Microprocessor

I

Introduction

Microprocessor, electronic circuit that functions as the central processing unit (CPU) of a computer, providing computational control. Microprocessors are also used in other advanced electronic systems, such as computer printers, automobiles, and jet airliners.

The microprocessor is one type of ultra-large-scale integrated circuit. Integrated circuits, also known as microchips or chips, are complex electronic circuits consisting of extremely tiny components formed on a single, thin, flat piece of material known as a semiconductor. Modern microprocessors incorporate transistors (which act as electronic amplifiers, oscillators, or, most commonly, switches), in addition to other components such as resistors, diodes, capacitors, and wires, all packed into an area about the size of a postage stamp.

A microprocessor consists of several different sections: The arithmetic/logic unit (ALU) performs calculations on numbers and makes logical decisions; the registers are special memory locations for storing temporary information much as a scratch pad does; the control unit deciphers programs; buses carry digital information throughout the chip and computer; and local memory supports on-chip computation. More complex microprocessors often contain other sections—such as sections of specialized memory, called cache memory, to speed up access to external data-storage devices. Modern microprocessors operate with bus widths of 64 bits (binary digits, or units of information represented as 1s and 0s), meaning that 64 bits of data can be transferred at the same time.

A crystal oscillator in the computer provides a clock signal to coordinate all activities of the microprocessor. The clock speed of the most advanced microprocessors allows billions of computer instructions to be executed every second.

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II

Computer Memory

Because the microprocessor alone cannot accommodate the large amount of memory required to store program instructions and data, such as the text in a word-processing program, transistors can be used as memory elements in combination with the microprocessor. Separate integrated circuits, called random-access memory (RAM) chips, which contain large numbers of transistors, are used in conjunction with the microprocessor to provide the needed memory. There are different kinds of random-access memory. Static RAM (SRAM) holds information as long as power is turned on and is usually used as cache memory because it operates very quickly. Another type of memory, dynamic RAM (DRAM), is slower than SRAM and must be periodically refreshed with electricity or the information it holds is lost. DRAM is more economical than SRAM and serves as the main memory element in most computers.

III

Microcontroller

A microprocessor is not a complete computer. It does not contain large amounts of memory or have the ability to communicate with input devices—such as keyboards, joysticks, and mice—or with output devices, such as monitors and printers. A different kind of integrated circuit, a microcontroller, is a complete computer on a chip, containing all of the elements of the basic microprocessor along with other specialized functions. Microcontrollers are used in video games, videocassette recorders (VCRs), automobiles, and other machines.

IV

Semiconductors

All integrated circuits are fabricated from semiconductors, substances whose ability to conduct electricity ranks between that of a conductor and that of a nonconductor, or insulator. Silicon is the most common semiconductor material. Because the electrical conductivity of a semiconductor can change according to the voltage applied to it, transistors made from semiconductors act like tiny switches that turn electrical current on and off in just a few nanoseconds (billionths of a second). This capability enables a computer to perform many billions of simple instructions each second and to complete complex tasks quickly.

The basic building block of most semiconductor devices is the diode, a junction, or union, of negative-type (n-type) and positive-type (p-type) materials. The terms n-type and p-type refer to semiconducting materials that have been doped—that is, have had their electrical properties altered by the controlled addition of very small quantities of impurities such as boron or phosphorus. In a diode, current flows in only one direction: across the junction from the p- to n-type material, and then only when the p-type material is at a higher voltage than the n-type. The voltage applied to the diode to create this condition is called the forward bias. The opposite voltage, for which current will not flow, is called the reverse bias. An integrated circuit contains millions of p-n junctions, each serving a specific purpose within the millions of electronic circuit elements. Proper placement and biasing of p- and n-type regions restrict the electrical current to the correct paths and ensure the proper operation of the entire chip.