typedef struct _KERNEL_STACK_CONTROL
{
     union
     {
          PKTRAP_FRAME PreviousTrapFrame;
          PVOID PreviousExceptionList;
     };
     ULONG StackControlFlags;
     ULONG PreviousLargeStack: 1;
     ULONG PreviousSegmentsPresent: 1;
     ULONG ExpandCalloutStack: 1;
     KERNEL_STACK_SEGMENT Previous;
} KERNEL_STACK_CONTROL, *PKERNEL_STACK_CONTROL;

Windows Vista Kernel Structures