UUO

 UUO (you-you-oh) [short for "Un-Used Operation"] n. A
       DEC-10 system monitor call. The term "Un-Used Operation"
       comes from the fact that, on DEC-10 systems, monitor calls are
       implemented as invalid or illegal machine instructions, which cause
       traps to the monitor (see TRAP). The SAIL manual describing the
       available UUO's has a cover picture showing an unidentified underwater
       object. See YOYO. [Note: DEC sales people have since decided that
       "Un-Used Operation" sounds bad, so UUO now stands for
       "Unimplemented User Operation".] Tenex and Twenex systems
       use the JSYS machine instruction (q.v.), which is halfway between a
       legal machine instruction and a UUO, since KA-10 Tenices implement it
       as a hardware instruction which can be used as an ordinary subroutine
       call (sort of a "pure JSR").