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<title>TIC54X-Subsyms - Using as</title>
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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<a name="TIC54X-Subsyms"></a>
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<a name="TIC54X_002dSubsyms"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="TIC54X_002dLocals.html#TIC54X_002dLocals">TIC54X-Locals</a>,
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Previous: <a rel="previous" accesskey="p" href="TIC54X_002dConstants.html#TIC54X_002dConstants">TIC54X-Constants</a>,
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Up: <a rel="up" accesskey="u" href="TIC54X_002dDependent.html#TIC54X_002dDependent">TIC54X-Dependent</a>
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<h4 class="subsection">9.43.5 String Substitution</h4>
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<p>A subset of allowable symbols (which we'll call subsyms) may be assigned
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arbitrary string values. This is roughly equivalent to C preprocessor
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#define macros. When <code>as</code> encounters one of these
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symbols, the symbol is replaced in the input stream by its string value.
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Subsym names <strong>must</strong> begin with a letter.
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<p>Subsyms may be defined using the <code>.asg</code> and <code>.eval</code> directives
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(See <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.asg</code></a>,
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See <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.eval</code></a>.
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<p>Expansion is recursive until a previously encountered symbol is seen, at
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which point substitution stops.
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<p>In this example, x is replaced with SYM2; SYM2 is replaced with SYM1, and SYM1
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is replaced with x. At this point, x has already been encountered
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and the substitution stops.
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<pre class="smallexample"> .asg "x",SYM1
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.asg "SYM1",SYM2
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.asg "SYM2",x
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add x,a ; final code assembled is "add x, a"
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</pre>
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<p>Macro parameters are converted to subsyms; a side effect of this is the normal
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<code>as</code> '\ARG' dereferencing syntax is unnecessary. Subsyms
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defined within a macro will have global scope, unless the <code>.var</code>
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directive is used to identify the subsym as a local macro variable
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see <a href="TIC54X_002dDirectives.html#TIC54X_002dDirectives"><code>.var</code></a>.
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<p>Substitution may be forced in situations where replacement might be
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ambiguous by placing colons on either side of the subsym. The following
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code:
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<pre class="smallexample"> .eval "10",x
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LAB:X: add #x, a
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</pre>
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<p>When assembled becomes:
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<pre class="smallexample"> LAB10 add #10, a
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</pre>
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<p>Smaller parts of the string assigned to a subsym may be accessed with
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the following syntax:
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<dl>
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<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>.
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<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
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length <var>length</var>.
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</dl>
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</body></html>
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