typedef struct _HEAP_SEGMENT
{
     HEAP_ENTRY Entry;
     ULONG SegmentSignature;
     ULONG SegmentFlags;
     LIST_ENTRY SegmentListEntry;
     PHEAP Heap;
     PVOID BaseAddress;
     ULONG NumberOfPages;
     PHEAP_ENTRY FirstEntry;
     PHEAP_ENTRY LastValidEntry;
     ULONG NumberOfUnCommittedPages;
     ULONG NumberOfUnCommittedRanges;
     WORD SegmentAllocatorBackTraceIndex;
     WORD Reserved;
     LIST_ENTRY UCRSegmentList;
} HEAP_SEGMENT, *PHEAP_SEGMENT;

Windows Vista Kernel Structures