Modern digital computers are all conceptually similar, regardless of size. Nevertheless, they can be divided into several categories on the basis of cost and performance: the personal computer or microcomputer, a relatively low-cost machine, usually of desk-top size (though "laptops" are small enough to fit in a briefcase, and "palmtops" can fit into a pocket); the workstation, a microcomputer with enhanced graphics and communications capabilities that make it especially useful for office work; the minicomputer, generally too expensive for personal use, with capabilities suited to a business, school, or laboratory; and the mainframe computer, a large, expensive machine with the capability of serving the needs of major business enterprises, government departments, scientific research establishments, or the like (the largest and fastest of these are called supercomputers).
A digital computer is not a single machine: rather, it is a system composed of five distinct elements: (1) a central processing unit; (2) input devices; (3) memory storage devices; (4) output devices; and (5) a communications network, called a bus, which links all the elements of the system and connects the system to the external world.
Central Processing Unit (CPU)
The CPU may be a single chip or a series of chips that perform arithmetic and logical calculations and that time and control the operations of the other elements of the system. Miniaturization and integration techniques made possible the development of the microprocessor, a CPU chip that incorporates additional circuitry and memory. The result is smaller computers and reduced support circuitry. Microprocessors are used in personal computers.
Most CPU chips and microprocessors are composed of four functional sections: (1) an arithmetic/logic unit; (2) registers; (3) a control section; and (4) an internal bus. The arithmetic/logic unit gives the chip its calculating ability and permits arithmetical and logical operations. The registers are temporary storage areas that hold data, keep track of instructions, and hold the location and results of these operations. The control section has three principal duties.
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The Science Of Computers
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