typedef struct _RTL_ACTIVATION_CONTEXT_STACK_FRAME
{
     PRTL_ACTIVATION_CONTEXT_STACK_FRAME Previous;
     _ACTIVATION_CONTEXT * ActivationContext;
     ULONG Flags;
} RTL_ACTIVATION_CONTEXT_STACK_FRAME, *PRTL_ACTIVATION_CONTEXT_STACK_FRAME;

Windows Vista Kernel Structures