Commit 880fbe03 authored by rtoy's avatar rtoy
Browse files

Add a note about vectors that the data portion of the header word is 

zero for normal vectors and non-zero for the key/value vector in a
hash table.
parent 87909e63

docs/internals/object.tex

+7 −1
Original line number Diff line number Diff line
@@ -193,7 +193,7 @@ data field of the header for ratio and complex numbers is two, one word each 
for the numerator and denominator or for the real and imaginary parts.



For vectors and data-blocks representing Lisp objects stored like vectors, the

system ignores the data portion of the header-word:

system (usually) ignores the data portion of the header-word:

\begin{verbatim}

----------------------------------------------------------------

| Unused Data (24 bits)   | Type (8 bits with low-tag) | 0 1 0 |
@@ -208,6 +208,12 @@ the header for complex arrays and vectors has a second word, following the 
header-word, the system uses for the fill pointer, so computing the length of

any array is the same code sequence.



For normal Lisp vectors, the data portion MUST be zero.  For hash

tables, a vector is used to store information about the hash key and

value, and the data portion is non-zero to indicate to GC that this is

the key/value vector for the hash table.  GENCGC uses this to

determine scavenge the key/value pairs correctly.  Cheney GC also uses

this to determine if rehashing (for EQ hash tables) is needed.





\section{Data-Blocks and Other-immediates Typing}