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.