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<a name="Bit_002dFields"></a>
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<a name="Bit_002dFields-1"></a>
<h3 class="section">13.11 Bit-Fields</h3>
<a name="index-bit_002dfields"></a>

<p>Special expression codes exist to represent bit-field instructions.
</p>
<dl compact="compact">
<dd><a name="index-sign_005fextract"></a>
<a name="index-BITS_005fBIG_005fENDIAN_002c-effect-on-sign_005fextract"></a>
</dd>
<dt><code>(sign_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt>
<dd><p>This represents a reference to a sign-extended bit-field contained or
starting in <var>loc</var> (a memory or register reference).  The bit-field
is <var>size</var> bits wide and starts at bit <var>pos</var>.  The compilation
option <code>BITS_BIG_ENDIAN</code> says which end of the memory unit
<var>pos</var> counts from.
</p>
<p>If <var>loc</var> is in memory, its mode must be a single-byte integer mode.
If <var>loc</var> is in a register, the mode to use is specified by the
operand of the <code>insv</code> or <code>extv</code> pattern
(see <a href="Standard-Names.html#Standard-Names">Standard Names</a>) and is usually a full-word integer mode,
which is the default if none is specified.
</p>
<p>The mode of <var>pos</var> is machine-specific and is also specified
in the <code>insv</code> or <code>extv</code> pattern.
</p>
<p>The mode <var>m</var> is the same as the mode that would be used for
<var>loc</var> if it were a register.
</p>
<p>A <code>sign_extract</code> can not appear as an lvalue, or part thereof,
in RTL.
</p>
<a name="index-zero_005fextract"></a>
</dd>
<dt><code>(zero_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt>
<dd><p>Like <code>sign_extract</code> but refers to an unsigned or zero-extended
bit-field.  The same sequence of bits are extracted, but they
are filled to an entire word with zeros instead of by sign-extension.
</p>
<p>Unlike <code>sign_extract</code>, this type of expressions can be lvalues
in RTL; they may appear on the left side of an assignment, indicating
insertion of a value into the specified bit-field.
</p></dd>
</dl>


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