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| <h4 class="subsection">9.13.3 AT&T Syntax versus Intel Syntax</h4> |
| |
| <p><a name="index-i386-intel_005fsyntax-pseudo-op-872"></a><a name="index-intel_005fsyntax-pseudo-op_002c-i386-873"></a><a name="index-i386-att_005fsyntax-pseudo-op-874"></a><a name="index-att_005fsyntax-pseudo-op_002c-i386-875"></a><a name="index-i386-syntax-compatibility-876"></a><a name="index-syntax-compatibility_002c-i386-877"></a><a name="index-x86_002d64-intel_005fsyntax-pseudo-op-878"></a><a name="index-intel_005fsyntax-pseudo-op_002c-x86_002d64-879"></a><a name="index-x86_002d64-att_005fsyntax-pseudo-op-880"></a><a name="index-att_005fsyntax-pseudo-op_002c-x86_002d64-881"></a><a name="index-x86_002d64-syntax-compatibility-882"></a><a name="index-syntax-compatibility_002c-x86_002d64-883"></a> |
| <code>as</code> now supports assembly using Intel assembler syntax. |
| <code>.intel_syntax</code> selects Intel mode, and <code>.att_syntax</code> switches |
| back to the usual AT&T mode for compatibility with the output of |
| <code>gcc</code>. Either of these directives may have an optional |
| argument, <code>prefix</code>, or <code>noprefix</code> specifying whether registers |
| require a ‘<samp><span class="samp">%</span></samp>’ prefix. AT&T System V/386 assembler syntax is quite |
| different from Intel syntax. We mention these differences because |
| almost all 80386 documents use Intel syntax. Notable differences |
| between the two syntaxes are: |
| |
| <p><a name="index-immediate-operands_002c-i386-884"></a><a name="index-i386-immediate-operands-885"></a><a name="index-register-operands_002c-i386-886"></a><a name="index-i386-register-operands-887"></a><a name="index-jump_002fcall-operands_002c-i386-888"></a><a name="index-i386-jump_002fcall-operands-889"></a><a name="index-operand-delimiters_002c-i386-890"></a> |
| <a name="index-immediate-operands_002c-x86_002d64-891"></a><a name="index-x86_002d64-immediate-operands-892"></a><a name="index-register-operands_002c-x86_002d64-893"></a><a name="index-x86_002d64-register-operands-894"></a><a name="index-jump_002fcall-operands_002c-x86_002d64-895"></a><a name="index-x86_002d64-jump_002fcall-operands-896"></a><a name="index-operand-delimiters_002c-x86_002d64-897"></a> |
| <ul> |
| <li>AT&T immediate operands are preceded by ‘<samp><span class="samp">$</span></samp>’; Intel immediate |
| operands are undelimited (Intel ‘<samp><span class="samp">push 4</span></samp>’ is AT&T ‘<samp><span class="samp">pushl $4</span></samp>’). |
| AT&T register operands are preceded by ‘<samp><span class="samp">%</span></samp>’; Intel register operands |
| are undelimited. AT&T absolute (as opposed to PC relative) jump/call |
| operands are prefixed by ‘<samp><span class="samp">*</span></samp>’; they are undelimited in Intel syntax. |
| |
| <p><a name="index-i386-source_002c-destination-operands-898"></a><a name="index-source_002c-destination-operands_003b-i386-899"></a><a name="index-x86_002d64-source_002c-destination-operands-900"></a><a name="index-source_002c-destination-operands_003b-x86_002d64-901"></a><li>AT&T and Intel syntax use the opposite order for source and destination |
| operands. Intel ‘<samp><span class="samp">add eax, 4</span></samp>’ is ‘<samp><span class="samp">addl $4, %eax</span></samp>’. The |
| ‘<samp><span class="samp">source, dest</span></samp>’ convention is maintained for compatibility with |
| previous Unix assemblers. Note that ‘<samp><span class="samp">bound</span></samp>’, ‘<samp><span class="samp">invlpga</span></samp>’, and |
| instructions with 2 immediate operands, such as the ‘<samp><span class="samp">enter</span></samp>’ |
| instruction, do <em>not</em> have reversed order. <a href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>. |
| |
| <p><a name="index-mnemonic-suffixes_002c-i386-902"></a><a name="index-sizes-operands_002c-i386-903"></a><a name="index-i386-size-suffixes-904"></a><a name="index-mnemonic-suffixes_002c-x86_002d64-905"></a><a name="index-sizes-operands_002c-x86_002d64-906"></a><a name="index-x86_002d64-size-suffixes-907"></a><li>In AT&T syntax the size of memory operands is determined from the last |
| character of the instruction mnemonic. Mnemonic suffixes of ‘<samp><span class="samp">b</span></samp>’, |
| ‘<samp><span class="samp">w</span></samp>’, ‘<samp><span class="samp">l</span></samp>’ and ‘<samp><span class="samp">q</span></samp>’ specify byte (8-bit), word (16-bit), long |
| (32-bit) and quadruple word (64-bit) memory references. Intel syntax accomplishes |
| this by prefixing memory operands (<em>not</em> the instruction mnemonics) with |
| ‘<samp><span class="samp">byte ptr</span></samp>’, ‘<samp><span class="samp">word ptr</span></samp>’, ‘<samp><span class="samp">dword ptr</span></samp>’ and ‘<samp><span class="samp">qword ptr</span></samp>’. Thus, |
| Intel ‘<samp><span class="samp">mov al, byte ptr </span><var>foo</var></samp>’ is ‘<samp><span class="samp">movb </span><var>foo</var><span class="samp">, %al</span></samp>’ in AT&T |
| syntax. |
| |
| <p>In 64-bit code, ‘<samp><span class="samp">movabs</span></samp>’ can be used to encode the ‘<samp><span class="samp">mov</span></samp>’ |
| instruction with the 64-bit displacement or immediate operand. |
| |
| <p><a name="index-return-instructions_002c-i386-908"></a><a name="index-i386-jump_002c-call_002c-return-909"></a><a name="index-return-instructions_002c-x86_002d64-910"></a><a name="index-x86_002d64-jump_002c-call_002c-return-911"></a><li>Immediate form long jumps and calls are |
| ‘<samp><span class="samp">lcall/ljmp $</span><var>section</var><span class="samp">, $</span><var>offset</var></samp>’ in AT&T syntax; the |
| Intel syntax is |
| ‘<samp><span class="samp">call/jmp far </span><var>section</var><span class="samp">:</span><var>offset</var></samp>’. Also, the far return |
| instruction |
| is ‘<samp><span class="samp">lret $</span><var>stack-adjust</var></samp>’ in AT&T syntax; Intel syntax is |
| ‘<samp><span class="samp">ret far </span><var>stack-adjust</var></samp>’. |
| |
| <p><a name="index-sections_002c-i386-912"></a><a name="index-i386-sections-913"></a><a name="index-sections_002c-x86_002d64-914"></a><a name="index-x86_002d64-sections-915"></a><li>The AT&T assembler does not provide support for multiple section |
| programs. Unix style systems expect all programs to be single sections. |
| </ul> |
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