VMS Help RTL Routines, MTH$, MTH$xLOG2, Returns  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    OpenVMS usage:floating_point
    type:         F_floating, D_floating, G_floating
    access:       write only
    mechanism:    by value

    The base 2 logarithm of floating-point-input-value. MTH$ALOG2
    returns an F-floating number. MTH$DLOG2 returns a D-floating
    number. MTH$GLOG2 returns a G-floating number.