Analytical engine

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The analytical engine, an important step in the history of computers, was the design of a mechanical general-purpose computer by English mathematician Charles Babbage. In its logical design the machine was essentially modern, anticipating the first completed general-purpose computers by about 100 years. It was first described in 1837. Babbage continued to refine the design until his death in 1871. Because of the complexity of the machine, the lack of project management, the expense of its construction, and the difficulty of assessing its value by Parliament relative to other projects being lobbied for, the engine was never built.

A partial construction of one of Babbage's machines was done by his son Henry and also more recently the construction of one of his simpler designs was done by the British Science Museum.[2] Indications are today that the machine could have been built successfully with the technology of the era if funding and political support had been stronger.



Babbage's first attempt at a mechanical computing device was the difference engine, a special-purpose calculator designed to tabulate logarithms and trigonometric functions by evaluating finite differences to create approximating polynomials. During this project he realized that a much more general design was possible and started work designing the analytical engine.

The input (programs and data) was to be provided to the machine via punched cards, a method being used at the time to direct mechanical looms such as the Jacquard loom. For output, the machine would have a printer, a curve plotter and a bell. The machine would also be able to punch numbers onto cards to be read in later. It employed ordinary base-10 fixed-point arithmetic.

There was to be a store (that is, a memory) capable of holding 1,000 numbers of 50 decimal digits each (ca. 20.7 kB). An arithmetical unit (the "mill") would be able to perform all four arithmetic operations, plus comparisons and optionally square roots. Initially it was conceived as a difference engine curved back upon itself, in a generally circular layout,[3] with the long store exiting off to one side. (Later drawings depict a regularized grid layout.)[4] Like the central processing unit (CPU) in a modern computer, the mill would rely upon its own internal procedures, to be stored in the form of pegs inserted into rotating drums called "barrels", to carry out some of the more complex instructions the user's program might specify.[5] (See microcode for the modern equivalent.)

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