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138 lines
6.1 KiB
HTML
138 lines
6.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>TIC54X-Subsyms (Using as)</title>
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<meta name="description" content="TIC54X-Subsyms (Using as)">
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<meta name="keywords" content="TIC54X-Subsyms (Using as)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="TIC54X_002dDependent.html#TIC54X_002dDependent" rel="up" title="TIC54X-Dependent">
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<link href="TIC54X_002dLocals.html#TIC54X_002dLocals" rel="next" title="TIC54X-Locals">
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<link href="TIC54X_002dConstants.html#TIC54X_002dConstants" rel="prev" title="TIC54X-Constants">
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</style>
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</head>
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<body lang="en">
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<a name="TIC54X_002dSubsyms"></a>
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<div class="header">
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<p>
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Next: <a href="TIC54X_002dLocals.html#TIC54X_002dLocals" accesskey="n" rel="next">TIC54X-Locals</a>, Previous: <a href="TIC54X_002dConstants.html#TIC54X_002dConstants" accesskey="p" rel="prev">TIC54X-Constants</a>, Up: <a href="TIC54X_002dDependent.html#TIC54X_002dDependent" accesskey="u" rel="up">TIC54X-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="String-Substitution"></a>
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<h4 class="subsection">9.44.5 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>
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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>
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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>
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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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</p>
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<div class="smallexample">
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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></div>
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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>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample"> .eval "10",x
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LAB:X: add #x, a
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</pre></div>
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<p>When assembled becomes:
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</p>
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<div class="smallexample">
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<pre class="smallexample">LAB10 add #10, a
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</pre></div>
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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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</p>
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<dl compact="compact">
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<dt><code><code>:<var>symbol</var>(<var>char_index</var>):</code></code></dt>
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<dd><p>Evaluates to a single-character string, the character at <var>char_index</var>.
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</p></dd>
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<dt><code><code>:<var>symbol</var>(<var>start</var>,<var>length</var>):</code></code></dt>
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<dd><p>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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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="TIC54X_002dLocals.html#TIC54X_002dLocals" accesskey="n" rel="next">TIC54X-Locals</a>, Previous: <a href="TIC54X_002dConstants.html#TIC54X_002dConstants" accesskey="p" rel="prev">TIC54X-Constants</a>, Up: <a href="TIC54X_002dDependent.html#TIC54X_002dDependent" accesskey="u" rel="up">TIC54X-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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