Description

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. In 1995 about 4 billion microprocessors were produced worldwide.
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 as many as ten million 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 is about 600 megahertz (MHz)-about 600 million cycles per second-allowing about a billion computer instructions to be executed every second.
Information is stored in a CPU memory location called a register. Registers can be thought of as the CPU's tiny scratchpad, temporarily storing instructions or data. When a program is run, one register called the program counter keeps track of which program instruction comes next. The CPU's control unit coordinates and times the CPU's functions, and it retrieves the next instruction from memory.
In a typical sequence, the CPU locates the next instruction in the appropriate memory device. The instruction then travels along the bus from the computer's memory to the CPU, where it is stored in a special instruction register. Meanwhile, the program counter is incremented to prepare for the next instruction. The current instruction is analyzed by a decoder, which determines what the instruction will do. Any data the instruction needs are retrieved via the bus and placed in the CPU's registers. The CPU executes the instruction, and the results are stored in another register or copied to specific memory locations.

Future Technology

The technology of microprocessors and integrated-circuit fabrication is changing rapidly. Currently, the most sophisticated microprocessors contain about ten million transistors. By the year 2001, advanced microprocessors are expected to contain more than 50 million transistors, and about 800 million by 2010.
Lithographic techniques will also require improvements. By the year 2001, minimum element size will be less than 0.2 microns. At these dimensions, even short-wavelength ultraviolet light may not reach the necessary resolution. Alternative possibilities include using very narrow beams of electrons and ions or replacing optical lithography with lithography that uses X rays of extremely short wavelength. Using these technologies, clock speeds could increase to more than 1000 MHz by 2010.
It is expected that the limiting factor in microprocessor performance will be the behavior of the electrons themselves as they are propelled through the transistors. At extremely small dimensions, quantum effects due to the wavelike nature of electrons could dominate the behavior of transistors and circuits. New devices and circuit designs may be necessary as microprocessors approach atomic dimensions. Techniques including molecular-beam epitaxy, in which semiconductors are layered one atom at a time in an ultra-high-vacuum chamber, and scanning tunneling microscopy, whereby single atoms can be viewed and even moved with atomic precision, may be the tools needed to produce future generations of microprocessors.