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| <h4 class="subsection">9.36.5 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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