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<a name="Constraints"></a>
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<a name="Constraints-for-asm-Operands"></a>
<h4 class="subsection">6.47.3 Constraints for <code>asm</code> Operands</h4>
<a name="index-operand-constraints_002c-asm"></a>
<a name="index-constraints_002c-asm"></a>
<a name="index-asm-constraints"></a>

<p>Here are specific details on what constraint letters you can use with
<code>asm</code> operands.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have.  Constraints can also require two operands to match.
Side-effects aren&rsquo;t allowed in operands of inline <code>asm</code>, unless
&lsquo;<samp>&lt;</samp>&rsquo; or &lsquo;<samp>&gt;</samp>&rsquo; constraints are used, because there is no guarantee
that the side effects will happen exactly once in an instruction that can update
the addressing register.
</p>

<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Simple-Constraints.html#Simple-Constraints" accesskey="1">Simple Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Basic use of constraints.
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<tr><td align="left" valign="top">&bull; <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="2">Multi-Alternative</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">When an insn has two alternative constraint-patterns.
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<tr><td align="left" valign="top">&bull; <a href="Modifiers.html#Modifiers" accesskey="3">Modifiers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">More precise control over effects of constraints.
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<tr><td align="left" valign="top">&bull; <a href="Machine-Constraints.html#Machine-Constraints" accesskey="4">Machine Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Special constraints for some particular machines.
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