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| <h4 class="subsection">9.13.8 Memory References</h4> |
| |
| <p><a name="index-i386-memory-references-946"></a><a name="index-memory-references_002c-i386-947"></a><a name="index-x86_002d64-memory-references-948"></a><a name="index-memory-references_002c-x86_002d64-949"></a>An Intel syntax indirect memory reference of the form |
| |
| <pre class="smallexample"> <var>section</var>:[<var>base</var> + <var>index</var>*<var>scale</var> + <var>disp</var>] |
| </pre> |
| <p class="noindent">is translated into the AT&T syntax |
| |
| <pre class="smallexample"> <var>section</var>:<var>disp</var>(<var>base</var>, <var>index</var>, <var>scale</var>) |
| </pre> |
| <p class="noindent">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&T syntax <em>must</em> |
| be preceded by a ‘<samp><span class="samp">%</span></samp>’. 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>Here are some examples of Intel and AT&T style memory references: |
| |
| <dl> |
| <dt>AT&T: ‘<samp><span class="samp">-4(%ebp)</span></samp>’, Intel: ‘<samp><span class="samp">[ebp - 4]</span></samp>’<dd><var>base</var> is ‘<samp><span class="samp">%ebp</span></samp>’; <var>disp</var> is ‘<samp><span class="samp">-4</span></samp>’. <var>section</var> is |
| missing, and the default section is used (‘<samp><span class="samp">%ss</span></samp>’ for addressing with |
| ‘<samp><span class="samp">%ebp</span></samp>’ as the base register). <var>index</var>, <var>scale</var> are both missing. |
| |
| <br><dt>AT&T: ‘<samp><span class="samp">foo(,%eax,4)</span></samp>’, Intel: ‘<samp><span class="samp">[foo + eax*4]</span></samp>’<dd><var>index</var> is ‘<samp><span class="samp">%eax</span></samp>’ (scaled by a <var>scale</var> 4); <var>disp</var> is |
| ‘<samp><span class="samp">foo</span></samp>’. All other fields are missing. The section register here |
| defaults to ‘<samp><span class="samp">%ds</span></samp>’. |
| |
| <br><dt>AT&T: ‘<samp><span class="samp">foo(,1)</span></samp>’; Intel ‘<samp><span class="samp">[foo]</span></samp>’<dd>This uses the value pointed to by ‘<samp><span class="samp">foo</span></samp>’ as a memory operand. |
| Note that <var>base</var> and <var>index</var> are both missing, but there is only |
| <em>one</em> ‘<samp><span class="samp">,</span></samp>’. This is a syntactic exception. |
| |
| <br><dt>AT&T: ‘<samp><span class="samp">%gs:foo</span></samp>’; Intel ‘<samp><span class="samp">gs:foo</span></samp>’<dd>This selects the contents of the variable ‘<samp><span class="samp">foo</span></samp>’ with section |
| register <var>section</var> being ‘<samp><span class="samp">%gs</span></samp>’. |
| </dl> |
| |
| <p>Absolute (as opposed to PC relative) call and jump operands must be |
| prefixed with ‘<samp><span class="samp">*</span></samp>’. If no ‘<samp><span class="samp">*</span></samp>’ is specified, <code>as</code> |
| always chooses PC relative addressing for jump/call labels. |
| |
| <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 (‘<samp><span class="samp">b</span></samp>’, ‘<samp><span class="samp">w</span></samp>’, ‘<samp><span class="samp">l</span></samp>’ or ‘<samp><span class="samp">q</span></samp>’, |
| respectively). |
| |
| <p>The x86-64 architecture adds an RIP (instruction pointer relative) |
| addressing. This addressing mode is specified by using ‘<samp><span class="samp">rip</span></samp>’ as a |
| base register. Only constant offsets are valid. For example: |
| |
| <dl> |
| <dt>AT&T: ‘<samp><span class="samp">1234(%rip)</span></samp>’, Intel: ‘<samp><span class="samp">[rip + 1234]</span></samp>’<dd>Points to the address 1234 bytes past the end of the current |
| instruction. |
| |
| <br><dt>AT&T: ‘<samp><span class="samp">symbol(%rip)</span></samp>’, Intel: ‘<samp><span class="samp">[rip + symbol]</span></samp>’<dd>Points to the <code>symbol</code> in RIP relative way, this is shorter than |
| the default absolute addressing. |
| </dl> |
| |
| <p>Other addressing modes remain unchanged in x86-64 architecture, except |
| registers used are 64-bit instead of 32-bit. |
| |
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