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| <h4 class="subsection">9.12.5 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-840"></a>These are the additional directives in <code>as</code> for the ESA/390: |
| |
| <dl> |
| <dt><code>.dc</code><dd>A small subset of the usual DC directive is supported. |
| |
| <br><dt><code>.drop </code><var>regno</var><dd>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. |
| |
| <br><dt><code>.ebcdic </code><var>string</var><dd>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. |
| |
| <br><dt><code>EQU</code><dd>The standard HLASM-style EQU directive is not supported; however, the |
| standard <code>as</code> directive .equ can be used to the same effect. |
| |
| <br><dt><code>.ltorg</code><dd>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. |
| |
| <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 |
| instructions. The <var>expr</var> will be evaluated to obtain the base address; |
| usually, <var>expr</var> will merely be ‘<samp><span class="samp">*</span></samp>’. |
| |
| <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>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. |
| |
| <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> |
| <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. |
| |
| </dl> |
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