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

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

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  62. <a name="i386_002dPrefixes"></a>

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  65. Next: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="n" rel="next">i386-Memory</a>, Previous: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="p" rel="prev">i386-Regs</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="Instruction-Prefixes"></a>

  69. <h4 class="subsection">9.16.6 Instruction Prefixes</h4>

  70. 

  71. <a name="index-i386-instruction-prefixes"></a>

  72. <a name="index-instruction-prefixes_002c-i386"></a>

  73. <a name="index-prefixes_002c-i386"></a>

  74. <p>Instruction prefixes are used to modify the following instruction.  They

  75. are used to repeat string instructions, to provide section overrides, to

  76. perform bus lock operations, and to change operand and address sizes.

  77. (Most instructions that normally operate on 32-bit operands will use

  78. 16-bit operands if the instruction has an &ldquo;operand size&rdquo; prefix.)

  79. Instruction prefixes are best written on the same line as the instruction

  80. they act upon. For example, the &lsquo;<samp>scas</samp>&rsquo; (scan string) instruction is

  81. repeated with:

  82. </p>

  83. <div class="smallexample">

  84. <pre class="smallexample">        repne scas %es:(%edi),%al

  85. </pre></div>

  86. 

  87. <p>You may also place prefixes on the lines immediately preceding the

  88. instruction, but this circumvents checks that <code>as</code> does

  89. with prefixes, and will not work with all prefixes.

  90. </p>

  91. <p>Here is a list of instruction prefixes:

  92. </p>

  93. <a name="index-section-override-prefixes_002c-i386"></a>

  94. <ul>

  95. <li> Section override prefixes &lsquo;<samp>cs</samp>&rsquo;, &lsquo;<samp>ds</samp>&rsquo;, &lsquo;<samp>ss</samp>&rsquo;, &lsquo;<samp>es</samp>&rsquo;,

  96. &lsquo;<samp>fs</samp>&rsquo;, &lsquo;<samp>gs</samp>&rsquo;.  These are automatically added by specifying

  97. using the <var>section</var>:<var>memory-operand</var> form for memory references.

  98. 

  99. </li><li> <a name="index-size-prefixes_002c-i386"></a>

  100. Operand/Address size prefixes &lsquo;<samp>data16</samp>&rsquo; and &lsquo;<samp>addr16</samp>&rsquo;

  101. change 32-bit operands/addresses into 16-bit operands/addresses,

  102. while &lsquo;<samp>data32</samp>&rsquo; and &lsquo;<samp>addr32</samp>&rsquo; change 16-bit ones (in a

  103. <code>.code16</code> section) into 32-bit operands/addresses.  These prefixes

  104. <em>must</em> appear on the same line of code as the instruction they

  105. modify. For example, in a 16-bit <code>.code16</code> section, you might

  106. write:

  107. 

  108. <div class="smallexample">

  109. <pre class="smallexample">        addr32 jmpl *(%ebx)

  110. </pre></div>

  111. 

  112. </li><li> <a name="index-bus-lock-prefixes_002c-i386"></a>

  113. <a name="index-inhibiting-interrupts_002c-i386"></a>

  114. The bus lock prefix &lsquo;<samp>lock</samp>&rsquo; inhibits interrupts during execution of

  115. the instruction it precedes.  (This is only valid with certain

  116. instructions; see a 80386 manual for details).

  117. 

  118. </li><li> <a name="index-coprocessor-wait_002c-i386"></a>

  119. The wait for coprocessor prefix &lsquo;<samp>wait</samp>&rsquo; waits for the coprocessor to

  120. complete the current instruction.  This should never be needed for the

  121. 80386/80387 combination.

  122. 

  123. </li><li> <a name="index-repeat-prefixes_002c-i386"></a>

  124. The &lsquo;<samp>rep</samp>&rsquo;, &lsquo;<samp>repe</samp>&rsquo;, and &lsquo;<samp>repne</samp>&rsquo; prefixes are added

  125. to string instructions to make them repeat &lsquo;<samp>%ecx</samp>&rsquo; times (&lsquo;<samp>%cx</samp>&rsquo;

  126. times if the current address size is 16-bits).

  127. </li><li> <a name="index-REX-prefixes_002c-i386"></a>

  128. The &lsquo;<samp>rex</samp>&rsquo; family of prefixes is used by x86-64 to encode

  129. extensions to i386 instruction set.  The &lsquo;<samp>rex</samp>&rsquo; prefix has four

  130. bits &mdash; an operand size overwrite (<code>64</code>) used to change operand size

  131. from 32-bit to 64-bit and X, Y and Z extensions bits used to extend the

  132. register set.

  133. 

  134. <p>You may write the &lsquo;<samp>rex</samp>&rsquo; prefixes directly. The &lsquo;<samp>rex64xyz</samp>&rsquo;

  135. instruction emits &lsquo;<samp>rex</samp>&rsquo; prefix with all the bits set.  By omitting

  136. the <code>64</code>, <code>x</code>, <code>y</code> or <code>z</code> you may write other

  137. prefixes as well.  Normally, there is no need to write the prefixes

  138. explicitly, since gas will automatically generate them based on the

  139. instruction operands.

  140. </p></li></ul>

  141. 

  142. <hr>

  143. <div class="header">

  144. <p>

  145. Next: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="n" rel="next">i386-Memory</a>, Previous: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="p" rel="prev">i386-Regs</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>

  146. </div>

  147. 

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