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<h4 class="section">Instruction Naming</h4>



   <p>Instruction mnemonics are suffixed with one character modifiers which

specify the size of operands.  The letters <code>b</code>, <code>w</code>, <code>l</code>

and <code>q</code> 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, <code>mov %ax, %bx</code> is equivalent

to <code>movw %ax, %bx</code>; also, <code>mov $1, %bx</code> is equivalent to

<code>movw $1, bx</code>.  Note that this is incompatible with the AT&amp;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&amp;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&amp;T

syntax.  Base names for sign extend and zero extend are

<code>movs...</code> and <code>movz...</code> in AT&amp;T syntax (<code>movsx</code>

and <code>movzx</code> 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, <code>movsbl %al, %edx</code> is AT&amp;T syntax for

"move sign extend <em>from</em> %al <em>to</em> %edx."  Possible suffixes,

thus, are <code>bl</code> (from byte to long), <code>bw</code> (from byte to word),

<code>wl</code> (from word to long), <code>bq</code> (from byte to quadruple word),

<code>wq</code> (from word to quadruple word), and <code>lq</code> (from long to

quadruple word).



   <p>The Intel-syntax conversion instructions



     <ul>

<li><code>cbw</code> -- sign-extend byte in <code>%al</code> to word in <code>%ax</code>,



     <li><code>cwde</code> -- sign-extend word in <code>%ax</code> to long in <code>%eax</code>,



     <li><code>cwd</code> -- sign-extend word in <code>%ax</code> to long in <code>%dx:%ax</code>,



     <li><code>cdq</code> -- sign-extend dword in <code>%eax</code> to quad in <code>%edx:%eax</code>,



     <li><code>cdqe</code> -- sign-extend dword in <code>%eax</code> to quad in <code>%rax</code>

(x86-64 only),



     <li><code>cdo</code> -- sign-extend quad in <code>%rax</code> to octuple in

<code>%rdx:%rax</code> (x86-64 only),

</ul>



<p>are called <code>cbtw</code>, <code>cwtl</code>, <code>cwtd</code>, <code>cltd</code>, <code>cltq</code>, and

<code>cqto</code> in AT&amp;T naming.  <code>as</code> accepts either naming for these

instructions.



   <p>Far call/jump instructions are <code>lcall</code> and <code>ljmp</code> in

AT&amp;T syntax, but are <code>call far</code> and <code>jump far</code> in Intel

convention.



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