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<h5 class="subsubsection">9.38.3.3 Instruction Operands</h5>
<p><a name="index-instruction-operands_002c-s390-1895"></a><a name="index-s390-instruction-operands-1896"></a>
Instruction operands can be grouped into three classes, operands located
in registers, immediate operands, and operands in storage.
<p>A register operand can be located in general, floating-point, access,
or control register. The register is identified by a four-bit field.
The field containing the register operand is called the R field.
<p>Immediate operands are contained within the instruction and can have
8, 16 or 32 bits. The field containing the immediate operand is called
the I field. Dependent on the instruction the I field is either signed
or unsigned.
<p>A storage operand consists of an address and a length. The address of a
storage operands can be specified in any of these ways:
<ul>
<li>The content of a single general R
<li>The sum of the content of a general register called the base
register B plus the content of a displacement field D
<li>The sum of the contents of two general registers called the
index register X and the base register B plus the content of a
displacement field
<li>The sum of the current instruction address and a 32-bit signed
immediate field multiplied by two.
</ul>
<p>The length of a storage operand can be:
<ul>
<li>Implied by the instruction
<li>Specified by a bitmask
<li>Specified by a four-bit or eight-bit length field L
<li>Specified by the content of a general register
</ul>
<p>The notation for storage operand addresses formed from multiple fields is
as follows:
<dl>
<dt><code>Dn(Bn)</code><dd>the address for operand number n is formed from the content of general
register Bn called the base register and the displacement field Dn.
<br><dt><code>Dn(Xn,Bn)</code><dd>the address for operand number n is formed from the content of general
register Xn called the index register, general register Bn called the
base register and the displacement field Dn.
<br><dt><code>Dn(Ln,Bn)</code><dd>the address for operand number n is formed from the content of general
regiser Bn called the base register and the displacement field Dn.
The length of the operand n is specified by the field Ln.
</dl>
<p>The base registers Bn and the index registers Xn of a storage operand can
be skipped. If Bn and Xn are skipped, a zero will be stored to the operand
field. The notation changes as follows:
<pre class="display">
<p><table summary=""><tr align="left"><th valign="top" width="30%">full notation </th><th valign="top" width="30%">short notation
<br></th></tr><tr align="left"><td valign="top" width="30%">Dn(0,Bn) </td><td valign="top" width="30%">Dn(Bn)
<br></td></tr><tr align="left"><td valign="top" width="30%">Dn(0,0) </td><td valign="top" width="30%">Dn
<br></td></tr><tr align="left"><td valign="top" width="30%">Dn(0) </td><td valign="top" width="30%">Dn
<br></td></tr><tr align="left"><td valign="top" width="30%">Dn(Ln,0) </td><td valign="top" width="30%">Dn(Ln)
<br></td></tr></table>
</pre>
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