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155 lines
6.4 KiB
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155 lines
6.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Symbol-table - Untitled</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Untitled">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="mmo.html#mmo" title="mmo">
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<link rel="prev" href="File-layout.html#File-layout" title="File layout">
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<link rel="next" href="mmo-section-mapping.html#mmo-section-mapping" title="mmo section mapping">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the BFD library.
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Copyright (C) 1991-2015 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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any later version published by the Free Software Foundation; with the
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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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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Symbol-table"></a>
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<a name="Symbol_002dtable"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>,
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Previous: <a rel="previous" accesskey="p" href="File-layout.html#File-layout">File layout</a>,
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Up: <a rel="up" accesskey="u" href="mmo.html#mmo">mmo</a>
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<hr>
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</div>
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<h4 class="subsection">3.5.2 Symbol table format</h4>
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<p>From mmixal.w (or really, the generated mmixal.tex) in the
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MMIXware package which also contains the <samp><span class="command">mmix</span></samp> simulator:
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“Symbols are stored and retrieved by means of a ‘<samp><span class="samp">ternary
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search trie</span></samp>’, following ideas of Bentley and Sedgewick. (See
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ACM–SIAM Symp. on Discrete Algorithms ‘<samp><span class="samp">8</span></samp>’ (1997), 360–369;
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R.Sedgewick, ‘<samp><span class="samp">Algorithms in C</span></samp>’ (Reading, Mass.
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Addison–Wesley, 1998), ‘<samp><span class="samp">15.4</span></samp>’.) Each trie node stores a
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character, and there are branches to subtries for the cases where
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a given character is less than, equal to, or greater than the
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character in the trie. There also is a pointer to a symbol table
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entry if a symbol ends at the current node.”
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<p>So it's a tree encoded as a stream of bytes. The stream of bytes
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acts on a single virtual global symbol, adding and removing
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characters and signalling complete symbol points. Here, we read
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the stream and create symbols at the completion points.
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<p>First, there's a control byte <code>m</code>. If any of the listed bits
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in <code>m</code> is nonzero, we execute what stands at the right, in
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the listed order:
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<pre class="example"> (MMO3_LEFT)
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0x40 - Traverse left trie.
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(Read a new command byte and recurse.)
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(MMO3_SYMBITS)
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0x2f - Read the next byte as a character and store it in the
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current character position; increment character position.
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Test the bits of <code>m</code>:
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(MMO3_WCHAR)
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0x80 - The character is 16-bit (so read another byte,
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merge into current character.
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(MMO3_TYPEBITS)
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0xf - We have a complete symbol; parse the type, value
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and serial number and do what should be done
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with a symbol. The type and length information
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is in j = (m & 0xf).
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(MMO3_REGQUAL_BITS)
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j == 0xf: A register variable. The following
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byte tells which register.
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j <= 8: An absolute symbol. Read j bytes as the
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big-endian number the symbol equals.
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A j = 2 with two zero bytes denotes an
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unknown symbol.
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j > 8: As with j <= 8, but add (0x20 << 56)
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to the value in the following j - 8
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bytes.
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Then comes the serial number, as a variant of
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uleb128, but better named ubeb128:
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Read bytes and shift the previous value left 7
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(multiply by 128). Add in the new byte, repeat
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until a byte has bit 7 set. The serial number
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is the computed value minus 128.
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(MMO3_MIDDLE)
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0x20 - Traverse middle trie. (Read a new command byte
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and recurse.) Decrement character position.
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(MMO3_RIGHT)
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0x10 - Traverse right trie. (Read a new command byte and
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recurse.)
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</pre>
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<p>Let's look again at the <code>lop_stab</code> for the trivial file
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(see <a href="File-layout.html#File-layout">File layout</a>).
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<pre class="example"> 0x980b0000 - lop_stab for ":Main" = 0, serial 1.
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0x203a4040
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0x10404020
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0x4d206120
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0x69016e00
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0x81000000
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</pre>
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<p>This forms the trivial trie (note that the path between “:” and
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“M” is redundant):
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<pre class="example"> 203a ":"
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40 /
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40 /
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10 \
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40 /
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40 /
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204d "M"
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2061 "a"
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2069 "i"
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016e "n" is the last character in a full symbol, and
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with a value represented in one byte.
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00 The value is 0.
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81 The serial number is 1.
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</pre>
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</body></html>
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