ip_value

    OpenVMS usage: IP value
    type:          quadword
    access:        read
    mechanism:     by reference

    Address of a location that contains the IP value. ip_value is
    used to find the unwind information block and the unwind
    information block operating system-specific data area address.

 unwind_ossd_p

    OpenVMS usage: address
    type:          quadword
    access:        write
    mechanism:     by reference

    Address of a quadword to receive the address of the operating
    system-specific data area. Note that the OSSD value is
    contained in the FDE unwind information and is therefore not
    writable. For more information, see the VSI Calling Standard
    guide.