mode bit
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.