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- <a name="MIPS-Macros"></a>
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- <p>
- Next: <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" accesskey="n" rel="next">MIPS Symbol Sizes</a>, Previous: <a href="MIPS-Options.html#MIPS-Options" accesskey="p" rel="prev">MIPS Options</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-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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- <hr>
- <a name="High_002dlevel-assembly-macros"></a>
- <h4 class="subsection">9.27.2 High-level assembly macros</h4>
-
- <p>MIPS assemblers have traditionally provided a wider range of
- instructions than the MIPS architecture itself. These extra
- instructions are usually referred to as “macro” instructions
- <a name="DOCF2" href="#FOOT2"><sup>2</sup></a>.
- </p>
- <p>Some MIPS macro instructions extend an underlying architectural instruction
- while others are entirely new. An example of the former type is <code>and</code>,
- which allows the third operand to be either a register or an arbitrary
- immediate value. Examples of the latter type include <code>bgt</code>, which
- branches to the third operand when the first operand is greater than
- the second operand, and <code>ulh</code>, which implements an unaligned
- 2-byte load.
- </p>
- <p>One of the most common extensions provided by macros is to expand
- memory offsets to the full address range (32 or 64 bits) and to allow
- symbolic offsets such as ‘<samp>my_data + 4</samp>’ to be used in place of
- integer constants. For example, the architectural instruction
- <code>lbu</code> allows only a signed 16-bit offset, whereas the macro
- <code>lbu</code> allows code such as ‘<samp>lbu $4,array+32769($5)</samp>’.
- The implementation of these symbolic offsets depends on several factors,
- such as whether the assembler is generating SVR4-style PIC (selected by
- <samp>-KPIC</samp>, see <a href="MIPS-Options.html#MIPS-Options">Assembler options</a>), the size of symbols
- (see <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>),
- and the small data limit (see <a href="MIPS-Small-Data.html#MIPS-Small-Data">Controlling the use
- of small data accesses</a>).
- </p>
- <a name="index-_002eset-macro"></a>
- <a name="index-_002eset-nomacro"></a>
- <p>Sometimes it is undesirable to have one assembly instruction expand
- to several machine instructions. The directive <code>.set nomacro</code>
- tells the assembler to warn when this happens. <code>.set macro</code>
- restores the default behavior.
- </p>
- <a name="index-at-register_002c-MIPS"></a>
- <a name="index-_002eset-at_003dreg"></a>
- <p>Some macro instructions need a temporary register to store intermediate
- results. This register is usually <code>$1</code>, also known as <code>$at</code>,
- but it can be changed to any core register <var>reg</var> using
- <code>.set at=<var>reg</var></code>. Note that <code>$at</code> always refers
- to <code>$1</code> regardless of which register is being used as the
- temporary register.
- </p>
- <a name="index-_002eset-at"></a>
- <a name="index-_002eset-noat"></a>
- <p>Implicit uses of the temporary register in macros could interfere with
- explicit uses in the assembly code. The assembler therefore warns
- whenever it sees an explicit use of the temporary register. The directive
- <code>.set noat</code> silences this warning while <code>.set at</code> restores
- the default behavior. It is safe to use <code>.set noat</code> while
- <code>.set nomacro</code> is in effect since single-instruction macros
- never need a temporary register.
- </p>
- <p>Note that while the <small>GNU</small> assembler provides these macros for compatibility,
- it does not make any attempt to optimize them with the surrounding code.
- </p>
- <div class="footnote">
- <hr>
- <h4 class="footnotes-heading">Footnotes</h4>
-
- <h3><a name="FOOT2" href="#DOCF2">(2)</a></h3>
- <p>The term “macro” is somewhat overloaded here, since
- these macros have no relation to those defined by <code>.macro</code>,
- see <a href="Macro.html#Macro"><code>.macro</code></a>.</p>
- </div>
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