typedef struct _IMAGE_SECURITY_CONTEXT
{
     union
     {
          PVOID PageHashes;
          ULONG Value;
          ULONG SecurityBeingCreated: 1;
          ULONG SecurityMandatory: 1;
          ULONG Unused: 1;
          ULONG PageHashPointer: 29;
     };
} IMAGE_SECURITY_CONTEXT, *PIMAGE_SECURITY_CONTEXT;

Windows Vista Kernel Structures