Computer is one of the most important invention humanity has ever created. And the first digital computer ever theorized was by Charles Babbage in 1830s. It was a mechanical computer and it laid down the foundation of modern computing so lets looks at its design.
a) If analytical engine was ever build it would look like this
On the analogy of a modern digital computer, the design principle of the Analytical Engine can be divided to:
1. Input. From 1836 on, punched cards (see the nearby photo) were the basic mechanism for feeding into the machine both numerical data and the instructions on how to manipulate them.
2. Output. Babbage’s basic mechanism was always a printing apparatus, but he had also considered graphic output devices even before he adopted punched cards for output as well as input.
3. Memory. For Babbage this was basically the number axes in the store, though he also developed the idea of a hierarchical memory system using punched cards for additional intermediate results that could not fit in the store.
4. Central Processing Unit. Babbage called this the Mill. Like modern processors it provided for storing the numbers being operated upon most immediately (registers); hardware mechanisms for subjecting those numbers to the basic arithmetic operations; control mechanisms for translating the user-oriented instructions supplied from outside into detailed control of internal hardware; and synchronization mechanisms (a clock) to carry out detailed steps in a carefully timed sequence. The control mechanism of the Analytical Engine must execute operations automatically and it consists of two parts: the lower level control mechanism, controlled by massive drums called barrels, and the higher level control mechanism, controlled by punched cards, developed by Jacquard for pattern-weaving looms and used extensively in the beginning of 1800s.
b) Analytical Engine Barrels
The sequence of smaller operations required to effect an arithmetical operation was controlled by massive drums called barrels (see the above figure). The barrels had studs fixed to their outer surface in much the same way as the pins of a music box drum or a barrel organ. The barrels orchestrated the internal motions of the engine and specify in detail how multiplication, division, addition, subtraction, and other arithmetic operations, are to be carried out. The barrel shown in the illustration has only several stud positions in each vertical row. In the actual machine, the barrels were much larger because they controlled and coordinated the interaction of thousands of parts. Each row could contain as many as 200 stud positions, and each barrel could have 50 to 100 separate rows. The overall machine had several different barrels controlling different sections. Naturally, the barrels had to be closely coordinated with one another. As a barrel turned, the studs activated specific motions of the mechanism and the position and arrangement of the studs determined the action and relative timing of each motion. The act of turning the drum thus automatically executed a sequence of motions to carry out the desired higher level operation. The process is internal to the Engine and logically invisible to the user. The technique is what in computing is now called a microprogram (though Babbage never used this term), which ensures that the lower level operations required to perform a function are executed automatically.
For higher level control mechanism, Babbage initially intended to use a big central barrel, to specify the steps of a calculation. This idea however seems impractical, because this will require changing the studs on the super barrel, which could be a cumbersome operation. The task of manually resetting studs in the central drum to tell the machine what to do was too cumbersome and error-prone to be reliable. Worse, the length of any set of instructions would be limited by the size of the drum.
His struggle with the problem of control led Babbage to a real breakthrough on June 30, 1836. He conceived of providing instructions and data to the engine not by turning number wheels and setting studs, but by means of punched card input, by means of cards, similar to these, used in the Jacquard looms. This did not render the central drum obsolete nor replace it. Punched cards provided a new top level of the control hierarchy that governed the positioning of the central drum. The central drum remained, but now with permanent sequences of instructions. It took on the function of micro-programming, as this of other barrels. If there were separate barrels for each operation, and a central barrel for controlling the operations drums, the punched card presents a way of instructing the machine (the central drum) as to which operations we wished to perform and in what order, i.e. high-level programming the Engine.
c) Punch cards used to program analytical engine
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