/sys$common/syshlp/helplib.hlb System Services, $PURGE WS (Alpha Only), Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 start_va_64

    OpenVMS usage:address
    type:         quadword address
    access:       read only
    mechanism:    by value
    The starting virtual address of the pages to be purged from the
    working set. The specified virtual address will be rounded down
    to a CPU-specific page boundary.

 length_64

    OpenVMS usage:byte count
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by value
    Length of the virtual address space to be purged from the working
    set. The specified length will be rounded up to a CPU-specific
    page boundary so that it includes all CPU-specific pages in the
    requested range.