Modern digital computers are well known to be extremely fast and accurate, but another attribute which is even more vital in the role of nerve centre to a modern nav./attack system is flexibility. The ground-based computer that carries out a different task every day of the week is now normal, but even in a military aircraft the computer is called upon to fulfil several roles: on the ground analysis and diagnosis, in the air navigation and weapons delivery, and often as flight director computer if the situation demands. Because of its characteristics of carrying out the computations serially in a central processor, a single decision point can alter the whole characteristics of the program; a single switch could make the computer carry out a fault diagnostic program or the navigation/attack program. By the action of one switch the meaning of every other switch and display can be altered. It is this characteristic above all that calls for special displays if full advantage is to be taken of the flexibility in the main computer interface.