<html lang="en"> <head> <title>Symbol-table - Untitled</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="Untitled"> <meta name="generator" content="makeinfo 4.7"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="mmo.html#mmo" title="mmo"> <link rel="prev" href="File-layout.html#File-layout" title="File layout"> <link rel="next" href="mmo-section-mapping.html#mmo-section-mapping" title="mmo section mapping"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <!-- This file documents the BFD library. Copyright (C) 1991, 2000, 2001, 2003, 2006, 2007 Free Software Foundation, Inc. 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Copies published by the Free Software Foundation raise funds for GNU development.--> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family: serif; font-weight: normal; } --></style> </head> <body> <div class="node"> <p> <a name="Symbol_002dtable"></a>Next: <a rel="next" accesskey="n" href="mmo-section-mapping.html#mmo-section-mapping">mmo section mapping</a>, Previous: <a rel="previous" accesskey="p" href="File-layout.html#File-layout">File layout</a>, Up: <a rel="up" accesskey="u" href="mmo.html#mmo">mmo</a> <hr><br> </div> <h4 class="subsection">3.5.2 Symbol table format</h4> <p>From mmixal.w (or really, the generated mmixal.tex) in <a href="http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz">http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz</a>): “Symbols are stored and retrieved by means of a <span class="samp">ternary search trie</span>, following ideas of Bentley and Sedgewick. (See ACM–SIAM Symp. on Discrete Algorithms <span class="samp">8</span> (1997), 360–369; R.Sedgewick, <span class="samp">Algorithms in C</span> (Reading, Mass. Addison–Wesley, 1998), <span class="samp">15.4</span>.) Each trie node stores a character, and there are branches to subtries for the cases where a given character is less than, equal to, or greater than the character in the trie. There also is a pointer to a symbol table entry if a symbol ends at the current node.” <p>So it's a tree encoded as a stream of bytes. The stream of bytes acts on a single virtual global symbol, adding and removing characters and signalling complete symbol points. Here, we read the stream and create symbols at the completion points. <p>First, there's a control byte <code>m</code>. If any of the listed bits in <code>m</code> is nonzero, we execute what stands at the right, in the listed order: <pre class="example"> (MMO3_LEFT) 0x40 - Traverse left trie. (Read a new command byte and recurse.) (MMO3_SYMBITS) 0x2f - Read the next byte as a character and store it in the current character position; increment character position. Test the bits of <code>m</code>: (MMO3_WCHAR) 0x80 - The character is 16-bit (so read another byte, merge into current character. (MMO3_TYPEBITS) 0xf - We have a complete symbol; parse the type, value and serial number and do what should be done with a symbol. The type and length information is in j = (m & 0xf). (MMO3_REGQUAL_BITS) j == 0xf: A register variable. The following byte tells which register. j <= 8: An absolute symbol. Read j bytes as the big-endian number the symbol equals. A j = 2 with two zero bytes denotes an unknown symbol. j > 8: As with j <= 8, but add (0x20 << 56) to the value in the following j - 8 bytes. Then comes the serial number, as a variant of uleb128, but better named ubeb128: Read bytes and shift the previous value left 7 (multiply by 128). Add in the new byte, repeat until a byte has bit 7 set. The serial number is the computed value minus 128. (MMO3_MIDDLE) 0x20 - Traverse middle trie. (Read a new command byte and recurse.) Decrement character position. (MMO3_RIGHT) 0x10 - Traverse right trie. (Read a new command byte and recurse.) </pre> <p>Let's look again at the <code>lop_stab</code> for the trivial file (see <a href="File-layout.html#File-layout">File layout</a>). <pre class="example"> 0x980b0000 - lop_stab for ":Main" = 0, serial 1. 0x203a4040 0x10404020 0x4d206120 0x69016e00 0x81000000 </pre> <p>This forms the trivial trie (note that the path between “:” and “M” is redundant): <pre class="example"> 203a ":" 40 / 40 / 10 \ 40 / 40 / 204d "M" 2061 "a" 2069 "i" 016e "n" is the last character in a full symbol, and with a value represented in one byte. 00 The value is 0. 81 The serial number is 1. </pre> </body></html>