From DECsystem-10 / DECSYSTEM-20 Processor Reference Manual (June, 1982)

    DMOVN    Double Move Negative

    Negate the doubleword from location E,E+1 interpreted in double
    precision floating point and move it to AC,AC+1.  If the memory
    doubleword is fixed point -2^70, set the Trap 1, Overflow and
    Carry 1 flags.  (Negating the equivalent floating point with
    fraction -1 and the maximum exponent sets the flags, but this is
    not a normalized number.)  If the memory doubleword is zero, set
    Carry 0 and Carry 1.

    [...]

    In the KI10 there is no overflow test as the KI10 lacks double
    precision fixed point instructions.  For floating point the
    overflow test is really unnecessary, as negatinv a correctly
    formatted floating point number cannot cause overflow.