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<h4 class="subsection">9.43.5 String Substitution</h4>
<p>A subset of allowable symbols (which we'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>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>Expansion is recursive until a previously encountered symbol is seen, at
which point substitution stops.
<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.
<pre class="smallexample"> .asg "x",SYM1
.asg "SYM1",SYM2
.asg "SYM2",x
add x,a ; final code assembled is "add x, a"
</pre>
<p>Macro parameters are converted to subsyms; a side effect of this is the normal
<code>as</code> '\ARG' 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>Substitution may be forced in situations where replacement might be
ambiguous by placing colons on either side of the subsym. The following
code:
<pre class="smallexample"> .eval "10",x
LAB:X: add #x, a
</pre>
<p>When assembled becomes:
<pre class="smallexample"> LAB10 add #10, a
</pre>
<p>Smaller parts of the string assigned to a subsym may be accessed with
the following syntax:
<dl>
<dt><code>:</code><var>symbol</var><code>(</code><var>char_index</var><code>):</code><dd>Evaluates to a single-character string, the character at <var>char_index</var>.
<br><dt><code>:</code><var>symbol</var><code>(</code><var>start</var><code>,</code><var>length</var><code>):</code><dd>Evaluates to a substring of <var>symbol</var> beginning at <var>start</var> with
length <var>length</var>.
</dl>
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