Untitled Document: Symbol-table

Untitled Document: Symbol-table

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.