A flag, usually in hardware, that selects between two (usually quITe different) modes of operation. The connotations are different from flag bIT in that mode bITs are mainly wrITten during a boot or set-up phase, are seldom explicITly read, and seldom change over the lifetime of an ordinary program. The classic example was the EBCDIC-vs.-ASCII bIT (#12) of the Program Status Word of the IBM 360. Another was the bIT on a PDP-12 that controlled whether IT ran the PDP-8 or the LINC instruction set. [Jargon File]