74 lines
3.8 KiB
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74 lines
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<title>i386-Jumps - Using as</title>
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<a name="i386-Jumps"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="i386_002dFloat.html#i386_002dFloat">i386-Float</a>,
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Previous: <a rel="previous" accesskey="p" href="i386_002dMemory.html#i386_002dMemory">i386-Memory</a>,
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Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
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<h4 class="subsection">9.16.8 Handling of Jump Instructions</h4>
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<p><a name="index-jump-optimization_002c-i386-1225"></a><a name="index-i386-jump-optimization-1226"></a><a name="index-jump-optimization_002c-x86_002d64-1227"></a><a name="index-x86_002d64-jump-optimization-1228"></a>Jump instructions are always optimized to use the smallest possible
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displacements. This is accomplished by using byte (8-bit) displacement
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jumps whenever the target is sufficiently close. If a byte displacement
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is insufficient a long displacement is used. We do not support
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word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump
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instruction with the ‘<samp><span class="samp">data16</span></samp>’ instruction prefix), since the 80386
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insists upon masking ‘<samp><span class="samp">%eip</span></samp>’ to 16 bits after the word displacement
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is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)
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<p>Note that the ‘<samp><span class="samp">jcxz</span></samp>’, ‘<samp><span class="samp">jecxz</span></samp>’, ‘<samp><span class="samp">loop</span></samp>’, ‘<samp><span class="samp">loopz</span></samp>’,
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‘<samp><span class="samp">loope</span></samp>’, ‘<samp><span class="samp">loopnz</span></samp>’ and ‘<samp><span class="samp">loopne</span></samp>’ instructions only come in byte
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displacements, so that if you use these instructions (<code>gcc</code> does
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not use them) you may get an error message (and incorrect code). The AT&T
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80386 assembler tries to get around this problem by expanding ‘<samp><span class="samp">jcxz foo</span></samp>’
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to
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<pre class="smallexample"> jcxz cx_zero
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jmp cx_nonzero
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cx_zero: jmp foo
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cx_nonzero:
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</pre>
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