typedef struct _DPH_BLOCK_INFORMATION
{
     ULONG StartStamp;
     PVOID Heap;
     ULONG RequestedSize;
     ULONG ActualSize;
     union
     {
          LIST_ENTRY FreeQueue;
          SINGLE_LIST_ENTRY FreePushList;
          WORD TraceIndex;
     };
     PVOID StackTrace;
     ULONG EndStamp;
} DPH_BLOCK_INFORMATION, *PDPH_BLOCK_INFORMATION;

Windows Vista Kernel Structures