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ELECTRONIC COMPUTING MACHINES
Before describing electronic machines it is useful to consider computing machines in general. There are two main classes of computing equipment known respectively as analogue and digital machines. In analogue machines numbers are represented by physical quantities such as the angular rotation of a shaft of the current in a resistor, the numbers being the measures of those physical quantities. Examples of analogue machines are the planimeter, the differential analyzer, resistance network, etc. Because they are dependent on the accuracy of physical measurements, analogue machines are strictly limited in regard to the precision of which they are capable. Even with a well designed planimeter, for example, it is difficult to calculate areas to more than four significant figures. For most physical problems this does not constitute a severe restriction and a far more serious limitation of analogue machines is that they are, in general, special purpose devices. It is known that digital machines, on the other hand, work directly with numbers in digital form and are capable of performing exactly the fundamental operations of arithmetic, addition and subtraction, and usually also multiplication and division. The central processing unit (CPU) or central processor is the centre of any digital computer system since it coordinates and controls the activities of all the other units and performs all the arithmetic and logic processes to be applied to data. All program instructions to be examined should be held within the CPU, and all the data to be processed should be loaded first into this unit. The general processor is known to have three separate hardware sections: an internal or main memory, the arithmetic and logic unit, and a control unit. As has been said, all computers require a program, or list of instructions, to guide their activity. Sometimes the program is designed, or resides, within the hardware of the computer and cannot be changed without redesigning the hardware. More often, the program is entered as software into memory and may be easily removed or altered. In fact, computers that do not have a hardware program usually have several programs available in the memory at the same time. Some of these programs are employed in the general operation of the computer — that is, they control the executive functions. These programs are part of what is referred to as the OPERATING SYSTEM. Modern computers actually have two sections to the operating system. The first and most primitive section usually is stored in a permanent memory in the hardware. This portion of the system provides setup and connections among basic computer elements, so that more general programs can be entered from a keyboard or loaded from secondary memory. In today's personal computers this primitive operating system is the BIOS (basic input/output system) that comes with the hardware. The second portion of the system is located in secondary storage for execution when required. These more complex systems, such as DOS and UNIX, perform a wide variety of file-handling tasks using a very structured command set. Yet another layer of operating system uses graphics rather than typed commands. Examples are Windows and Apple products. All these levels have the common goal of making the machine elements interact for the purpose of transferring data. The microcomputers were the first computers to use a single microprocessor chip as the processor. Personal computers and small business computers are microcomputers. 8-bit microcomputers usually support only one user terminal and have a maximum of 64 KB of memory. 16-bit microcomputers may support several user terminals and usually have a maximum of one million bytes of memory. As the power of microcomputers increases, they can be used in two ways. They can be used either as a central computer (providing processing for several user terminals), or as a more powerful single computer for an individual user. A microprocessor is the tiny processor used, as mentioned above, in microcomputers. The microprocessor requires a power supply and memory to be a complete computer. Microprocessors are also used in minicomputers, mainframes and peripheral devices, as well as in all industrial and consumer products which use a computer. (3 500)
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