typedef struct _RTL_STACK_TRACE_ENTRY
{
     union
     {
          PRTL_STACK_TRACE_ENTRY HashChain;
          PRTL_STACK_TRACE_ENTRY FreeChain;
     };
     ULONG TraceCount: 11;
     ULONG BlockDepth: 5;
     WORD IndexHigh;
     WORD Index;
     WORD Depth;
     VOID * BackTrace[32];
} RTL_STACK_TRACE_ENTRY, *PRTL_STACK_TRACE_ENTRY;

Windows Vista Kernel Structures