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mode bit

mode bit: n. 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.