<br><dt>(long displacement)<dd>
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<pre class="example"> <var>OPCODE</var> ..., foo ;
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brb bar ;
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foo: jmp <var>destination</var> ;
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bar:
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</pre>
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</dl>
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<br><dt><code>aobleq</code><dt><code>aoblss</code><dt><code>sobgeq</code><dt><code>sobgtr</code><dd>
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<dl>
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<dt>(byte displacement)<dd><var>OPCODE</var><kbd> ...</kbd>
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<br><dt>(word displacement)<dd>
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<pre class="example"> <var>OPCODE</var> ..., foo ;
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brb bar ;
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foo: brw <var>destination</var> ;
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bar:
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</pre>
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<br><dt>(long displacement)<dd>
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<pre class="example"> <var>OPCODE</var> ..., foo ;
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brb bar ;
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foo: jmp <var>destination</var> ;
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bar:
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</pre>
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</dl>
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</dl>
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</body></html>
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