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Node:<a name="MMIX-mmixal">MMIX-mmixal</a>,

Previous:<a rel="previous" accesskey="p" href="MMIX-Syntax.html#MMIX-Syntax">MMIX-Syntax</a>,

Up:<a rel="up" accesskey="u" href="MMIX-Dependent.html#MMIX-Dependent">MMIX-Dependent</a>

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<h4 class="section">Differences to <code>mmixal</code></h4>



   <p>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-base.html#GREG-base">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 <code>mmo</code> 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>&amp;</code>, the fractional division operator

<code>//</code>, the logical not operator <code>!</code> and the modulus operator

<code>%</code> are not available.



   <p>Symbols are not global by default, unless the option

<code>--globalize-symbols</code> is passed.  Use the <code>.global</code> 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

<code>0x2000000000000000</code> or higher, maps to the <code>.data</code> section and

lower addresses map to the <code>.text</code> section (see <a href="MMIX-loc.html#MMIX-loc">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>

   <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

<code>IS</code> and <code>GREG</code> pseudo-operations must be specified in

upper-case characters.



   <p>There's no unicode support.



   <p>The following is a list of programs in <code>mmix.tar.gz</code>, available at

&lt;<code>http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html</code>&gt;, 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 <code>Done</code>. 

<br><dt><code>test.mms</code>

     <dd>Uses the serial operator <code>&amp;</code>. 

</dl>



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