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40.9 Symbols and NIL

Symbol data-block has the following format:

-------------------------------------------------------
|     5 (data-block words)     | Symbol Type (8 bits) |
-------------------------------------------------------
|                       Value Descriptor              |
-------------------------------------------------------
|  Hash Value (x86/amd64/sparc) Unused (other arch.)  |
-------------------------------------------------------
|                        Property List                |
-------------------------------------------------------
|                          Print Name                 |
-------------------------------------------------------
|                           Package                   |
-------------------------------------------------------

All of these slots are self-explanatory given what symbols must do in Common Lisp.

The issues with nil are that we want it to act like a symbol, and we need list operations such as CAR and CDR to be fast on it. CMU Common Lisp solves this by putting nil as the first object in static space, where other global values reside, so it has a known address in the system:

-------------------------------------------------------  <-- space
|     6 (data-block words)     |         0            |      start
-------------------------------------------------------
|     0 (data-block words)     | Symbol Type (8 bits) |
-------------------------------------------------------  <-- nil
|                           Value/CAR                 |
-------------------------------------------------------
|                         Hash Value/CDR              |
-------------------------------------------------------
|                         Property List               |
-------------------------------------------------------
|                           Print Name                |
-------------------------------------------------------
|                            Package                  |
-------------------------------------------------------
|                              ...                    |
-------------------------------------------------------

In addition, we make the list typed pointer to nil actually point past the header word of the nil symbol data-block. This has usefulness explained below. The value and hash-value of nil are nil. Therefore, any reference to nil used as a list has quick list type checking, and CAR and CDR can go right through the first and second words as if nil were a cons object.

When there is a reference to nil used as a symbol, the system adds offsets to the address the same as it does for any symbol. This works due to a combination of nil pointing past the symbol header-word and the chosen list and other-pointer type tags. The list type tag is four less than the other-pointer type tag, but nil points four additional bytes into its symbol data-block.


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