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125 lines
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<title>ESA/390 Directives - Using as</title>
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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<a name="ESA_002f390-Directives"></a>Next: <a rel="next" accesskey="n" href="ESA_002f390-Opcodes.html#ESA_002f390-Opcodes">ESA/390 Opcodes</a>,
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<h4 class="subsection">9.12.5 ESA/390 Assembler Directives</h4>
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<p><code>as</code> for the ESA/390 supports all of the standard ELF/SVR4
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assembler directives that are documented in the main part of this
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documentation. Several additional directives are supported in order
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to implement the ESA/390 addressing model. The most important of these
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are <code>.using</code> and <code>.ltorg</code>
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<p><a name="index-ESA_002f390_002donly-directives-813"></a>These are the additional directives in <code>as</code> for the ESA/390:
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<dl>
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<dt><code>.dc</code><dd>A small subset of the usual DC directive is supported.
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<br><dt><code>.drop </code><var>regno</var><dd>Stop using <var>regno</var> as the base register. The <var>regno</var> must
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have been previously declared with a <code>.using</code> directive in the
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same section as the current section.
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<br><dt><code>.ebcdic </code><var>string</var><dd>Emit the EBCDIC equivalent of the indicated string. The emitted string
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will be null terminated. Note that the directives <code>.string</code> etc. emit
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ascii strings by default.
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<br><dt><code>EQU</code><dd>The standard HLASM-style EQU directive is not supported; however, the
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standard <code>as</code> directive .equ can be used to the same effect.
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<br><dt><code>.ltorg</code><dd>Dump the literal pool accumulated so far; begin a new literal pool.
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The literal pool will be written in the current section; in order to
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generate correct assembly, a <code>.using</code> must have been previously
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specified in the same section.
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<br><dt><code>.using </code><var>expr</var><code>,</code><var>regno</var><dd>Use <var>regno</var> as the base register for all subsequent RX, RS, and SS form
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instructions. The <var>expr</var> will be evaluated to obtain the base address;
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usually, <var>expr</var> will merely be <span class="samp">*</span>.
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<p>This assembler allows two <code>.using</code> directives to be simultaneously
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outstanding, one in the <code>.text</code> section, and one in another section
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(typically, the <code>.data</code> section). This feature allows
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dynamically loaded objects to be implemented in a relatively
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straightforward way. A <code>.using</code> directive must always be specified
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in the <code>.text</code> section; this will specify the base register that
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will be used for branches in the <code>.text</code> section. A second
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<code>.using</code> may be specified in another section; this will specify
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the base register that is used for non-label address literals.
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When a second <code>.using</code> is specified, then the subsequent
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<code>.ltorg</code> must be put in the same section; otherwise an error will
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result.
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<p>Thus, for example, the following code uses <code>r3</code> to address branch
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targets and <code>r4</code> to address the literal pool, which has been written
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to the <code>.data</code> section. The is, the constants <code>=A(some_routine)</code>,
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<code>=H'42'</code> and <code>=E'3.1416'</code> will all appear in the <code>.data</code>
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section.
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<pre class="example"> .data
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.using LITPOOL,r4
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.text
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BASR r3,0
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.using *,r3
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B START
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.long LITPOOL
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START:
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L r4,4(,r3)
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L r15,=A(some_routine)
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LTR r15,r15
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BNE LABEL
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AH r0,=H'42'
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LABEL:
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ME r6,=E'3.1416'
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.data
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LITPOOL:
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.ltorg
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</pre>
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<p>Note that this dual-<code>.using</code> directive semantics extends
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and is not compatible with HLASM semantics. Note that this assembler
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directive does not support the full range of HLASM semantics.
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</dl>
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