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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

  2. <html>

  3. <!-- This file documents the GNU Assembler "as".

  4. 

  5. Copyright (C) 1991-2020 Free Software Foundation, Inc.

  6. 

  7. Permission is granted to copy, distribute and/or modify this document

  8. under the terms of the GNU Free Documentation License, Version 1.3

  9. or any later version published by the Free Software Foundation;

  10. with no Invariant Sections, with no Front-Cover Texts, and with no

  11. Back-Cover Texts.  A copy of the license is included in the

  12. section entitled "GNU Free Documentation License".

  13.  -->

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  15. <head>

  16. <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

  17. <title>i386-Jumps (Using as)</title>

  18. 

  19. <meta name="description" content="i386-Jumps (Using as)">

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  25. <link href="AS-Index.html#AS-Index" rel="index" title="AS Index">

  26. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  27. <link href="i386_002dDependent.html#i386_002dDependent" rel="up" title="i386-Dependent">

  28. <link href="i386_002dFloat.html#i386_002dFloat" rel="next" title="i386-Float">

  29. <link href="i386_002dMemory.html#i386_002dMemory" rel="prev" title="i386-Memory">

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  60. 

  61. <body lang="en">

  62. <a name="i386_002dJumps"></a>

  63. <div class="header">

  64. <p>

  65. Next: <a href="i386_002dFloat.html#i386_002dFloat" accesskey="n" rel="next">i386-Float</a>, Previous: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="p" rel="prev">i386-Memory</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> &nbsp; [<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>

  66. </div>

  67. <hr>

  68. <a name="Handling-of-Jump-Instructions"></a>

  69. <h4 class="subsection">9.16.8 Handling of Jump Instructions</h4>

  70. 

  71. <a name="index-jump-optimization_002c-i386"></a>

  72. <a name="index-i386-jump-optimization"></a>

  73. <a name="index-jump-optimization_002c-x86_002d64"></a>

  74. <a name="index-x86_002d64-jump-optimization"></a>

  75. <p>Jump instructions are always optimized to use the smallest possible

  76. displacements.  This is accomplished by using byte (8-bit) displacement

  77. jumps whenever the target is sufficiently close.  If a byte displacement

  78. is insufficient a long displacement is used.  We do not support

  79. word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump

  80. instruction with the &lsquo;<samp>data16</samp>&rsquo; instruction prefix), since the 80386

  81. insists upon masking &lsquo;<samp>%eip</samp>&rsquo; to 16 bits after the word displacement

  82. is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)

  83. </p>

  84. <p>Note that the &lsquo;<samp>jcxz</samp>&rsquo;, &lsquo;<samp>jecxz</samp>&rsquo;, &lsquo;<samp>loop</samp>&rsquo;, &lsquo;<samp>loopz</samp>&rsquo;,

  85. &lsquo;<samp>loope</samp>&rsquo;, &lsquo;<samp>loopnz</samp>&rsquo; and &lsquo;<samp>loopne</samp>&rsquo; instructions only come in byte

  86. displacements, so that if you use these instructions (<code>gcc</code> does

  87. not use them) you may get an error message (and incorrect code).  The AT&amp;T

  88. 80386 assembler tries to get around this problem by expanding &lsquo;<samp>jcxz foo</samp>&rsquo;

  89. to

  90. </p>

  91. <div class="smallexample">

  92. <pre class="smallexample">         jcxz cx_zero

  93.          jmp cx_nonzero

  94. cx_zero: jmp foo

  95. cx_nonzero:

  96. </pre></div>

  97. 

  98. 

  99. 

  100. 

  101. </body>

  102. </html>