This is an ASCII diagram of the data structure used to check pointervalidity.  It was provided by Dave Barrett <barrett@asgard.cs.colorado.edu>,and should be of use to others attempting to understand the code.The data structure in GC4.X is essentially the same.   -HB		Data Structure used by GC_base in gc3.7:			      21-Apr-94			 			    63                  LOG_TOP_SZ[11]  LOG_BOTTOM_SZ[10]   LOG_HBLKSIZE[13]   +------------------+----------------+------------------+------------------+ p:|                  |   TL_HASH(hi)  |                  |   HBLKDISPL(p)   |   +------------------+----------------+------------------+------------------+    \-----------------------HBLKPTR(p)-------------------/    \------------hi-------------------/                       \______ ________/ \________ _______/ \________ _______/                             V                   V                  V                             |                   |                  |           GC_top_index[]    |                   |                  |  ---      +--------------+   |                   |                  |    ^       |              |   |                   |                  |     |       |              |   |                   |                  |    TOP      +--------------+<--+                   |                  |       _SZ   +-<|      []      | *                     |                  |     (items)|  +--------------+  if 0 < bi< HBLKSIZE  |                  |      |    |  |              | then large object     |                  |      |    |  |              | starts at the bi'th   |                  |      v    |  |              | HBLK before p.        |             i    |     ---   |  +--------------+                       |          (word-  |           v                                         |         aligned) |       bi= |GET_BI(p){->hash_link}->key==hi          |                  |          v                                         |                  |           |   (bottom_index)  \ scratch_alloc'd     |                  |           |   ( struct  bi )  / by get_index()      |                  |     ---   +->+--------------+                       |                  |      ^       |              |                       |                  |  ^       |              |                       |                  | BOTTOM   |              |   ha=GET_HDR_ADDR(p)  |                  |_SZ(items)+--------------+<----------------------+          +-------+  |   +--<|   index[]    |                                  |           |   |   +--------------+                      GC_obj_map: v                |   |   |              |              from      / +-+-+-----+-+-+-+-+  ---   v   |   |              |              GC_add   < 0| | |     | | | | |   ^   ---  |   +--------------+             _map_entry \ +-+-+-----+-+-+-+-+   |        |   |   asc_link   |                          +-+-+-----+-+-+-+-+ MAXOBJSZ      |   +--------------+                      +-->| | |  j  | | | | |  +1         |   |     key      |                      |   +-+-+-----+-+-+-+-+   |        |   +--------------+                      |   +-+-+-----+-+-+-+-+   |       |   |  hash_link   |                      |   | | |     | | | | |   v       |   +--------------+                      |   +-+-+-----+-+-+-+-+  ---      |                                         |   |<--MAX_OFFSET--->|         |                                         |         (bytes)HDR(p)| GC_find_header(p)                       |   |<--MAP_ENTRIES-->|       |                           \ from        |    =HBLKSIZE/WORDSZ         |    (hdr) (struct hblkhdr) / alloc_hdr() |    (1024 on Alpha)      +-->+----------------------+              |    (8/16 bits each)GET_HDR(p)| word   hb_sz (words) |              |                    +----------------------+              |               | struct hblk *hb_next |              |          +----------------------+              |                 |mark_proc hb_mark_proc|              |          +----------------------+              |          | char * hb_map        |>-------------+          +----------------------+                     | ushort hb_obj_kind   |                     +----------------------+                     |   hb_last_reclaimed  |            ---      +----------------------+                  ^       |                      | MARK_BITS|       hb_marks[]     | *if hdr is free, hb_sz + DISCARD_WORDS_SZ(words)|                      |  is the size of a heap chunk (struct hblk)  v       |                      |  of at least MININCR*HBLKSIZE bytes (below), ---      +----------------------+  otherwise, size of each object in chunk.Dynamic data structures above are interleaved throughout the heap in blocks of size MININCR * HBLKSIZE bytes as done by gc_scratch_alloc which cannot befreed; free lists are used (e.g. alloc_hdr).  HBLKs's below are collected.	      (struct hblk)       ---      +----------------------+ < HBLKSIZE ---         ---          DISCARD_  ^       |garbage[DISCARD_WORDS]|   aligned   ^           ^ HDR_BYTES WORDS  |       |                      |             |           v (bytes)   (words)  |       +-----hb_body----------+ < WORDSZ    |          ---   ---     |       |                      |   aligned   |           ^     ^  |       |      Object 0        |             |           hb_sz |  |       |                      |           i |(word-    (words)|  |       |                      |      (bytes)|aligned)   v     |  |       + - - - - - - - - - - -+ ---         |          ---    |  |       |                      |  ^          |           ^     |  n *     |                      |  j (words)  |          hb_sz BODY_SZ  HBLKSIZE |      Object 1        |  v          v           |   (words) (bytes)  |                      |---------------          v   MAX_OFFSET  |       + - - - - - - - - - - -+                        ---  (bytes)  |       |                      | !All_INTERIOR_PTRS      ^     |  |       |                      | sets j only for       hb_sz   |  |       |      Object N        | valid object offsets.   |     |  v       |                      | All objects WORDSZ      v     v ---      +----------------------+ aligned.               ---   ---DISCARD_WORDS is normally zero.  Indeed the collector has not been testedwith another value in ages.