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<h4 class="section">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>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><code></code>

     <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><code></code>

     <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><code></code>

     <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 <code>*</code>.



     <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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