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.

