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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").