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<h4 class="subsection">9.13.4 Instruction Naming</h4>
<p><a name="index-i386-instruction-naming-887"></a><a name="index-instruction-naming_002c-i386-888"></a><a name="index-x86_002d64-instruction-naming-889"></a><a name="index-instruction-naming_002c-x86_002d64-890"></a>
Instruction mnemonics are suffixed with one character modifiers which
specify the size of operands. The letters <span class="samp">b</span>, <span class="samp">w</span>, <span class="samp">l</span>
and <span class="samp">q</span> specify byte, word, long and quadruple word operands. If
no suffix is specified by an instruction then <code>as</code> tries to
fill in the missing suffix based on the destination register operand
(the last one by convention). Thus, <span class="samp">mov %ax, %bx</span> is equivalent
to <span class="samp">movw %ax, %bx</span>; also, <span class="samp">mov $1, %bx</span> is equivalent to
<span class="samp">movw $1, bx</span>. Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing mnemonic suffix implies long
operand size. (This incompatibility does not affect compiler output
since compilers always explicitly specify the mnemonic suffix.)
<p>Almost all instructions have the same names in AT&T and Intel format.
There are a few exceptions. The sign extend and zero extend
instructions need two sizes to specify them. They need a size to
sign/zero extend <em>from</em> and a size to zero extend <em>to</em>. This
is accomplished by using two instruction mnemonic suffixes in AT&T
syntax. Base names for sign extend and zero extend are
<span class="samp">movs...</span> and <span class="samp">movz...</span> in AT&T syntax (<span class="samp">movsx</span>
and <span class="samp">movzx</span> in Intel syntax). The instruction mnemonic suffixes
are tacked on to this base name, the <em>from</em> suffix before the
<em>to</em> suffix. Thus, <span class="samp">movsbl %al, %edx</span> is AT&T syntax for
“move sign extend <em>from</em> %al <em>to</em> %edx.” Possible suffixes,
thus, are <span class="samp">bl</span> (from byte to long), <span class="samp">bw</span> (from byte to word),
<span class="samp">wl</span> (from word to long), <span class="samp">bq</span> (from byte to quadruple word),
<span class="samp">wq</span> (from word to quadruple word), and <span class="samp">lq</span> (from long to
quadruple word).
<p><a name="index-conversion-instructions_002c-i386-891"></a><a name="index-i386-conversion-instructions-892"></a><a name="index-conversion-instructions_002c-x86_002d64-893"></a><a name="index-x86_002d64-conversion-instructions-894"></a>The Intel-syntax conversion instructions
<ul>
<li><span class="samp">cbw</span> — sign-extend byte in <span class="samp">%al</span> to word in <span class="samp">%ax</span>,
<li><span class="samp">cwde</span> — sign-extend word in <span class="samp">%ax</span> to long in <span class="samp">%eax</span>,
<li><span class="samp">cwd</span> — sign-extend word in <span class="samp">%ax</span> to long in <span class="samp">%dx:%ax</span>,
<li><span class="samp">cdq</span> — sign-extend dword in <span class="samp">%eax</span> to quad in <span class="samp">%edx:%eax</span>,
<li><span class="samp">cdqe</span> — sign-extend dword in <span class="samp">%eax</span> to quad in <span class="samp">%rax</span>
(x86-64 only),
<li><span class="samp">cqo</span> — sign-extend quad in <span class="samp">%rax</span> to octuple in
<span class="samp">%rdx:%rax</span> (x86-64 only),
</ul>
<p class="noindent">are called <span class="samp">cbtw</span>, <span class="samp">cwtl</span>, <span class="samp">cwtd</span>, <span class="samp">cltd</span>, <span class="samp">cltq</span>, and
<span class="samp">cqto</span> in AT&T naming. <code>as</code> accepts either naming for these
instructions.
<p><a name="index-jump-instructions_002c-i386-895"></a><a name="index-call-instructions_002c-i386-896"></a><a name="index-jump-instructions_002c-x86_002d64-897"></a><a name="index-call-instructions_002c-x86_002d64-898"></a>Far call/jump instructions are <span class="samp">lcall</span> and <span class="samp">ljmp</span> in
AT&T syntax, but are <span class="samp">call far</span> and <span class="samp">jump far</span> in Intel
convention.
<h4 class="subsection">9.13.5 AT&T Mnemonic versus Intel Mnemonic</h4>
<p><a name="index-i386-mnemonic-compatibility-899"></a><a name="index-mnemonic-compatibility_002c-i386-900"></a>
<code>as</code> supports assembly using Intel mnemonic.
<code>.intel_mnemonic</code> selects Intel mnemonic with Intel syntax, and
<code>.att_mnemonic</code> switches back to the usual AT&T mnemonic with AT&T
syntax for compatibility with the output of <code>gcc</code>.
Several x87 instructions, <span class="samp">fadd</span>, <span class="samp">fdiv</span>, <span class="samp">fdivp</span>,
<span class="samp">fdivr</span>, <span class="samp">fdivrp</span>, <span class="samp">fmul</span>, <span class="samp">fsub</span>, <span class="samp">fsubp</span>,
<span class="samp">fsubr</span> and <span class="samp">fsubrp</span>, are implemented in AT&T System V/386
assembler with different mnemonics from those in Intel IA32 specification.
<code>gcc</code> generates those instructions with AT&T mnemonic.
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