Mode Bit: N. A {flag}, Usually In Hardware, That Selects Between Two (usually Quite Different) Modes Of Operation.

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: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.
-- The AI Hackers Dictionary