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| Next: <a href="MMIX_002dSyntax.html#MMIX_002dSyntax" accesskey="n" rel="next">MMIX-Syntax</a>, Previous: <a href="MMIX_002dOpts.html#MMIX_002dOpts" accesskey="p" rel="prev">MMIX-Opts</a>, Up: <a href="MMIX_002dDependent.html#MMIX_002dDependent" accesskey="u" rel="up">MMIX-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Instruction-expansion-1"></a> |
| <h4 class="subsection">9.28.2 Instruction expansion</h4> |
| |
| <a name="index-instruction-expansion_002c-MMIX"></a> |
| <p>When <code>as</code> encounters an instruction with an operand that is |
| either not known or does not fit the operand size of the instruction, |
| <code>as</code> (and <code>ld</code>) will expand the instruction into |
| a sequence of instructions semantically equivalent to the operand fitting |
| the instruction. Expansion will take place for the following |
| instructions: |
| </p> |
| <dl compact="compact"> |
| <dt>‘<samp>GETA</samp>’</dt> |
| <dd><p>Expands to a sequence of four instructions: <code>SETL</code>, <code>INCML</code>, |
| <code>INCMH</code> and <code>INCH</code>. The operand must be a multiple of four. |
| </p></dd> |
| <dt>Conditional branches</dt> |
| <dd><p>A branch instruction is turned into a branch with the complemented |
| condition and prediction bit over five instructions; four instructions |
| setting <code>$255</code> to the operand value, which like with <code>GETA</code> must |
| be a multiple of four, and a final <code>GO $255,$255,0</code>. |
| </p></dd> |
| <dt>‘<samp>PUSHJ</samp>’</dt> |
| <dd><p>Similar to expansion for conditional branches; four instructions set |
| <code>$255</code> to the operand value, followed by a <code>PUSHGO $255,$255,0</code>. |
| </p></dd> |
| <dt>‘<samp>JMP</samp>’</dt> |
| <dd><p>Similar to conditional branches and <code>PUSHJ</code>. The final instruction |
| is <code>GO $255,$255,0</code>. |
| </p></dd> |
| </dl> |
| |
| <p>The linker <code>ld</code> is expected to shrink these expansions for |
| code assembled with ‘<samp>--relax</samp>’ (though not currently implemented). |
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