Library /sys$common/syshlp/dbg$help.hlb DEBUG, Messages, IFLTUND  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 floating underflow at or near opcode_name

    Facility: DEBUG, VMS Debugger

    Explanation: While performing the arithmetic operation, a
    floating-point value became less than the smallest representable
    value for that data type.

    User Action: You should understand that the result of the
    operation is imprecise and may not be exactly what you expect.