typedef struct _DPH_HEAP_BLOCK
{
     union
     {
          PDPH_HEAP_BLOCK pNextAlloc;
          LIST_ENTRY AvailableEntry;
          RTL_BALANCED_LINKS TableLinks;
     };
     UCHAR * pUserAllocation;
     UCHAR * pVirtualBlock;
     ULONG nVirtualBlockSize;
     ULONG nVirtualAccessSize;
     ULONG nUserRequestedSize;
     ULONG nUserActualSize;
     PVOID UserValue;
     ULONG UserFlags;
     PRTL_TRACE_BLOCK StackTrace;
     LIST_ENTRY AdjacencyEntry;
     UCHAR * pVirtualRegion;
} DPH_HEAP_BLOCK, *PDPH_HEAP_BLOCK;

Windows Vista Kernel Structures