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Define the word mode bit

mode bit n. [common] 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.

mode bit
     
        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]