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.