Untitled Document: Symbol-table

Next: mmo section mapping, Previous: File layout, Up: mmo [Contents][Index]

#### 3.5.2 Symbol table format

From mmixal.w (or really, the generated mmixal.tex) in the MMIXware package which also contains the `mmix` simulator: “Symbols are stored and retrieved by means of a ‘`ternary search trie`’, following ideas of Bentley and Sedgewick. (See ACM–SIAM Symp. on Discrete Algorithms ‘`8`’ (1997), 360–369; R.Sedgewick, ‘`Algorithms in C`’ (Reading, Mass. Addison–Wesley, 1998), ‘`15.4`’.) 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.”

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.

First, there’s a control byte `m`. If any of the listed bits in `m` is nonzero, we execute what stands at the right, in the listed order:

     (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 m:

            (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.)

Let’s look again at the `lop_stab` for the trivial file (see File layout).

     0x980b0000 - lop_stab for ":Main" = 0, serial 1.
     0x203a4040
     0x10404020
     0x4d206120
     0x69016e00
     0x81000000

This forms the trivial trie (note that the path between “:” and “M” is redundant):

     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.

Next: mmo section mapping, Previous: File layout, Up: mmo [Contents][Index]