Mode Bit N. [common] A Flag, Usually In Hardware, That Selects Between Two (usually Quite Different) Modes Of Operation.

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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.