typedef struct _NT_TIB
{
     PEXCEPTION_REGISTRATION_RECORD ExceptionList;
     PVOID StackBase;
     PVOID StackLimit;
     PVOID SubSystemTib;
     union
     {
          PVOID FiberData;
          ULONG Version;
     };
     PVOID ArbitraryUserPointer;
     PNT_TIB Self;
} NT_TIB, *PNT_TIB;

Windows Vista Kernel Structures