151 lines
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151 lines
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<title>Symbol-table - Untitled</title>
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<p>
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<a name="Symbol_002dtable"></a>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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</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
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<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>):
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“Symbols are stored and retrieved by means of a <span class="samp">ternary
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search trie</span>, following ideas of Bentley and Sedgewick. (See
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ACM–SIAM Symp. on Discrete Algorithms <span class="samp">8</span> (1997), 360–369;
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R.Sedgewick, <span class="samp">Algorithms in C</span> (Reading, Mass.
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Addison–Wesley, 1998), <span class="samp">15.4</span>.) 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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