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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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<title>s390 Operands (Using as)</title>
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<a name="s390-Operands"></a>
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<div class="header">
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<p>
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Next: <a href="s390-Formats.html#s390-Formats" accesskey="n" rel="next">s390 Formats</a>, Previous: <a href="s390-Mnemonics.html#s390-Mnemonics" accesskey="p" rel="prev">s390 Mnemonics</a>, Up: <a href="s390-Syntax.html#s390-Syntax" accesskey="u" rel="up">s390 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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</div>
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<hr>
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<a name="Instruction-Operands"></a>
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<h4 class="subsubsection">9.40.3.3 Instruction Operands</h4>
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<a name="index-instruction-operands_002c-s390"></a>
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<a name="index-s390-instruction-operands"></a>
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<p>Instruction operands can be grouped into three classes, operands located
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in registers, immediate operands, and operands in storage.
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</p>
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<p>A register operand can be located in general, floating-point, access,
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or control register. The register is identified by a four-bit field.
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The field containing the register operand is called the R field.
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</p>
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<p>Immediate operands are contained within the instruction and can have
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8, 16 or 32 bits. The field containing the immediate operand is called
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the I field. Dependent on the instruction the I field is either signed
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or unsigned.
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</p>
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<p>A storage operand consists of an address and a length. The address of a
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storage operands can be specified in any of these ways:
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</p>
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<ul>
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<li> The content of a single general R
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</li><li> The sum of the content of a general register called the base
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register B plus the content of a displacement field D
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</li><li> The sum of the contents of two general registers called the
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index register X and the base register B plus the content of a
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displacement field
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</li><li> The sum of the current instruction address and a 32-bit signed
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immediate field multiplied by two.
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</li></ul>
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<p>The length of a storage operand can be:
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</p>
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<ul>
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<li> Implied by the instruction
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</li><li> Specified by a bitmask
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</li><li> Specified by a four-bit or eight-bit length field L
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</li><li> Specified by the content of a general register
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</li></ul>
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<p>The notation for storage operand addresses formed from multiple fields is
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as follows:
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</p>
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<dl compact="compact">
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<dt><code>Dn(Bn)</code></dt>
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<dd><p>the address for operand number n is formed from the content of general
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register Bn called the base register and the displacement field Dn.
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</p></dd>
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<dt><code>Dn(Xn,Bn)</code></dt>
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<dd><p>the address for operand number n is formed from the content of general
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register Xn called the index register, general register Bn called the
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base register and the displacement field Dn.
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</p></dd>
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<dt><code>Dn(Ln,Bn)</code></dt>
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<dd><p>the address for operand number n is formed from the content of general
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register Bn called the base register and the displacement field Dn.
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The length of the operand n is specified by the field Ln.
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</p></dd>
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</dl>
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<p>The base registers Bn and the index registers Xn of a storage operand can
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be skipped. If Bn and Xn are skipped, a zero will be stored to the operand
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field. The notation changes as follows:
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</p>
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<div class="display">
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<table>
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<thead><tr><th width="30%"><pre class="display">full notation</pre></th><th width="30%"><pre class="display">short notation</pre></th></tr></thead>
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<tr><td width="30%"><pre class="display">Dn(0,Bn)</pre></td><td width="30%"><pre class="display">Dn(Bn)</pre></td></tr>
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<tr><td width="30%"><pre class="display">Dn(0,0)</pre></td><td width="30%"><pre class="display">Dn</pre></td></tr>
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<tr><td width="30%"><pre class="display">Dn(0)</pre></td><td width="30%"><pre class="display">Dn</pre></td></tr>
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<tr><td width="30%"><pre class="display">Dn(Ln,0)</pre></td><td width="30%"><pre class="display">Dn(Ln)</pre></td></tr>
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</table>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="s390-Formats.html#s390-Formats" accesskey="n" rel="next">s390 Formats</a>, Previous: <a href="s390-Mnemonics.html#s390-Mnemonics" accesskey="p" rel="prev">s390 Mnemonics</a>, Up: <a href="s390-Syntax.html#s390-Syntax" accesskey="u" rel="up">s390 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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</div>
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