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| <h4 class="subsection">9.40.5 VAX Branch Improvement</h4> |
| |
| <p><a name="index-VAX-branch-improvement-1924"></a><a name="index-branch-improvement_002c-VAX-1925"></a><a name="index-pseudo_002dops-for-branch_002c-VAX-1926"></a>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 ‘<samp><span class="samp">j</span></samp>’ for ‘<samp><span class="samp">b</span></samp>’ 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>‘<samp><span class="samp">Jsb</span></samp>’ is already an instruction mnemonic, so we chose ‘<samp><span class="samp">jbsb</span></samp>’. |
| <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><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><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><dd><var>X</var> may be one of <code>b d f g h l w</code>. |
| <dl> |
| <dt>(word displacement)<dd><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><dd><var>YYY</var> may be one of <code>lss leq</code>. |
| <br><dt><code>jsob</code><var>ZZZ</var><dd><var>ZZZ</var> may be one of <code>geq gtr</code>. |
| <dl> |
| <dt>(byte displacement)<dd><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><dt><code>aoblss</code><dt><code>sobgeq</code><dt><code>sobgtr</code><dd> |
| <dl> |
| <dt>(byte displacement)<dd><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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