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| <title>Using as: i386-Variations</title> |
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| <a name="i386_002dVariations"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="i386_002dChars.html#i386_002dChars" accesskey="n" rel="next">i386-Chars</a>, Up: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="u" rel="up">i386-Syntax</a> [<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> |
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| <hr> |
| <a name="AT_0026T-Syntax-versus-Intel-Syntax"></a> |
| <h4 class="subsubsection">9.15.3.1 AT&T Syntax versus Intel Syntax</h4> |
| |
| <a name="index-i386-intel_005fsyntax-pseudo-op"></a> |
| <a name="index-intel_005fsyntax-pseudo-op_002c-i386"></a> |
| <a name="index-i386-att_005fsyntax-pseudo-op"></a> |
| <a name="index-att_005fsyntax-pseudo-op_002c-i386"></a> |
| <a name="index-i386-syntax-compatibility"></a> |
| <a name="index-syntax-compatibility_002c-i386"></a> |
| <a name="index-x86_002d64-intel_005fsyntax-pseudo-op"></a> |
| <a name="index-intel_005fsyntax-pseudo-op_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-att_005fsyntax-pseudo-op"></a> |
| <a name="index-att_005fsyntax-pseudo-op_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-syntax-compatibility"></a> |
| <a name="index-syntax-compatibility_002c-x86_002d64"></a> |
| |
| <p><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>%</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"></a> |
| <a name="index-i386-immediate-operands"></a> |
| <a name="index-register-operands_002c-i386"></a> |
| <a name="index-i386-register-operands"></a> |
| <a name="index-jump_002fcall-operands_002c-i386"></a> |
| <a name="index-i386-jump_002fcall-operands"></a> |
| <a name="index-operand-delimiters_002c-i386"></a> |
| |
| <a name="index-immediate-operands_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-immediate-operands"></a> |
| <a name="index-register-operands_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-register-operands"></a> |
| <a name="index-jump_002fcall-operands_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-jump_002fcall-operands"></a> |
| <a name="index-operand-delimiters_002c-x86_002d64"></a> |
| <ul> |
| <li> AT&T immediate operands are preceded by ‘<samp>$</samp>’; Intel immediate |
| operands are undelimited (Intel ‘<samp>push 4</samp>’ is AT&T ‘<samp>pushl $4</samp>’). |
| AT&T register operands are preceded by ‘<samp>%</samp>’; Intel register operands |
| are undelimited. AT&T absolute (as opposed to PC relative) jump/call |
| operands are prefixed by ‘<samp>*</samp>’; they are undelimited in Intel syntax. |
| |
| </li><li> <a name="index-i386-source_002c-destination-operands"></a> |
| <a name="index-source_002c-destination-operands_003b-i386"></a> |
| <a name="index-x86_002d64-source_002c-destination-operands"></a> |
| <a name="index-source_002c-destination-operands_003b-x86_002d64"></a> |
| AT&T and Intel syntax use the opposite order for source and destination |
| operands. Intel ‘<samp>add eax, 4</samp>’ is ‘<samp>addl $4, %eax</samp>’. The |
| ‘<samp>source, dest</samp>’ convention is maintained for compatibility with |
| previous Unix assemblers. Note that ‘<samp>bound</samp>’, ‘<samp>invlpga</samp>’, and |
| instructions with 2 immediate operands, such as the ‘<samp>enter</samp>’ |
| instruction, do <em>not</em> have reversed order. <a href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>. |
| |
| </li><li> <a name="index-mnemonic-suffixes_002c-i386"></a> |
| <a name="index-sizes-operands_002c-i386"></a> |
| <a name="index-i386-size-suffixes"></a> |
| <a name="index-mnemonic-suffixes_002c-x86_002d64"></a> |
| <a name="index-sizes-operands_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-size-suffixes"></a> |
| In AT&T syntax the size of memory operands is determined from the last |
| character of the instruction mnemonic. Mnemonic suffixes of ‘<samp>b</samp>’, |
| ‘<samp>w</samp>’, ‘<samp>l</samp>’ and ‘<samp>q</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>byte ptr</samp>’, ‘<samp>word ptr</samp>’, ‘<samp>dword ptr</samp>’ and ‘<samp>qword ptr</samp>’. Thus, |
| Intel ‘<samp>mov al, byte ptr <var>foo</var></samp>’ is ‘<samp>movb <var>foo</var>, %al</samp>’ in AT&T |
| syntax. |
| |
| <p>In 64-bit code, ‘<samp>movabs</samp>’ can be used to encode the ‘<samp>mov</samp>’ |
| instruction with the 64-bit displacement or immediate operand. |
| </p> |
| </li><li> <a name="index-return-instructions_002c-i386"></a> |
| <a name="index-i386-jump_002c-call_002c-return"></a> |
| <a name="index-return-instructions_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-jump_002c-call_002c-return"></a> |
| Immediate form long jumps and calls are |
| ‘<samp>lcall/ljmp $<var>section</var>, $<var>offset</var></samp>’ in AT&T syntax; the |
| Intel syntax is |
| ‘<samp>call/jmp far <var>section</var>:<var>offset</var></samp>’. Also, the far return |
| instruction |
| is ‘<samp>lret $<var>stack-adjust</var></samp>’ in AT&T syntax; Intel syntax is |
| ‘<samp>ret far <var>stack-adjust</var></samp>’. |
| |
| </li><li> <a name="index-sections_002c-i386"></a> |
| <a name="index-i386-sections"></a> |
| <a name="index-sections_002c-x86_002d64"></a> |
| <a name="index-x86_002d64-sections"></a> |
| The AT&T assembler does not provide support for multiple section |
| programs. Unix style systems expect all programs to be single sections. |
| </li></ul> |
| |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="i386_002dChars.html#i386_002dChars" accesskey="n" rel="next">i386-Chars</a>, Up: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="u" rel="up">i386-Syntax</a> [<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> |
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