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.
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