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73 lines
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<title>MMIX-Expand - Using as</title>
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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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<a name="MMIX_002dExpand"></a>Next: <a rel="next" accesskey="n" href="MMIX_002dSyntax.html#MMIX_002dSyntax">MMIX-Syntax</a>,
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Previous: <a rel="previous" accesskey="p" href="MMIX_002dOpts.html#MMIX_002dOpts">MMIX-Opts</a>,
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Up: <a rel="up" accesskey="u" href="MMIX_002dDependent.html#MMIX_002dDependent">MMIX-Dependent</a>
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<h4 class="subsection">9.23.2 Instruction expansion</h4>
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<p><a name="index-instruction-expansion_002c-MMIX-1263"></a>When <code>as</code> encounters an instruction with an operand that is
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either not known or does not fit the operand size of the instruction,
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<code>as</code> (and <code>ld</code>) will expand the instruction into
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a sequence of instructions semantically equivalent to the operand fitting
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the instruction. Expansion will take place for the following
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instructions:
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<dl>
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<dt><span class="samp">GETA</span><dd>Expands to a sequence of four instructions: <code>SETL</code>, <code>INCML</code>,
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<code>INCMH</code> and <code>INCH</code>. The operand must be a multiple of four.
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<br><dt>Conditional branches<dd>A branch instruction is turned into a branch with the complemented
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condition and prediction bit over five instructions; four instructions
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setting <code>$255</code> to the operand value, which like with <code>GETA</code> must
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be a multiple of four, and a final <code>GO $255,$255,0</code>.
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<br><dt><span class="samp">PUSHJ</span><dd>Similar to expansion for conditional branches; four instructions set
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<code>$255</code> to the operand value, followed by a <code>PUSHGO $255,$255,0</code>.
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<br><dt><span class="samp">JMP</span><dd>Similar to conditional branches and <code>PUSHJ</code>. The final instruction
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is <code>GO $255,$255,0</code>.
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</dl>
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<p>The linker <code>ld</code> is expected to shrink these expansions for
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code assembled with <span class="samp">--relax</span> (though not currently implemented).
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