typedef struct _HEAP_LIST_LOOKUP
{
     PHEAP_LIST_LOOKUP ExtendedLookup;
     ULONG ArraySize;
     ULONG ExtraItem;
     ULONG ItemCount;
     ULONG OutOfRangeItems;
     ULONG BaseIndex;
     PLIST_ENTRY ListHead;
     ULONG * ListsInUseUlong;
     PLIST_ENTRY * ListHints;
} HEAP_LIST_LOOKUP, *PHEAP_LIST_LOOKUP;

Windows Vista Kernel Structures