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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-Notes (Using as)</title>

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

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  65. Previous: <a href="i386_002dBugs.html#i386_002dBugs" accesskey="p" rel="prev">i386-Bugs</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="Notes-2"></a>

  69. <h4 class="subsection">9.16.18 Notes</h4>

  70. 

  71. <a name="index-i386-mul_002c-imul-instructions"></a>

  72. <a name="index-mul-instruction_002c-i386"></a>

  73. <a name="index-imul-instruction_002c-i386"></a>

  74. <a name="index-mul-instruction_002c-x86_002d64"></a>

  75. <a name="index-imul-instruction_002c-x86_002d64"></a>

  76. <p>There is some trickery concerning the &lsquo;<samp>mul</samp>&rsquo; and &lsquo;<samp>imul</samp>&rsquo;

  77. instructions that deserves mention.  The 16-, 32-, 64- and 128-bit expanding

  78. multiplies (base opcode &lsquo;<samp>0xf6</samp>&rsquo;; extension 4 for &lsquo;<samp>mul</samp>&rsquo; and 5

  79. for &lsquo;<samp>imul</samp>&rsquo;) can be output only in the one operand form.  Thus,

  80. &lsquo;<samp>imul %ebx, %eax</samp>&rsquo; does <em>not</em> select the expanding multiply;

  81. the expanding multiply would clobber the &lsquo;<samp>%edx</samp>&rsquo; register, and this

  82. would confuse <code>gcc</code> output.  Use &lsquo;<samp>imul %ebx</samp>&rsquo; to get the

  83. 64-bit product in &lsquo;<samp>%edx:%eax</samp>&rsquo;.

  84. </p>

  85. <p>We have added a two operand form of &lsquo;<samp>imul</samp>&rsquo; when the first operand

  86. is an immediate mode expression and the second operand is a register.

  87. This is just a shorthand, so that, multiplying &lsquo;<samp>%eax</samp>&rsquo; by 69, for

  88. example, can be done with &lsquo;<samp>imul $69, %eax</samp>&rsquo; rather than &lsquo;<samp>imul

  89. $69, %eax, %eax</samp>&rsquo;.

  90. </p>

  91. 

  92. 

  93. 

  94. 

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