VMS Help RTL Routines, LIB$, LIB$FLT_UNDER, Returns  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    OpenVMS usage:longword_unsigned
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

    The old floating-point underflow enable setting (the previous
    contents of the SF$W_PSW[PSW$V_FU] in the caller's frame).