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.