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| <h4 class="subsection">9.25.4 Differences to <code>mmixal</code></h4> |
| |
| <p><a name="index-mmixal-differences-1378"></a><a name="index-differences_002c-mmixal-1379"></a> |
| The binutils <code>as</code> and <code>ld</code> combination has a few |
| differences in function compared to <code>mmixal</code> (see <a href="mmixsite.html#mmixsite">mmixsite</a>). |
| |
| <p>The replacement of a symbol with a GREG-allocated register |
| (see <a href="GREG_002dbase.html#GREG_002dbase">GREG-base</a>) is not handled the exactly same way in |
| <code>as</code> as in <code>mmixal</code>. This is apparent in the |
| <code>mmixal</code> example file <code>inout.mms</code>, where different registers |
| with different offsets, eventually yielding the same address, are used in |
| the first instruction. This type of difference should however not affect |
| the function of any program unless it has specific assumptions about the |
| allocated register number. |
| |
| <p>Line numbers (in the ‘<samp><span class="samp">mmo</span></samp>’ object format) are currently not |
| supported. |
| |
| <p>Expression operator precedence is not that of mmixal: operator precedence |
| is that of the C programming language. It's recommended to use |
| parentheses to explicitly specify wanted operator precedence whenever more |
| than one type of operators are used. |
| |
| <p>The serialize unary operator <code>&</code>, the fractional division operator |
| ‘<samp><span class="samp">//</span></samp>’, the logical not operator <code>!</code> and the modulus operator |
| ‘<samp><span class="samp">%</span></samp>’ are not available. |
| |
| <p>Symbols are not global by default, unless the option |
| ‘<samp><span class="samp">--globalize-symbols</span></samp>’ is passed. Use the ‘<samp><span class="samp">.global</span></samp>’ directive to |
| globalize symbols (see <a href="Global.html#Global">Global</a>). |
| |
| <p>Operand syntax is a bit stricter with <code>as</code> than |
| <code>mmixal</code>. For example, you can't say <code>addu 1,2,3</code>, instead you |
| must write <code>addu $1,$2,3</code>. |
| |
| <p>You can't LOC to a lower address than those already visited |
| (i.e., “backwards”). |
| |
| <p>A LOC directive must come before any emitted code. |
| |
| <p>Predefined symbols are visible as file-local symbols after use. (In the |
| ELF file, that is—the linked mmo file has no notion of a file-local |
| symbol.) |
| |
| <p>Some mapping of constant expressions to sections in LOC expressions is |
| attempted, but that functionality is easily confused and should be avoided |
| unless compatibility with <code>mmixal</code> is required. A LOC expression to |
| ‘<samp><span class="samp">0x2000000000000000</span></samp>’ or higher, maps to the ‘<samp><span class="samp">.data</span></samp>’ section and |
| lower addresses map to the ‘<samp><span class="samp">.text</span></samp>’ section (see <a href="MMIX_002dloc.html#MMIX_002dloc">MMIX-loc</a>). |
| |
| <p>The code and data areas are each contiguous. Sparse programs with |
| far-away LOC directives will take up the same amount of space as a |
| contiguous program with zeros filled in the gaps between the LOC |
| directives. If you need sparse programs, you might try and get the wanted |
| effect with a linker script and splitting up the code parts into sections |
| (see <a href="Section.html#Section">Section</a>). Assembly code for this, to be compatible with |
| <code>mmixal</code>, would look something like: |
| <pre class="smallexample"> .if 0 |
| LOC away_expression |
| .else |
| .section away,"ax" |
| .fi |
| </pre> |
| <p><code>as</code> will not execute the LOC directive and <code>mmixal</code> |
| ignores the lines with <code>.</code>. This construct can be used generally to |
| help compatibility. |
| |
| <p>Symbols can't be defined twice–not even to the same value. |
| |
| <p>Instruction mnemonics are recognized case-insensitive, though the |
| ‘<samp><span class="samp">IS</span></samp>’ and ‘<samp><span class="samp">GREG</span></samp>’ pseudo-operations must be specified in |
| upper-case characters. |
| |
| <p>There's no unicode support. |
| |
| <p>The following is a list of programs in ‘<samp><span class="samp">mmix.tar.gz</span></samp>’, available at |
| <a href="http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html">http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html</a>, last |
| checked with the version dated 2001-08-25 (md5sum |
| c393470cfc86fac040487d22d2bf0172) that assemble with <code>mmixal</code> but do |
| not assemble with <code>as</code>: |
| |
| <dl> |
| <dt><code>silly.mms</code><dd>LOC to a previous address. |
| <br><dt><code>sim.mms</code><dd>Redefines symbol ‘<samp><span class="samp">Done</span></samp>’. |
| <br><dt><code>test.mms</code><dd>Uses the serial operator ‘<samp><span class="samp">&</span></samp>’. |
| </dl> |
| |
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