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| <hr> |
| <a name="Symbol-table-format"></a> |
| <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 ‘<samp>ternary |
| search trie</samp>’, following ideas of Bentley and Sedgewick. (See |
| ACM–SIAM Symp. on Discrete Algorithms ‘<samp>8</samp>’ (1997), 360–369; |
| R.Sedgewick, ‘<samp>Algorithms in C</samp>’ (Reading, Mass. |
| Addison–Wesley, 1998), ‘<samp>15.4</samp>’.) 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> |
| <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> |
| <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: |
| </p> |
| <div class="example"> |
| <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></div> |
| |
| <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>). |
| </p> |
| <div class="example"> |
| <pre class="example"> 0x980b0000 - lop_stab for ":Main" = 0, serial 1. |
| 0x203a4040 |
| 0x10404020 |
| 0x4d206120 |
| 0x69016e00 |
| 0x81000000 |
| </pre></div> |
| |
| <p>This forms the trivial trie (note that the path between “:” and |
| “M” is redundant): |
| </p> |
| <div class="example"> |
| <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></div> |
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