vax-branch.html
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<h4 class="section">VAX Branch Improvement</h4>
<p>Certain pseudo opcodes are permitted. They are for branch
instructions. They expand to the shortest branch instruction that
reaches the target. Generally these mnemonics are made by
substituting <code>j</code> for <code>b</code> at the start of a DEC mnemonic.
This feature is included both for compatibility and to help
compilers. If you do not need this feature, avoid these
opcodes. Here are the mnemonics, and the code they can expand into.
<dl>
<dt><code>jbsb</code>
<dd><code>Jsb</code> is already an instruction mnemonic, so we chose <code>jbsb</code>.
<dl>
<dt>(byte displacement)
<dd><kbd>bsbb ...</kbd>
<br><dt>(word displacement)
<dd><kbd>bsbw ...</kbd>
<br><dt>(long displacement)
<dd><kbd>jsb ...</kbd>
</dl>
<br><dt><code>jbr</code>
<dd><dt><code>jr</code>
<dd>Unconditional branch.
<dl>
<dt>(byte displacement)
<dd><kbd>brb ...</kbd>
<br><dt>(word displacement)
<dd><kbd>brw ...</kbd>
<br><dt>(long displacement)
<dd><kbd>jmp ...</kbd>
</dl>
<br><dt><code>j</code><var>COND</var><code></code>
<dd><var>COND</var> may be any one of the conditional branches
<code>neq</code>, <code>nequ</code>, <code>eql</code>, <code>eqlu</code>, <code>gtr</code>,
<code>geq</code>, <code>lss</code>, <code>gtru</code>, <code>lequ</code>, <code>vc</code>, <code>vs</code>,
<code>gequ</code>, <code>cc</code>, <code>lssu</code>, <code>cs</code>.
<var>COND</var> may also be one of the bit tests
<code>bs</code>, <code>bc</code>, <code>bss</code>, <code>bcs</code>, <code>bsc</code>, <code>bcc</code>,
<code>bssi</code>, <code>bcci</code>, <code>lbs</code>, <code>lbc</code>.
<var>NOTCOND</var> is the opposite condition to <var>COND</var>.
<dl>
<dt>(byte displacement)
<dd><kbd>b</kbd><var>COND</var><kbd> ...</kbd>
<br><dt>(word displacement)
<dd><kbd>b</kbd><var>NOTCOND</var><kbd> foo ; brw ... ; foo:</kbd>
<br><dt>(long displacement)
<dd><kbd>b</kbd><var>NOTCOND</var><kbd> foo ; jmp ... ; foo:</kbd>
</dl>
<br><dt><code>jacb</code><var>X</var><code></code>
<dd><var>X</var> may be one of <code>b d f g h l w</code>.
<dl>
<dt>(word displacement)
<dd><kbd></kbd><var>OPCODE</var><kbd> ...</kbd>
<br><dt>(long displacement)
<dd>
<pre class="example"> <var>OPCODE</var> ..., foo ;
brb bar ;
foo: jmp ... ;
bar:
</pre>
</dl>
<br><dt><code>jaob</code><var>YYY</var><code></code>
<dd><var>YYY</var> may be one of <code>lss leq</code>.
<br><dt><code>jsob</code><var>ZZZ</var><code></code>
<dd><var>ZZZ</var> may be one of <code>geq gtr</code>.
<dl>
<dt>(byte displacement)
<dd><kbd></kbd><var>OPCODE</var><kbd> ...</kbd>
<br><dt>(word displacement)
<dd>
<pre class="example"> <var>OPCODE</var> ..., foo ;
brb bar ;
foo: brw <var>destination</var> ;
bar:
</pre>
<br><dt>(long displacement)
<dd>
<pre class="example"> <var>OPCODE</var> ..., foo ;
brb bar ;
foo: jmp <var>destination</var> ;
bar:
</pre>
</dl>
<br><dt><code>aobleq</code>
<dd><dt><code>aoblss</code>
<dd><dt><code>sobgeq</code>
<dd><dt><code>sobgtr</code>
<dd>
<dl>
<dt>(byte displacement)
<dd><kbd></kbd><var>OPCODE</var><kbd> ...</kbd>
<br><dt>(word displacement)
<dd>
<pre class="example"> <var>OPCODE</var> ..., foo ;
brb bar ;
foo: brw <var>destination</var> ;
bar:
</pre>
<br><dt>(long displacement)
<dd>
<pre class="example"> <var>OPCODE</var> ..., foo ;
brb bar ;
foo: jmp <var>destination</var> ;
bar:
</pre>
</dl>
</dl>
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