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In every case, the object is the same: to translate information —letters, numbers, images, sounds, marks or simply magnetized ink on a check—into patterns of electrical pulses that are comprehensible to the computer.
Memory-Babbage dubbed this unit the store, and it does just that; it stores information until it is needed by other parts of the machine. For nearly two decades the most popular memory in modern computers has been the magnetic core variety. It consists of thousands of tiny iron rings, each one encircling an intersection of two wires in a rectangular grid made up of thousands of wires. Depending on the direction of current in the two wires that pass through its hole, each doughnut is magnetized in either a clockwise or counterclockwise direction. This represents either a 1 or a 0—a "bit" (for binary digit) of information. Because each core has a specific location in the precisely designed grid, it can be "addressed" almost instantly: information can be read from any doughnut by means of a third wire passing through each core. These fragile and expensive core memories are now being replaced by semiconductor memories on chips. In addition to such "random access" memories, as they are called, computers have auxiliary memories in the form of magnetic tape or discs. These have the advantage of large capacity and low cost, and are used to store information in bulk.
Arithmetic and Logic. To handle, direct and process the flood of information, the computer relies on this unit, which Babbage dubbed the mill. It is here that the computer does its number crunching and data manipulation.
Control. This is the computer's traffic cop. It gets instructions stored in the memory section and interprets them; it regulates the memory and arithmetic-logic sections and the flow of information between them, and orders processed data to move from the memory to the output section.
Output. Processed data are translated by this section into electrical impulses that can control an almost endless variety of devices. Thus the output may take the form of words or numbers "read out" on high-speed printers or glowing cathode-ray tubes. It can also emerge as an artificial voice, commands to an airplane's steering mechanism or even directions to another computer.
While Babbage's engine also included the concept of programmed instructions, today's machines are significantly different as a result of a refinement proposed in the 1940s by the Hungarian-born mathematical genius John von Neumann. After seeing ENIAC, he suggested "writing" both the data to be handled by the computer and the instructions for doing the job in the same memory and using the same code. It was a key innovation in computer theory, for it meant that the machine could cope with instructions just as if they were data. As Texas Instruments' William C. Holton explains, "A program can therefore alter another program or even itself."