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<a name="i386_002dMemory"></a>
<div class="header">
<p>
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<hr>
<a name="Memory-References"></a>
<h4 class="subsection">9.16.7 Memory References</h4>

<a name="index-i386-memory-references"></a>
<a name="index-memory-references_002c-i386"></a>
<a name="index-x86_002d64-memory-references"></a>
<a name="index-memory-references_002c-x86_002d64"></a>
<p>An Intel syntax indirect memory reference of the form
</p>
<div class="smallexample">
<pre class="smallexample"><var>section</var>:[<var>base</var> + <var>index</var>*<var>scale</var> + <var>disp</var>]
</pre></div>

<p>is translated into the AT&amp;T syntax
</p>
<div class="smallexample">
<pre class="smallexample"><var>section</var>:<var>disp</var>(<var>base</var>, <var>index</var>, <var>scale</var>)
</pre></div>

<p>where <var>base</var> and <var>index</var> are the optional 32-bit base and
index registers, <var>disp</var> is the optional displacement, and
<var>scale</var>, taking the values 1, 2, 4, and 8, multiplies <var>index</var>
to calculate the address of the operand.  If no <var>scale</var> is
specified, <var>scale</var> is taken to be 1.  <var>section</var> specifies the
optional section register for the memory operand, and may override the
default section register (see a 80386 manual for section register
defaults). Note that section overrides in AT&amp;T syntax <em>must</em>
be preceded by a &lsquo;<samp>%</samp>&rsquo;.  If you specify a section override which
coincides with the default section register, <code>as</code> does <em>not</em>
output any section register override prefixes to assemble the given
instruction.  Thus, section overrides can be specified to emphasize which
section register is used for a given memory operand.
</p>
<p>Here are some examples of Intel and AT&amp;T style memory references:
</p>
<dl compact="compact">
<dt>AT&amp;T: &lsquo;<samp>-4(%ebp)</samp>&rsquo;, Intel:  &lsquo;<samp>[ebp - 4]</samp>&rsquo;</dt>
<dd><p><var>base</var> is &lsquo;<samp>%ebp</samp>&rsquo;; <var>disp</var> is &lsquo;<samp>-4</samp>&rsquo;. <var>section</var> is
missing, and the default section is used (&lsquo;<samp>%ss</samp>&rsquo; for addressing with
&lsquo;<samp>%ebp</samp>&rsquo; as the base register).  <var>index</var>, <var>scale</var> are both missing.
</p>
</dd>
<dt>AT&amp;T: &lsquo;<samp>foo(,%eax,4)</samp>&rsquo;, Intel: &lsquo;<samp>[foo + eax*4]</samp>&rsquo;</dt>
<dd><p><var>index</var> is &lsquo;<samp>%eax</samp>&rsquo; (scaled by a <var>scale</var> 4); <var>disp</var> is
&lsquo;<samp>foo</samp>&rsquo;.  All other fields are missing.  The section register here
defaults to &lsquo;<samp>%ds</samp>&rsquo;.
</p>
</dd>
<dt>AT&amp;T: &lsquo;<samp>foo(,1)</samp>&rsquo;; Intel &lsquo;<samp>[foo]</samp>&rsquo;</dt>
<dd><p>This uses the value pointed to by &lsquo;<samp>foo</samp>&rsquo; as a memory operand.
Note that <var>base</var> and <var>index</var> are both missing, but there is only
<em>one</em> &lsquo;<samp>,</samp>&rsquo;.  This is a syntactic exception.
</p>
</dd>
<dt>AT&amp;T: &lsquo;<samp>%gs:foo</samp>&rsquo;; Intel &lsquo;<samp>gs:foo</samp>&rsquo;</dt>
<dd><p>This selects the contents of the variable &lsquo;<samp>foo</samp>&rsquo; with section
register <var>section</var> being &lsquo;<samp>%gs</samp>&rsquo;.
</p></dd>
</dl>

<p>Absolute (as opposed to PC relative) call and jump operands must be
prefixed with &lsquo;<samp>*</samp>&rsquo;.  If no &lsquo;<samp>*</samp>&rsquo; is specified, <code>as</code>
always chooses PC relative addressing for jump/call labels.
</p>
<p>Any instruction that has a memory operand, but no register operand,
<em>must</em> specify its size (byte, word, long, or quadruple) with an
instruction mnemonic suffix (&lsquo;<samp>b</samp>&rsquo;, &lsquo;<samp>w</samp>&rsquo;, &lsquo;<samp>l</samp>&rsquo; or &lsquo;<samp>q</samp>&rsquo;,
respectively).
</p>
<p>The x86-64 architecture adds an RIP (instruction pointer relative)
addressing.  This addressing mode is specified by using &lsquo;<samp>rip</samp>&rsquo; as a
base register.  Only constant offsets are valid. For example:
</p>
<dl compact="compact">
<dt>AT&amp;T: &lsquo;<samp>1234(%rip)</samp>&rsquo;, Intel: &lsquo;<samp>[rip + 1234]</samp>&rsquo;</dt>
<dd><p>Points to the address 1234 bytes past the end of the current
instruction.
</p>
</dd>
<dt>AT&amp;T: &lsquo;<samp>symbol(%rip)</samp>&rsquo;, Intel: &lsquo;<samp>[rip + symbol]</samp>&rsquo;</dt>
<dd><p>Points to the <code>symbol</code> in RIP relative way, this is shorter than
the default absolute addressing.
</p></dd>
</dl>

<p>Other addressing modes remain unchanged in x86-64 architecture, except
registers used are 64-bit instead of 32-bit.
</p>
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