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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> |
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| 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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