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| <title>Using as: ESA/390 Directives</title> |
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| Next: <a href="ESA_002f390-Opcodes.html#ESA_002f390-Opcodes" accesskey="n" rel="next">ESA/390 Opcodes</a>, Previous: <a href="ESA_002f390-Floating-Point.html#ESA_002f390-Floating-Point" accesskey="p" rel="prev">ESA/390 Floating Point</a>, Up: <a href="ESA_002f390_002dDependent.html#ESA_002f390_002dDependent" accesskey="u" rel="up">ESA/390-Dependent</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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| <a name="ESA_002f390-Assembler-Directives"></a> |
| <h4 class="subsection">9.14.5 ESA/390 Assembler Directives</h4> |
| |
| <p><code>as</code> for the ESA/390 supports all of the standard ELF/SVR4 |
| assembler directives that are documented in the main part of this |
| documentation. Several additional directives are supported in order |
| to implement the ESA/390 addressing model. The most important of these |
| are <code>.using</code> and <code>.ltorg</code> |
| </p> |
| <a name="index-ESA_002f390_002donly-directives"></a> |
| <p>These are the additional directives in <code>as</code> for the ESA/390: |
| </p> |
| <dl compact="compact"> |
| <dt><code>.dc</code></dt> |
| <dd><p>A small subset of the usual DC directive is supported. |
| </p> |
| </dd> |
| <dt><code>.drop <var>regno</var></code></dt> |
| <dd><p>Stop using <var>regno</var> as the base register. The <var>regno</var> must |
| have been previously declared with a <code>.using</code> directive in the |
| same section as the current section. |
| </p> |
| </dd> |
| <dt><code>.ebcdic <var>string</var></code></dt> |
| <dd><p>Emit the EBCDIC equivalent of the indicated string. The emitted string |
| will be null terminated. Note that the directives <code>.string</code> etc. emit |
| ascii strings by default. |
| </p> |
| </dd> |
| <dt><code>EQU</code></dt> |
| <dd><p>The standard HLASM-style EQU directive is not supported; however, the |
| standard <code>as</code> directive .equ can be used to the same effect. |
| </p> |
| </dd> |
| <dt><code>.ltorg</code></dt> |
| <dd><p>Dump the literal pool accumulated so far; begin a new literal pool. |
| The literal pool will be written in the current section; in order to |
| generate correct assembly, a <code>.using</code> must have been previously |
| specified in the same section. |
| </p> |
| </dd> |
| <dt><code>.using <var>expr</var>,<var>regno</var></code></dt> |
| <dd><p>Use <var>regno</var> as the base register for all subsequent RX, RS, and SS form |
| instructions. The <var>expr</var> will be evaluated to obtain the base address; |
| usually, <var>expr</var> will merely be ‘<samp>*</samp>’. |
| </p> |
| <p>This assembler allows two <code>.using</code> directives to be simultaneously |
| outstanding, one in the <code>.text</code> section, and one in another section |
| (typically, the <code>.data</code> section). This feature allows |
| dynamically loaded objects to be implemented in a relatively |
| straightforward way. A <code>.using</code> directive must always be specified |
| in the <code>.text</code> section; this will specify the base register that |
| will be used for branches in the <code>.text</code> section. A second |
| <code>.using</code> may be specified in another section; this will specify |
| the base register that is used for non-label address literals. |
| When a second <code>.using</code> is specified, then the subsequent |
| <code>.ltorg</code> must be put in the same section; otherwise an error will |
| result. |
| </p> |
| <p>Thus, for example, the following code uses <code>r3</code> to address branch |
| targets and <code>r4</code> to address the literal pool, which has been written |
| to the <code>.data</code> section. The is, the constants <code>=A(some_routine)</code>, |
| <code>=H'42'</code> and <code>=E'3.1416'</code> will all appear in the <code>.data</code> |
| section. |
| </p> |
| <div class="example"> |
| <pre class="example">.data |
| .using LITPOOL,r4 |
| .text |
| BASR r3,0 |
| .using *,r3 |
| B START |
| .long LITPOOL |
| START: |
| L r4,4(,r3) |
| L r15,=A(some_routine) |
| LTR r15,r15 |
| BNE LABEL |
| AH r0,=H'42' |
| LABEL: |
| ME r6,=E'3.1416' |
| .data |
| LITPOOL: |
| .ltorg |
| </pre></div> |
| |
| |
| <p>Note that this dual-<code>.using</code> directive semantics extends |
| and is not compatible with HLASM semantics. Note that this assembler |
| directive does not support the full range of HLASM semantics. |
| </p> |
| </dd> |
| </dl> |
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