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.