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11 KiB
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<!-- This file documents the BFD library.
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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Invariant Sections being "GNU General Public License" and "Funding
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Free Software", the Front-Cover texts being (a) (see below), and with
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the Back-Cover Texts being (b) (see below). A copy of the license is
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included in the section entitled "GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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<link href="BFD-Index.html#BFD-Index" rel="index" title="BFD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="mmo.html#mmo" rel="up" title="mmo">
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</head>
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<body lang="en">
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<a name="File-layout"></a>
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<div class="header">
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<p>
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Next: <a href="Symbol_002dtable.html#Symbol_002dtable" accesskey="n" rel="next">Symbol-table</a>, Previous: <a href="mmo.html#mmo" accesskey="p" rel="prev">mmo</a>, Up: <a href="mmo.html#mmo" accesskey="u" rel="up">mmo</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="File-layout-2"></a>
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<h4 class="subsection">3.5.1 File layout</h4>
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<p>The mmo file contents is not partitioned into named sections as
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with e.g. ELF. Memory areas is formed by specifying the
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location of the data that follows. Only the memory area
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‘<samp>0x0000…00</samp>’ to ‘<samp>0x01ff…ff</samp>’ is executable, so
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it is used for code (and constants) and the area
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‘<samp>0x2000…00</samp>’ to ‘<samp>0x20ff…ff</samp>’ is used for
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writable data. See <a href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>.
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</p>
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<p>There is provision for specifying “special data” of 65536
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different types. We use type 80 (decimal), arbitrarily chosen the
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same as the ELF <code>e_machine</code> number for MMIX, filling it with
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section information normally found in ELF objects. See <a href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>.
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</p>
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<p>Contents is entered as 32-bit words, xor:ed over previous
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contents, always zero-initialized. A word that starts with the
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byte ‘<samp>0x98</samp>’ forms a command called a ‘<samp>lopcode</samp>’, where
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the next byte distinguished between the thirteen lopcodes. The
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two remaining bytes, called the ‘<samp>Y</samp>’ and ‘<samp>Z</samp>’ fields, or
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the ‘<samp>YZ</samp>’ field (a 16-bit big-endian number), are used for
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various purposes different for each lopcode. As documented in
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<a href="http://mmix.cs.hm.edu/doc/mmixal.pdf">http://mmix.cs.hm.edu/doc/mmixal.pdf</a>,
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the lopcodes are:
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</p>
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<dl compact="compact">
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<dt><code>lop_quote</code></dt>
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<dd><p>0x98000001. The next word is contents, regardless of whether it
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starts with 0x98 or not.
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</p>
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</dd>
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<dt><code>lop_loc</code></dt>
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<dd><p>0x9801YYZZ, where ‘<samp>Z</samp>’ is 1 or 2. This is a location
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directive, setting the location for the next data to the next
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32-bit word (for <em>Z = 1</em>) or 64-bit word (for <em>Z = 2</em>),
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plus <em>Y * 2^56</em>. Normally ‘<samp>Y</samp>’ is 0 for the text segment
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and 2 for the data segment. Beware that the low bits of non-
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tetrabyte-aligned values are silently discarded when being
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automatically incremented and when storing contents (in contrast
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to e.g. its use as current location when followed by lop_fixo
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et al before the next possibly-quoted tetrabyte contents).
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</p>
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</dd>
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<dt><code>lop_skip</code></dt>
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<dd><p>0x9802YYZZ. Increase the current location by ‘<samp>YZ</samp>’ bytes.
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</p>
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</dd>
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<dt><code>lop_fixo</code></dt>
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<dd><p>0x9803YYZZ, where ‘<samp>Z</samp>’ is 1 or 2. Store the current location
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as 64 bits into the location pointed to by the next 32-bit
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(<em>Z = 1</em>) or 64-bit (<em>Z = 2</em>) word, plus <em>Y *
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2^56</em>.
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</p>
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</dd>
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<dt><code>lop_fixr</code></dt>
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<dd><p>0x9804YYZZ. ‘<samp>YZ</samp>’ is stored into the current location plus
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<em>2 - 4 * YZ</em>.
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</p>
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</dd>
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<dt><code>lop_fixrx</code></dt>
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<dd><p>0x980500ZZ. ‘<samp>Z</samp>’ is 16 or 24. A value ‘<samp>L</samp>’ derived from
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the following 32-bit word are used in a manner similar to
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‘<samp>YZ</samp>’ in lop_fixr: it is xor:ed into the current location
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minus <em>4 * L</em>. The first byte of the word is 0 or 1. If it
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is 1, then <em>L = (<var>lowest 24 bits of word</var>) - 2^Z</em>, if 0,
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then <em>L = (<var>lowest 24 bits of word</var>)</em>.
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</p>
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</dd>
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<dt><code>lop_file</code></dt>
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<dd><p>0x9806YYZZ. ‘<samp>Y</samp>’ is the file number, ‘<samp>Z</samp>’ is count of
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32-bit words. Set the file number to ‘<samp>Y</samp>’ and the line
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counter to 0. The next <em>Z * 4</em> bytes contain the file name,
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padded with zeros if the count is not a multiple of four. The
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same ‘<samp>Y</samp>’ may occur multiple times, but ‘<samp>Z</samp>’ must be 0 for
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all but the first occurrence.
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</p>
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</dd>
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<dt><code>lop_line</code></dt>
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<dd><p>0x9807YYZZ. ‘<samp>YZ</samp>’ is the line number. Together with
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lop_file, it forms the source location for the next 32-bit word.
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Note that for each non-lopcode 32-bit word, line numbers are
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assumed incremented by one.
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</p>
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</dd>
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<dt><code>lop_spec</code></dt>
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<dd><p>0x9808YYZZ. ‘<samp>YZ</samp>’ is the type number. Data until the next
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lopcode other than lop_quote forms special data of type ‘<samp>YZ</samp>’.
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See <a href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>.
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</p>
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<p>Other types than 80, (or type 80 with a content that does not
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parse) is stored in sections named <code>.MMIX.spec_data.<var>n</var></code>
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where <var>n</var> is the ‘<samp>YZ</samp>’-type. The flags for such a
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sections say not to allocate or load the data. The vma is 0.
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Contents of multiple occurrences of special data <var>n</var> is
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concatenated to the data of the previous lop_spec <var>n</var>s. The
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location in data or code at which the lop_spec occurred is lost.
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</p>
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</dd>
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<dt><code>lop_pre</code></dt>
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<dd><p>0x980901ZZ. The first lopcode in a file. The ‘<samp>Z</samp>’ field forms the
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length of header information in 32-bit words, where the first word
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tells the time in seconds since ‘<samp>00:00:00 GMT Jan 1 1970</samp>’.
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</p>
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</dd>
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<dt><code>lop_post</code></dt>
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<dd><p>0x980a00ZZ. <em>Z > 32</em>. This lopcode follows after all
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content-generating lopcodes in a program. The ‘<samp>Z</samp>’ field
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denotes the value of ‘<samp>rG</samp>’ at the beginning of the program.
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The following <em>256 - Z</em> big-endian 64-bit words are loaded
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into global registers ‘<samp>$G</samp>’ … ‘<samp>$255</samp>’.
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</p>
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</dd>
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<dt><code>lop_stab</code></dt>
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<dd><p>0x980b0000. The next-to-last lopcode in a program. Must follow
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immediately after the lop_post lopcode and its data. After this
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lopcode follows all symbols in a compressed format
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(see <a href="Symbol_002dtable.html#Symbol_002dtable">Symbol-table</a>).
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</p>
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</dd>
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<dt><code>lop_end</code></dt>
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<dd><p>0x980cYYZZ. The last lopcode in a program. It must follow the
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lop_stab lopcode and its data. The ‘<samp>YZ</samp>’ field contains the
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number of 32-bit words of symbol table information after the
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preceding lop_stab lopcode.
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</p></dd>
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</dl>
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<p>Note that the lopcode "fixups"; <code>lop_fixr</code>, <code>lop_fixrx</code> and
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<code>lop_fixo</code> are not generated by BFD, but are handled. They are
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generated by <code>mmixal</code>.
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</p>
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<p>This trivial one-label, one-instruction file:
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</p>
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<div class="example">
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<pre class="example"> :Main TRAP 1,2,3
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</pre></div>
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<p>can be represented this way in mmo:
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</p>
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<div class="example">
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<pre class="example"> 0x98090101 - lop_pre, one 32-bit word with timestamp.
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<timestamp>
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0x98010002 - lop_loc, text segment, using a 64-bit address.
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Note that mmixal does not emit this for the file above.
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0x00000000 - Address, high 32 bits.
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0x00000000 - Address, low 32 bits.
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0x98060002 - lop_file, 2 32-bit words for file-name.
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0x74657374 - "test"
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0x2e730000 - ".s\0\0"
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0x98070001 - lop_line, line 1.
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0x00010203 - TRAP 1,2,3
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0x980a00ff - lop_post, setting $255 to 0.
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0x00000000
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0x00000000
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0x980b0000 - lop_stab for ":Main" = 0, serial 1.
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0x203a4040 See <a href="Symbol_002dtable.html#Symbol_002dtable">Symbol-table</a>.
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0x10404020
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0x4d206120
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0x69016e00
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0x81000000
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0x980c0005 - lop_end; symbol table contained five 32-bit words.
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Symbol_002dtable.html#Symbol_002dtable" accesskey="n" rel="next">Symbol-table</a>, Previous: <a href="mmo.html#mmo" accesskey="p" rel="prev">mmo</a>, Up: <a href="mmo.html#mmo" accesskey="u" rel="up">mmo</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="BFD-Index.html#BFD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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