File:BabbageKeyValueCard.tiff

English: Tabular data card proposed for Charles Babbage's w:Analytical Engine, from page 121 of Babbage's Passages from the Life of a Philosopher. The rectangular punched card has two numeric fields, 2303 and 3622939 and would have represented one entry in a table of logarithms. Note that .3622939 is the base 10 logarithm of 2.303. As Babbage explained it, the cards comprising tables of special functions, such as logarithms, would be initially punched by the Analytic Engine. When the computer subsequently needed a logarithm, it would stop, ring a bell and display the number whose log was required. An attendant would retrieve that number's card and place it in the engine. The engine would then check that the correct card was loaded and "continually ringing a loud bell" if not. Babbage also observed that it might turn out to be be faster for the engine to compute special functions when needed rather than resort to tables.