1     A CRT display from the Ferranti Mark 1

This display shows the contents of the PERM page (page 2) in a program running under Scheme A.

Although a CRT storage tube used an array of phosphor spots to store information, it could not in general be used to observe its current set of values, as the screen would be covered by a pick-up plate (to "read" the values) and the whole tube would be contained in a metal box to screen it from outside electrical interference. So display tubes were provided to show the current state of a selected storage tube, but laid out in a more useful way for viewing. (See display detail.)