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<title>MIPS Macros - Using as</title>
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This file documents the GNU Assembler "as".
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<a name="MIPS-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">MIPS Symbol Sizes</a>,
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Previous: <a rel="previous" accesskey="p" href="MIPS-Options.html#MIPS-Options">MIPS Options</a>,
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Up: <a rel="up" accesskey="u" href="MIPS_002dDependent.html#MIPS_002dDependent">MIPS-Dependent</a>
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<h4 class="subsection">9.27.2 High-level assembly macros</h4>
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<p>MIPS assemblers have traditionally provided a wider range of
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instructions than the MIPS architecture itself. These extra
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instructions are usually referred to as “macro” instructions
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<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
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<p>Some MIPS macro instructions extend an underlying architectural instruction
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while others are entirely new. An example of the former type is <code>and</code>,
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which allows the third operand to be either a register or an arbitrary
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immediate value. Examples of the latter type include <code>bgt</code>, which
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branches to the third operand when the first operand is greater than
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the second operand, and <code>ulh</code>, which implements an unaligned
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2-byte load.
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<p>One of the most common extensions provided by macros is to expand
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memory offsets to the full address range (32 or 64 bits) and to allow
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symbolic offsets such as ‘<samp><span class="samp">my_data + 4</span></samp>’ to be used in place of
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integer constants. For example, the architectural instruction
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<code>lbu</code> allows only a signed 16-bit offset, whereas the macro
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<code>lbu</code> allows code such as ‘<samp><span class="samp">lbu $4,array+32769($5)</span></samp>’.
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The implementation of these symbolic offsets depends on several factors,
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such as whether the assembler is generating SVR4-style PIC (selected by
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<samp><span class="option">-KPIC</span></samp>, see <a href="MIPS-Options.html#MIPS-Options">Assembler options</a>), the size of symbols
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(see <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>),
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and the small data limit (see <a href="MIPS-Small-Data.html#MIPS-Small-Data">Controlling the use of small data accesses</a>).
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<p><a name="index-g_t_0040code_007b_002eset-macro_007d-1486"></a><a name="index-g_t_0040code_007b_002eset-nomacro_007d-1487"></a>Sometimes it is undesirable to have one assembly instruction expand
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to several machine instructions. The directive <code>.set nomacro</code>
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tells the assembler to warn when this happens. <code>.set macro</code>
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restores the default behavior.
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<p><a name="index-g_t_0040code_007bat_007d-register_002c-MIPS-1488"></a><a name="index-g_t_0040code_007b_002eset-at_003d_0040var_007breg_007d_007d-1489"></a>Some macro instructions need a temporary register to store intermediate
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results. This register is usually <code>$1</code>, also known as <code>$at</code>,
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but it can be changed to any core register <var>reg</var> using
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<code>.set at=</code><var>reg</var>. Note that <code>$at</code> always refers
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to <code>$1</code> regardless of which register is being used as the
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temporary register.
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<p><a name="index-g_t_0040code_007b_002eset-at_007d-1490"></a><a name="index-g_t_0040code_007b_002eset-noat_007d-1491"></a>Implicit uses of the temporary register in macros could interfere with
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explicit uses in the assembly code. The assembler therefore warns
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whenever it sees an explicit use of the temporary register. The directive
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<code>.set noat</code> silences this warning while <code>.set at</code> restores
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the default behavior. It is safe to use <code>.set noat</code> while
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<code>.set nomacro</code> is in effect since single-instruction macros
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never need a temporary register.
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<p>Note that while the <span class="sc">gnu</span> assembler provides these macros for compatibility,
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it does not make any attempt to optimize them with the surrounding code.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> The term “macro” is somewhat overloaded here, since
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these macros have no relation to those defined by <code>.macro</code>,
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see <a href="Macro.html#Macro"><code>.macro</code></a>.</p>
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<hr></div>
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