mode bit

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.
    

mode bit

   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.

   [{Jargon File}]