typedef struct _SEGMENT
{
     PCONTROL_AREA ControlArea;
     ULONG TotalNumberOfPtes;
     ULONG NonExtendedPtes;
     ULONG NumberOfCommittedPages;
     UINT64 SizeOfSegment;
     union
     {
          PMMEXTEND_INFO ExtendInfo;
          PVOID BasedAddress;
     };
     EX_PUSH_LOCK SegmentLock;
     SEGMENT_FLAGS SegmentFlags;
     ULONG u1;
     ULONG u2;
     PMMPTE PrototypePte;
     MMPTE ThePtes[1];
} SEGMENT, *PSEGMENT;

Windows Vista Kernel Structures