| <html lang="en"> |
| <head> |
| <title>i386-Notes - Using as</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using as"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="i386_002dDependent.html#i386_002dDependent" title="i386-Dependent"> |
| <link rel="prev" href="i386_002dBugs.html#i386_002dBugs" title="i386-Bugs"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU Assembler "as". |
| |
| Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, |
| 2000, 2001, 2002, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 |
| or any later version published by the Free Software Foundation; |
| with no Invariant Sections, with no Front-Cover Texts, and with no |
| Back-Cover Texts. A copy of the license is included in the |
| section entitled ``GNU Free Documentation License''. |
| |
| --> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="i386-Notes"></a> |
| <a name="i386_002dNotes"></a> |
| <p> |
| Previous: <a rel="previous" accesskey="p" href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>, |
| Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">9.13.16 Notes</h4> |
| |
| <p><a name="index-i386-_0040code_007bmul_007d_002c-_0040code_007bimul_007d-instructions-994"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-i386-995"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-i386-996"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-x86_002d64-997"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-x86_002d64-998"></a>There is some trickery concerning the ‘<samp><span class="samp">mul</span></samp>’ and ‘<samp><span class="samp">imul</span></samp>’ |
| instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding |
| multiplies (base opcode ‘<samp><span class="samp">0xf6</span></samp>’; extension 4 for ‘<samp><span class="samp">mul</span></samp>’ and 5 |
| for ‘<samp><span class="samp">imul</span></samp>’) can be output only in the one operand form. Thus, |
| ‘<samp><span class="samp">imul %ebx, %eax</span></samp>’ does <em>not</em> select the expanding multiply; |
| the expanding multiply would clobber the ‘<samp><span class="samp">%edx</span></samp>’ register, and this |
| would confuse <code>gcc</code> output. Use ‘<samp><span class="samp">imul %ebx</span></samp>’ to get the |
| 64-bit product in ‘<samp><span class="samp">%edx:%eax</span></samp>’. |
| |
| <p>We have added a two operand form of ‘<samp><span class="samp">imul</span></samp>’ when the first operand |
| is an immediate mode expression and the second operand is a register. |
| This is just a shorthand, so that, multiplying ‘<samp><span class="samp">%eax</span></samp>’ by 69, for |
| example, can be done with ‘<samp><span class="samp">imul $69, %eax</span></samp>’ rather than ‘<samp><span class="samp">imul |
| $69, %eax, %eax</span></samp>’. |
| |
| <!-- Copyright 2000, 2003 Free Software Foundation, Inc. --> |
| <!-- This is part of the GAS manual. --> |
| <!-- For copying conditions, see the file as.texinfo. --> |
| </body></html> |
| |