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139 lines
6.7 KiB
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<title>MMIX-mmixal - Using as</title>
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<link rel="up" href="MMIX_002dDependent.html#MMIX_002dDependent" title="MMIX-Dependent">
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 Free Software Foundation, Inc.
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<a name="MMIX-mmixal"></a>
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<a name="MMIX_002dmmixal"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="MMIX_002dSyntax.html#MMIX_002dSyntax">MMIX-Syntax</a>,
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Up: <a rel="up" accesskey="u" href="MMIX_002dDependent.html#MMIX_002dDependent">MMIX-Dependent</a>
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<hr>
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</div>
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<h4 class="subsection">9.28.4 Differences to <code>mmixal</code></h4>
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<p><a name="index-mmixal-differences-1625"></a><a name="index-differences_002c-mmixal-1626"></a>
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The binutils <code>as</code> and <code>ld</code> combination has a few
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differences in function compared to <code>mmixal</code> (see <a href="mmixsite.html#mmixsite">mmixsite</a>).
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<p>The replacement of a symbol with a GREG-allocated register
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(see <a href="GREG_002dbase.html#GREG_002dbase">GREG-base</a>) is not handled the exactly same way in
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<code>as</code> as in <code>mmixal</code>. This is apparent in the
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<code>mmixal</code> example file <code>inout.mms</code>, where different registers
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with different offsets, eventually yielding the same address, are used in
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the first instruction. This type of difference should however not affect
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the function of any program unless it has specific assumptions about the
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allocated register number.
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<p>Line numbers (in the ‘<samp><span class="samp">mmo</span></samp>’ object format) are currently not
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supported.
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<p>Expression operator precedence is not that of mmixal: operator precedence
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is that of the C programming language. It's recommended to use
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parentheses to explicitly specify wanted operator precedence whenever more
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than one type of operators are used.
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<p>The serialize unary operator <code>&</code>, the fractional division operator
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‘<samp><span class="samp">//</span></samp>’, the logical not operator <code>!</code> and the modulus operator
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‘<samp><span class="samp">%</span></samp>’ are not available.
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<p>Symbols are not global by default, unless the option
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‘<samp><span class="samp">--globalize-symbols</span></samp>’ is passed. Use the ‘<samp><span class="samp">.global</span></samp>’ directive to
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globalize symbols (see <a href="Global.html#Global">Global</a>).
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<p>Operand syntax is a bit stricter with <code>as</code> than
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<code>mmixal</code>. For example, you can't say <code>addu 1,2,3</code>, instead you
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must write <code>addu $1,$2,3</code>.
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<p>You can't LOC to a lower address than those already visited
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(i.e., “backwards”).
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<p>A LOC directive must come before any emitted code.
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<p>Predefined symbols are visible as file-local symbols after use. (In the
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ELF file, that is—the linked mmo file has no notion of a file-local
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symbol.)
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<p>Some mapping of constant expressions to sections in LOC expressions is
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attempted, but that functionality is easily confused and should be avoided
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unless compatibility with <code>mmixal</code> is required. A LOC expression to
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‘<samp><span class="samp">0x2000000000000000</span></samp>’ or higher, maps to the ‘<samp><span class="samp">.data</span></samp>’ section and
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lower addresses map to the ‘<samp><span class="samp">.text</span></samp>’ section (see <a href="MMIX_002dloc.html#MMIX_002dloc">MMIX-loc</a>).
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<p>The code and data areas are each contiguous. Sparse programs with
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far-away LOC directives will take up the same amount of space as a
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contiguous program with zeros filled in the gaps between the LOC
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directives. If you need sparse programs, you might try and get the wanted
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effect with a linker script and splitting up the code parts into sections
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(see <a href="Section.html#Section">Section</a>). Assembly code for this, to be compatible with
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<code>mmixal</code>, would look something like:
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<pre class="smallexample"> .if 0
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LOC away_expression
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.else
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.section away,"ax"
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.fi
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</pre>
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<p><code>as</code> will not execute the LOC directive and <code>mmixal</code>
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ignores the lines with <code>.</code>. This construct can be used generally to
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help compatibility.
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<p>Symbols can't be defined twice–not even to the same value.
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<p>Instruction mnemonics are recognized case-insensitive, though the
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‘<samp><span class="samp">IS</span></samp>’ and ‘<samp><span class="samp">GREG</span></samp>’ pseudo-operations must be specified in
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upper-case characters.
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<p>There's no unicode support.
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<p>The following is a list of programs in ‘<samp><span class="samp">mmix.tar.gz</span></samp>’, available at
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<a href="http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html">http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html</a>, last
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checked with the version dated 2001-08-25 (md5sum
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c393470cfc86fac040487d22d2bf0172) that assemble with <code>mmixal</code> but do
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not assemble with <code>as</code>:
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<dl>
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<dt><code>silly.mms</code><dd>LOC to a previous address.
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<br><dt><code>sim.mms</code><dd>Redefines symbol ‘<samp><span class="samp">Done</span></samp>’.
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<br><dt><code>test.mms</code><dd>Uses the serial operator ‘<samp><span class="samp">&</span></samp>’.
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</dl>
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