VMS Help SET, TERMINAL, Qualifiers, /ALTYPEAHD  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Causes the terminal driver to create a permanent, alternate type-
    ahead buffer. The system parameter TTY_ALTYPEAHD determines the
    size of the type-ahead buffer. This specification is effective
    at your next login and stays in effect until you reboot your VAX
    computer.

    To enable /ALTYPEAHD, you must also set the qualifier /TYPE_
    AHEAD.

    You should specify SETTERMINAL/PERMANENT/ALTYPEAHD in
    SYS$STARTUP:SYSTARTUP_VMS.COM for those communication lines that
    require this capability.

    To use this feature interactively, specify SET
    TERMINAL/PERMANENT/ALTYPEAHD. This specification is effective
    at your next login.