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185 lines
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This file documents the BFD library.
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Copyright (C) 1991, 2000, 2001, 2003, 2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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Free Software'', the Front-Cover texts being (a) (see below), and with
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<div class="node">
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<p>
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<a name="File-layout"></a>Next: <a rel="next" accesskey="n" href="Symbol_002dtable.html#Symbol_002dtable">Symbol-table</a>,
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Previous: <a rel="previous" accesskey="p" href="mmo.html#mmo">mmo</a>,
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Up: <a rel="up" accesskey="u" href="mmo.html#mmo">mmo</a>
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<hr><br>
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</div>
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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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<span class="samp">0x0000...00</span> to <span class="samp">0x01ff...ff</span> is executable, so
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it is used for code (and constants) and the area
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<span class="samp">0x2000...00</span> to <span class="samp">0x20ff...ff</span> 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>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>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 <span class="samp">0x98</span> forms a command called a <span class="samp">lopcode</span>, where
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the next byte distinguished between the thirteen lopcodes. The
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two remaining bytes, called the <span class="samp">Y</span> and <span class="samp">Z</span> fields, or
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the <span class="samp">YZ</span> 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://www-cs-faculty.stanford.edu/~knuth/mmixal-intro.ps.gz">http://www-cs-faculty.stanford.edu/~knuth/mmixal-intro.ps.gz</a>,
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the lopcodes are:
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<dl>
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<dt><code>lop_quote</code><dd>0x98000001. The next word is contents, regardless of whether it
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starts with 0x98 or not.
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<br><dt><code>lop_loc</code><dd>0x9801YYZZ, where <span class="samp">Z</span> 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 Z = 1) or 64-bit word (for Z = 2),
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plus Y * 2^56. Normally <span class="samp">Y</span> is 0 for the text segment
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and 2 for the data segment.
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<br><dt><code>lop_skip</code><dd>0x9802YYZZ. Increase the current location by <span class="samp">YZ</span> bytes.
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<br><dt><code>lop_fixo</code><dd>0x9803YYZZ, where <span class="samp">Z</span> 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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(Z = 1) or 64-bit (Z = 2) word, plus Y *
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2^56.
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<br><dt><code>lop_fixr</code><dd>0x9804YYZZ. <span class="samp">YZ</span> is stored into the current location plus
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2 - 4 * YZ.
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<br><dt><code>lop_fixrx</code><dd>0x980500ZZ. <span class="samp">Z</span> is 16 or 24. A value <span class="samp">L</span> derived from
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the following 32-bit word are used in a manner similar to
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<span class="samp">YZ</span> in lop_fixr: it is xor:ed into the current location
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minus 4 * L. The first byte of the word is 0 or 1. If it
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is 1, then L = (<var>lowest 24 bits of word</var>) - 2^Z, if 0,
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then L = (<var>lowest 24 bits of word</var>).
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<br><dt><code>lop_file</code><dd>0x9806YYZZ. <span class="samp">Y</span> is the file number, <span class="samp">Z</span> is count of
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32-bit words. Set the file number to <span class="samp">Y</span> and the line
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counter to 0. The next Z * 4 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 <span class="samp">Y</span> may occur multiple times, but <span class="samp">Z</span> must be 0 for
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all but the first occurrence.
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<br><dt><code>lop_line</code><dd>0x9807YYZZ. <span class="samp">YZ</span> 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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<br><dt><code>lop_spec</code><dd>0x9808YYZZ. <span class="samp">YZ</span> is the type number. Data until the next
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lopcode other than lop_quote forms special data of type <span class="samp">YZ</span>.
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See <a href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>.
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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.</code><var>n</var>
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where <var>n</var> is the <span class="samp">YZ</span>-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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<br><dt><code>lop_pre</code><dd>0x980901ZZ. The first lopcode in a file. The <span class="samp">Z</span> 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 <span class="samp">00:00:00 GMT Jan 1 1970</span>.
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<br><dt><code>lop_post</code><dd>0x980a00ZZ. Z > 32. This lopcode follows after all
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content-generating lopcodes in a program. The <span class="samp">Z</span> field
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denotes the value of <span class="samp">rG</span> at the beginning of the program.
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The following 256 - Z big-endian 64-bit words are loaded
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into global registers <span class="samp">$G</span> <small class="dots">...</small> <span class="samp">$255</span>.
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<br><dt><code>lop_stab</code><dd>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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<br><dt><code>lop_end</code><dd>0x980cYYZZ. The last lopcode in a program. It must follow the
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lop_stab lopcode and its data. The <span class="samp">YZ</span> 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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</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>This trivial one-label, one-instruction file:
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<pre class="example"> :Main TRAP 1,2,3
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</pre>
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<p>can be represented this way in mmo:
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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>
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