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<a name="TIC54X_002dSubsyms"></a>
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<a name="String-Substitution"></a>
<h4 class="subsection">9.45.5 String Substitution</h4>
<p>A subset of allowable symbols (which we&rsquo;ll call subsyms) may be assigned
arbitrary string values.  This is roughly equivalent to C preprocessor
#define macros.  When <code>as</code> encounters one of these
symbols, the symbol is replaced in the input stream by its string value.
Subsym names <strong>must</strong> begin with a letter.
</p>
<p>Subsyms may be defined using the <code>.asg</code> and <code>.eval</code> directives
(See <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.asg</code></a>,
See <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.eval</code></a>.
</p>
<p>Expansion is recursive until a previously encountered symbol is seen, at
which point substitution stops.
</p>
<p>In this example, x is replaced with SYM2; SYM2 is replaced with SYM1, and SYM1
is replaced with x.  At this point, x has already been encountered
and the substitution stops.
</p>
<div class="smallexample">
<pre class="smallexample"> .asg   &quot;x&quot;,SYM1
 .asg   &quot;SYM1&quot;,SYM2
 .asg   &quot;SYM2&quot;,x
 add    x,a             ; final code assembled is &quot;add  x, a&quot;
</pre></div>

<p>Macro parameters are converted to subsyms; a side effect of this is the normal
<code>as</code> &rsquo;\ARG&rsquo; dereferencing syntax is unnecessary.  Subsyms
defined within a macro will have global scope, unless the <code>.var</code>
directive is used to identify the subsym as a local macro variable
see <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.var</code></a>.
</p>
<p>Substitution may be forced in situations where replacement might be
ambiguous by placing colons on either side of the subsym.  The following
code:
</p>
<div class="smallexample">
<pre class="smallexample"> .eval  &quot;10&quot;,x
LAB:X:  add     #x, a
</pre></div>

<p>When assembled becomes:
</p>
<div class="smallexample">
<pre class="smallexample">LAB10  add     #10, a
</pre></div>

<p>Smaller parts of the string assigned to a subsym may be accessed with
the following syntax:
</p>
<dl compact="compact">
<dt><code><code>:<var>symbol</var>(<var>char_index</var>):</code></code></dt>
<dd><p>Evaluates to a single-character string, the character at <var>char_index</var>.
</p></dd>
<dt><code><code>:<var>symbol</var>(<var>start</var>,<var>length</var>):</code></code></dt>
<dd><p>Evaluates to a substring of <var>symbol</var> beginning at <var>start</var> with
length <var>length</var>.
</p></dd>
</dl>

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