INTEGRATED CIRCUIT (q.v.; IC) containing the central processing unit (CPU) of a COMPUTER, (q.v.), which can also be used independently in a wide range of applications. A microprocessor is built onto a single piece of silicon, called a chip, that is commonly no longer than 2.0 cm (0.8 in) along one side and no more than 0.1 cm (0.04 in) thick. Despite its small size, a microprocessor may be programmed to perform a great number of information-handling tasks. It can serve as a general-purpose computing machine for arithmetic operations, or for data- or word-processing use.

Personal computers (see PERSONAL COMPUTER,) usually contain one microprocessor. The more powerful computers used in engineering or graphics applications may have up to 10 microprocessors. The largest computers used in scientific and mathematical calculations contain hundreds or even thousands of microprocessors operating in parallel to speed up operation (see SUPERCOMPUTER,).

A microcontroller is an IC similar to a microprocessor that is used primarily in specific embedded applications. Microcontrollers are found in most electronic appliances from washing machines to cellular phones and cash registers. They are also used to control manufacturing equipment and processes.

Development.

The advent of the microprocessor was made possible by the progressive miniaturization of the IC and by advances in SEMICONDUCTOR, (q.v.) technology. The development of the first microprocessor began in 1969, when Intel engineer Marcian Edward “Ted” Hoff (1937–    ) proposed a single-chip, general-purpose CPU that could be programmed to perform most desired functions. The result was the first microprocessor, the 4004, announced by Intel in 1971. This microprocessor evolved into a series of increasingly powerful Intel chips—the 286, 386, 486, and in 1993, the Pentium—for the International Business Machines (IBM) Corp. personal computer (PC) and IBM-compatible PCs. Meanwhile, Motorola Corp. developed the 68000 series of chips for the Macintosh personal computer made by Apple Computer, Inc.

Structure.

Like any IC, a microprocessor is made of transistors, capacitors (qq.v.), and resistors connected by narrow patterned lines of metal called interconnects. A microprocessor may incorporate as few as 1000 or as many as several million transistors on its single chip. The components are combined into a number of more complex functional units. Programming software instructs the microprocessor where to find the data and what operation to perform (see COMPUTER OPERATING SYSTEM,). Additional software programs called compilers and assemblers are often needed to translate the computer program into a form that the microprocessor can understand. After the operation is complete, the microprocessor stores the result or sends it to its output terminals for transfer to another part of the system.

Performance.

One measure of the computing power of a microprocessor is its clock speed, measured in millions of cycles per second (MHz). It usually takes from one to seven cycles of a microprocessor’s internal clock to fully process an instruction. The faster the internal clock, the more instructions can be processed per unit of time. For the microprocessors in laptop and desktop computers, clock speeds are usually greater than 100 MHz. The fastest microprocessors can run at more than 500 MHz.

Another measure of performance is the number of instructions that can be processed per second, referred to as MIPS, for millions of instructions per second. The MIPS rating of a microprocessor depends on both the clock speed and the number of instructions that can be executed per clock cycle. Simple microprocessors can execute a maximum of one instruction per clock cycle. Advanced microprocessors can execute up to six or eight instructions per clock cycle. The relationship between clock speed and MIPS is not straightforward, however, because some instructions may take more than one clock cycle to execute, depending on the program. The MIPS value may therefore be less than the product of clock speed and the number of instructions that can be executed per cycle.

Still another indication of microprocessor speed is the length of a “word” it can accept, measured as the number of “bits” of information that can be transferred into it simultaneously. Long words allow the microprocessor to handle data and perform complex tasks more efficiently. The number of bits per word has been steadily increasing with the growth of circuit technology. Thus 4-, 8-, 16-, 32-, and 64-bit microprocessors and microcontrollers are now common. Most personal computers use 32-bit microprocessors. More powerful computers may use 64-bit chips. The 4-, 8-, or 16-bit devices are usually employed in simple embedded applications, such as microwave ovens, electric shavers, and televisions.

Memory.

A microprocessor chip typically contains memory for holding data and instructions. Data and instructions that do not change are often stored in Read-Only Memory (ROM). Most microprocessors also contain Random-Access Memory (RAM) to hold data and instructions used in a current application. This memory is often called “on-chip cache.” The circuits that operate on the data are called arithmetic/logic units. Also present are registers for holding data, instructions, and their locations, or addresses, while they are being processed. Interfaces connect the microprocessor with external memories and other systems as needed.

High-density computer memories, although themselves not microprocessors, are made using the same techniques. By the late 1990s, the density commonly attained was about 16 million bits (Mb) storage per individual circuit, but 64-Mb memories had already appeared and billion-bit units were being developed. Further advances will be determined by theoretical and practical limits on minimum transistor size.        L.G.,LINDA GEPPERT, Ph.D.

For further information on this topic, see the Bibliography, sections 541. Computer history–542. Computer applications.