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<title>Macro - Using as</title>
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<link rel="up" href="Pseudo-Ops.html#Pseudo-Ops" title="Pseudo Ops">
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This file documents the GNU Assembler "as".
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<div class="node">
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<a name="Macro"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MRI.html#MRI">MRI</a>,
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Previous: <a rel="previous" accesskey="p" href="Long.html#Long">Long</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<hr>
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</div>
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<h3 class="section">7.58 <code>.macro</code></h3>
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<p><a name="index-macros-400"></a>The commands <code>.macro</code> and <code>.endm</code> allow you to define macros that
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generate assembly output. For example, this definition specifies a macro
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<code>sum</code> that puts a sequence of numbers into memory:
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<pre class="example"> .macro sum from=0, to=5
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.long \from
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.if \to-\from
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sum "(\from+1)",\to
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.endif
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.endm
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</pre>
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<p class="noindent">With that definition, ‘<samp><span class="samp">SUM 0,5</span></samp>’ is equivalent to this assembly input:
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<pre class="example"> .long 0
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.long 1
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.long 2
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.long 3
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.long 4
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.long 5
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</pre>
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<dl>
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<dt><code>.macro </code><var>macname</var><a name="index-g_t_002emacro-_0040var_007bmacname_007d-401"></a><dt><code>.macro </code><var>macname</var> <var>macargs</var><code> ...</code><a name="index-g_t_002emacro-_0040var_007bmacname_007d-_0040var_007bmacargs_007d-_0040dots_007b_007d-402"></a><dd><a name="index-g_t_0040code_007bmacro_007d-directive-403"></a>Begin the definition of a macro called <var>macname</var>. If your macro
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definition requires arguments, specify their names after the macro name,
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separated by commas or spaces. You can qualify the macro argument to
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indicate whether all invocations must specify a non-blank value (through
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‘<samp><span class="samp">:</span><code>req</code></samp>’), or whether it takes all of the remaining arguments
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(through ‘<samp><span class="samp">:</span><code>vararg</code></samp>’). You can supply a default value for any
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macro argument by following the name with ‘<samp><span class="samp">=</span><var>deflt</var></samp>’. You
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cannot define two macros with the same <var>macname</var> unless it has been
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subject to the <code>.purgem</code> directive (see <a href="Purgem.html#Purgem">Purgem</a>) between the two
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definitions. For example, these are all valid <code>.macro</code> statements:
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<dl>
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<dt><code>.macro comm</code><dd>Begin the definition of a macro called <code>comm</code>, which takes no
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arguments.
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<br><dt><code>.macro plus1 p, p1</code><dt><code>.macro plus1 p p1</code><dd>Either statement begins the definition of a macro called <code>plus1</code>,
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which takes two arguments; within the macro definition, write
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‘<samp><span class="samp">\p</span></samp>’ or ‘<samp><span class="samp">\p1</span></samp>’ to evaluate the arguments.
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<br><dt><code>.macro reserve_str p1=0 p2</code><dd>Begin the definition of a macro called <code>reserve_str</code>, with two
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arguments. The first argument has a default value, but not the second.
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After the definition is complete, you can call the macro either as
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‘<samp><span class="samp">reserve_str </span><var>a</var><span class="samp">,</span><var>b</var></samp>’ (with ‘<samp><span class="samp">\p1</span></samp>’ evaluating to
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<var>a</var> and ‘<samp><span class="samp">\p2</span></samp>’ evaluating to <var>b</var>), or as ‘<samp><span class="samp">reserve_str
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,</span><var>b</var></samp>’ (with ‘<samp><span class="samp">\p1</span></samp>’ evaluating as the default, in this case
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‘<samp><span class="samp">0</span></samp>’, and ‘<samp><span class="samp">\p2</span></samp>’ evaluating to <var>b</var>).
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<br><dt><code>.macro m p1:req, p2=0, p3:vararg</code><dd>Begin the definition of a macro called <code>m</code>, with at least three
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arguments. The first argument must always have a value specified, but
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not the second, which instead has a default value. The third formal
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will get assigned all remaining arguments specified at invocation time.
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<p>When you call a macro, you can specify the argument values either by
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position, or by keyword. For example, ‘<samp><span class="samp">sum 9,17</span></samp>’ is equivalent to
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‘<samp><span class="samp">sum to=17, from=9</span></samp>’.
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</dl>
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<p>Note that since each of the <var>macargs</var> can be an identifier exactly
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as any other one permitted by the target architecture, there may be
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occasional problems if the target hand-crafts special meanings to certain
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characters when they occur in a special position. For example, if the colon
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(<code>:</code>) is generally permitted to be part of a symbol name, but the
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architecture specific code special-cases it when occurring as the final
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character of a symbol (to denote a label), then the macro parameter
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replacement code will have no way of knowing that and consider the whole
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construct (including the colon) an identifier, and check only this
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identifier for being the subject to parameter substitution. So for example
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this macro definition:
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<pre class="example"> .macro label l
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\l:
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.endm
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</pre>
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<p>might not work as expected. Invoking ‘<samp><span class="samp">label foo</span></samp>’ might not create a label
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called ‘<samp><span class="samp">foo</span></samp>’ but instead just insert the text ‘<samp><span class="samp">\l:</span></samp>’ into the
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assembler source, probably generating an error about an unrecognised
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identifier.
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<p>Similarly problems might occur with the period character (‘<samp><span class="samp">.</span></samp>’)
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which is often allowed inside opcode names (and hence identifier names). So
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for example constructing a macro to build an opcode from a base name and a
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length specifier like this:
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<pre class="example"> .macro opcode base length
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\base.\length
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.endm
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</pre>
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<p>and invoking it as ‘<samp><span class="samp">opcode store l</span></samp>’ will not create a ‘<samp><span class="samp">store.l</span></samp>’
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instruction but instead generate some kind of error as the assembler tries to
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interpret the text ‘<samp><span class="samp">\base.\length</span></samp>’.
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<p>There are several possible ways around this problem:
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<dl>
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<dt><code>Insert white space</code><dd>If it is possible to use white space characters then this is the simplest
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solution. eg:
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<pre class="example"> .macro label l
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\l :
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.endm
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</pre>
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<br><dt><code>Use ‘</code><samp><span class="samp">\()</span></samp><code>’</code><dd>The string ‘<samp><span class="samp">\()</span></samp>’ can be used to separate the end of a macro argument from
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the following text. eg:
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<pre class="example"> .macro opcode base length
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\base\().\length
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.endm
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</pre>
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<br><dt><code>Use the alternate macro syntax mode</code><dd>In the alternative macro syntax mode the ampersand character (‘<samp><span class="samp">&</span></samp>’) can be
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used as a separator. eg:
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<pre class="example"> .altmacro
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.macro label l
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l&:
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.endm
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</pre>
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</dl>
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<p>Note: this problem of correctly identifying string parameters to pseudo ops
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also applies to the identifiers used in <code>.irp</code> (see <a href="Irp.html#Irp">Irp</a>)
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and <code>.irpc</code> (see <a href="Irpc.html#Irpc">Irpc</a>) as well.
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<br><dt><code>.endm</code><a name="index-g_t_002eendm-404"></a><dd><a name="index-g_t_0040code_007bendm_007d-directive-405"></a>Mark the end of a macro definition.
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<br><dt><code>.exitm</code><a name="index-g_t_002eexitm-406"></a><dd><a name="index-g_t_0040code_007bexitm_007d-directive-407"></a>Exit early from the current macro definition.
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<p><a name="index-number-of-macros-executed-408"></a><a name="index-macros_002c-count-executed-409"></a><br><dt><code>\@</code><a name="index-g_t_005c_0040_0040-410"></a><dd><samp><span class="command">as</span></samp> maintains a counter of how many macros it has
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executed in this pseudo-variable; you can copy that number to your
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output with ‘<samp><span class="samp">\@</span></samp>’, but <em>only within a macro definition</em>.
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<br><dt><code>LOCAL </code><var>name</var><code> [ , ... ]</code><a name="index-LOCAL-_0040var_007bname_007d-_005b-_002c-_0040dots_007b_007d-_005d-411"></a><dd><em>Warning: </em><code>LOCAL</code><em> is only available if you select “alternate
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macro syntax” with ‘</em><samp><span class="samp">--alternate</span></samp><em>’ or </em><code>.altmacro</code><em>.</em>
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See <a href="Altmacro.html#Altmacro"><code>.altmacro</code></a>.
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</dl>
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</body></html>
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